Surfactant chains

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

An important application of foams arises in foam displacement, another means to aid enhanced oil recovery. The effectiveness of various foams in displacing oil from porous media has been studied by Shah and co-workers [237, 238]. The displacement efficiency depends on numerous physicochemical variables such as surfactant chain length and temperature with the surface properties of the foaming solution being an important determinant of performance. 

FIG. 22 Side view snapshots of a simulation of a 16-carbon hydrogenated surfactant chain with a carboxylate-like head group on a water surface at 300 K. The view iu (a) (top) is au area of 0.21 um molecule (b) (bottom) is at 0.21 um molecule . These two areas roughly bracket a first-order trausitiou with some features of the LE-LC transition. See also Figure 23 for the correspoudiug pressure-area isotherm. (Reproduced with permission from Ref. 364. Copyright 1992 American Chemical Society.)... 

Electrochemical redox studies of electroactive species solubilized in the water core of reverse microemulsions of water, toluene, cosurfactant, and AOT [28,29] have illustrated a percolation phenomenon in faradaic electron transfer. This phenomenon was observed when the cosurfactant used was acrylamide or other primary amide [28,30]. The oxidation or reduction chemistry appeared to switch on when cosurfactant chemical potential was raised above a certain threshold value. This switching phenomenon was later confirmed to coincide with percolation in electrical conductivity [31], as suggested by earlier work from the group of Francoise Candau [32]. The explanations for this amide-cosurfactant-induced percolation center around increases in interfacial flexibility [32] and increased disorder in surfactant chain packing [33]. These increases in flexibility and disorder appear to lead to increased interdroplet attraction, coalescence, and cluster formation. 

Fromherz s model considers a spherical micelle where the surfactants are arranged in parallel forming a packaging without tensions and without contact with the water, in which the heads of the surfactants are as separated as possible [20], The surfactant chains, in the region of the heads, are bent to lower electrostatic repulsion as much as possible. Figure 3 shows a cross-section of this model of micelle. 

It was mentioned previously that the narrow range of concentrations in which sudden changes are produced in the physicochemical properties in solutions of surfactants is known as critical micelle concentration. To determine the value of this parameter the change in one of these properties can be used so normally electrical conductivity, surface tension, or refraction index can be measured. Numerous cmc values have been published, most of them for surfactants that contain hydrocarbon chains of between 10 and 16 carbon atoms [1, 3, 7], The value of the cmc depends on several factors such as the length of the surfactant chain, the presence of electrolytes, temperature, and pressure [7, 14], Some of these values of cmc are shown in Table 2. 

Krafft point, Tyaff, Surfactant chain length, l Increases with increasing l... 

Zhong et al. (2003) studied the apparent solubility of trichloroethylene in aqueous solutions, where the experimental variables were surfactant type and cosolvent concentration. The surfactants used in the experiment were sodium dihexyl sulfo-succinte (MA-80), sodium dodecyl sulfate (SDS), polyoxyethylene 20 (POE 20), sorbitan monooleate (Tween 80), and a mixture of Surfonic- PE2597 and Witconol-NPIOO. Isopropanol was used as the alcohol cosolvent. Eigure 8.20 shows the results of a batch experiment studying the effects of type and concentration of surfactant on solubilization of trichloroethylene in aqueous solutions. A correlation between surfactant chain length and solubilization rate may explain this behavior. However, the solubilization rate constants decrease with surfactant concentration. Addition of the cosolvent isopropanol to MA-80 increased the solubility of isopropanol at each surfactant concentration but did not demonstrate any particular trend in solubilization rate of isopropanol for the other surfactants tested. In the case of anionic surfactants (MA-80 and SDS), the solubility and solubilization rate increase with increasing electrolyte concentration for all surfactant concentrations. 

K. Thalberg, B. Lindman, and G. Karlstrom Phase Behavior of Cationic Surfactant and Anionic Polyelectrolyte Influence of Surfactant Chain Length and Polyelectrolyte Molecular Weight. J. Phys. Chem. 95, 3370 (1991). 

K. Hayakawa and J.C.T. Kwak Surfacatant-Polyelectrolyte Interactions. 4. Surfactant Chain Length Dependence on the Binding of Alkylpyridinium Cations to Dextran Sulfate. J. Phys. Chem. 88, 1930 (1984). 

The interaction that arises from the overlapping of the surfactant chains is due to a complex interplay between enthalpic and entropic effects involving surfactant chain segments (monomer units) and solvent molecules. The enthalpic part of the... 

As for direct emulsions, the presence of excess surfactant induces depletion interaction followed by phase separation. Such a mechanism was proposed by Binks et al. [ 12] to explain the flocculation of inverse emulsion droplets in the presence of microemulsion-swollen micelles. The microscopic origin of the interaction driven by the presence of the bad solvent is more speculative. From empirical considerations, it can be deduced that surfactant chains mix more easily with alkanes than with vegetable, silicone, and some functionalized oils. The size dependence of such a mechanism, reflected by the shifts in the phase transition thresholds, is... 

Figure 2. Plot of interlayer distance versus carbon number (exclusive of head groups) of surfactant chain for mesostructured MoS-L materials.

Figure 3. Schematic representation of two different hexagonal arrangements in mesostructured inorganic / surfactant composites the hydrophobic chains are drawn as straight lines for simplicity, (a) The normal structure with a fully-connected inorganic network (dark area), (b) Inverse surfactant assemblies with single domains of the inorganic material enclosed in the centres. In the latter case the hydrophobic surfactant chains are allowed more space for their distribution, leading to a smaller d spacing. In this picture they are also interpenetrating each other.

Figure 4 Short-chain surfactant molecules are introduced into the interlayers as spacers. The interlayer distance can be modulated by varying the surfactant chain length.

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