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}Kmic (where Kmic 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}Kmic]
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 N�methy and Scheraga's
estimate of the free energy of hydrophobic interaction between two methylene
groups (-1.40 kJ/mol).
Effect of hydrocarbon chain length on adsorption of cationic surfactants onto clinoptilolite
