Coagel

The stability of the gel phase and the transitions of coagel and gel phases to liquid crystal in the dioctadecyldimethylammonium bromide-water system were determined by... 

In a study of pressure effects on sodium and potassium decanoate micelles, the frequency of the CH2 stretching band was found to increase with pressure, with a discontinuous drop at the critical coagelization pressure (75). These results indicate that external pressure applied to micelles induces hydrocarbon tail disordering, even at pressures as high as 20 kbar, followed by a large increase in tail ordering upon coagel formation. 

The peak height ratio between the antisymmetric and symmetric CHj stretching modes H-2880/H-2850 is plotted as a function of pressure in Figure 7. This parameter has been shown to reflect the magnitude of interchain interactions in systems that contain polymethylene chains [20]. An abrupt increase in this ratio, and thus an increase in the interchain interactions, is observed at the pressure-induced transition from the micellar to the coagel phase. Furthermore, it is also evident from Figure 7 that the transition has a considerable pressure hysteresis. Interestingly, pressure [76] and temperature [79] have different effects on the H-2880/H-2850 ratio in the micellar phase of... 

The utility of FTIR spectroscopy in studies of phase transitions involving micellar surfactant solutions has been increasingly demonstrated in recent years (1-7). The familiar concentration-dependent monomer to micelle transition (cmc), the thermally induced hydrated solid to micelle transition (cmt), and the micelle to coagel transition achieved at high pressure (ccp) have all been investigated (Figure 1). These transitions can be monitored by shifts in frequency (= 5 cm"1) of the CH2 stretching bands which appear in the spectra. 

Figure 1 Pictoral phase diagram for a typical ionic surfactant. Micellar phases exist at temperatures above the critical micellization temperature (cmt), and concentrations above the critical micellization concentration (cmc). "pseudophase" transition from spherical to rodlike micelles may also occur at low temperatures or high surfactant concentrations. Also shown are regions where hydrated solid (gel or coagel) phases and liquid crystals (lamellar or hexagonal) appear (artwork courtesy of Linda Briones).

Methylene Tail Modes. Figure 4 presents representative spectra in the C-H stretching region of 70 mM SDS as a function of added NaCl (T=25°C). Also plotted are several difference spectra, obtained by subtraction of die spectrum of SDS in water from the spectra of the SDS samples containing added electrolyte, and a spectrum of a SDS coagel phase (T=6°C). 

Figure 4 Plot of the C-H stretching region of 70 mM SDS solutions under various conditions (T = 25°C, except where noted). From top 0 M NaCl and 0.6 M NaCl micelle solutions, 0 M NaCl (T = 6°C, coagel), and three difference spectra 0.6-0 M, 0.4-0 M,. 05-0 M. The inset shows a plot of v, CH2 as a function of added NaCl.

The difference spectra provide addidonal information. The 0.8 cm 1 decrease in frequency of the v, CH2 band upon the addition of 0.6 M NaCl results from the relative increase in intensity of a highly shifted CH2 band, which appears in the difference spectrum near 2852 cm 1. This relatively low frequency is comparable to that found in the spectrum of the coagel phase of SDS in electrolyte-free water at 6 °C (2852.5 cm 1). In the latter, the methylene chains are present in the more ordered, all-trans conformation. 

The absolute differences in the CH2 stretching frequencies in the spectra of the coagel phases of the three different systems reflect the differences in packing of the methylene chains adopted at low temperatures. The CH2 scissoring band is doubled in the spectrum of the SDS/0.3M NaCl system below the cmt, suggesting that an orthorhombic subcell is readily adopted. Examination of the S-0 stretching bands of... 

The viscosity is very much higher than that of the medium they set to jelly-like masses, often termed gels (or coagels). Examples sols of silicic acid, tin(IV), gelatin, starches, and proteins. 

During heating the surface layer of glutenin molecules in a gel structure is cross-linked by disulfide bridges, and the layer breaks up into rubber-like units a coagel. Tbe polar lipids take over at the interface during this process as indicated in Fig. 8.7. 

Figure 8.7 Structure of bread illustrated by lamellae between gas cells/pores. The dotted regions denote coagel formed by gluten proteins [20].

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