ABSTRACT
I. ADSORPTION AND CMC A. Surfactant Molecules in Aqueous Phase Materials such as short-chain fatty acids and alcohols are soluble in both water and oil (e.g., paraffin hydrocarbon) solvents. The hydrocarbon part of the molecule is responsible for its solubility in oil, whereas the ionic or polar head group has sufficient affinity to water to drag the nonpolar hydrocarbon chain into aqueous solution with it. This occurs also for longer homologs, driven by hydration of the hydrophilic head group. A small attractive contribution also arises from van der Waals forces occurring all along the lipophilic tail. In this particular case, cohesive forces are dipole-induced dipole in nature, between the polarizable hydrocarbon bonds and surrounding water molecules. The relative intensities of cohesive forces indicate that attractive interactions are in the order hydrocarbon-hydrocarbon (dispersion) < hydrocarbon-water (induced polar-polar) < water-water (polar-polar). Powerful energetic forces of solvation are able to carry the whole molecule into solution, even against the unfavorable process of transferring the alkyl group of the surfactant molecule into an aqueous phase that constitutes a hostile environment. It is hostile because the hydrocarbon chain locally disrupts the hydrogenbond network that surrounding water molecules share normally with each other in the bulk solution, and it creates a "cavity" in the structure of water. Thus, the "hydration" of an alkane molecule transferred from the vapor phase to water would be expected to be accompanied by a more positive entropy change than for the corresponding transfer to a nonpolar solvent. In fact, the hydration entropy of hydrocarbons is found to be large and negative. This points to the water "structure" being reinforced rather than disrupted. Water molecules are left with an unexhausted H-bonding capacity that they want to share and that concurs to reinforce the arrangement of the water shelf around this cavity. The concept that water adjacent to hydrocarbon chains in solution is more hydrogen-bonded and hence more structured than ordinary water was first proposed by Frank and Evans [1]. In their original article, they suggest that when a nonpolar molecule dissolves in water at room temperature, modification of the water structure is in the direction of greater "crystallinity," and that water, so to speak, builds a microscopic iceberg around the dissolved molecule. Many authors like Tadros commented on this orientation of water molecules around the hydrocarbon chain and agree with an "ice-like" structure of the ordered water molecules [2]. Recent molecular dynamics computer simulations performed by Grigera et al. [3] support this mechanism. They took two systems, both composed of a number of water
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molecules together with a layer of either purely hydrophilic molecules (an ice-like structure) or purely hydrophobic molecules (stated as neutral atoms) restrained in mobility so as to form walls. Among several parameters, they studied the hydrogen-bond network and found that the distribution of H bonds of water when in the presence of the hydrophilic "wall" is almost the same as the distribution for pure water, but a clear shift to higher number of H bonds per molecules for the case of the hydrophobic "wall." Furthermore, the hydrophobic wall has a repelling effect on water molecules, with a lower density profile perpendicular to the wall than for the hydrophilic wall.
