Tunable Wetting of Surfaces with Ionic Functionalities
D Vanzo and D Bratko and A Luzar, JOURNAL OF PHYSICAL CHEMISTRY C, 116, 15467-15473 (2012).
Surface charges can dramatically improve the wettability of solid surfaces by water and aqueous solutions. This effect is common in nature, as in ion channel proteins, and can be used to facilitate fluid transport in microfluidic applications. While it is often possible to make reliable predictions of contact angle reduction due to a uniform electrode charge, we address the effect of discrete charge distribution on a surface carrying a pattern of ionic functionalities. We perform atomistic molecular dynamics simulations to investigate the wetting regimes at the nanoscale on molecular-brush-coated graphane surfaces. We tune hydrophilicity by covering the surface with a mixture of covalently bonded butyl and potassium butyrate (or propylammonium chloride) chains at different densities of ionizing groups. We use thermodynamic integration to determine the relation between wetting free energy and the amount of discrete charges on the substrate. We show that nanopores with oppositely charged walls in neat water feature a Lippmann-like quadratic dependence of the cosine of contact angle on the surface charge density while discretely charged surfaces surrounded by neutralizing counterions exhibit a linear dependence reminiscent of the Cassie-Baxter relation. Nonuniform surfaces show a strong dependence on the distribution pattern, with charge effects on maximally segregated surfaces about 4 times weaker than for the uniform distribution. The findings provide guidance for the design of nanopatterned materials with tailored wettability.
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