Tuning Electrostatic Interactions of Colloidal Particles at Oil-Water Interfaces with Organic Salts

Monolayers of colloidal particles at oil-water interfaces readily crystallize owing to electrostatic repulsion, which is often mediated through the oil. However, little attempts exist to control it using oil-soluble electrolytes. We probe the interactions among charged hydrophobic microspheres confined at a water-hexadecane interface and show that repulsion can be continuously tuned over orders of magnitude upon introducing nanomolar amounts of an organic salt into the oil. Our results are compatible with an associative discharging mechanism of surface groups at the particle-oil interface, similar to the charge regulation observed for charged colloids in nonpolar solvents.

Figure 1

Illustration of the experiments. (a) Scheme (not to scale) of the cell holding the water-hexadecane interface, with a hexagonally packed colloidal monolayer. (b) After monolayer formation, the cell is flipped by 90° and allowed to equilibrate for $>8\mathrm{h}$. (c) Representative plot of the particle position as a function of height ($z$) in a vertical monolayer of PS particles ($d=2.80\mathrm{\mu }\mathrm{m}$) after 15 h, and corresponding dark field microscopy images. $z=0\mathrm{mm}$ is assigned to the uppermost particle in the monolayer, as illustrated in the top dark-field image, and particles are color-coded according to their average interparticle distance $r$. Scale bars: $10\mathrm{\mu }\mathrm{m}$.

Figure 2

Particle vertical position $z$ versus interparticle distance $r$. Data for three sets of PS particles, showing the mean $r$ in $50\mathrm{\mu }\mathrm{m}$-high horizontal bins. The error bars indicate the standard deviation of the interparticle distance. Solid lines represent the fits to Eq. (2).

Figure 3

The effect of adding IL to the organic phase. (a)–(d) Representative dark-field images of the $d=2.80\mathrm{\mu }\mathrm{m}$ PS monolayers at the bottom of the experimental cell (i.e., $z\approx 0.1–0.3\mathrm{mm}$) for different IL concentrations. Scale bars: $20\mathrm{\mu }\mathrm{m}$. (e) $z$ vs $r$ for different IL concentrations. Error bars show the standard deviation of $r$ in height bins of $50\mathrm{\mu }\mathrm{m}$. (f) $a_2$ as a function of the concentration of IL $c$. The gray area marks the range of values reported in literature [31, 32, 33, 34, 41]. (g) Average number of charges at the particle-oil interface $⟨N⟩$ as a function of $c$. Line indicates the fit to Eq. (5). Error bars in (f) and (g) indicate the error measured over 3 independent experiments. Inset: Sketch of the adsorption of positive ions of the organic electrolyte to the negative charges on the particle surface exposed to the oil side.