Advances in Colloid and Interface Science (v.123-126, #C)
Editorial Board (iii).
by P. Kumar (1-4).
Extended DLVO theory: Electrostatic and non-electrostatic forces in oxide suspensions by M. Boström; V. Deniz; G.V. Franks; B.W. Ninham (5-15).
According to classical DLVO theory all ions of background salt solution with the same ionic charge should result in the same effective force between colloidal particles. However, the relative effectiveness of different ions in influencing forces between ceramic oxide surfaces follows either a reversed Hofmeister sequence or a direct Hofmeister sequence depending on the type of oxide and if the pH is above or below the isoelectric point (iep). This ion specificity is inexplicable in classical double layer theory that deals only with pure electrostatic forces acting between the ions and the colloidal particles. A theoretical explanation is given here. At, and above, biological salt concentrations other, non-electrostatic (NES) ion specific forces act that are ignored in such modeling. In this overview we present the basic theory for the double layer near a single oxide surface and for the extended DLVO forces between oxide colloidal particles that accounts for these NES forces. We will demonstrate that ion specificity can be understood to a large degree once NES forces are included consistently in the non-linear theory.
Synthesis, functionalisation and characterisation of mesoporous materials and sol–gel glasses for applications in catalysis, adsorption and photonics by Gisle Øye; Wilhelm R. Glomm; Torbjørn Vrålstad; Sondre Volden; Heléne Magnusson; Michael Stöcker; Johan Sjöblom (17-32).
In this review, synthesis and functionalisation approaches for ordered mesoporous materials and sol–gel glasses are described. Catalytic and adsorption applications are emphasised for the ordered mesoporous materials, while optical applications are the focus for sol–gel glasses.
Keywords: Ordered mesoporous materials; Sol–gel glasses; Functionalisation; Catalytic materials; Photonic materials;
The influence of surface-active media on the mechanical properties of materials by Eugene D. Shchukin (33-47).
This survey summarizes studies dealing with the influence of surface-active media on the mechanical behavior of materials that have been carried out at the Institute of Physical Chemistry of the Russian Academy of Sciences, at the Department of Chemistry of Moscow State University and at the Department of Geography and Environmental Engineering of the Johns Hopkins University, and presented partially at the SIS Symposia. The phenomena of the environment-sensitive mechanical behavior take place for all kinds of solids under the effect of adsorption layers and due to the contact with liquid phases containing some specific components which are physically–chemically akin with respect to a given solid and cause lowering of the surface energy of the solid and weakening of the cohesive forces in the surface (interfacial) layer. Studies of these effects and their atomic-molecular mechanisms provide basis for control, prevention or utilization of these phenomena.
Keywords: Adsorption; Surfactants; Mechanical properties;
Foam formation and mitigation in a three-phase gas–liquid–particulate system by Krishna Vijayaraghavan; Alex Nikolov; Darsh Wasan (49-61).
Foaming is of great concern in a number of industrial processes involving three-phase gas–liquid-finely divided solid systems such as those encountered in the vitrification of highly radioactive nuclear waste slurries and sludges. Recent work has clearly shown that the surface properties of the particles such as hydrophilicity, hydrophobicity or biphilicity (i.e. partially wetted by water) are the cause of foamability and foam stability. The literature data on particles causing foaminess and foam stability in the absence of any surfactant are rather scarce. This paper presents experimental observations on aqueous foams with polyhedral structures containing over 90% air generated due to the presence of irregularly-shaped fine crystalline particles of sodium chloride which were modified into amphiphilic particles by physical adsorption of a cationic surfactant. Cross-polarized light microscopy was used to visualize the physical adsorption of the surfactant on the crystal surface. It is shown that these biphilic or amphiphilic particles attach to the air bubble surface and prevent the coalescence of bubbles, thereby extending the life of the foam. The foaming power of solid particles increases with an increase in the concentration of amphiphilic particles, and a maximum in foaminess is observed which is due to two competing effects. Amphiphilic particles promote foamability by attachment to the bubble surfaces as individual particles and foam inhibition due to the clustering or flocculation of particles in the bulk at high particle concentrations. We studied the adsorption of amphiphilic particles at a planar air–water surface and found that the degree of foamability correlates well with the particle coverage (i.e. adsorption density) at the air–liquid surface. An exploratory study was also conducted using an antifoam recently developed by IIT researchers to mitigate foaming in particle-laden gas–liquid systems.
Keywords: Adsorption; Crystal modification; Amphiphilic particle; Foaminess; Flocculation; Antifoaming;
The selective partitioning of the oligomers of polyethoxylated surfactant mixtures between interface and oil and water bulk phases by Alain Graciaa; José Andérez; Carlos Bracho; Jean Lachaise; Jean-Louis Salager; Laura Tolosa; Fredy Ysambertt (63-73).
Because their affinities for the oil and water phases vary considerably with the number of ethylene oxide units in their hydrophilic group, the ethoxylated nonionic species occurring in commercial products tend to behave in a non-collective way, with the low ethoxylation oligomers partitioning mostly in the oil phase. This results in a surfactant mixture at the interface which is more hydrophilic than the one which was introduced in the system in the first place. The pseudophase model is used to study the partitioning in Winsor III type systems, and to estimate the deviation of the interfacial mixture composition from the overall one. New results indicate that the selective partitioning into the oil phase increases when the oil phase becomes aromatic, when the total surfactant concentration decreases and when the water-to-oil ratio decreases.
Keywords: Formulation; Nonionic; Surfactant; Partitioning; Coefficient;
Aggregation of water-soluble block copolymers in aqueous solutions: Recent trends by Kenichi Nakashima; Pratap Bahadur (75-96).
