Comparison between inhomogeneous adsorption of charged surfactants on air-water and on solid-water interfaces by self-consistent field theory

We use a realistic molecular model to study the interfacial behavior of hydrocarbon sulfate surfactants within a self-consistent field model and consider the adsorption both at the air-water interface and at a hydrophobic solid-water interface. We focus on the structural properties of the hemimicelles at the critical interface aggregation concentration (CIAC) for the air-water system and the critical surface aggregation concentration (CSAC) for the solid-water system. The major difference between the two systems is that the liquid interface is penetrable but the solid surface is intrinsically impenetrable for the molecular species. At the LG interface the hemimicelles have a lens shape with their centers of mass positioned slightly toward the aqueous side and feature an aspect ratio of approximately 2, with the long dimension parallel to the interface. Hemimicelle formation occurs below a critical (interfacial) area per molecule and above a critical surface pressure depending on tail length and ionic strength. Hemimicelles are not expected at air-water interfaces for a surfactant with a tail length ( t) lower than 15 CH2 units. In contrast, at a hydrophobic solid the formation of laterally inhomogeneous micelles even takes place for surfactants with the tail length as short as t = 12. This difference is attributed to the screening of the lateral interactions in the vapor phase. The shape of surface hemimicelles is caplike (or half-lens) with an aspect ratio lower than 2 and the long dimension parallel to the solid surface. The tail length, the ionic strength, the adsorption energies, and the surfactant concentration have an effect on the surface micelle properties such as the aggregation number and size and shape.     

Molecular interpretation of electrokinetic potentials

A discussion is given of the electrokinetic, or ζ-potential in terms of the slip process and the composition of the electric double layer. Electrokinetically, only the outer parts of double layers are active. The existence of a stagnant part is generally observed for aqueous solutions adjacent to solid surfaces. It is claimed that this stagnancy is caused by the spontaneous structuring of fluids near solid surfaces. Hence, it is a ubiquitous phenomenon and the thickness of the stagnant layer does not significantly depend on the wettability and the surface charge of the surfaces.     

Compression/expansion rheology of oil/water interfaces with adsorbed proteins. Comparison with the air/water surface

Dynamic interfacial tensions and surface dilational moduli were measured for four proteins at three fluid interfaces, as a function of time and concentration. The proteins-beta-casein, beta-lactoglobulin, bovine serum albumin, and ovalbumin-were adsorbed from aqueous solution against air, n-tetradecane, and a triacylglycerol oil. The sinusoidal interfacial compression/expansion, at frequencies ranging from 0.005 to 0.5 Hz, was effected in a dynamic drop tensiometer suited to viscous oil phases. Generally, at interfacial pressures up to 15 mN/m, dilational moduli were purely elastic at frequencies from 0.1 Hz. In this elastic range, in-surface relaxation either was essentially completed or had not yet started within a time on the order of 10 s. Within this time span, protein exchange with the bulk solution was negligible. In cases where in-surface relaxation was completed in the imposed time, the moduli depended only on the equilibrium Pi(Gamma) relationship. We interpret these results in terms of a simple two-dimensional solution model, based on a Gibbs dividing surface, accounting for nonideal mixing to the first order with respect to both entropy and enthalpy. Interfacial mixing enthalpy is shown to have a major effect on the elasticity, with both quantities increasing in the sequence triacylglycerol < tetradecane < air. We also suggest a strong correlation between enthalpy and clean-interface tension that increases in the same order. At each interface, the enthalpy increases with increasing molecular rigidity: beta-casein < beta-lactoglobulin < bovine serum albumin < ovalbumin. Best agreement with the experimental data was obtained with a recently extended version of the model accounting for proteins adopting smaller molecular areas with increasing surface pressure. For interfacial pressures above 15 mN/m, the moduli were generally no longer purely elastic, with viscous loss angles ranging up to 36 degrees. In this range of high pressures, the moduli depended on relaxation mechanisms for which specific kinetic models must be developed.     

Polymeradsorption und ihre Wirkung auf die Stabilit?t hydrophober Kolloide. III. Kinetik der Ausflockung von Silberiodid-Solen

Ohne Zusammenfassung     

Driving Forces for Enzyme Adsorption at Solid-Liquid Interfaces: 2. The Fungal Lipase Lipolase

Lipolase is the trade name for a fungal lipase that can catalyze the hydrolysis of ester bonds in, for example, triacylglycerol molecules. An important characteristic of this enzyme is that it is water-soluble whereas its substrate is water-insoluble. The adsorption of Lipolase was studied on several polystyrene latices and glass in order to determine the effect of the nature of the solid phase and to determine the interactions which are of importance for the adsorption. Electrostatic interaction and dehydration of hydrophobic surfaces are the main driving forces for Lipolase adsorption. Under attractive electrostatic conditions between the surface and the enzyme, the plateau value of the isotherm corresponds to saturated monolayer coverage. Under conditions where dehydration of the hydrophobic surface is almost compensated for by electrostatic repulsion the lateral repulsion between the adsorbed enzymes becomes also important and contributes to the surface coverage. The adsorption mechanism of Lipolase is similar to that of the protein Savinase. However, Lipolase adsorbs much less on hydrophobic interfaces under electrostatic repulsive conditions than proteins examined in the literature, indicating that the dehydrated contact area between enzyme and surface is relatively small and that consequently the enzyme does not unfold significantly upon adsorption.