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Advances in Colloid and Interface Science (v.128-130, #)
Recent advances in nanoparticle synthesis with reversed micelles by Julian Eastoe; Martin J. Hollamby; Laura Hudson (pp. 5-15).
Synthesis of nanoparticles in microemulsions is an area of considerable current interest. This subject can be broadly divided into two sections defined by the nature of the host microemulsion reaction medium.Water-in-oil microemulsions have been used to prepare nanoparticles for more than two decades, and a wide variety of materials has been synthesised by these methods. Control parameters have been elucidated for influencing both nanoparticle concentration and morphology, allowing for tailored syntheses with various applications.More recently, the ability to synthesise nanoparticles in water/supercritical fluid microemulsions was realised. This method promises to be a highly useful route for controlled nanoparticle synthesis due to the added control variables afforded by tuneability of the solvent quality (density) through pressure and temperature.This review presents the current state-of-the-art in both fields.
Keywords: Nanoparticles; Nanoparticle synthesis; Reversed micelles; Microemulsions; Supercritical fluids
Composite interfacial layers containing micro-size and nano-size particles by R. Miller; V.B. Fainerman; V.I. Kovalchuk; D.O. Grigoriev; M.E. Leser; M. Michel (pp. 17-26).
Surface layers of micro- and nanoparticles at fluid/liquid interfaces in absence and presence of surfactants are of large importance in the process of re-discovering Pickering systems, i.e. emulsions and foams stabilized by particles. The surface pressure/area isotherms of such layers can provide information about the properties of the used particles (dimensions, interfacial contact angles), the structure of interfacial layers, the interactions between the particles as well as about relaxation processes within the layers. For a correct description of Π-A isotherms of composite surface layers containing particles the significant difference in size of these particles to that of solvent and surfactant molecules should be taken into account. Corresponding equations can be derived on the basis of the two-dimensional solution theory. The gained equations provide satisfactory agreement with experimental data and predict realistic values for the area of particles at the interface. Also equations of state and of the dilational elasticity for composite surface layers containing particles can be obtained in the framework of the presented methodology.
Keywords: Surface pressure isotherm; Nanoparticles; Surfactants; Composite surface layers; Thermodynamic models; Dilational elasticity
Polymerization of and in mesophases by Feng Yan; John Texter (pp. 27-35).
The use of surfactant mesophases such as vesicular and lyotropic mesophases as templates for the preparation of nanostructured polymers by polymerization is reviewed. Recent developments using polymerizable and polymeric surfactants in mesophase formation and polymerization are also represented with examples. The formations of various novel materials including nanocapsules, vesicle-polymer architecture, mesoporous polymers and functional nano-composites which would be unobtainable through conventional techniques are highlighted. The effects of reaction thermodynamics and kinetics on templated polymerizations are also discussed.
Keywords: Surfactant templating; Vesicles; Lyotropic mesophases; Polymerization; Polymerizable surfactants; Lipids
The role of surfactants in dispersion of carbon nanotubes by Linda Vaisman; H. Daniel Wagner; Gad Marom (pp. 37-46).
The discovery of carbon nanotubes offers exciting opportunities for the development of novel high property materials. Disaggregation and uniform dispersion are critical challenges that must be met to successfully produce such high property materials, since carbon nanotubes tend to self-associate into micro-scale aggregates. This results in products with inferior mechanical and electric performance. Recognizing this problem, extensive research has been reported in the literature on development of dispersion technologies based on both mechanical and chemical approaches. Here, we review recent progress and advances that have been made on dispersion of carbon nanotubes in aqueous and organic media by non-covalent adsorption of surfactants and polymers. Carbon nanotube structure, properties and mainly self-assembly are discussed in detail.
Keywords: Carbon nanotubes; Dispersion; Stabilization; Surfactants; Polymers
Microemulsions as carriers for drugs and nutraceuticals by Aviram Spernath; Abraham Aserin (pp. 47-64).
