Reaction Rates, Catalysis and Surfactants

Foam films are usually used as a model in the study of various physicochemical processes, such as thinning, expansion and contraction of films, formation of black spots, film rupture, molecular interactions in films. Thus, it is possible to model not only the properties of a foam but also the processes undergoing in it. These studies allow to clarify the mechanism of these processes and to derive quantitative dependences for foams, O/W type emulsions and foamed emulsions, which in fact are closely related by properties to foams. Furthermore, a number of theoretical and practical problems of colloid chemistry, molecular physics, biophysics and biochemistry can also be solved. Several physico-technical parameters, such as pressure drop, volumetric flow rate (foam rotameter) and rate of gas diffusion through the film, are based on the measurement of some of the foam film parameters. For instance, Dewar [1] has used foam films in acoustic measurements. The study of the shape and tension of foam bubble films, in particular of bubbles floating at a liquid surface, provides information that is used in designing pneumatic constructions [2], Given bellow are the most important foam properties that determine their practical application. The processes of foam flotation of suspensions, ion flotation, foam accumulation and foam separation of soluble surfactants as well as the treatment of waste waters polluted by various substances (soluble and insoluble), are based on the difference in the compositions of the initial foaming solution and the liquid phase in the foam. Due ro this difference it is possible to accelerate some reactions (foam catalysis) and to shift the chemical equilibrium of some reactions in the foam. The low heat... 

A fascinating area is micellar autocatalysis reactions in which surfactant micelles catalyse the reaction by which the surfactant itself is synthesized. Thus synthesis of dimethyldoceylamino oxide (reaction between dimethyl dodecyl amine and H2O2) benefits from this strategy. Here an aqueous phase can be used and an organic solvent can be avoided. Synthesis of mesoporous molecular sieves benefit through micellar catalysis and silicate polymerization rates have been increased by a factor 2000 in the presence of cetyltrimethyl ammonium chloride (Rathman, 1996). 

Another solution to the problem of catalyst/product separation is the biphasic catalysis. The liquid biphasic catalysis became an attractive technology for potential commercial application of enantioselective homogeneous catalysis. The most important features of such systems are related to the fact that both reaction rate and e.s. may be influenced by the number of ionic groups in water-soluble ligand or by addition of surfactants. Descriptions of water-soluble ligands and the recent results in the rapidly progressing area of biphasic enantioselective catalysis are available in recent reviews [255,256],... 

The rate constants for micelle-catalyzed reactions, when plotted against surfactant concentration, yield approximately sigmoid-shaped curves. The kinetic model commonly used quantitatively to describe the relationship of rate constant to surfactant, D, concentration assumes that micelles, D , form a noncovalent complex (4a) with substrate, S, before catalysis may take place (Menger and Portnoy, 1967 Cordes and Dunlap, 1969). An alternative model... 

It is tempting to speculate about the reasons for the observation that surfactant aggregates often do not appear to be as effective as hoped. In the author s opinion, the reasons for this could well be (1) the choice of reactions and (2) the way in which reaction rates are compared. Starting with the first point, it appears as if micellar and vesicular catalysis is often studied for reactions for which water is intrinsically a good solvent, that is, a better solvent than less polar organic solvents. By using the less polar pseudophase formed by surfactant aggregates as a base for catalysis, part of... 

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

The surfactant mass fraction in a microemulsion defines the size of the interfacial area between the water and oil. The reaction rate of organic reactions in microemulsions can be dramatically enhanced by increasing the specific interfacial area [95]. Enzyme catalysis in microemulsions is usually not influenced by the size of the interfacial area because only a small fraction of the reverse micelles are hosting a bio-molecule. Most investigations published so far were made with low enzyme concentrations resulting in a low population of enzymes per reverse micelle. 

Since phosphates and sulfates with long chain alkyl substituents form micelles at concentrations above their CMC, the hydrolysis of these esters can be subject to micellar catalysis thereby providing a simplified system in which micelle formation and structure are not alfected by the presence of a foreign solubilizate. The hydrolysis of such surfactants must be considered, however, in investigations of their effects on reaction rates. Fortunately, the rate constants for the neutral hydrolysis of esters such as sodium dodecyl sulfate are extremely slow at 90° = 296 days at pH = 8-63), and the acid-catalyzed hydrolysis of the same ester is some three orders of magnitude faster and thus is still negligible in most cases (Kurz, 1962). 

The reactions of a phosphate triester, p-nitrophenyl diphenyl phosphate with hydroxide and fluoride ions has been demonstrated to be catalyzed strongly by cationic surfactants and inhibited by NaLS and a non-ionic surfactant (Bunton and Robinson, 1969a Bunton et al., 1969, 1970). Hexadecyltrimethylammonium bromide (CTAB) increased the second-order rate constant for the reaction ofp-nitrophenyl diphenyl phosphate with hydroxide ion by a maximum factor of approximately 11 and that with fluoride ion by a maximum factor of approximately 33 at CTAB concentrations of 3 x 10 m and 2 x 10 m respectively. At higher detergent concentrations the catalysis became progressively less pronounced (Fig. 11). This behavior does not fit equation (10) (Bunton and Robinson, 1969a). However, a number of other micelle-catalyzed reactions between anions and neutral molecules have been found to... 

