Proton NMR Studies on the Structure of Water at Interfaces of Aqueous Micelles. Part 4: Effects of Cationic and Zwitterionic Headgroups

Abstract

Fr.actionation factors, rpM' of interfacial water at surfaces of cationic and zwitterionic surfactants were determined from the dependence of the 1H chemical shift of water on the isotopic composition of the solvent (H°p lus O 2°) .~alue~of rpM are greater than unity, showing that interfacial water is more structured than bulk water who.se fractionation factor = 1, by defin.itio.n. The structuring increases with increasing bulk of the headgroup: but IS unaffected by the length of the catiomc surfactant hydrophobic tail. The effect of zwitterionic sulfobetaine s~r~actants on the structure of their water of hydration is less than that of cationic surfactants that have otherwise slm.ll~r .headgrou~s. Values of rpM for cationic micelles are: cetyldimethylphenylammonium chloride, 1.08; cetylp~ndln1Um chlonde, 1.06; and dodecyltrimethylammonium bromide, 1.06. Values of QlM for sulfobetaine micelles are: 3-N-tetradecyl-N,N-di-n-butylammonio-I-propane sulfonate, 1.07, and 3-N-tetradecyl-N,N-diethylammonio-I-propane sulfonate, 1.03.7]. These methods often involve hydrophobic solutes whose (average) solubilization site in the micellar pseudophase is a matter of discussion, and they may perturb aggregate structure [8, 9]. In the present work, we examine effects of the nature of surfactant headgroups on the structure of interfacial water by measuring the deuterium solvent isotope effect on the 1H chemical shift of the solvent (H 20 plus D 20). Isotopic substitution in the solvent involves minimum perturbation of the micellar system. The resulting secondary isotope effect is, however, small, but 1H NMR spectroscopy is a very sensitive technique and is, therefore, suitable for measurement of such small effects [10][11][12][13][14][15][16].It is convenient to describe isotope effects in terms of fractionation factors, rp, defined for a micelle, M, as:That is, for a micelle in H 20-D20, the fractionation factor for interfacial water, rpM, describes the isotopic composition at the aggregate/water interface relative to that in bulk solvent [14][15][16].We are interested in exploiting the value of rpM to probe effects of the charge and nature of headgroups on the degree of structure of interfacial water. To date, effects of micelles of various ionic, zwitterionic and nonionic surfactants have been examined [14][15][16]. In the present work we examine effects due to aromatic or bulky alkyl headgroups in cationic and zwitterionic sulfobetaine micelles, as well as variation in the length of the surfactant hydrophobic tail.Earlier results showed that hydrophobic headgroups (phenyl or n-butyl) enhance the structure of interfacial water; electrostriction by the charged interface and hydrophobic hydration of the bulky group being the main factors.

Specific Ion Effects on Zwitterionic Micelles Are Independent of Interfacial Hydration Changes

“…Zwitterionic micelles without salt are less hydrated than cationic micelles with chloride, a weekly binding anion, as counterion with the same chain length. , Our results are in line with those obtained by Phukon and Sahu because DRS of cationic micelles of N -dodecyl- N , N -trimethylammonium chloride, DTAC, yielded a value of 26 for the Z t of DTAC micelles, compared to a Z t of 19 found here for DPS. For other counterions in cationic systems of DTAX, where X is (Cl, Br, Ms, or Tfl), Z t is lower, and this will be discussed in the next session.…”

Fluorescence and Light-Scattering Studies of the Aggregation of Cationic Surfactants in Aqueous Solution: Effects of Headgroup Structure

“…The present study is a part of our interest in effects of the structure of the surfactant headgroup on the aggregation and catalytic properties of organized assemblies. − We have studied micelles of the cationic surfactants shown in Chart by four independent techniques, namely, surface tension, conductance, static and quasi-elastic light scattering, LS, and fluorescence.

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“…Previously, extensive work has been carried out on cationic surfactants with the general structure RN + R‘R‘ ‘R‘ ‘‘ X - , where X - = halide ion, R = octyl to octadecyl, and R‘, R‘ ‘, and R‘ ‘‘ generally represent identical alkyl groups, e.g., trimethyl to tri- n -pentyl. Alternatively, a number of studies have employed R‘ and R‘ ‘ = methyl and R‘ ‘‘ = alkyl group (e.g., ethyl to n -octyl, or 2-hydroxyethyl), or R‘ = methyl and R‘ ‘ and R‘ ‘‘ = alkyl group (e.g., methyl to n -butyl, or 2-hydroxyethyl). ,,, Of the surfactants bearing a phenyl headgroup (see Chart ), only CMe 2 BzACl has been studied in detail because it is a major component of “benzalkonium chloride”, a product widely used as an antiseptic in pharmaceutical preparations , and of Zana's and Lang's work on the formation of water-in-oil microemulsions by CMe 2 PhACl, CMe 2 BzACl, and CMe 2 PhEtACl, there appear to be no published data on the properties (e.g., α, N agg ) of aqueous micelles of CMe 2 PhACl, CMe 2 PhEtACl, and CMe 2 PhPrACl, respectively.…”

DSC Analysis of Surfactant-Based Microstructures

“…Water in very small volumes plays a dominant role as the medium that controls structure, function, dynamics, and thermodynamics near biological membranes or in other confined regions of space [171]. Interfacial water is involved in, among other things, solvation of reactants and transition states, solvation of surfactant headgroups and counterions, and protons [178]. Water in microstructured systems may be roughly classified into four categories: free water, interstitial water, surface or interfacial water (physically and/or chemically adsorbed), and internal water (chemically bound water).…”