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Ion at air-water interface enhances capillary wave fluctuations: Energetics of ion adsorption Yanbin Wang, Shayandev Sinha, Parth Rakesh Desai, Haoyuan Jing, and Siddhartha Das J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06205 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018
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Journal of the American Chemical Society
Ion at air-water interface enhances capillary wave fluctuations: Energetics of ion adsorption
YanbinWang, Shayandev Sinha, Parth Rakesh Desai, Haoyuan Jing, and Siddhartha Das*
Department of Mechanical Engineering, University of Maryland, College Park, MD 20742
*Email:
[email protected] 1
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Abstract Recent simulations provide the energetics of ion adsorption at the air-water (a/w) interface: the presence of the ion at the interface suppresses the fluctuations of the capillary waves (CWs) reducing the entropy and also displaces the weakly interacting water molecules to the bulk causing a reduction in the enthalpy. Here we provide atomistic simulation-based evidence that the inferences of the existing studies stem from considering a small simulation volume that pins the CWs. For an appropriate size of the simulation system, ion at the a/w interface enhances the CW fluctuations. Furthermore, we discover that the characteristics of the waves governing these enhanced CW fluctuations ensure a significant decrease in the pressure-volume work causing the enthalpy decrease, while the same wave characteristics of the CWs become responsible for an entropy decrease. Overall, the paper revisits the free energy picture of this fundamental problem of ion adsorption at the a/w interface.
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INTRODUCTION The problem of ion adsorption at air-water (a/w) interface is significant to better understand a large number of events ranging from Hoffmeister effects in protein chemistry (1), light-induced conversion of hailde ions to halogen atoms on the surface of a water drop (2,3), use of the a/w interface for promoting certain reactions (4,5), regulating the size and composition of atmospheric aerosols that play a critical role in thunderstorm activity, lightning production, and precipitation (6,7), hydrolysis of SO2 in cloud droplets responsible for producing acid rain (8), oxidation of chloride ion to chlorine gas contributing to the ozone formation in the polluted marine boundary layer (9,10), dictating the preferential ion adsorption to proteins at the air-water interface (11), improve the efficiency of the fabrication of semiconductor thin-film transistors (12), and many more. Extensive studies over the past couple of decades have opened up a large number of issues that dictate the preferential adsorption/desorption of a variety of different ions at the air water interface (13-30). Several of these studies attempted to provide a comprehensive understanding of the ion adsorption/desorption process by specifically studying the changes in the internal energy, enthalpy, and entropy, and the resulting changes in the Helmholtz and Gibbs free energy as the ion moves from the bulk to the interface (26-30). Two key inferences emerge from these analyses: (a) the presence of the ions at the interface dampens the fluctuations of the capillary waves (CWs) thereby decreasing the entropy (i.e., ΔS10σ and l > lzone. For the present case, we satisfy this condition by choosing l=5.2 nm (see the Materials and Methods section) and obviously, the longest wavelength that dictates the ion-CW interactions ~ l. Given that the periodic boundary condition of the system would always imply that a second ion is present at a distance of l from the first ion, in case l < lzone, the water molecules within the simulation box would be influenced by this second ion yielding a wrong estimate of all the quantities.
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Predominance of the contribution of a mode of a given wavelength (see Fig. 3) ensures that the pressure fluctuations characterizing the CWs are significantly reduced with the ion at the interface. This is established in Fig. S2 showing a much smaller (quantified by a larger negative value) of ΔP as the ion approaches the a/w interface. On the other hand, the CWs with the ion in the bulk has a uniform distribution of the contribution from the constituting modes (in other words, there is a uniform distribution of Aξ/A0 ratio corresponding to different modes with the ion in the bulk) (see Fig. 3). This results in a larger value of pressure fluctuations, evidenced by a larger value of ΔP with the ion in the bulk (see Fig. S2). This large difference in ΔP as the ion approaches the a/w interface from the bulk causes a significant lowering of the pressure-volume work Δ(PV)=VΔP+PΔV≈ VΔP (since the change in volume is negligible as the ion moves from the bulk to the a/w interface, see Fig. S1). This significantly negative Δ(PV) ensures a negative (or favorable) enthalpy change, i.e., ΔH=ΔU+Δ(PV)0) [see Fig. S7(b) and compare with Fig. 5 below] and –TΔS (–TΔS
