A.E Alexander

Kinetics of adsorption of proteins at interfaces. Part I. The role of bulk diffusion in adsorption

Abstract Owing to their low diffusion constants and the (in general) irreversibility of their adsorption, proteins can be used with advantage in elucidating the general mechanism of adsorption at interfaces. In the present study of adsorption at the air/water interface this has been followed firstly, in the absence of an appreciable film pressure, that is in the very initial stages, and secondly, in the later stages when an appreciable film pressure exists, due either to the protein itself or to a spread monolayer. In the former case bulk diffusion accounts very reasonably for the observed rates; in the latter the observed rates are much slower than calculated, suggesting the building up of a barrier associated with the film pressure.

Surface properties and flow behavior of kaolinite. Part I: Electrophoretic mobility and stability of kaolinite sols

The electrophoretic mobility of kaolinite sols has been studied as a function of electrolyte concentration in NaHCO3-Na2CO3 buffer at pH 10.4. The positive charges which occur on the edges of the kaolinite particles in acid solution are no longer present at this pH and the particles are negatively charged owing to defects in the crystal lattice. Most of the exchangeable cations are evidently quite firmly attached to the particles in a Stern layer. n nThe calculated ζ-potentials are considered to give a measure of the potential (Ψδ) at the outer edge of the Stern layer (i.e., the diffuse double layer potential), and this is used to calculate the potential energy of repulsion between the particles as a function of distance. Rapid coagulation of the sol by a uni-univalent electrolyte at this pH occurs at an ionic strength of about 0.016 g. ion/l.; this is consistent with a value for the van der Waals constant, A, of 2 × 10−12 erg, in agreement with the values previously suggested by Street and Buchanan, Ottewill and Watanabe, and Overbeek.

The effect of sucrose on protein films I. Spread monolayers

Although much work has been done in the past on protein films (1), comparatively little attention has been paid to the effect of agents which influence hydrogen bonding, in view of the great importance of this bond in proteins. The effect of urea has been studied to some extent (2) but mainly as regards surface pressure changes, which, however, are much less sensitive than the mechanical properties (surface viscosity and elasticity) for detecting changes in molecular bonding. Sucrose offers the advantage of high solubility in water without affecting the pH or ionic strength, factors which are known to influence protein films. The study of the effects of sucrose on protein films is also relevant to certain technological systems in which foams and emulsions are stabilized by proteins in the presence of sugars. In this work, protein films have been studied as spread monolayers and as films adsorbed from solution, together with some bulk foam measurements.

The surface chemistry of wheat gluten II. Measurements of surface viscoelasticity

The dependence of surface viscoelasticity of gluten films at the interface on the pH and the ionic strength of the substrate was studied by means of an oscillating needle surface torsion pendulum. Some experiments were carried out at the interface between oil and 10% sodium salicylate or 24% urea. n nGluten was spread from dispersion in anhydrous chloroethanol containing 0.1 M hydrogen chloride. Film behavior showed a marked dependence on the pH and the ionic strength of the substrate. The highest viscoelasticity was observed around pH 7.5, which may be regarded as the surface isoelectric point of gluten when spread from chloroethanol. The decrease in viscoelasticity away from the isoelectric point was more pronounced on the acid side. The viscoelasticity, at the same area and pH, decreased with an increase in the ionic strength. n nExtrapolation of the regression of the reciprocal loss angles on area gave the area at which elasticity begins to appear as about 1.0 m^2/mg. This agrees well with the area of minimum compressibility as obtained from the π-A isotherms. n nThe viscoelasticity of gluten films spontaneously increased with time after spreading while the interfacial pressure remained constant. This increase is believed to reflect intermolecular bond formation. Extrapolation of the regression of the rate constants K of the time increase of the surface viscosity on the area to zero K indicated that bonding begins to occur at about 1.54 m^2/mg. This is identical with the area of close-packing found from an extrapolation of the π-A curves to zero pressure. n nUrea had little effect but films spread under sodium salicylate showed hardly any viscoelasticity. These findings, together with the effect produced by a change in the pH of the substrate, point to an interplay between hydrogen bonds and ionic linkages.

Surface properties and flow behavior of kaolinite. Part III: Flow of kaolinite sols through a silica column

Abstract The influence of various physical and chemical conditions on the flow of a colloidal suspension of kaolinite through a silica column has been investigated. A stable (i.e., slowly coagulating) sol, upon entering the column, causes an appreciable reduction in permeability but an equilibrium value is soon established, after which the remainder of the sol passes through unchanged. The equilibrium permeability is decreased by increase in electrolyte concentration, by decrease in the size of the silica particles, and by reversal of the electric charge on the silica surfaces. Increase in time of contact of the sol with the column causes an initial decrease in equilibrium permeability followed by an increase if the contact time is prolonged. The temperature of the column and the ζ-potential of the clay particles also influence the process. The mass of the deposited material was found to be linearly related to the equilibrium permeability. In the model proposed, the above effects are ascribed to a balance between the opposing effects of the yield strength of the sol and the shear stress applied to the deposited clay by the flowing liquid.

