Ions at Water Surfaces 1.2 M Aqueous Sodium Halide Solutions Top View Side View Jungwirth, P.; Tobias, D.J. Chem. Rev.2006, 106, 1259-1281.
Salty Glycerol Solutions What happens when gaseous DCl collides with pure glycerol? Near-Interfacial DH Exchange (3%) Incident Molecule Inelastic Scattering DCl DCl HCl DCl Non-Reactive Desorption (10%) HCl Desorption After Bulk Solvation Glycerol Vapor Pressure = 10-4 Torr Viscosity = 1400 cP Dielectric Constant = 44 Surface Tension = 63 dyn cm-1 No surface dangling hydrogen bonds H+ H+ / D+ Cl- Bulk Solvation (87%) Bulk residence time of HCl = 0.2 s Cl- Salts alter non-reactive and reactive pathways Concentration Cation Charge Identity 1.2, 2.6, 3.9 M NaI 0.7, 1.3 M CaI2 NaI, NaBr, KI, LiI, and KF…a surprise! Na+ (1.16 Å) Ca2+ (1.14 Å)
Molecular Beam Scattering Experiments Liquid Reservoir 90 kJ mol-1 DCl Beam Nozzle Skimmer Post-Chopper Time-of-Flight Wheel Scraper Exiting HCl / DCl Molecules Rotating Glass Wheel Mass Spectrometer Pure or Salty Glycerol Solution Double-Decker Reservoir System
Ar Scattering Pathways Ar TD IS 45 45 Pure / Salty Glycerol Are Ions Present at the Surface? Inelastic Scattering (IS) Surface Mass Ratios Experimental Data “Planar Cut” Prediction Ar Signal Pure 3.9 M NaI Maxwell-Boltzmann fits at 291 K ThermalDesorption (TD) 0 Arrival Time (µs) 800
Ions Enhance Non-Reactive Pathways Inelastic Scattering (IS) More DCl scatters from salt solutions DCl Scattering Pathways TD 90 kJ mol-1 (36RT) IS 45 45 Pure / Salty Glycerol DCl Signal More DCl undergoes non-reactive desorption from salt solutions Pure 2.6 M NaI Thermal Desorption (TD) 0 Arrival Time (µs) 800
Isolating Near-Interfacial Exchange DClHCl Exchange in Glycerol HCl Scattering Pathways 2.6 M NaI in Glycerol 90 kJ mol-1 Interfacial Exchange 45 50 µs pulses = 10-6 s = 10-5 s HCl Signal Pure / Salty Glycerol Pure Glycerol Maxwell-Boltzmann fits convoluted with a characteristic bulk residence time, , at 291 K 0 1200 Arrival Time (µs)
DCl HCl DCl H+ Cl- DCl DCl Changing Cation & Anion Identity 1.0 Note: All salt solutions are ~2.6 M Bulk Solvation 0.8 Non-Reactive Desorption 0.6 Near-Interfacial Exchange Fraction of Trapped DCl Molecules 0.4 0.2 PureKI NaI LiI NaBr PureKI NaI LiI NaBr PureKI NaI LiI NaBr The addition of alkali halide salts to pure glycerol increases the fraction of DCl molecules that thermalize on the surface and leave as DCl or undergo an near-interfacial DH exchange. Muenter, A.H.; DeZwaan, J.L.; Nathanson, G.M. J. Phys. Chem. C2007.
Concentration & Charge Effects Non-Reactive DCl Desorption Near-Interfacial DH Exchange CaI2 CaI2 NaI NaI Fraction of Trapped DCl Fraction of Trapped DCl Salt Concentration (Molar) Salt Concentration (Molar) Bulk Solvation Near linear trends for NaI implies no changes in interactions in the interface at high concentrations NaI When [I-] = 2.6 M, Ca2+ has a larger effect than Na+ Fraction of Trapped DCl CaI2 Salt Concentration (Molar)
Cl- H+ C H O Interfacial Interactions Incoming DCl molecules need 3 hydrogen bonding sites to form a contact ion pair and dissociate Ions can take up these bonding sites, enhancing DCl desorption and scattering Chorny, I.; Benjamin, I. J. Phys. Chem. B, 2004, 108, 995. Ions can interrupt the proton relay, increasing near-interfacial DH exchange Once trapped, DCl molecules can dissociate:
Eliminating Interfacial Reactivity CaI2, NaI, LiI, KI, NaBr have all been shown to enhance interfacial reactivity, but how can we eliminate it? For a 4 M KF solution KF eliminates near-interfacial DClHCl exchange Incident Molecule Near-Interfacial Exchange Inelastic Scattering DCl DCl HF DCl Non-Reactive Desorption (5%) X HF Desorption F- H+ D+ Cl- K+ Bulk Solvation (95%) F- Cl- H+ + F- HF
KF Suppresses DCl Desorption! Inelastic Scattering (IS) Non-Reactive DCl Desorption Fraction of Trapped DCl DCl Signal KF Concentration (M) Pure 4 M KF 1.3 M KF Thermal Desorption (TD) 0 800 Arrival Time (µs)
Interfacial DCl Scavenging by F- A possible mechanism… DCl HF Cl- D+ Cl- F- F- DF K+ Hydrogen Bond Strengths1 DH Exchange Bond kJ mol-1 F-…HF 163 F-…HOCH3 125 Cl-…HCl 96 F-…HCl 92 I- …HCl 58 HF H+ + F- Ka(H2O) = 1 10-3 1Klepeis, N. E. et al. J. Phys. Chem. 1993,99, 3865.
Future Work with Cold, Salty Water Ar Scattering from 7.05 m aqueous LiBr at -60 C Ar IS 90 kJ mol-1 (36RT) TD 45 45 Ar Signal Br- Li+ 0 Arrival Time (µs) 1400