Thermodynamics and micellization of cetyltrimethyl ammonium bromide in the presence of lysozyme

To study the structural changes in lysozyme (cationic protein) on the addition of N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB) and to understand the mechanism underlying aggregation of resulting protein-surfactant complex, surface tension, density and sound velocity measurements of lysozyme-CTAB solutions were carried out over a temperature range of 25-40 °C. The critical micelle concentration (cmc) of CTAB has been calculated from surface tension measurements and is found to increase with increase in lysozyme concentration as well as with temperature. Surface tension data have been further used to calculate the interfacial parameters; maximum surface excess concentration (Γmax), minimum area per molecule (Amin), standard free energy of adsorption (ΔGoad), surface pressure at cmc (Πcmc) and standard free energy of transfer (ΔGotr), that have direct bearing on the consequences of such interactions at the molecular level. The negative values of standard Gibbs energy changes indicate spontaneity of micellization. Apparent molar volume (ϕv) and apparent molar adiabatic compressibility (ϕκ) have also been calculated using density and velocity of sound. These parameters have been examined as a function of surfactant concentration, lysozyme concentration and temperature to understand the consequences of interactions between these two components. A good qualitative correlation is found to exist with regard to the surfactant-lysozyme interaction obtained from the experimental data from different techniques.