MICELLIZATION OF SODIUM DODECYL-SULFATE IN SODIUM-ACETATE ACETIC-ACID BUFFER - A CONDUCTIVITY STUDY

Electrical conductivity of sodium dodecyl sulfate (SDS) in sodium acetate-acetic acid buffer solutions was measured as functions of concentration, temperature and pH. First and second critical micelle concentrations (cmc1 and cmc2) of these solutions were estimated. Plot of cmc1 versus pH passes through a minimum in the pH range 5.2 to 5.6. The decrease in cmc1 with increasing pH is shown to be due to the Na+ counterion and is described quantitatively by an analytical expression which also provides the value of counterion binding constant. The increasing tendency of cmc1 with increase in pH is, on the other hand, caused by the acetate co-ion. The enthalpy and standard free energy changes due to micellization of SDS were estimated. The slope of the specific conductivity versus concentration plot above cmc2 is found to decrease with increasing concentration of sodium acetate which is contrary to the trend reported in sodium chloride solution. This study has revealed the effect of the nature of co-ion on micellization.