Quaternary N-alkylaldonamide-brine-decane-alcohol systems. Part I: phase behaviour and microemulsions

N-alkylaldonamides belong to a family of surfactants which differ by the length of their hydrophilic and hydrophobic part and by the branching of their polar head. The purpose of this work was to search for microemulsions stabilized by these molecules in ternary N-alkylaldonamide-water-decane systems and to study the corresponding phase diagrams and the microstructure. This paper first describes a new methodology to determine the conditions required for the formation of microemulsions. This is based on the study of binary systems and their miscibility gap and simplifies the determination of the phase behaviour. N-alkylaldonamide-water and N-alkylaldonamide-decane phase diagrams did not show any critical temperature in the temperature interval investigated and the interplay of the miscibility gaps in the phase prism could not lead to a triphasic equilibrium. N-alkylaldonamide could not stabilize microemulsion in a ternary N-alkylaldonamide-water-decane system because it was too miscible with water, but not soluble enough in decane. The critical temperatures were modified by addition of a lyotropic salt (NaCl) in water and of alcohol in decane so that the microemulsion in the triphasic equilibrium was found. It was, thus, shown that N-alkylaldonamides formed microemulsions in the quaternary N-alkylaldonamide-brine-decane-alcohol system. In the second part of this paper, corresponding pseudo-ternary phase diagrams, keeping the water-to-decane ratio constant, were established and compared. They present different kinds of equilibrium and a characteristic "fish shape" phase separation. The effect of the nature of the surfactant and of the alcohol on the phase separation was analysed. The branching of the polar head of the surfactant and the length of the alcohol have a large influence on the extent of the three-phase body in the phase diagram. These phase diagrams are the important basis of the study of the microstrucure of the microemulsion phases. (C) 1998 Elsevier Science B.V. All rights reserved.