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City College of the City University of New York

Molecular dynamics study of the wetting of hydrophobic substrates by aqueous trisiloxane and polyethoxylate surfactant solutions. An ongoing doctoral research project by. Jonathan D. Halverson 1. With the faculty advisement of. J. Koplik 2,3 , A. Couzis 1 , C. Maldarelli 1,3.

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City College of the City University of New York

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  1. Molecular dynamics study of the wetting of hydrophobic substrates by aqueous trisiloxane and polyethoxylate surfactant solutions An ongoing doctoral research project by Jonathan D. Halverson1 With the faculty advisement of J. Koplik2,3, A. Couzis1, C. Maldarelli1,3 Department of Chemical Engineering1, Department of Physics2 The Benjamin Levich Institute for Physico-chemical Hydrodynamics3 City College of the City University of New York New York, NY American Chemical Society Colloid and Surface Science Symposium Clarkson University, Potsdam, NY 15 June 2005

  2. Wetting phenomena According to hydrodynamic theory, a drop on a flat surface assumes the shape of a spherical cap: The Young equation relates the contact angle  of a sessile drop to the interfacial tensions: where SVis the solid-vapor tension, is the liquid-vapor tension, and SLis the solid-liquid tension.

  3. Motivation In the application of paint, ink, a herbicide solution, or a coating to a hydrophobic surface it is important for the fluid to completely wet the surface. Surfactants may be used to enhance the wetting of aqueous solutions on hydrophobic substrates.

  4. Objectives • Elucidate the mechanism by which surfactants enhance the spreading of aqueous solutions on hydrophobic solid substrates. • Offer an explanation based on molecular properties as to why some surfactant molecules are more effective than others. • Use the results of this work to suggest forms of new surfactants or mixtures.

  5. Polyoxyethylene surfactants Polyoxyethylene (POE) compounds are the most important nonionic surfactants in commercial use. POE surfactants with an alkyl ether link have the chemical formula CiEj, where Ci is CH3(CH2)i-1 and Ej is (OCH2CH2)jOH. C12E4 A methyl-capped polymethylene chain serves as the hydrophobic moiety. A hydroxyl-terminated polyoxyethylene chain serves as the hydrophilic moiety. C12E4 is capable of reducing the water contact angle to 40º on highly hydrophobic surfaces.

  6. Trisiloxane surfactants Trisiloxane alkoxylate surfactants are known as superspreaders. They have been shown to increase the wetted area of a sessile drop by a factor of 25 in comparison to conventional organic surfactants. Trisiloxane ethoxylate surfactants consist of oxyethylene groups (-OCH2CH2-) which act as the hydrophile while the nonpolar trisiloxane group (-SiOSiOSi-) serves as the hydrophobe. The trisiloxanes are the only class of surfactant to give the complete wetting of water on highly hydrophobic hydrocarbon substrates. TSE4 TSE4

  7. Surfactants in action Superwetting solution: >60 wt.% methyl (propylhydroxide, ethoylated) bis(trimethylsiloxy) silane, 15 - 40 wt.% polyethylene oxide monoallyl ether, <= 9 wt.% polyethylene glycol with 20:1 water Alkyl polyethoxylate/alcohol solution: C12E8 at 7 times the CMC and C12E0 at 21 times the solubility limit.

  8. Superspreading A consistent theory of superspreading has been emerging since it was first studied during the 1960’s. • Lowering of LV and SL • Molecular structure of the trisiloxane tailgroup • Rapid adsorption kinetics • Marangoni effect • Phase behavior and molecular aggregates • Humid environment

  9. SPC/E water potential The interaction potential between a pair of SPC/Ec water molecules is There is one Lennard-Jones interaction and nine Coulomb interactions between each pair of water molecules. The cutoff distance is taken as rc = 13 Å. cH. J. C. Berendsen, J. R. Grigera, T. P. Straatsma, J. Phys. Chem., 91, 6269 (1987).

  10. Wetting of graphite by water A cluster of water molecules spontaneously takes on the shape of a sphere in free space (Step 1). The equilibrated drop of 2197 SPC/E water molecules at 298 K is placed in the vicinity of two graphene sheets (Step 2): The contact angle of the sessile drop is seen to fluctuate. A soft potential maintains a vapor pressure.

