Effect of the headgroup structure on counterion binding in adsorbed surfactant films investigated by total reflection x-ray absorption fine structure spectroscopy

Yosuke Imai, Yuhei Tokiwa, Shusaku Ueno, Hajime Tanida, Iwao Watanabe, Hiroki Matsubara, Takanori Takiue, Makoto Aratono

Research output: Contribution to journalArticlepeer-review

Abstract

Competitive binding of binary mixed counterions to the headgroups of adsorbed surfactant films has been investigated at solution surfaces by total reflection X-ray absorption fine structure (XAFS) spectroscopy. The obtained extended XAFS spectra for bromide counterions are linear combinations of the spectra of fully hydrated bromide ions (free Br) and partially dehydrated bromide ions bound to the headgroups of the surfactant ions (bound Br). From the fraction of bound Br in counterion mixed systems, two series of the relative strengths of counterion binding are proposed for the trimethylammonium (TA+) and 3-methylimidazolium (MIM+) headgroups: (a) TASO4 < TACl < TABr < TABF4 and (b) MIMBr < TABr < TABF4 < MIMBF4. For the TA headgroup, matching the hydration of the headgroups and counterions gives series (a) according to Collins' law, which states that the tendency of contact ion pair formation becomes larger when the absolute values of the hydration enthalpies of the ions match. For the MIM headgroup, the number of binding sites of hydrogen bonds between the MIM headgroup and counterion is essential, which leads to series (b) because of competition between the counterion and water for interaction with the MIM headgroup.

Original languageEnglish
Pages (from-to)1487-1494
Number of pages8
JournalBulletin of the Chemical Society of Japan
Volume91
Issue number10
DOIs
Publication statusPublished - Jan 1 2018

All Science Journal Classification (ASJC) codes

  • Chemistry(all)

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