62nd Annual Report on Research 2017

This project involves characterizing the pH and size of water-in-oil emulsion droplets as a function of surfactant type and concentration for different oils. Reverse nanoemulsions (water-in-oil) are thermodynamically unstable while at the same time notoriously difficult to break. This complicates attempts to characterize them using conventional techniques. This year we focused on understanding and controlling the evolution in time of these emulsion, in an effort to reliably measure and control pH as a function of droplet size.

Various instabilities that cause changes in droplet size over time were identified and characterized.

(1) Molecular diffusion (Ostwald ripening) is the main contributor to droplet growth. The Laplace pressure of a small droplet is larger than that of a large droplet. Consequently, the chemical potentials of water molecules inside and near the interface of a small droplet is also larger. To minimize the system's energy, water molecules will diffuse away from small droplets and join into large droplets. This process continues until the osmotic pressure inside the small droplet balances the Laplace pressure. Minimum size therefore depends on and can be controlled by the ionic strength of the droplet phase.

(2) Coalescence of smaller, fast-diffusing droplets, can be problematic if their concentration is high. This effect was minimized by working at lower water:oil ratios (smaller than 1:200 by volume for a typical surfactant concentration of 0.5% by weight in the oil). For these cases, the inter-droplet distance is large enough that coalescence is effectively eliminated on the time scale of our measurements. In addition, non-ionic surfactants such as span 80 were used to increase droplet stability and inhibit coalescence. At small water:oil ratios droplets are far apart and coalescence is minimal. In dynamic light scattering measurements, coalescence shows up both as the onset of a larger diameter droplet population, and also as an onset of micelle formation as the decreasing surface area releases surfactant into the continuous phase.

(3) Sedimentation of larger droplets effectively (slowly) removes this population from the measurement and so can contributes to an apparent change in the droplet size with time. This is a slow enough effect that it does not generally affect our measurements.

Span 80 and other non-ionic and mixed surfactants, all with hydrophilic-lipophilic balance (HLB) below 9, were added to the hydrocarbons (dodecane, nonane, squalane, and hexadecane) to reduce the interfacial tension of the water droplets. Dodecane was the easiest to work with and made relatively stable emulsions, so most of our data was acquired using dodecane. Conventional wisdom dictates that increasing the surfactant concentration should decrease the droplet size, however, coalescence made droplet size unpredictable if the emulsion was too dense. As noted above, working at water:oil ratio below 1:200 appears to solve this problem.

Using dodecane, ionic strength of the droplet phase was varied from zero to 200 mM with no systematic change in size noted. Emulsions were made by sonication in a bath sonicator; sonication time also had little effect on droplet size.

The only reliable method for changing droplet size in a controlled manner was to change the volume fraction of the aqueous phase in the emulsion at constant surfactant concentration and ionic content. Higher water:oil ratio made larger droplets but also resulted in larger droplet polydispersity. We attribute this to a combination of coalescence and Ostwald ripening as discussed above. Nonetheless, a study of droplet sizes revealed monatonically increasing diameters (from 125 to 250 nm) upon changing the water:oil ratio from 1:200 to 1:50.

Droplet pH was monitored in separate experiments on emulsions formulated in the same way as those characterized using dynamic light scattering. To measure pH, we use pH sensing dyes in a ratiometric fluorimeter measurement that was described in last year’s report. We observe an increase in pH with time that tracks the increase of droplet diameter, and an overall increase in pH with increase in average droplet size. We are still working to quantify the modification of pH as a function of droplet size and surfactant type and concentration.

One postdoc and one undergraduate student received training under this grant in the last year. The postdoc was formerly a theorist who received extensive training in experimental techniques that have dramatically broadened her area of expertise. The undergraduate, who did many of the early DLS measurements and reported on his work as reported last year, has gone on to graduate school for a PhD in Aerospace Engineering at Notre Dame.