title: Does the optimum hydrophilic lipophilic balance condition affect the physical properties of mixed reverse micelles? A spectroscopic investigation.
abstract: Synergism in several physical properties as realized in many mixed surfactant reverse micellar (RM) systems often manifests optimum hydrophilic-lipophilic balance (HLB), interdroplet interaction, or both. Such synergism is often desired for specific applications of RM systems; however, a proper rationale on the effect of such phenomenon imparted on the structure, dynamics, and activity of water molecules in RM waterpool is strongly demanded. In the present contribution we have investigated how the optimum HLB condition of mixed RM composed two nonionic surfactants (Igepal 210 and Igepal 630) affects the physical properties of entrapped water molecules in the RM waterpool. The studied mixed RM exhibits synergistic water solubilization behavior as a function of the mixing ratio with a maximum in solubilization capacity being reached at X(Ig630) = 0.3. Dynamic light scattering (DLS) studies show a bimodal distribution of droplet size in this region, whereas it is monomodal in terminal compositions. Fourier transform infrared spectroscopy (FTIR) study in the 3000-3800 cm(-1) region identifies a linear trend in which the content of "bound" water increases at the expense of the "network" water as the content of the hydrophilic surfactant Igepal 630 is increased in the mixture. Subnanosecond relaxation dynamics of the entrapped water as revealed by the fluoroprobe coumarin 500 corroborates a similar linear trend as observed in the FTIR measurements as the rotational diffusion gets retarded with the increase of ethylene oxide chain length of Igepal. Reaction kinetics of solvolysis of benzoyl chloride reaction, however, does not offer any linear trend as it gets slower in the optimum HLB region, the nonlinearity being a consequence of the distribution of the substrate in the different phases.
authors.affiliation: Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences , Block JD, Sector III, Salt Lake, Kolkata, West Bengal 700098, India.
authors.forename: Arindam
authors.initials: A
authors.lastname: Das
authors.forename: Rajib Kumar
authors.initials: RK
authors.lastname: Mitra