Microemulsions phase transitions

S. A. Safran, Theory of Structure and Phase Transitions in Globular Microemulsions, in Micellar Solutions and Microemulsions, S. H. Chen and R. Rajagopalan, eds.. Springer-Verlag, New York, 1990, Chapter 9. 

The behavior of the internal energy, heat capacity, Euler characteristic, and its variance ( x ) x) ) the microemulsion-lamellar transition is shown in Fig. 12. Both U and (x) jump at the transition, and the heat capacity, and (x ) - (x) have a peak at the transition. The relative jump in the Euler characteristic is larger than the one in the internal energy. Also, the relative height of the peak in x ) - x) is bigger than in the heat capacity. Conclude both quantities x) and x ) - can be used to locate the phase transition in systems with internal surfaces. 

FIG. 12 The behavior of the internal energy U (per site), heat capacity Cy (per site), the average Euler characteristic (x) and its variance (x") — (x) close to the transition line and at the transition to the lamellar phase for/o = 0. The changes are small at the transition and the transition is very weakly first-order. The weakness of the transition is related to the proliferation of the wormhole passages, which make the lamellar phase locally very similar to the microemulsion phase (Fig. 13). Note also that the values of the energy and heat capacity are not very much different from their values (i.e., 0.5 per site) in the Gaussian approximation of the model [47]. (After Ref. 49.)... 

Figure 7. Topological fluctuations of the lamellar phase at different points of the phase diagram, (a) Single fusion between the lamellae by a passage (this configuration is close to the topological disorder line), (b) Configuration close to the transition to the disordered microemulsion phase the Euler characteristic is large and negative.

As for direct emulsions, the presence of excess surfactant induces depletion interaction followed by phase separation. Such a mechanism was proposed by Binks et al. [ 12] to explain the flocculation of inverse emulsion droplets in the presence of microemulsion-swollen micelles. The microscopic origin of the interaction driven by the presence of the bad solvent is more speculative. From empirical considerations, it can be deduced that surfactant chains mix more easily with alkanes than with vegetable, silicone, and some functionalized oils. The size dependence of such a mechanism, reflected by the shifts in the phase transition thresholds, is... 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

Activity and stability are often comparable to values in aqueous media. Many substrates which cannot be made to react in water or in pure organic solvents such as hexane owing to lack of solubility can be brought to reaction in microemulsions. Whereas enzyme structure and mechanism do not seem to change upon transition from water to the microemulsion phase (Bommarius, 1995), partitioning effects often are very important. Besides an enhanced or diminished concentration of substrates in the vidnity of microemulsion droplets and thus of enzyme molecules, the effective pH values in the water pool of the droplets can be shifted in the presence of charged surfactants. Frequently, observed acceleration or deceleration effects on enzyme reactions can be explained with such partitioning effects (Jobe, 1989). 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

A thermodynamic treatment, similar to that used for microemulsions, as well as an approximate statistical mechanical one, are developed to explain the phase transition in monolayers of insoluble surfactants [3.8], A similar thermodynamic approach is applied to multilamellar liquid crystals, and it is shown that, for a given set of interactions and bending moduli, only narrow ranges of the thicknesses of the water and oil layers are allowed [3.9]. 

Microemulsions are thermodynamically stable dispersions of oil and water stabilized by a surfactant and, in many cases, also a cosurfactant.1-4 The microemulsions can be of the droplet type, either with spherical oil droplets dispersed in a continuous medium of water (oil-in-water microemulsions, O/W) or with spherical water droplets dispersed in a continuous medium of oil (water-in-oil microemulsions, W/O). The droplet-type microemulsions can be either a single-phase system or part of a two-phase system wherein the microemulsion phase coexists with an excess dispersed phase (an upper phase of excess oil in the case of O/W and a lower phase of excess water in the case of W/O microemulsions). There are also nondroplet-type microemulsions, referred to as middle-phase microemulsions. In this case, the microemulsion phase is part of a three-phase system with the microemulsion phase in the middle coexisting with an upper phase of excess oil and a lower phase of excess water. One possible structure of this middle-phase microemulsion, characterized by randomly distributed oil and water microdomains and bicontinuity in both oil and water domains, is known as thebiccntinuous microemulsion. Numerous experimental studies have shown1 2 4 that one can achieve a transition... 

To illustrate the present theory, calculations were carried outfor a system consisting of an anionic surfactant, oil, water, alcohol, and electrolyte. Obviously, this model can be readily applied to simpler microemulsions free of alcohol, wherein the phase transitions are caused by varying the electrolyte concentration or the temperature. 

Micellar aggregates are considered in chapter 3 and a critical concentration is defined on the basis of a change in the shape of the size distribution of aggregates. This is followed by the examination, via a second order perturbation theory, of the phase behavior of a sterically stabilized non-aqueous colloidal dispersion containing free polymer molecules. This chapter is also concerned with the thermodynamic stability of microemulsions, which is treated via a new thermodynamic formalism. In addition, a molecular thermodynamics approach is suggested, which can predict the structural and compositional characteristics of microemulsions. Thermodynamic approaches similar to that used for microemulsions are applied to the phase transition in monolayers of insoluble surfactants and to lamellar liquid crystals. 

