Microemulsions phase inversion

The phase inversion temperature (PIT) method is helpful when ethoxylated nonionic surfactants are used to obtain an oil-and-water emulsion. Heating the emulsion inverts it to a water-and-oil emulsion at a critical temperature. When the droplet size and interfacial tension reach a minimum, and upon cooling while stirring, it turns to a stable oil-and-water microemulsion form. " ... 

Wadle, A., Forster, Th. and von Rybinski, W. (1993) Influence of the microemulsion phase structure on the phase inversion temperature emulsification of polar oils. Colloids and Surfaces A Physicochemical and Engineering Aspects, 76, 51-57. 

The packing ratio also explains the nature of microemulsion formed by using nonionic surfactants. If v/a 1 increases with increase of temperature (as a result of reduction of a ), one would expect the solubilisation of hydrocarbons in nonionic surfactact to increase with temperature as observed, until v/a l reaches the value of 1 where phase inversion would be expected. At higher temperatures, va l > 1 and water in oil microemulsions would be expected and the solubilisation of water would decrease as the temperature rises again as expected. 

A. Wadle, T. Forster, and W. Von Rybinski Influence of the Microemulsion Phase Structure on the Phase Inversion Temperature Emulsiflcation of Polar Oils. Colloid and Surfaces A Physicochem. Eng. Aspects 76, 51 (1993). 

The temperature (or salinity) at which optimal temperature (or optimal salinity), because at that temperature (or salinity) the oil—water interfacial tension is a minimum, which is optimum for oil recovery. For historical reasons, the optimal temperature is also known as the HLB (hydrophilic—lipophilic balance) temperature (42,43) or phase inversion temperature (PIT) (44). For most systems, all three tensions are very low for Tlc < T < Tuc, and the tensions of the middle-phase microemulsion with the other two phases can be in the range 10 5—10 7 N/m. These values are about three orders of magnitude smaller than the interfacial tensions produced by nonmicroemulsion surfactant solutions near the critical micelle concentration. Indeed, it is this huge reduction of interfacial tension which makes micellar-polymer EOR and its SEAR counterpart physically possible. 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

Subtraction of the spectrum of liquid water, even of moderate band intensity, can also be complicated by solute-water interactions which cause a shift in the H-O-H bending bands, making a complete nulling of the band in the difference spectrum impossible (23). As discussed further below, in bulk phase samples such as microemulsions or inverse micelles of moderate water content, significant information about aggregate structure is obtained from shifts in the water bands. 

There are other variations of this approach that involve the phase inversion temperature (PIT) (see Section 3.6.1). In one method an emulsion is formed at a temperature a few degrees lower than the PIT, where the interfacial tension is quite low and small droplets can be formed. The emulsion can then be quickly cooled. Another method uses a controlled temperature change to cause an emulsion to suddenly change from a coarse oil-in-water (O/W) emulsion, through a microemulsion phase, and into a fine water-in-oil (W/O) emulsion [432]. 

The ultralow interfacial tension can be produced by using a combination of two surfactants, one predominantly water soluble (such as sodium dodecyl sulfate) and the other predominantly oil soluble (such as a medium-chain alcohol, e.g., pentanol or hexanol). In some cases, one surfactant may be sufficient to produce the microemulsion, e.g., Aerosol OT (dioctyl sulfosuccinate), which can produce a W/O microemulsions. Nonionic surfactants, such as alcohol ethoxylates, can also produce O/W microemulsions, within a narrow temperature range. As the temperature of the system increases, the interfacial tension decreases, reaching a very low value near the phase inversion temperature. At such temperatures, an O/W microemulsion may be produced. 

At low temperatures an O/W microemulsion (0/Wm) is formed which is in equilibrium with an excess oil phase. This condition is termed a Winsor I system. At high temperatures the headgroup requires less space on the interface and, thus, a negative curvature can result. A phase inversion o ccurs and a W/O microemulsion (W/Om) is formed which is in equilibrium with an excess water phase. This situation is termed a Winsor II system. At intermediate temperatures three phases - a water phase, a microemulsion D and an oil phase - are in equilibrium. This is called a Winsor III system. Here the curvature of the interfaces is more or less zero. Hence, the interfacial tension is minimum as depicted in Figure 3.24 (right) for the system C12E5, tetradecane and water. 

For this system the temperature of phase inversion (PIT) is between 45°C and 55°C. Variation of both the temperature and the surfactant concentration in a system with a fixed ratio of water and oil leads to a phase diagram that is called informally the Kahlweit fish due to the shape of the phase boundaries that resemble a fish. In Figure 3.24 (left), this diagram is given for the system water/tetradecane/CnEs. For small surfactant concentrations (<15%), the phases already discussed occur but, at higher emulsifier concentrations, the surfactant is able to solubilise all the water and the hydrocarbon which results in a one-phase microemulsion D or a lamellar phase La. 

First a coarse O/W emulsion is prepared and, on heating, phase inversion occurs. After cooling down through the microemulsion zone, the finely dispersed nature of the microemulsion is partially retained and emulsions with drop sizes of about 100 nm result [28-30]. They show considerable long-term stability as a consequence of the Brownian motion of the oil droplets [31] and pump sprayable deodorants are one of the cosmetic products based on this technology. 