This review summarizes recent literature and some of our own results on aggregation behavior on water-soluble block copolymers belonging to three different classes viz. hydrophilic–hydrophobic (AB, ABA and BAB) block copolymers, double hydrophilic block copolymers (DHBCs) and ABC triblock copolymers. In the case of amphiphilic copolymers, special attention has been focussed on aggregation of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymers (Pluronics®) and their aggregation in aqueous solutions at different temperatures as well as in the presence of various additives. Recent studies based on modern techniques viz. scattering (static and dynamic light scattering and small angle neutron scattering), spectral methods, e.g., fluorescence (static and time resolved), nuclear magnetic resonance and Fourier transform infrared spectroscopies, thermal methods e.g., differential scanning calorimetry and isothermal titration calorimetry, cryotransmission electron microscopy, ultrasonic absorption along with general physical properties like surface tension, viscosity and dye solubilization are summarized. For the DHBCs where one of the blocks is usually a polyion, complex formation by adding oppositely charged ions induces the formation of nanoaggregates. Characterization of such nanoaggregates of polyion complexes of DHBCs and their potential use for incorporation of ionic solutes in the micellar core are reviewed. The formation and characteristics of core-shell-corona micelles of ABC triblock copolymers and their applications as vehicles for controlled drug release are also discussed.
Keywords: Block copolymers; Micelles; Nanoaggregates; Pluronics;
Effect of salts and organic additives on the solubility of proteins in aqueous solutions by Eli Ruckenstein; Ivan L. Shulgin (97-103).
The goal of this review is to examine the effect of salts and organic additives on the solubility of proteins in aqueous mixed solvents. The focus is on the correlation between the aqueous protein solubility and the osmotic second virial coefficient or the preferential binding parameter. First, several approaches which connect the solubility and the osmotic second virial coefficient are presented. Most of the experimental and theoretical results correlate the solubility and the osmotic second virial coefficient in the presence of salts. The correlation of the aqueous protein solubility with the osmotic second virial coefficient when the cosolvent is an organic component requires additional research. Second, the aqueous protein solubility is correlated with the preferential binding parameter on the basis of a theory developed by the authors of the present review. This theory can predict (i) the salting-in or -out effect of a cosolvent and (ii) the initial slope of the solubility curve. Good agreement was obtained between theoretical predictions and experimental results.
Keywords: Protein solubility; Osmotic second virial coefficient; Preferential binding parameter; Mixed solvent; Salting-in and salting-out;
Formation of polyelectrolyte–surfactant complexes on surfaces by Tommy Nylander; Yulia Samoshina; Björn Lindman (105-123).
The interfacial behavior of polyelectrolytes, mainly cationic with varying content of amphiphilic groups, and their complexes with oppositely charged surfactant are discussed. Both the kinetics and the reversibility aspect of the adsorption are considered. The structure of adsorbed layer formed was found to be dependent not only on the bulk solution phase behavior, but also on the pre-applied conditions, i.e., the path used to obtain a particular solution condition (e.g., by changing pH and concentration of salt, surfactant or polymer). Polyelectrolyte adsorption appears only partly reversible, due to its high affinity to the surface, which slows down the adsorption process. In general, relaxation occurs more easily if the direction of the process is from low to high surface coverage. Association of the surfactant with the polymer, which depends on the surfactant concentration, can completely alter the interfacial behavior. Maximum adsorption occurs generally at a surfactant concentration just before the expected phase separation region, while the complex in some cases could desorb from the surface at high enough surfactant concentration (above the cmc). Different results were obtained for coadsorption of amphiphilic polyelectrolytes when surfactant was added to the preadsorbed polymer layers and when complexes were pre-formed in the solution prior to exposing the surface to the polymer–surfactant solution.
Keywords: Polyelectrolyte adsorption; Polyelectrolyte–surfactant interaction; Kinetics of polymer adsorption; Reversibility of polymer adsorption; Coacervation;
Self-assembly of polar food lipids by Martin E. Leser; Laurent Sagalowicz; Martin Michel; Heribert J. Watzke (125-136).
Polar lipids, such as monoglycerides and phospholipids, are amphiphilic molecules commonly used as processing and stabilization aids in the manufacturing of food products. As all amphiphilic molecules (surfactants, emulsifiers) they show self-assembly phenomena when added into water above a certain concentration (the critical aggregation concentration). The variety of self-assembly structures that can be formed by polar food lipids is as rich as it is for synthetic surfactants: micelles (normal and reverse micelles), microemulsions, and liquid crystalline phases can be formulated using food-grade ingredients.In the present work we will first discuss microemulsion and liquid crystalline phase formation from ingredients commonly used in food industry. In the last section we will focus on three different potential application fields, namely (i) solubilization of poorly water soluble ingredients, (ii) controlled release, and (iii) chemical reactivity. We will show how the interfacial area present in self-assembly structures can be used for (i) the delivery of functional molecules, (ii) controlling the release of functional molecules, and (iii) modulating the chemical reactivity between reactive molecules, such as aromas.
A critical evaluation of the binary and ternary solid–oil–water and solid–water–oil interaction by M. Järn; B. Granqvist; J. Lindfors; T. Kallio; J.B. Rosenholm (137-149).
When determining the surface energies of solids the most frequently utilised method is to measure contact angles for particular probe liquids. The measured contact angles (usually measured in air) are then combined with published values of surface- and interfacial tensions of the liquids to give the surface energy of the solid. It is, however, very seldom that sufficient attention is paid to the impurities, to the experimental (e.g. saturated vapour) conditions chosen and to the heterogeneities (chemical and structural) of the solid surface. In this study five typical probe liquids: octane, hexadecane, diiodomethane, α-bromonaphthalene and water have been used to establish the dispersion component of the surface energy of the solid and the hydrophobic interaction occurring at the interface of four solids: hydrophobic/hydrophilic SiO2 and hydrophobic/hydrophilic TiO2. Two (solid/liquid) and three (solid/liquid/liquid) component systems were investigated. The results are compared with previously published results when Alkenyl Succinic Anhydride (ASA) was brought into contact with SiO2 under water. The preferential surface vapour pressure and liquid spreading of the one liquid over the solids in the absence and presence of a saturated liquid vapour were evaluated as sources of errors.