Microemulsions are potentially excellent carriers for bioactive molecules. They offer the advantage of spontaneous formation, ease of manufacture, thermodynamic stability, and improved solubilization of bioactive materials. This review explores some of the new trends in microemulsion research through analysis of some representative studies. The solubilization of different classic drugs, peptides, and nutraceuticals in various oral microemulsion compositions and microstructures was reviewed. It was found that even W/O microemulsions, which are expected to break upon dilution in the digestive tract, increase the permeability and bioavailability of drugs. Thus, it seems that component selection (the use of molecules that can act as permeability enhancers) is of great importance. Structures such as self-microemulsifying drug delivery systems (SMEDDS), W/O, bicontinuous, and O/W microemulsions, were closely examined, and their potential to serve as drug carriers was evaluated.It seems that for microemulsion systems to be used as vehicles for bioactive materials, the formulations should be based on SMEEDS, which form O/W upon dilution to a specific water content. U-type microemulsions are actually an improved SMEDDS formulation, capable of being diluted with any given water concentration.Much research is being conducted on microemulsion microstructure since there is an obvious direct connection between the microstructure, solubilization capacity, and bioavailability of the active molecules.
Keywords: Microemulsions; Oral drug delivery; Nutraceuticals; SMEDDS; Microstructure
Comparative analysis of complex liquids based on a multi-experimental approach by Donatella Senatra (pp. 65-75).
A parallel analysis is here presented between the results gathered by the author by means of three different experimental approaches – Dielectric analysis, Thermally Stimulated Depolarization (TSD) and Differential Scanning Calorimetry (DSC) – on some multi-component complex liquids consisting of association structures of self-assembling amphiphiles as, water-in-oil microemulsions. The highly dispersed nature of these systems was evidenced with the dielectric study: a Maxwell-Wagner relaxation was found to characterize this type of systems. The structural evolution of the system, against water addition, was well described by the orientational processes induced in the liquid samples by a nearly static electric field as in the TSD method. The investigation by DSC, besides offering a basic picture of the system's thermodynamics, allowed the measure of the specific heat, at constant pressure, at the higher order percolative phase transition. The comparison between the results obtained on samples processed with identical thermal cycles, once under the action of an impressed electric field as in TSD, and, once in the absence of any impressed electric field as in DSC, did help distinguish three main concentration intervals within which structurally different systems form namely, isotropic microemulsions and two liquid crystalline structures of lyotropic type.
Properties, main applications and perspectives of worm micelles by S. Ezrahi; E. Tuval; A. Aserin (pp. 77-102).
This tutorial review deals with one of the most remarkable forms of surfactant aggregates, described as having a flexible, elongated cylindrical shape. Three structural scale lengths are pertinent to the flexibility and mobility of worm micelles: the cross-sectional radius, rcs, the overall (contour) length, L, and the persistence length, lp. The diversity of lp values in amphiphilic systems is demonstrated as well as the relation between L and lp. The review also discusses the viscoelasticity of worm micelles and the relaxation mechanisms underlying this dominant property. Many aspects of viscoelasticity – such as non-linearity, shear banding, flow-induced phase transition, rheochaos – are only shortly described.The prevailing application of worm micelles, namely as fracture fluids and drag reducing agents are discussed in detail, stressing the effect of variations in the surfactant molecular structure on the efficacy of worm micelles. The vague possibility of using “smart” worm micelles in the foreseeable future is tersely outlined.
Silicone emulsions by P. Somasundaran; Somil C. Mehta; Parag Purohit (pp. 103-109).
