Adsorption of thiocyanate ions to the dodecanol/water interface characterized by UV second harmonic generation

129

162

Uncertainties in quantifying the kinetics of evaporation and condensation of water from atmospheric aerosol are a significant contributor to the uncertainty in predicting cloud droplet number and the indirect effect of aerosols on climate. The influence of aerosol particle surface composition, particularly the impact of surface active organic films, on the condensation and evaporation coefficients remains ambiguous. Here, we report measurements of the influence of organic films on the evaporation and condensation of water from aerosol particles. Significant reductions in the evaporation coefficient are shown to result when condensed films are formed by monolayers of long-chain alcohols [C n H (2n+1) OH], with the value decreasing from 2.4 × 10 −3 to 1.7 × 10 −5 as n increases from 12 to 17. Temperature-dependent measurements confirm that a condensed film of long-range order must be formed to suppress the evaporation coefficient below 0.05. The condensation of water on a droplet coated in a condensed film is shown to be fast, with strong coherence of the long-chain alcohol molecules leading to islanding as the water droplet grows, opening up broad areas of uncoated surface on which water can condense rapidly. We conclude that multicomponent composition of organic films on the surface of atmospheric aerosol particles is likely to preclude the formation of condensed films and that the kinetics of water condensation during the activation of aerosol to form cloud droplets is likely to remain rapid.water transport | organic monolayers | mass accommodation | surface films T he transport of volatile and semivolatile components between the condensed and gas phases in aerosol during evaporation or condensation is important for understanding cloud microphysics, atmospheric chemistry, the delivery of drugs to the lungs, and the applications of aerosols in materials and combustion science. For example, the uptake of chemical species to atmospheric aerosol particles facilitates chemical reactions (1), the condensation of water on cloud condensation nuclei allows cloud droplets to form (2), the uptake of water on pharmaceutical aerosol can affect their deposition pattern within the lungs (3), and the evaporation rates of solvents in spray drying processes influence the structure and morphology of the final microparticle (4). The rate of mass transport to or from an aerosol particle can be considered to involve the interplay of three mechanistic steps: gas phase diffusion, interfacial transfer, and bulk condensed phase diffusion. Different limiting regimes can be identified under which the rate of evaporation or condensation is determined by any one of these steps (5).The Knudsen number is the ratio of the mean free path of gas phase molecules to the particle radius. At low Knudsen number, in the limits of large particle size and high gas pressure, gas phase diffusion usually determines evaporation and condensation rates. Bulk phase diffusion is limiting when the viscosity of the particle impedes transport to or from the surfac...

Section: Resultssupporting

confidence: 88%

Influence of organic films on the evaporation and condensation of water in aerosol

Davies

Miles

Haddrell

et al. 2013

129

162

“…The general approach and apparatus used in our resonanceenhanced UV second harmonic generation (SHG) experiments are described elsewhere in connection with our previous studies of thiocyanate as a prototype for the behavior of chaotropic ions (8,22,23). For details see Methods.…”

Section: Resultsmentioning

confidence: 99%

“…1 to a Langmuir adsorption model (22,23), determining a Gibbs free energy of adsorption (ΔG) at each temperature (see Fig. 2).…”

Section: Resultsmentioning

confidence: 99%

Elucidating the mechanism of selective ion adsorption to the liquid water surface

Otten

Shaffer²,

Geissler³

et al. 2012

Self Cite

187

342

Adsorption of aqueous thiocyanate ions from bulk solution to the liquid/vapor interface was measured as a function of temperature by resonant UV second harmonic generation spectroscopy. The resulting adsorption enthalpy and entropy changes of this prototypical chaotrope were both determined to be negative. This surprising result is supported by molecular simulations, which clarify the microscopic origins of observed thermodynamic changes. Calculations reveal energetic influences of adsorbed ions on their surroundings to be remarkably local. Negative adsorption enthalpies thus reflect a simple repartitioning of solvent density among surface, bulk, and coordination regions. A different, and much less spatially local, mechanism underlies the concomitant loss of entropy. Simulations indicate that ions at the interface can significantly bias surface height fluctuations even several molecular diameters away, imposing restrictions consistent with the scale of measured and computed adsorption entropies. Based on these results, we expect an ion’s position in the Hofmeister lyotropic series to be determined by a combination of driving forces associated with the pinning of capillary waves and with a competition between ion hydration energy and the neat liquid’s surface tension.

“…Furthermore, using UV SHG spectroscopy, Onorato et al (25) measured similar free energies for SCN − adsorption to the dodecanol/water interface. Based on interfacial affinities alone, it would appear that the effect of a hydrophobic boundary is generic, regardless of whether the boundary is a rigid material like graphene or instead a coexisting vapor phase.…”

Section: Discussionmentioning

confidence: 90%

Mechanism of ion adsorption to aqueous interfaces: Graphene/water vs. air/water

McCaffrey

Nguyen

Cox

et al. 2017

Self Cite

The adsorption of ions to aqueous interfaces is a phenomenon that profoundly influences vital processes in many areas of science, including biology, atmospheric chemistry, electrical energy storage, and water process engineering. Although classical electrostatics theory predicts that ions are repelled from water/hydrophobe (e.g., air/water) interfaces, both computer simulations and experiments have shown that chaotropic ions actually exhibit enhanced concentrations at the air/water interface. Although mechanistic pictures have been developed to explain this counterintuitive observation, their general applicability, particularly in the presence of material substrates, remains unclear. Here we investigate ion adsorption to the model interface formed by water and graphene. Deep UV second harmonic generation measurements of the SCN − ion, a prototypical chaotrope, determined a free energy of adsorption within error of that for air/water. Unlike for the air/water interface, wherein repartitioning of the solvent energy drives ion adsorption, our computer simulations reveal that direct ion/graphene interactions dominate the favorable enthalpy change. Moreover, the graphene sheets dampen capillary waves such that rotational anisotropy of the solute, if present, is the dominant entropy contribution, in contrast to the air/water interface.specific ion effects | graphene | SHG spectroscopy | molecular dynamics | adsorption O ver a decade ago, detailed simulations predicted that certain simple ions would exhibit enhanced concentrations at the air/water interface (1), reinvigorating a century-old debate primarily addressing the origin of the famous Hofmeister effects in protein chemistry (2). Experiments subsequently verified these predictions (3, 4). Surface enhancement of simple ions has since been widely studied [e.g., see the review by Jungwirth and Tobias (5)] and attributed to various properties of the ions including size, polarizability, dispersion forces, hydration free energy (6-8), and the degree of interfacial roughness (9-11). Previous temperaturedependent deep UV (DUV) second harmonic generation (SHG) experiments from the Saykally group have determined the enthalpic and entropic contributions to the adsorption of the prototypical pseudohalide, the thiocyanate (SCN − ) ion, showing that a negative enthalpy change drives the ion adsorption, whereas a negative entropy change impedes it (9). Using molecular simulations performed in the same study, the following underlying mechanism was proposed: first, when the ion moves from the bulk to the interface, weakly interacting water molecules are displaced from both the surface and the ion solvent shell into the bulk solution, where they form stronger water-water bonds, leading to the negative enthalpy change. Second, the presence of an ion at the interface dampens its capillary wave fluctuations, leading to the negative entropy change. Although a consensus has yet to be reached regarding the complete theory of interfacial ion adsorption, these collective, detailed studie...