ACIDITY AT A LIQUID-LIQUID INTERFACE

By using porous polypropylene as a support, we have measured the contact angle of buffered aqueous droplets on surfaces which were over half bis(2-ethylhexyl)hydrogen phosphate, HA. From the Variation of the contact angle with the pH of the aqueous solution the degree of dissociation, alpha, of HA has been evaluated as a function of pK. By defining the standard states of the interfacial HA and its conjugate base at the 50% ionized aqueous-organic interface, a pK(a) is obtained for the interfacial acid, equal to the pH of the aqueous solution which is in equilibrium with the 50% ionized interfacial acid. Following Katchalsky and Spitnik(14) the ratio of the activity coefficient of the conjugate base gamma(A)- to the activity coefficient of the acid, gamma(HA) is equated to [alpha/(1 - alpha)((n-1))] for alpha values near 0.5. This permits the evaluation of the empirical parameter, n, and leads to a relation between pH and alpha.