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The phase inversion of polymeric water-inoil emulsions has been systematically studied by employing nonylphenol and alcohol ethoxylates with various chemistries as well as physical chemical characteristics. A combination of thermodynamics, phase diagrams, and rheometry were used to investigate the behavior of the inverting surfactants as well as the inverted, acrylamide-based, cationic emulsions. Polymeric inverse-emulsions containing the inverting surfactant showed no evidence of low-shear thinning, though they did thin as hydrodynamic forces increased (0.01 to 100 s À1 ) prior to reaching a chemistry-and concentration-independent plateau, as is typical for emulsions. The viscosity of emulsions containing inverting surfactants reached a minimum at 1.2% of the ''emulsion breaker''. The efficiency of inversion was optimized at 2 wt % of nonylphenols, expressed as a percentage of the total emulsion mass, and increased with the degree of ethoxylation. Interestingly, the viscosity of the polymer inverted in water was maximized at an inverting-surfactant level corresponding to the CMC of the pure surfactant in water. The alcohol ethoxylates required a higher concentration for inversion (3 wt %), though they provided a higher ultimate inverse viscosity of the polymeric emulsion in water. Therefore, while the inversion process was less efficient with alcohol ethoxylates, the ultimate dilution solution properties of the polyelectrolytes liberated were improved relative to the nonylphenols. Overall, the process of adding a water-in-oil emulsion, containing an emulsion breaker, to an excess of water involves a catastrophic inversion mechanism. To be effective under such circumstances, an inverting surfactant should have a partition coefficient between the aqueous an organic phases greatly exceeding unity as well as a hydrophilic-lipophilic balance (HLB) above 12. Effectiveness increases linearly with the partition coefficient.

The influence of the interfacial chemistry on the phase inversion of polymerized water-in-oil emulsions has been investigated. For copolymerizations of acrylamide with cationic monomers, the effect of substituting of fatty acid esters and ethoxylated fatty acid esters with ABA block type stabilizers, on the kinetics and extent of phase inversion, were examined. It was determined that the solution viscosity was a valid metric to identify the mechanism by which inversion occurs, while conductivity provided a means to quantify inversion efficiency, Therefore, the interfacial chemistry was found to influence not only the plateau value of the viscosity of the polymer solution but also its kinetics. The most suitable inversion was observed with a polymer emulsion stabilized with low block copolymer stabilizer levels in the blend (8 wt %), relative to traditional fatty acid esters and ethoxylated fatty acid esters. This provided an ultimate solution viscosity 30% higher than for a polymer synthesized under identical conditions though with higher levels of the ABA block stabilizer. Overall, the optimal formulation (8% ABA) was found to liberate 88% of the latent viscosity. Given that the options in regards to inverting surfactants can be, legislatively, limited, the present work makes a case for the selection of the interfacial composition not only for its stability during reaction, and the molecular weight of the synthesized polymer, but also for the extent and rapidity of inversion. The formulation-composition map approach provided an understanding of phase inversion applied to polymer emulsion and was a useful fingerprint to qualitatively describe the catastrophic mechanism of inversion. The surfactant affinity difference applied to a blend of surfactant was found to be a convenient formulation parameter which allowed us to locate the representative point on the map of the polymer emulsion stabilized with different surfactant blend composition.

The stability of polyacrylamide-based inverse emulsions, prepared with surfactant blends consisting of mixtures of fatty acid esters, ethoxylated fatty acid esters, and ABA triblock copolymeric stabilizers, was investigated by means of oscillatory shear measurements. “Milky-like” inverse lattices were obtained from inverse-emulsion polymerization of acrylamide. The surfactant blend composition and type were varied at a fixed concentration of 3 wt %, with the elastic modulus followed as a function of time for a period not exceeding 30 days. The data indicated that the logarithm of G‘ as a function of time was linear for at least 15 days. The slope of log G‘ versus time was validated as an index of stability, such that the higher the slope, the less stable the system. A threshold with regard to this indicator was also established at 0.02. As two surfactant types (fatty acid esters and block copolymers) were used to stabilize the emulsion, a threshold stability was also observed as a function of the stabilizer ratio, with emulsions reported to be stable below 40% fatty acid ester levels. In contrast, for emulsions stabilized with three types of surfactants (the aforementioned two plus ethoxylated fatty acid esters), nonsettling emulsions were obtained, regardless of the blend composition. Oscillatory shear measurements were further compared with the level of separated oil phase obtained from accelerated settling tests and shelf life. The results were found to be in close accordance and to indicate the efficiency of long-range interparticle interaction toward emulsion stability. Similarly to the range and magnitude of the interaction forces, the dispersed-phase volume fraction was observed to influence emulsion stability. For volume fractions less than 0.55, all inverse emulsions were unstable.

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