The liquid crystalline phases formed by linear-dodecylbenzene sulphonic acid during neutralisation with sodium carbonate

The liquid crystals and other phases formed when dodecyl-p-benzene sulphonic acid (DBSA) is “dry-neutralised” with sodium carbonate to form sodium dodecylbenzene sulphonate (NaDBS) have been investigated using optical microscopy, X-ray diffraction and differential scanning calorimetry. In addition, the phases formed by DBSA with water have also been investigated using the same methods. For the DBSA/water system, liquid phases are formed for the surfactant-rich and water-rich samples, whilst a lamellar phase occurs over an intermediate composition range of ca. 35/65-90/10 DBSA/water (by weight). There are no solid hydrates or “acid/soap” solid phases present. In the “dry-neutralised” system, CO2 is lost during the reaction, but half a mole of water per mole of NaDBS remains in the system. The fully neutralised NaDBS is a soft solid that contains at least three different mesophase structures at ambient temperature, probably of the lamellar type. These phases exist down to ca. 80% neutralisation and appear to coalesce into a single phase above ca. 80 °C, and take several days to re-form on cooling. Several low-intensity, high-angle X-ray reflections are also observed at low temperatures implying that at least one of the phases has more order than a simple lamellar phase. Over ca. 30–55% neutralisation there is a single lamellar phase above 0 °C. Between ca. 60% and ca. 80% neutralisation the phase structure is much more complex. Several layer phases are present, with numerous low-intensity, high-angle reflections. These phases do not fully coalesce by 120 °C, and do not immediately revert to the original structure on cooling. It is suggested that the more complex X-ray patterns indicate the possible presence of separate “acid/soap”-type phases. The presence of multiple phases and the slow relaxation to more stable structures after heating will have an important influence on the formulation of low water content products with NaDBS.