Presence of electrolyte promotes wetting and hydrophobic gating in nanopores with residual surface charges

Abstract

Hydrophobic nanopores provide a model system to study hydrophobic interactions at the nanoscale. Such nanopores could also function as a valve since they halt the transport of water and all dissolved species. It has recently been found that a hydrophobic pore can become wetted i.e. filled with condensed water or an aqueous solution of salt when a sufficiently high electric field is applied across the membrane. The wetting process is reversible thus when the voltage is lowered or switched off, the pore comes back to a closed state due to water evaporation in the pore. In this manuscript we present experimental studies on how the switching between conducting and non-conducting states can be regulated by the electrolyte concentration. Transport properties of single nanopores modified with alkyl chains of different lengths were recorded in salt concentrations between 10 mM and 1 M KCl. Nanopores modified with propyl chains exhibited gating in 10 mM KCl and were open for ionic transport for all voltages at higher salt concentrations. Nanopores modified with decyl chains did not conduct current in 10 mM and exhibited repeatable hydrophobic gating in 100 mM and 1 M KCl. The results are explained in the context of Maxwell stress in confined geometry with local surface charges, which change the shape of the water-vapor interface and promote wetting.

Nonlinear Ion Transport through Ultrathin Metal–Organic Framework Nanosheet

“…The shape of I − V curves and the magnitude of ionic current barely changed with the different KCl concentrations of 0.01, 0.1, 0.2, 0.5, 1 and 2 m in acid condition (Figure S14, Supporting Information), which was different from the concentration‐dependent wetting behavior with the contact angles from 75° to 79°. [ 52 ] The concentration‐independent I − V curves further confirmed there was weak charge effect in hydrophobic nanopore at pH 2 due to the extreme hydrophobicity with the contact angle of 175°. In contrast, the dramatic changes were observed in basic environment with the changes of KCl concentrations (Figure S15, Supporting Information).…”

Gating of Hydrophobic Nanopores with Large Anions

“…The locally enhanced ion concentration could promote wetting, because previous experiments from our group indicated that wetting of hydrophobic nanopores is enhanced in high ionic strength solutions. 23 In order to elucidate which of these phenomena dominates, we prepared a nanopore with one entrance containing a negatively charged polyelectrolyte while the other entrance remained hydrophobic. This system was created by subjecting a nanopore, as schematically shown in Figure 1F, to a solution of polyglutamic acid (PGA).…”

Microstructural and associated chemical changes during the composting of a high temperature biochar: Mechanisms for nitrate, phosphate and other nutrient retention and release

“…9 indicate that when water enters the internal pores of BC it becomes constrained and that cations and anions will be tightly held. Innes et al (2015) noted that salt solutions can be tightly held in nanopores depending on the surface charge and the type and concentration of functional groups on the pore surface. XPS results indicated that nitrates and phosphates were detected in BCC after crushing (which allows analysis of the inner surfaces of the particle).…”