Surface activity coefficients of spread monolayers of behenic acid salts at air–water interface by D.K. Chattoraj; E. Halder; K.P. Das; A. Mitra (151-161).
The pressure–area isotherms of ionized monolayers of behenic acid at air–water interface at pH 12.0 have been obtained from the Langmuir film balance experiments under various physico-chemical conditions. The value of the measured surface pressure at a given area per molecule is equal to the sum of the ideal pressure, cohesive pressure and electrical pressure. The electrical pressure term is regarded as the sum of the pressure originating from the Gouy–Chapman double layer including discrete ion effect, ion binding and monolayer hydration effect. At a given area, the deviation of the measured surface pressure from its ideal value has been calculated in terms of the apparent surface compressibility coefficients, surface fugacity coefficients for gaseous monolayer and surface activity coefficients of solute forming two-dimensional solutions in the monolayer phase respectively. Values of all these coefficients have been calculated for different compositions of the monolayer using non-ideal gas model and Raoult's and Henry's laws modified for two-dimensional non-ideal solutions respectively. Values of these coefficients may be higher or lower than unity depending upon ionic strengths and nature of inorganic salts present in the sub-phase. Using these values of surface activity coefficients, the standard free energies of formation, of spread monolayers of salts of behenic acid have been calculated at different standard states of reference.
Keywords: Surface activity coefficients; Surface fugacity coefficients; Apparent compressibility coefficients; Free energy change for spread monolayer; Equations of state for neutral and charged monolayer; Ionized monolayers; Free energy change;
Reversibility and irreversibility of adsorption of surfactants and proteins at liquid interfaces by V.B. Fainerman; R. Miller; J.K. Ferri; H. Watzke; M.E. Leser; M. Michel (163-171).
It is shown experimentally that the desorption of sodium decyl sulphate from the liquid/air interface is purely diffusion controlled, while the desorption of higher surface active surfactants such as the non-ionic surfactants Triton X-100 and tridecyl dimethyl phosphine oxide obeys a mixed mechanism. The desorption kinetics of β-lactoglobulin (BLG) and β-casein is, however, determined by a barrier mechanism. From the analysis of the BLG and β-casein desorption kinetics at different temperatures the activation energy of desorption is calculated. The values obtained are rather close to the free energy of adsorption. The theoretical model of desorption kinetics predicts that these two energetic parameters are similar if the adsorption activation energy is low. This explains why substances with a higher adsorption activity have a lower desorption rate. Adsorption kinetics studies for β-casein with and without forced convection show the same equilibrium surface tension values. This leads to the conclusion that the protein adsorption at liquid interfaces is thermodynamically reversible, although the slow desorption kinetics would allow to assume it to be an irreversible process.
Keywords: Adsorption kinetics; Surfactants; Proteins; Reversibility; Desorption barrier; Activation energy;
Nucleation in monolayers by D. Vollhardt (173-188).
Two-dimensional monolayer material of a large number of amphiphiles is transformed into overgrown three-dimensional structures in a state of metastability and supersaturation. This process is described by a theoretical concept and characterised by sensitive experimental techniques. Two theoretical models compatible with each other are based on nucleation and growth of the formed 3D nuclei: (i) the model for limiting cases of nucleation and overgrowth of 3D nuclei with assumed shapes, and (ii) the generalised model under formation of lenticular nuclei. The latter model allows determination of the nucleation rate constant which renders possible the determination of the critical nucleus size and the free energy for the formation of critical nucleus in dependence on the surface pressure. A theoretical model developed on the basis of the classical nucleation theory is applied for this. An effective double-surface pressure-step method is introduced to distinguish between the nucleation and growth processes. Consequently, a critical surface pressure, the limiting surface pressure for the formation of 3D nuclei, and an equilibrium surface pressure, at which the 3D nuclei cease to grow, can be defined and experimentally determined.Despite the different boundary conditions, the constant surface pressure relaxation data can be theoretically described by these two consistent nucleation-growth models. In the most cases, the limiting case of progressive nucleation occurs but also instantaneous nucleation is observed.Direct evidence has been provided by AFM studies that constant surface pressure relaxation of monolayers is caused by nucleation-growth of 3D nuclei from the monolayer material. The quantitative analysis of growth and distribution of the 3D micrograins by AFM supports the mechanism obtained by application of the nucleation-growth theories. New instrumental techniques such as, BAM, AFM, GIXD and X-ray reflectivity provide detailed information on the characteristics of the 3D structures overgrown on the monolayer during constant surface pressure relaxation on microscopic and molecular scales.
Adsorption and structure of the adsorbed layer of ionic surfactants by Ivan B. Ivanov; Kavssery P. Ananthapadmanabhan; Alex Lips (189-212).
