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The critical micelle concentrations (cmcs) of chlorhexidine digluconate (CG) in aqueous solution were determined over the temperature range 15-40 "C by a method based on deconvolution into Gaussians of the second derivative of the conductivity/concentration data. The mass-action model was modified in order to calculate the thermodynamic parameters (Ac",, & , ,As",) of micelle formation. To verify the theoretical predictions the enthalpy of micelle formation at 25 "C was calorimetrically determined.

SynopsisA sedimentation analysis has been used to determine the proportion of protein present as monomer and aggregate in 0.5 and 1.0 g/dl solutions of 0-casein A in pH 7 phosphate buffer over the temperature range 10-40°C. The amount and molecular weight of the aggregate increase with temperature; under the conditions used, the aggregation number ( n ) of &casein is given approximately by n = 0.6t + 2 with t in degrees centigrade. The concentration of p-casein in monomeric and aggregated states a t different temperatures is used to calculate the standard enthalpy of aggregation AH" (Van't Hoff) by assuming that p-casein undergoes a cooperative, two-state, micellization process; aggregation is an endothermic process and AH" = 66.0 f 2.6 kJ mol-'. Combination of this AHo with the amount of protein calculated to dissociate when 1 g/dl solutions are diluted isothermally to 0.5 g/dl gives the heat of dilution a t various temperatures. These calculated heats of dilution are compared with the experimental values obtained by carrying out the same dilutions in a microcalorimeter. The heat of dilution decreases linearly with &casein concentration, but the extrapolated zero-concentration value of 65.8 f 1.6 kJ mol-' is the same as the Van't Hoff enthalpy. This agreement in the enthalpy values indicates that the micellization of &casein occurs cooperatively. The effect of modifying the hydrophobic/hydrophilic balance of the system on the micellization of 6-casein A has been investigated. The hydrophobic interaction between the protein molecules is decreased by removing the three C-terminal residues (Ileu Ileu Val) with carboxypeptidase-A. This modification drastically reduces the ability of the p-casein molecule to form micelles. Substitution of 2H20 for HzO at constant temperature perturbs the monomer-micelle equilibrium in favor of micelles because of enhanced hydrophobic interactions in the former solvent. The results are consistent with P-casein micellization involving a delicate balance of the hydrophobic forces favoring aggregation and electrostatic forces opposing it.

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