Surface properties and flow behavior of kaolinite. Part II. Electrophoretic studies of anion adsorption

Abstract Electrophoretic measurements on kaolinite at pH 7.4 in mixtures of NaCl and phosphate buffer have been used to calculate ζ-potential and surface charge density. These results provide confirmatory evidence for the occurrence of a specific phosphate adsorption process. Chloride ion is also adsorbed at pH 7.4, possibly by a physical process. The extent of chloride adsorption calculated from electrophoretic measurements agrees well with that arrived at by the direct analytical method, provided it is assumed that the ζ-potential is a direct measure of the Stern potential. An equation which allows the analytical results to be corrected for “negative adsorption” is derived for the case when the surface charge density is low.

Some notes on the measurement of electrokinetic potentials

Abstract The solution of a number of problems which arise in the measurement of ζ-potentials, by the microelectrophoretic and streaming potential methods, is discussed. For microelectrophoretic measurements a simplified statement of the Henry optical correction is derived, and two methods are described for determining the voltage gradient in a two-tube microelectrophoresis cell. For streaming potential measurements a method is suggested for overcoming the effects of “asymmetry potentials” on the electrodes.

Surface films of vinyl stearate, cetyl-vinyl ether, and their polymers

Abstract Monolayers of vinyl stearate and cetyl-vinyl ether, spread at an air/water interface on substrates of different acidity (0.005 N -5 N HCl), have been examined by the techniques of surface pressure, surface potential, and surface moment. The vinyl compounds were chosen as representative of monomers subject in the one case to free radical polymerization, and in the other to cationic polymerization. Parallel studies of the corresponding saturated compounds, ethyl stearate and cetyl-ethyl ether, permitted an analysis of the influence of the double bonds in the vinyl compounds. All four compounds gave condensed films at 25°C. As compared with the corresponding saturated compound vinyl stearate was found to have a substantially higher and cetyl vinyl ether a substantially lower , surface moment. In the case of cetyl vinyl ether the surface moment was very sensitive to the acidity of the substrate. These results show that the vinyl groups affect the properties of the monolayers in a way which can be related to the interaction of the adjacent polar groups with the double bonds. Polymers of the two monomer species were also prepared and examined as monolayers, the results being interpreted in terms of the structure and orientation of the monomer units.

The surface chemistry of wheat gluten I. Surface pressure measurements

Wheat gluten dispersed in a 0.1 M solution of hydrogen chloride in anhydrous chloroethanol was spread at the A/W and the O/W interface and the surface pressure π of the film measured as a function of the area (A) over a wide range of pH (1-13) at the ionic strengths of 0.02 and 0.1. Some measurements were carried out at the interface between oil and 10 % sodium salicylate or 24 % urea. nGluten films were found to be more compressible and more stable than those of most other proteins. Pressures as high as 32 dynes/cm. (A/W) and 36 dynes/cm. (O/W) were obtained at an area of 0.2 m^2/mg, while the minimum compressibility was 0.05 cm./dyne. The extrapolated area of close-packing was 1.33 m^2/mg, at the A/W and 1.54 m^2/mg, at the O/W interface. The area of minimum compressibility was 1.1 m^2/mg, at both interfaces. nAt the O/W interface variation in the pH of the substrate caused differences in the degree of expansion of the films at higher areas but all the π-A isotherms between pH 1 and 12 met at about the area of minimum compressibility. The area at constant pressure (1 dyne/era.) as a function of pH gave a W-shaped curve showing minima at about pH 5.5 and 8.5 and a maximum at about pH 7.5. Variation in ionic strength from 0.02 to 0.1 had no effect on the minima; at the lower ionic strength the curve was flatter at the maximum and at the extreme ends. At pH 13 the π-A isotherm was quite different, indicating reduced cohesion in the film. The isotherm at the oil/aqueous salicylate interface was very similar to the pH 13 curve. At the oil/aqueous urea interface the isotherm was much expanded at the higher areas but met the control curve at about the area of minimum compressibility. nIt is concluded that spread gluten forms strongly bonded, coherent films, parts of which, when compressed below the area at which the films may be regarded as a close-packed monolayer, are displaced from the interface with relative ease and are folded up into loops. The unusually strong cohesion shown by these films may be due in part to hydrogen bonding between the numerous glutamine side chains, modified by electrostatic attraction. The rather high areas of close-packing and of minimum compressibility are probably due to the relatively high proline content of gluten.