2, 9) and decreases the cloud point of zwitterionic (10) 8-Anilinonaphthalene-1-sulfonate (ANS) was used as fluoressurfactants; (E) increases the local viscosity and decreases cence probe to study the effects of urea on properties of sodium the local polarity at the micellar interface (4, 11, 13, 17); dodecyl sulfate micellar solutions. The critical micelle concentra-(F) alters the permeability of solutes (2); (G) decreases the tion (CMC) of the surfactant, the degree of counterion association partitioning of additives such as aromatic hydrocarbons (12) to the micelles, and the partition constant of ANS between the and alkanols (5); and (H) decreases the selectivity of differmicelles and the external medium in the vicinity of the CMC were ent counterions for the binding at the surface of ionic miobtained at different urea concentrations ranging from 0 to 8 M. celles (15, 16). Recently, the effects of urea on monolayers Complementary results regarding the effects of urea on the micelle (19) and micelle formation of fluorocarbon surfactants (20) aggregation number, the E T (30) parameter, and the partitioning of ANS between n-octanol and water or water-plus-urea mixtures and block copolymers (21) have also been investigated.
were also obtained to allow critical evaluation of the ANS fluores-Two mechanisms have been proposed to explain the denacence data. It is concluded that the effect of urea on ANS fluoresturant action of urea . One is an indirect mechanism cence at intermediate surfactant concentrations is due mainly to in which urea acts only at the level of the solvent, altering increased stability of the probe in the aqueous medium. Extrapolathe structure of water in a way that facilitates the dissolution tion of the data at total ANS incorporation into the micelles allows of hydrophobic species [i.e., by invoking the capacity of the conclusion that the intramicellar emission increases slightly urea to act as a ''water structure breaker,'' so decreasing on urea addition. This result is attributed to an increase in the the free energy required for cavity formation ( )]. The microviscosity at the micellar surface resulting from urea-micelle other is a direct mechanism in which urea participates in the interactions. ᭧ 1997 Academic Press solvation of hydrophobic species by replacing some water