Urea effect on aggregation and adsorption of sodium dioctylsulfosuccinate in water.

Understanding the mechanism that controls the folding/unfolding of proteins in the presence of urea continues to be a subject of research, and since micelles mimic biological aggregates, equal importance has been given to the study of surfactants in the presence of urea. Despite several studies on the effect of urea on the behavior of reverse micelles and microemulsions based on sodium dioctylsulfosuccinate (AOT), the urea effect on AOT regular micelles has not been investigated and hence it is studied herein by using surface tension, steady-state fluorescence, and dynamic light scattering methods. The effect of urea on the behavior of AOT is found to be different below and above 1.0 mol kg(-1) urea (c(u)). The critical micelle concentration (cmc) is almost independent of urea concentration below c(u), whereas it increases with increasing urea amount above c(u). In AOT+urea aqueous solution below c(u), added NaCl at a particular critical concentration (c*) induces sudden increase in the values of (i) counterion binding constant, (ii) aggregation number, (iii) fluorescence intensity ratio of pyrene excimer to monomer, and (iv) hydrodynamic diameter of AOT aggregate, whereas such changes are suppressed by urea above c(u). NaCl-induced shape change in AOT micelle takes place if urea concentration is below c(u), but hindered above c(u). The adsorption behavior of AOT at the air-solution interface as a function of NaCl is also found to be different below and above c(u). The urea effect is explained in terms of increase in the polarity of the medium, better solvation of head groups and counterions, and weakening of head group-head group and head group-counterion interactions.