Micellar solubilisation

Micelles forming above the c.m.c. incorporate hydrophobic molecules in addition to those dissolved in the aqueous phase, which results in apparently increased aqueous concentrations. It has to be noted, however, that a micelle-solubilised chemical is not truly water-dissolved, and, as a consequence, is differently bioavailable than a water-dissolved chemical. The bioavailability of hydrophobic organic compounds was, for instance, reduced by the addition of surfactant micelles when no excess separate phase compound was present and water-dissolved molecules became solubilised by the micelles [69], In these experiments, bacterial uptake rates were a function of the truly water-dissolved substrate concentration. It seems therefore that micellar solubilisation increases bioavailability only when it transfers additional separate phase substrate into the aqueous phase, e.g. by increasing the rates of desorption or dissolution, and when micelle-solubilised substrate is efficiently transferred to the microorganisms. Theoretically, this transfer can occur exclusively via the water phase, involving release of substrate molecules from micelles, molecular diffusion through the aqueous phase and microbial uptake of water-dissolved molecules. This was obviously the case, when bacterial uptake rates of naphthalene and phenanthrene responded directly to micelle-mediated lowered truly water-dissolved concentrations of these chemicals [69]. These authors concluded from their experiments that micellar naphthalene and phenanthrene had to leave the micellar phase and diffuse through the water phase to become... 

Solubilisation by surface-active agents is discussed in Chapter 6. Alternatives to micellar solubilisation (or solubilisation in vesicles) include the use of the cyclodextrin family. When the first edition of this book was published in 1981 (and a diagram of a cyclo-dextrin-dmg complex was used to adorn the cover), the use of cyclodextrins was in its infancy. Attention was then focused around a-, P- and yavailable commercially for pharmaceutical use. Ten per cent of this cyclodextrin can enhance the solubility of betamethasone 118 times, of diazepam 21 times and of ibuprofen 55 times. 

Table 6.11 Micellar solubilisation parameters for steroids in r>alkyl polyoxyethylene surfactants C E (where n = alkyl (C) chain length and m = polyoxyethylene (E) chain length) at 25°C°...

In order to use supersolubilisation for DNAPL extraction, the reduction of interfacial tension must be well controlled. The critical level of interfacial tension is dependent on size and heterogeneity of the pore space. For example, a value of 4 mN m 1 was found for soil from a contaminated site [47]. Since supersolubilising systems exhibit lower interfacial tension, they cannot be directly applied for contaminant extraction. Therefore, a salinity gradient was used for column experiments in preparation for a field test [47,63]. When the salinity was increased in two steps from 0 to 0.6 wt.% and 1 wt.% CaC, a mixture of a sul-phated alkyl propoxylate (Isalchem 145-4P0-S04) and a twin-head aromatic sulphonate (Dowfax 8390) exhibited the usual micellar solubilisation, supersolubilisation and formation of bicontinuous micro emulsions with perchloroethylene. Applying this three-step gradient to soil columns contaminated with PCE yielded high extraction values and no mobilisation of DNAPL [47]. 

The presence of physiological surface-active agents in the stomach and small intestine will influence the solubility and the dissolution of sparingly soluble drugs by improved wetting of solid particle surface areas and by micellar solubilisation. This has been reviewed in more detail by Gibaldi and Feldman (1970) and Charman et al. (1997). 

Another advantage demonstrated is that all those semiquinones, which show some stability over a pH zone, can be generated in situ cleanly by a combination of micellar solubilisation and photolytic irradiation [51,52]. 

Chandy, T. and Sharma C. P. 1990. Chitosan-as a biomaterial. Biomater. Artif. Cells Artif. Org. 18 1-24. Chen, X., Ding, S., Qu, G., and Zhang, C. 2008. Synthesis of novel chitosan derivatives for micellar solubilisation of cyclosporine A. J. Bioact. Compat. Polym. 23 563. 

The problem of obtaining zero interfacial tension is also one of formulation, i.e., choosing one or more surfactant molecules with the right solubilities relative to the two phases. These must be such as to induce a high degree of adsorption at the interface (rather than a micellar solubilisation in one of the solvents), by creating a high film pressure. Box III, entitled Enhanced Petroleum Recovery, sums up the industrial research which led to discovery of microemulsions. 

Stilbs, P. 1986. A comparative study of micellar solubilisation for combination of surfactants and solubilisates using Fourier transform pulsed-gradient spin-echo NMR multicomponent self-diffusion technique. Journal of Colloid and Interface Science, 94(2), 463-469. 

Laguerre, M., V. Hugouvieux, N. Cavusoglu, F. Aubert, A. Lafuma, H. Fulcrand, and C. Poncet-Legrand. 2014. Probing the micellar solubilisation and inter-micellar exchange of polyphenols using the DPPH free radical. Food Chemistry 149 114-120. 

