Electrification at water–hydrophobe interfaces

The mechanisms leading to the electrification of water when it comes in contact with hydrophobic surfaces remains a research frontier in chemical science. A clear understanding of these mechanisms could, for instance, aid the rational design of triboelectric generators and micro- and nano-fluidic devices. Here, we investigate the origins of the excess positive charges incurred on water droplets that are dispensed from capillaries made of polypropylene, perfluorodecyltrichlorosilane-coated glass, and polytetrafluoroethylene. Results demonstrate that the magnitude and sign of electrical charges vary depending on: the hydrophobicity/hydrophilicity of the capillary; the presence/absence of a water reservoir inside the capillary; the chemical and physical properties of aqueous solutions such as pH, ionic strength, dielectric constant and dissolved CO2 content; and environmental conditions such as relative humidity. Based on these results, we deduce that common hydrophobic materials possess surface-bound negative charge. Thus, when these surfaces are submerged in water, hydrated cations form an electrical double layer. Furthermore, we demonstrate that the primary role of hydrophobicity is to facilitate water-substrate separation without leaving a significant amount of liquid behind. These results advance the fundamental understanding of water-hydrophobe interfaces and should translate into superior materials and technologies for energy transduction, electrowetting, and separation processes, among others.


Supplementary Section 2: Experimental setup
Supplementary Figure 2. Experimental set-up for investigating the behaviors of aqueous (pendant) droplets formed using hydrophobic and hydrophilic capillaries inside a capacitor. The capacitor facilitated uniform electric field strengths. Once the droplet was formed, voltage was applied to the capacitor plates, and it was gradually increased until the pendant droplet detached from the tip. Images were recorded using a high-speed camera and analyzed for further analyzed. Correlation between the electrical charges carried by water droplets dispensed from polypropylene capillaries/pipettes and the contact area between water and tip's surface. To study the effect of the liquid-solid interfacial area on the electrification, we compared the electrical charges of 20-50 μL water droplets (green bars) with those of two-to-five 10 μL droplets (blue bars) using the electrometer set up. (B) Another elucidation of the effect of liquid-solid interfacial area on the electrification by dispensing 50 μL of deionized water in five aliquots of 10 μL from cylindrical FDTScoated glass capillaries and comparing it against conical polypropylene pipettes. The differences are statistically significant only for the conical capillaries; for cylindrical capillaries the liquid-solid interfacial area is the same for 50 μL and 5×10 μL drops. Error bars represent the standard deviation of ten measurements. Charge (nC) Single droplet (50 mL) Multiple droplets (5´10 mL)

Supplementary Section 3: Investigation of the roles of water-polypropylene and water-air interactions
We compared the extents of electrification when a volume V 1 of water was dispensed from conical polypropylene capillaries by (i) dispensing the entire volume in a single unloading, or (ii) dispensing n portions of volume V 2 = 10 μL, such that n×V 2 = V 1 . Here, the liquid-solid area was fixed, but the air-water interfacial area was larger in the second case. We found that the average electrical charges in both the scenarios were similar. Therefore, we considered the contribution of the air-water interface on the electrification to be significantly lower than the water-polypropylene interface.
Supplementary Table 1. The comparison of the charges of water droplets after dispensing from polypropylene tip either discharged at once or dispensed in smaller portions.

Volume of water (μL)
Charge of the water droplet dispensed at once ( Charges carried by water drops were measured by a Faraday cup connected to an electrometer. (B) Correlation between the total electrical charge carried by 200 µL water dispensed from PTFE tubes of inner diameters of 0.5 mm (red dots) and 1 mm (blue dots) as a function of the rate of dispensing (controlled by a syringe pump). The larger liquidsolid contact area in the tube of 0.5 mm diameter led to higher electrical charging in comparison with the tube with 1 mm diameter. (C) When the total charge in either scenario is normalized by the liquid-solid interfacial area, similar charge density is obtained. This is expected because the material composition of the tubes is the same. Error bars in each panel represent the standard deviation of five measurements. Using an electrometer, we quantified the electrical charges of tiny aliquots (50 μL) from aqueous reservoirs (1 mL) whose ionic strength was adjusted by KBr, NaCl, NaOH, and HCl, using polypropylene pipettes. The electrical charges of the reservoirs were after the withdrawals (red) were always equal and opposite to those of the aliquots (blue). The magnitude of the electrification was quite similar for KBr (1mM), NaCl (1mM), and NaOH (pH 11), but it was significantly lower for HCl (pH 3). After the experiments with pH 3 solutions, when we used the same pipettes for water, the electrification corresponded to the original surface charge density of polypropylene. These results demonstrate that acids do not necessarily "neutralize" the surface charge. (Note: the surface charge densities were obtained by normalizing the observed charges by the solid-liquid interfacial areas inside the pipettes prior to dispensing.) Error bars in each panel represent the standard deviation of ten measurements.

Derivation of an equation relating the dependence of the tilting angles, , of pendant water droplets on the excess charges, , carried by them and the applied electric field strengths, E, through a variational analysis
We estimated the excess charge of the droplets (q) as: where m is the mass of the drop, g is the acceleration due to gravity, α is a tilting angle, and E is the electric field inside the capacitor.

tan = [2]
If we express this expression in logarithmic form, we get: This equation on differentiation gives, where ∆ is the change in the tilting angle as the electric field is changed by ∆ and ∆ refers to the ion-exchange between the drop and the water reservoir above (Supplementary Figure 2, Figures 2A and 2C) This result is same as derived in Equation [10].

Supplementary Figure 7.
Electrical charges of single drops of diiodomethane, diiodomethane containing 1mM NaCl, water, and water containing 1mM NaCl, manually dispensed from polypropylene capillaries at the rate of ~3 mL min -1 . Due to the low dielectric constant of diiodomethane (ε r = 5.3), even the addition of 1 mM salt did not enhance its electrification during pipetting. In contrast, significantly higher electrification was observed at the water-polypropylene interface due to water's intrinsic ions and higher dielectric constant (ε r = 80). Addition of 1mM NaCl increased the electrification for water. Error bars represent the standard deviation of five measurements.  Figure 9. Electrical charges of water droplets manually dispensed at the rate of ~3 mL min -1 from new polypropylene capillaries (red) and after the capillaries were washed with (A) acetone or (B) methanol and dried overnight. We performed the cycle of washing with a solvent, drying and dispensing water droplets four times for each pipette. The last set of bars represents the average results of performed measurements. The charges were measured by a Faraday cup connected to an electrometer. Error bars in each panel represent the standard deviation of ten measurements.