Physicochemical investigation of biocompatible mixed surfactant reverse micelles: II. Dynamics of conductance percolation, energetics of droplet clustering, effect of additives and dynamic light scattering studies

•Percolation temperature depends on contents of water, nonionic surfactant and [NaCl].•Acetyl modified amino acids retard the temperature-induced percolation process.•The clustering process is spontaneous, endothermic and entropically driven.•Enthalpy and entropy of clustering process increase with increasing nonionic content.•Chemical structures of oils influence thermodynamic parameters of droplet clustering.

Temperature-induced percolation behavior in mixed reverse micellar systems (RMs) comprising sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and polyoxyethylene(20) sorbitan trioleate (Tween-85) in polar lipophilic oils e.g., ethyl oleate (EO), isopropyl myristate (IPM) and isopropyl palmitate (IPP) was studied at a total surfactant concentration (ST) of 0.25 ⋅ 103 mol ⋅ m−3 in absence and presence of the additives (acetyl modified amino acids (MAA) of different chemical structures). The threshold temperature of percolation (Tp) of these systems was found to be dependent on water content (ω), content of nonionic surfactant (XTween-85), and concentration of NaCl. The standard free energy change (ΔGcl0), enthalpy change (ΔHcl0) and entropy change (ΔScl0) of cluster formation were evaluated based on an association model at different physicochemical environments. The clustering process was spontaneous and found to be endothermic and entropically driven. Scaling laws for the temperature-induced percolation demonstrated dynamic nature of the percolation process. The activation energy, Ep of these systems was estimated both in absence and presence of additives. Droplet sizes of these systems were measured using dynamic light scattering (DLS) technique at different physicochemical environments (comparable to conductivity measurements) to underline the factors governed the percolation process. The chemical structure of the oils played an important role in influencing Tp, Ep, thermodynamic parameters and droplet size of these systems in absence and presence of the additives. An attempt has been made to underline the microstructures of these systems in the light of percolation of conductance vis-à-vis thermodynamics of droplet clustering and droplet dimensions.

Graphical abstractTp/°C: Threshold temperature of percolation; ω: molar ratio of water to surfactant; IPM: isopropyl myristate; EO: ethyl oleate; IPP: isopropyl palmitate.Figure optionsDownload full-size imageDownload as PowerPoint slide