Emulsifying ionic apolar polymer in water: understanding the process

Binders for paints or adhesives are frequently polymerized as melt or in solution. To use these apolar polymers in water-based formulations, they should first be emulsified in water with the help of an incorporated ionic emulsifier (surfactant). Here, we aim for a deeper understanding of this emulsification process. On mixing oil, water and surfactant, the mixture is expected to evolve into its thermodynamic equilibrium state, denoted by the phrase ‘microemulsion.’ For unclear reasons, however, the mixture frequently becomes entrapped into an arrested structure, before reaching equilibrium. Then, a so-called metastable emulsion is attained that might remain stable over many years. This study focuses on the underlying reason for oil/water/surfactant mixtures to become entrapped into such a metastable state. From an engineering perspective, this is essential to know since the tools available to control the size of the emulsion droplets are entirely different for microemulsions, on the one hand, and metastable emulsions, on the other hand. First, a generic classification scheme is proposed to distinguish between emulsification processes and resulting emulsion structures from a thermodynamic perspective. Second, emulsions are studied by mixing an acetone solution of apolar ionic polymer with varying amounts of water. First, we examined the rate at which microemulsion structures were assembled by ‘thermodynamics.’ This was done by measuring the response of the emulsion turbidity, on a stepwise change of the water/acetone (w/a) ratio. Upon a stepwise reduction of the w/a ratio down to 0.7, spontaneous assembly of the equilibrium structure appeared to be already finished in less than 6 min. At increasing w/a ratio, however, the time required to reach equilibrium strongly increased. At a w/a ratio of 2.2, spontaneous assembly even appeared to be practically blocked, indicating that a metastable emulsion was attained. We suggest that the assembly rate declines with increasing w/a ratio due to decreasing solubility of polymer in the water-enriched phase of the emulsions. Next, we determined the equilibrium phase diagram and the composition line where inversion occurs from ‘water-in-oil’ (w/o) into ‘oil-in-water’ (o/w) microemulsions. In practice, the polymer solution in acetone is emulsified by gradually dosing water to the stirred solution, up to a w/a ratio in the range of 2.0–2.5. The results of our study clarify that the success of this process is likely related to the very moment the thermodynamically driven assembly comes to a halt: either before or after inversion. If spontaneous assembly only comes to a halt after inversion, an arrested o/w microemulsion will be obtained, with a polymer particle size being independent on both stirring speed and water dosing rate. However, if spontaneous assembly already stops before effectuation of inversion, then ‘hydrodynamics’ will take the lead at inverting the w/o emulsion into an o/w emulsion. Consequently, inversion will then be effectuated by the less effective mechanism of mechanical rupture, resulting in particles that might be large and rapidly sediment. Consequently, the emulsion might be judged as being unstable.