Specific Ion Effects at the Vapor-Formamide Interface: A Reverse Hofmeister Series in Ion Concentration Depth Profiles

Anand Kumar; Vincent S J Craig; Hayden Robertson; Alister J Page; Grant B Webber; Erica J Wanless; Valerie D Mitchell; Gunther G Andersson

doi:10.1021/acs.langmuir.3c01286

Specific Ion Effects at the Vapor-Formamide Interface: A Reverse Hofmeister Series in Ion Concentration Depth Profiles

Langmuir. 2023 Sep 12;39(36):12618-12626. doi: 10.1021/acs.langmuir.3c01286. Epub 2023 Aug 29.

Authors

Anand Kumar¹, Vincent S J Craig², Hayden Robertson³, Alister J Page³, Grant B Webber³, Erica J Wanless³, Valerie D Mitchell⁴, Gunther G Andersson^{1

5}

Affiliations

¹ Flinders Institute of Nanoscale Science and Technology, College of Science and Engineering, Flinders University, Adelaide, SA 5042, Australia.
² Department of Materials Physics, Research School of Physics, The Australian National University, Canberra, ACT 2601, Australia.
³ College of Science, Engineering, and Environment, University of Newcastle, Callaghan, NSW 2308, Australia.
⁴ Australian Synchrotron, ANSTO, Clayton, VIC 3168, Australia.
⁵ Flinders Microscopy and Microanalysis, College of Science and Engineering, Flinders University, Adelaide, SA 5042, Australia.

PMID: 37642667
DOI: 10.1021/acs.langmuir.3c01286

Abstract

Employing neutral impact collision ion scattering spectroscopy (NICISS), we have directly measured the concentration depth profiles (CDPs) of various monovalent ions at the vapor-formamide interface. NICISS provides CDPs of individual ions by measuring the energy loss of neutral helium atoms backscattered from the solution interface. CDPs at the vapor-formamide interface of Cl^-, Br^-, I^-, Na⁺, K⁺, and Cs⁺ are measured and compared to elucidate the interfacial specific ion trends. We report a reverse Hofmeister series in the presence of inorganic ions (anion and cation) at the vapor-formamide interface relative to the water-vapor interface, and the CDPs are found to be independent of the counterion for most ions studied. Thus, ions at the surface of formamide follow a "Hofmeister paradigm" where the counterion does not impact the ion series. These specific ion trends are complemented with surface tension and X-ray absorption near-edge structure (XANES) measurements on formamide electrolyte solutions.