Studies On The Thermodynamics Of Molecular Organized Assemblies Formed By Alkyl Aryl Sulfonates

In this paper, a series of alkyl aryl sulfonates which have definite structure and high purity were finely synthesized from benzene and its homologues, acyl chloride andα-bromoalkane, the symbols areФC14-5S, MФC14-5S, EФC14-5S, DMФC14-5S, DMФC12-5S, DMФC16-5S, DMФC14-3S and DMФC14-7S, respectively. The content of surfactant in the products were all more than 96.0% determined by two-phase titration.The molecular organized assemblies such as micelle, reverse micelle and microemulsion formed by these alkyl aryl sulfonates in different conditions were investigated by using surface tension, iodine spectrometric, water solubilization, quasi-ternary phase diagram and Winsor phase diagram methods. The influences of molecular structures, temperature, solvents and various additives on these molecular organized assemblies were discussed.Results show that the micellization of alkyl aryl sulfonates in aqueous solutions is spontaneous and entropy-driven. As the temperature rises, the micellization is easy initialy but then becomes more difficult. The contribution of entropy change to the change in Gibbs free energy tends to decrease but the contribution of enthalpy change to the change in Gibbs free energy tends to increase. The micellization is enthalpy-entropy compensated. The compensation temperature (Tc) is found to be (306±2)K, which is independent of the molecular structure of the alkyl aryl sulfonates. The formation ability and the stability of the micelles increase when the carbon atoms of the short or long alkyl chains on the aromatic rings increase, but decrease as the aromatic rings shift from the edge to the middle of the long carbonic chains.Water or mixed polar solvents (EG-water) are unfavorable for micelle formation but that reverse micelle formation in non-polar solvents (n-heptane) is favorable when the branching coefficients of the lipophilic group in the tetradecyl aryl sulfonate molecules increase. A decrease in solvent polarity causes the surfactant solution to transform from a micellar solution to a monomer solution and then to a reverse micellar solution. Upon addition of inorganic salts or n-propanol, n-butanol and n-pentanol, micelle formation of DMФC14-5S in aqueous solutions is favored. Together with higher counter-ion valence states or alcohol alkyl carbon atoms, micelle formation increases even further. Upon addition of ethanol, polyethylene glycol or non-aqueous polar solvents, micelle formation is unfavored. It's difficulty for micelle formation when cetyl trimethylammonium bromide and DMФC14-5S mixed in aqueous solutions. Results also show that the region area of microemulsion formed by DMФC12-5S, DMФC14-5S and DMФC16-5S in the surfactant/ alcohol/ n-octane/ water systems was the bigest when the optimum cosurfactants were n-butanol, n-pentanol and n-pentanol respectively. Temperature is a important factor for middle phase microemulsion formation of alkyl aryl sulfonates. As the concentration of inorganic salt increases, the surfactant/ n-butanol/ n-octane/ NaCl/ water microemulsion systems transform from Winsor I to Winsor III and then to Winsor II at specific temperatures. The optimum salinity c*(NaCl) and the salt width of the middle phase microemulsion formation ?c(NaCl) decrease but the optimum volume fraction of middle phase microemulsion (φM*) and solubilization capacity (S*) increase when the branching coefficients or the alkyl chain lengths of the lipophilic group in the alkyl aryl sulfonate molecules increase.