Water adsorption on hydrophilic and hydrophobic self-assembled monolayers as proxies for atmospheric surfaces. A grand canonical Monte Carlo simulation study

Abstract

Grand canonical Monte Carlo simulations are used to determine water adsorption on prototypical organic surfaces as a function of relative humidity at 300 K. Three model surfaces formed by well-ordered self-assembled monolayers (SAMs) of alkanethiolate chains on gold are investigated: (i) a smooth hydrophobic surface of methyl-terminated C(7)-CH(3) SAM; (ii) a rough hydrophobic surface of randomly mixed two-component SAM, composed of equal fractions of C(5)-CH(3) and C(7)-CH(3) chains (C(5)/C(7)-CH(3) SAM); and (iii) a smooth hydrophilic surface of carboxyl-terminated C(7)-COOH SAM. The all atom CHARMM22 force field is used for the SAM chains together with the SPC/E model for water. No noticeable water adsorption is observed on the smooth hydrophobic surface up to saturation. The mild surface roughness introduced by the uneven chain length of the two components constituting the C(5)/C(7)-CH(3) SAM has no significant effect on the surface hydrophobicity, and the rough hydrophobic surface also remains dry up to the point when water condensation occurs. In contrast, water readily adsorbs onto the hydrophilic surface by forming hydrogen bonds with the COOH groups of the substrate. In addition, hydrogen bonding with pre-adsorbed water molecules contributes to the mechanism of water uptake. Under low humidity conditions, water is present on the hydrophilic surface as individual molecules or small water clusters and, with increasing relative humidity, the surface coverage grows continuously beyond a monolayer formation. The adsorbed water film is observed to be rather inhomogeneous with patches of bare surface exposed. The amount of water constituting a stable adsorption layer prior to condensation is estimated to consist of about 2-5 molecular layers. Detailed analysis of the simulation results is used to obtain important insights into the structure and energetics of water adsorbed on highly oxidized organic surfaces exposed to ambient air of increasing relative humidity.

Nanoscale condensation of water on self-assembled monolayers

“…These results are consistent with molecular dynamics simulations of Szori and coworkers where no noticeable water adsorption was observed up to the point of saturation for both smooth and slightly roughened (mixtures of -C 6 H 13 /-C 8 H 17 ) methyl-terminated SAMs. 35 The chloro-terminated monolayer showing an increase in thickness due to the adsorbed water. Fig.…”

“…Recently published molecular dynamics calculations by Szori and co-workers for SAMs of -C 7 -COOH also support this observation. 35,36 For small chemical potentials (vapour pressures), water adsorption isotherms for this surface were found to increase exponentially in a continuous fashion beyond the point of saturation of the first molecular layer of water. These calculations also indicate that already adsorbed water molecules contribute to the driving force that governs the adsorption of further water molecules.…”

“…In the absence of extensive experimental investigations, computational studies such as those by Szori et al 35,36 have shed significant light on the interactions between hydrophobic and hydrophilic thiol-based SAMs on gold, with atmospheric water at a range of vapour pressures at 300 K. Their grand canonical Monte Carlo calculations revealed essentially no water adsorption on hydrophobic alkyl-terminated surfaces over the entire range of chemical potentials up to the point of saturation, where water condensation occurred. In contrast, they found that water readily adsorbed on a hydrophilic C7-COOH SAM even at low vapour pressures.…”

Effects of relative humidity and particle and surface properties on particle resuspension rates

“…In the past decade, adhesion force studies using atomic force microscope (AFM) have yielded a more complex picture of capillary forces (Bocquet et al 1998;Halsey and Levine 1998;Ata et al 2002;Jones et al 2002;Rabinovich et al 2002;Farshchi-Tabrizi et al 2006;Paajanen et al 2006;Farshchi-Tabrizi et al 2008;Butt and Kappl 2009;Szori et al 2010). These studies show that in addition to RH, capillary force strength is influenced by surface chemistry and geometry (Jones et al 2002), especially the nanoscale roughness of both the particles and the surfaces Hubbard et al 2012).…”

Adsorption of Methylene Fluoride and Methylene Chloride at the Surface of Ice under Tropospheric Conditions: A Grand Canonical Monte Carlo Simulation Study

“…Thus, performing a set of GCMC simulations in which the chemical potential of the adsorbate molecules is systematically varied, the number of the adsorbed molecules per surface unit vs. the chemical potential, i.e., the adsorption isotherm can easily be determined from extremely low pressures up to the point of condensation. Indeed, the GCMC method has been successfully applied to calculate the adsorption isotherms of water and other small molecules at various different solid surfaces, such as at carbonaceous materials, 15-23 self-assembled monolayers, 24,25 covalent organic frameworks, [26][27][28] protein crystals, 29 metal oxides, [30][31][32][33] zeolites, [34][35][36][37][38][39][40][41] kaolinite, [42][43][44] and ice. [45][46][47][48][49][50][51][52][53] Further, besides the adsorption isotherms themselves, the structure and energetics of the adsorption layer can also be analyzed in detail in such simulations.…”