Our goal in this study was to investigate theoretically and experimentally the adsorption of ionic surfactants and the role of different factors in the mechanism of adsorption, the adsorption parameters and the structure of the adsorbed layer. We used available literature data for the interfacial tension, σ, vs. concentration, C s, for sodium dodecyl sulfate (SDS) in three representative systems with Air/Water (A/W), Oil/Water (O/W) and Oil/Water + 0.1 M NaCl (O/WE) interfaces. We derived 6 new adsorption isotherms and 6 new equations of state (EOS) based on the adsorption isotherms for non-ionic surfactants of Langmuir, Volmer and Helfand–Frisch–Lebowitz (HFL) with interaction term βθ 2 / 2 in the EOS, θ = αΓ being the degree of coverage, with Γ — adsorption and α — minimum area per molecule. We applied Gouy equation for high surface potentials and modified it to account for partial penetration of the counterions in the adsorbed layer. The equations were written in terms of the effective concentration C = [C s(C s + C el)]1 / 2, where C s and C el are, respectively concentrations of the surfactant and the electrolyte. We showed that the adsorption constant K was model independent and derived an equation for the effective thickness of the adsorbed layer, δ s . We found also that the minimum area per molecule, α, is larger than the true area, α 0, which depends on the adsorption model and is a function of the adsorption Γ. The interaction term βθ 2 / 2 in the Langmuir EOS was found to be exact for small β ≪ 1, but for the Volmer EOS it turned out to be only a crude approximation. Semi-quantitative considerations about the interaction between adsorbed discrete charges revealed that at A/W interface part of the adsorbed surfactant molecules are partially immersed in water, which leads to decreased repulsion and increased adsorption Γ. At O/W the larger adsorption energy keeps the surfactant molecules on the surface, so that the electrostatic repulsion is stronger, which translates into negative β's, larger α's and smaller adsorption. The addition of electrolyte partly screens the repulsion at O/W, leading to decreased α and increased adsorption. We determined K, α and β by a three-parameter fit. The constant K was found to be model independent and smaller for A/W than for O/W, because of the smaller adsorption energy. The values of α were larger for O/W than for A/W and decreased for O/W upon addition of electrolyte in agreement with the theory. For the Volmer model α was smaller than for Langmuir's model and both were found to increase with decreasing Γ — again in agreement with the theoretical predictions. It turned out that θ never exceeds 0.5 i.e. the adsorbed layer is never saturated. We tried to determine which adsorption model gave better results by calculating theoretically the Gibbs elasticity, but it turned out that when the results were plotted vs. an experimental variable, say C, all curves collapsed in a single one, which coincided with the respective experimental curve. This means that it is impossible to determine the adsorption model by using only interfacial tension data.
Advances in adsorption of surfactants and their mixtures at solid/solution interfaces by Rui Zhang; P. Somasundaran (213-229).
Surfactants and their mixtures can drastically change the interfacial properties and hence are used in many industrial processes such as dispersion/flocculation, flotation, emulsification, corrosion inhibition, cosmetics, drug delivery, chemical mechanical polishing, enhanced oil recovery, and nanolithography. A review of studies on adsorption of single surfactant as well as mixtures of various types (anionic–cationic, anionic–nonionic, cationic–nonionic, cationic–zwitterionic and nonionic–nonionic) is presented here along with mechanisms involved. Results obtained using techniques such as zeta potential, flotation, AFM, specular neutron reflectivity, small angle neutron scattering, fluorescence, ESR, Raman spectroscopy, ellipsometry, HPLC and ATR-IR are reviewed along with those from traditional techniques to elucidate the mechanisms of adsorption and particularly to understand synergistic/antagonistic interactions at solution/liquid interfaces and nanostructures of surface aggregates. In addition, adsorption of several mixed surfactant systems is considered due to their industrial relevance. Finally an attempt is made to derive structure–property relationships to provide a solid foundation for the design and use of surfactant formulations for industrial applications.
Keywords: Adsorption; Surfactant; Mixtures; Aggregate structure; Adsorption model;
Water-in-diesel emulsions and related systems by Anna Lif; Krister Holmberg (231-239).
Water-in-diesel emulsions are fuels for regular diesel engines. The advantages of an emulsion fuel are reductions in the emissions of nitrogen oxides and particulate matters, which are both health hazardous, and reduction in fuel consumption due to better burning efficiency. An important aspect is that diesel emulsions can be used without engine modifications. This review presents the influence of water on the emissions and on the combustion efficiency. Whereas there is a decrease in emissions of nitrogen oxides and particulate matters, there is an increase in the emissions of hydrocarbons and carbon monoxide with increasing water content of the emulsion. The combustion efficiency is improved when water is emulsified with diesel. This is a consequence of the microexplosions, which facilitate atomization of the fuel. The review also covers related fuels, such as diesel-in-water-in-diesel emulsions, i.e., double emulsions, water-in-diesel microemulsions, and water-in-vegetable oil emulsions, i.e., biodiesel emulsions. A brief overview of other types of alternative fuels is also included.
Keywords: Emulsion; Diesel; Emission; NO x ; Particulate matter; Hydrocarbon; Carbon monoxide; Engine; Combustion;
Solubilization rates of oils in surfactant solutions and their relationship to mass transport in emulsions by Alejandro A. Peña; Clarence A. Miller (241-257).
Information on solubilization rates of oils in aqueous micellar solutions is reviewed. For ionic surfactants electrostatic repulsion prevents close approach of micelles to the oil–water interface, so that solubilization results from oil molecules dissolving individually in the solution and being taken up by micelles during and/or after transport across a diffusion boundary layer to the bulk solution. Experiments with SDS solutions and single oil drops having low (but not negligible) solubility indicate that mass transfer is often not rate-controlling. Instead phenomena near the oil–water interface including, but not limited to, the rates of micellar uptake of oil from the aqueous solution seem to control the solubilization rate. In contrast, Ostwald ripening experiments involving multiple oil drops in SDS solutions are often interpreted in terms of molecular dissolution and diffusion alone since ripening rates are typically only slightly different from those observed in the absence of surfactant micelles, where this mechanism is considered to hold.For many nonionic surfactant systems and oils of low or negligible solubility the principal mechanism of solubilization is incorporation of surfactant at the oil–water interface from micelles, which coalesce or “adsorb” at the interface or else dissociate nearby, permitting individual surfactant molecules to be adsorbed. Subsequently the excess surfactant is emitted as oil-containing micelles. Most experiments have indicated that this process, which appears in the analyses as an interfacial resistance, is rate controlling. New results are presented here supporting this model and showing that resistance to mass transfer is often quite low because natural convection can arise near an oil drop owing to the density change produced by solubilized oil in micelles near the drop surface. Provided that polydispersity of drop sizes is properly accounted for, experiments on solubilization and compositional ripening in emulsions stabilized with nonionic surfactants can be interpreted using the interfacial resistance model with values of resistance obtained from single-drop experiments. However, it is unclear whether mass transfer, interfacial resistance or perhaps some combined mechanism controls the rate of Ostwald ripening. One uncertainty limiting predictions of the interfacial resistance model is the lack of information on the oil-to-surfactant ratio in micelles when the concentration of individually dissolved oil molecules slightly exceeds the equilibrium value for a plane oil–water interface, the situation during Ostwald ripening.
