Infrared and far-infrared spectroscopic studies on the structure of water in lyotropic liquid crystals

Abstract

Studies on the infrared and far-infrared spectra of water in lyotropic liquidcrystalline phases of dodecyltrimethyl ammonium chloride (and related materials) have shown the existence of at least two severely perturbed types of water molecule -both very different from bulk water. Band shifts, half-widths and relative intensities change rapidly over the concentration range, but show no discontinuities at phase boundaries (including the gel/lamellar boundary). The water molecules therefore probe only the short-range interactions between head group and counterion and do not reflect changes in long-range micellar interactions in the different mesophases. These data support the equilibrium binding model for water-surfactant interactions at polar organic interfaces and tend to refute the existence of different polarized layers. As such, they may eventually help to cast light on the nature of hydration forces between the bilayers in lamellar phases. IntroductionThe binding of water at polar charged organic interfaces has been a topic of considerable interest among surface chemists, spectroscopists and biochemists (and biophysicists) for the past 20 years. The state or structure of water in ordered phases (micelles, mesophases, microemulsions) has been perceived to be important: for example, for the stabilization of the bilayer structures (lamellar phases) from which biomembranes are formed [l-111 and for the adhesion [8] and fusion [7] of such membranes. Accordingly, there have been a range of spectroscopic measurements aimed at distinguishing biological (i.e. perturbed or bound) water from bulk water. The most extensive studies have been made using N.M.R. techniques, notably *H and "0 quadrupolar splitting [12-221 and 'H, 'H, "0 nuclear relaxation times [15][16][17][18]. These have been employed to monitor the ordering and motional freedom of the water molecules at organic interfaces, through the well-known order and relaxation parameters [I 8, 191. However, there have also been microwave (dielectric) [3 1-35], far-infrared [3&38], mid-infrared [39-541 and Raman [55] spectroscopic measurements aimed at finding structural and motional changes caused by water molecules interacting at either.head group or counterion (or both). It is clear from the literature that the interpretation of such spectra is far from straightforward or conclusive and often different models will apparently fit the same data. Certainly, a consensus of opinion on the crucial question as to whether water molecules in mesophases form perturbed and distinct layers [lo, 111, or whether there is simply an equilibrium

Effect of the Cationic Surfactant Moiety on the Structure of Water Entrapped in Two Catanionic Reverse Micelles Created from Ionic Liquid‐Like Surfactants

“…[41] In the liquid phase, this band not only corresponds to the OÀH stretching but also to the vibrational coupling of the HÀOÀH bonds. [42,43] This phenomenon is also observed for deuterated water (D 2 O), for which the band recorded in the 2570-2350 cm À1 region, also broad, corresponds to the OÀD stretching mode and the vibrational couplings of the DÀOÀD bonds. In our FTIR studies we decided to use monodeuterated water (HDO), which exhibits a narrow band at around 2570-2350 cm À1 that can be assigned only to the OÀD stretching band (n OÀD ).…”

FTIR and1H NMR Studies of the Solubilization of Pure and Aqueous 1,2-Ethanediol in the Reverse Aggregates of Aerosol-OT

“…Figure B shows three conformations of AOT at the oil/water interface. In all conformers H1 is in the oil pseudo-phase, whereas the two H1‘ protons are mostly in the aqueous pseudo-phase because rotational isomer III predominates (I: II: III = 0.21:0.04:0.75) 16e. Therefore, the chemical shift of the former proton may be essentially affected by changes in the electron density of the sulfonate group due to its solvation.…”

“…Conclusions with regard to the types of water present within RMs and W/O μEs are usually based on the number of peaks obtained by curve fitting of ν OH (of solubilized H 2 O) or ν OD (of solubilized D 2 O). However, the basic premise involved in this assumption, i.e., that each band obtained by curve fitting may be attributed to a different type of water, seems possibly suspect because these bands may originate from coupled water molecule vibrations, and from a bending overtone often reported in the spectrum of liquid water. 16a-d On the other hand, deconvolution of ν OH or ν OD vibrations of HOD is straightforward because both frequencies are essentially decoupled, provided that D 2 O ≤ 10% . This advantage has been recognized both for the bulk aqueous phase, , and for reverse aggregates. ,, Finally, we employed isooctane as a solvent in the 1 H NMR experiment because its peaks spread over a narrower δ range than those of n -heptane.…”

How the Type of Cosurfactant Impacts Strongly on the Size and Interfacial Composition in Gemini 12-2-12 RMs Explored by DLS, SLS, and FTIR Techniques

“…It is well-known that water and alcohols molecules exhibit a broad band in the 3500–3200 cm –1 region, which is assigned to the O–H stretching . In liquid phase, this band not only corresponds to the O–H stretching but also to the vibrational coupling of the H–O–H bonds. , This phenomenon is also observed for deuterated water (D 2 O) in which the band recorded in the 2570–2350 cm –1 region, also broad, corresponds to the O–D stretching mode and the vibrational couplings of the D–O–D bonds. In RMs, the most common method to obtain information is the curve fitting of the different IR peaks, e.g., v O–H of entrapped water or v O–D of entrapped D 2 O. ,− However, the basic premise involved in this assumption, i.e., that each band obtained by curve fitting may be attributed to a different type of water, is open to question because these bands possibly originate from coupled water molecule vibrations, and from a bending overtone often reported in the spectrum of liquid water .…”