Microemulsion solubilization parameter

Fig. 13 Effect of electrolyte concentration on Winsor microemulsion type (saliniu scan), w water o oil m microemulsion solubilization parameter volume of oil solubilized per mass of surfactant.

It is widely recognized that the system IFT reaches a minimum in the middle phase microemulsion region. At the same time, the solubilization parameter (a), defined as mass of oil solubilized per unit mass of surfactant, is maximized in middle phase microemulsion systems (see Figure 1). This inverse relationship between the solubilization parameter (c)and IFT (y)has been defined by the Chun-Huh equation (Huh 1979, Sunwoo et. al. 1992, Abe et. al. 1987) ... 

Figure (3) shows the solubilization parameters as functions of water concentration for SDS/2- entanol ratios of 0.25 and 0.40 at 25 C. The solubilization parameters are defined as Vo/Vs and Vw/Vs, where Vo, Vs and Vw are the volumes of organic phase, surfactant and aqueous phase in the microemulsions. The parameters are related to the drop size and also interfacial torsions f7.23). The bicontinuous phase is located around the composition range corresponding to equal values of solubilization parameters. The solubilization parameters are dependent on the initial surfactant and/or cosurfactant concentration. Similar dependence has been observed in other systems as a function of salinity and pH (7.231. Conductivity measurements performed as a function of water content indicate an S-shaped curve as shown in Figure (4). This is typical of microemulsions showing transition from oil-continuous to bicontinuous to water-continuous microstructure with increasing water content. 

Huh developed a theoretical relationship between the solubilization parameter and IFT for a middle-phase microemulsion (type III). His equations are... 

We start with the equations by Huh (1979) who developed a theoretical relationship between the solubilization parameter and IFT for a middle-phase microemulsion (type 111). His equations are shown as Eqs. 7.77 and 7.78. According to Eq. 7.96,... 

FIGURE 9.3 Interfacial tensions and solubilization parameters for microemulsions in a system containing a synthetic petroleum sulfonate, an alcohol cosurfactant, a mixture of refined oils, and NaCl brine. (From Reed, R.L. and Healy, R.N., in Improved Oil Recovery by Surfactant and Polymer Flooding, Shah, D.O. and Schechter, R.S., Eds., Academic Press, New York, 1977. With permission.)... 

There are two bulk interfaces in middle phase microemulsions and one in lower or upper phase microemulsions. Thus, one or three values of interfacial tension (IFT) may be measured depending on system composition (1) ymo between microemulsion and excess oil phase, (2) between microemulsion and excess brine phase, and (3) >Vm between excess oil and brine phases. Phase volumes and consequently the volumes of oil (Vo) and brine () solubilized in the microemulsion depend on the variables that control the phase behavior. The solubilization parameters are defined as Vg/Vs and V JV, where Vs is the volume of the surfactant in the microemulsion phase. These parameters are easily determined from phase volume measurements if all the surfactant is assumed to be in the microemulsion phase. The magnitude of decreases as Vg/Vs increases, i.e., as more oil is solubilized. Similarly, the magnitude of decreases as Vg/Vs increases. The salinity at which the values of ymo and are equal is known as the optimal salinity based on IFT. Similarly, the intersection of Vg/Vs and V. /Vs defines the optimal salinity based on phase behavior. The optimal salinity concept is very important for enhanced oil recovery. 

The first step in selecting an optimal micellar system for any field application is to define a complete one-phase microemulsion without any oil or water phase separation, whose viscosity is in the range of the equivalent viscosity of the oil-bank, and which can be prepared from a minimum amount of surfactant, corresponding to the smallest multiphase area in phase diagrams or to the highest solubilization parameters. Such a system involves a surfactant... 

In the previous relationship, the solubilization parameters SP refer to the Vo/Vs and Vw/l s ratios of the volume of oil and water solubilized in the microemulsion phase (the surfactant-rich phase) per volume of surfactant. If the surfactant is a solid, and as a more general definition, the solubilization parameters may be taken as Vo/nts and V ulfn, where ms is the mass of surfactant. The volumes can be deduced firom the measurement of the microemulsion-phase volume at equilibrium mid the amount of surfactant, oil, and water that were added to the system in the first place. It is helpful to note that in two- or three-phase systems, the excess phase(s) contain(s) at most the critical micelle concentration of the surfactant, which is, in general, a negligible amount in the mass balance. Thus, all the surfactants can be assumed to be in the microemulsion phase, and the excess phase can be assumed to be a pure component. 

