Probing the relationship between interfacial concentrations and lipase activity in cationic W/O microemulsions: a quantitative study by chemical trapping

Interfacial concentrations and/or space: which one is the predominant factor in regulating lipase activity at the water−oil interface? This work is an endeavor toward probing the relationship between lipase activity and interfacial concentrations in cationic water-in-oil (W/O) microemulsions through quantitative study by a chemical trapping method. The interfacial concentrations of water ([H2Oi]), bromide ([Bri-]), and n-hexanol ([HexOHi]) were estimated in the W/O microemulsions of six surfactants with varying headgroup architecture and hydrophilicity across a wide Wo ([H2O]/[surfactant]) range. The surfactants were prepared by the replacement of methyl groups of cetyltrimethylammonium bromide (1) by n-propyl (2−4), one hydroxyethyl (5), and one methoxyethyl (6) group. The estimated [H2Oi] was found not to change much (30.0−36.7 M) with the variation in headgroup hydrophilicity or size from 1−5. However, [Bri-] was found to increase with a decrease in the degree of dissociation (α), being higher for 1 and 5 (2.4−3.3 M) and relatively lower (0.9−1.9 M) for others depending on Wo. Interestingly, [H2Oi] was found to be little higher (41.5−42.2 M) in the case of 6. The present study elucidates the importance of interfacial water and counterion concentrations in modulating the lipase activity in reverse micelles. In our previous report, the lipase activity was found to increase from 1−4 and in 6, whereas that observed in 5 was comparable with 1, being largely regulated by the surfactant head group size (Das, D.; Roy, S.; Mitra, R. N.; Dasgupta, A.; Das, P. K. Chem. Eur. J. 2005, 11, 4881). The only other parameter that increased distinctly with lipase activity is the headgroup size, not [H2Oi]. Thus, the role of [H2Oi] in comparison to the surfactant's headgroup size is not found to be that significant. Moreover, the lower [Bri-] in 2−4 and 6 perhaps enhances the probability of enzyme and substrate localization at the interface, leading to higher lipase activity