Thermodynamics of mixed monolayer formation at the air—water interface

Unusual excess free energies of mixing in mixtures of partially fluorinated and hydrocarbon surfactants at the air-water interface: correlation with the structure of the layer

The air-water interface of three mixtures of partially fluorinated surfactants and hydrocarbon surfactants, C4F9C11H22N(CH3)3Br (fC4hC11TAB) with hexadecyltrimethylammonium bromide (C16TAB), (CF3)2C3F6C10H20N(CH3)3Br (fC5hC10TAB) with C16TAB, and C8F17C6H12N(CH3)3Br (fC8hC6TAB) with C18TAB, have been investigated using surface tension (ST) and neutron reflection (NR). Using the composition of the layer determined by NR, the pseudophase separation model was used to fit the variation of concentration for a specific ST to a free energy of mixing, G(E), that included adjustable quadratic, cubic and quartic terms. In all three cases, G(E) was found to be highly unsymmetrical, being approximately ideal at low surface fractions of hydrocarbon surfactant and repulsive at high fractions with a maximum value of 0.2-0.3RT. The corresponding structure of the layer was also determined by NR and showed that the initial ideal behavior of G(E) probably results from a balance of a gain in energy from a reduced immersion of the fluorocarbon chain, brought about by screening of the fluorocarbon from water by the hydrocarbon surfactant, and a loss from increased fluorocarbon-hydrocarbon repulsion. At higher concentration, there is no space in the layer for further screening and the fluorocarbon-hydrocarbon repulsion leads to the expected positive G(E). The calculated G(E) also indicated that there should be phase separation of the two components in the interface over a bulk composition range of about 60-95% hydrocarbon surfactant. However, experiment indicates no phase separation. It is suggested that there are a number of possible additional negative contributions to G(E) close to a phase transition, which are not possible for a true bulk phase separation, and which prevent surface phase separation unless it is strongly favored.

The effect of surfactants on oxygen mass-transfer through the air-water interface

The aim of the current study was to investigate the oxygen transfer rate through the water-air interface in the presence of different concentrations of various surfactants. The surfactants used in the current study were methanol, ethanol, 1-propanol and 1-butanol. The measurements were performed with an apparatus based on the electrochemical oxygen sensor. Theresults obtained showed that the impediment to oxygen mass-transfer through the water-air interface depends on the molecular structure and concentration of the surfactant used. A simple mathematical model was proposed, which allowed us to calculate the characteristic constants of surfactants--K' and deltak starting from the experimental results. K' is the reciprocal of the equilibrium constant of adsorption process and deltak characterizes the influence of the surfactant to oxygen mass-transfer through the air-water interface.

Nonideal Mixing of Decyl Methyl Sulfoxide and Decyldimethylphosphine Oxide in Adsorbed Films and Micelles

The surface tension of aqueous solution of decyl methyl sulfoxide (DeMS) and decyldimethylphosphine oxide (DePO) mixture was measured as a function of the total concentration of the surfactants and the composition in the surfactants. The compositions of the adsobed film and micelle were evaluated by applying thermodynamic equations to the surface tension and the phase diagrams of adsorption and micelle formation were drawn. The DeMS and DePO mixture showed a negative deviation from the ideal mixing in adsorbed film and a positive deviation in micelles. The activity coefficients of surfactant in the adsorbed film and micelle were calculated from the phase diagrams and the nonideality of mixing in the adsorbed film and micelle was clarified in terms of the excess thermodynamic quantities of adsorption and micelle formation. Judging from the negative values of the excess Gibbs free energy and area of adsorption and a positive one of the excess Gibbs free energy of micelle formation, it was concluded that the difference in head group between DeMS and DePO molecules causes a favorable packing in the adsorbed film and an unfavorable one in the micelle. Copyright 1998 Academic Press.

Kinetics of Adsorption of Cationic Surfactants at Silica-Water Interface

The kinetics of adsorption of cationic surfactants (CTAB, MTAB and DTAB) at silica surface has been studied at various values of bulk surfactant concentration (Ct<INF POS="STACK">2), pH, ionic strength, and temperature and in presence of different electrolytes and urea. The adsorption process has been found to follow a two-step first-order kinetic rate equation with two different rate constants k1 and k2. From the variation of k1 and k2 with temperature, values of energies of activation Ea1 and Ea2 for both the kinetic steps have been evaluated. The corresponding values of enthalpies of activation (DeltaH<INF POS="STACK">1# and DeltaH<INF POS="STACK">2#), entropies of activation (DeltaS<INF POS="STACK">1# and DeltaS<INF POS="STACK">2#) have been evaluated using Eyring's equation for absolute reaction rate. It has been found that for both the kinetic steps, DeltaH<INF POS="STACK">1# < TavDeltaS<INF POS="STACK">1# and DeltaH<INF POS="STACK">2# < TavDeltaS<INF POS="STACK">2#, which means that activation reaction is largely entropy controlled. Again, for both kinetic steps, DeltaH# varies linearly with TavDeltaS#, and DeltaG<INF POS="STACK">1# and DeltaG<INF POS="STACK">2# vary between 70 and 88 kJ/mole of surfactant, respectively. Thus there is a entropy-enthalpy compensation effect in the adsorption process. Copyright 1998 Academic Press.