Interfacial composition, thermodynamic properties, and structural parameters of water-in-oil microemulsions stabilized by 1-pentanol and mixed surfactants

The present study is focused on the evaluation of the interfacial composition, thermodynamic properties, and structural parameters of water-in-oil mixed surfactant microemulsions [(cetylpyridinium chloride, CPC + polyoxyethylene (20) cetyl ether, Brij-58 or polyoxyethylene (20) stearyl ether, Brij-78)/1-pentanol/n-heptane, or n  -decane] under various physicochemical environments by the Schulman method of cosurfactant titration of the oil/water interface. The estimation of the number of moles of 1-pentanol at the interface (nai) and bulk oil (nao) and its distribution between these two domains at the threshold level of stability have been emphasized. The thermodynamics of transfer of 1-pentanol from the continuous oil phase to the interface have been evaluated. nai,nai, standard Gibbs free energy (ΔGt0), standard enthalpy (ΔHt0), and standard entropy (ΔGt0) of transfer process have been found to be dependent on the molar ratio of water to surfactant (ω), type of nonionic surfactant and its content (XBrij-58 or Brij-78), oil and temperature. A correlation between (ΔHt0) and (ΔSt0) is examined at different experimental temperatures. Bulk surfactant composition dependent temperature insensitive microemulsions have been reported. Associated structural parameters, such as droplet dimensions and aggregation number of surfactant and cosurfactant at the droplet interface have been evaluated using a mathematical model after suitable modifications for mixed surfactant systems. In light of these parameters, the prospect of using these microemulsion systems for the synthesis of nanoparticles and the modulation of enzyme activity has been discussed. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.

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► Dilution method evaluates interfacial compositions in mixed surfactant water-in-oil microemulsions.
► Energetic parameters depend on water content, nonionic surfactant and its content, temperature and oil.
► Spontaneity of pentanol transfer process favors at high temperatures in both oils.
► With increasing pentanol concentration at the oil/water interface, radii of droplet and water pool decrease.
► Aggregation number of mixed surfactants increases with increasing water content.