Interfacial Interactions of DCl with Salty Glycerol Solutions of KI, NaI, LiI, and NaBr

Abstract

Gas−liquid scattering experiments are used to explore the role of interfacial ions in controlling collisions and reactions of DCl molecules with ∼2.5 M KI, NaI, LiI, and NaBr glycerol solutions. DCl molecules that thermalize upon collision with the surface of each solution follow one of three pathways: they may desorb immediately as DCl, undergo D → H exchange in the near-interfacial region and desorb as HCl, or dissolve and dissociate in the bulk. We find that each salt amplifies the differences among the channels: the fraction of DCl molecules that desorb intact increases from 9% for pure glycerol to 18% on average for the salt solutions, interfacial DCl → HCl exchange increases from 3 to 7%, and bulk dissolution drops from 87 to 75%. Dissolved ions may enhance nonreactive DCl desorption and interfacial D → H exchange by tying up glycerol OH groups, which will impede both DCl−OH bonding and migration of D+ and H+ away from Cl- after DCl ionization. Despite differences in the sizes of the ions and their interactions with the solvent, the enhancements are found to be similar for NaI, LiI, and NaBr and slightly smaller for KI. These similarities may arise from an interplay between ion pairing, the strength and number of ion−solvent bonds, and ion exposure at the surface.

Long range interactions in nanoscale science

“…Neilson et al, 2001;Muenter et al, 2007͒. Neutron wide-angle, small-angle, and ultrasmall-angle scattering has been extensively used to identify structures ͑scattering density contrasts͒ in complex fluids, polymer blends, and solid phases angstrom to millimeter length scales.…”

Atomic and Molecular Collisions at Liquid Surfaces

“…Relative to pure glycerol, solutions containing NaI and CaI 2 slightly increase the peak translational energy of IS, inhibit DCl trapping at high collision energies, and promote trapping-desorption and trapping-exchange-desorption at the expense of trapping-solvation. These observations suggest that NaI, CaI 2 , and NaBr make the glycerol surface mechanically stiffer and smoother, and it is hypothesized that these salts modulate the post-trapping pathways of DCl by decreasing the availability of the glycerol hydroxyl groups that promote DCl dissociation and H + /D + transport (15,105,106). Like NaI, CaI 2 , and NaBr, glycerol solutions of the ionic surfactant tetrahexylammonium bromide (THABr) also promote DCl trapping-desorption and trapping-exchangedesorption at the expense of trapping-solvation.…”

The Roles of Salt Concentration and Cation Charge in Collisions of Ar and DCl with Salty Glycerol Solutions of NaI and CaI2

“…These values yield branching fractions of p trap-desorb /p trap ) 10.2 ( 0.9%, p ied /p trap ) 2.7 ( 0.2%, and p solv /p trap ) 87.1 ( 0.9%, which differ slightly from those reported earlier. 17,18 They show that ∼9 out of 10 DCl molecules that adsorb on the surface enter glycerol as molecular DCl or as Cl -and D + /H + after dissociating first at the surface. Separate experiments demonstrate that, regardless of their form upon entry, DCl exists within solution as dissociated ions, which recombine and desorb as HCl on a 0.1 s time scale after D + becomes scrambled among the protic H atoms.…”

“…An analogous mechanism may operate in our experiments if DCl molecules bind directly to exposed Na + and Ca 2+ ions, which could capture DCl long enough for D f H exchange to occur between it and a neighboring OH group. 18 In separate 17 O and 1 H NMR measurements, the average proton exchange time between water molecules in aqueous salt solutions was shown to vary with the addition of salts. 64,65 The exchange rate dropped by 10-fold from 0 to 8 m NaI, from which it was inferred that the ions disrupt water-water hydrogen bonds and therefore impede proton shuttling, as also inferred in our studies.…”