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Wetting and Freezing of Hexadecane on an Aqueous Surfactant Solution: Triple Point in a 2-D film H. Matsubara,*,† E. Ohtomi,† M. Aratono,† and Colin D. Bain*,‡ Department of Chemistry, UniVersity of Durham, South Road, Durham, DH1 3LE, United Kingdom and Department of Chemistry, Faculty of Sciences, Kyushu UniVersity, Fukuoka 812-8581, Japan ReceiVed: March 11, 2008; ReVised Manuscript ReceiVed: June 2, 2008
Wetting of water by hexadecane has been investigated by ellipsometry as a function of the concentration of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) in the aqueous phase and temperature. Three phases are identified: a 2-D gas of hexadecane molecules and DTAB molecules, a 2-D liquid comprising a mixed monolayer of hexadecane and DTAB, and a 2-D ‘solid’ phase. Evidence is presented to support the hypothesis that the liquid-solid phase transition is actually a wetting transition in which a surface-frozen layer of pure hexadecane wets the liquid-like mixed monolayer of hexadecane and DTAB. The triple point, at which the three phases coexist, is located at a temperature of 17.3 °C and DTAB concentration of 0.75 mmol kg-1. The slopes of the three phase boundaries are analyzed thermodynamically. Introduction For the simplicity of their molecular structure, linear alkanes have remarkably rich and complex phase behavior. For moderate chain lengths, alkanes crystallize in orthorhombic, monoclinic, and triclinic unit cells as well as a variety of stable or metastable rotator phases with the packing depending on temperature, chain length, and whether n is even or odd.1 Alkanes are almost unique in showing surface freezing, in which a crystalline monolayer forms on top of the liquid bulk at temperatures above the bulk freezing point, Tb.2,3 For 16 < n < 30, these monolayers are hexagonally packed with upright chains and exist at temperatures up to 3 °C above Tb, depending on n.4-10 The wetting behavior of alkanes on water is also complex. If a drop of oil is placed on a water surface, there are, at first glance, two possible outcomes: the drop either spreads out to form a film of uniform thickness covering the whole of the surface (known as complete wetting) or it does not spread and remains as a lens (partial wetting). However, the partial wetting state is divided further into two classes when the short- and long-range surface forces across the film are taken into account. Brochard et al. quantified the short-range force by the initial spreading coefficient, Si.11 If Si > 0, the free energy of the system is lowered by replacing the air-water interface by air-oil and oil-water: that is, spreading will be favored by the short-range force. The opposite is true for Si < 0. Since the long-range contribution of liquid alkane films on water is attractive (represented by a positive Hamaker constant, A) and the free energy decreases as the oil film becomes thinner,12 the competition between a positive initial spreading coefficient and attractive long-range force gives rise to a minimum in the interfacial free energy at a finite thickness (pseudopartial wetting). In the partial wetting state, an oil droplet placed on water does not spread even on the molecular level and remains as a lens as a result of both short- and long-range forces inhibiting spreading (Si < 0, A > 0). If the short-range * To whom correspondence should be addressed. H.M: e-mail
[email protected]. C.D.B.: e-mail c.d.bain@ durham.ac.uk. † Kyushu University. ‡ University of Durham.
