Wetting simulations of water with surfactant on solid substrates

J. D. Halverson1, J. Koplik2, A. Couzis1, C. Maldarelli1

The City College and The Graduate Center of

The City University of New York

New York, NY

Department of Chemical Engineering1, Department of Physics2

The Benjamin Levich Institute of Physico-chemical Hydrodynamics

Motivation

Many industrial processes rely on surfactants to enhance the spreading of aqueous solutions on hydrophobic surfaces.

A greater understanding of the effects of molecular structure and chemical composition of the surfactant on these wetting processes is needed. The mechanism by which wetting occurs in these systems is also of interest.

Molecular dynamics simulation is ideally suited for the study of surfactant systems at the molecular scale.

Outline

• Wetting simulation of water on an atomic surface

• Comparison of results for water on graphite

• Water-ethanol wetting simulation

• Water-poly(oxyethylene) glycol wetting simulation

• Conclusions

TIP3P water modelThe TIP3P model is empirical, pairwise, and rigid. In a shifted-force representation:

Each atom is assigned a partial charge: qO = -0.834 e, qH = 0.417 e. The bond angle of HOH = 104.52 and the O-H bond length of 0.9572 Å are kept fixed using RATTLE. Group-based interactions are computed.

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Water on Lennard-Jones substrateAn equilibrated drop of 1000 TIP3P water molecules at 298 K is placed in the vicinity of an atomic solid.

The interaction potential between a TIP3P oxygen atom and a Lennard-Jones atom in the solid is

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The wettability of the solvent is determined by COS.

Water on Lennard-Jones substrateThe microscopic contact angle may be measured once equilibrium has been achieved.

Contact angle is found to be 117 for COS = 1.0. For a value of COS = 1.1, = 100. The contact angle is found by assuming the drop forms an ideal spherical cap.

Water on graphite (validation)Other workers have studied the interaction of

water and graphite. Lennard-Jones parameters have been found for the SPC/E water model that reproduce the equilibrium contact anglea. Good agreement is observed.

The left imagea features 2000 SPC/E water molecules on graphite while the right imageb shows 900 TIP3P water molecules on a graphene sheet.

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).

a

Simplified substrate interactionComparison of real versus half space solids:

The solid is assigned the Lennard-Jones parameters of a TraPPE CH2 united atom (S = 3.95 Å, S = 0.382 kJ/mole). The number density is taken as S = 3 O-3.

Simplified substrate interactionThe interaction potential between a Lennard-

Jones atom and a semi-infinite continuous Lennard-Jones solid (or half space) of densityS is

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where according to the Lorentz-Berthelot combining rules,

Water-ethanol simulationWhile ethanol is fully soluble in water the alcohol preferentially adsorbs at the liquid-vapor and liquid-solid interfaces (NH20 = 1000, NCH3CH2OH =

168).

The amphiphilic species is found to decease the contact angle by lowering LV and SL.

Surfactant model

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Valence angle potential:

Dihedral angle potential:

The united atom approximation is applied to each CH2 and CH3 group. Partial charges are assigned to the nine atoms of the surfactant head group.

The polyethoxylate, CH3(CH2)11(OCH2CH2)2OH, or C12E2 is sparingly soluble in water.

Water-C12E2 simulationThe animations below feature 2000 TIP3P water molecules and 36 C12E2 molecules interacting on a continuous Lennard-Jones solid at 298 K.

(side view)

(bottom view)

Water-C12E2 simulationSurfactant molecules are radially distributed around the drop at the contact line. The head group of each surfactant molecule is directed into the drop.

SummaryMolecular dynamics simulations have been used to investigate the wetting of water containing surfactant on various solid substrates.

The water-C12E2 simulation gave physically correct behavior for small molecule numbers. Larger systems must be studied before insight into macroscopic systems can be gained.

Acknowledgements• NSF IGERT Graduate Research Fellowship

• Graphics by PyMOL (http://pymol.sourceforge.net/)