Structure of the Surfactant

Bivalent metal alkyl sulfates appear to show greater solubilizing power than the corresponding sodium salts for hydrocarbons, probably reflecting the greater micellar aggregation numbers, asymmetry, and volumes of the former compared to the latter (Satake, 1963). 

In aqueous solutions of POE nonionics, the extent of solubilization of aliphatic hydrocarbons at a given temperature appears to increase as the length of the 

Polymeric quaternary ammonium surfactants, made from w-dodecyl bromide and poly(2-vinylpyridine), are better solubilizers for aliphatic and aromatic hydrocarbons than AMaurylpyridinium chloride, with the extent of solubilization increasing as the alkyl content of the polymeric quaternary is increased (Strauss, 1951 Inoue, 1964). 

Consistent with the above, in aqueous solutions of two different surfactants that interact strongly with each other (Chapter 11, Table 11-1), mixed micelle formation is unfavorable for the solubilization of polar solubilizates that are solubilized in the palisade layer and favorable for the solubilization of nonpolar ones that are solubilized in the micellar inner core. This is due to the reduction of a() and the sphere-to-cylindrical micelle transition and the increase in aggregation number resulting from the interaction (Treiner, 1990). 

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Another important interaction that needs to be considered is the hydrophobic interaction. This can be most easily thought of in terms of two immiscible liquids such as oil and water being induced to mix by adding surfactants, to form (micro) emulsions. The exact structure of the phase formed depends heavily on the relative compositions of the various phases and the structure of the surfactant (see Figure 6.4). 

These assumptions have been expanded upon (Shah and Capps, 1968 Lucassen-Reynders, 1973 Rakshit and Zografi, 1980), especially in regard to the application of the ideal mixing relationship in gaseous films (Pagano and Gershfeld, 1972). It has been pointed out that water may contribute to the energetics of film compression if the molecular structures of the surfactants are sufficiently different (Lucassen-Reynders, 1973). It must be noted that this treatment assumes that the compression process is reversible and the monolayer is truly stable thermodynamically. It must therefore be applied with considerable reservation in view of the hysteresis that is often found for II j A isotherms. 

Lundquist and the Stenhagens concentrated their efforts on the physical aspects of monolayer chemistry and did not elaborate then-work much in the direction of structural variation of the surfactant molecules. Their results show clearly, however, that the response of chiral monolayers to changes in surface pressure and temperature is sharply dependent on both the molecular structure of the surfactant and the optical purity of the sample. The Stenhagens were keenly aware of the possible application of the monolayer technique to stereochemical and other structural problems (72) however, they failed to exploit the full potential suggested by their initial results and, instead, pursued the field of mass spectrometry, to which they made substantial contributions. 

The formation of mixed micelles in surfactant solutions which contain two or more surfactant components can be significantly affected by the structures of the surfactants involved. The observed critical micelle concentration (cmc) is often significantly lower than would be expected based on the erne s of the pure surfactants. This clearly demonstrates that interactions between different surfactant components in the mixed micelles are taking place. 

Adsorption of a surfactant on solids is dependent, among other things, on the structure of both the hydro-phobic and hydrophilic portions of it. There are a number of mechanisms proposed for surfactant adsorption and an understanding of the effects of the structure of the surfactant can help in elucidating the role of these mechanisms. In this study, the effect on adsorption on alumina of some structure variations of sulfonates (chain length and the branching and the presence of ethyoxyl, phenyl, disulfonate and dialkyl groups) is examined above and below CMC as a function of surfactant concentration, pH and salinity. Co-operative action between an ionic alkyl sulfonate and a nonionic ethoxylated alcohol is also studied. 

The important role of the structure of the surfactants in determining adsorption is evident. Some of the surfactants discussed above can produce low interfacial tension and some others have excellent salt tolerance. A knowledge of the structure of such surfactants in adsorption can be helpful in developing surfactants that will meet different requirements simultaneously for special applications such as in enhanced oil recovery. 

As discussed previously, the biodegradation pathways using non-selected bacterial inocula for nonionic ethoxylates may be divided into two distinct areas, each dependent on the structure of the surfactant. 

The molecular structure of the surfactant influences the form of the aggregate, and there are some geometrical empirical rules (Israelachvili et al, 1977, Israelachvili, 1992) illustrated in Figure 9.2, based on the geometrical parameters of the surfactant molecule. In particular the volume V occupied by the surfactant, the head area... 

In general, it may be said that the variety of architecture built by surfactants and lipids in particular is extremely rich, that small variations in chemical structure of the surfactant may bring about significant changes in the supramolecular structure of the... 

Discuss the observed differences in permeability between (a) the two alcohols at the same film pressure and (b) the two 18-carbon surfactants at different pressures. In your comments include comparisons of the molecular structure of the surfactants and the efficiencies of these monolayers in retarding evaporation. 

One can extend the above analysis to come up with the values of the packing parameter for which different shapes of aggregates are favored (Israelachvili 1991). These are summarized in Table 8.2. The results shown also serve as rules of thumb for what one can expect as one changes the chemical conditions of the solvent or the structure of the surfactant or for controlling the shape or aggregation number. For example,... 

In this context it is instructive to ruminate on the structure of the surfactant phase. A representative composition of the phase would be 10% emulsifier and equal amounts of water and hydrocarbon. The conclusions giving a layer structure (31, 32, 33) appear to be a reasonable basis for discussing the energy conditions implied in the structure. If an area per molecule of 10-18 m2 is considered reasonable (39), the water and oil layers are approximately 1.2 X 10"8 m thick. Low angle x-ray determinations have shown that the structure does not consist of regular layers with constant spacings a structure which would accommodate the factors which determine stability would be difficult to envision. Further, since the phase is an isotropic liquid, a regularly layered structure is excluded. 

