Hydrophobic Interactions in Model Systems

Abstract

Hydrophobic interactions in model systems have been studied. Cationic and non-ionic aqueous surfactant systems were used as model systems. The physico-chemical properties of surfactant solutions studied, in order to evaluate hydrophobic interaction, were critical micelle concentrations, hydrodynamics, conductance, partial molal volumes, light scattering and the thermodynamic data of micellization. The observed hydrodynamic micellar properties of cationic surfactants indicate that the intrinsic viscosity of micelles is affected by hydration, the electroviscous effect and the micellar shape by means of hydrophobic interactions. The micellar properties are changed when aliphatic alcohol is added to the system. The intrinsic viscosity of the micelles does not increase significantly, although the relative viscosity of the solution is higher compared with the aqueous surfactant solution. The increase in viscosity has been discussed in terms of hydrophobic interactions and the change in micellar shape. The observed decrement in intrinsic viscosity shows that the hydration and electroviscous effect decreases with increasing concentration of alcohol. The critical micelle concentrations of cationic surfactant solutions containing alcohols were discussed considering the structure promoting effect of alcohols at low concentration, the reverse effect at higher concentration; solubility; hydrophobic interactions between possible orientated configurations, and their effect on the surface charge of the micelles. The positive B coefficients of alkylammonium bromides confirm the structure maker effect of the nonpolar groups of the surfactants. This behaviour is considerably influenced by the addition of alcohol to the system. At low concentration of alcohol the B coefficient is positive, but as the concentration of alcohol is increased it shifts to a negative value. This is due to destruction of water clusters around the nonpolar groups, and the breaking of structured water molecules in the bulk liquid. The volumetric properties of cationic surfactants below the critical micelle concentration, and above the critical micelle concentration show that the partial molal volume of the surfactant increases with increasing alkyl chain length. The micelle formation is accompanied by an increase in volume. When aliphatic alcohol is added to the system, it contributes to the volume change and increases the partial molal volume of the micelles. The effect of aromatic alcohols, ureas and urethane on micellar properties of cationic systems has been examined by measuring the conductance of the micelles. The observed conductance of the micelles indicate that the critical micelle concentration increases when 0.01M alcohol is added to the system. As the concentration of alcohol is increased the critical micelle concentration value shifts to lower values. The increment in critical micelle concentration has been discussed in terms of hydrophobic interactions between alcohol, surfactant and water molecules, taking into account the solubility of alcohols and their structure promoting behaviour at low concentration. However, the decrement in critical micelle concentration has been attributed to the structure breaking behaviour of alcohol at high concentration. On the addition of urea and urethane the critical micelle concentration increases. The increment in specific conductance at the critical micelle concentration has been attributed to the cooperative effect of the solutes on water structure. It is believed that ureas and urethane behave as structure promoting, since they take an active part in mixed cluster formation, which are big enough to accommodate nonpolar groups. Light scattering data on micelles of cationic surfactants indicates that the micellar molecular weight increases as the chain length increases. The addition of aliphatic alcohol to the system first increased the apparent molecular weight, then it decreased with increasing concentration. The aliphatic alcohols decreased the scattering intensity, and the turbidity of the solution in terms of the density, and concentration fluctuations in refractive index of the solution near the surfactant molecule. The effective charge on the micelles increased, then decreased with increasing concentration of the alcohol. Hydrophobic interactions in non-ionic surfactant systems of the polyoxyethylene n-alkanol type were also studied in terms of the hydrodynamic, volumetric, light scattering and thermodynamic data of micellar properties. The intrinsic viscosity of the micelles increases as the ethylene oxide chain length increases. The increment in relative viscosity, and in intrinsic viscosity of the micelles is due to the hydration of micelles, micellar size and shape. The observed critical micelle concentrations indicate that the critical micelle concentration decreases as the ethylene oxide chain length increases. This behaviour has been discussed by means of the decrement in solubility of non-ionics in aqueous solution, when the ethylene oxide chain length is increased, and the increment in degree of coiling as the hydrophilic chain length increases. It is likely that the hydrophilic chain coils tightly at the end of the hydrocarbon chain, this is entropically favourable allowing hydrogen bonded water molecules to be released, this type of configuration will increase the hydrophobic surface area of the molecule. Consequently, the critical micelle concentration decreases with increasing hydrophobic character of the non-ionic surfactant. The volumetric properties of micelles show that a volume change occurs during the micelle formation. The partial molal volume change decreases with increasing hydrophilic chain length. The addition of aromatic alcohols to the system shows similar behaviour to that observed with cationic systems. Critical micelle concentration is increased when 0.01M aromatic alcohol is added to the system, then the effect of additive on critical micelle concentration tends to the reverse direction as the concentration increases. The observed phenomenon is due to the selective adsorption of additive on the surfactant molecule, the change in density, and concentration fluctuations in the refractive index of the solution near the surfactant molecule, and a complex type of association of surfactant with alcohol through hydrophobic interactions. The scattering intensity, and turbidity of the solution containing alcohol is higher compared with the aqueous surfactant solution. The change in micellar properties of non-ionics in aqueous urea, and urethane, show that the critical micelle concentration increases. The increases arise from the active participation of urea, and urethane in mixed cluster formation in terms of hydrophobic interactions. The increment in scattering intensity and turbidity of the solution also supports the idea that these substances behave as structure makers in aqueous surfactant solution. The thermodynamic investigation of cationic and non-ionic micellization shows that micelle formation is an entropy directed process. The free energy of micellization decreases as the chain length, and ethylene oxide chain length increases. This decrement is due to the reduction of structural order of water molecules, which arises from the minimization of hydrocarbon water interface, this causes the transference of monomers to the nonpolar environments. The decrement in enthalpy (cationics) is caused by the change in the translational energy of monomers, in water structure and the effect of hydrated head groups. The observed entropy change is governed by hydrophobic interactions between nonpolar, polar and structural water molecules. The enthalpy and entropy of micellization of non-ionic surfactants is always positive showing that micelle formation is governed by the gain in entropy. Entropy increases during the micelle formation, which contributes largely to the free energy. The increase in entropy is due to the hydrophobic interactions between nonpolar groups, and structured water molecules which is accompanied by a release of energy, due to the structural melting of water molecules, when the nonpolar groups move to the nonpolar environment. The reorientation of the solvent molecules contributes also to an increase in entropy.