Coalescence stability of emulsions containing globular milk proteins by Slavka Tcholakova; Nikolai D. Denkov; Ivan B. Ivanov; Bruce Campbell (259-293).
This review summarizes a large set of related experimental results about protein adsorption and drop coalescence in emulsions, stabilized by globular milk proteins, β-lactoglobulin (BLG) or whey protein concentrate (WPC). First, we consider the effect of drop coalescence on the mean drop size, d 32, during emulsification. Two regimes of emulsification, surfactant-rich (negligible drop coalescence) and surfactant-poor (significant drop coalescence) are observed in all systems studied. In the surfactant-rich regime, d 32 does not depend on emulsifier concentration and is determined mainly by the interfacial tension and the power dissipation density in the emulsification chamber, ε. In the surfactant-poor regime and suppressed electrostatic repulsion, d 32 is a linear function of the inverse initial emulsifier concentration, 1 / C INI, which allows one to determine the threshold emulsifier adsorption needed to stabilize the oil drops during emulsification, Γ ⁎ (the latter depends neither on oil volume fraction nor on ε). Second, we study how the BLG adsorption on drop surface changes while varying the protein and electrolyte concentrations, and pH of the aqueous phase. At low electrolyte concentrations, the protein adsorbs in a monolayer. If the pH is away from the isoelectric point (IEP), the electrostatic repulsion keeps the adsorbed BLG molecules separated from each other, which precludes the formation of strong intermolecular bonds during shelf-storage as well as after heating of the emulsion. At higher electrolyte concentration, the adsorption Γ increases, as a result of suppressed electrostatic repulsion between the protein molecules; monolayer or multilayer is formed, depending on protein concentration and pH. The adsorption passes through a maximum (around the protein IEP) as a function of pH. Third, the effect of various factors on the coalescence stability of “fresh” emulsions (up to several hours after preparation) was studied. Important conclusion from this part of the study is the establishment of three different cases of emulsion stabilization: (1) electrostatically-stabilized emulsions with monolayer adsorption, whose stability is described by the DLVO theory; (2) emulsions stabilized by steric repulsion, created by protein adsorption multilayers — a simple model was adapted to describe the stability of these emulsions; and (3) emulsions stabilized by steric repulsion, created by adsorption monolayers. Fourth, we studied how the emulsion stability changes with storage time and after heating. At high electrolyte concentrations, we find a significant decrease of the coalescence stability of BLG-emulsions after one day of shelf-storage (aging effect). The results suggest that aging is related to conformational changes in the protein adsorption layer, which lead to formation of extensive lateral non-covalent bonds (H-bonds and hydrophobic interactions) between the adsorbed molecules. The heating of BLG emulsions at high electrolyte concentration leads to strong increase of emulsion stability and to disappearance of the aging effect, which is explained by the formation of disulfide bonds between the adsorbed molecules. The emulsion heating at low electrolyte concentration does not affect emulsion stability — this result is explained with the electrostatic repulsion between the adsorbed molecules, which keeps them separated so that no intermolecular disulfide bonds are formed. Parallel experiments with WPC-stabilized emulsions show that these emulsions are less sensitive to variations of pH and thermal treatment; no aging effect is detected up to 30 days of storage. The observed differences between BLG and WPC are explained with the different procedures of preparation of these protein samples (freeze-drying and thermally enhanced spray-drying, respectively). Our data for emulsion coalescence stability are compared with literature results about the flocculation stability of BLG emulsions, and the observed similarities/differences are explained by considering the structure of the protein adsorption layers.
Keywords: Protein stabilized emulsions; Coalescence stability of emulsions; Protein adsorption; Emulsification; Osmotic pressure of emulsions; β-lactoglobulin; Whey protein;
On the formation and stability of high internal phase O/W emulsions by Jerzy Kizling; Bengt Kronberg; Jan Christer Eriksson (295-302).
High internal phase o/w emulsions have been investigated with respect to stability. A series of aliphatic hydrocarbons were used as the oil component. By matching the refractive index of both phases, transparent, concentrated emulsions were produced and these emulsions were found to have the highest long-term stability. The long-term stability of transparent emulsions is attributed to a minimum in free energy at the equilibrium thickness, which, in turn, is related to a reduced attraction over the thin aqueous lamellae. Another factor that contributes to the stability is the absence of the destabilizing mechanisms commonly encountered for ordinary emulsions and foams.
Determining α-tocopherol distributions between the oil, water, and interfacial regions of macroemulsions: Novel applications of electroanalytical chemistry and the pseudophase kinetic model by K. Gunaseelan; Laurence S. Romsted; Maria-Jose Pastoriza Gallego; Elisa González-Romero; Carlos Bravo-Díaz (303-311).