The effects of macromolecules other than surfactants on the rates of organic reactions have been investigated extensively (Morawetz, 1965). In many cases, substrate specificity, bifunctional catalysis, competitive inhibition, and saturation (Michaelis-Menten) kinetics have been observed, and therefore these systems also serve as models for enzyme-catalyzed reactions and, in these and other respects, resemble micellar systems. Indeed, in some macromolecular systems micelle formation is very probable or is known to occur, and in others mixed micellar systems are likely. Recent books and reviews should be consulted for a more detailed description of macromolecular systems and for their applicability as models for enzymatic catalysis and other complex interactions (Morawetz, 1965 Bruice and Benkovic, 1966 Davydova et al., 1968 Winsor, 1968 Jencks, 1969 Overberger and Salamone, 1969). 

Micellar media have been extensively used to affect rates of numerous organic and inorganic reactions.16-21 Catalysis or inhibition of solubilized species involve many kinds of interactions and may vary with the nature of the surfactant. The kinetic studies performed so far in solutions of micelle-forming surfactants can be grouped into two categories the first includes those cases in which a surfactant micelle acts only as a medium for the reaction, and the second includes reactions in which the surfactant participates directly either as a catalyst or as a substrate. This feature of surfactants determines their usefulness in practical processes. 

Most of the work concerned with micellar catalysis of nucleophilic substitution refers to reactions of the Aac2 and SN2 types and will not be reviewed here. To date only a few systems have been examined in which a micellar medium affects the partitioning of solvolytic reactions between unimolecular and bimolecular mechanisms. The effects of cationic (hexadecyltrimethylammonium bromide = CTAB) and anionic (sodium lauryl sulfate = NaLS) micelles on competitive SN1 and SN2 reactions of a-phenylallyl butanoate 193) have been investigated189. The rate of formation of the phenylallyl cation 194) is retarded by both surfactants probably as a consequence of the decreased polarity of the micellar pseudo phase. The bimolec-... 

The kinetics and electronic mechanisms of conventional chemical catalysts- are contrasted with those in enzymes. The analogy between certain attributes of surfactants and phase-transfer catalysis and enzyme active sites are made and the limitations of surface catalysts and zeolites are pointed out. The principle features that give enz3nnes their unusual rate enhancements and remarkable specificity are discussed and ways in which these can be realized in man-made catalysts are proposed. The catalytic activation of CO2 by both enzymatic and non-enzymatic means, including a detailed analysis of the electronic reaction sequence for the metalloenzyme carbonic anhydrase, is used to illustrate the above themes. 

In addition to catalysis of small molecule transformations and biocatalysis, non-functionalized LLC phases used as reaction media have also been found to accelerate polymerization reactions as well. For example, the L and Hi phases of the sodium dodecylsulfate/n-pentanol/sulfuric acid system have been found to lower the electric potential needed to electropolymerize aniline to form the conducting polymer, polyaniline [110]. In this system, it was also found that the catalytic efficiency of the L phase was superior to that of the Hi phase. In addition to this work, the Ii, Hi, Qi, and L phases of non-charged Brij surfactants (i.e., oligo(ethylene oxide)-alkyl ether surfactants) have been observed to accelerate the rate of photo-initiated radical polymerization of acrylate monomers dissolved in the hydrophobic domains [111, 112]. The extent of polymerization rate acceleration was found to depend on the geometry of the LLC phase in these systems. Collectively, this body of work on catalysis with non-functionalized LLC phases indicates that LLC phase geometry and system composition have a large influence on reaction rate. 

A further technique to overcome the mass transport limitations in biphasic catalysis is the method to work in micellar [187] or reverse micellar [188] systems, that means to enhance the surface area decisively via addition of surfactants. Ren-ken found higher reaction rates and selectivities than in non-micellar systems and could hydroformylate also olefins with a long hydrocarbon chain up to C16 (see also Section 4.5). 

With the growth of PTC, various new technologies have been developed where PTC has been combined with other methods of rate enhancement. In some cases, rate enhancements much greater than the sum of the individual effects are observed. Primary systems studied involving the use of PTC with other rate enhancement techniques include the use of metal co-catalysts, sonochemistry, microwaves, electrochemistry, microphases, photochemistry, PTC in single electron transfer (SET) reactions and free radical reactions, and PTC reactions carried out in a supercritical fluid. Applications involving the use of a co-catalyst include co-catalysis by surfactants (Dolling, 1986), alcohols and other weak acids in hydroxide transfer reactions (Dehmlow et al., 1985,1988), use of iodide (traditionally considered a catalyst poison, Hwu et... 

---