Volumetric and Compressibility Studies of Salt Induced Hydrophobic Interactions in Pre–Micellar Region of Sodium Dodecyl Sulfate

Apparent molar volume,  _v and apparent molar adiabatic compressibility,  _k of sodium dodecyl sulphate (SDS) in water and in 0.1 mol dm^–3 aqueous solutions of LiCl and NaCl have been determined form density and speed of sound measurements. Both these parameters are found to be highly sensitive to the presence electrolyte, in the concentration range ∼0.001–0.01 mol kg^–1, which corresponds to the pre—micellar region of SDS; it indicates extensive intermolecular hydrophobic interaction in the presence of electrolytes. From a comparison between the two electrolytes in affecting the  _v and  _k values, NaCl is found to be more effective than LiCl. Above ∼0.01 mol kg^–1, the data indicate that solution loses its hydrophobic hydration character due to the micellization of surfactant. The constancy in  _v and  _k values further suggested that no structural—transition occurs under these experimental conditions. Classic structural hypothesis of hydrophobic hydration proposed by 'Frank and Evans' [1(a)] has been invoked to account for the negative  _v and  _k values of SDS observed in aqueous solutions of LiCl and NaCl. All these observations are ultimately found to justify that salt induced hydrophobic interaction is much more effective than solvent induced, however, the contribution of hydration characteristic of the ions is also found to play its own role.