Scale invariants in the preparation of reverse high internal phase ratio emulsions

Abstract

Reverse high internal phase ratio emulsions (HIPRE) are prepared with the system water/Span80/dodecane to identify the influence of the process variables on droplet size and polydispersity, and to study the influence of the process scale-up. After discussing the most common scale invariants used in emulsification processes, we prepare emulsions at two different scales following a complete factorial experimental design. Using the empirical models obtained with the statistical analysis of the results, we determine the scale invariants for the system. Results from a previous study indicated that the scale invariant for the preparation of HIPRE could be the stirring rate. However, the results obtained in this work show that the scale invariant, based on droplet size, could change depending on the conditions of emulsification. Only when the stirring rate is high enough (> 1500 rpm), or when the surfactant-to-oil ratio is high and the total addition time low, the scale invariant is indeed the stirring rate. The elevated shear stress inside the mixer when the stirring rate is high is responsible of the small droplet size obtained on both scales. However, for other values of the process variables, the scale-invariant can be expressed as N¿D^a, where 0 < a < 0.5, and depends on other two factors: the addition flow rate of dispersed phase or addition time, and the surfactant-to-oil ratio. We propose a methodology to determine these scale invariants and to calculate the threshold values of surfactant-to-oil ratio and addition flow rate from which the stirring rate is the scale invariant (when a → 0).