Accumulating CO2

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Science  13 May 2011:
Vol. 332, Issue 6031, pp. 768-769
DOI: 10.1126/science.332.6031.768-c

Even a milliliter of water contains so many trillions upon trillions of molecules that a proton concentration range spanning 14 orders of magnitude (the standard pH scale) is rather easily sampled and measured. What happens, though, when water droplets shrink down to a few thousand molecules or less? At that point, it's hard to even define the pH range, let alone measure it. Levinger et al. confronted this dilemma in exploring the potential of CO2 to penetrate and react within reverse micelles—nanometer-scale pools of water bounded by surfactants within a hydrophobic solvent. They relied on tracking the extent to which vanadium oxide ions dissolved in the water pools linked up with one another after CO2 was introduced (51V nuclear magnetic resonance spectroscopy can clearly distinguish and quantify the different metal clusters). The known pH dependence of this oligomerization equilibrium in more conventional environments then allowed them to estimate the effective acidity within the micelles before and after CO2 absorption. The results indicated significant absorption of CO2 into the water pools, even under ambient conditions, with concomitant acidification presumably associated with carbonic acid formation. The study implies that analogous acidification of atmospheric aerosols by CO2 may be a more important process than previously appreciated.

J. Am. Chem. Soc. 133, 7205 (2011).

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