The acidity of the water surface sensed by a colliding gas is determined in experiments in which the protonation of gaseous trimethylamine (TMA) on aqueous microjets is monitored by online electrospray mass spectrometry as a function of the pH of the bulk liquid (pH_(BLK)). TMAH^+ signal intensities describe a titration curve whose equivalence point at pH_(BLK) 3.8 is dramatically smaller than the acidity constant of trimethylammonium in bulk solution, pK_A(TMAH^+) = 9.8. Notably, the degree of TMA protonation above pH_(BLK) 4 is enhanced hundred-fold by submillimolar LiCl or NaCl and weakly inhibited at larger concentrations. Protonation enhancements are associated with the onset of significant direct kinetic solvent hydrogen isotope effects. Since TMA(g) can be protonated by H_2O itself only upon extensive solvent participation, we infer that H3O^+ emerges at the surface of neat water below pH_(BLK) 4

Colussi, A. J.

Enami, Shinichi

Hoffmann, Michael R.

English

Caltech Authors - Main

 1
Supporting Information for 
Proton Availability at the Air/Water Interface 
Shinichi Enami, Michael R. Hoffmann, Agustín J. Colussi* 
W. M. Keck Laboratories, California Institute of Technology, California 91125, USA 
 
jz-2010-00322w-R2 
 
 
 2
EXPERIMENTAL METHODS 
 Our experiments involve the generation of aqueous microjets in the spraying chamber of 
an electrospray ionization mass spectrometer (ESI-MS) that is simultaneously flushed with 
TMA(g)/N2(g) mixtures at 1 atm, 293 K1,2 (Fig. S1). TMAH+ ions already present on the 
surface of the water microjets by acidifying TMAH+Cl-, or those produced in situ during brief 
exposure to TMA(g), are detected and quantified by online ESI-MS within 1 ms. Solutions 
pumped through a grounded stainless steel needle, which is coaxial with a sheath issuing 
nebulizer N2 gas, surge as a microjet into the chamber.3 The fast nebulizer gas (vg ~ 2.5 × 104 
cm s-1) rapidly shreds the interfacial layers of the slower liquid microjet (vj ~ 10 cm s-1) into 
charged microjets that carry anion or cation excesses following to a normal probability 
distribution function.4,5 On average, the nebulizer gas shreds a 1 μm radius droplet from 
interfacial liquid layers δ ≤ 1 nm thick in a few microseconds. Microjets subsequently lose 
mass, but retain their excess charges ze, via solvent evaporation in the dry, warm N2(g) 
emanating from the electrically polarized inlet to the mass spectrometer. Hence, microjets 
will be drawn to the inlet with increasing acceleration: a = (ze/m) E. The strong direct 
correlation between residence time and droplet size ensures that most reactive events occur in 
TMA(g) collisions with the liquid microjets and nascent microdroplets. The degree of (the 
relatively slow kd = KA kb ~ 10-9.8 M × 1010 M-1 s-1 ~ 1 s-1) TMAH+ acid dissociation in the 
microjets is therefore preserved prior to detection ~ 1 ms later.6 The ESI-MS signals 
 3
generated by our instrument are linear transfer functions of the composition of the interfacial 
layers of the liquid microjet.7 The inference is that the cationic excess carried by the 
positively charged nascent microjets and detected by mass spectrometry is proportional to the 
[cation] in the interfacial layers of the microjet. Similar considerations apply to the titration 
of TMAH+Cl- followed by positive ion ESI-MS (see below). Further experimental details and 
validation tests may be found in previous publications from our laboratory.1,2,8 
 TMA/N2 gas mixtures were introduced into the spraying chamber in a direction 
perpendicular to a stainless steel needle injector and mass spectrometer, respectively. This 
geometry was chosen to prevent unwanted TMA losses on the wall of the spraying chamber. 
The TMA concentration in the chamber was calculated from the combined TMA/N2 mixture 
and N2 drying gas flow rates. The [TMA(g)] values reported in the figures throughout 
correspond to the concentrations actually sensed by microjets in the spraying chamber, which 
are ~13 times smaller than the values determined from the flow rates due to further dilution 
by the N2 drying gas. Gas flows were regulated by calibrated mass flow controllers (MKS) 
and a needle valve. Typical instrumental parameters were as follows: drying gas flow rate, 13 L 
min-1; drying gas temperature, 340 oC; nebulizer pressure, 2 atm; collector capillary voltage, -3.5 
kV; fragmentor voltage, 22 V. TMA gas (1 % or 0.01 % in UHP N2) was obtained from 
Matheson-Tri-Gas. D2O (> 99.9 %) and LiCl (> 99.9 %) were obtained from Sigma-Aldrich. NaCl 
(> 99.9 %) was obtained from J.T. Baker Chem. Co. H3PO4 (85 %) was obtained from Fisher 
 4
Scientific. All solutions were prepared in deionized water from a Millipore Milli-Q gradient water 
purification system. Solution pHBLK was adjusted by adding HCl/NaOH and measured with a 
calibrated pH meter (VWR). We verified that our solutions did not contain detectable amounts of 
dissolved atmospheric CO2 from the absence of HCO3- (m/z = 61) via negative ion ESI-MS. 
 5
Captions to Figures 
Fig. S1 Schematic diagram of the present experimental setup and TMA(g) injection system. 
MFC stands for mass flow controller. 
Fig. S2 Plot of the m/z = 60 signal intensity vs. [TMA·HCl(aq)] in water solution (red circle) 
and in 100 mM LiCl solution (blue square) at pHBLK ~ 7. 
Fig. S3 Positive ion mass spectra of water microjets at various pHBLK’s adjusted with 
HCl/NaOH and exposed to 1.5 ppmv TMA(g) (A) or 3 ppmv TMA(g) (B, C and D). 
Fig. S4. ESI-MS TMAH+ (m/z = 60) signal intensities as functions of pHBLK. Blue downward 
triangles: water microjets. Yellow upward triangles: 1.5 mM sodium phosphate buffer 
microjets. Red downward triangles: 15 mM sodium phosphate buffer microjets in 
experiments under 1 ppmv TMA(g). Signals normalized to TMAH+ = 1 at pHBLK = 1. 
Fig. S5. ESI-MS TMAH+ (m/z = 60, blue downward triangles) and TMALi+ (m/z = 66, red 
upward triangles) signal intensities as functions of log [LiCl] upon exposing LiCl solution 
microjets at pHBLK 2.9 to 1 ppmv TMA(g). 
Fig. S6. ESI-MS TMAH+ (m/z = 60, blue downward triangles) and TMALi+ (m/z = 66, red 
upward triangles) signal intensities as functions of pHBLK upon exposing 100 mM aqueous 
LiCl solution microjets to 1 ppmv TMA(g). 
 6
 