Figure 8. The possibility of a non-regular structure of the surfactant phase containing both oil ana water dispersed and continuous is attractive but does not appear probable...

A micelle is a dynamic structure. Surfactants leave the micelle and go into solution while other surfactants enter the micelle from solution. The timescales involved depend critically on the specific structure of the surfactant, in particular on the length of the hydrocarbon chain. For example, the residence time of a single dodecylsulfate (CH3(CH2)h0S03 ) in a SDS micelle at 25° C is 6 /xs [525], If we reduce the chain length by two methylene units to decyl sulfate (CH3(CH2)g0S03 ) the residence time decreases to roughly 0.5 /us. Tetradecyl sulfate (CH3(CH2)i30S03 ), which has two methylene units more than dodecylsulfate, typically remains 83 /its in a micelle. 

Table I summarizes properties and gives structures of the surfactants discussed in this chapter. Aerosol OT (AOT, 100% solid) was purchased from Fisher Scientific. TWEEN 80 (80.0% active) was purchased from Uniquema (Wilmington, DE). Tetrachloroethylene (PCE, 99% liquid), dodecane (99% liquid), and hexadecane (99% liquid) were purchased from Aldrich Co. (Milwaukee, WI). All chemicals were used as received.

Although we have presented here only two examples of pressure tuning vibrational spectroscopic studies with aqueous surfactants, we hope that they are sufficient to demonstrate the uniqueness of pressure as a physical parameter in the investigation of the structural and dynamic properties of aqueous surfactants. For many systems, the vibrational spectra at atmospheric pressure are very similar. Yet, the pressure tuning of these spectra will be able to provide additional information about the structure of the surfactant molecules and about their aggregation in water. 

Mixtures of low molecular weight silicon-based surfactants and cosurfactants have been used to prepare a self-dispersing microemulsion of silicone agents applied to building materials to impart water repellency [54, 55]. The structure of the surfactants used was not disclosed but they are described as being themselves reactive so that they bind to the surfaces of the building materials and become part of the water-repellancy treatment. 

Finally, one word about the lattice theories of microemulsions [30 36]. In these models the space is divided into cells in which either water or oil can be found. This reduces the problepi to a kind of lattice gas, for which there is a rich literature in statistical mechanics that could be extended to microemulsions. A predictive treatment of both droplet and bicontinuous microemulsions was developed recently by Nagarajan and Ruckenstein [37], which, in contrast to the previous theoretical approaches, takes into account the molecular structures of the surfactant, cosurfactant, and hydrocarbon molecules. The treatment is similar to that employed by Nagarajan and Ruckenstein for solubilization [38]. 

The authors have studied the effects of some surfactants on the rate of interfacial transfer this work, which has not been published, has shown that the rate may be increased or decreased. The structure of the surfactant is important, and the effects apply both above and below the critical micelle concentration of the surfactant. The effect in certain cases was to impede transfer totally, which suggests that much more work is required to build up an understanding of the properties of biological systems, and of the effects of surfactants on them. Surfactants are commonly present in pesticide formulations. 

Figure 14.2 summarizes the structures of the surfactants employed in this report. These are all anionic surfactants with phosphoric or sulfosuccinate groups and different types of hydrophobic chains. 

In aqueous solutions the micellar assembly structure allows sparingly soluble or water-insoluble chemical species to be solubilized, because they can associate and bind to the micelles. The interaction between surfactant and analyte can be electrostatic, hydrophobic, or a combination of both [76]. The solubilization site varies with the nature of the solubilized species and surfactant [77]. Micelles of nonionic surfactants demonstrate the greatest ability for solubilization of a wide group of various compounds for example, it is possible to solubilize hydrocarbons or metal complexes in aqueous solutions or polar compounds in nonpolar organic solutions. As the temperature of an aqueous nonionic surfactant solution is increased, the solution turns cloudy and phase separation occurs to give a surfactant-rich phase (SRP) of small volume containing the analyte trapped in micelle structures and a bulk diluted aqueous phase. The temperature at which phase separation occurs is known as the cloud point. Both CMC and cloud point depend on the structure of the surfactant and the presence of additives. Table 6.10 gives the values of CMC and cloud point for the surfactants most frequently applied in the CPE process. 

It should be noted that high concentrations of ionic species can alter the phase stability of microemulsions based upon ionic surfactant systems. Nonionic surfactant systems are much less susceptible to this effect. The curvature of the interfacial film of the microemulsion droplet is determined by a balance between the electrostatic interactions of the head groups and repulsive interactions of the surfactant tail group. Addition of ionic solutes can upset this delicate balance and induce phase separation. By changing the structure of the surfactant or through the addition of cosurfactants one can restore this balance and thus allow the dissolution of high concentrations of ionic species. 

We now know that emulsion polymerization is not just another polymer synthesis method and that the complexity of the interactions, whether chemical or physical, must he considered before any control is possiUe over the outcome of the reaction. The creation and nucleation of particles, for example, is not necessarily and simply explained by the presence or or absence of micelles, but needs the understanding of interactions of all the ingredients present. Variables such as hydrophilic and hydrophobic associations or repulsions, polarity of the monomers, chemical structure of the surfactants, have to lx taken into account. 

The chemical structure of the surfactant is an important factor in detersive effectiveness. When relating detersive power to chemical constitution, within limited series, certain regularities can be observed, but few if any general principles apply to the... 

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