Partitioning, cosurfactants

Despite this donble complexity, it is possible to use enzymes in surfactant solutions to get a deeper insight in their structure and dynamics. In this chapter, we showed that enzymes can yield useful information on surfactant hydration, interfacial film rigidity, and partitioning of cosurfactants. Enzymes are also useful as an independent check for pH in reverse microemulsions. We could also show that for some of the properties of microemulsions such as cosurfactant partition coefficients, semi-qnantitative results can be obtained. 

MEEKC is a CE mode similar to MEKC, based on the partitioning of compounds between an aqueous and a microemulsion phase. The buffer solution consists of an aqueous solution containing nanometer-sized oil droplets as a pseudo-stationary phase. The most widely used microemulsion is made up of heptane as a water-immiscible solvent, SDS as a surfactant and 1-butanol as a cosurfactant. Surfactants and cosurfactants act as stabilizers at the surface of the droplet. 

In the second part of the paper, it will be shown that the partition of the alcohol between the phases can explain the deviations from the ideal dilution law for a water/ oil/surfactant/cosurfactant lyotropic lamellar liquid crystal. 

A pentanol/potasslum oleate ratio of 15 that Is typical of the inverse micellar solution gives the corresponding value 1.02. Formally the two values are straddling the value v/a l = 1 in the correct directions, but it is obvious that they are extremely similar and the application of the zeroth order approachto these systems must be viewed with caution. The pronounced influence of a partition of cosurfactants between the Interface and the organic bulk is evident. 

The alcohol cosolvents or cosurfactants may partition between aqueous and oil phases in different proportions than the primary surfactant, and therefore, grouping these components in the surfactant pseudocomponent is inappropriate. Chromatographic separation of the components may occur during flow in the reservoir, and unwanted phases may form. 

A significant amount of work has demonstrated the feasibility and the interest of reversed micelles for the separation of proteins and for the enhancement or inhibition of specific reactions. The number of micellar systems presently available and studied in the presence of proteins is still limited. An effort should be made to increase the number of surfactants used as well as the set of proteins assayed and to characterize the molecular mechanism of solubilization and the microstructure of the laden organic phases in various systems, since they determine the efficiency and selectivity of the separation and are essential to understand the phenomena of bio-activity loss or preservation. As the features of extraction depend on many parameters, particular attention should be paid to controlling all of them in each phase. Simplified thermodynamic models begin to be developed for the representation of partition of simple ions and proteins between aqueous and micellar phases. Relevant experiments and more complete data sets on distribution of salts, cosurfactants, should promote further developments in modelling in relation with current investigations on electrolytes, polymers and proteins. This work could be connected with distribution studies achieved in related areas as microemulsions for oil recovery or supercritical extraction (74). In addition, the contribution of physico-chemical experiments should be taken into account to evaluate the size and structure of the micelles. 

Four different emulsifier selection methods can be applied to the formulation of microemulsions (i) the hydrophilic-lipophilic-balance (HLB) system (ii) the phase-inversion temperature (PIT) method (iii) the cohesive energy ratio (CER) concept and (iv) partitioning of the cosurfactant between the oil and water phases. The first three methods are essentially the same as those used for the selection of emulsifiers for macroemulsions. However, with microemulsions attempts should be made to match the chemical type of the emulsifier with that of the oil. A summary of these various methods is given below. 

Measurement of the partition of the cosurfactant between the oil and the interface is not easy. A simple procedure to select the most efficient cosurfactant is to determine the oil/water interfacial tension as a function of cosurfactant concentration. In this case, the lower the percentage of cosurfactant required to reduce /q/w rn the better is the candidate. 

Surface active additives (cosurfactants, demulsifiers, etc.), such as fatty alcohols in the case of ionic surfactants, may affect the emulsifier partitioning between the phases and its adsorption, thereby changing the Gibbs elasticity and the interfacial tension. The surface-active additive may also change the surface charge (mainly by increasing the... 

Recently, MEEKC has been applied for the determination of catechins. In a similar fashion to MEKC, MEEKC uses a pseudo-stationary phase for separation of analytes. Instead of having micelles, surfactant-stabilized oil droplets in microemulsion solution serve as the pseudo-stationary phase for the partitioning of analytes. The formation of a microemulsion involves an organic solvent, SDS as a surfactant, and an alcohol as a cosurfactant. Pomponio et al. first developed a MEEKC method for the analysis of six catechins (C, EC, GC, ECG, EGC, and EGCG). This involved the use of heptane (1.36%, w/v), SDS (2.31%, w/v), and 1-butanol (9.72%, w/v), which serve, respectively, for the three functions mentioned above. The medium was a 50-mM phosphate buffer (86.61%, w/v) at pH 2.5. Electro-osmotic force (EOF) was almost absent, and analytes, due to their interactions with SDS-coated oil droplets. 