A typical variation of the interfacial tension and solubility parameters is shown in Fig. 20 [63] 7mo and 7mw refer to the interfacial tensions between the microemulsion (water phase) and the excess oil, and between the microemulsion (oil phase) and the excess water, respectively. The solubilization parameters are also defined for two- and three-phase systems. In the last case, the two solubilization parameters can be measured at the same time, Vo and Vw being the amount of oil and the amount of water, respectively, that are solubilized into the middle-phase microemulsion. 

As seen in Fig. 20, the solubilization parameter curves cross over at exactly the same formulation (here salinity) where the interfacial tension curves do it, which is expected if the Hu relationship strictly applies. At this point, which is by definition the optimum formulation of the scan, the microemulsion is solubilizing the same amoimt of oil and water (as V = Vw) and its representative point on a SOW diagram is exactly at the same distance from the W and O vertices. As seen in Fig. 20, the solubilization increases from both sides when the formulation tends to the optimum one. The value of the solubilization parameter at optimum is generally referred to as SP (the maximum of the minimum of the Vq/Vs and Vw/Vs ratios that is attained at Vq/Vs = It is worth noting that SP ... 

Part II starts with the possibilities of ACE for characterizing the relevant physicochemical properties of drugs such as lipophilicity/hydrophilicity as well as thermodynamic parameters such as enthalpy of solubilization. This part also characterizes interactions between pharmaceutical excipients such as amphiphilic substances (below CMC) and cyclodextrins, which are of interest for influencing the bioavailability of drugs from pharmaceutical formulations. The same holds for interactions of drugs with pharmaceutical vehicle systems such as micelles, microemulsions, and liposomes. 

This effect can be of great importance, because it is susceptible to considerable alteration of the surfactant interaction between oil and water, and the solubilization in microemulsion (as in the so-called lipophilic and hydrophihc linker mechanisms). The role of the linker molecules is to extend the reach of the surfactant in the bulk phase and in practice to somehow modify the oil and water phases close to the interface, so that their characteristic parameters are altered [66-69]. 

It is well known that the aqueous phase behavior of surfactants is influenced by, for example, the presence of short-chain alcohols [66,78]. These co-surfactants increase the effective value of the packing parameter [67,79] due to a decrease in the area per head group and therefore favor the formation of structures with a lower curvature. It was found that organic dyes such as thymol blue, dimidiiunbromide and methyl orange that are not soluble in pure supercritical CO2, could be conveniently solubihzed in AOT water-in-C02 reverse microemulsions with 2,2,3,3,4,4,5,5-octafluoro-l-pentanol as a co-surfactant [80]. In a recent report [81] the solubilization capacity of water in a Tx-lOO/cyclohexane/water system was foimd to be influenced by the compressed gases, which worked as a co-surfactant. 

The physicochemical aspects of micro- and macroemulsions have been discussed in relation to enhanced oil recovery processes. The interfacial parameters (e.g. interfacial tension, interfacial viscosity, interfacial charge, contact angle, etc.) responsible for enhanced oil recovery by chemical flooding are described. In oil/brine/surfactant/alcohol systems, a middle phase microemulsion in equilibrium with excess oil and brine forms in a narrow salinity range. The salinity at which equal volumes of brine and oil are solubilized in the middel phase microemulsion is termed as the optimal salinity. The optimal salinity of the system can be shifted to a desired value hy varying the concentration and structure of alcohol. 

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. 

A schematic of change in the type of microemulsion with the salinity is shown in Figure 7.8, and a volume fraction diagram of the data presented in Table 7.2 is shown in Figure 7.9. The volume fraction information can also be represented by a solubility plot, as shown in Figure 7.10 (see page 254). We will see later that the solubilization ratio is a very important parameter in interfacial tension calculation. 

Run a batch simulation for many injection pore volumes. Check whether the solubilization ratios match the experimental data. The solubilization ratios can be calculated from the concentrations in the microemulsion phase (.COMP ME). If matched, the input parameters are correct, and these input parameters can be used in other simulation studies. Otherwise, repeat steps 2 and 3 with new values of those seven parameters. Sometimes, Csei, and... 

