Neat surfactants

Howard [27] determined dissolved aluminium in seawater by the micelle-enhanced fluorescence of its lumogallion complex. Several surfactants (to enhance fluorescence and minimise interferences), used for the determination of aluminium at very low concentrations (below 0.5 pg/1) in seawaters, were compared. The surfactants tested in preliminary studies were anionic (sodium lauryl sulfate), non-ionic (Triton X-100, Nonidet P42, NOPCO, and Tergital XD), and cationic (cetyltrimethylammonium bromide). Based on the degree of fluorescence enhancement and ease of use, Triton X-100 was selected for further study. Sample solutions (25 ml) in polyethylene bottles were mixed with acetate buffer (pH 4.7, 2 ml) lumogallion solution (0.02%, 0.3 ml) and 1,10-phenanthroline (1.0 ml to mask interferences from iron). Samples were heated to 80 °C for 1.5 h, cooled, and shaken with neat surfactant (0.15 ml) before fluorescence measurements were made. This procedure had a detection limit at the 0.02 pg/1 level. The method was independent of salinity and could therefore be used for both freshwater and seawater samples. 

In the case of liquid detergents, surfactants are almost always present. At low to intermediate concentration, most neat surfactant solutions have low viscosity and are close to Newtonian in flow. Only at higher surfactant concentrations, when structured micellar bilayers and other complex phases are formed, do systems tend to differ greatly from Newtonian. This behavior also helps drive the viscosity of finished formulations. In the great majority of liquid detergent formulations, concentrations of surfactant are such that little structure is developed by the surfactants themselves, resulting in formulations of low viscosity. As such, thickeners and/or rheology modifiers are often required to obtain the desired viscosity and flow characteristics. 

Figure 3 Simulated fluorescence decay data illustrating the changes in quenching behavior with the confinement structure. The influence of natural decay has been eliminated by multiplication with the factor exp(A oO- Curve a, without quencher b-e, quenching in zero to three dimensions (spherical micelles, rods, bilayers, neat surfactant). The parameters were chosen to mimic C12E6 in water, with pyrene as probe and dimethylbenzophenone as quencher. (From Ref. 15.)...

The detailed phase diagrams of the selected surfactant with water or formamide exhibit a variety of lyotropic liquid crystalline phases. While there is only a monotropic cholesteric phase in the neat surfactant, the addition of either one of the solvents leads to the induction of the following enantiotropic phases cholesteric, lamellar L , high and low temperature two-dimensional monoclinic M , and SmC analog. Remarkably, the lyotropic SmC analog phase occurs only at elevated solvent concentrations, which shows that this is a tme lyotropic phase. 

The substituents of the ammonium cations may also influence the thermal stability of neat surfactants as well as of corresponding organoclays. In general, methyl and benzyl groups in these materials were susceptible at elevated temperatures to nucleophilic attack by the halide anions [16,30],... 

As the concentration of a surfactant in water increases, there is a critical point, called the critical micelle concentration (CMC), at which micelles form, and there is, at this point, a sharp change in physical properties such as viscosity and refractive index (Figure 16.4). As the solution becomes more concentrated, other phases are formed including cylindrical micelles and bilayers. In a neat surfactant phase, the main structure is a bilayer bilayers are commonly found when... 

Surface tension is usually predicted using group additivity methods for neat liquids. It is much more difficult to predict the surface tension of a mixture, especially when surfactants are involved. Very large molecular dynamics or Monte Carlo simulations can also be used. Often, it is easier to measure surface tension in the laboratory than to compute it. 

