Daniel Schuhmann

A study of the interfacial behavior of polyoxyethylene at a mercury—aqueous solution interface: Compact and diffuse layers

Abstract The adsorption of polyoxyethylene (POE) at a mercury-solution interface has been studied, using NaF, KF, and KCl as supporting electrolytes and polymers over a molecular weight range from 400 to 4000. In the first part, a flat configuration of the adsorbed macromolecule is put forward from an analysis of data concerning three characteristic properties of the adsorbed layer. In the second part, information on the diffuse layer in the presence of a saturated layer of POE is obtained from experiments at various salt concentrations. While the classical models of Gouy-Chapman (GC) and Stern-Grahame for the diffuse and global double layer are well verified for concentrations higher than 10−2 M, a deviation from the theoretical expectation is found for more dilute solutions, increasing with the absolute value of the surface charge. It seems difficult to interpret this effect in terms of an artifact due to the experimental techniques or to solution impurities. Similar deviations from the GC model are expected in the case of electrophoretic measurements. A comprehensive explanation is not given but the results show that it is related to the particular nature of the double layer in the presence of polymer.

Electrical properties of adsorbed or spread films: The effective value of permittivities in the Helmholtz equation (plane distribution of point dipoles)

Abstract In contrast to the Helmholtz formula (HF), surface potential data obtained with spread or adsorbed films formed with multipolar molecules were recently analyzed assuming different dielectric constants for the potential components due to the various dipoles involved with each molecule. It is shown that HF is valid in special cases only (isolated dipoles at a metal-vacuum or air interface). Imaging effects generally should be taken into account. A calculation for foliated plane systems based on the Fourier-Bessel transform is reported. It provides a better theoretical basis than HF for analyzing data with mono- or multipolar molecules.

On the influence of the adsorbent, the hydrophobic, and the oxyethylenic chain lengths on nonionic surfactant adsorption

Abstract A discussion of previously published results concerned mainly with adsorption of copolymers of the type CmH2m+1O(CH2CH2O)pH as a function of m (including m = 0), p, and of the adsorbent is reported. The maximum surface excess and the corresponding molecular area could be determined only from the experiments. The results taken as a whole can be explained by a two-state adsorption model, the former state being determined by hydrophobic interactions and the latter by specific interactions between the surface and the hydrophilic tail. The ratio of the adsorption equilibrium constants corresponding to each state is determined by comparison between the predictions of the model and the available data, this ratio varies according to m and temperature. Differences from some conclusions on the results previously reported are pointed out. Some implications of the information derived from this work are discussed.

The effect of electrical nonhomogeneities of particles on mean surface potential

Abstract When a surface charge is not uniformly distributed on the surface of a particle and when the linearized Poisson-Boltzmann equation cannot be used (high values of the zeta potential), the mean surface charge density calculated from the surface potential may be different from the value derived using a classical formula. This can be expressed as a coupling effect between the diffuse layers corresponding to different uniform areas. The fundamental term of the distortion is derived from a model based on a double Fourier expansion. Computations show that the distortion is negligible in some cases but can be important in other cases, for example, in the case of irreversible ionic adsorption on nonuniform mineral particles. Repercussions of the coupling effect on electrophoretic measurements are discussed in reference to J. L. Andersons treatment ( J. Colloid Interface Sci. 105, 45 (1985)).

On the impedance of the interface between two immiscible electrolytic solutions in the presence of an adsorbed ion soluble in both media

Abstract A theoretical study of the interfacial impedance between two immiscible liquids is undertaken assuming that, before being transferred from one medium to the other, the ion considered can be adsorbed at the interface. The simple corresponding model is applicable within the whole range of frequencies explored for one of the two quaternary ammonium ion studied. In the case of the other, additional complications must be taken into consideration to explain the behavior observed at low frequencies. This could be explained by the existence of two adsorption states corresponding possibly to two different orientations of the adsorbed ion. This second model forecasts a behavior approximately similar to the previous one, except in the case of the lower frequencies. The determination of several kinetic parameters with the help of the first model leads to very small values for the apparent transfer coefficient which are perfectly compatible with the results previously obtained for these type of systems. Further information could be gained by extending the region of frequencies studied toward the lower values and by comparing the spectra thus given with the concentration of the ion which is transferred.

An example of specific interactions between ions and adsorbed polymers: polyethylene oxide and selenite

Abstract It is shown that the adsorption of selenite on a mercury electrode is doubled when polyethylene oxide (PEO) 400 is adsorbed as a compact monolayer. This polymer has no effect on other ions and another polymer, polyvinyl alcohol (PVA) 15,000, does not enhance the adsorption of selenite. This selective interaction is explained by the high hydrophilicity of PEO giving structured water near the interface and by the equality of the distance between the oxygen atoms in two neighboring water molecules and in the oxyanion.

Preliminary noteThe nature of specific adsorption: Adsorption of chloride and iodide ions at mercury in the presence of polyethylene oxide

It is shown that some of our recent experiments, reported herein, carried out with halide solutions in the presence of a dissolved polymer (polyethylene oxide, PEO) throw more light on this subject

Interaction of yeast 3-phosphoglycerate kinase with negatively charged carriers

The aim of this study was to investigate the possibility of an interaction of yeast 3-phosphoglycerate kinase with negatively charged carriers such as polyanionic agents or a polarized electrode. Various polyanions were found to promote enzyme aggregation as judged by ultracentrifugation measurements and chemical modification. The data obtained suggest that these interactions are mediated through the N-terminal domain of the protein. However, the most striking property of 3-phosphoglycerate kinase described here is concerned with its significant dipolar moment as evidenced by electrocapillary measurements, which allows an orientation of the macromolecule in an electric field. Further, the enzyme could be absorbed by a negatively charged surface, first by hydrophobic links and then oriented perpendicularly to the surface. Therefore, the intrinsic properties of yeast 3-phosphoglycerate kinase agree with the formation of an enzyme-membrane complex and afford the ability for a specific orientation of the molecule at the lipid bilayer surface or in the cytoplasm.

The variation of the superficial excess of entropy in relation to the structure of water in sodium fluoride solutions

Abstract The mercury-NaF aqueous solutions interfacial tension has been measured at varying concentrations and temperatures. These results, associated with those of other authors [1,2], enable one to determine, without ambiguity, and for all accessible values of the electrode charge, the sign and the order of magnitude of the variation with the temperature of the superficial excess of entropy. The curves showing the variations of the superficial excess of concentration with the charge indicate a very slight specific adsorption of the fluoride ions at low temperatures which leads to a contribution of about 0.007 erg K−1 cm−2 to the entropy. These results have been compared with those forecast theoretically by Bockris and Habib [3] who schematically consider the interfacial layer of water as being composed of monomers and dimers, the latter being unaffected by the electrical field. Taking into account the variation with the temperature of the various factors contributing to the interfacial entropy, as envisaged by these authors, one finds a variation of −0.04 erg Kt-1 cm−2 as compared with +0.14 erg K−1 cm−2 found experimentally. This reveals the shortcomings of the proposed model in spite of the good agreement obtained at one temperature for the variations with the charge. This model can therefore only be considered as acceptable if supplementary hypotheses are made as for example assuming interactions between the different molecular forms. One can also consider that this type of dimer or even larger aggregate such as those envisaged by Frumkin [4] and Parsons [5] for the interfacial water, consistutes a somewhat over simplified approach of the problem. The experimental results obtained and their examination in the light of recent theories show that the important problems of the structure of the interfacial water still remains unsolved. There is therefore a need for a statistical approach to these questions based the theories suggested for the volume, and also taking into account the influence of the substratum.