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Understanding the structure and thermodynamic properties of the free surface of fluorocarbons is an area of fundamental and current interest with numerous technological implications[1]. The properties of the free surface are in general governed by surface tension which in turn is related to the difference between the intermolecular interaction in the bulk and at the surface[2]. Thus, a theory that exactly describes intermolecular interactions is essential to accurately predict the properties of the free surface. To date, there exist few theoretical studies of the free surface of fluorocarbons despite their great promise for nanotechnological and biomedical applications. From both theoretical and experimental perspective, our knowledge of the surface of fluorocarbons is very limited.

Fluorocarbons are found to be very sensitive to molecular details and an all-atom description of the molecular structure, where the fluorine atoms are treated explicitly, may be required in order to capture, in principle, all of the required properties. An advantage of using an all-atom representation of the molecules rather than a united-atom lies in the ability to examine the role of molecular flexibility and internal vibrations in affecting the molecular orientation in the system. Since the fluorine atoms are treated explicitly in all-atom models, the requirement in terms of computer time is significantly larger than for united-atom models. This has been yet another reason for the limited number of atomistic simulation studies of fluorocarbons. To the best of our knowledge, only A few computational studies have been conducted which qualitatively describe the interactions of fluorocarbon systems [3-8].

In order to elucidate the molecular basis for the structure and surface tension properties of the surfaces of fluorocarbons, an all-atom molecular dynamics study using two commonly used atomistic force fields optimized for simulation of perfluorinated alkanes has been carried out. The two force fields are: the Optimized Parameter for Liquid Simulation-All Atom (OPLS-AA) force field of Watkins and Jorgensen [1] (hereafter referred to as OPLS force field) and the force field of Borodin et al. (hereafter referred to as exp-6 force field) [4,9]. Both force fields are found to produce accurate descriptions of densities and heat of vaporization for perfluorinated alkanes. However, the OPLS force field is found to be less accurate in describing conformational energies of short chains, [4] thus bringing into question the transferability of the force field to longer chains. The transferability of the exp-6 potential to longer chains has been demonstrated [4]. At present the exp-6 force field is expected to perform better than the OPLS force field in describing structural and dynamic properties of a range of fluorinated alkane chains.

RESULTS for SURFACE TENSION and SURFACE ORIENTATION of PERFLUORINATED ALKANES
i)   Longer simulation time of at least 7 ns was required in order to obtain reliable surface tension values.
ii)  Both force fields yield good agreement with experimental data.
iii) At the surface, the chain segments were found to point perpendicular to the surface with the
     -CF3 end groups at the surface.
iV)  The amount of surface orientation of the chains strongly depends on temperature and also on chain length, where at a given temperature the ordering increased with increasing chain length.

RESULTS for LIQUID-LIQUID INTERFACES OF SEMIFLUORINATED ALKANE DIBLOCK COPOLYMERS with WATER, ALKANES, and PERFLUORINATED ALKANES
i)   Aqueous interfaces of semifluorinated alkanes as well as those of protonated and perfluorinated alkanes are found to be sharp.
ii)  For semifluorinated alkane-water interfaces, hydrogen concentration enhancement is observed through increased density and residence time in the interfacial region, in agreement with results from studies of Langmuir monolayers of semifluorinated at the water-air interface and in contrast to the observed fluorine enhancement seen at the semifluorinated-vapor interface.
iii) Hydrogen enhancement is found to be an increasing function of temperature.
iv)  In contrast to the sharp interfaces between semifluorinated alkanes and water, for the chain lengths we studied, semifluorinated alkanes are found to be miscible with alkanes and perfluorinated alkanes.
v)   Miscibility is expected to decrease with increasing chain length due to the compounded effect of the
     energetic incompatibility of hydrogenated and fluorinated segments over a large number of units.

IMPACT OF THE PROJECT ON MY CAREER:
The funding has substantial impact on my career. It has helped me to (1) attend several national conferences, (2) attract graduate students and (3) increase my publication record. Above all, the preliminary results I managed to generate using this funding about a year ago has helped me to win the National Science Foundation's CAREER award.

IMPACT OF THE PROJECT ON THE STUDENTS PARTICIPATING IN THE PROJECT
Most of the funding is used to support two graduate students (Gary Leuty and Chrianjivi Lamsal) in the summer of 2008 and summer of 2009. The fund has given a tremendous opportunity for these two students to focus more on research and less on teaching. In March of 2009, both of them had a chance to present their findings at the American Physical Society March Meeting in Pittsburgh, PA. Their expenses were fully covered by the project. Thanks to the funding, both students are now writing their Master thesis and will be defending soon.

References
[1]. B. Ameduri, B. Boutevin, "Well-Architectured Fluoropolymers: Synthesis, Properties and Applications", Elsevier Science: New York, 2004.
[2]. T. Sakka, Y. H. Ogata, Journal of Fluorine Chemistry, 126, 371 (2005).
[3]. E. K. Watkins, W. L. Jorgensen, J. Phys. Chem. A, 105, 4118 (2001).
[4]. O Borodin, G. D. Smith, D. Bedrov, J. Phys. Chem. B, 106, 9912 (2002).
[5]. S. Shin, N. Collazo, S. A. Rice, J. Chem. Phys., 96, 1352 (1992).
[6]. N. Collazo, S. Shin, S. A. Rice, J. Chem. Phys., 96, 4735 (1992).
[7]. S. Shin, N. Collazo, S. A. Rice, J. Chem. Phys., 98, 3469 (1993).
[8]. T. S. Cui, J. I. Siepmann, H. D. Cochran, P. T. Cummings, Fluid Phase Equilibra, 146, 51 (1998).
[9]. O Borodin, G. D. Smith, D. Bedrov, J. Phys. Chem. B, 110, 6279 (2006).