Add time:08/22/2019         Source:sciencedirect.com

The critical micelle concentrations (CMC) of three double long-chain salts, octyltrimethylammonium (cas 15461-38-8) ortyl sulfate (C8–C8), decyl sulfate (C8–C10) and dodecyl sulfate (C8–C12), were determined at 298.15 K as a function of the pressure up to 200 MPa. With respect to C8–C8, the CMCs at 293.15 and 303.15 K were also determined as a function of the pressure. The CMCs for a homologous series of double long-chain salts decreased logarithmically with an increase in the alkyl chain length. The standard free energy change for the micelle formation of the double long-chain salt was—3.4 kJ mol−2 per CH2 group. The presence of a maximum In the CMC-pressure curve was presented. Below the CMC the double long-chain salt behaved as a strong 1:1 electrolyte, while above the CMC the micelle was almost unchanged even at high pressure up to 200 MPa. The partial molar volume change on micellization (ΔVm) calculated from the pressure dependence of the CMC was 7.6, 8.5 and 11.3 cm1 mol−1 for C8–C8, C8–C10 and C8–C12, respectively. Δ V̄m decreased monotonously with an increase in the pressure and changed its sign from plus to minus at about 75 MPa. The partial molar entropy change on micellization of (ΔSm)C8–C8, which was determined from the temperature dependence of the CMC, increased initially with an increase in the pressure up to 75 MPa and then decreased with the pressure. The dependence of ΔVm and Δ Sm upon the pressure were interpreted by the differences in the partial molar compressibilities and expansibilities of the double long-chain salts respectively, between the singly dispersed state and the micellar state.

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