Ionic Liquid Induced Changes in the Properties of Aqueous Zwitterionic Surfactant Solution

Abstract

Modifying properties of aqueous surfactant solutions by addition of external additives is an important area of research. Unusual properties of ionic liquids (ILs) make them ideal candidates for this purpose. Changes in important physicochemical properties of aqueous zwitterionic N-dodecyl- N, N-dimethyl-3-ammonio-1-propanesulfonate (SB-12) surfactant solution upon addition of hydrophilic IL 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF 4], are reported. Dynamic light scattering results indicate a dramatic reduction in the average micellar size in the presence of [bmim][BF 4]; micellar (or micelle-like) aggregation in the presence of as high as 30 wt % [bmim][BF 4] is confirmed. Responses from fluorescence probes are used to obtain critical micelle concentration (cmc), aggregation number ( N agg), and dipolarity and microfluidity of the micellar pseudophase of aqueous SB-12 in the presence of [bmim][BF 4]. In general, increasing the amount of [bmim][BF 4] to 30 wt % results in decrease in N agg and increase in cmc. Increase in the dipolarity and the microfluidity of the probe cybotactic region within the micellar pseudophase is observed on increasing [bmim][BF 4] concentration in the solution. It is attributed to increased water penetration into the micellar pseudophase as [bmim][BF 4] is added to aqueous SB-12. It is proposed that IL [bmim][BF 4] behaves similar to an electrolyte and/or a cosurfactant when present at low concentrations and as a polar cosolvent when present at high concentrations. Electrostatic attraction between cation of IL and anion of zwitterion, and anion of IL and cation of zwitterion at low concentrations of [bmim][BF 4] is evoked to explain the observed changes. Presence of IL as cosolvent appears to reduce the efficiency of micellization process by reducing the hydrophobic effect.

Catanionic surfactant formation from the interaction of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and the ionic liquid 1-butyl-3-methylimidazolium octyl sulfate (bmim-octyl SO4) in aqueous solution

“…Binary mixtures of CTAB and bmim-octylSO 4 at different CTAB mole fractions ( CTAB ranging from 0.05 to 0.9) were prepared by mixing the appropriated volumes of bulk solutions of CTAB and bmim-octylSO 4 at two total concentrations, 0.5 mM and 10 mM. Samples were allowed to stand for a week to reach steady state.…”

“…A sample of 0.448 g of CTAB was mixed with 0.285 g of octylSO 4 Na in 25 ml of water. The reaction mixture was stirred vigorously at 60ºC for 4 hours.…”

“…The effect of the ionic liquid on the surfactant properties depends on the IL structure and concentration. Behera and Pandey [1,4,8] reported that at low concentration some ILs act similarly to an electrolyte decreasing the surfactant cmc whereas at high concentration they behave as a cosolvent increasing this parameter. In the case of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) the effect of ILs on the surfactant cmc confirms the trend above described [8][9][10][11][12][13].…”

“…In some cases the IL acts as an aqueous cosolvent and modifying some surfactant properties such as the critical micelle concentration (cmc) with respect to the value in neat water because of the weaker solvophobic interaction between the alkyl chain of the surfactant and the IL aqueous solution [1][2][3][4][5][6][7]. The effect of the ionic liquid on the surfactant properties depends on the IL structure and concentration.…”

“…Therefore, moreover the catanionic surfactant, CTA-octylSO 4 , and the salt resulting from the corresponding counterions, bmim-Br, the excess of one of the components, CTAB or bmim-octylSO 4 , will be present in the aqueous solution for non equimolar mixtures.…”