The interfacial tension y at the planar interface has a minimum near the temperature Indeed, at the latter temperature r is small, A/jt0 = 0 and because d ij w/d J and dfi /dT have opposite signs and s is also small (because T is small), dy/d I 0. The temperature T0 is provided by Eq. (25) and is independent of the concentration of surfactant. In other words, the two minima of Fig. 4 which correspond to the phase inversion temperatures of a macroemulsion (the curve with a positive minimum) and microemulsion (the curve with a negative minimum) are the same. When emulsions are generated from a microemulsion and its excess phase, the emulsion is of the same kind as the microemulsion, the phase inversion temperature is obviously located in the middle of the middle phase microemulsion range and the above conclusion remains valid. The above results explain the observation of Shinoda and Saito [6,7] that the phase inversion temperature (PIT) of emulsions can be provided by the ternary equilibrium phase diagram. 

In conclusion, thermodynamics provides some information about the overall behavior of macroemulsions, suggests to correlate the stability of macroemulsions to the surface excess, relates the phase inversion temperature or the phase inversion HLB to the minimum of the interfacial tension at a planar interface between oil and water and explains why the phase inversion temperature is the same for macroemulsions and the corresponding microemulsions. 

Microemulsions should be formed near or at the phase inversion temperature (PIT) or HLB temperature for a given nonionic surfactant, since the solubilization of oil (or water) in an aqueous (or nonaqueous) solution of nonionic surfactant shows a maximum at this temperature. 

Light scattering measurements and theoretical treatment strongly support the idea that attractive interactions between inverse micelles play an important role in the stability of oil rich microemulsions. In the system containing pentanol, attractions between (i)/o micelles can be sufficient to give rise to a phase separation between two microemulsion phases. 

Chan, K.S., Shah, D.O., 1979. The effect of surfactant partitioning on the phase behavior and phase inversion of the middle phase microemulsions. Paper SPE 7869 presented at the SPE International Symposium on Oilfield and Geothermal Chemistry, Houston, 22-24 January. 

The effect of surfactants on the interfacial tension between water and supercritical fluids is a key property for describing emulsions and microemulsions (8), as shown in Figure 2. The v axis may be any formulation variable that influences surfactant partitioning between the phases such as the pressure or temperature. A minimum in y is observed at the phase inversion point, where the system is balanced with respect to the partitioning of the surfactant... 

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. 

Mixtures containing 1 wt% of the pure nonionic surfactant C,2E5 in water were contacted with pure n-hexadecane and n-tetradecane at various temperatures between 25 and 60°C using the vertical cell technique. Similar experiments were performed with C,2E4 and n-hexadecane between about 15 and 40°C. In both cases the temperature ranged from well below to well above the phase inversion temperature (PIT) of the system, i.e., the temperature where hydrophilic and lipophilic properties of the surfactant are balanced and a middle phase microemulsion forms (analogous to the optimal salinity for ionic surfactants mentioned above). The different intermediate phases that were seen at different temperatures and the occurrence of spontaneous emulsification in some but not all of the experiments could be understood in terms of known aspects of the phase behavior, e.g., published phase diagrams for the C12E 5-water-n-tetradecane system, and diffusion path theory. That is, plausible diffusion paths could be found that showed the observed intermediate phases and/or spontaneous emulsification for each temperature. 

At the oil-rich side, the phase behaviour is inverted temperature-wise as can be seen in the T( wA)-section provided in Fig. 1.7(c). Thus, the near-critical phase boundary 2 —1 starts at low temperatures from the lower n-octane-QoEs miscibility gap (below <0°C) and ascends steeply upon the addition of water. With increasing wA, this boundary runs through a maximum and then decreases down to the upper critical endpoint temperature Tu. The emulsification failure boundary 1 —r 2 starts at high temperatures and low values of wA, which means that only small amounts of water can be solubilised in a water-in-oil (w/o) microemulsion at temperatures far above the phase inversion. Increasing amounts of water can be solubilised by decreasing the temperature, i.e. by approaching the phase inversion. At Tu the efb intersects the near-critical phase boundary and the funnel-shaped one-phase region closes. 

In the preceding sections, the phase behaviour of rather simple ternary and quaternary non-ionic microemulsions have been discussed. However, the first microemulsion found by Schulman more than 50 years ago was made of water, benzene, hexanol and the ionic-surfactant potassium oleate [1, 3]. Winsor also used the ionic-surfactant sodium decylsulphate and the co-surfactant octanol to micro-emulsify water/sodium sulphate and petrol ether [2], In the last 30 years, in-depth studies on ionic microemulsions have been carried out [7, 8, 65, 66]. It toned out that nearly all ionic surfactants which contain one single hydrocarbon chain are too hydrophilic to build up microemulsions. Such systems can only be driven through the phase inversion if an electrolyte and a co-surfactant is added to the mixture (see below and Fig. 1.11). 

Ionic surfactants with only one alkyl chain are generally extremely hydrophilic so that strongly curved and thus almost empty micelles are formed in ternary water-oil-ionic surfactant mixtures. The addition of an electrolyte to these mixtures results in a decrease of the mean curvature of the amphiphilic film. However, this electrolyte addition does not suffice to drive the system through the phase inversion. Thus, a rather hydrophobic cosurfactant has to be added to invert the structure from oil-in-water to water-in-oil [7, 66]. In order to study these complex quinary mixtures of water/electrolyte (brine)-oil-ionic surfactant-non-ionic co-surfactant, brine is considered as one component. As was the case for the quaternary sugar surfactant microemulsions (see Fig. 1.9(a)) the phase behaviour of the pseudo-quaternary ionic system can now be represented in a phase tetrahedron if one keeps temperature and pressure constant. 

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