Silicone polymers are a class of hybrid organic/inorganic polymers, that show desirable surface properties such as low surface energy and high flexibility, which enables even a very high molecular weight chain to achieve optimal orientation at the interface. They have excellent physical properties such as water repellency, heat stability, and high resistance to chemical and UV attack.Silicone polymers have dual characteristics, because of which they can either be used as emulsifiers or act as the continuous/dispersed phase of the emulsion. The results of an anionically modified silicone polymer indicate that it can stabilize an emulsion of water in cyclic silicone oil (D5) only in a narrow range of compositions around 80% water and 20% oil, formulated at low shear rates. A silicone emulsion stabilized by hydrocarbon emulsifiers shows drastic changes in their electrokinetic and optical properties under external perturbations, for example pH change. Advanced analytical tools such as atomic force microscopy (AFM) illustrated that a coating of silicone emulsion causes a solid substrate like fabric to smoothen out. This article further discusses the various types of silicone emulsions and their applications.
Keywords: Silicone; Emulsion; Poly(dimethyl siloxane); PDMS; Zeta potential; Turbidimetric analysis; Phase diagram; HLB
Molecular recognition on the supported and on the air/water interface-spread protein monolayers by Adam Baszkin (pp. 111-120).
Targeting of proteins at interfaces via affinity ligands or specific antibodies is important for the understanding of protein functioning in biological membranes. This review brings together a great number of research works accomplished in this field in the past decade by a variety of analytical methods. It highlights two simple in situ techniques of monitoring molecular recognition processes at interfaces recently developed in the author's laboratory. The first of these techniques is based on the measurements of surface pressure increments of a protein monolayer spread at the air/water interface at a constant area resulting from the interaction with its specific ligands injected into the aqueous subphase beneath the preformed protein monolayer. The second technique takes advantage of the feature of [14C]-labeled proteins that enable in situ measurements of surface density changes of adsorbed protein molecules on a solid support resulting from the interaction with its specific antibody.
Keywords: Proteins; Specific interaction; Monolayers; Interface; Surface pressure
Surface modification for aluminium pigment inhibition by Philip Karlsson; Anders E.C. Palmqvist; Krister Holmberg (pp. 121-134).
This review concerns surface treatment of aluminium pigments for use in water borne coatings. Aluminium pigments are commonly used in coatings to give a silvery and shiny lustre to the substrate. Such paints and inks have traditionally been solvent borne, since aluminium pigment particles react with water. For environmental and health reasons solvent borne coatings are being replaced by water borne and the aluminium pigments then need to be surface modified in order to stand exposure to water. This process is called inhibition and both organic and inorganic substances are used as inhibiting agents. The organic inhibiting agents range from low molecular weight substances, such as phenols and aromatic acids, via surfactants, in particular alkyl phosphates and other anionic amphiphiles, to high molecular weight compounds, such as polyelectrolytes. A common denominator for them all is that they contain a functional group that interacts specifically with aluminium at the surface. A particularly strong interaction is obtained if the inhibiting agent contains functional groups that form chelating complex with surface Al(III). Encapsulation of the pigment can be made by in situ polymerization at the surface of the pigment and a recent approach is to have the polymerization occur within a double layer of adsorbed surfactant. The inorganic route is dominated by coating with silica, and recent progress has been made using an alkoxide, such as tetraethoxysilane as silica precursor. Such silica coated aluminium pigments are comparable in performance to chromate inhibited pigments and thus offer a possible heavy metal-free alternative. There are obvious connections between surface modifications made to prevent the pigment to react with water and inhibition of corrosion of macroscopic aluminium surfaces.
Keywords: Aluminium; Pigment; Coating; Paint; Ink; Surface treatment; Surface modification; Inhibition; Alumina; Surfactant; Polyelectrolyte; Silica
The influence of surface active molecules on the crystallization of biominerals in solution by M. Dutour Sikirić; H. Füredi-Milhofer (pp. 135-158).
