Molecular origins of fluorocarbon hydrophobicity

Abstract

We have undertaken atomistic molecular simulations to systematically determine the structural contributions to the hydrophobicity of fluorinated solutes and surfaces compared to the corresponding hydrocarbon, yielding a unified explanation for these phenomena. We have transformed a short chain alkane, n-octane, to n-perfluorooctane in stages. The free-energy changes and the entropic components calculated for each transformation stage yield considerable insight into the relevant physics. To evaluate the effect of a surface, we have also conducted contact-angle simulations of water on self-assembled monolayers of hydrocarbon and fluorocarbon thiols. Our results, which are consistent with experimental observations, indicate that the hydrophobicity of the fluorocarbon, whether the interaction with water is as solute or as surface, is due to its "fatness." In solution, the extra work of cavity formation to accommodate a fluorocarbon, compared to a hydrocarbon, is not offset by enhanced energetic interactions with water. The enhanced hydrophobicity of fluorinated surfaces arises because fluorocarbons pack less densely on surfaces leading to poorer van der Waals interactions with water. We find that interaction of water with a hydrophobic solute/surface is primarily a function of van der Waals interactions and is substantially independent of electrostatic interactions. This independence is primarily due to the strong tendency of water at room temperature to maintain its hydrogen bonding network structure at an interface lacking hydrophilic sites.solubility | hydration | wetting P erfluorinated alkanes which have all the hydrogen (H) atoms of an alkane replaced with fluorine (F) atoms are known to have much lower water solubilities than the corresponding hydrocarbons (1, 2) while at the same time showing high lipophobicity (2) and an extraordinary affinity for carbon dioxide (3, 4). Enzyme inhibitors with fluorinated moieties show stronger binding than their nonfluorinated analogues (5), even in cases when the group on the inhibitor is not among those that bind to the active site (6)-consistent with the expectation from higher hydrophobicity. One intriguing observation regarding the greater hydrophobicity is that the free energy of hydration per unit hydrophobic surface area is similar for hydrocarbons and fluorocarbons (6). The puzzling aspect of this similarity is that, because the C-F bond has a much greater dipole moment than does the C-H bond, a stronger binding with dipolar water might be expected. Further, although the polarizability of F in the C-F bond is relatively low considering its position in the periodic table, it is not lower than that in the C-H bond (7), so that the dispersion interactions of C-F with water are reasonably expected to be more attractive than those of C-H with water. Therefore, a fluorocarbon surface could be argued to be more hydrophilic than that of the corresponding hydrocarbon. A plausible resolution could be that the fluorocarbon with a molecular cross-section of 28.3 Å 2 (8) occupi...

Hydration dynamics at fluorinated protein surfaces

“…Fluorinated compounds are more hydrophobic than hydrogenated compounds of equal carbon number (4,5,(17)(18)(19)(20)(21), and the increase in hydrophobic character of fluorocarbons has been ascribed to their increased molecular size (18,56). This interpretation appears to be consistent with the observation that the melting temperature of A1-Tfl is 13°C higher than that of A1-Leu (11), while T m for A1-Hil is increased by 17°C in comparison to A1-Leu.…”

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

“…For the SAMs derived from the F8H10SH, the contact angle values are higher than those obtained from the C18SH film, which is consistent with previously observed values on SAMs derived from partially fluorinated adsorbates with the same degree of terminal fluorination [81,84]. The low surface energy enjoyed by fluorinated interfaces compared to hydrocarbon interfaces is consistent with the higher contact angle values observed on the F8H10SH SAM [85][86][87]. …”

Engineering Hydrophobic Organosilica Nanoparticle-Doped Nanofibers for Enhanced and Fouling Resistant Membrane Distillation

“…Besides, experimental results showed an increase of the WCA from 123 to 142 degrees by increasing the percent of PMO NPs doping from 5 to 10 wt% due to the hydrophobic ethylene-pentafluorophenylene-based particle composition ( Figure 3). Previous studies [56][57][58] combined with the increase of liquid entry pressures (Table S2) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 high porosities on the order of 77 to 86 % with mean pore sizes at around one micron ( Figure S8). …”