Novel stabilisation of emulsions with polyelectrolyte complexes

Bago Rodríguez, Ana María

Abstract

The concept of a novel stabiliser of oil-water emulsions has been put forward, being the polyelectrolyte complex (PEC) formed between oppositely charged water-soluble polymers in cases where either polymer alone is incapable of stabilising an emulsion. Four oppositely charged synthetic polyelectrolytes (strong and weak) are selected, which allowed four polymer mixtures to be studied. The behaviour of their mixtures in water is correlated with that of emulsions after addition of oil.

Aqueous polymer mixtures are investigated via dynamic light scattering to determine the size of the aggregates. Moreover, various optical techniques are used to identify the type of associative phase separation (precipitation or complex coacervation) and their shape. The effects of polyelectrolyte (PEL) mixing ratio, pH, [PEL] and salt content are studied in detail. In general, PEC particles are obtained as a result of a strong electrostatic interaction while complex coacervates arise from weak interactions. Around equal mole fractions of the two polymers, the zeta potential of the aggregates reverses in sign. Spherical complexes of diameters of few hundreds nanometres are obtained at low polyelectrolyte concentration. However, by increasing the initial [PEL], primary particles aggregate. Aggregated PEC particles have an irregular shape while coacervate droplets, which contain high amounts of water, are spherical and have no special internal structure, as observed from TEM images. Under specific conditions, coacervate droplets completely coalesce giving rise to the formation of the so-called coacervate phase. The effect of increasing the salt concentration is comparable in both PEC precipitates and coacervates and causes an initial destabilisation of the aqueous dispersion due to complex aggregation, followed by dissolution of the electrostatic complex at high salt concentrations.

For the emulsion study, the same parameters as for aqueous PEC dispersions are evaluated, as well as the oil volume fraction (ϕo). The complete study is carried out with dodecane despite oils of different chemistry and polarity have also been considered throughout this thesis. The most stable emulsions to both creaming and coalescence are prepared with aqueous PEC dispersions containing complexes of almost neutral charge. By increasing the polyelectrolyte concentration, emulsions become more stable. However, at high [PEL], aggregation levels are relatively high and emulsion stability is slightly worse as big particles can easily be dislodged from the oil-water interface compared to smaller ones. From cryo-SEM images, close-packed particle layers are detected at drop interfaces as well as particle aggregation in the continuous phase. By increasing the oil volume fraction in the emulsion, the droplet diameter increases constantly up until a point where oil droplets appear to be deformed and the viscosity of the emulsion increases substantially. This suggests the formation of high internal phase emulsions (HIPEs), which is rare in particle-stabilised systems, where catastrophic phase inversion is the usual outcome. Taking advantage of the intrinsic fluorescence of the used PEL, confocal microscopy turns out to be a useful technique to visualise where PEC particles are placed upon homogenisation. At high oil volume fractions, particles are only detected around oil droplets, whereas at low oil volume fractions, excess particles remain at the continuous aqueous phase providing extra stability against coalescence. As for aqueous PEC dispersions, the concentration of salt has a remarkable effect on emulsion stability. For emulsions stabilised with PEC particles, by increasing the aggregation level, emulsions become completely unstable. However, at a relatively high salt content, emulsions re-stabilise due to adsorption of uncharged individual polymer molecules. Emulsions with coacervate droplets can be prepared by the addition of oil stepwise and multiple homogenisation steps. However, unlike PEC particles, the system is sensible to the oil type. The feasibility of the coacervate phase to spread at the oil-water interface is discussed in terms of the relevant spreading coefficients and predictions are compared with experiments for a range of oils. We encounter oils whose drops become engulfed by the coacervate phase as well as oils where no engulfing occurs.

Therefore, from the findings obtained from four different polyelectrolyte combinations, we can claim that emulsion stability is given by the presence of PEC at the oil-water interface as individual PEL are not surface-active on their own. Despite this work being a complete starting point for the basic understanding of emulsions stabilised by mixtures of oppositely charged polymers, we are not yet in a position to predict definite rules of behaviour in both aqueous PEC dispersions and emulsions containing them. Further investigation of other polyelectrolyte combinations is required to develop a better understanding of this area.

Citation

Bago Rodríguez, A. M. (2018). Novel stabilisation of emulsions with polyelectrolyte complexes. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4221229

• Thesis Type: Thesis
• Deposit Date: Jan 15, 2019
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4221229
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Sep 1, 2018