Macroemulsions inversion

Figure 9.3 shows the most clear-cut image of the macroemulsion inversion as a function of temperature equal volumes of oil and water are emulsified at various temperatures. Five hours after preparation, the macroemulsions sediment completely. Below the balanced temperature (HLB temperature), a stable O/W macroemulsion is formed, whereas above the balanced temperature a stable W/O emulsion is formed. Close to the balanced point (60-68 °C), a three-phase equilibrium is observed and neither O/W or W/O emulsions are stable. 

PIT behavior can be readily observed in test-tube experiments. Close to the balanced state, the samples of macroemulsions can be readily pr ared by hand because of the low interfacial tension between oil and water. Moreover, the surfactant purity is not very important. Impurities just shift the position of the macroemulsion inversion point, leaving the general inversion trend unchanged. In fact, the PIT trend itself was established not with pure surfactants but with a polydisperse surfactant product. On the other hand, good temperature control is necessary, in particular close to the balanced point. 

Figure 7.11 Macroemulsion inversion as a function of monolayer spontaneous curvature. The spontaneous curvature is varied by changing the ratio of two surfactants in mixture (a), by varying the equivalent alkane carbon number (EACN), i.e. by mixing two oils in different ratios (b). and by the addition of alcohol (c). All the system are studied over the 2—3—2 transition range see the bars at the top of the plots. The left-hand axis shows the macroemulsion lifetime, denoted as the time for the separation of two-thirds of the disperse phase by volume. The right-hand axis shows the macroemulsion conductivity. All emulsions were prepared at a 1 1 oil-to-water ratio other details of the compositions are shown on the plots. DDS and WITCO TRS10-80 are commercial surfactants (Reproduced by pemiission of Marcel Dekker Inc. from ref. 78)...

For a constant amount of nonionic surfactant, the interfacial tension at the planar oil-water interface, for the same amounts of oil and water, passes through a minimum when plotted against the hydrophilic-lipophilic balance (HLB). The emulsion stability passes through maxima in the W/O and O/W ranges and through a minimum between the two at the phase inversion point. The minima in the two cases coincide. These observations are explained on the basis of thermodynamics. The stability of macroemulsions can be correlated with the surface excess of surfactant, which also passes through two maxima and a minimum between them [2.11]. 

Keywords Macroemulsion stability Hydrophilic-hydrophobic balance (HLB) Interfacial tension vs. HLB Surface excess vs. HLB Surface excess vs. temperature Phase inversion temperature Bancroft rule... 

The scope of the present review is to emphasize that thermodynamics can explain the above experimental observations. The next section (Section 2), which is based on ref. [10], will be concerned with the effects of HLB (denoted in what follows h) on the interfacial tension and on the stability of macroemulsions, the goal being to explain the observations of Boyd et al. [5] and of Berger et al. [4]. Section 3, which is based on ref. [11], will examine the effect of temperature on the interfacial tension at the oil-water interface by assuming that no microemulsion or emulsion is formed, as well as its effect on the stability of emulsions. Shinoda and Saito s observations regarding the equality of the two inversion temperatures will be thus explained. Finally, the Bancroft rule [8,9], and some of the violations of this rule, will be examined in the spirit of ref. [12],... 

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. 

To synthesize water-soluble or swellable copolymers, inverse heterophase polymerization processes are of special interest. The inverse macroemulsion polymerization is only reported for the copolymerization of two hydrophilic monomers. Hernandez-Barajas and Hunkeler [62] investigated the copolymerization of AAm with quaternary ammonium cationic monomers in the presence of block copoly-meric surfactants by batch and semi-batch inverse emulsion copolymerization. Glukhikh et al. [63] reported the copolymerization of AAm and methacrylic acid using an inverse emulsion system. Amphiphilic copolymers from inverse systems are also successfully obtained in microemulsion polymerization. For example, Vaskova et al. [64-66] copolymerized the hydrophilic AAm with more hydrophobic methyl methacrylate (MMA) or styrene in a water-in-oil microemulsion initiated by radical initiators with different solubilities in water. However, not only copolymer, but also homopolymer was formed. The total conversion of MMA was rather limited (<10%) and the composition of the copolymer was almost independent of the comonomer ratio. This was probably due to a constant molar ratio of the monomers in the water phase or at the interface as the possible locus of polymerization. Also, in the case of styrene copolymerizing with AAm, the molar fraction of AAm in homopolymer compared to copolymer is about 45-55 wt% [67], which is still too high for a meaningful technical application. 

The selection of the organic phase has a substantial effect on the minimum surfactant concentration necessary to obtain the inverse microemulsion. Nevertheless, there is a threshold for the minimum surfactant amount needed to form the small microemulsion droplets (d a 5-10 nm). Calculations give a limiting value of approximately 10% of all other components lower concentrations of surfactant lead to conventional macroemulsions. 

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. 

Perrin P, Monfreux N, Thierry F, Lafuma F, Lequeux F. Concentrated direct and inverse macroemulsions stabilized by amphiphilic polyelectrolytes. Proc ACS Div Polym Mater Sci Eng 1999 81 492-494. 

If the driving force (Ay) is less than zero an oil-in-water dispersion forms, while if Ay is of the opposite sign an inverse (water-in-oil) dispersion is produced. The stability threshold represents a critical emulsifier concentration below which a kinetically stable macroemulsion is produced. These can be transformed,... 

Over the past decade there has been extensive interest in the kinetics and colloidal behavior of water-in-oil polymerizations. However, these efforts have focused on the elucidation of a general set of phenomena to describe water-in-oil processes, without distinguishing inverse-emulsion and inverse-suspension sub-domains. A confounding factor is certainly the inconsistent nomenclature inverse-suspensions (Ila) are within the inverse-macroemulsion polymerization domain (II), and are often described as inverse-emulsions, where the prefix macro has been omitted for brevity. However, inverse-emulsion (lib) is itself a... 

Heterophase processes should be primarily distinguished based on their emulsion structure (oil-in-water or water-in-oil) and type of stability (kinetic or thermodynamic). This identifies four mutually independent polymerization regimes, each with unique colloidal and chemical behavior. L Macroemulsion, II. Inverse-Macroemulsion, HI. Microemulsion, IV. Inverse-Microemulsion. The macroemulsion and inverse-macroemulsion domains can be further subdivided into Suspension (la), Emulsion (lb), Inverse-suspension (Ha) and Inverse-emulsion (lib) subdomains based on a transition at the critical micelle concentration. 

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