Collisions of DCl with Pure and Salty Glycerol:  Enhancement of Interfacial D → H Exchange by Dissolved NaI

Abstract

The fate of DCl molecules striking pure glycerol and a 2.6 M NaI-glycerol solution is investigated using scattering, uptake, and residence time measurements. We find that dissolved Na+ and I- ions alter every gas-liquid pathway from the moment of contact of DCl with the surface to its eventual emergence as HCl. In particular, the salt enhances both trapping-desorption of DCl and interfacial DCl --> HCl exchange at the expense of DCl entry into the bulk solution. The reduced entry and enhanced desorption of thermalized DCl molecules are interpreted by assuming that Na+ and I- ions bind to interfacial OH groups and tie up surface sites that would otherwise capture incoming DCl molecules. These ion-glycerol interactions may also be responsible for enhancing interfacial D --> H exchange by disrupting the interfacial hydrogen bond network that carries the newly formed H+ ion away from its Cl- pair. This disruption may increase the fraction of interfacial Cl- and H+ that recombine and desorb immediately as HCl before the ions separate and diffuse deeply into the bulk.

Microjets and coated wheels: versatile tools for exploring collisions and reactions at gas–liquid interfaces

“…These entry probabilities can be readily measured when the incident gas beam is contained within the larger exposed area of the coated wheel. 33,38 Second, the typical velocity of a 15 mm radius jet is 30 m s À1 , permitting a viewing time of 100 ms over a 3 mm wide detection window. In contrast, the wetted wheel rotates slowly, allowing measurements of desorbing species over times of 0.05 to 1 s, or up to 10 4 times longer than for the microjet.…”

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

“…The ratios of the evaporation signals from the salty and pure solutions are equal to the ratio of vapor pressures P and therefore equal to the solvent activity P salt /P pure graphed in panel b. 17 The dashed line is the prediction P salt ) x solv P pure based on ideal mixing of the individual ions, where the solvent mole fraction is x solv ) n solv / (n solv + n cation + n anion ). The activities of glycerol drop steadily as the solvent mole fraction decreases, reaching 0.18 for the 4.3 M NaI glycerol solution (x solv ) 0.57).…”

“…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.…”

“…Most DCl recoil directly from the surface at collision energies exceeding 40RT (channel (a) in the scheme below). 17 Under thermal collision conditions, where <E inc > ) 2RT, nearly all impinging DCl molecules are momentarily trapped in the interfacial region (b). 19 For pure glycerol at 292 K, we find that 10% of the thermally equilibrated DCl molecules desorb immediately back into the gas phase before they react (c), while 87% dissolve and dissociate into Cl -and D + , followed by D f H exchange and slow desorption of HCl and HI(d).…”