The assumptions of the pseudophase model for chemical reactivity in homogeneous microemulsions are used to determine the distribution of α-tocopherol (TOC) in macroemulsions from changes in the observed rate constant (k obs) for reaction between 4-hexadecylarenediazonium ion (16-ArN2 +) probe and TOC with increasing surfactant concentration. Two partition constants are needed to describe the distribution of TOC or other antioxidant (AO) or polar uncharged molecule between the oil and interfacial (P O I) and the water and interfacial (P W I) regions of stirred fluid emulsions. The observed rate constants are measured electrochemically. Here we report values of P O I and P W I for the distribution of TOC in octane/acidic water/C12E6 (hexaethylene glycol monododecyl ether) and octane/acidic water/C12E4 (Brij 30, tetraethylene glycol dodecyl ether) emulsions obtained by fitting two kinetic data sets with an equation based on the pseudophase model and solving two equations in two unknowns. The partition constants were used to estimate the %TOC in each region of the emulsions. In 1:1 oil:water C12E6 emulsions, at 2% volume fraction of C12E6, 73% of TOC is in the interfacial region, 26% in the octane and about 1% in the water. The distributions of TOC in C12E4 emulsions are similar. The combined electrochemical-pseudophase model approach is applicable to any AO or other compound that reacts with 16-ArN2 +. The second-order rate constant, k I, for reaction in the interfacial region of the emulsions is also estimated from the kinetic data and is about the same for both surfactants (k I ≈ 0.1 − 0.2 M− 1s− 1) showing that the medium properties of the interfacial regions of C12E6 and C12E4 emulsions are similar. Comparison of these rate constants for a variety of AOs may provide a scale of AO efficiency that is independent of AO distribution between the oil, interfacial and aqueous regions of emulsions.
Keywords: Pseudophase model; Nonionic surfactant; Emulsions; α-tocopherol; Electrochemical kinetics;
Some pertinent factors in skin care emulsion by Abeer Al-Bawab; Stig E. Friberg (313-322).
Emulsion structures are reviewed with special consideration given to the conditions in emulsions for topical applications with more phases than the traditional two liquids. The fundamentals of emulsions containing liquid crystals and vesicles are described, focussing on the dependence of the volume ratios of liquid crystals and vesicles on the surfactant content.
Keywords: Emulsion; Liquid crystal; Vesicle; Evaporation; Skin lotion;
Junctions and end-caps in self-assembled non-ionic cylindrical micelles by N. Dan; S.A. Safran (323-331).
Cylindrical micelles are known to exhibit two types of morphologies: branched networks and linear, worm-like (or thread-like) micelles. These structures correspond to two types of topological defects: end-caps and junction points. Although either type of defect increases the micelle energy (when compared to the cylindrical sections), they are stabilized by an increase in the translational (end-caps) or configurational (junctions) entropy. End-caps reduce the length of the cylindrical micelles, resulting in a suspension of linear, worm-like micelles. Y-junction branch points cause the formation of a network structure that may percolate and coexist thermodynamically with a “sol” of finite cylinders with end-caps. In this paper, we review current experimental and theoretical studies of non-ionic cylindrical micelles in aqueous solutions. We focus on single and multicomponent amphiphiles, and consider both small molecules and macromolecules (polymers), in order to identify the driving forces that determine the type of topological ‘defect’ and the resulting system morphology.
Keywords: Worm-like; Rod; Thread-like; Micelles; Surfactant; Amphiphile;
The dependence of micellar rate effects upon reaction mechanism by Clifford A. Bunton (333-343).
In the pseudophase treatment of reactivities in aqueous surfactants, water and the micelles are treated as discrete reaction media. Provided that transport of reactants between water and micelles is faster than the chemical reactions the overall reaction rate depends on local concentration(s) in water and micelles and rate constants in each medium. Only substrate binding has to be considered for spontaneous reactions, and observed first-order rate constants are interpreted in terms of reaction mechanism on the assumption that the micellar interfacial reaction region is less polar than water and has (for ionic micelles) a high electrolyte content. Rate effects on bimolecular reactions depend on second-order rate constants and concentrations of both reactants in the aqueous and micellar pseudophases. Second-order rate constants in the micellar pseudophase, relative to those in water, are also as expected for a reaction region that is less polar than water. For reactions of anionic nucleophiles and bases in cationic micelles local second-order rate constants are generally not very different from those in water, but are much lower for oxygen transfers from anionic oxidants. These rate effects are understandable in terms of properties of the interfacial region, reaction mechanism and consequent charge redistributions in formation of the transition state.
Keywords: Micelles; Pseudophase model; Organic reaction mechanisms; Reactivity;
Dynamics of micelles of the triblock copolymers poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) in aqueous solution by Raoul Zana; Carlos Marques; Albert Johner (345-351).
A number of results reported on the kinetics of exchange of triblock copolymers poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide), PEO–PPO–PEO, between micelles and the intermicellar aqueous solution are reviewed and analyzed to extract the rate constants k + for the entry of a copolymer into a micelle and k − for the exit of a copolymer from a micelle. Contrary to what is generally observed for conventional surfactants, the rate constant for the entry of a copolymer into a micelle is slower to much slower than for a diffusion-controlled process and decreases as the degree of polymerization of the PO block, n PO, increases. The effect of the degree of polymerization of the EO block, n EO, on the two rate constants is significant only for low values of n EO. The variation of k − with n PO strongly suggests that the free copolymer molecule adopts a conformation where the PO block is tightly coiled with little contact with water and not a fully extended and hydrated conformation, in contrast to what is commonly assumed when analyzing the dependence of the cmc on the polymerization degree of the hydrophobic block
Mechanism of formation of inorganic and organic nanoparticles from microemulsions by C. Destrée; J. B.Nagy (353-367).
This chapter essentially deals with the preparation of nanoparticles using microemulsions. The preparation of inorganic nanoparticles — Ni2B, Pt, Au, Pt–Au, AgX — and the synthesis of organic nanoparticles — cholesterol, rhovanil, rhodiarome — are systematically studied as a function of the concentration of the precursor molecules, the size of the inner water cores, and the manner of mixing the various solutions. Two different behaviors are observed in the various systems. The first case shows a dependence of the nanoparticle size on the various physicochemical parameters. Either a monotonous increase of the size or the presence of a minimum is observed as a function of the concentration of the precursor molecules. This case can be easily explained following the LaMer diagram, where the nucleation of the nanoparticles is separated from the particle growth. The second case does not show any dependence of the nanoparticle size on the physicochemical parameters. The size remains constant in all experimental conditions. The constant character of the size can be explained only by thermodynamic stabilization, where particles with a certain size are better stabilized. It should be emphasized that the size distribution is small in all the cases studied. Finally, the aging of the nanoparticles was also checked, especially for the organic nanoparticles. It is concluded that these particles remain stable for months in the microemulsion.
