Dependence upon surfactant

The above discussion has been based upon the assumption that aqueous and micellar pseudophases are distinct reaction media, i.e. a reactant in water does not attack a micellar-bound reactant. Recently this view has been challenged by Nome and coworkers who have shown that rate constants for attack of OH- upon hydrophobic organic chlorides, such as DDT and related compounds (10), have the predicted dependence upon [surfactant] or... 

Foam exhibits higher apparent viscosity and lower mobility within permeable media than do its separate constituents.(1-3) This lower mobility can be attained by the presence of less than 0.1% surfactant in the aqueous fluid being injected.(4) The foaming properties of surfactants and other properties relevant to surfactant performance in enhanced oil recovery (EOR) processes are dependent upon surfactant chemical structure. Alcohol ethoxylates and alcohol ethoxylate derivatives were chosen to study techniques of relating surfactant performance parameters to chemical structure. These classes of surfactants have been evaluated as mobility control agents in laboratory studies (see references 5 and 6 and references therein). One member of this class of surfactants has been used in three field trials.(7-9) These particular surfactants have well defined structures and chemical structure variables can be assigned numerical values. Commercial products can be manufactured in relatively high purity. 

The moisture-absorbing capacity of certain surfactants may enhance the activation of water-soluble compounds the capacity to do so appears to depend upon surfactant chemical class, E content, and ambient relative humidity. Hydrophilic surfactants of high E content are the most hygroscopic, and under field conditions they may act as condensation nuclei just below water vapor saturation. Under these circumstances, a water droplet may re-form over the original droplet, thus enhancing uptake of the a.i. 

The effectiveness of DAF depends upon efficient air oxidation and the attachment of bubbles to the oil, VOCs, surfactants, and other particles that are to be removed from the influent water stream. Flotation can be induced in at least three ways ... 

Public information about the specific chemical identity of the surfactants and stabilizers in use is scant(353-355) (Figure 11). Performance of foamed fluids is heavily dependent upon the size and distribution of the individual foam cells that are present, therefore the generator, testing apparatus, pressure and procedures employed are critical parts of the evaluation and the observed results. Contaminants (salts, acids, alkalies, etc) in the liquid phase also can cause drastic changes in foam performance. 

The EACN values for toluene were found to vary depending upon the nature of the surfactant molecular structure, but sulphonate systems confirm an EACN equivalence of 0. 

Provided that equilibrium is maintained between the aqueous and micellar pseudophases (designated by subscripts W and M) the overall reaction rate will be the sum of rates in water and the micelles and will therefore depend upon the distribution of reactants between each pseudophase and the appropriate rate constants in the two pseudophases. Early studies of reactivity in aqueous micelles showed the importance of substrate hydropho-bicity in determining the extent of substrate binding to micelles for example, reactions of a very hydrophilic substrate could be essentially unaffected by added surfactant, whereas large effects were observed with chemically similar, but hydrophobic substrates (Menger and Portnoy, 1967 Cordes and Gitler, 1973 Fendler and Fendler, 1975). 

Equation (1) is generally used to estimate the rate constant, kin the micellar pseudophase, but for inhibited bimolecular reactions it provides an indirect method for estimation of otherwise inaccessible rate constants in water. Oxidation of a ferrocene to the corresponding ferricinium ion by Fe3 + is speeded by anionic micelles of SDS and inhibited by cationic micelles of cetyltrimethylammonium bromide or nitrate (Bunton and Cerichelli, 1980). The variation of the rate constants with [surfactant] fits the quantitative treatment described on p. 225. Oxidation of ferrocene by ferricyanide ion in water is too fast to be easily followed kinetically, but the reaction is strongly inhibited by anionic micelles of SDS which bind ferrocene, but exclude ferricyanide ion. Thus reaction occurs essentially quantitatively in the aqueous pseudophase, and the overall rate depends upon the rate constant in water and the distribution of ferrocene between water and the micelles. It is easy therefore to calculate the rate constant in water from this micellar inhibition. 

Some examples of micellar rate enhancements of bimolecular reactions of electrophiles are shown in Table 5. Generally the surfactant was SDS with added electrophile, e.g. H30+ or a metal ion, but sulfonic acids were also used so that HaO+ was the counterion and there was no interionic competition. The maximum rate enhancements, knl, depend upon the specific conditions of the experiment, and, as predicted by the pseudophase ion-exchange model, generally decrease with increasing concentration of the electrophilic ion. In some cases the reactions were too fast for measurement... 

An impressive body of evidence supports these generalizations. This evidence has been reviewed (Romsted, 1984) and it does not seem necessary to discuss it in detail here, but some examples will be given and some exceptions to these generalizations will be mentioned. Some reactions of OH- are shown in Table 3 for both inert and reactive ion surfactants, and Table 4 gives data for reactions of other hydrophilic ions. Reactions of hydrophobic nucleophiles are shown in Table 2. For all these reactions second-order rate constants in the micellar pseudophase are compared with those in water. For some reactions we also give values of krcl, i.e. the rate constant relative to that in water. These values depend upon the reactant concentration and are included merely to provide an indication of the micellar rate effects. Other examples of micellar rate effects are given in the Appendix. 

The structure of vesicles formed from a given surfactant depends upon the extent of sonication, and over a period of time vesicles fuse and separation of phases occurs. The ease of fusion depends upon vesicular charge and the extent to which it is neutralized by added electrolyte. 

A microemulsion droplet is a multicomponent system containing oil, surfactant, cosurfactant, and probably water therefore there may be considerable variation in size and shape depending upon the overall composition. The packing constraints which dictate size and shape of normal micelles (Section 1) should be relaxed in microemulsions because of the presence of cosurfactant and oil. However, it is possible to draw analogies between the behavior of micelles and microemulsion droplets, at least in the more aqueous media. 

Depending upon the physical properties of a surfactant (component), removal from the mixed liquor is further possible through precipitation of insoluble salts and adsorption onto solids or bacterial floes, which, in turn, are subsequently withdrawn with the excess sludge [53]. In particular, intact or partly degraded low water-soluble surfactants are eliminated by this route. 

Moss et al. (1979) examined the micelle-catalyzed hydrolysis of ll- and DL-N-carbobenzyloxyalanylproline p-nitrophenyl esters. They found that the kLL/kDL ratio changes from 0.62 to 4.33 depending upon the surfactant micelles, and the catalytic efficiency was rationalized in terms of difference in the binding mode. 

For the thermally initiated (potassium persulphate) emulsion polymerisation of styrene, we have observed [73] a twofold increase in the initial polymerisation rate in the presence of ultrasound (20 kHz), the increase being dependent upon the level of surfactant employed. Several workers have suggested that possible explanations for the observed increase in rate are ... 

Since we still observed increased rates in the absence of inhibitor and in the presence of ultrasound (Fig. 5.35) we have explained the reduction or lack of an induction period in the presence of ultrasound in terms of two factors namely, a greater radical production both from enhanced initiator breakdown [74], and/or degradation of the polymer, and also from the creation of a far more stable emulsion. This latter point was confirmed visually. Depending upon the irradiation power and surfactant level employed, we were able to observe up to 40 % increase in initiator breakdown and a de-... 

Surfactant adsorption on solids from aqueous solutions plays a major role in a number of interfacial processes such as enhanced oil recovery, flotation and detergency. The adsorption mechanism in these cases is dependent upon the properties of the solid, solvent as well as the surfactant. While considerable information is available on the effect of solid properties such as surface charge and solubility, solvent properties such as pH and ionic strength (1,2,3), the role of possible structural variations of the surfactant in determining adsorption is not yet fully understood. 

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