The oil/water interfacial behavior of microgels used for enhancing oil recovery: A comparative study on microgel powder and microgel emulsion

The use of particles as foam and emulsion stabilising species, with or without surfactants, has received great interest in recent years. The majority of work has studied the effects of particles as stabilisers in emulsion systems, but recent successes has widened consideration into foams, where industries such as flotation and food processing have encountered the effects of particle stabilisation for many years. This review seeks to clarify studies into emulsions, highlighting new research in this area, and relate similarities and differences to foam systems. Past research has focused on defining the interaction mechanisms of stability, such as principles of attachment energies, particle-particle forces at the interface and changes to the interfilm, with a view to ascertain conditions giving optimum stability. Studied conditions include effects of particle contact angle, aggregation formations, concentration, size and interactions of other species (i.e. surfactant). Mechanisms can be complex, but overall the principle of particles creating a steric barrier to coalescence, is a straitforward basis of interaction. Much research in emulsions can be applied to foam systems, however evidence would suggest foam systems are under a number of additional constraints, and the stability 'window' for particles is smaller, in terms of size and contact angle ranges. Also, because of increased density differences and interfilm perturbations in foam systems, retardation of drainage is often as important to stability as inhibiting coalescence.

Temperature- and pH-sensitive microgels from cross-linked poly(N-isopropylacrylamide)-co-methacrylic acid are utilized for emulsion stabilization. The pH- and temperature-dependent stability of the prepared emulsion was characterized. Stable emulsions are obtained at high pH and room temperature. Emulsions with polar oils, like 1-octanol, can be broken by either addition of acid or an increase of temperature, whereas emulsions with unpolar oils do not break upon these stimuli. However, complete phase separation, independent of oil polarity, can be achieved by successive acid addition and heating. This procedure also offers a way to recover and recycle the microgel from the sample. Interfacial dilatational rheology data correlate with the stimuli sensitivity of the emulsion, and a strong dependence of the interfacial elastic and loss moduli on pH and temperature was found. The influence of the preparation method on the type of emulsion is demonstrated. The mean droplet size of the emulsions is characterized by means of flow particle image analysis. The type of emulsion [water in oil (w/o) or oil in water (o/w)] depends on the preparation technique as well as on the microgel content. Emulsification with high shear rates allows preparation of both w/o and o/w emulsions, whereas with low shear rates o/w emulsions are the preferred type. The emulsions are stable at high pH and low temperature, but instable at low pH and high temperature. Therefore, we conclude that poly(N-isopropylacrylamide)-co-methacrylic acid microgels can be used as stimuli-sensitive stabilizers for emulsions. This offers a new and unique way to control emulsion stability.