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| Critical adsorption & wetting |
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Ali Zarbakhsh, James Bowers, Hugo K. Christenson, Ian A. McLure, John R. P. Webster
Critical and multilayer adsorption from alkane + perfluoroalkane mixtures to chemically modified silicon substrates / PHY-04-0977 / V6 / 7
Motivation: measurement of the theoretically predicted critical adsorption behaviour of binary liquids with high spacial resolution
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The wetting of surfaces in binary liquid mixtures has provided a rich area for fundamental studies of surface critical phenomena [1,2]. Hydrocarbon-fluorocarbon interactions are both of academic interest and practical importance with applications such as non-stick coatings, anti-graffiti paints and specialized lubricants and surfactants. Despite the absence of dipolar effects and hydrogen-bonding, alkane-perfluoroalkane systems are characterized by significant non-ideality and this gives rise to a miscibility gap with an upper critical solution temperature Tc. Close to the critical point long-range correlations in the bulk fluid dominate the adsorption behavior at a surface. Here, the density profile is expected to show universal behaviour [3], decaying as an inverse power law close to the surface and showing exponential decay for distances exceeding the bulk correlation length ξ.
Unlike the case with most liquid-vapour systems, the critical point in binary liquid systems is often at a convenient temperature and ambient pressure, thus providing ideal testing ground for theoretical predictions [1-5]. However, there are few experimental studies of wetting and adsorption in liquid-liquid mixtures away from the critical regime [2], in particular in the complete wetting regime, where the adsorption is expected to be governed by long-range van der Waals forces. In our experiment at HMI we carried out a neutron reflection study of the n-hexane and perfluoro-n-hexane system at a volume fraction of n-hexane ΦH = 0.25 (in the complete wetting regime, far from the critical ΦH = 0.50).
We have for the first time [7] determined the complete composition profile of such a film, and find a composition different from bulk out to distances of z ≈ 100-120 nm near coexistence T0. The surface excess Γ decays as a power law with exponent –1/3 close to T0, but more steeply near Tc.
The reflectivity data displayed in Figure 2 show significant changes close to the critical edge (inset). The volume fraction profiles derived from the multilayer fits in Figure 2 are shown in Figure 3a. Far from coexistence (high t0, i.e. T/Tc > 1) there is a thin adsorbed film with hexane content decaying exponentially towards the bulk. As T approaches Tc the decay length of the exponential increases and the hexane content increases. For T < Tc the decay length remains constant but the thickness and n-hexane content of the film increase with decreasing T. Furthermore, the advancement of the critical edge indicates the presence of a long-range effect on the density profile. There must be an extensive region of n-hexane content greater than that of bulk, but different from that of the wetting film that emerges at coexistence. Only in the inner region, next to the surface, does the composition of the film approach that of the n-hexane-rich phase which phase separates at T0 (Figure 1). The horizontal regions of the profiles for small z result from minimising the number of parameters in the fitting procedure.
The reflectivity data were fitted using three modeling strategies, resulting in profiles displaying the same characteristic features (Figure 3). The profiles in Figures 3b and 3c give rise to the same high-quality fit to the reflectivity data as that of Figure 2, which corresponds to the profile in Figure 3a. With three separate modeling strategies the essential features emerge, i.e. the growth of a wetting layer close to the interface followed by a long decay to bulk composition. These features all occur on length scales greater than the nominal resolution (2π/Qmax) of 60 Å. The ln–ln plot in Figure 4 allows the surface excess to be represented as and the exponent m to be determined. At small t0 for T < Tc, m is close to -1/3 consistent with van der Waals complete wetting. For larger t0, m >> -1/3, indicating a crossover in wetting behaviour. We note that the latter statements have to be read with care as the Lifshitz theory of van der Waals forces in its strict sense cannot be applied to a system in which the density profile decays so gradually into the bulk. In such a case it is not possible to define an interface between the bulk solution and the adsorbed region. Moreover, it is not clear to what extent the wall and the bulk fluid at large z compete in determining the strength of the z-3 potential.
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 Experiment at BENSC, Germany
Technique: Reflectometry
- Ali Zarbakhsh
School of Biological & Chemical Sciences, Queen Mary, University of London, Mile End Road,
London E1 4NS. U.K.
- James Bowers
Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD. U.K.
- Hugo K. Christenson
School of Physics and Astronomy, The University of Leeds, Leeds LS2 9JT. U.K.
- Ian A. McLure
Department of Chemistry, The University of Sheffield, Sheffield S3 7HF. U.K.
- John R. P. Webster
ISIS Facility, CLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX. U.K.
- Roland Steitz
Berlin Neutron Scattering Center, Hahn-Meitner-Institut, Glienicker Strasse 100, D-14109 Berlin.
References:
1. Bonn, D.; Ross, D. Rep. Prog. Phys. 64, (2001).
2. Law, B. D. Prog. Surf. Sci. 66, (2001).
3. Fisher, M. E.; De Gennes, P. G. C. R. Acad. Sci. (Paris) Ser. B 287, 207,(1978); Rowlinson, J. S.; Widom, B. Molecular Theory of Capillarity; Dover: Mineola, NY, 2002.
4. Zhao, H; Penninckx-Sans, A.; Lee, L.-T.; Beysens, D.; Jannink, G. Phys. Rev. Lett. 75, (1995).
5. Fenistein, D.; Bonn, D.; Rafaï, S.; Wegdam, G. H.; Meunier, J.; Parry, A. O.; Telo da Gama, M.; Phys. Rev. Lett. 89, 096101, (2002)
6. Bowers, J.; Zarbakhsh, A.; Christenson, H. K.; McLure, I. A.; Webster, J. R. P.; Steitz, R. Phys. Rev. E. 72 (2005) 041606 Neutron reflectivity studies of critical adsorption: Behavior of the surface scaling function
7. Bowers, J.; Zarbakhsh, A.; McLure, I. A.; Webster, J. R. P.; Steitz, R.; Christenson, H. K. J. Phys. Chem. C 111, 5568 (2007).
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Supported by the EU through FP5, FP6 and FP7