In the following article studies pertaining to “ in situ” interactions of growing biogenic crystals (calcium phosphates, carbonates and oxalates) with, soluble, surface active molecules, including small, highly charged organic molecules, natural and synthetic polymers and synthetic surfactants, are discussed. Such interactions are at the roots of crystallization processes occurring in nature (biological mineralization) and in the controlled production of materials with well defined crystal structure, morphology and phase composition. The main characteristics of the crystals, including crystallographic data, and of the organic molecules, including their molecular structures, are briefly described. Most of the model crystals are crystal hydrates, whose dominant crystal planes are covered with continuous layers of structural water molecules (hydrated layer). The experimental methods reviewed include kinetic experiments determining induction times and/or the rates and rate controlling mechanisms of seeded and unseeded crystallization, techniques for the characterization of the nascent solid phase(s), and techniques, suitable for the assessment of interactions on the molecular level.Numerous examples show that the dominant mechanism underlying host crystal/additive interactions is selective adsorption of the additive at the crystal/solution interface, with the main driving forces ranging from purely electrostatic to highly specific recognition of crystal faces by the additive.Selective electrostatic interactions take place between growing crystals and flexible, highly charged small and macromolecules and/or surfactants because of differing ionic structures and charges of the crystal planes, some of them being shielded by hydrated layers. As in solution, surfactant molecules at high concentrations self-assemble into various superstructures (hemimicelles, bilayers) at the crystal/solution interface.Recognition of crystal planes by rigid small molecules and macromolecules with partial β-sheet conformation (such as proteins or polyelectrolytes) is highly specific. It requires a dimensional fit between the distances of constituent ions protruding from the affected crystal plane(s) and the distances between functional groups that are part of the additive molecules. The consequences of selective additive/crystal interactions range from changes in crystal growth morphology to changes in the composition of the crystallizing phase. Examples showing the dual role of macromolecules as initiators and retarders of crystallization are discussed.
Keywords: Biominerals; Crystallization; Polymers; Small molecules; Surfactants
Proteins at fluid interfaces: Adsorption layers and thin liquid films by Galina Yampolskaya; Dimo Platikanov (pp. 159-183).
A review in which many original published results of the authors as well as many other papers are discussed. The structure and some properties of the globular proteins are shortly presented, special accent being put on the α-chymotrypsin (α-ChT), lysozyme (LZ), human serum albumin (HSA), and bovine serum albumin (BSA) which have been used in the experiments with thin liquid films. The behaviour of protein adsorption layers (PAL) is extensively discussed. The dynamics of PAL formation, including the kinetics of adsorption as well as the time evolution of the surface tension of protein aqueous solutions, are considered. A considerable place is devoted to the surface tension and adsorption isotherms of the globular protein solutions, the simulation of PAL by interacting hard spheres, the experimental surface tension isotherms of the above mentioned proteins, and the interfacial tension isotherms for the protein aqueous solution/oil interface. The rheological properties of PAL at fluid interfaces are shortly reviewed. After a brief information about the experimental methods for investigation of protein thin liquid (foam or emulsion) films, the properties of the protein black foam films are extensively discussed: the conditions for their formation, the influence of the electrolytes and pH on the film type and stability, the thermodynamic properties of the black foam films, the contact angles film/bulk and their dynamic hysteresis. The next center of attention concerns some properties of the protein emulsion films: the conditions for formation of emulsion black films, the formation and development of a dimpling in microscopic, circular films. The protein–phospholipid mixed foam films are also briefly considered.
Keywords: Protein adsorption layers; Protein foam films; Protein emulsion films; α-Chymotrypsin; Lysozyme; Human and bovine serum albumins
Disjoining pressure of thin films stabilized by nonionic surfactants by Krassimir D. Danov; Ivan B. Ivanov; Kavssery P. Ananthapadmanabhan; Alex Lips (pp. 185-215).
