Interfacial properties of semifluorinated alkane diblock copolymers

Abstract

The liquid-vapor interfacial properties of semifluorinated linear alkane diblock copolymers of the form F(3)C(CF(2))(n-1)(CH(2))(m-1)CH(3) are studied by fully atomistic molecular dynamics simulations. The chemical composition and the conformation of the molecules at the interface are identified and correlated with the interfacial energies. A modified form of the Optimized Parameter for Liquid Simulation All-Atom (OPLS-AA) force field of Jorgensen and co-workers [J. Am. Chem. Soc. 106, 6638 (1984); 118, 11225 (1996); J. Phys. Chem. A 105, 4118 (2001)], which includes specific dihedral terms for H-F blocks-and corrections to the H-F nonbonded interaction, is used together with a new version of the exp-6 force field developed in this work. Both force fields yield good agreement with the available experimental liquid density and surface tension data as well as each other over significant temperature ranges and for a variety of chain lengths and compositions. The interfacial regions of semifluorinated alkanes are found to be rich in fluorinated groups compared to hydrogenated groups, an effect that decreases with increasing temperature but is independent of the fractional length of the fluorinated segments. The proliferation of fluorine at the surface substantially lowers the surface tension of the diblock copolymers, yielding values near those of perfluorinated alkanes and distinct from those of protonated alkanes of the same chain length. With decreasing temperatures within the liquid state, chains are found to preferentially align perpendicular to the interface, as previously seen.

Systems Involving Hydrogenated and Fluorinated Chains: Volumetric Properties of Perfluoroalkanes and Perfluoroalkylalkane Surfactants

“…Liquid densities were also calculated for F5H5 and found to be in excellent agreement with the simulations of Pierce et al 56 The results are presented in Table 6 and plotted in Figure 8, from which a number of conclusions can be drawn. As can be seen from the figure, the simulations are able to predict the liquid densities in excellent agreement with the experimental results.…”

SAFT-γ force field for the simulation of molecular fluids: 8. Hetero-segmented coarse-grained models of perfluoroalkylalkanes assessed with new vapour–liquid interfacial tension data

“…Pierce et al [18] used two different force fields to study the interfacial behaviour of a number of PFAAs, alkanes, and perfluoroalkanes: the optimized OPLS-AA model based on ab initio calculations by Pádua [19]; and the exponential-6 force field based on the ab initio model for perfluoroalkanes developed by Borodin et al [20] The predicted surface tensions of PFAAs were sometimes lower than both the n-alkane and perfluoro-n-alkane with the same number of carbon atoms. Pierce et al [18] again found segregation of the fluorinated segments to the surface, with a normal orientation of the molecules relative to the interface. However, the simulated densities and surface tensions were essentially compared with experimental data for the n-alkanes.…”

“…Comprehensive computational studies focusing on the interfacial properties of the PFAA molecules have been performed by Hariharan and Harris [17] and by Pierce et al [18]. A united-atom approach was used by Hariharan and Harris [17] to examine the surfaces of n-decane, perfluoro-n-decane, F 5 H 5 , and F 10 H 10 in terms of surface tension, density profiles and molecular orientations; their study indicated that in PFAA molecules the perfluorinated segments tend to segregate to the surface and are preferentially oriented perpendicularly to the interface.…”

Liquid-vapor equilibria and interfacial properties of n-alkanes and perfluoroalkanes by molecular simulation

“…Hariharan and Harris 23 presented a UA-based study in which their model could not capture surface-tension differences between decane and perfluorodecane. References [24][25][26] to that work 23 seem to suggest that this is caused by an inherent limitation of the UA method. However, the results in this work demonstrate that this is not the case; rather, the issue is with the specific UA model used and its parameterization.…”