Advances in Colloid and Interface Science (v.183-184, #C)

Foam–oil interaction in porous media: Implications for foam assisted enhanced oil recovery by R. Farajzadeh; A. Andrianov; R. Krastev; G.J. Hirasaki; W.R. Rossen (1-13).
The efficiency of a foam displacement process in enhanced oil recovery (EOR) depends largely on the stability of foam films in the presence of oil. Experimental studies have demonstrated the detrimental impact of oil on foam stability. This paper reviews the mechanisms and theories (disjoining pressure, coalescence and drainage, entering and spreading of oil, oil emulsification, pinch-off, etc.) suggested in the literature to explain the impact of oil on foam stability in the bulk and porous media. Moreover, we describe the existing approaches to foam modeling in porous media and the ways these models describe the oil effect on foam propagation in porous media.Further, we present various ideas on an improvement of foam stability and longevity in the presence of oil. The outstanding questions regarding foam–oil interactions and modeling of these interactions are pointed out.Display Omitted► The theories to explain the effect of oil on foam stability are not conclusive. ► Studies on pseudoemulsion films is important for optimization of foam assisted EOR. ► Modeling of foam flow for EOR is still a challenge for petroleum industry. ► The current modeling issues are discussed in the paper.
Keywords: Foam; Porous media; Enhanced oil recovery; Disjoining pressure; Limiting capillary pressure; Pseudoemulsion film;

The development and study of nano–bio hybrid materials engineered from membrane proteins (the key functional elements of various biomembranes) and nanoheterostructures (inorganic colloidal nanoparticles, transparent electrodes, and films) is a rapidly growing field at the interface of materials and life sciences. The mainspring of the development of bioinspired materials and devices is the fact that biological evolution has solved many problems similar to those that humans are attempting to solve in the field of light-harvesting and energy-transferring inorganic compounds. Along this way, bioelectronics and biophotonics have shown considerable promise. A number of proteins have been explored in terms of bioelectronic device applications, but bacteriorhodopsin (bR, a photosensitive membrane protein from purple membranes of the bacterium Halobacterium salinarum) and bacterial photosynthetic reaction centres have received the most attention. The energy harvesting in plants has a maximum efficiency of 5%, whereas bR, in the absence of a specific light-harvesting system, allows bacteria to utilize only 0.1–0.5% of the solar light. Recent nano-bioengineering approaches employing colloidal semiconductor and metal nanoparticles conjugated with biosystems permit the enhancement of the light-harvesting capacity of photosensitive proteins, thus providing a strong impetus to protein-based device optimisation.Fabrication of ultrathin and highly oriented films from biological membranes and photosensitive proteins is the key task for prospective bioelectronic and biophotonic applications. In this review, the main advances in techniques of preparation of such films are analyzed. Comparison of the techniques for obtaining thin films leads to the conclusion that the homogeneity and orientation of biomembrane fragments or proteins in these films depend on the method of their fabrication and increase in the following order: electrophoretic sedimentation < Langmuir–Blodgett and Langmuir–Schaefer methods < self-assembly and layer-by-layer methods. The key advances in the techniques of preparation of the assemblies or complexes of colloidal nanocrystals with bR, purple membranes, or photosynthetic reaction centres are also reviewed. Approaches to the fabrication of the prototype photosensitive nano–bio hybrid materials with advanced photovoltaic, energy transfer, and optical switching properties and future prospects in this field are analyzed in the concluding part of the review.Display Omitted► Making assemblies and ordered films is a key step in designing nano–bio hybrid devices. ► Approaches to designing films and assemblies of photosensitive proteins are reviewed. ► Self-assembly and layer-by-layer methods provide highly ordered and homogeneous films. ► Colloid nanocrystals enhance light harvesting and photovoltaic biological functions. ► Nano–bio complexes with enhanced biological function are reviewed.
Keywords: Langmuir films; Layer-by-layer; Self-assembly; Bacteriorhodopsin; Purple membranes; Photosynthetic reaction centres; Colloidal nanocrystals; Nano–bio hybrid materials;

