A polarity-sensitive probe was used to assess the thermal stability of aggregates of six nonionic surfactants in aqueous solution. The I1/I3 peak ratio of the pyrene emission spectrum, which varies with solvent polarity, was taken as an indicator of the probe environment and hence of the surfactant aggregates in which this probe was sequestered. Aggregate stabilities of surfactants in the Triton, Igepal, and Brij series in premicellar, micellar, and supramicellar (clouded) solutions were considered. Temperatureinduced clouding was not found to have a measurable effect on the environment of the sequestered probe. Fully developed micelles exhibited a near-linear decrease in pyrene I1/I3 with temperature, similar to the changes observed in pure benzene solvent. In the premicellar and early micellar stages, however, thermal stabilities of aggregates varied significantly with the structure and size of both the hydrophobic and hydrophilic moieties of the surfactant molecules. The least stable assemblies were formed by surfactants with short polyoxyethylene chains and highly branched aliphatic segments.

The clouding behavior of mixed surfactant (Triton X-114/sodium dodecyl sulfate) systems, as manifested by solution turbidity, was studied over a range of concentrations and temperatures. The results were correlated with particle size data obtained from dynamic light scattering measurements. Fluctuating behavior, involving clouding at relatively low temperatures, (partial) clarification upon further heating ("declouding"), and definitive clouding at still higher temperatures, was observed. The temperature ranges and solution compositions producing declouding were found to coincide roughly with a previously reported preclouding phenomenon. In this, clouding occurs in two stages, starting with a relatively stable colloidal suspension displaying a Tyndall effect, and proceeding to full clouding at higher temperatures. Both preand declouding were found to be promoted by the presence of anionic sodium dodecyl sulfate in the micellar Triton X-114 solution. A modified charge accumulation mechanism is proposed to account for the fluctuations.

Fo ¨rster energy transfer from excited surfactant species was used to monitor their association with micellized perylene at different temperatures. Fluorescence from the perylene acceptor was obtained through sensitization by surfactant monomers and aggregates, the latter being identified as spectroscopic rather than physical. Surfactant monomer fluorescence was subject to a strong inner filter effect inside the micelles, indicating an abundance of spectroscopically monomeric species there. Temperature increases caused reductions in native surfactant fluorescence through thermal quenching, but sensitized fluorescence intensities generally increased with temperature. This was ascribed to a temperature-induced approach between the perylene acceptor and both monomeric and aggregated surfactant donors. Clouding of the solutions showed little evidence of organizational changes at the micellar level.

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