Phase diagram of sodium dodecyl sulfate-water system: 1. A calorimetric study

Abstract

The phase diagram of sodium dodecyl sulfate (SDS)-water system is established from differential calorimetric measurements. The SDS purification and the absence of degradation was checked using NMR analysis before and after the calorimetric recordings. Thermodynamic equilibrium of mixtures was achieved by using low heating rates (0.08°C/min). A quasi-continuous exploration of the phase diagram (0.2 wt% per step) with accurate temperature and enthalpy measurements allows precise determination of the complicated intermediate mesophase region. Six invariant reactions are identified. Temperatures of five of them are separated by small intervals (from 0.2 to 5°C). Tammann representations of enthalpies precisely determine the eutectoid and monotectoid compositions and invariant limits. In the surfactant-rich region, all the invariant limits appear at the same molar ratio: SDS/2 D 2O, while the high hydration limits are located within a small composition range (from 57.1 to 66.4% SDS). Their enthalpies and entropies are discussed in relation to the mesophase structures and topology changes. The description of the surfactant-rich region is attempted. A dihydrate not previously reported is found. The freezing of water is also examined, and three types of water interactions are observed.