Thermal properties of nanofluids by John Philip; P.D. Shima (30-45).
Colloidal suspensions of fine nanomaterials in the size range of 1–100 nm in carrier fluids are known as nanofluids. For the last one decade, nanofluids have been a topic of intense research due to their enhanced thermal properties and possible heat transfer applications. Miniaturization and increased operating speeds of gadgets warranted the need for new and innovative cooling concepts for better performance. The low thermal conductivity of conventional heat transfer fluid has been a serious impediment for improving the performance and compactness of engineering equipments. Initial studies on thermal conductivity of suspensions with micrometer-sized particles encountered problems of rapid settling of particles, clogging of flow channels and increased pressure drop in the fluid. These problems are resolved by using dispersions of fine nanometer-sized particles. Despite numerous experimental and theoretical studies, it is still unclear whether the thermal conductivity enhancement in nanofluids is anomalous or within the predictions of effective medium theory. Further, many reports on thermal conductivity of nanofluids are conflicting due to the complex issues associated with the surface chemistry of nanofluids. This review provides an overview of recent advances in the field of nanofluids, especially the important material properties that affect the thermal properties of nanofluids and novel approaches to achieve extremely high thermal conductivities. The background information is also provided for beginners to better understand the subject.Display Omitted► Nanofluids have significant potential as heat transfer fluids. ► Magnetic nanofluids offer tunable and enhanced thermal properties. ► Studies show that microconvection has no major role in k enhancement. ► The agglomerates and their conformation dictate the k enhancement in nanofluids. ► Response stimuli nanofluids are promising candidates for heat transfer applications.
Keywords: Nanofluids; Dispersions; Thermal conductivity; Heat transfer; Agglomeration;

Niosomes: Novel sustained release nonionic stable vesicular systems — An overview by N.B. Mahale; P.D. Thakkar; R.G. Mali; D.R. Walunj; S.R. Chaudhari (46-54).
Vesicular systems are novel means of delivering drug in controlled manner to enhance bioavailability and get therapeutic effect over a longer period of time. Niosomes are such hydrated vesicular systems containing nonionic surfactants along with cholesterol or other lipids delivering drug to targeted site which are non toxic, requiring less production cost, stable over a longer period of time in different conditions, so overcomes drawbacks of liposome. Present review describes history, all factors affecting niosome formulation, manufacturing conditions, characterization, stability, administration routes and also their comparison with liposome. This review also gives relevant information regarding various applications of niosomes in gene delivery, vaccine delivery, anticancer drug delivery, etc.Display Omitted► Vesicular system introduction and types ► History of niosomes ► Classification, impact, uses and drawbacks of surfactant in short and compiled ► Impact of properties of niosomes such as chain length, HLB value, etc. ► Use of cholesterol, surfactants and there impacts ► Methods of preparation ► Evaluation of niosomes by various methods for different parameters ► Application of niosomes such as in delivery of NSAIDs, genes, vaccines, anticancer drugs, etc.
Keywords: Nonionic surfactant; Niosome; Non toxic; Cholesterol impact; Drug delivery;

Determination of the aggregation number and charge of ionic surfactant micelles from the stepwise thinning of foam films by Svetoslav E. Anachkov; Krassimir D. Danov; Elka S. Basheva; Peter A. Kralchevsky; Kavssery P. Ananthapadmanabhan (55-67).
The stepwise thinning (stratification) of liquid films, which contain micelles of an ionic surfactant, depends on the micelle aggregation number, N agg, and charge, Z. Vice versa, from the height of the step and the final film thickness one can determine N agg, Z, and the degree of micelle ionization. The determination of N agg is based on the experimental fact that the step height is equal to the inverse cubic root of the micelle concentration. In addition, Z is determined from the final thickness of the film, which depends on the concentration of counterions dissociated from the micelles in the bulk. The method is applied to micellar solutions of six surfactants, both anionic and cationic: sodium dodecylsulfate (SDS), cetyl trimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC), sodium laurylethersulfates with 1 and 3 ethylene oxide groups (SLES-1EO and SLES-3EO), and potassium myristate. The method has the following advantages: (i) N agg and Z are determined simultaneously, from the same set of experimental data; (ii) N agg and Z are determined for each given surfactant concentration (i.e. their concentration dependence is obtained), and (iii) N agg and Z can be determined even for turbid solutions, like those of carboxylates, where the micelles coexist with acid-soap crystallites, so that the application of other methods is difficult. The results indicate that the micelles of greater aggregation number have a lower degree of ionization, which can be explained with the effect of counterion binding. The proposed method is applicable to the concentration range, in which the films stratify and the micelles are spherical. This is satisfied for numerous systems representing scientific and practical interest.Display Omitted► At a given surfactant concentration, the heights of the steps are equal. ► From the measured step height, the micelle aggregation number is determined. ► The final film thickness h 0 is affected by the concentration of dissociated counterions. ► From the experimental h 0, the micelle charge (degree of ionization) is determined. ► The obtained micelle charge and aggregation number agree well with literature data.
Keywords: Charge of surfactant micelles; Micelle aggregation number; Degree of micelle ionization; Stepwise thinning of foam films; Stratification;