In this article an attempt is made to derive a comprehensive theory of the disjoining pressure of thin liquid films, stabilized by low molecular nonionic surfactants. We accounted for effects playing a role in the case of surfactants with spherical hydrophilic heads: (i) The thermal fluctuations of the adsorbed surfactant molecules, due to the fact that the energy of adsorption of a –CH2– group is approximately equal to the average thermal energy kB T; (ii) The contribution of the collisions between molecules adsorbed on different surfaces; (iii) The restriction imposed on the fluctuation of the molecules by the presence of a second surface situated at a small distance h from the interface where the molecules are adsorbed; (iv) The volume of the hydrophilic heads, which expels part of the water molecules from the film region; (v) The equilibrium between the molecules adsorbed at the film surfaces and at the menisci surrounding the film. The adsorption on the film surfaces has two main effects. First, the concentration of solute inside the film region becomes larger than in the bulk solution and this will push the solvent toward the film thus creating an osmotic pressure (the disjoining pressure), which tends to increase the film thickness. Second, the higher concentration inside the film and the collisions between the polar heads lead to higher chemical potential, which pushes the surfactant toward the meniscus. We treated these effects by modifying adequately the Hildebrand–Scatchard theory for the osmotic pressure of concentrated solutions. The partition function of the surfactant, needed for this calculation, was found by deriving an expression for the configurational integral, based on virial expansion. The surface equations of state of Helfand, Frisch and Lebowitz and Volmer were critically analyzed and then generalized, by using the partition function obtained by virial expansion, to permit the derivation of partition functions of the surfactant molecules in the film. A simple thermodynamic approach was developed and applied to derive expressions for the disjoining pressure, Π, and the chemical potential of the surfactant molecules in the film, μ. They were used to calculate numerically Π and μ and analyze their dependence on the film thickness h and the surface coverage θ. It turned out that Π has completely different behavior above and below h=2 d, where d is the diameter of the hydrophilic head. For thick films, with h>2 d, the decay of Π is initially exponential (due mainly to the thermal fluctuations of the adsorbed molecules), followed by a long tail, proportional to h−2, due to the contribution of the osmotic pressure of the displaced solvent molecules. At h<2 d the collisions between the molecules adsorbed at different surfaces are hindered, which leads to a steady decrease of the contribution due the interaction between the molecules. The overall result of these effects is the appearance of a maximum of Π at h=2 d. It is very large (it may reach 1000 atm and even more) and depends strongly on the surfactant adsorption. To facilitate the application and the analysis of the theory, we derived several simpler asymptotic expressions. One of them is virial expansion, which is valid for small surface coverage and has the advantage of being independent of the adsorption model. The other asymptotic expression is applicable at h>2 d, which is the region where the stabilization of the film occurs. We compared our theory with the simpler theory of Israelachvili and Wennerström. It turned out that while both theories lead to decay of Π vs. h, the numerical results and the shape of the curves are usually very different. The experimental data, which could be used to verify our theory, are scarce, but we found reasonable agreement with the data of Lyle and Tiddy for bilayers of C12EO4. The data of Parsegian et al. for lipid bilayers also confirmed qualitatively some of our theoretical conclusions.
Keywords: Disjoining pressure; Nonionic surfactants; Thin liquid films; Surface equation of state; Osmotic pressure
Supercritical fluid processing of polymer thin films: An X-ray study of molecular-level porosity by Tadanori Koga; J. Jerome; M.H. Rafailovich; B. Chu; J. Douglas; S. Satija (pp. 217-226).
This paper reviews our recent experimental results that address the effects of solvent density inhomogeneities in supercritical carbon dioxide (scCO2) on polymer thin film processing. The key phenomenon is excess sorption of CO2 molecules into polymer thin films even when the bulk miscibility with CO2 is very poor. We have found that the amount of the excess sorption is attributed to the large density fluctuations in scCO2 near the critical point. Further, taking advantage of the vitrification process of polymer chains through quick evaporation of CO2, we can preserve the “expanded” structures as they are. The resultant films have large degree of molecular-level porosity that is very useful in producing coatings with low dielectric constants, enhanced adhesion, and metallization properties. These characteristics can be achieved in an environmentally “green” manner, without organic solvents, and are not specific to any class of polymers.
