CALORIMETRIC OBSERVATIONS OF THE SPHERE ROD TRANSITION OF SODIUM DODECYL-SULFATE - EFFECTS OF ELECTROLYTES AND NONELECTROLYTES AT 25-DEGREES-C

Incremental calorimetric titrations of small amounts of alcohols into solutions containing 3.5% (w/w) sodium dodecyl sulfate and added electrolytes reveal a sharp break in the partial molar enthalpy of the alcohol. The end of this break corresponds to the micellar sphere-rod transition, as observed by light scattering and other techniques. Unlike other techniques, however, calorimetric measurements show a well-defined beginning and end of the transition in some solutions. This technique has been used to determine the effects of various electrolytes and nonelectrolytes on the transition at 25-degrees-C. The effect of electrolytes and nonelectrolytes is generally complementary, but p-dioxane and urea retard the transition while benzyl alcohol and 2-pentanol have little effect. The effects of electrolytes are determined primarily by the cations, decreasing in the series K+ > NH4+ > Na+ > Li+. The effect of alcohols increases with chain length and decreases with branching.