Frequency Domain Fluorescence Measurements of the Aggregation Properties of CnEm Surfactants in Agarose Gels

Abstract

The micellar properties of C12E6, C12E8, and C12E10 surfactants in agarose gels were investigated. The aggregation number and the aggregate polarity were determined as a function of temperature, gel fiber volume fraction, and surfactant type by using steady-state fluorescence and frequency domain fluorescence quenching techniques. Application of the latter technique to the study of micellar properties in solution is described, including a method for accounting for the excitation light scattered by the gel fibers. The aggregation behavior in gels of surfactants which form spherical micelles in water is compared to those which form cylindrical micelles, and the results are interpreted by using a thermodynamic model for micelle formation in a gel matrix. For surfactants which form small spherical micelles in solution, there is little difference between the aggregation number and micelle polarity in an agarose gel and in pure solution. For surfactants which form large cylindrical micelles in solution, the aggregation number decreases with increasing gel concentration while the micelle polarity remains constant. The thermodynamic model shows that, if the agarose gel restricts the micelle size distribution, then the weight average aggregation number decreases for large cylindrical micelles and remains unchanged for small spherical micelles. The model results are consistent with the experimental results for the aggregation number of C12E6 in an agarose gel.