Condensed structure formation in mixed monolayers of anionic surfactants and 2-hydroxyethyl laurate at the air–water interface

A first-order phase transition has been studied by Brewster angle microscopy in mixed monolayers of 2-hydroxyethyl laurate (2-HEL) and two anionic surfactants, sodium 3,6,9,12-tetraoxa-octacosanoate (TOOCNa) and sodium 3,6,9,12-tetraoxa-triacontanoate (TOTCNa) at the air–water interface. Condensed phase domains are observed on the surface of a mixed solution containing 1.0×10−5 M each of 2-HEL and of TOOCNa or TOTCNa up to 20 °C. This concentration of 2-HEL is much lower than that required for the phase transition in the pure system at the studied temperatures. The phase transition pressures (πt) of all the systems increase with increasing temperature. The πt of 2-HEL-TOOCNa system almost corresponds to that of pure 2-HEL at a definite temperature. In contrast, the πt of 2-HEL-TOTCNa system is lower than the corresponding value of 2-HEL. The shapes and the textures of the domains formed in the mixed system are different to those formed in the pure system. The formed domains in the mixed system have a circular shape with internal segments whereas those of 2-HEL have a fingering pattern with an optical isotropy at the same temperature. A large variety of defects are found in the domains formed in the mixed system. Furthermore, these domains undergo fusion while touching each other.