Retention in Porous Media. Anionic surfactants can be lost in porous media in a number of ways adsorption at the solid—liquid interface, adsorption at the gas—liquid interface, precipitation or phase-separation due to incompatibility of the surfactant and the reservoir brine (especially divalent ions), partitioning or solubilization of the surfactant into the oil phase, and emulsification of the aqueous phase (containing surfactant) into the oil. The adsorption of surfactant on reservoir rock has a major effect on foam propagation and is described in detail in Chapter 7 by Mannhardt and Novosad. Fortunately, adsorption in porous media tends to be, in general, less important at elevated temperatures 10, 11). The presence of ionic materials, however, lowers the solubility of the surfactant in the aqueous phase and tends to increase adsorption. The ability of cosurfactants to reduce the adsorption on reservoir materials by lowering the critical micelle concentration (CMC), and thus the monomer concentration, has been demonstrated (72,13). 

Nonpolar solutes are located in the nonpolar organic phase, and polar solutes are found in the aqueous phase of the microemulsion. It should be noted that the interphase surfactant/cosurfactant can represent a significant part of the total mass or volume of the microemulsion. The (]) , value in a microemulsion system includes the oil, the surfactant and the cosurfactant, thus, <]) , values as high as 90% are possible in these systems. Solutes with intermediate polarity can partition between the two phases and/or be located in the interphase. Solute location in physicochemical structures stabilized by surfactants is the subject of continuous research [39-42]. 

Polymerisation of vinyl toluene in quaternary microemulsions containing cetyltrimethylammonium bromide as the cationic surfactant was studied using laser Raman spectroscopy and dilatometry. The influences of water soluble (potassium peroxodisulphate, ammonium peroxodisulphate) and oil-soluble (azobisisobutyronitrile, benzoyl peroxide) initiators, monomer, surfactant, cosurfactants (n-alcohol and bifunctional alcohols) and temperature on the rates of polymerisation, energy of activation, particle diameter, number of polymer particles, molecular weight of polyvinyl toluene and number of polymer chains per latex particle were investigated. The dependencies of the kinetic and latex size parameters on the initiators and co-surfactants are discussed in terms of the efficiency of the initiators in initiating the polymerisation and on the interfacial partitioning behaviour of various co-surfactants. 19 refs. 

The following mechanism was put forward [31] to explain this autocatalysis (1) permeation by cosurfactant (amide) of the water-AOT-toluene interfacial regions as a result of partitioning equilibria with concomitant increase in polarity and dielectric constant in these regions (2) diffusion of swollen micelle to proximity of electrode surface (3) collision of swollen micelle with the electrode surface (de facto hemimicelle formation) or with a hemi-micelle on the electrode surface and diffusion of amide through the AOT interfacial region within the electron transfer distance of the electrode (4) irreversible oxidation of amide. 

The SANS technique was further adopted to study the partitioning of monomer between microemulsion droplets and polymer particles for various monomers (ST, w-butyl methacrylate, -butyl methacrylate, and CeMA) (34). It was found that, during microemulsion polymerization, the partitioning of monomer is strongly dependent on the composition of microemulsion, especially on the distance to the phase boundary in the pseudo three-phase diagram of the surfactant/cosurfactant-oil-water system. For example, the monomer partitioning is linear in nature and the concentration of monomer in polymer particles is quite low if the initial microemulsion composition is far away from the phase boundary. In contrast, the monomer partitioning is essentially nonlinear and the... 

The lipase-catalyzed hydrolysis of p-nitrophenyl butyrate (p-NPB) was used as a model reaction. It was found that the hydrolysis rate was faster in the water-in-IL microemnlsions than in the water-in-isooctane microemulsions. Hie intrinsic activity of lipase in the IL microemulsion was about three times higher than that of water/ AOT/isooctane microemulsions of AOT under the given experimental conditions. The enhanced catalytic activity of lipase in water-in-IL microemulsions may be due to (i) aqueous microenvironmental changes, (ii) the partition of the substrate or other molecules involved in the reaction between water and IL phases, and (iii) the existence of 1-hexanol as a cosurfactant. 

As a typical example of the different structuring of alcohols in direct micellar systems, we will discuss the ternary system water/Ci2E023/alcohol with different alcohols and at different compositions. Two exemplary phase diagrams are given in Figure 12.3, [5]. It is well known that some alcohols are good cosurfactants, whereas others do not incorporate into the micellar structure. Therefore, it can be expected that water-soluble enzymes with a low affinity to interfaces, such as HLADH, will interact only with the amount of alcohols that is partitioned in the aqueous pseudophase. This is indeed the case, as can be concluded from Figure 12.4. 

As a conclusion of this part we can state that enzymatic reactivity is a very sensitive measure of (1) stractural changes, (2) completion of hydration (of surfactant and cosurfactant), (3) partitioning of the various components, (4) interfacial film rigidity, and, not to forget, (5) pH. 

---