How Interfaces Affect the Acidity of the Anilinium Ion. Issue 11 (16th February 2016)

• Record Type: Journal Article
• Authors: Sripradite, Jarukorn; Miller, Susannah A.; Johnson, Michael D.; Tongraar, Anan; Crans, Debbie C.
• Is Part Of: Chemistry. Volume 22:Issue 11(2016)
• Page Start: 3873
• Page End: 3880
• Publication Date: 2016-02-16
• Subjects: acidity -- anilinium ion -- interface effects -- micelles -- pKa; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201504804
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: The acidity of a compound is a fundamental property that dictates molecular speciation and reactivity in solution. Measurements of acidity of simple molecules in interfacial environments are rarely carried out but assumptions often are made that the difference is sufficiently small that the change can be ignored. The effect of oil–surfactant–water interfaces in reverse micellar systems on the p K a value of the anilinium ion was measured using titrations by NMR spectroscopy as the size of the bis(2‐ethylhexyl)sulfosuccinate (AOT)/isooctane reverse micelles decreased. The p K a was observed to drop from 4.85±0.02 to 4.62±0.02 in water as the reverse micelle decreased from w 0 10 to 4 (that is down to a reverse micellar radius of about 2 nm). NOSEY experiments demonstrated that the aniline moiety resides within the surfactant interface with the amine/ammonium moiety protruding into the waterpool bridging the interface. The presence of the aniline was found to have modest and variable effect on the size of the reverse micelles as observed using dynamic light scattering. Our experimental results provide information important to theoretical studies, which explore interface phenomena and provide a framework for information on such simple molecules. These studies quantitate the small but significant effect on the p K a values upon placement of an aromatic amine molecule at a hydrophilic–hydrophobic interface. Abstract : On the interface of it all : The effect of oil–surfactant–water interfaces in reverse micellar systems on the p K a value of the anilinium ion as the size of reverse micelles decrease was measured using titrations by NMR spectroscopy (see figure). The p K a was observed to drop in water. NOSEY experiments demonstrated that the aniline moiety resides within the surfactant interface with the amine/ammonium moiety protruding into the waterpool bridging the interface.