Steady-state droplet size in montmorillonite stabilised emulsions

Abstract

The formation of hexadecane-in-water emulsions stabilised by montmorillonite platelets was studied. In this system the platelets form a monolayer around the droplets and the droplet size decreases with increasing platelet volume fraction. However, the number of platelets present exceeds that required for monolayer coverage. The kinetics of emulsification were investigated and coalescence of droplets during turbulent mixing was found to continue even after the droplets had reached their ultimate size. Non-spherical droplets, resulting from arrested coalescence, were not observed suggesting that particles may be desorbing from the interface during the turbulent flow. A kinetic model based on a competition between droplet break-up and coalescence, mediated by particle adsorption and desorption, reproduces experimental trends in droplet diameter. The model can be used to predict the most efficient formulation to minimise droplet diameters for given materials and mixing conditions and sheds light on the processes occurring during emulsification in this system.

Controlling the Rheology of Montmorillonite Stabilized Oil-in-Water Emulsions

“…These mechanisms are depicted in Figure 8. Both could occur in this system as montmorillonite platelets are well-known to form gels in aqueous suspension, 15,32 and previous work has estimated the adsorption energy of a single platelet at a hydrocarbon−oil interface to be of order 10 5 k B T. 10 Figure 2 shows that emulsions both with and without excess particles in the continuous phase do not show any appreciable change in D 32 over time. In the absence of a continuous phase network D 32 is slightly higher, and this is attributed to some initial coalescence occurring too rapidly to be recorded.…”

“…An interesting further study would be to test the limits of stability of these emulsions under controlled shear as we have recently shown that particles desorb during high shear mixing. 10 The rheological data presented in the previous section show that NaCl can enhance rheological properties of montmorillonite emulsion gels, Na 4 P 2 O 7 can reduce rheological parameters, and the presence of droplets provides an enhancement to the rheology of the continuous phase gel. To understand these effects, we need to establish the mechanisms by which the droplets enhance the rheology of the continuous phase gels and those by which the additives modify the continuous phase resulting in the observed 3 orders of magnitude change in rheological parameters.…”

“…The droplets for all three sets of conditions show birefringence at the edges when viewed through crossed polarizers which was previously reported to indicate the adsorption of particles parallel to the interface. 6,10 This suggests that modification of particle interactions after emulsification does not significantly alter the configuration of the particles at the interface.…”

“…Above 0.1 M NaCl extensive aggregation of the particles and coalescence of the droplets was observed (as has been reported previously 34 ), and Na 4 P 2 O 7 concentrations above 20 μmol g −1 have previously been shown to have minimal further measurable effect. 32 Previous work on montmorillonite stabilized oil-in-water emulsions has shown that emulsions can be prepared at particle concentrations in the continuous phase as low as 0.5 wt %; however, the minimization of droplet size 10 and formation of a gel 15,32 require higher concentrations. The montmorillonite concentration in the continuous phase was therefore fixed at 3 wt % for the main part of the study.…”

“…3,4 These include methods for controlling particle wettability, 5 the effects of particle shape, 6−8 and also the impact of preparation methods. 9,10 Many different types of soft materials can be prepared using Pickering stabilization including simple emulsions, 3 high internal phase emulsions, 11 arrested bicontinuous systems, 12 and droplet embedded gels. 13 These droplet embedded gels (herein referred to as emulsion gels) arise due to an excess of stabilizing particles remaining in the continuous phase after emulsification which form a gel due to competing attractive and repulsive interactions.…”