The impact of non-polar solvent choice on the formation and structural properties of reverse micelles created by the non-ionic surfactant Tergitol 15-S-7

Abstract

This study explores the solubilization capabilities and properties of reverse micelles (RMs) formed by the non-ionic surfactant Tergitol 15-S-7 (TG7) in various non-polar solvents, including n-heptane (HP), toluene, and isopropyl myristate (IPM), both in the absence and presence of water (quantified as W₀ = [water]/[TG7]). The findings reveal that TG7 forms stable, transparent solutions with HP and IPM, exhibiting a greater capacity for water solubilization (W₀ of 6 and 5, respectively) compared to toluene (W₀ = 0.5). Dynamic light scattering measurements indicate the formation of RMs, with sizes increasing as water content increases. The sizes observed range from 9 nm to 38 nm for systems formulated in HP, while those in IPM exhibit sizes range from 8 nm to 19 nm. This suggests that water molecules are being entrapped and interacting with the surfactant monolayer. Additionally, spectroscopic studies using two highly sensitive probes demonstrate structural variations and differing properties at the micellar interfaces, influenced by the choice of non-polar solvent. Interestingly, we found that the HP/TG7/water RMs exhibit a microenvironment similar to that of n-pentanol. In contrast, the IPM/TG7/water RMs displayed characteristics resembling those of 3-propanone or chloroform at the interface. Proton NMR analysis of entrapped water and TG7's polar head group reveals distinct external solvent penetration, emphasizing the impact of solvent interactions on micellar properties. Therefore, in the HP/TG7 RMs, the protons associated with the entrapped water shift from 4.27 ppm to 4.78 ppm as W₀ changes. In contrast, within a similar water content for the IPM/TG7 system, the values shift from 3.96 ppm to 4.32 ppm. NOESY experiments indicated that IPM penetrates the interface more significantly than HP, leading to changes in the interface in both RMs. Overall, this research clarifies the fundamental behavior of TG7-based RMs and underscores their potential applications in Green Chemistry.