Microemulsions as transdermal drug delivery vehicles by Anna Kogan; Nissim Garti (369-385).
Microemulsions are clear, stable, isotropic mixtures of oil, water, and surfactant, frequently in combination with a cosurfactant. Microemulsions have been intensively studied during the last decades by many scientists and technologists because of their great potential in many food and pharmaceutical applications. The use of microemulsions is advantageous not only due to the facile and low cost preparation, but also because of the improved bioavailability. The increased absorption of drugs in topical applications is attributed to enhancement of penetration through the skin by the carrier. Saturated and unsaturated fatty acids serving as an oil phase are frequently used as penetration enhancers. The most popular enhancer is oleic acid. Other permeation enhancers commonly used in transdermal formulations are isopropyl myristate, isopropyl palmitate, triacetin, isostearylic isostearate, R(+)-limonene and medium chain triglycerides. The most popular among the enhancing permeability surfactants are phospholipids that have been shown to enhance drug permeation in a different mode. l-α-phosphatidylcholine from egg yolk, l-α-phosphatidylcholine 60%, from soybean and dioleylphosphatidyl ethanolamine which are in a fluid state may diffuse into the stratum corneum and enhance dermal and transdermal drug penetration, while distearoylphosphatidyl choline which is in a gel-state has no such capability. Other very commonly used surfactants are Tween 20®, Tween 80®, Span 20®, Azone®, Plurol Isostearique® and Plurol Oleique®. As cosurfactants commonly serve short-chain alkanols such as ethanol and propylene glycol. Long-chain alcohols, especially 1-butanol, are known for their enhancing activity as well. Decanol was found to be an optimum enhancer among other saturated fatty alcohols that were examined (from octanol to myristyl alcohol). Many enhancers are concentration-dependent; therefore, optimal concentration for effective promotion should be determined. The delivery rate is dependent on the type of the drug, the structure and ingredients of the carrier, and on the character of the membrane in use. Each formulation should be examined very carefully, because every membrane alters the mechanism of penetration and can turn an enhancer to a retarder.Various potential mechanisms to enhance drug penetration through the skin include directly affecting the skin and modifying the formulation so the partition, diffusion, or solubility is altered. The combination of several enhancement techniques such as the use of iontophoresis with fatty acids leads to synergetic drug penetration and to decrease in skin toxicity.Selected studies of various microemulsions containing certain drugs including retinoic acid, 5-fluorouracil, triptolide, ascorbic acid, diclofenac, lidocaine, and prilocaine hydrochloride in transdermal formulations are presented in this review.In conclusion, microemulsions were found as an effective vehicle of the solubilization of certain drugs and as protecting medium for the entrapped of drugs from degradation, hydrolysis, and oxidation. It can also provide prolonged release of the drug and prevent irritation despite the toxicity of the drug. Yet, in spite of all the advantages the present formulations lack several key important characteristics such as cosmetic-permitted surfactants, free dilution in water capabilities, stability in the digestive tracts and sufficient solubilization capacity.
Keywords: Microemulsions; Drug delivery system; Transdermal;
Importance of head group polarity in controlling aggregation properties of cationic gemini surfactants by Mahendra S. Borse; Surekha Devi (387-399).
Cationic gemini surfactants have been extensively studied in the recent past and the effect of chain length, spacer length and nature on aggregation behavior has been examined. But the effect of variation in head group polarity on micellization has not been examined. Hence, the effect of head group polarity of the butane-1,4-bis(dodecyldimethylammonium bromide) surfactants on aggregation properties is studied through conductance, surface tension, viscosity, and small-angle neutron scattering (SANS) measurements. The critical micellar concentration (cmc), average degree of micelle ionization (β ave), minimum area per molecule of surfactant at air–water interface (A min), surface excess concentration (Γ max) and Gibbs free energy change of micellization (ΔG°mic) of the surfactants were determined from conductance and surface tension data. The aggregation numbers (N), dimension of micelle (b/a), effective fractional charge per monomer (α) were determined from SANS and hydration of micelle (h m) from viscosity data. The increasing head group polarity of gemini surfactant having spacer chain length of 4 methylene units promotes micellar growth, leading to decrease in cmc, β ave, ΔG°mic and increase in N and b/a. This is well supported by the observed increase in hydration (h m) of micelle with increase in aggregation number (N) and dimension (b/a) of micelle. The Kraft temperature (kT), foamability and foam stability as a function of head group polarity of gemini surfactants were also examined.
Keywords: Gemini surfactants; SANS; Hydration of micelles; Head group polarity; Intrinsic viscosity;
Wormlike micelles in mixed surfactant solutions by Durga P. Acharya; Hironobu Kunieda (401-413).
Small micellar aggregates of some surfactants exhibit enormous growth in one dimension and form very long and flexible wormlike micelles. Depending on the nature of the surfactant, such micellar growth can be induced in different ways, for example by adding cosurfactants or salts. Above a system-dependent concentration of surfactant, these giant micelles are entangled to form a transient network, and exhibit viscoelastic behavior analogous to a flexible polymer solution. However, unlike polymers in solutions, wormlike micelles undergo breaking and recombination, and, therefore, exhibit complex rheological behavior. Information on the evolution of aggregate morphology can be obtained from rheological study. In this article formation of wormlike micelles and the evolution of rheological properties in different mixed surfactant systems is discussed. Besides, a brief overview on the salt-induced micellar growth in ionic surfactant systems and reverse micellar systems induced by adding certain polar additives has also been presented.
