The relations between surface p H, ion concentrations and interfacial tension

Publisher Website
Google Scholar
Abstract
The form of the curves of variation of interfacial tension with pH, between long-chain acids in brombenzene and aq. solns., resembles those for the dissociation of the acid in the bulk phase, but the curves are shifted about 3 pH units, suggesting that the pH in the surface layer may differ from that in the bulk phase. With surface layers of oleic and palmitic acids in brombenzene, increasing alkalinity produced the usual fall in I.T. between pH''s 5 and 9. the neutral substances brombenzene and cholesterol showing only a slight fall, perhaps attributable to acid impurities. Dilution with water of the phosphate and borate buffers, made up 0.4 M for cation, resulted in a shift of the curves for I.T. with the brombenzene-acid about 1.14 pH units towards the alkaline for each 20-fold dilution; dilution with 0.4 M NaCl resulted in slight displacement only. In buffers at pH 8.4, decreasing the Na/Ca ratio, with the total cation concn. maintained constant, resulted in an increase of I.T. At pH 7.5 changing the cation ratio had little effect, at pH 6.9 the I.T. decreased slightly. Assuming that the amt. of lowering of I.T. due to ionization is proportional to the extent of ionization the effect of dilution is shown to be due to a Donnan equilibrium. Assigning probable values for the thickness of the surface phase, the number of car-boxyl groups per sq. cm. and the no. of ions contributed to the surface layer by the buffer soln., a method of calcu- lating the pH in the surface phase is developed. Under physiological conditions, this difference between bulk and surface phases may be of the order of 2 pH units. The main factors involved in the mechanism of reduction of I.T. are the number of molecules adsorbed per sq. cm. (Traube''s rule does not hold for oil-water interfaces), lateral adhesion between the molecules, and the area occupied by a molecule at the interface. Evidences from other sources that the I.T. changes here discussed are primarily due to changes in the ionization of the end groups of the surface-active molecules, and applications of the differences in pH of bulk and surface phases to biological problems, are discussed.

This publication has 2 references indexed in Scilit: