NOTE
Effect of a Weak Electrolyte on the Critical Micellar Concentration of Sodium Dodecyl Sulfate were always prepared using bidistilled water and used without any further treatment throughout experiments. The concentration of SDS stock solutions
The critical micellar concentration of sodium dodecyl sulfate is was well above the CMC (Γ8 1 10 03 M in water) and ranged from 48 to strongly altered by tris(hydroxy-methyl)methylammonium ions. 200 1 10 03 M. The concentration of 1,8-ANS was lower than Γ1.4 1
The effect of buffer solutions containing this weak electrolyte as 10 03 M, as judged using a molar absorptivity coefficient of 6,800 M 01 the counterion source has been studied using various concentracm 01 at 374 nm (6). Aqueous buffer solutions, whose total concentration tions of the acid-base system as well as modifying the pH. Results ranged from 10 to 600 1 10 03 M, were prepared using either tris(hydroxy- show that counterion concentrations ranging from 0 to Γ340 1 methyl)aminomethane (Tris) or ammonium hydroxide (28.0-30.0% NH 3 ), 10 03 M induce an appreciable diminution of the critical micellar both from Sigma Chemical Co. Their pH after dissolution was left un- concentration from Γ8 to Γ0.7 1 10 03 M. The analysis of data changed. In a number of experiments the pH was adjusted by adding small suggests that the critical micellar concentration of sodium dodecyl aliquots of a concentrated solution of HCl, so that the only source of sulfate depends on the concentration of weak electrolytes in a way counterions was the buffer itself. In any case, pH values were used to evaluate the counterion concentration using a pK a of 8.21 and 9.40 at 20ΠC very similar to that of strong electrolytes. α§ 1998 Academic Press for Tris/TrisH / and NH 3 /NH / 4 , respectively. A pK a of 7.77 was used for