Preclouding in mixed micellar solutions

Intermediate phase behavior (preclouding) in micellar solutions of Triton X-114 containing a minor proportion of sodium dodecyl sulfate (SDS) has been investigated. Solutions with a low SDS content were found to cloud in the conventional manner, with slightly elevated cloud points. Solutions with higher proportions of SDS exhibited preclouding, involving the formation of a stable colloidal suspension, before macroscopic clouding set in at a much higher temperature. Solutions with intermediate amounts of SDS showed alternating phase behavior, clouding after initial heating, then preclouding at a higher temperature, and eventually clouding after still further heating. The evolution of aggregate sizes was monitored by photon correlation spectroscopy, which showed that two major populations existed in preclouded suspensions. Fluorescence anisotropy measurements of a perylene probe were used to construct Perrin plots, which were nonlinear, indicating anomalous viscosity changes during the preclouding process. A tentative mechanism is proposed, in which it is suggested that preclouded suspensions consist of small, charge-stabilized aggregates.