With appropriate values of the various parameters, this model yielded predictions in good agreement with experiment on such phenomena as the amounts of various pure hydrocarbons solubilized in micelles of sodium dodecyl sulfate SDS as well as the amounts of benzene and -hexane solubilized in the same micelles from various mixtures of the two hydrocarbons. It was also able to predict transformation of rodlike micelles to spherical microemulsion droplets as a result of hydrocarbon solubilization, an effect that has been observed experimentally. In the absence of hydrocarbon, films of these surfactants can attain their preferred curvatures only by forming cylindrical micelles, as micelle radius is limited to the extended length of a surfactant molecule. However, when considerable hydrocarbon is present, this constraint no longer applies, and spherical microemulsion droplets can grow until the preferred curvature is reached. 

Liquid soil is usually removed by roll-up, emulsification, direct solubilization, and possibly formation of microemulsion or liquid crystalline phases. The oil emulsification capability of the surfactant solution and the oil-water interfacial tension are relevant physicochemical parameters. 

The thermodynamic modeling of microemulsions has taken various lines and gave conflicting results in the period before the thermodynamic stability and microstructure were established. It was early realized that a maximal solubilization of oil and water simultaneously could be discussed in terms of a balance between hydrophilic and lipophilic interactions the surfactant (surfactant mixture) must be balanced. This can be expressed in terms of the HLB balance of Shinoda,Winsor s R value, and a critical packing parameter (or surfactant number), as introduced to microemulsions by Israelachvili et al. [37], Mitchell and Ninham [38], and others. The last has become very popular and useful for an understanding of surfactant aggregate structures in general. 

Parameters that may be used to tune the phase behavior of microemulsions include salinity, surfactant type and concentration, cosolvent type and concentration, pH, oil composition, temperature, and pressure. As salinity increases, there is a steady progression from lower phase to middle phase to upper phase microemulsions. This reflects a continuous evolution of the preferred curvature of the surfactant film and corresponds to an increase in hydrophobicity with added electrolyte such as NaCl. At low salinity the droplet size in the water-continuous lower phase increases with increasing salinity. This corresponds to an increase in the solubilization of oil and is reflected in increased light scattering. As salinity increases further, the middle phase appears and is initially water-continuous. 

By varying several parameters such as the W/O ratio, one can induce an inversion from an O/W to a W/O microemulsion and vice versa. The type of structure in the inversion domain depends essentially on the bending constant a characteristic of the elasticity of the surfactant layer [7]. If Ke is on the order of kT (where k is the Boltzmann constant and T absolute temperature), the persistence length of the film (i.e., the distance over which the film is locally flat) is microscopically small. The interfacial film is flexible and is easily deformed under thermal fluctuations. The phase inversion occurs through a bicontinuous structure formed of water and oil domains randomly interconnected [8,9]. The system is characterized by an average curvature around zero, and the solubilization capacity is maximum. When K kT, is large and the layers are flat over macroscopic distances. The transition occurs through a lamellar phase. 

The presence of a chromophore group in the hydrophilic or hydrophobic moieties in the surfactant molecular structure makes it sensitive to different physical responses, in particular, for the control of physicochemical parameters of colloidal systems such as surface activity, aggregation structure, viscosity, microemulsion separation, and solubilization. 

Experimental studies concerning crystallization from W/O microemulsions use thermal analysis methods to characterize the microemulsions themselves, to determine thermodynamic parameters of crystallization, and to characterize the final products. A large number of studies are concerned with the state of water in ionic [109] and nonionic [110] W/O microemulsions. It has been shown that because of the close proximity of the interface, the properties of the water molecules are quite different from those of water in the bulk, and this difference in itself may have a profound effect on the solubilization and crystallization of solutes. The problem is discussed in detail in two other chapters (by Schulz et al. and by Garti et al.) in this book and will not be reiterated here. In this presentation we describe (1) calorimetric studies of the formation of nanosized inorganic crystallites and (2) the use of TG and DSC in the characterization of a water-soluble organic compound crystallized in a W/O microemulsion. 

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