Formation, stability and properties of multilayer emulsions for application in the food industry by Demet Guzey; D. Julian McClements (pp. 227-248).
The formation of multilayered interfaces around oil droplets in oil-in-water emulsions provides food technologists with a novel means of improving the quality and stability of many food products, as well as the ability to develop novel encapsulation and delivery systems. This article reviews the basic principles of multilayer emulsion formation, discusses the factors that influence the characteristics of the interfaces formed, and highlights the relationship between interfacial properties and emulsion functionality. Finally, it highlights some potential applications of the multilayer emulsion technology in the food industry for improving the stability of emulsions to environmental stresses or for developing controlled or triggered release systems.
Keywords: Layer-by-layer deposition; Emulsions; Electrostatic attraction; Colloidal dispersions; Multilayer formation; Bridging flocculation; Depletion flocculation
Synergisms between yellow mustard mucilage and galactomannans and applications in food products — A mini review by Steve W. Cui; Michael A.N. Eskin; Ying Wu; Shaodong Ding (pp. 249-256).
Yellow or white mustard ( Sinapis alba L.) is unique in the mustard family by containing large amounts of mucilaginous material in the seed coat. This material was shown to exhibit similar rheological properties to xanthan gum such as shear thinning flow behavior and weak gel structure. This review will discuss the synergistic interactions between yellow mustard mucilage (YMM) and galactomannans, particularly locust bean gum (LBG), and its potential food applications. In addition, synergistic interactions between YMM, with or without LBG, on starch paste viscosity and syneresis will also be reviewed. The thickening, texturizing and stabilizing properties of YMM, and its ability to form gels at very low concentration in the presence of LBG, could lead to many food and industrial applications.
Quantitative study on the microstructure of colloidal fat crystal networks and fractal dimensions by Dongming Tang; Alejandro G. Marangoni (pp. 257-265).
This paper highlights the most recent progress in the quantitative study on the microstructure and the rheological properties of colloidal fat crystal networks. Several physical models describing the structural hierarchy of the colloidal fat crystal networks are reviewed here, with particular emphasis on fractal model, which can be used to explain the scaling behavior of fat crystal networks. The concept of the fractal dimension has been extensively used in the quantitative study of the microstructure of fat crystal networks and other colloidal networks; however, the relationship between the fractal dimension value and microstructural characteristics remains somewhat nebulous. Recent computer simulation work from our laboratory will be presented relating simulated microstructural features to values of particular measure of fractality including the box-counting, particle counting and Fourier-transform fractal dimensions.
Keywords: Microstructure; Fractal dimension; Fat crystal network
Review: Mechanism of oil uptake during deep-fat frying and the surfactant effect-theory and myth by Dina Dana; I. Sam Saguy (pp. 267-272).
Three mechanisms have been previously proposed to explain the complex process of oil uptake during deep-fat frying. The mechanisms reviewed are water replacement, cooling-phase effect and surface-active agents. The former mechanism describes mainly oil uptake of relatively large voids in the fried food created due to water evaporation. The second mechanism furnishes an explanation for the significant amount of oil absorbed when the food is removed from the fryer. At this point, product surface characteristics and oil viscosity play paramount roles. Surface-active agents’ formation provides only a limited explanation for the increased oil uptake during prolonged frying. Reviewed literature, theory and new data show contradicting values and do not support the myth that during extended frying time the surfactants generated reduce the contact angle and/or the interfacial tension, and consequently, influence oil uptake significantly. Higher oil uptake after extended frying time is probably related to higher oil viscosity caused by polymerization reactions and oil adherence to the product surface. Further research is needed for establishing the interrelationships between surface-active agents’ formation and their effect on fried product oil uptake and quality to resolve this myth.
Keywords: Contact angle; Surface-active agents; Viscosity; Interfacial tension