Cloud point and thermodynamic parameters of a non-ionic surfactant heptaoxyethylene dodecyl ether (C12E7) in presence of various organic and inorganic additives

Cloud points (CPs) of heptaoxyethylene dodecyl ether (C12E7) in the presence of varying concentrations of additives such as inorganic salts, alkanols, diols, glycols and their ethers, hydroptopes and short chain tetraalkylammonium salts are measured and the increase/decrease in CP is discussed. While inorganic anions decrease the CP due to the “salting out” (dehydration) effect and follow the Hofmeister series, an increase in CP by CNS and I ions is governed by the “salting in” effect. Short chain alcohols (< C4), diols (< C6) and ethylene glycol ethers, diethylene glycol, propylene glycol, dipropylene glycol (< C4) increase the CP whereas a decrease is seen in the case of higher homologues. This increasing/decreasing effect of alcohols and diols and their monoalkyl ethers is ascribed to their function as a cosolvent/cosurfactant for C12E7 micellar systems. Both hydrotropes and short chain tetraalkylammonium salts increased CP of C12E7 to 100 °C at very low concentration. The thermodynamic parameters of these mixtures are also calculated at varying additive concentrations. As the solubility of non-ionic surfactant containing heptapolyoxyethylene hydrophilic chain is highest at the cloud point, the thermodynamic parameters are calculated at this temperature. The results show that the standard Gibbs free energy change ( o cp G ∆ ), the enthalpy ( o cp H ∆ ) and the entropy ( o cp S ∆ ) at the cloud point are positive or negative depending on the nature of the additives.