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Langmuir 2007, 23, 1792-1803
Stability and Disintegration of Ultrathin Heptane Films in Water: Molecular Dynamics Simulations Tetyana Kuznicki,†,‡ Jacob H. Masliyah,‡ and Subir Bhattacharjee*,† Department of Mechanical Engineering, UniVersity of Alberta, Edmonton, Alberta, T6G 2G8, and Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, T6G 2G6, Canada ReceiVed July 24, 2006. In Final Form: October 23, 2006 Molecular dynamics simulations of ultrathin heptane films (less than 5 nm in thickness) in water were conducted to study their stability and disintegration behavior. The density distributions of heptane and water molecules across the film were determined for different equilibrium film thicknesses ranging from 1.5 to 4 nm. The potential energy of the system was computed as a function of the heptane number fraction, and the results were employed to determine the excess energy of mixing of heptane in water. The diffusion coefficients of heptane and water obtained from the MD simulations were also compared with experimental data. A good agreement was found between the heptane self-diffusivity obtained from the MD simulations and its literature reported value. Following an analysis of the equilibrium properties of the heptane films and associated structures, we performed simulations where the shapes of the heptane films were initially perturbed. Different perturbations of these ultrathin films led to formation of various associated structures, including cylindrical rodlike heptane droplets, films with holes, and intact films. The different shapes are formed in systems with the same heptane/water composition. An analysis of this behavior is presented showing the possibility of multiple associated structures with similar total energy in these highly confined systems.
1. Introduction Insight regarding the stability of water-in-oil and oil-in-water emulsions is of great significance in diverse industrial applications,1 ranging from food processing, petrochemicals, pharmaceuticals, paints and coatings, to oil recovery.2 The stability of the dispersed phase (droplets) in such emulsions is primarily dictated by the stability of thin liquid films separating the dispersed phases.3-5 For water-in-oil emulsions, the film comprises an oil layer sandwiched between two considerably larger expanses of water. An accurate understanding of the interfacial behavior of such films is critically important in assessing their stability, molecular structure, drainage, and rupture mechanisms.6-13 Analysis of thin liquid films employing continuum approaches is generally applied to so-called “thick thin films”, where the film thickness is greater than 10 nm.3 Continuum analysis of * Corresponding author. Tel: (780) 492 6712; Fax: (780) 492 2200; Email:
[email protected]. † Department of Mechanical Engineering. ‡ Department of Chemical and Materials Engineering. (1) Holmberg, K., Shah, D., Schwuger, M., Eds. Handbook of Applied Surface and Colloid Chemistry; John Wiley & Sons: Baffins Lane, UK, 2002; Vols. 1-2. (2) Fink, J. K. Oil Field Chemicals; Gulf Professional Publishing, Elsevier: Burlington, MA, 2003. (3) Kralchevsky, P. A.; Nagayama, K. Particles at fluids interfaces and membranes. Attachment of colloid particles and proteins to interfaces and formation of two-dimensional arrays. In Studies in Interface Science; Elsevier: Amsterdam, 2001; Vol. 10. (4) Volkov, A., Ed. Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications; Surfactant Science Series 95; Marcel Dekker: New York, 2001. (5) van Aken, G. A. Colloids Surf., A 2003, 213, 209-219. (6) Georges, J. M.; Millot, S.; Loubet, J. L.; Tonck, A. J. Chem. Phys. 1993, 98, 7345-7360. (7) Forcada, M. L.; Mate, C. M. Nature (London) 1993, 363, 527-529. (8) Evers, L. J.; Shulepov, S. Y.; Frens, G. Faraday Discuss. 1996, 335-344. (9) Weinstein, A.; Safran, S. A. Europhys. Lett. 1998, 42, 61-66. (10) Yu, C. J.; Richter, A. G.; Datta, A.; Durbin, M. K.; Dutta, P. Phys. ReV. Lett. 1999, 82, 2326-2329. (11) Evers, L. J.; Nijman, E. J.; Frens, J. Colloids Surf., A 1999, 149, 521-527. (12) Dimitrova, T. D.; Leal-Calderon, F. Langmuir 1999, 15, 8813-8821. (13) Evmenenko, G.; Dugan, S. W.; Kmetko, J.; Dutta, P. Langmuir 2001, 17, 4021-4024.
thinning and breakup of such films,14,15 often performed using the long-wave approximations,14,16-18 generally makes certain assumptions regarding the film rupture. These approaches introduce a minimum cutoff thickness of the film; when the film thins to achieve this minimum thickness, the continuum theories assume the films to be ruptured.19,20 The arbitrariness of the cutoff distance is manifested in films where only van der Waals interactions are present. While, in reality, these attractive forces eventually lead to disintegration of the film,21 continuum calculations must be terminated as soon as this cutoff distance is attained to prevent divergence of the interaction energy. In the presence of antagonistic attractive/repulsive interactions, such films might attain a stable thickness that is greater than the minimum cutoff thickness.15 Furthermore, the density fluctuations, structural modifications, and property changes of ultrathin films observed in many experimental studies6,7,9,12,13 are not explicitly considered in these continuum theories. Consequently, continuum approaches may not be suitable for assessing the behavior of ultrathin liquid films, where the discrete molecular architecture of the film starts to interfere with the continuum assumptions. In ultrathin films, often referred to as Newton black films, the film thickness is typically in the 1-5 nm range.11 Quite often, these films consist of at best two to three layers of constituent molecules. Such films are difficult to study either on the basis of continuum theory or experimentally. The granularity of the molecules becomes important as the film drains to attain thicknesses of
