Supervisor: Dr Chi Phan, Dept of Chemical Engineering

1. Quantifying the influence of alcohol structure on ions adsorption at air/water interface by molecular simulations William Foskett, Curtin University Supervisor: Dr Chi Phan, Dept of Chemical Engineering Curtin University

2. Overview • Examine effect of alcohol structure on its influence on properties at an air/water interface • In this study: • 1-Butanol • Isobutanol • Both pure water/alcohol and electrolyte solution/alcohol • Results modelled with molecular simulator GROMACS 1-Butanol Isobutanol

3. Overview • Surface active agents (surfactants) used in chemical process industry • Ability to alter interfacial properties – affects efficiency • Quantifying influences not well defined • Surface Potential (ΔV) • Surface Tension

4. Overview • Affect of MIBC and 1-hexanol recently been quantified (Nguyen, 2013) • Investigated 1-butanol and isobutanol through molecular simulations • Surface adsorption • Effect of structural branching • Effect of tail length Isobutanol 1-Butanol

5. Methodology • Multiple simulations with different parameters • Structural (topology) files adjusted to remove degree of freedom Molecular Dynamic Simulations

6. Methodology • Rectangular box 3 x 3 x 30nm used • Contains solution and two air regions • Solution initially pure water then NaCl solution at varied concentrations Figure 1: a simplified model of box setup used for simulation

7. Methodology • Initial series of small simulations (NVT and NPT) (Phan, 2012) • Visual inspection • Run final 20ns production run to equilibrium on compacted box under NVT conditions (T = 298K)

8. Simulation a) b) Figure 2. Molecular arrangement of 1-butanol at the air/water interface; a) before simulation, b) following 20ns simulation

9. Methodology Energy/Surface Analysis • Surface tension and surface potential at 0.01nm intervals along 30nm box • RDF analysis on individual atoms within alcohol molecules

10. Results • Surface Tension • General increasing trend as concentration of NaCl increases

11. Results • Surface Potential • Adding in electrolyte initially decreases magnitude of potential • Increasing concentration increases potential

12. Results

13. Limitations • Various assumptions made to reduce degrees of freedom. • Water molecules modelled with SPC/E (Berendsen, 1987) • Molecular interaction cut-off distance (1nm) • Length of simulation set at 20ns

14. Future Work • Continuing with this work for research project as part of my undergraduate degree • Complete isobutanol simulations • Compare isobutanol and 1-butanol results for effect of branching • Compare 1-butanol and 1-hexanol results for effect of tail length • Vary number of molecules per box to observe effect of alcohol concentration • Change charge distribution of molecules

15. Acknowledgements The author gratefully acknowledges the resources provided by and support from iVEC Supercomputing, Western Australia , as well as the supervisor Dr. Chi Phan and Cuong Van Nguyen for this work.

16. References Nguyen, C.V. et al., 2013. “Surface potential of 1-hexanol solution: a comparison with methyl isobutyl carbinol.” J. Phys. Chem. B, vol 117 pp. 7615-7620. Phan, Chi M., Nakahara, Hiromichi, Shibata, Osamu, Moroi, Yoshikiyo, and Ha M. Ang. 2012. “Surface Potential of Methyl Isobutyl Carbinol Adsorption Layer at the Air/Water Interface.” J Phys. Chem. B, vol 116 pp 980-986. Berendsen, H.J.C., Grigera, J.R, and T.P. Straatsma. 1987. “The Missing Term in Effective Pair Potentials.” J. Phys. Chem, vol 91 pp. 6269-6271.