bond with water molecules, but a hydrophobic group cannot. We present a theory developed to describe the diffuse double When a hydrophobic group is in aqueous solution it becomes layer of electrolytic soap films. The hydrophobic nature of the more difficult for surrounding water molecules to hydrogen soap molecules is modeled, in an effective field theory for the bond with each other for geometric reasons. In reality it is system, by the introduction of a free energetic potential favoring believed that water molecules do successfully hydrogen the presence of the soap molecules at the film surface. This potenbond around hydrophobic groups by becoming more orgatial can take various forms, depending on how accurately we wish nized and forming an ordered cage about the hydrophobic to model the geometry of the system. The resulting theory is anagroup. However this organization leads to a decrease in enlyzed and leads to the estimation of the electrostatic interactions tropy and at room temperature this leads to a net increase at the mean field level. A whole range of physical observables is predicted including the electrostatic component of the disjoining in the free energy of the system. Hence there is an overall pressure isotherm and the Gibbs adsorption isotherm of the systendency for hydrophobic groups to escape from the aqueous tem. We compare the theory with accurate disjoining pressure solution in order to reduce the free energy of the system.
measurements (using X-ray specular reflectivity) carried out on
One may estimate the cost in free energy for having a tail systems composed of sodium dodecyl sulphate (SDS) with two inside the aqueous medium; empirically (see for example different monovalent electrolytes on a wide range of salinity. The the discussion in (2)) it has been noticed that this free energy theory appears to account particularly well for the experimental l ร w 1 effective area of molecule, where w typically takes data. A comparison with the DLVO theory is given. Finally, we values between 40 and 50 mJ/m 2 at 25ะC. l is also known discuss the limitations of the theory and its modification due to as the adhesion energy (3). Making a rough calculation the inclusion of dispersion forces.