The theory of the discrete-ion effect for adsorbed counter-ions in the electric double layer is applied to fully ionized monolayers at A-W and O-W interfaces. This effect may be described in terms of a two-dimensional self-atmosphere or fluctuation potential acting at an adsorbed ion and is responsible for a negative term in the adsorption energy of the counter-ions which increases with their surface density. If the Stern theory is assumed, this term appears as a variation in the specific (chemical) adsorption potential with surface charge and this has been calculated for monolayers of long-chain sulfates. A second phenomenon predicted by the discrete-ion theory is the occurrence of a maximum in the magnitude of the potential at the plane of adsorbed ions when the surface charge is increased at constant ionic strength. A corresponding maximum is found at the slipping plane defining the £ potential and this behavior in the £ potential of O-W emulsions stabilized by SDS and DTAB monolayers has been observed. The presence of a maximum in the interfacial potential with variation in surface charge, observed for monolayers of long-chain sulfates, also can be attributed to the discrete-ion effect. The calculation of the "adsorption-plane" self-atmosphere potential depends on the model used for the "inner region" at the interface. In the case of an ionized monolayer, the adsorbed counter-ions and primary head-group ions are assumed to be situated in the same plane, embedded in an inner region having a dielectric constant smaller than that of the aqueous substrate. The various phenomena described above are reproduced by the theory if the specific binding between counter-ions and head-group ions is assumed small, so that the counterions are mobile. A detailed comparison is made between the theory and the experiments of Mingins and Pethica on surface potentials of sodium octadecyl sulfate at the A-W interface in the presence of 0.01 M NaCl.
Using compression techniques, surface pressure (II) against area ( A ) isotherms are measured as a function of temperature for insoluble monolayers of 1,2-distearoyl-sn-glycero-3-phosphorylcholine spread at the n-heptane/aqueous sodium chloride interface. Problems of spreading are resolved and accurate results at moderate and high IT are presented. The results show an almost first order phase change which is very temperature sensitive. Various forms of the Clapeyron analysis are applied to the data to calculate the heats of the phase change and to assess variations arising from the choice of a close-packed area. These heats vary with temperature but not with salt concentration or pH over the range studied. Results on 1,2-dioleoyl-sn-glycero-3-phosphorylcholine show no such phase changes.
We have previously found that saturated phospholipids such as phosphatidylethanolamines can, in certain cases, adopt as many as three different inverse bicontinuous cubic phases in water, of probable space groups Ia3d (No 230), Im3m (No 229) and Pn3m (No 224). We found that these cubic phases could be induced to appear by reducing the chain length, or by increasing the hydrophilicity of the headgroup of the phospholipid molecule. All of these cubic phases are located in the phase diagrams between the lamellar and the inverse hexagonal (HIJ phases. We now report the observation of a novel inverse facecentred cubic phase, of probable space group Fd3m (No 227), in two different systems of hydrated binary lipid mixtures. One of these systems consists of mixtures of phosphatidylcholine with diacylglycerol; the other is an acid-soap mixture of an unsaturated fatty acid with its alkali salt. This Fd3m cubic phase in both systems occurs between the inverse hexagonal (H1J phase and the inverse micellar solution (L2). with increasing concentration of the lipid component with the less strongly hydrophilic headgroup. We surmise that the average mean curvature of the polar/non-polar interface in this Fd3m cubic phase is more negative than that of the neighbouring HI, phase; this is quite different from the inverse bicontinuous cubic phases, where it has a value intermediate between those of the lamellar and HII phases. We conclude that the structure of this Fd3m cubic phase most probably consists solely of closed inverse micellar aggregates.
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