Temperature dependence of molecular motion in smectic liquid crystals of hydrated sodium 4-(1-heptylnonyl)benzenesulfonate

Abstract

The surfactant sodium 4-(1'-heptylnonyl)benzenesulfonate (SHBS), upon hydration to form a smectic liquid crystalline phase, exhibits a thermal transition of 1.62 +- 0.22 kcal per mole of SHBS centered at -70/sup 0/C. From measurements of the temperature dependence of the carbon-13 NMR spectra and from the calorimetric studies, transition was identified as the freezing-in of motion of the aliphatic chains and can thus be labeled as the so-called gel-liquid crystal transition observed in phospholipids. The enthalpy and the entropy associated with the transition correspond to a change of approximately one trans-gauche rotation per hydrocarbon tail. A more intense, broad thermal transition with a heat of 15 +- 3 cal per gram of smectic phase is observed from ca. -10 to -50/sup 0/C and shown to come primarily from the feezing-in of bilayer water. At ca. -23/sup 0/C water associated with the ionic groups, as well as the motion of the SHBS head, becomes frozen-in. Several comparisons between SHBS and similar studies on synthetic phosphatidylcholine are made.