An upper critical point in a lamellar liquid crystalline phase

An equilibrium between two lamellar phases is observed to occur in the binary system sodium dodecyl-5-p-benzenesulfonate/water. The coexistence region, extending from ∼25 to 68%, is characterized by an upper critical (or consolute) temperature of ∼39 o C. Deuterium NMR of D 2 O is a particularly suitable technique for revealing the existence of two lamellar phases in such systems. Samples in the above composition range have also a «white, milky» appearance typical of all emulsions. We attribute the coexistence of two lamellar phases to the presence of an interbilayer attractive component caused by a specific Na ion-head group interaction