The problem is considered theoretically of dynamics of current-induced concentration polarization of interfaces between ideally perm-selective and non-ideally perm-selective (“leaky”) ion-exchange media in binary electrolyte solutions under galvanostatic conditions and at negligible volume flow. In contrast to the previous studies, the analysis is systematically carried out in terms of local thermodynamic equilibrium in the approximation of local electric neutrality in virtual solution. For macroscopically homogeneous media, this enables one to obtain model-independent results in quadratures for the stationary state as well as an approximate scaling-form solution for the transient response to the step-wise increase in electric-current density. These results are formulated in terms of such phenomenological properties of the “leaky” medium as ion transport numbers, diffusion permeability to salt and specific chemical capacity. An easy-to-solve numerically 1D PDE is also formulated in the same terms.A systematic parametric study is carried out within the scope of fine-pore model of “leaky” medium in terms of such properties as volumetric concentration of fixed electric charges and diffusivities of ions of symmetrical electrolyte. While previous studies paid principal attention to the shape and propagation rate of the so-called deionization “shocks”, we also consider in detail the time evolution of voltage drop and interface salt concentration. Our analysis confirms the previously predicted pattern of propagating deionization “shocks” within the “leaky” medium but also reveals several novel features. In particular, we demonstrate that the deionization-shock pattern is really pronounced only at intermediate ratios of fixed-charge concentration to the initial salt concentration and at quite high steady-state voltages where the model used in this and previous studies is applicable only at relatively early stages of concentration-polarization process.Display Omitted► Local-equilibrium analysis of over-limiting currents and deionization “shocks” ► Deionization-shock sharpness is generally moderate. ► Shock pattern is most pronounced at high stationary voltages. ► Two stages in time evolution: propagation and depletion
Keywords: Concentration polarization; Over-limiting current; Ion-exchange medium; Deionization shock; Propagation; Local equilibrium;

Lipid monolayer collapse and microbubble stability by James J. Kwan; Mark A. Borden (82-99).
Microbubbles are micrometer-size gaseous particles suspended in water, and they are often stabilized by a lipid monolayer shell. Natural microbubbles are found in freshwater and saltwater systems, and engineered microbubbles have a variety of applications in food sciences, biotechnology and medicine. Lipid-coated microbubbles are found to have remarkable stability and mechanical behavior owing to the resistance of the lipid monolayer encapsulation to collapse. The purpose of this review is to tie in recent observations of lipid-coated microbubble dissolution and gas exchange with current literature on the physics of lipid monolayer collapse in the context of lung surfactant. Based on this analysis, we conclude that microbubble shells collapse through the nucleation of microscopic folds, which then catalyze the formation and aggregation of new folds, leading to macroscopic folding events. This process results in a cyclic behavior of crumple-to-smooth transitions, which can be modulated through lipid composition. Eventually, the microbubbles stabilize at 1–2 μm diameter, regardless of initial size or lipid composition, and various mechanisms for this stabilization are postulated. Our ultimate goal is to inspire the reader to consider lipid monolayer collapse as the main long-term stabilizing mechanism for lipid-coated microbubbles, and to stimulate the use of microbubbles as a platform for studying monolayer collapse phenomena.Display Omitted► Lipid-coated microbubbles and their applications are introduced. ► Microbubble generation, structure and stability are discussed. ► Current theory on lipid monolayer collapse is reviewed. ► New data on microbubble dissolution are analyzed. ► Mechanisms for stabilization at a size of 1–2 μm are proposed.
Keywords: Phospholipid; Perfluorocarbon; Lung surfactant; Langmuir isotherm; Ultrasound contrast agent; Interfacial rheology;