Hydrophobic groups, of surfactants

Considering a system of two immiscible phases (e.g., heptane and water), a surface-active molecule that is adsorbed at the interface between the two liquids will tend to orient itself with its hydrophilic end toward the more polar liquid (water), and its hydrophobic end toward the less polar liquid (heptane). Thus the surfactant molecules replace water and/or heptane molecules of the original interface. The interaction across the interface is then between the hydrophilic group of the surfactant and the water molecules on one side of the interface, and between the hydrophobic group of surfactant and heptane on the other side of the interface. These interactions are much stronger than the original interactions between the unlike molecules of heptane and water therefore the interfaciai... 

Surfactant Pore sizes of mesoporous sihca materials mainly depend on the hydrophobic groups of surfactants. Cationic quaternary surfactants with longer alkyl chains can yield larger pore sizes. The chain is, however, limited to C22, because surfactants with long alkyl chains are insoluble in water and lead to the formation of disordered products. As for the conventional PEO-PPO-PEO triblock copolymers, the pore... 

Fig. 4.1 Emulsified particle demonstrating hydrophilic and hydrophobic end groups of surfactant...

Table 17.2 shows values of the CMC for a series of alkylsulfate surfactants. It reflects the influence of hydrophobicity on the CMC. Within homologous groups of surfactants the CMC decreases with increasing length of the hydrocarbon tail. Regression of log CMC against the number of C-atoms yields a slope of 0.3. 

A potential limitation of surfactant-enhanced desorption is the observation that sorbed surfactant molecules can increase the sorption of hydrophobic organic contaminants (Edwards et al. 1994 Sun et al. 1995 Ko et al. 1998). Sun et al. (1995) reported that the nonionic surfactant Triton X-100 increased the sorption of p,p -DDT, 2,2 ,4,4 ,5,5 -PCB, and 1,2,4-trichlorobenzene to a soil (joc= 0.001) at concentrations below CMC. At concentrations above CMC, the distribution coefficients (Kp) of the DDT and PCB studied were reduced to levels below their respective values in pure water. However, at a surfactant concentration of five times CMC, the Kp of 1,2,4-trichlorobenzene was still a factor of three higher than Kp in pure water. Edwards et al. (1994) and Ko et al. (Ko et al. 1998) reported similar results for different groups of surfactants. 

For the dispersion of polar or ionic solids in aqueous media the hydrophobic group of the surfactant should contain polar groups, such as ether linkages, or polarizable groups, such as aromatic rings, capable of adsorbing onto the solid. 

In non-polar solvents, hydrophilic head groups interact due to dipole-dipole attractions and produce aggregates called reverse micelles. With this structure, head groups of surfactant molecules orientate towards the interior and the hydrophobic tails orientate towards the nonpolar solvents. In the absence of additives such as water, the aggregation numbers of reverse micelles are small (mostly less than 10). On the other hand, in polar solvents such as glycol,... 

The ratio of the hydrophilic and the hydrophobic groups of the surfactant molecules, that is, their hydrophile-lipophile balance (HLB), is also important in determining interfacial him curvature and consequently the structure of the ME. The HLB system has been used for the selection of surfactants to formulate MEs and accordingly the HLB of the candidate surfactant blend should match the required HLB of the oily component for a particular system furthermore a match in the lipophilic part of the surfactant used with the oily component is favorable [7],... 

In the presence of anionic surfactants, it is reasonable to expect that the hydrophobic groups of the poljrmer and of the surfactant would combine to form a mixed film at the liquid-air interface. The interactions between the cationic groups of the polymer and the anionic groups of the surfactant would further strengthen the interactions in the monolayer. These effects can be expected to increase the surface and sub-solution viscosity in lamellae and in turn enhance their stability. 

Some surfactants form reversed micelles in hydrophobic solvents containing a small amount of water. In these structures, the water is dispersed as microdroplets surrounded by a shell of polar head groups [12], When the water/surfactant ratio is less than 12, most of the water molecules are strongly hydrated to the ions or polar head groups of surfactant molecules, giving the aqueous core a viscosity and polarity very different from that of bulk water [13]. 

