Collision dynamics and uptake of water on alcohol-covered ice

Abstract

Molecular scattering experiments are used to investigate water interactions with methanol and n-butanol covered ice between 155 K and 200 K. The inelastically scattered and desorbed products of an incident molecular beam are measured and analyzed to illuminate molecular scale processes. The residence time and uptake coefficients of water impinging on alcohol-covered ice are calculated. The surfactant molecules are observed to affect water transport to and from the ice surface in a manner that is related to the number of carbon atoms they contain. Butanol films are observed to reduce water uptake by ice by 20%, whereas methanol monolayers pose no significant barrier to water transport. Water colliding with methanol covered ice rapidly permeates the alcohol layer, but on butanol has mean surface lifetimes of ≲0.6 ms, enabling some molecules to thermally desorb before reaching the water ice underlying the butanol. These observations are put into the context of cloud and atmospheric scale processes, where such surfactant layers may affect a range of aerosol processes, and thus have implications for cloud evolution, the global water cycle, and long term climate

Surface Transformations and Water Uptake on Liquid and Solid Butanol near the Melting Temperature

“…We have studied the dynamics and kinetics of D 2 O interactions with solid and liquid nbutanol from 160 to 200 K. The experimental results consist of TOF spectra that are further analyzed to determine desorption rate constants and the probabilities for inelastic scattering, thermal desorption and bulk uptake of water on butanol surfaces prepared by different procedures. previously been observed for D 2 O collisions with butanol and methanol monolayers on water ice surfaces, 12,21 as well as in gas collisions with water ice surfaces. 39,40 Figure 2 shows the absolute TD probability and the relative IS intensity as a function of temperature.…”

“…The angular distribution for the TD component is described by a symmetric cosine distribution and measurements in a single direction are sufficient to calculate the total desorbing flux. 21 The absolute IS intensity can on the other hand not be determined since (Figures 2a and 3). Thus, passing the bulk melting point does not have a singular effect, rather changes appear gradually between 180 and 190 K.…”

“…19,20 Recent molecular beam experiments show that the effect of a methanol monolayer on ice is negligible, while a butanol layer on ice reduces uptake by 20-40% compared to pure ice. 21 Molecular dynamics simulations of butanol-covered liquid water suggest that water condensation is reduced by a factor of three at 300 K. 22 However, other studies on the water evaporation from supercooled sulfuric acid through butanol films show little to no effect of the butanol. 23,24 These differing results suggest that the water interactions are quite sensitive to the detailed chemical and physical properties of adsorbed alcohol surface layers.…”

Rapid Water Transport through Organic Layers on Ice

“…Atmospheric droplets and ice particles are often coated with organic compounds, 1 which potentially change chemical reactivity 2−6 and condensation and evaporation processes 7,8 and thereby influence the properties and actions of aerosols and clouds. 9,10 Such interfacial organic layers interrupt the molecular interactions between water vapor and the water surface and may thereby influence water accommodation on ice and droplets characterized by mass accommodation coefficient (α), 11 i.e., the ratio of the number of molecules absorbed into bulk water to the number of impinging molecules. An important factor determining the barrier effect is the size and the structure of the organic molecules.…”

“…Recently, several environmental molecular beam (EMB) studies pointed out that even though the apparent water accommodation coefficient on ice with an organic coating, e.g., n-butanol or n-hexanol, does not differ from that of clean ice, the mechanisms of the sorption and mass exchange may be significantly different. 11,20 Gas water molecules are efficiently thermalized once they collide with a clean ice surface, 21 but before being incorporated into a strongly bonded state, the molecules are in a precursor state where about 1−2 hydrogen bonds are formed. These weakly bound molecules may desorb from an ice surface in milliseconds around 200 K. 21 However, when the ice is covered by an n-butanol film, a fraction of the impinging water molecules trap and rapidly desorb while the majority become more strongly bound and remain on the ice.…”