As Figure 1.18 shows, the solubilisation rate and the micellar solubilisation capacity increase as the chain length of the hydrocarbon decreases, and both are... 

Solubilisation is usually treated in terms of the pseudophase model, in which the bulk aqueous phase is regarded as one phase and tire micellar pseudophase as another. This allows the affinity of the solubilisate for the micelle to be quantified by a partition coefficient P. Different definitions of P can be found in the literature, differing in their description of the micellar phase. Frequently P is... 

Calculations usirig this value afford a partition coefficient for 5.2 of 96 and a micellar second-order rate constant of 0.21 M" s" . This partition coefficient is higher than the corresponding values for SDS micelles and CTAB micelles given in Table 5.2. This trend is in agreement with literature data, that indicate that Cu(DS)2 micelles are able to solubilize 1.5 times as much benzene as SDS micelles . Most likely this enhanced solubilisation is a result of the higher counterion binding of Cu(DS)2... 

In retrospect, this study has demonstrated the limitations of two commonly accepted methods of analysing solubilisation and micellar catalysis, respectively. It has become clear that solubilisate ririg-current induced shifts need to be interpreted with due caution. These data indicate a proximity of solubilisate and parts of the surfactant and, strictly, do not specify the location within the micelle where the encounter takes place. Also the use of the pseudophase model for bimolecular reactions requires precaution. When distribution of the reactants over the micelle is not comparable, erroneous results are likely to be obtained... 

For fluorescence PAH determination in tap water acid-induced cloud point extraction was used. This kind of extraction based on the phase separation into two isotropic liquid phases a concentrated phase containing most of the surfactant (surfactant-rich phase), where the solubilised solutes are exttacted, and an aqueous phase containing a surfactant concenttation closes to the critical micellar concentration. 

M. Baviere and T. Rouaud. Solubilization of hydrocarbons in micellar solutions Influence of structure and molecular weight (solubilisation des hydrocarbures dans les solutions micellaires influence de la structure et de la masse moleculaire). Rev Inst Franc Petrol, 45(5) 605-620, September-October 1990. 

A new cholesterol flow injection analysis biosensor has also been described as an application of the H2O2 ECL sensor56. In that work, the luminol electrochemiluminescence, previously studied in aqueous media, was implemented in Veronal buffer added of 0.3% triton X-100 (v/v), 0.3% PEG and 0.4% cholate to enable the solubilisation of the cholesterol and then its efficient oxidation catalyzed by the immobilized cholesterol oxidase. The ECL reaction occurred thus in a micellar medium and the performances of the H2O2 ECL sensor were investigated. 

Edwards, D. A., Luthy, R. G. and Liu, Z. (1991). Solubilisation of polycyclic aromatic hydrocarbons in micellar nonionic surfactant solutions, Environ. Sci. Technol., 25, 127-133. 

The levels of PAF synthesis and release are also modulated by levels of extracellular albumin. In the absence of albumin, neutrophils (stimulated with fMet-Leu-Phe) synthesise only low levels of PAF within 1-2 min of stimulation. In the presence of 0.25% albumin, PAF synthesis is increased, and up to half of this may be released with 5% albumin, rates of synthesis and release are increased further and sustained over a 30-min period. Newly-synthesised PAF is reincorporated by neutrophils into membrane lipids and is therefore poorly soluble in aqueous media. Thus, extracellularly added albumin will bind to cell-associated PAF and effectively solubilise it at concentrations below its critical micellar concentration (CMC). This will effectively enhance the PAF release rate, which will decrease the concentration of cell-associated PAF thus, the rate of biosynthesis will be sustained. 

Solubilisation is important in the formulation of pharmaceutical drugs containing water insoluble ingredients, in detergency (removal of oily soil), in emulsion polymerisation and in micellar catalysis. 

In order to obtain a thermodynamically stable micro emulsion, the analysis of the phase behaviour is indispensable. With bovine serum albumin instead of an enzyme (because of the cost of the bio-catalyst) phase behaviour studies are shown in Fig. 2. A strong shift of the phase boundary is observed, yielding a system that solubilises much less water in the presence of the protein. In case of hydrophobic enzymes, the addition of dry lyophilised protein to an already prepared reverse micellar solution can also work well [53]. 

In a previous publication ( ), results were presented on the micellar properties of binary mixtures of surfactant solutions consisting of alkyldimethylamine oxide (C12 to Cig alkyl chains) and sodium dodecyl sulfate. It was reported that upon mixing, striking alteration in physical properties was observed, most notably in the viscosity, surface tension, and bulk pH values. These changes were attributed to 1) formation of elongated structures, 2) protonation of amine oxide molecules, and 3) adsorption of hydronium ions on the mixed micelle surface. In addition, possible solubilisation of a less soluble 1 1 complex, form between the protonated amine oxide and the long chain sulfate was also considered. 

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