Simulated Solvation of Organic Ions II: Study of Linear Alkylated Carboxylate Ions in Water Nanodrops and in Liquid Water. Propensity for Air/Water Interface and Convergence to Bulk Solvation Properties

Abstract

We investigated the solvation of carboxylate ions from formate to hexanoate, in droplets of 50 to 1000 water molecules and neat water, by computations using standard molecular dynamics and sophisticated polarizable models. The carboxylate ions from methanoate to hexanoate show strong propensity for the air/water interface in small droplets. Only the ions larger than propanoate retain propensity for the interface in larger droplets, where their enthalpic stabilization by ion/water dispersion is reduced there by 3 kcal mol(-1) per CH2 group. This is compensated by entropy effects over +3.3 cal mol(-1) K(-1) per CH2 group. On the surface, the anionic headgroups are strongly oriented toward the aqueous core, while the hydrophobic alkyl chains are repelled into air and lose their structure-making effects. These results reproduce the structure-making effects of alkyl groups in solution, and suggest that the hydrocarbon chains of ionic headgroups and alkyl substituents solvate independently. Extrapolation to bulk solution using standard extrapolation schemes yields absolute carboxylate solvation energies. The results for formate and acetate yield a proton solvation enthalpy of about 270 kcal mol(-1), close to the experiment-based value. The largest carboxylate ions yield a value smaller by about 10 kcal mol(-1), which requires studies in much larger droplets.

Reactions of Criegee Intermediates with Alcohols at Air–Aqueous Interfaces

“…times smaller than those at m/z 431 (and 433) from OA at [1-octanol] = [OA] = 100 mM reveals that 1-octanol is much less reactive than OA toward CIs, although both species are expected to have similar affinities for aqueous surfaces. 25,32,43,[69][70] The C 23 ethers and esters produced in each case, by having similar mass and structures (see This finding is in line with gas-phase reaction rate constants of CIs + HCOOH/CH 3 COOH, which approach collisionally controlled values: k ≥ 10 -10 cm 3 molecule -1 s -1 , 22 vs. the much smaller values for CIs + methanol/2-propanol. 48,50,68 Tobias and Ziemann have reported that rate constants of gas-phase C 13 CIs reactions with various species increase in the order: water < methanol < 2-propanol < formaldehyde < formic acid < heptanoic acid, over a 10 4…”

Improving the description of solvent pairwise interactions using local solute/solvent three‐body functions. The case of halides and carboxylates in aqueous environment

“…This arises from an inaccurate description of the water pairwise interactions corresponding to a particular water dimer conformation. In particular that yields the proton hydration enthalpy, Δ H hyd (H + ), extrapolated from halide or carboxylate/water nanodroplet data, to be from 3 to 8 kcal mol –1 weaker than the well accepted experimental range of values, while our TCPE/2013‐based Δ H hyd (H + ) estimate extrapolated from methylated ammonium/water nanodroplets is in much better agreement with experiment …”

“…/water interactions are modeled accordingly, i.e., we use the above dispersion term instead of a specific U mbp one (the above second sum then runs on ammonium centers). The carboxylate methyl and ammonium dispersion parameters are provided in our former studies …”

“…This model is shown to accurately model pure water systems (gas‐phase clusters and liquid water under a large domain of temperature and pressure conditions). It was applied to study the hydration processes of organic cations (like methylated ammonium cations and guanidinium) and of anions (like halides, from fluoride, F − to astatide, At –, and linearly alkylated carboxylates). We provide Supporting Information Appendix S1 as a comparison between the TCPE/2013 predictions regarding small gas‐phase water clusters and bulk liquid water and those derived from the above mentioned water models.…”

Shifted equilibria of organic acids and bases in the aqueous surface region

“…Organic molecules with long alkyl chains will reside more readily in the surface region, despite attached ionic functional groups. 7 , 39 The longer the alkyl chain, the more important hydrophobic interactions become that outweigh the hydrophilic–water interactions, resulting in a more similar surface enrichment of the charged and non-charged species, as seen for A6/A6H + , A4/A4H + and C5 – /C5H in Table 1 . For up to three carbon atoms in an alkyl chain, the hydrophilic interactions dominate the overall hydration behavior, yielding relatively large differences in the surface enrichment factors of a carboxylic acid and its conjugated carboxylate ion, see results on C4 – /C4H in Table 1 .…”