A wide variety of this group of surfactants is commercially available. They include some of the Tritons (alkyl phenol additives, as far as they are non-ionic). Tweens (sorbitan fatty acid ester polyoxyethylene ethers). Spans (sorbitan fatty acid esters) and alkyl polyoxyethylenes (C Ej, where n and x stand for the number of CH2- or CH3-. and CHjO-groups in the hydrophobic and hydrophilic parts of the molecule, respectively). Given our interest in the fundamentals we shall emphasize only the last-mentioned group, and only when n and x are sharply defined emd the two moieties are either linear or branched in a defined way. Unless specified otherwise, there is an OH-group at the end of the E-chain. Notwithstanding the non-ionic nature of these molecules, micelles sometimes appear to cany a (low) charge probably caused by preferential uptake of ionic species. 

When nonionic surfactant is applied to a soil-aqueous system, the surfactant can exist as dissolved monomers, sorbed molecules on the soil, or aggregated groups of molecules called micelles. Molecules of HOCs in such a system can be solubilized in surfactant micelles, dissolved in the surrounding solution, sorbed directly on the soil, or sorbed in association with sorbed surfactant. The presence of nonionic surfactant micelles in the bulk solution of the system results in the partitioning of the HOC between two bulk solution compartments, commonly referred to as pseudophases. The micellar pseudophase consists of the hydrophobic interiors of surfactant micelles, whereas the aqueous pseudophase consists mainly of dissolved surfactant monomers and water. Micelles form when the bulk solution concentration exceeds the surfactant CMC. 

Figure 2. Schematic illustration of three-dimensional network formed by binding of affinity surfactant to multi-binding site biomolecule and subsequent hydrophobic aggregation of surfactant tail groups.

Hydrophobic Bonding. Occurs when the combination of mutual attraction between hydrophobic groups of the surfactant molecules and their tendency to escape from an aqueous environment becomes large enough to permit them to adsorb onto the solid adsorbent by aggregating their chains (Wakamatsu, 1968 Dick, 1971 Giles, 1974, Gao, 1987). 

Since the cross-sectional area of an aliphatic chain oriented perpendicular to the interface is about 20 A2 and that of a benzene ring oriented in the same fashion is about 25 A2, it is apparent that the hydrophobic chains of surfactants adsorbed at the aqueous solution-air or aqueous solution hydrocarbon interfaces are generally not in the close-packed arrangement normal to the interface at saturation adsorption. On the other hand, since the cross-sectional area of a —CH2 group lying flat in the interface is about 7 A2, the chains in the usual ionic surfactant with a hydrophilic group at one end of the molecule are not lying flat in the interface either, but are somewhat tilted with respect to the interface. 

Change in the length of the hydrophobic group of straight-chain ionic surfactants beyond 10 carbon atoms appears to have almost no effect on the effectiveness of adsorption at the aqueous solution-heptane interface and very little effect on the effectiveness at the aqueous solution-air interface. 

A theory of micellar structure, based upon the geometry of various micellar shapes and the space occupied by the hydrophilic and hydrophobic groups of the surfactant molecules, has been developed by Israelachvili, Mitchell, and Ninham (1976, 1977) and Mitchell and Ninham (1981). The volume Vh occupied by the hydrophobic groups in the micellar core, the length of the hydrophobic group in the core /,., and the cross-sectional area ao occupied by the hydrophilic group at the micelle-solution interface are used to calculate a packing parameter, Vn/hao, which determines the shape of the micelle. 

Depression of the CMC appears to be greater for straight-chain compounds than for branched ones and increases with chain length to a maximum when the length of the hydrophobic group of the additive approximates that of the surfactant. An explanation for these observations (Schick, 1957) is that those molecules that are most effective at reducing the CMC are solubilized in the outer portion of the micelle core and are there under lateral pressure tending to force them into the inner... 

Saturated aliphatic and alicyclic hydrocarbons and other types of molecules that are not polarized or not easily polarizable are solubilized in aqueous medium in the inner core of the micelle between the ends of the hydrophobic groups of the surfactant molecules. Their UV and NMR spectra indicate a completely nonpolar environment on solubilization. 

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