Abstract
Although perfluorination is known to enhance hydrophobicity and change protein activity, its influence on hydration-shell structure and thermodynamics remains an open question. Here we address that question by combining experimental Raman multivariate curve resolution spectroscopy with theoretical classical simulations and quantum mechanical calculations. Perfluorination of the terminal methyl group of ethanol is found to enhance the disruption of its hydration-shell hydrogen bond network. Our results reveal that this disruption is not due to the associated volume change but rather to the electrostatic stabilization of the water dangling OH···F interaction. Thus, the hydration shell structure of fluorinated methyl groups results from a delicate balance of solute-water interactions that is intrinsically different from that associated with a methyl group.
Highlights
The perfluorination of hydrocarbons or alkyl substituents often increases their hydrophobicity as evidenced, for example, by the decreased solubility of methane (CH4) upon fluorination[1] and the increased aqueous contact angle of polyethylene (PE) upon fluorination.[2]
Our results indicate that the mechanism behind the formation of dangling OH structures is inherently different for EtOH and TFE, which suggests that are −CF3 and −CH3 chemically different and they are intrinsically different with regards to their hydrophobicity
The measured Raman spectra of pure water shown in panel A contains peaks arising from the water OH stretch, bend, Article relative low intensity of the 3200 cm−1 shoulder in the TFE hydration shell indicates that these water molecules are less tetrahedral than pure water
Summary
The perfluorination of hydrocarbons or alkyl substituents often increases their hydrophobicity as evidenced, for example, by the decreased solubility of methane (CH4) upon fluorination (to CF4)[1] and the increased aqueous contact angle of polyethylene (PE) upon fluorination (to PTFE).[2]. Previous infrared spectroscopic studies have provided intriguing hints regarding the influence of fluorination on hydration,[23,24] they have not quantified the associated changes in hydration-shell structure and thermodynamics. We do so by combining Raman multivariate curve resolution (Raman-MCR) spectroscopy with classical molecular dynamics (MD) simulations and ab initio interaction energy calculations. Our Raman-MCR spectra reveal striking differences between the hydration-shell structures of ethanol (EtOH) and 2,2,2-trifluoroethanol (TFE) dissolved in water. Comparisons of those experimental results with molecular dynamics simulation predictions obtained using classical force fields facilitate the definitive assignment of the observed fluorination-induced hydration-shell structure changes, quantitatively link those changes to hydration thermodynamics, and establish the electrostatic origin of these changes
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