Surfactant hydrophobic effect

The effectiveness of surfactants in overcoming the hydrophobic effect of magnesium stearate may not be a result solely of an increase in the wetting properties of the bulk phase. Compared to putting the surfactant in the dosage form, Botzolakis [71] and Wang and Chowhan [135] found that adding an equivalent amount of surfactant to the dissolution medium was not effective. The possible impact of the surfactant at... 

Although the notion of monomolecular surface layers is of fundamental importance to all phases of surface science, surfactant monolayers at the aqueous surface are so unique as virtually to constitute a special state of matter. For the many types of amphipathic molecules that meet the simple requirements for monolayer formation it is possible, using quite simple but elegant techniques over a century old, to obtain quantitative information on intermolecular forces and, furthermore, to manipulate them at will. The special driving force for self-assembly of surfactant molecules as monolayers, micelles, vesicles, or cell membranes (Fendler, 1982) when brought into contact with water is the hydrophobic effect. 

The binding constants between the anionic substrates and cationic micelles are large because of the combination of coulombic and hydrophobic effects so rate enhancements may be large even with dilute surfactant. There is binding with non-ionic and zwitterionic micelles despite the absence of coulombic attraction (Bunton et al., 1975). 

N-Alkylhydroxamic acid hydrolysis Methyl Violet + OH" Cl C12H25S03Na + H30+, CTABr + OH". An attempt made to separate electronic and hydrophobic effects on the micellar reaction Anionic and cationic micelles. Effect of surfactant structure examined Berndt el at., 1984 Malaviya and Katiyar, 1984... 

Most commercial products are mixtures because of the way they are manufactured. For instance many surfactant hydrophobes come from assorted products such as petroleiun alkylate cuts or triglyceride oils, with a molecular weight distribution that could be narrow or wide. Usually, a purification and separation of single isomeric species would be too costly and, in most cases, pointless. Moreover, the synthesis reactions involved in the surfactant manufacturing might be the intrinsic reason of the production of a mixture, such as in the case of polycondensation of ethylene oxide which results in an often wide spread ethylene oxide munber (EON) distribution. A residual content of some intermediates or by-products might also be a significant cause for mixture effects. 

Spreading of an insoluble (or temporarily insoluble) surfactant monolayer effectively produces a two-dimensional surface phase. This thin molecular layer exerts a lateral film pressure , which can be simply demonstrated by covering a water surface with a uniform layer of finely divided hydrophobic talc and placing a droplet of surfactant solution (0.003M CTAB solution) in its centre. The effect of the film pressure of the spreading surfactant is dramatic, as seen in Figures 8.8 and 8.9. 

Tanford, C., Ben Franklin Stilled the Waves, Duke University Press, Durham, NC, 1989. (A nontechnical, popular introduction to surfactant films and their applications in biology by a pioneer in the so-called hydrophobic effect.)... 

An engaging discussion of the history of Benjamin Franklin s experiment and a relatively nontechnical treatment of monolayers and bilayers of surfactants and their implications to biochemistry and biology are presented by Tanford, a pioneer of what is known as the hydrophobic effect and the biological applications of mono- and multilayers (Tanford 1989). Almost all of the material discussed in this highly readable volume is relevant to the focus of this chapter. 

This explanation for the entropy-dominated association of surfactant molecules is called the hydrophobic effect or, less precisely, hydrophobic bonding. Note that relatively little is said of any direct affinity between the associating species. It is more accurate to say that they are expelled from the water and —as far as the water is concerned —the effect is primarily entropic. The same hydrophobic effect is responsible for the adsorption behavior of amphi-pathic molecules and plays an important role in stabilizing a variety of other structures formed by surfactants in aqueous solutions. 

Surfactant aggregation in an anhydrous, nonpolar medium differs in several important respects from aggregation in water. The most apparent of these differences is that the hydrophobic effect plays no role in the formation of reverse micelles. The amphipathic species are relatively passive in aqueous micellization, being squeezed out of solution by the water. In contrast, surfactant molecules play an active role in the formation of reverse micelles, which are held together by specific interactions between head groups in the micellar core. 

Tanford, C., The Hydrophobic Effect. The Formation of Micelles and Biological Membranes, 2d ed., Wiley, New York, 1980. (Undergraduate level. A classic reference by a pioneer on the hydrophobic effect on the relevance of surfactants to biological membranes.)... 

Both adsorption from solution and micellization occur as a result of the hydrophobic effect. To test the correspondence between these two effects. Rosen assembled AG° values for adsorption at the air-water interface and for micellization of a number of linear and branched surfactants. The following is a selection of these data ... 

