Fourier transform surfactant systems

A disadvantage of the PMR technique, in particular, if applied to nonpolar surfactant systems is its low sensitivity. Although this has been considerably improved since the introduction of the Fourier transform technique it is still difficult to study very low concentration regions and, thus, to detect reliably critical concentrations. 

It is possible, for example, to use the method of Fourier transformations to solve the boundary problems for the system of differential equations (5.198), (5.199). In this case we have jmax linear differential equations of the second order. Because jmax is usually of the order of hundred, a further analytical investigation of Eqs. (5.198), (5.199) is senseless. However, the problem can be essentially simplified if the surfactant diffusion is treated separately for time scales comparable with the relaxation times of the fast and slow steps of micellisation, respectively. 

Recently, the same behavior was demonstrated for the system water (NaCl 8%) + decane 1 1-butanol-A-octylribonamide (CsNg), by using H chemical shift and relaxation time data. At saturation, the molar ratio of bound water to OH groups is again about 1 [135]. This is somewhat surprising, as usually the water solubilization behavior revealed by spectroscopic techniques is entirely different. Thus, NMR [14] (Fig. 17), time domain dielectric spectroscopy (TDS) [136], ESR [137], and Fourier transform infrared (FTIR) [15] measurements indicate that upon the addition of even a small amount of water, an equilibrium between free and bound water is established. This apparent discrepancy is readily understood because the spectroscopic techniques sense the water molecules most near the surfactant. 

Sabo, M., J. Gross, I. E. Rosenberg, Anionic surfactants in aqueous systems via Fourier transform IR spectroscopy,/. Soc. Cosmet. Chem., 1984,35,207-220. 

PFPE-based surfactants have been studied extensively by Johnston and coworkers [26-29], Fourier transform infrared (FTIR) measurements on a w = 10 microemulsion based on ammonium carboxylate PFPE [CF30(CF2CF(CF3)0) CF2C00NH4] (MW = 740) indicated the presence of bulk water domains. Water solubility up to w = 14 was reported at 55°C and around 177 bar for 1.4wt% surfactant [26]. Time-resolved fluorescence and electron paramagnetic resonance (EPR) studies suggested the presence of anisotropic or nonspherical micelles. Zielinski et al. [30] studied the phase stability of this system at 35°C, as shown in Fig. 2 SANS was then employed to study droplet structure. Observations with PFPE were similar to the earlier study in H7E7 [11] discrete water droplets of around 25 A radius were found to be present, and as indicated in Fig. 3 the droplet size increased with added water. 

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