Keywords: Wormlike micelles; Micellar growth; Viscoelastic micellar solution; Phase behavior; Rheology; Rheological behavior;
Dielectric analysis and Differential Scanning Calorimetry of water-in-oil microemulsions by Donatella Senatra (415-424).
The system investigated is a multicomponent thermodynamically stable liquid consisting of water spherical droplets, (≈ 10 nm diameter), stabilized by a surfactant shell and dispersed in a continuous oil phase (microemulsions). A comparative analysis is carried out among the results obtained with three different experimental approaches, namely, dielectric analysis, Thermally Stimulated Depolarization, and Differential Scanning Calorimetry. The different measurement procedures are briefly described. The attention was focused on the system water concentration. Two microemulsions, differing only in the oil used as continuous phase, were studied. For any concentration a comparison between the results obtained with the different experimental methods was undertaken. The evolution with water addition of the water/oil interphase region was analysed in some detail.
Keywords: Microemulsions; Dielectric analysis; Disperse systems; Differential Scanning Calorimetry; Thermally Stimulated Depolarization;
Nanotechnology in action: Overbased nanodetergents as lubricant oil additives by L.K. Hudson; J. Eastoe; P.J. Dowding (425-431).
The synthesis and study of oil-soluble metal carbonate colloids are of interest in the area of lubricant additives. These surfactant-stabilised nanoparticles are important components in marine and automotive engine oils. Recently introduced, environmentally driven legislation has focused on lowering of gaseous emissions by placing limits on the levels of phosphorous sulphur and ash allowed in engine oil systems. These chemical limits, coupled with improved engine performance and extended oil drainage intervals, have lead to renewed interest in the production of stable, efficient nanodetergent systems. To date, this has resulted in modification of existing surfactant structures and development of new generations of surfactants. This review covers the current state of research in the area of nanodetergents.
Keywords: Lubricant additives; Lubricants; Overbased; Detergents; Nanoparticles; Non-aqueous colloids;
Application of emulsifiers/stabilizers in dairy products of high rheology by Shane N.D. Lal; Charmian J. O'Connor; Laurence Eyres (433-437).
The role played by low molecular weight emulsifiers (mono- and di-glycerides) and non-dairy stabilizers (alginates, carrageenans, gums and gelatins) in the formation and stabilization of liquid milk (and specifically a functionalized milk containing omega-3), yoghurt and ice cream has been reviewed. Attention is given to the interactions that may occur between the reactive sites on polysaccharide stabilizers and milk proteins and other milk components, and to the desirable characteristics, e.g., viscosity/consistency, appearance and mouthfeel, body and texture, imparted to yoghurt and ice cream by addition of emulsifiers and gums.
Keywords: Emulsifiers; Stabilizers; Milk; Yoghurt; Ice cream;
Spreading on and penetration into thin, permeable print media: Application to ink-jet printing by Richard C. Daniel; John C. Berg (439-469).
This paper examines spreading and penetration of surfactant-laden drops on thin-permeable media with reference to ink-jet printing. A detailed review of the interaction of both pure liquids and surfactant containing solutions with porous substrates is given for individual spreading and penetration and for the combined processes. A new model based on energy arguments is derived and compared to current hydrodynamic equations used to describe simultaneous spreading and penetration. Three studies of how surfactant solutions interact with thin commercial ink-jet photographic quality papers are presented. Here, two relevant systems are examined: Tergitol 15-S-5 and 1,2-octanediol. The first study examines the spreading and penetration profiles for surfactant solutions over a range of concentrations spanning their critical micelle concentration. As expected, these profiles depend on the concentration of surfactant and the chemistry of the medium with which it interacts. In many cases, partial vertical penetration of the region directly beneath the drop dominates at low interaction times and will be significant in ink-jet applications. The second study consists of a parametric investigation of the energy-based model derived herein. It shows that the model can capture all of the behaviors observed in the first study. In the final study, the ability of the energy-based model to fully predict the spreading behavior of Tergitol 15-S-5 solutions is tested. It is found that the model produces good quantitative agreement at the highest concentrations and, as such, will be useful in screening spreading dynamics concentrated systems like ink-jet inks. Agreement at low to intermediate concentrations is often limited by finite induction periods prior to significant spreading and penetration. Possible corrections that could improve the agreement for weakly concentrated solutions are discussed, and directions for future studies of simultaneous spreading and penetration are proposed.
Keywords: Spreading; Penetration; Surfactant-assisted spreading; Print media; Ink-jet printing;
Nanoparticles for bioimaging by Parvesh Sharma; Scott Brown; Glenn Walter; Swadeshmukul Santra; Brij Moudgil (471-485).
The emergence of synthesis strategies for the fabrication of nanosized contrast agents is anticipated to lead to advancements in understanding biological processes at the molecular level in addition to progress in the development of diagnostic tools and innovative therapies. Imaging agents such as fluorescent dye-doped silica nanoparticles, quantum dots and gold nanoparticles have overcome many of the limitations of conventional contrast agents (organic dyes) such as poor photostability, low quantum yield, insufficient in vitro and in vivo stability, etc. Such particulates are now being developed for absorbance and emission in the near infrared region, which is expected to allow for real time and deep tissue imaging via optical routes. Other efforts to facilitate deep tissue imaging with pre-existing technologies have lead to the development of multimodal nanoparticles which are both optical and MRI active. The main focus of this article is to provide an overview of properties and design of contrast agents such as dye-doped silica nanoparticles, quantum dots and gold nanoparticles for non-invasive bioimaging.
Keywords: Non-invasive imaging; Fluorescence; Magnetic resonance imaging; Silica; Gold; Quantum dots; Multimodal; Nanoparticles;