A DSC study of water behavior in water-in-oil microemulsions stabilized by sucrose esters and butanol

Sub-zero temperature differential scanning calorimetry (SZT-DSC) has been applied to a model nonionic water-in-oil microemulsion system based on: sucrose esters/water/1-butanol/n-alkanes (C₁₂-C₁₆). The maximum water solubilization was 40, 56 and 80 wt.% for the systems containing n-dodecane, n-tetradecane and n-hexadecane as the oil phase, respectively. Two types of solubilized water have been detected. The so-called `bulk' (free) water present in the core of the microemulsion and the `interfacial' (bound) water attached at the interface to the surfactant (and or butanol). The internal distribution of the water within the microemulsions was determined along two dilution lines (with 32 and 43 wt.% of the initial surfactant). It was found that for the n-dodecane system the maximum `interfacial' (bound) water is 12 and 14 wt.% along the two dilution lines, respectively. Above this water content a core of `bulk' (free) water is formed. The type of the oil and the butanol interfacial participation strongly affect the water internal distribution. Both the temperature of fusion, T_f, of the `bulk' (free) water and of the `interfacial' (bound) water are strongly affected by the butanol and the oil. The nature of the surfactant, its fatty chain length and its HLB also affect the binding capabilities and capacity of water in microemulsion systems. For both n-dodecane and n-hexadecane, 11-13 molecules of water can be bound to the surfactant at the interface.