In previous studies, the solubilization of hydrophobic organic contaminants using surfactants has been shown to increase the rate of contaminant desorption from soil to water (Deitsch and Smith 1995 Yeom et al. 1995 Tiehm et al. 1997). A 3,000 mg/L solution of Triton X-100 (CMC = 140 mg/L) increased the rate of desorption of laboratory-contaminated TCE from a peat soil (Deitsch and Smith 1995). However, the solubilization effect was secondary compared to the surfactant s effect on the desorption rate coefficient. Yeom et al (1995) developed a model that satisfactorily predicted the extent of polycyclic aromatic hydrocarbon solubilization from a coal tar-contaminated soil. Only at high surfactant dosages did the model fail to accurately predict the ability of different surfactants to solubilize polycyclic aromatic hydrocarbons. It was hypothesized that mass-transfer limitations encountered by the polycyclic aromatic hydrocarbons in the soil caused the observed differences between the data and the model simulations. In another study (Tiehm et al. 1997), two nonionic surfactants, Arkopal N-300 and Saogenat T-300, increased the rate of polycyclic aromatic hydrocarbon desorption from a field-contaminated soil. The primary mechanism for the enhanced desorption of polycyclic aromatic hydrocarbons was attributed to surfactant solubilization of the polycyclic aromatic hydrocarbons. 

Increasing the hydrophobic part of the surfactant molecules favours micelle formation (see Table 4.3). In aqueous medium, the c.m.c. of ionic surfactants is approximately halved by the addition of each CH2 group. For non-ionic surfactants this effect is usually even more pronounced. This trend usually continues up to about the C16 member. Above the C18 member the c.m.c. tends to be approximately constant. This is probably the result of coiling of the long hydrocarbon chains in the water phase50. 

Measurement of AH0 and AS0 showed that the former is small and positive and the second is large and positive. This implies that micelle formation is entropy driven and is described in terms of the hydrophobic effect (14). Then hydrophobic chains of the surfactant monomers tend to reduce their contact with water, whereby the latter form icebergs by hydrogen bonding. This results in reduction of the entropy of the whole system. Flowever, when the monomers associate to from micelles, these icebergs tend to melt (hydrogen bonds are broken), and this results in an increase in the entropy of the whole system. 

To find out whether a hydrophobizing effect can be obtained by surfactant adsorption, photoresist layers processed with exposure doses between 50% and 120% of the threshold dose have been investigated by the captive bubble method. Their receding contact angle was first... 

The experiment described above does surely not yield the contact angles that determine the capillary forces since the surfactant layer is desorbed partially in water. It proves, however, that a noticeable hydrophobizing effect after surfactant adsorption can be found for the rather hydrophilic photoresist surfaces processed with the threshold dose. 

This observation confirms that a hydrophobizing effect is indeed found mainly for photoresists processed with the threshold dose having an initial contact angle < 60°. For photoresists exposed with lower doses, the contact angle remains constant or decreases as has been found in inverse ADSA measurements in surfactant solutions. 

The use of reversed micelles in the selective recovery and concentration of low and high molecular weight bioproducts from dilute aqueous streams appears to be a promising new avenue for innovative research and applications. To date, it has been shown that electrostatic interactions between the charged solute residues and the surfactant headgroups, as well as hydrophobic effects, can play a significant role in determining the selectivity of this process for one protein over another. Moreover, there appears to be some latitude in the selection of surfactants and cosurfactants that enables enhancements in selectivity to be made over and above those already inherent in the process. 

The Span 80 with an HLB (hydrophilic-lipophilic balance number) of 4,3, which is an oil soluble liquid, was used as surfactant. The effect of the continuous medium was investigated by employing 1,1,2,2-tetrachloroethane, toluene, or decane, which have various degrees of hydrophobicity. The amounts of the components used are listed in Table 3. At room temperature (20 °C), concen-... 

For the analysis of water-soluble polymers (such as surfactants, oligosaccharides, PEGS, lignosulfonates, polyacrylates, polysaccharides, PVA, cellulose derivatives, PEG, polyacrylic acids, polyacrylamides, hyaluronic acids, CMC, starches, gums) and for separations of oligomers and small molecules, columns that are comprised of macroporous material with hydrophilic functionalities may be used. The requirement for these columns in SEC mode is to eliminate or minimize ionic and hydrophobic effects that make aqueous SEC (otherwise known as GFC) very demanding. The interaction of analytes with neutral, ionic, and hydrophobic moieties must be suppressed. It is often necessary to modify the eluent (addition of salt) in order to avoid sample-to-sample and sample-to-column interactions that can result in poor aqueous SEC separations and low recoveries. 

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