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.
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.
CTP-Centre Thermodynamique des Procédés, MINES ParisTech, PSL Research University , 35 rue Saint-Honoré, 77300 Fontainebleau, France.
Céline
C
Houriez
Department of Chemistry, Virginia Commonwealth University , Richmond, Virginia 23284-2006, United States.|Department of Chemistry, University of Canterbury , Christchurch, New Zealand 8001.
Michael
M
Meot-Ner Mautner
Laboratoire de Biologie Structurale et Radiobiologie, Service de Bioénergétique, Biologie Structurale et Mécanismes, Institut de Biologie et de Technologies de Saclay, CEA Saclay , F-91191 Gif sur Yvette Cedex, France.
Michel
M
Masella
D000388:Air; D002208:Caproates / Q000737:chemistry*; D019277:Entropy; D005561:Formates / Q000737:chemistry*; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D049329:Nanostructures / Q000737:chemistry*; D044367:Phase Transition; D012997:Solvents / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
D002208:Caproates; D005561:Formates; D012997:Solvents; D014867:Water; C030544:formic acid; C037652:hexanoic acid