Microemulsions temperature considerations

Oh et al. [16] have demonstrated that a microemulsion based on a nonionic surfactant is an efficient reaction system for the synthesis of decyl sulfonate from decyl bromide and sodium sulfite (Scheme 1 of Fig. 2). Whereas at room temperature almost no reaction occurred in a two-phase system without surfactant added, the reaction proceeded smoothly in a micro emulsion. A range of microemulsions was tested with the oil-to-water ratio varying between 9 1 and 1 1 and with approximately constant surfactant concentration. NMR self-diffusion measurements showed that the 9 1 ratio gave a water-in-oil microemulsion and the 1 1 ratio a bicontinuous structure. No substantial difference in reaction rate could be seen between the different types of micro emulsions, indicating that the curvature of the oil-water interface was not decisive for the reaction kinetics. More recent studies on the kinetics of hydrolysis reactions in different types of microemulsions showed a considerable dependence of the reaction rate on the oil-water curvature of the micro emulsion, however [17]. This was interpreted as being due to differences in hydrolysis mechanisms for different types of microemulsions. 

Schomacker compared the use of nonionic microemulsions with phase transfer catalysis for several different types of organic reactions and concluded that the former was more laborious since the pseudo-ternary phase diagram of the system had to be determined and the reaction temperature needed to be carefully monitored [13,29]. The main advantage of the microemulsion route for industrial use is related to the ecotoxicity of the effluent. Whereas nonionic surfactants are considered relatively harmless, quaternary ammonium compounds exhibit considerable fish toxicity. 

The method developed originally for microemulsion formulation (Section II above) has been adapted (Salager, 1983, 2000) to macroemulsion formation. In this method, the value of the left-hand side of equation 8.10 or 8.11 is called the hydrophilic-lipophilic deviation (HLD). When the value equals zero, as in Section II, a microemulsion is formed when the value is positive, a W/O macroemulsion is preferentially formed when it is negative, an O/W macroemulsion is preferentially formed. The HLD is similar in nature to the Winsor R ratio (equation 5.2) in that when the HLD is larger than, smaller than, or equal to 0, R is larger than, smaller than, or equal to 1. The value of the HLD method is that, on a qualitative basis, it takes into consideration the other components of the system (salinity, cosurfactant, alkane chain length, temperature, and hydrophilic and hydrophobic groups of the surfactant). On the other hand, on a quantitative basis, it requires the experimental evaluation of a number of empirical constants. 

The phenomenon of microemulsification is mainly governed by factors such as (1) nature and concentration of the oil, surfactant, co-surfactant and aqueous phase, (2) oil/surfactant and surfactant/co-surfactant ratio, (3) temperature, (4) pH of the environment and (5) physicochemical properties of the API such as hydrophilicity/lipophilicity, plformulating microemulsions. From a pharmaceutical perspective, one of the most important factors to be considered is acceptability of the oil, surfactant and co-surfactant for the desired route of administration. This factor is very important while developing micro emulsions for parenteral and ocular delivery as there is only limited number of excipients which are approved for the parenteral and ocular route. In Chapter 3 of this book a more general overview of formulating microemulsions is given and formulation considerations with respect to the components of microemulsions used in pharmaceutical applications are discussed below. 

If large quantities are used for technical processes, e.g. for cleaning, the recovery and reuse of the microemulsion or at least of a considerable amount of the most expensive components is desired. Therefore, strategies are needed to separate contaminants from the organic microemulsion components. Separation is usually more complicated than from ordinary solvents and often requires several steps [39, 40]. In particular, the separation of waste materials from the surfactants is usually very difficult or often even impossible. The temperature-dependent phase behaviour of bicontinuous microemulsions, however, can sometimes be beneficially used for separation [41]. Easy separation, at least from the unpolar solvent, can be achieved from microemulsions with supercritical liquids [42]. 

Mixtures of anionic and nonionic surfactants were proposed to provide temperature-insensitive systems [37], a suggestion that has considerable practical interest not only for microemulsion systems but also in emulsion polymerization and enhanced oil recovery It was recently shown that since both the anionic and nonionic surfactants can be selected, this double degree of freedom can be used to attain both temperature insensitivity and mixture composition insensitivity so that the formulation is a particularly robust one as far as the applications are concerned [39]. 

In order for microemulsion-based materials synthesis to be feasible, surfactant/oil/water formulations that give stable microemulsions must be identified. Phase diagrams already available in the literature [122-124] provide a useful starting point. Frequently, however, these published diagrams do not extend to conditions directly relevant to materials synthesis, e.g., in terms of the specific metal salt, base, acid, and temperature. Of important consideration, therefore, are investigations into the effects of the reactants... 

Most likely there will be a considerable mismatch between academic research with its well-defined, highly optimized systems and the need for chemical systems in process industry. For example, the highly optimized bicontinuous microemulsion systems seem to be far too sensitive to contamination, process settings, temperature, and retention effects to become an industrial success. Hence in the future one can expect the industrial benefit from these systems to include solubilized W/O or O/W systems. These systems exist, as we know, over rather broad compositional intervals and also for large temperature intervals. 

At temperatures significantly below the PIT (e.g., in the Winsor I phase region), the microemulsion can solubilize much less hydrocarbon (see Figure 4.25). The rate of solubilization is also slower (Miller and Raney, 1993) and interfacial tension is higher. As a result, soil removal is considerably lower, as may be seen from Figure 4.32. Well below the cloud point temperature, no... 

If we take also into consideration the work of Shinoda and others (9,10) on the composition of emulsions and microemulsions, we see a drastic change of the composition at a given temperature, the phase inversion temperature PIT. The problems connected with the thermostatting of the measurement apparatus may then easily be imagined. 

The applications of ILs are broadening with the advent of IL-based microemulsions. The IL-based nonaqueous microemulsions, particularly the IL/O ternary systems, have been considerably studied and subsequently used in industries. This class of mieroemulsion systems offers several advantages over the corresponding water-in-oil systems, due to the tunable size of the polar droplets, their wide range of temperature stability, and the ease of preparation for specific tasks. 

Crooks, G.E., Rees, G.D., Robinson, B.H., Svensson, M., Stephenson, G.R. 1995. Comparison of hydrolysis and esterification behavior of Humicola lanuginosa and Rhizomucor miehei lipases in AOT-stabilized water-in-oil microemulsions 11. Effect of temperature on reaction kinetics and general considerations of stability and prodnctivity. Biotechnol. Bioeng. 48, 190-196. 

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