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How to explain microemulsions formed by solvent mixtures without conventional surfactants.

ABSTRACT: Ternary solutions containing one hydrotrope (such as ethanol) and two immiscible fluids, both being soluble in the hydrotrope at any proportion, show unexpected solubilization power and allow strange but yet unexplained membrane enzyme activity. We study the system ethanol-water-octanol as a simple model of such kinds of ternary solutions. The stability of "detergentless" micelles or microemulsions in such mixtures was proposed in the pioneering works of Barden and coworkers [Smith GD, Donelan CE, Barden RE (1977)J Colloid Interface Sci60(3):488-496 and Keiser BA, Varie D, Barden RE, Holt SL (1979)J Phys Chem83(10):1276-1281] in the 1970s and then, neglected, because no general explanation for the observations was available. Recent direct microstructural evidence by light, X-ray, and neutron scattering using contrast variation reopened the debate. We propose here a general principle for solubilization without conventional surfactants: the balance between hydration force and entropy. This balance explains the stability of microemulsions in homogeneous ternary mixtures based on cosolvents.

SUBMITTER: Zemb TN

PROVIDER: S-EPMC4843454 | biostudies-other | 2016 Apr

REPOSITORIES: biostudies-other

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Project description:The aggregation behavior of two cationic surfactants, i.e., cetyldimethylethanolammonium bromide (CDMEAB) and cetyltributylphosphonium bromide (CTBPB), within an aqueous deep eutectic solvent (DES) is studied. The synthesized DES is composed of 1:2 mole ratio of choline chloride and glycerol and is further characterized by Fourier transform infrared (FTIR) and 1H NMR spectroscopy techniques. The critical micellar concentration (CMC), micellar size, and intermolecular interaction in surfactants within Gly-based DES solutions are investigated by various techniques including surface tension, conductivity, fluorescence, dynamic light scattering (DLS), FTIR, 1H NMR, and two-dimensional (2D) nuclear Overhauser effect spectroscopy (NOESY). The various interfacial properties and thermodynamic parameters are determined in the presence of 5 wt % glyceline (Gly)-based DES in an aqueous solution. The CMC, aggregation number (N agg), and Stern-Volmer constant (K sv) have also been determined by a steady-state fluorescence method. DLS is used to obtain information regarding the size of the aggregates formed by the cationic surfactants in DES solutions. FTIR spectroscopy is used to study the surfactant-DES interactions that tune the micellar structure of the surfactants within the Gly-based DES solution. The functional groups involved in the interactions (H-bonding and electrostatic) are the head groups (HO-CH2-CH2-N+ ion for CDMEAB and quaternary phosphonium (P+) ion for CTBPB) of the surfactants with the -OH-containing Gly DES. The hydrophobic moieties are involved in the hydrophobic interactions. The 1H NMR data show that differences in chemical shifts can provide significant information about the interactions taking place within the system. 1H NMR and NOESY techniques are further employed to strengthen our claim on the feasible structural arrangements within the aqueous surfactant-DES self-assembled structures. It is observed that both the cationic surfactants, i.e., CDMEAB and CTBPB, form self-assembled nanostructures in the Gly-based DES solutions. The present results are expected to be useful for colloidal solutions of DES and their mixtures with water.

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How to explain microemulsions formed by solvent mixtures without conventional surfactants.

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