  11. Contact angle measurement The liquid-vapor interface occurs where the density falls to one half of the bulk liquid value. The contact angle is found to be 82.6.

  12. Water on graphite Several contact angle measurements have been made for water on graphite. aT. Werder, J. H. Walther, R. L. Jaffe, T. Halicioglu, P. Koumoutsakos, J. Phys. Chem. B, 107, 1345 (2003). bM. Lundgren, N. L. Allen, T. Cosgrove, N. George, Langmuir, 18, 10462 (2002).

  13. Water/hexanol simulation Surfactant molecules tend to the contact line where the headgroups interact strongly with water.

  14. Water/hexanol simulation Hexanol molecules at the contact line are found to align with neighboring surfactant molecules. (Nwater = 4096, Nhexanol = 240, R = 4.5 nm)

  15. Water/hexanol simulation Few surfactant molecules are found at the solid-liquid interface. (Nwater = 4096, Nhexanol = 240, R = 4.5 nm)

  16. Surfactant distributions Two distinct peaks are seen in the vertical distribution of surfactant molecules. Alcohols have a higher first peak.

  17. Wetting results A plot of the center-of-mass vertical coordinate versus time reveals that negligible increased wetting is observed.

  18. Simulation challenge The radius of curvature of the sessile drop must be much greater than the thickness of the liquid-vapor and solid-liquid interfaces. Cases (b), (c) and (d) are sufficiently large for the surfactants considered in this work.

  19. Nanodrop of water/TSE4 Nwater = 55,073; Nsurfactant = 2143; N = 227,366; R = 9.5 nm Cross-section of initial condition (t = 0 ps)

  20. Nanodrop of water/TSE4 Consider adding TSE4 surfactant molecules to a spherical drop of liquid water with a radius of 9.5 nm: molecules on surface = surface area / maximum packing = 4R2 / (53.4 Å2 / molecule) = 2123 molecules TSE4 molecules in bulk = CMC  volume = (0.11 mole / m3)  4/3R3 = 0.2 molecules TSE4 At 100  CMC there are 20 molecules in the bulk.

  21. Nanodrop of water/TSE4 Nwater = 55,073; Nsurfactant = 2143; N = 227,366; R = 9.5 nm Cross-section of initial condition (t = 0 ps)

  22. Nanodrop of water/C12E4 Consider adding C12E4 surfactant molecules to a spherical drop of liquid water with a radius of 9.5 nm: molecules on surface = surface area / maximum packing = 4R2 / (38.0 Å2 / molecule) = 2983 molecules C12E4 molecules in bulk = CMC  volume = (0.05 mole / m3)  4/3R3 = 0.1 molecules C12E4 At 100  CMC there are 10 molecules in the bulk.

  23. Nanodrop of water/C12E4 Nwater = 39,060; Nsurfactant = 2993; N = 194,998; R = 9.5 nm Cross-section of initial condition (t = 0 ps)

  24. Nanodrop of water/C12E4 Nwater = 39,060; Nsurfactant = 2993; N = 194,998; R = 9.5 nm Cross-section of initial condition (t = 0 ps)

  25. J. Board et al. Long-range interactions The fast multipole algorithm (FMA) of Greengard and Rokhlin (1987) may be used to compute long-range interactions towithin round-off error. The computational complexity of the method is O(N). The basic idea is a particle interacts with the local multipole expansion of its box instead of the individual particles in distant boxes. Interactions are computed directly between particles in neighboring boxes. The mutual electrostatic forces between 200,000 particles can be calculated to within 1% relative error using the FMA in 506.7 s versus 6722.8 s for the direct method (over four time integration steps).

  26. Summary The wetting of graphite by aqueous surfactant droplets consisting of O(103) molecules has been simulated at 298 K. Physically consistent behavior has been observed. Parallel computing techniques and the fast multipole algorithm will be used to study the wetting of hydrophobic substrates by aqueous trisiloxane and polyethoxylate surfactant droplets consisting of O(104) molecules.

  27. Acknowledgements • Nitin Kumar and Makonnen Payne • National Energy Research Scientific Computing Center (NERSC) Funding • National Science Foundation IGERT Graduate Research Fellowship in Multiscale Phenomena of Soft Materials (2003-2005) • National Science Foundation IGERT Graduate Research Fellowship in Nanostructured Materials and Devices (2002) • NASA

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