title: How Interfaces Affect the Acidity of the Anilinium Ion.
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 pKa 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 pKa 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 pKa values upon placement of an aromatic amine molecule at a hydrophilic-hydrophobic interface.
authors.affiliation: School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand.|Department of Chemistry, Colorado State University, Fort Collins, CO, 80523, USA.|College of Industrial Technology, King Mongkut's University of Technology North Bangkok, Bangkok, 10800, Thailand.
authors.forename: Jarukorn
authors.initials: J
authors.lastname: Sripradite
authors.affiliation: Department of Chemistry, Colorado State University, Fort Collins, CO, 80523, USA.
authors.forename: Susannah A
authors.initials: SA
authors.lastname: Miller
authors.affiliation: Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, NM, 88003, USA.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Johnson
authors.affiliation: School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand.
authors.forename: Anan
authors.initials: A
authors.lastname: Tongraar
authors.affiliation: Department of Chemistry, Colorado State University, Fort Collins, CO, 80523, USA. crans@lamar.colostate.edu.
authors.forename: Debbie C
authors.initials: DC
authors.lastname: Crans
keywords: acidity; anilinium ion; interface effects; micelles; pKa