Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface

Soft anions exhibit surface activity at the air/water interface that can be probed using surface-sensitive vibrational spectroscopy, but the structural implications of this surface activity remain a matter of debate. Here, we examine the nature of anion–water interactions at the air/water interface...

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Autores principales:	John M. Herbert, Suranjan K. Paul
Formato:	article
Lenguaje:	EN
Publicado:	MDPI AG 2021
Materias:	air–water interface Hofmeister series hydrogen bonding charge transfer symmetry-adapted perturbation theory noncovalent interactions Organic chemistry QD241-441
Acceso en línea:	https://doaj.org/article/79a8b1047bb74dfeb300a3199a423643
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spelling	oai:doaj.org-article:79a8b1047bb74dfeb300a3199a4236432021-11-11T18:39:49ZInteraction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface10.3390/molecules262167191420-3049https://doaj.org/article/79a8b1047bb74dfeb300a3199a4236432021-11-01T00:00:00Zhttps://www.mdpi.com/1420-3049/26/21/6719https://doaj.org/toc/1420-3049Soft anions exhibit surface activity at the air/water interface that can be probed using surface-sensitive vibrational spectroscopy, but the structural implications of this surface activity remain a matter of debate. Here, we examine the nature of anion–water interactions at the air/water interface using a combination of molecular dynamics simulations and quantum-mechanical energy decomposition analysis based on symmetry-adapted perturbation theory. Results are presented for a set of monovalent anions, including Cl<sup>−</sup>, Br<sup>−</sup>, I<sup>−</sup>, CN<sup>−</sup>, OCN<sup>−</sup>, SCN<sup>−</sup>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mi>NO</mi><mn>2</mn><mo>−</mo></msubsup></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mi>NO</mi><mn>3</mn><mo>−</mo></msubsup></semantics></math></inline-formula>, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mi>ClO</mi><mi>n</mi><mo>−</mo></msubsup></semantics></math></inline-formula> (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>n</mi><mo>=</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mn>3</mn><mo>,</mo><mn>4</mn></mrow></semantics></math></inline-formula>), several of which are archetypal examples of surface-active species. In all cases, we find that average anion–water interaction energies are systematically larger in bulk water although the difference (with respect to the same quantity computed in the interfacial environment) is well within the magnitude of the instantaneous fluctuations. Specifically for the surface-active species Br<sup>−</sup>(aq), I<sup>−</sup>(aq), <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mi>ClO</mi><mn>4</mn><mo>−</mo></msubsup></semantics></math></inline-formula>(aq), and SCN<sup>−</sup>(aq), and also for ClO<sup>−</sup>(aq), the charge-transfer (CT) energy is found to be larger at the interface than it is in bulk water, by an amount that is greater than the standard deviation of the fluctuations. The Cl<sup>−</sup>(aq) ion has a slightly larger CT energy at the interface, but <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mi>NO</mi><mn>3</mn><mo>−</mo></msubsup></semantics></math></inline-formula>(aq) does not; these two species are borderline cases where consensus is lacking regarding their surface activity. However, CT stabilization amounts to <20% of the total induction energy for each of the ions considered here, and CT-free polarization energies are systematically larger in bulk water in all cases. As such, the role of these effects in the surface activity of soft anions remains unclear. This analysis complements our recent work suggesting that the short-range solvation structure around these ions is scarcely different at the air/water interface from what it is in bulk water. Together, these observations suggest that changes in first-shell hydration structure around soft anions cannot explain observed surface activities.John M. HerbertSuranjan K. PaulMDPI AGarticleair–water interfaceHofmeister serieshydrogen bondingcharge transfersymmetry-adapted perturbation theorynoncovalent interactionsOrganic chemistryQD241-441ENMolecules, Vol 26, Iss 6719, p 6719 (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	air–water interface Hofmeister series hydrogen bonding charge transfer symmetry-adapted perturbation theory noncovalent interactions Organic chemistry QD241-441
spellingShingle	air–water interface Hofmeister series hydrogen bonding charge transfer symmetry-adapted perturbation theory noncovalent interactions Organic chemistry QD241-441 John M. Herbert Suranjan K. Paul Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface
description	Soft anions exhibit surface activity at the air/water interface that can be probed using surface-sensitive vibrational spectroscopy, but the structural implications of this surface activity remain a matter of debate. Here, we examine the nature of anion–water interactions at the air/water interface using a combination of molecular dynamics simulations and quantum-mechanical energy decomposition analysis based on symmetry-adapted perturbation theory. Results are presented for a set of monovalent anions, including Cl<sup>−</sup>, Br<sup>−</sup>, I<sup>−</sup>, CN<sup>−</sup>, OCN<sup>−</sup>, SCN<sup>−</sup>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mi>NO</mi><mn>2</mn><mo>−</mo></msubsup></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mi>NO</mi><mn>3</mn><mo>−</mo></msubsup></semantics></math></inline-formula>, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mi>ClO</mi><mi>n</mi><mo>−</mo></msubsup></semantics></math></inline-formula> (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>n</mi><mo>=</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mn>3</mn><mo>,</mo><mn>4</mn></mrow></semantics></math></inline-formula>), several of which are archetypal examples of surface-active species. In all cases, we find that average anion–water interaction energies are systematically larger in bulk water although the difference (with respect to the same quantity computed in the interfacial environment) is well within the magnitude of the instantaneous fluctuations. Specifically for the surface-active species Br<sup>−</sup>(aq), I<sup>−</sup>(aq), <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mi>ClO</mi><mn>4</mn><mo>−</mo></msubsup></semantics></math></inline-formula>(aq), and SCN<sup>−</sup>(aq), and also for ClO<sup>−</sup>(aq), the charge-transfer (CT) energy is found to be larger at the interface than it is in bulk water, by an amount that is greater than the standard deviation of the fluctuations. The Cl<sup>−</sup>(aq) ion has a slightly larger CT energy at the interface, but <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mi>NO</mi><mn>3</mn><mo>−</mo></msubsup></semantics></math></inline-formula>(aq) does not; these two species are borderline cases where consensus is lacking regarding their surface activity. However, CT stabilization amounts to <20% of the total induction energy for each of the ions considered here, and CT-free polarization energies are systematically larger in bulk water in all cases. As such, the role of these effects in the surface activity of soft anions remains unclear. This analysis complements our recent work suggesting that the short-range solvation structure around these ions is scarcely different at the air/water interface from what it is in bulk water. Together, these observations suggest that changes in first-shell hydration structure around soft anions cannot explain observed surface activities.
format	article
author	John M. Herbert Suranjan K. Paul
author_facet	John M. Herbert Suranjan K. Paul
author_sort	John M. Herbert
title	Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface
title_short	Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface
title_full	Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface
title_fullStr	Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface
title_full_unstemmed	Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface
title_sort	interaction energy analysis of monovalent inorganic anions in bulk water versus air/water interface
publisher	MDPI AG
publishDate	2021
url	https://doaj.org/article/79a8b1047bb74dfeb300a3199a423643
work_keys_str_mv	AT johnmherbert interactionenergyanalysisofmonovalentinorganicanionsinbulkwaterversusairwaterinterface AT suranjankpaul interactionenergyanalysisofmonovalentinorganicanionsinbulkwaterversusairwaterinterface
_version_	1718431770876051456

Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface

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