An example for a partially known ternary phase diagram is the sodium octane 1 -sulfonate/ 1-decanol/water system [61]. Figure 34 shows the isotropic areas L, and L2 for the water-rich surfactant phase with solubilized alcohol and for the solvent-rich surfactant phase with solubilized water, respectively. Furthermore, the lamellar neat phase D and the anisotropic hexagonal middle phase E are indicated (for systematics, cf. Ref. 62). For the quaternary sodium octane 1-sulfonate (A)/l-butanol (B)/n-tetradecane (0)/water (W) system, the tricritical point which characterizes the transition of three coexisting phases into one liquid phase is at 40.1°C A, 0.042 (mass parts) B, 0.958 (A + B = 56 wt %) O, 0.54 W, 0.46 [63]. For both the binary phase equilibrium dodecane... 

K. Okamoto and M. Okamoto also investigated the biodegradability of neat PBS before and after nanocomposite preparation with three different types of OMLF. They used alkylammonium or alkylphosphonium salts for the modification of pristine layered silicates, and these surfactants are toxic for microorganisms [56]. 

A modified rare earth catalyst (30) which is based on a polystyrene backbone as depicted in Scheme 4.15 can be applied even in neat water. It is attached via a hydrophobic oligomeric linker which creates a nonpolar reaction environment and acts as a surfactant for the substrates. The reaction of 4-phenyl-2-butanone with tetraallyltin in water using 1.6 mol% of the scandium catalyst (30) afforded the corresponding homoallylalcohol in a yield of 95%. Interestingly, when using other solvents (dichloromethane, acetonitrile, benzene, ethanol, DMF) the yields decreased drastically, indicating a much higher reaction rate in water [98]. 

Similar to the case of Suzuki couplings (6.1.2), ally lie alkylations can also be run in neat water as solvent in the presence of surfactants. In addition to the general solubihzation effect, the amphiphiles may also have a specific influence on the reaction rate. For example, the reaction of the P-ketoester substrate on Scheme 6.22 with allyl acetate, catalyzed by [Pd(PPh3)4] was only slightly accelerated by the anionic SDS (1.5 h, 18 % yield), however, the reaction rate dramatically increased in the presence of the cationic CTAB and the neutral Triton X-100 detergents, leading to 74 % and 92% yields in 1.5 h and 5 min ( ), respectively [51]. Several other carbonucleophiles were alkylated in such emulsions with excellent yields. 

Albert Einstein derived a simple equation for the viscosity of a solution of spherical particles, and from this result it is obvious that if we could make the polymer in small colloidal-sized balls, then the solution would be much less viscous. Also, if we could use surfactants to stabilize (e.g. by charging) the polymer particles in water, then there would be no need for organic solvents. Both these conditions are neatly obtained in the emulsion polymerization process, which is schematically explained in Figure 5.3. A polymer latex is produced by this process and can contain up to 50% polymer in the form of 0.1-0.5 im size spherical particles in water. A typical starting composition is ... 

At relatively low concentrations of surfactant, the micelles are essentially the spherical structures we discussed above in this chapter. As the amount of surfactant and the extent of solubilization increase, these spheres become distorted into prolate or oblate ellipsoids and, eventually, into cylindrical rods or lamellar disks. Figure 8.8 schematically shows (a) spherical, (b) cylindrical, and (c) lamellar micelle structures. The structures shown in the three parts of the figure are called (a) the viscous isotropic phase, (b) the middle phase, and (c) the neat phase. Again, we emphasize that the orientation of the amphipathic molecules in these structures depends on the nature of the continuous and the solubilized components. 

FIG. 8.8 Schematic representations of surfactant structures in (a) viscous isotropic, (b) middle, and (c) neat liquid crystal phases. 

Effect of addition of homopolymer, salt or conventional surfactants The effect of addition of PEO and PPO homopolymers on the gel formation of Pluronic F127 (defined in Fig. 4.3) in aqueous solution has been studied by Malm-sten and Lindman (1993). The structure, studied via SANS, and rheology of neat F127 solutions in the concentration range 10-20% has been probed by Prud homme et al. (1996). Addition of PEO can reduce the gel region and/or eliminate it at sufficiently high PEO concentration. The amount of PEO required to melt the gel depends on the copolymer concentration and decreases with... 

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