ESMS
Chamber
MFC
MFC
N 2
Teflon 
Tube
1 % 
TMA
TMA/N2 from MFC
Solution from syringe pump
Teflon Tube
Teflon
Stainless steel 
Nebulizer
To MS
N2
Trap 
at -78oC
Fig. S1
 7
Fig. S2
 8
 
Fig. S3
 9
Fig. S4
 10
 
Fig. S5
 11
 
 Fig. S6
 12
REFERENCES 
 (1) Enami, S.; Hoffmann, M. R.; Colussi, A. J. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 
7365. 
 (2) Enami, S.; Hoffmann, M. R.; Colussi, A. J. J. Phys. Chem. A 2009, 113, 7002. 
 (3) Kebarle, P.; Peschke, M. Anal. Chim. Acta 2000, 406, 11. 
 (4) Manisali, I.; Chen, D. D. Y.; Schneider, B. B. Trends in Analytical Chemistry 2006, 
25, 243. 
 (5) Zilch, L. W.; Maze, J. T.; Smith, J. W.; Ewing, G. E.; Jarrold, M. F. J. Phys. Chem. A 
2008, 112, 13352. 
 (6) Grunwald, E.; Cocivera, M. Discuss. Faraday Soc. 1965, 105. 
 (7) Cheng, J.; Psillakis, E.; Hoffmann, M. R.; Colussi, A. J. J. Phys. Chem. A 2009, 113, 
8152. 
 (8) Enami, S.; Vecitis, C. D.; Cheng, J.; Hoffmann, M. R.; Colussi, A. J. J. Phys. Chem. 
A 2007, 111, 13032. 
 
 


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