Conductometric study of hydrophobic interactions in aqueous solutions of double long-chain electrolytes

Abstract

We report measurements of the equivalent conductance (Λ) of aqueous solutions of a series of decyl- and hexadecyl-trimethylammonium carboxylates (acetate to undecanoate). In the decyl series, measurements were made on both sides of the critical micelle concentration (CMC). The CMC decreased when the hydrocarbon chain length of the carboxylate ion increased. In the hexadecyl series, measurements were confined to micellar solutions.     The results obtained below the CMC suggest that hydrophobic interaction between the hydrocarbon chains of decyltrimethylammonium undecanoate, decanoate, and nonanoate leads to the formation of ion- pairs. There was no evidence, however, for formation of ion-pairs by the shorter carboxylates.     By assuming that micellar ions obey the Onsager equation, we have derived a theoretical relationship between Λ and concentration above the CMC. An excellent fit of theoretical curve to experimental points can be obtained by adjusting the value of a single parameter. This parameter is {(m+1)/n}K_mic (where K_mic is the association constant for binding the carboxylate ion to the micelle and m/n is the ratio of tetra-alkylammonium ions to carboxylate ions in the micelle).     Plots of -RT ln[{(m+1)/n}K_mic] against the number of methylene groups on the carboxylate ion are linear for both series, which suggests that m/n is independent of the chain length of the carboxylate ion. Both lines have a slope equal to Némethy and Scheraga's estimate of the free energy of hydrophobic interaction between two methylene groups (-1.40 kJ/mol).