Relaxation time surfactant effects

It was determined, for example, that the surface tension of water relaxes to its equilibrium value with a relaxation time of 0.6 msec [104]. The oscillating jet method has been useful in studying the surface tension of surfactant solutions. Figure 11-21 illustrates the usual observation that at small times the jet appears to have the surface tension of pure water. The slowness in attaining the equilibrium value may partly be due to the times required for surfactant to diffuse to the surface and partly due to chemical rate processes at the interface. See Ref. 105 for similar studies with heptanoic acid and Ref. 106 for some anomalous effects. 

Gasljevic and Matthys expressed DR as a function of both u and They showed that polymer DR as a function of V is independent of diameter. However, diameter scale-up was only valid for Type A DR. Usui, Itoh, and Saeki suggested that the fluid relaxation time was essential to surfactant DR. Their scaling law for DR as a function of either u or V (mean flow velocity) included the effect of shear thinning behavior of their surfactant solution. 

FIGURE 6.10 Effect of surfactant concentration on elastic modulus and relaxation time. Source Data from Kang (2001). 

The diffusion coefficients that can be measured with the PGSE method cover the range from fast diffusion of small molecules in solutions with D values typically around 10" m /s to very slow diffusion of, for instance, polymers in the semidilute concentration regime, where D values down to 10 m /s can be measured [19]. Measurements of such very slow diffusion requires gradients of extreme magnitudes and places severe demands on the actual experimental setup [9]. What often limits the lowest value of D that can be measured is the value of spin-spin relaxation time, T2. As a general rule, slow diffusion is often found in systems that also show rapid transverse relaxation. As a consequence, the echo intensity gets severely damped by T2 relaxation in such systems. For microemulsion systems, such problems are virtually nonexistent for the solvents, while for the surfactant molecules the accuracy is often reduced because of T2 effects. 

Dressing surfactant density Magnetic core density Effective relaxation time of magnetization Brownian relaxation time Neelian relaxation time Hydrodynamic volume fraction of the surfactant-dressed nanoparticles Suspension volume fraction of magnetic material embedded in the nanoparticles y-phase-side values of the variable eg on K-phase-side values of the variable eg on Spatial fluctuation of the bounded piecewise continuous scalar field cp around the intrinsic average in u... 

The effect of the change of the salt concentration on the relaxation time distribution at SDS/PEO 1 9 w/w, shown in Fig. 14, appears larger than may be expected from the change in the phase ctiagram. At both salt concentrations is composition is far beyond the saturation limit. The amplitudes of the scattering peaks should, however, dii tly reflect the relative amounts of material present as free micelles in the polymer-surfactant complex when the main peak areas are equal. 

Rotational dynamics of a fluorescent dye adsorbed at the interface provides useful information concerning the rigidity of the microenvironment of liquid-liquid interface in terms of the interfacial viscosity. The rotational relaxation time of the rhodamine B dye was studied by the time-resolved total internal reflection fluorescent anisotropy. In-plane rotational relaxation time of octadecylrhodamine B cation was evaluated under the presence or absence of a surfactant [26]. Table 2.8 shows that by adding a surfactant, the relaxation time and the interfadal viscosity increased. Anionic surfactants SDS and HDHP (hydrogen dihexadecylphosphate) were more effective in reducing the rotational motion, because of the electrostatic interaction. HDHP with double long chains hindered the interfacial rotation more [40]. 

The intermolecular association of polymer-bound dodecyl groups is evidenced by an effect of a surfactant molecule added to polymer solutions. Hgure 7 compares relaxation time distributions for the copolymers with, X)d = 2.5 and 10 mol % at a 10.0 g/L polymer concentration in the presence of varying concentrations of n-dodecyl hexaoxyethylene glycol monc tiier (C12E6). In the cai of the copolymer with j >od =... 

Studies concerning the micelle formation/breakdown in mixed micellar solutions are few. Folger et al. showed that small amounts of STS (mole fraction 2-5%) significantly affected the value of T2 for SDS, particularly at C close to the cmc (see Figure 3.5). This is expected since micelle formation/breakdown is similar to a nucleation process. Patist et ai 160 reported that the slow relaxation process in solutions of SDS became considerably slower upon the addition of alkyl-trimethylammonium bromides. The largest effect was obtained with dodecyltrimethylammonium bromide, and the authors interpreted the results in terms of chain compatibility. Measurements of the slow relaxation time have been used to show that solutions of mixtures of some hydrocarbon and perfluorocarbon surfactants contain two types of mixed micelles, one rich in hydrocarbon surfactant, the other rich in perfluorocarbon surfactant. 

Other studies considered specifically the effect of the alcohol on the kinetics of micelle formation/break-down.92>i i -i 2 Spectacular changes of the relaxation time T2 were evidenced. Note, however, that the three relaxation processes expected for a mixed surfactant + alcohol system were evidenced and investigated for only the tetradecyltrimethy-lammonium bromide (TTAB)/l-pentanol system. ... 

The reciprocal of the relaxation time T12 characterizing the surfactant exchange varied linearly with the surfactant concentration at constant alcohol concentration (see Figure 3.13). The cosurfactant was found to have relatively little effect on the exchange rate constants and k" of the surfactant. Some examples are given in Table 3.3. 

There have been few investigations of the effect of aromatic or alkyl solubilizates on the micellar dynamics. Alkanes have been found to have almost no effect on the value of the relaxation time for the surfactant exchange in micellar solutions of sodium heptylsulfate and hexylammonium chloride. Likewise, cyclohexane has very little effect on the exchange in micellar solutions of sodium octylsulfate. In these studies the amount of solubilizate was relatively small. In contrast. 

In Equation 3.26, T is the equilibrium surface excess, C the bulk concentration, t the time, and D the surfactant monomer diffusion coefficient. Eastoe et al. have measured the time dependence of the DST and the relaxation time %2 for solutions of many surfactants nonionic, dimeric, and zwitterionic. In all instances the fitting of the data to Equation 3.26 with the experimentally determined value of %2 was poor. The authors concluded that the micelle dissociation may have an effect on the measured DST only if the concentration of monomeric surfactant in the subsurface diffusion layer is limiting or when the micelle lifetimes are extremely long. No surfactant for which this last condition is fulfilled was evidenced by the authors. They also concluded that the rapid dissociation of monomers from micelles present in the subsurface was not likely to limit the surfactant adsorption and thus the DST. 

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