The bulk properties of clay-water dispersions, particularly with montmorillonites, depend on the very large surface area of the materials and their plate-like form. Owing to isomorphous substitution in the clay lattice the plates are charged and hence interact electrostatically with each other. The form of the interaction was tested using macroscopic clay surfaces in the form of cleaved mica and the results compared with those obtained by compression of concentrated aqueous dispersions of various montmorillonites in a homoionic form. The results obtained with mica and lithium montmorillonite were comparable suggesting the latter disperses as single plates. When sodium, potassium and caesium were used as the counter-ions for montmorillonite, differences were observed which were attributed to face-face association. The consequences of isomorphous substitution in either the tetrahedral or the octahedral layer were also examined.
Application of an electric field to insoluble monolayers at an air-water interface is shown to induce changes in surface pressure and monolayer density which correlate with surface potentials in accordance with a recently published thermodynamic analysis using Gibbsian methods. For twocomponent monolayers, the interdependence of local fields and composition has been demonstrated qualitatively. The results provide experimental proof that the thermodynamic treatments of the Volta effect due to Kelvin, Lorentz and Bridgman are incorrect. These treatments essentially ignore the dependence of the interfacial structure of oriented dipoles on the applied field. Correspondingly, the experimental results open up novel approaches for relating these interfacial structures to interfacial potentials.
Double layer theories of ionized monolayers have mostly been developed for the adsorption of soluble ionic surfactants at the air/water and oil/ water interfaces, with much disagreement, due in part to the adoption of nonthermodynamic assumptions and the lack of solution activity data for estimating surface excess densities from surface tension experiments. Both surface pressure and density can be measured directly with insoluble spread monolayers of ionized surfactants, allowing simpler analysis of the several contributions to the surface properties. Data are presented on the surface pressureÀsurface area isotherms for very dilute spread monolayers of sodium octadecyl sulfate (NaODS) at the air/water interface at two temperatures and three NaCl concentrations. The results indicate that the basic Gouy model fails to account for the contribution of the ionic double layer to the surface pressure in the low density region at low NaCl concentrations, both conditions favorable to the model. Of the possible corrections, the discreteness of charge effect, interionic van der Waals forces, headgroup immersion, and ion association with the charged monolayer head groups all appear to be relevant factors. The results indicate that the electrostatic and van der Waals contributions to the surface pressure are not independent additive terms. Further complex electrostatic modifications to traditional ionic double layer models are considered unpromising. Recently developed methods in molecular dynamics and lattice summation methods which avoid the nonphysical PoissonÀBoltzmann equation and include ionÀion van der Waals forces have shown more constructive insights for advancing an understanding of ionic double layers.
As part of a collaborative programme to establish criteria for surface manometry a detailed study was made of the spreading pressures for tetradecanoic, pentadecanoic and hexadecanoic acid crystals prepared from melts and solutions. The spreading pressures vary considerably for a given acid. The removal of solvents from recrystallised acids and the solubilities of chosen crystal samples in hexane were measured. The spreading pressures were then correlated for the various crystal preparations. Optical and scanning electron microscopy were used to characterise crystal geometries, and the crystallographic forms were determined by X-ray diffraction. It was established that solvent retention in the crystals can lower the spreading pressure, and that crystals with microscopic points and edges show high spreading pressures. The results can be interpreted simply by thermodynamic arguments. Data on the effect of temperature on the spreading pressures and solubilities are also presented, and the enthalpies of spreading and solution are calculated.Many lipids spread spontaneously when placed in solid or liquid form at the air/water (A/W) interface to give monolayers in apparent equilibrium with the excess bulk lipid phase. The corresponding monolayer surface pressure (n) is generally known as the equilibrium spreading pressure (e.s.p.). The e.s.p. is, in principle, useful for obtaining thermodynamic information on the crystal-monolayer transition. Unfortunately the reported values for a range of lipids are diverse beyond reasonable experimental error, as shown in table 1 for several fatty-acid homologues. We have recently examined in some depth the conditions for reproducible surface manometry using several manometric techniques, identifying experimental artefacts and showing the large effects given by small impurity le~els.~-ll Using the techniques developed it was found that the spreading pressure of a given lipid varies significantly from pure sample to pure sample, whereas the pressures for the liquid-expanded-liquid-condensed and the liquid-vapour phase transitions for the same samples did not. These various spreading pressures are often the metastable values for non-equilibrium solid
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