The effect of position of ester group on critical micelle concentration of ester-linked sulfonates

The critical micelle concentration (CMC) of sodium alkyl sulfoacetates and β-sulfopropionates, and sodium salt of 2-sulfo ethyl ester, 3-sulfo propyl ester and 4-sulfo butyl ester of fatty acids have been determined by the electrical conductance of each aqueous solution. The relation between CMC value and number of total methylene groups (N) for the C_n ^*H_2n ^*+₁COO(CH₂)₃ SO₃Na and C₉H₁₉COO(CH₂)_n ^**SO₃Na (n^*=9, 10 and 11. n^**=2, 3 and 4) can be formulated as follows. $$\begin{gathered} \log {\text{CMC = - 0}}{\text{.293N + 1}}{\text{.778}} \hfill \\ {\text{for C}}_{\text{n}} *{\text{H}}_{{\text{2n}}} *_{ + ^1 } {\text{COO (CH2) 3SO3Na}} \hfill \\ {\text{log CMC = - 0}}{\text{.147 N + 0}}{\text{.011}} \hfill \\ {\text{for C9H19 COO (CH2) n **SO3Na}} \hfill \\ \end{gathered} $$ From these equations it was determined that the methylene unit situated between ester and sulfonate groups is equivalent to 0.5 methylene groups in its effect on CMC. For a given number of carbon atoms in the alkyl chain, the log CMC value increased regularly with a change in the ester group away from the terminal position to more central positions in the hydrocabon chain. The two different types of ester-linkages (RCOO-and ROCO-) have no apparent effect on the CMC value.