contributions give oscillations of free energy of the film as a function of thickness, a wetting transition between two pseudopartial wetting states becomes possible.13 Bonn and co-workers experimentally verified that short-chain alkanes exhibit a firstorder transition from the adsorbed film with single molecular thickness to the one with several molecular thickness followed by a continuous increase in thickness with increasing temperature (critical wetting).14-18 They used the term “partial wetting” for the former case and “frustrated complete wetting” for the latter case. The critical transition to the complete wetting occurs when the Hamaker constant changes sign. We have previously shown that adsorption of surfactant molecules (such as tetramethylammonium dodecyl sulfate or dodecyltrimethylammonium bromide (DTAB)) at the air-water interface induces the first-order wetting transition of hexadecane from the partial wetting state to pseudopartial wetting state.19-22 In pseudopartial wetting, hexadecane molecules form a liquidlike mixed monolayer with DTAB in which the composition of the mixed monolayer depends on the DTAB concentration in aqueous solution (m). Cooling of this mixed monolayer of surfactant and alkane leads to another first-order phase transition whose character depends on the relative lengths of the hydrocarbon chains of the surfactant and alkane. In a detailed study of the surfactant hexadecyltrimethylammonium bromide (CTAB), Wilkinson et al. and Sloutskin et al. showed that for alkanes with 11 e n e 17 the low-temperature phase was a hexagonally packed solid phase with upright, conformationally ordered chains.23,24 For n g 18, an unusual bilayer phase was formed in which a solid layer of pure alkane rested on a liquid-like lower phase.25 The crystallographic structure of the ordered phase was in each case essentially the same as that of the surface-frozen phase of alkanes. Sloutskin et al. proposed that the mixed monolayer in the pseudopartial wetting phase behaves like a liquid alkane that is wet by the solid alkane phase at temperatures below the surface freezing point, Ts, of the alkane.25 For alkanes with chain lengths similar to or shorter than the surfactant this wetting transition is pre-empted by freezing of the mixed monolayer. Whether the ordered phase is of the monolayer or bilayer type can be readily determined by ellipsometry: the change in the coefficient of ellipticity at the
10.1021/jp802108v CCC: $40.75 2008 American Chemical Society Published on Web 08/26/2008
Triple Point of Wetting Film
J. Phys. Chem. B, Vol. 112, No. 37, 2008 11665
Figure 1. (a) Ellipticity vs molality curve in the absence (filled squares) and presence of liquid hexadecane (open squares), measured at 25.0 °C. The initial spreading coefficient is plotted on the right-hand axis (open circles). (b) Ellipticity vs molality curve in the absence (filled squares) and presence of solid hexadecane (open squares) measured at 14.9 °C.
phase-transition temperature is roughly three times as large in the latter case. The three phases that exist for alkane molecules on a surfactant solution can be compared with the three phases of a 2-D fluid: gas (partial wetting), liquid (pseudopartial wetting at high temperatures), and solid (pseudopartial wetting at low temperatures). For brevity we will use the terminology G, L, and S to identify these three phases. To date, the G/L transition has been identified by variation of the bulk surfactant concentration, m, at fixed temperature and the L/S phase by variation of the temperature at fixed m.23,25 Thermodynamically, one would expect these two phase boundaries to meet at a triple point (Tt, mt). In this paper we locate the triple point for the system DTAB + hexadecane at Tt ) 17.3 °C and mt ) 0.75 mmol kg-1 and show that a G/S phase boundary exists below Tt. Since the alkane chain length (n ) 16) is four carbons longer than the surfactant chain (n ) 12 for DTAB), the S phase is predicted to be of the bilayer type, which is confirmed by ellipsometry. The slope of the phase boundaries is interpreted in terms of the thermodynamic parameters associated with the phase transitions. Experimental Section Materials. DTAB (Tokyo Kasei Co., Ltd., 99.5%) was recrystallized twice from a mixture of acetone and ethanol and dried in vacuum. The purity was confirmed by the absence of a minimum on the surface tension vs concentration curve around the cmc. Water for measurements was distilled three times from dilute alkaline permanganate solution. n-Hexadecane (Kanto Chemical Co., Inc., >98%) was distilled fractionally under reduced pressure (bp 133-134 °C at 3.0 mmHg). The purity was estimated to be >99.5% by GC. Interfacial Tensiometry. Interfacial tensions were determined as a function of the molality of DTAB, m, in aqueous solutions at 25.0 °C by analyzing the shape of a pendant drop hanging on a glass capillary tip.26 The experimental error in the interfacial measurements was
