Micelle hydrotrope

C. Tanford, in The Hydrotropic Effect Formation of Micelles and Biological Membranes, 2nd Ed., John Wiley and Sons, New York, 1980, pp. 72-78. 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

In recent studies, Friberg and co-workers (J, 2) showed that the 21 carbon dicarboxylic acid 5(6)-carboxyl-4-hexyl-2-cyclohexene-1-yl octanoic acid (C21-DA, see Figure 1) exhibited hydrotropic or solubilizing properties in the multicomponent system(s) sodium octanoate (decanoate)/n-octanol/C2i-DA aqueous disodium salt solutions. Hydrotropic action was observed in dilute solutions even at concentrations below the critical micelle concentration (CMC) of the alkanoate. Such action was also observed in concentrates containing pure nonionic and anionic surfactants and C21-DA salt. The function of the hydrotrope was to retard formation of a more ordered structure or mesophase (liquid crystalline phase). 

Self-aggregating amphiphiles can broadly be divided into hydrotropes and surfactants. The main difference between hydrotropes and surfactants lies in the fact that hydrotropes are typically not sufficiently hydrophobic to cooperatively self-aggregate and form organized structures, whereas surfactants form distinct aggregates such as micelles and vesicles above their critical aggregation concentrations. 

The most elusive of self-aggregation processes is presented by the so-called hydrotropes amphiphilic compounds with the hydrophobic moiety being typically too small to induce micelle formation. Examples of hydro tropes are butylmonogly cosulfate (BMGS),/7-toluenesulfonate (PTS), and cumenesulfonate (CS) (Scheme 1). 

Contrary to hydrotropes, micelle-forming surfactants spontaneously self-aggregate cooperatively above the critical micelle concentration (cmc) even in the absence of solubilizate. Typical examples of micelle-forming surfactants include sodium dode-cylsulfate (SDS), dodecyltrimethylammonium bromide (DTAB), cetyltrimethyl-ammonium bromide (CTAB), and heptaoxyethylene dodecyl ether (C12E7) (Scheme 2). 

Unfortunately, information about the activity of hydrotrope molecules in the concentration range of interest is not available. The only determination in existence, to our knowledge, is concerned with a more complex associated system [55], This is in contrast to the case for traditional long-chain surfactants, which have been thoroughly investigated [56-59], the results of which justified the approach to use concentrations instead of activities in the common plot of surface tension to determine critical micellization concentrations. The closest to hydrotrope molecules should be bile salts, which have been investigated [60],... 

The initial publications did not emphasize the specific action of the hydrotrope molecules in different applications. Instead they considered the structural modification of aqueous micelles by the addition of hydrotrope. Assessing the results from this point of view [61-64] the conclusion was that the reduction of electrostatic repulsion is the main cause of the modification of surfactant micelles from spherical to cylindrical shape after addition of a hydrotrope with opposite charge. 

Although this research on ionic surfactant micelles is of high quality and fundamentally relevant, the research on nonionic micellar systems has a more direct bearing on the application of hydrotropes. One essential function of hydrotropic... 

The recent investigation [71] of a nonionic system, hexaoxyethylene dodecyl ether and water, showed a hydrotrope molecule to be introduced into the micelle first at concentrations at which the hydrotrope self-associates.This increase of the minimum concentration at which the hydrotrope molecule enters the micelle from the values in ionic systems [61-66] is in all probability due to electrostatic effects. One essential result of the investigations into nonionic systems [71] is that the presence of the hydrotrope reduces the size of the micelle i.e., the radius of the curvature toward the hydrophobic region is reduced and, hence, the cloud point is enhanced in accordance with the views of Shinoda and Arai [70], Investigations of block copolymer systems [72-76] may now be interpreted in a similar manner and the coupling or linking action of a hydrotrope in a nonionic system is given a simple explanation in the form of a modified micellar structure. 

Hydrotropy When there are strong chain-chain and head-head interactions between surfactant molecules (due to long, straight chains and close-packed heads), either insoluble crystal formation (low Krafft point, p. 214) or liquid-crystal formation (Chapter 3, Section IIC) may occur. Since there is much less space available for solubilization in rigid liquid-crystal structures than in the more flexible types of micelles, the onset of crystal formation usually limits the solubilization capacity of the solution. The tendency to form crystalline structures can be reduced by the addition of certain nonsurfactant organic additives called hydrotropes. 

Miscellaneous strategies use of light, micelles, microwaves, supercritical fluids, hydrotropes, plus several combinatorial strategies... 

Hydrotropes have many features in common with micelles. The most important is the presence of a minimum hydrotrope concentration (CHC) analogous to the minimum micellar concentration (CMC) described earlier (Balasubramanian et al., 1989). The most important difference is that in hydrotropes, the dissolved solute is precipitated on dilution, whereas with surfactants dilution leads to emulsification with consequent problems of separation. Another difference is that surfactants show solubility enhancements at low concentrations, usually in the millimolar range, whereas hydrotropic solubilization occurs in the molar concentration range. Yet another difference is that, unlike micellar solubilization which is general and nonselective, hydrotropes do not solubilize all hydrotropic substances and are hence selective. This is obviously an advantage where reactant selectivity is important. 

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