Rajendra P. Borwankar

Equilibrium and dynamics of adsorption of surfactants at fluid-fluid interfaces

Abstract Surfactant adsorption at fluid-fluid interfaces is modeled with respect to both equilibrium and dynamic behavior. A thermodynamic treatment is developed wherein the surface concentration of ionic surfactants is distinguished from their surface excess concentration by the contribution from the electrical double layer. Only the surface concentrations are assigned to the dividing surface and the electrical double layer is considered to be a part of the aqueous phase. A general adsorption isotherm is derived by including non-idealities in the bulk as well as at the dividing surface. The corresponding equation of state is derived using the Gibbs adsorption isotherm. Several well-known adsorption isotherms and equations of state arise as limiting cases of the general equations of this model. The model is shown to correlate with several published surface and interfacial tension measurements on ionic surfactant systems with or without indifferent electrolytes. Ideal behavior is observed at the interfaces of all liquid-liquid systems investigated. Surface non-idealities are observed only in gas-liquid systems. It is concluded that the non-idealities do not arise from the electrical interactions but rather from the attractive interactions between the hydrophobic tails of the adsorbed surfactant. This approach is superior to the approaches based on a Gibbsian dividing surface since it is readily applicable to the treatment of electrostatic and electrokinetic phenomena as demonstrated by a remarkable correlation achieved between the calculated values of interfacial potentials and observed electrophoretic mobilities. The model is extended using phenomenological considerations to the case of systems not at equilibrium. A continuum treatment of adsorption dynamics is formulated wherein surfactant transport in the bulk phases is described using transport equations with constant coefficients. Activation energy barriers to solute exchange between the bulk and the dividing surface are represented by kinetics of a reversible reaction. The kinetic expression corresponding to the general adsorption isotherm is derived from phenomenological considerations.

The kinetics of adsorption of surface active agents at gas-liquid surfaces

Abstract A general model of adsorption of surfactants at a gas-liquid surface has been developed, which accounts for both the diffusion in the bulk and the barrier to adsorption. In this model, the activation energy barrier to adsorption is accounted for by means of a kinetic expression. A numerical scheme for the solution of the resulting equation of the diffusion-kinetic model is illustrated for the case of the Frumkin isotherm. The model is used to estimate the extent of the adsorption barrier from the dynamic surface tension data of two aqueous surfactant systems reported in the literature. A simplification of the diffusion-kinetic model is proposed which accounts for diffusion in the bulk using diffusion-penetration theory. The simplified model compares very well with the exact model, especially for high barrier resistances.

Pair interaction energy between deformable drops and bubbles

The potential energy of interaction between two deformed particles, having the shape of spherical segments and separated by a planar film, is considered. An exact explicit expression for the van der Waals interaction energy between such deformed particles of arbitrary size and deformation is derived by using the microscopic approach of Hamaker [Physica 4, 1058 (1937)]. By means of Derjaguin’s approximation [Kolloid Z. 69, 155 (1934)] explicit expressions for the electrostatic, steric, depletion, and other types of interaction are derived. The relative contributions of the interaction across the planar film and between the spherical surfaces surrounding the film, are analyzed for different cases. The surface deformation energy caused by an increase of the interfacial area during the particle deformation is also considered. The implication of the obtained formulas for treatment of the particle interactions in some particular systems like emulsions, microemulsions, and vesicles, as well as for the adhesion o...

Emulsion stability : kinetics of flocculation and coalescence

Abstract A mathematical model that accounts for the kinetics of flocculation and coalescence or emulsion drops was developed in order to quantify the kinetic stability of emulsions. The model is similar to that developed by van den Tempel with a correction to account for the coalescence in small flocs. The model identifies the conditions under which either the flocculation or coalescence rate controls the kinetics. The model also demonstrates that the rate-controlling mechanism could change from coalescence-rate controlling to flocculation-rate controlling during the course of the emulsion life. An extension to the model for concentrated emulsions was also developed. The model was used to fit two sets of experimental data to determine the kinetic constants that characterize the coalescence in the emulsions.

The role of additives for the behaviour of thin emulsion films stabilized by proteins

Abstract Experimental results obtained with thin aqueous films of emulsion type stabilized by bovine serum albumin (BSA) and β-casein are presented. The film behaviour is time dependent. The contact angle increases with ageing and exhibits pronounced hysteresis. With BSA one observes slow reversible aggregation on the surface (but not in the bulk) and the protein lumps are gradually squashed by the capillary pressure as the film thins. The findings can be explained by slow surface denaturation, accompanied by developing attraction and partial entanglement of the BSA molecules. These processes are promoted by oleic acid dissolved in the oil phase. Electrostatic interactions were found to be important: without salt the films remain thick, whereas in the presence of 0.15 M NaCl one obtains Newton black films whose contact angle depends upon the molecular charge. A marked difference in the surface mobility is observed with foam and emulsion films stabilized by BSA. Lenses, containing protein aggregates and liquid, when surrounded by an area which has reached the black film stage, remain entrapped in foam films but are slowly squeezed out in emulsion films. Hydrophobization of the protein molecules may be responsible for this behaviour. With β-casein, ageing effects in films are observed only at the isoelectric point. This protein strongly aggregates in the bulk, but the lumps are readily flattened on the film interfaces. Addition of Ca 2+ ions leads to a decrease in film thickness, depending on the concentration.