Figure 8. Self-diffusion coefficients of the components of a microemulsion of sodium dodecyl sulfate (SDS), butanol, toluene, and NaCl brine. Vertical lines denote 2,3 and 3,2 phase transitions. Reprinted with permission from P. Guering and B. Lindman, Langmuir 1,464 (1985) [14]. Copyright 1985 American Chemical Society.

Several microemulsion inkjet inks have been described in the literature. An inkjet phase transition ink in the form of a microemulsion consists of an organic vehicle phase having a colorant dispersed therein, where the vehicle phase is preferably liquid while jetting at temperatures above 70°C and solid upon keeping the substrate at room temperature (22-25°C). This formulation undergoes a phase transition from a microemulsion phase to a lamellar phase upon heating, which allows build up of several layers of inks on the surface of the paper. In a similar concept for phase transition, an ink comprised of an aqueous phase, an oil phase. 

The influence of sodium acetate on the phase equilibria of acrylamide microemulsions has been investigated (Holtzscherer, C. Candau, F. J. Colloid Interface Sci., in press). The interfacial tensions of the systems preequilibrated are reported versus the salt concentration in Figure 6. It can be seen that addition of sodium acetate induces a phase transition HI - H III which occurs for S = 1.2H. The intercept of the two curves which occurs in the Vinsor III domain defines an optimal salinity for the formation of bicontinuous microemulsions. 

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. 

Formation and Structure of Middle Phase Microemulsion. The 1 - m - u transitions of the microemulsion phase as a function of various parameters are shown in Figure 4. Chan and Shah (31) compared the phenomenon of the formation of middle phase microemulsion with that of the coacervation of micelles from the aqueous phase. They concluded that the repulsive forces between the micelles decreases due to the neutralization of surface charge of micelles by counterions. The reduction in repulsive forces enhances the aggregation of micelles as the attractive forces between the micelles become predominant. This theory was verified by measuring the surface charge density of the equilibrated oil droplets in the middle phase (9). 

Figure 4. Schematic illustration of the l- mm transition of the microemulsion phase by several variables.

The term "microemulsion" will be used as defined by Healy and Reed (2) "a stable translucent micellar solution of oil, water that may contain electrolytes, and one or more amphiphilic compounds (surfactants, alcohols, etc.)". Microemulsions have been classified as lower phase (A), upper phase (u), or middle phase (m) in equilibrium with excess oil, excess water, or both excess oil and water respectively. On increasing salinity, phase transitions take place in the direction of lower middle - upper phase microemulsions. 

When the NaOH-free aqueous solutions were equilibrated with equal amounts of dodecane, the resulting systems contained two phases, an upper phase microemulsion and an excess brine phase, at salinities exceeding 0.8 gm/dl NaCl. Three phases were observed at 0.8 gm/dl NaCl, an excess-oil phase, a middle-phase microemulsion and an excess-brine phase. Small aliquots of concentrated NaOH were added to the other systems to observe the uppers middle lower phase transitions where possible. 

The effect of pH on phase behavior of microemulsions has been discussed in a different paper (4). In general, an increase in pH by addition of NaOH at constant salinity makes surfactant more hydrophilic by ionizing the carboxylic acid. Therefore, under appropriate conditions, the effect of salinity which is to make the surfactant hydrophobic, can be counterbalanced by an appropriate change in pH. The amount of NaOH, or equivalently, the pH needed for an upper phase microemulsion to shift to a middle phase increases with increasing salinity. Thus, the concentrations are 0.03M and 0.1M NaOH for 2 and 7 gm/dl NaCl, respectively. The upper + middle + lower phase transitions were observed with pH adjustment for salinities less than 5 gm/dl NaCl. For higher salinities, the microemulsion remained as a middle phase even with an excess of NaOH. All the surfactant molecules are ionized in such a situation, and the salinity is too high to be counterbalanced by pH adjustment only. 

The amount of cosurfactant necessary for optimum microemulsion formation can be reduced by changing the chain length of the carboxylic acid to slightly lower values. Thus, middle phases were obtained with 5 gm/dl TRS 10-410, 5 gm/dl IBA and 1 gm/dl octanoic acid by adjusting the pH and salinity. Figure 15 illustrates the phase behavior at 1.5 gm/dl NaCl with varying concentrations of NaOH. The upper middle - lower phase transitions were observed as expected. Middle phases were obtained, but not lower phases, for... 

It is possible to extend the middle phase region by increasing the concentration of carboxylic acid, coupled with an increase in alcohol concentration. The phase behavior of a mixed microemulsion system containing 5 gm/dl TRS 10—410, 1.5 gm/dl octanoic acid and 8 gm/dl IBA at 22°C is shown in Figure 17. Between 1.5 and 4 gm/dl NaCl, if only NaOH concentration is varied then upper - middle - lower phase transitions occur, apparently due to an increase in the ionization of the surfactant. Extension of salinity and NaOH concentrations in Figure 17 is expected to show upper - middle - lower + middle, and other phase transitions as observed in the oleic acid system (4). 

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