The adsorption of iodide ion at mercury has been studied from two systems at constant ionic strength, namely, xMnormalNaI+false(0.25−xfalse)MnormalNaF and xMKI+false(0.25−xfalse)MKF . The surface excess of adsorbed iodide ions was calculated from differential capacity data using a modified version of the Hurwitz‐Parsons analysis which takes into consideration variation in ionic activity coefficients with solution composition. The iodide surface excess was found to depend on the nature of the cation for a given bulk concentration and electrode charge density; comparison of the present data with those previously published shows a marked dependence of adsorbed surface excess on ionic strength. The effect of counterion nature on ionic adsorption is discussed within the framework of an electrostatic model for adsorption which takes the discrete nature of the charge at the interface into consideration. Counterion effects at charged monolayers, bilayer lipid membranes, and real membranes are compared with those observed at mercury.

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amination of the literature reveals that counterion effects have been observed in studies of the electrical properties of other charged interfaces (27). From a biological point of view, the physical properties of charged monotayers and bilayer lipid membranes (BLM) are often studied in the hope that the experimental results will help elucidate the behavior of membranes in living systems. However, it must be emphasized that counterion effects at artificial membranes do not necessarily correspond to those in living systems. In the following discussion only those effects due to cations as counterions will be considered. In this regard one may distinguish two cation effects at negatively charged monolayers and BLM. The first is the selectivity for cation transport which is often observed in these systems and which depends both on the chemical nature of the film and also on the presence of lipid soluble carriers (28-30). The second effect and that with Which this paper is concerned is the nature of the average and local potential profiles at the interfaces of the system and its influence on adsorption of other ions and dipolar molecules and on transport through the film.Goddard and co-workers (31-34) have studied the effects of counterion nature on the surface pressure and potential at both positively and negatively charged monolayers. For negatively charged head groups, these properties depend both on the nature of the head group and on the nature of the cation in solution. Examination of the literature on cation effects at negatively charged coIi:oids reveals that cation interaction depends very much on the nature of the anionic group and that more than one interaction sequence for alkali metal cations is observed experimentally (35). Cation interaction with BLNI does not appear to have been studied in any detail (29, 30) but electrical properties such as BLM capacitance have been found to depend on the nature and concentration of the electrolyte in the surrounding solution (29).Erlander (35) has shown that cation interaction sequences at negatively charged colloids can be rationalized on the basis of the Frank and Wen model (36) of water structure surrounding the interacting ions with consideration of their effective dielectric constants, AccOrdingly, ions with high charge to radius ratios are surrounded by A-type water which is strongly bound to the ion by ion-dipole interactions. Most ions are also surrounded by B-type water in which the normal structure is broken and neither ion-dipole nor dipole-dipole interactions predominate. The normal water structure which is determined by dipole-dipole interactions and hydrogen bonding is designated C-type. Thus, Li +, Mg ++, Ca ++ , Sr++, and Ba ++ ions are surrounded immediately by A water with a layer of B water beyond the tightly bound dipoles. Cs +, Rh +, K +, CI-, Br-, and I-do not have any ordering effect and thus are surrounded by a B region only. Na + and Fare considered to have an A region but no B region. Erlander has estimated the effective dielectric constant...

The relative merits of three acrylic based resins, cured either chemically or by gamma radiation, are examined for use in the impregnation of soils for thin section analysis. The effect of several soil variables (moisture content, pH, texture, presence of expanding minerals or transition metals) upon these resins is discussed to aid in the selection of a suitable resin for the impregnation of various soils.