Competitive adsorption of monoglycerides and lecithin at the vegetable oil—water interface

Abstract Understanding the competitive adsorption behavior of monoglycerides and lecithins is critical, especially in the development and processing of low-fat spreads. In the present study two commercial oil blends, containing different levels of native surface-active impurities, were used. Our data show that lecithin has higher surface activity than the impurities in the oil blends, which in turn are more surface active than the monoglyceride. Hence, in the presence of lecithin the surface-active impurities are excluded from the interface. At high levels of the monoglyceride, lecithin gets incorporated in the reverse micelles and is also removed from the interface. Experiments at different temperatures with soybean oil containing native surface-active impurities also support this hypothesis.

Experimental investigations on model emulsion systems stabilized with non-ionic surfactant blends

Abstract We have performed experimental research into model oil-in-water emulsion systems stabilized with non-ionic surfactant blends: thin aqueous films between oil phases and oil drops coalescing against their homophase. Xylene was chosen as the oil phase, and Tween 20 and Span 20, alone or in mixtures at different molar ratios, were used as stabilizers. The roles of the surfactant mole ratio, the electrolyte concentration and pH were studied. It was shown that there is considerable electrostatic repulsion within the aqueous films. The results obtained on thin film stability and on drop lifetime at constant electrolyte concentration indicate that Tween 20 (when present) is the emulsifier that predominantly determines the stability of the systems. This conclusion can be related to the higher surface activity and higher adsorption of Tween as deducted from interfacial tension data. Some evidence of synergism was observed only at an electrolyte concentration of 10 −3 M (NaCl). We also carried out stability tests on shaken and homogenized batch emulsion systems and the results show good correlation with the data from the model investigations.

Foam film rheology and thickness stability of foam-based food products

Abstract The foam film rheology and foam film thickness stability produced from fat-in-water emulsions were investigated by a novel film rheometer. Several important properties, such as dynamic film tension, foam film elasticity and critical film expansion area, were obtained. It is found that the film elasticity and foam critical expansion area for 20 wt.% fat foam film are 125.9 mN m−1, 66% respectively, while those for 12 wt.% fat foam film are 104.2 mN m−1, 46% respectively. The higher the foam film elasticity and critical expansion area, the more stable the foam-based products.

Flocculation of food dispersions by gums : isotropic/anisotropic dispersion separation by xanthan gum

Creaming (flocculation) of a model food dispersion in the presence of xanthan and guar gums was studied using phase separation measurements, back-light scattering studies and microscopic observations. Xanthan gum was a much more effective flocculent than guar gum, especially at lower gum concentrations. Dispersion separation with xanthan gum also showed interesting features. The degree of separation gradually decreased with decreasing xanthan gum concentration and creaming could be detected as low as 10 ppm xanthan. Moreover, separation by xanthan gum was non-specific on the nature of the dispersion (fat, silica or latex dispersion) and had a phase separation character: most of the macromolecules migrated out of the flocculated dispersion into the aqueous phase. Based on these observations, it is suggested that the mechanism of dispersion separation by xanthan gum is a geometrical incompatibility between the anisotropic, worm-shaped xanthan molecules and the isotropic dispersed particles. The effects of xanthan gum molecular mass and dispersion concentration were in agreement with the theories on isotropic/anisotropic phase separations.

Composition of mixed adsorption layers of non-ionic surfactants an oil/water interfaces

Abstract The subject of this work is the adsorption of binary mixtures of non-ionic surfactants whose molecules differ considerably in size. Since the surface layers are known to behave ideally, we investigate the role of the configurational entropy of mixing. We adopt the Gibbs model for the interface and propose a statistical treatment of the Flory—Huggins type for the case of incomplete coverage. Expressions for the surface chemical potentials are derived from the canonic partition function. The solvent-surfactant interactions are also discussed briefly and it is confirmed that for non-ionic surfactants on the oil/water boundary the mixing entropy effects are predominant. The theory allows us to develop a method for obtaining the composition of the adsorption layer using data for the interfacial tension isotherms in mixed systems and for the individual surfactants. We study xylene—water systems containing Span 20, Tween 20 and their mixtures in proportions of 1:1 and 9:1 (molar ratio). From the equilibrium interfacial tensions we deduce the surface composition in a wide range of concentrations. It turns out that the adsorption layer is always enriched with Tween 20 owing to its higher surface activity. The results can be helpful in connection with emulsion stability.