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Wetting of Mercury Surfaces by Halide Electrolyte Solutions Zhenghe Xu,* Qingxia Liu, Jinghong Ling, and Allen Summers Department of Mining and Metallurgical Engineering, McGill University, Montreal, Quebec, Canada H3A 2A7 Received July 31, 1995. In Final Form: September 25, 1995X The contact angle, contact angle hysteresis, and interfacial tension were measured using a model system consisting of mercury and halide aqueous solutions. The measured contact angle hysteresis was discussed in terms of the surface roughness, chemical heterogeneity, and mechanical instability. The experimental results of contact angle measurements were analyzed using the surface force theory of wetting films, which demonstrated the presence of hydrophobic forces. The origin of the hydrophobic force observed in the present system was discussed briefly. The importance of hydrated halide anions in controlling the wettability of mercury surfaces was demonstrated. Finally, a correlation between the interfacial tension and hydrophobic force was illustrated.
Introduction Wetting and dewetting are fundamental subjects which have a variety of industrial applications, including painting, welding, adhesion, lubrication, tertiary oil recovery, detergents, pesticides, materials engineering, and flotation. A useful measure of wetting characteristics is contact angle analysis, particularly in the case of partial wetting. Considered as a thermodynamic quantity, the contact angle has been related to the intermolecular interactions between various phases in contact. The surface properties, such as the type and state of molecules in the surface region, have been identified as controlling parameters for the desired wettability. Recent advances in understanding the molecular mechanisms of wettability and its connection with contact angles have made contact angle measurements a convenient method for characterizing polymer surfaces.1 Three fundamental equations are often used in relating surface and interfacial tensions to measured macroscopic contact angles as described in the following. Young’s equation relates the cosine of equilibrium contact angle (θe) to three interfacial tensions by:2
cos θe )
γsv - γsl - πe γlv
(1)
where γ is the surface or interfacial tension, and subscripts sv (12), lv (32), and sl (13) represent solid-vapor, liquidvapor, and solid-liquid interfaces, respectively. πe in the above equation is the equilibrium spreading pressure defined as (γs - γsv). In most cases of partial wetting (i.e., θe > 0), πe can be considered negligible.3 Young’s equation implies that a strong interaction between a solid and a liquid, i.e., a small value of γsl, results in a low contact angle. A nonzero πe is characteristic of the presence of an adsorbed film on the solid surface. * Corresponding author. E-mail:
[email protected]. Telephone: (514) 398-8373. Fax: (514) 398-4492. X Abstract published in Advance ACS Abstracts, December 15, 1995. (1) Lee, L. H. J. Adhes. Sci. Technol. 1993, 7, 583 also in Contact Angle, Wettability and Adhesion; Mittal, K. L., Ed.; VSP: Utrecht, 1993; p 45. (2) Good, R. J. In Contact Angle, Wettability and Adhesion; Mittal, K. L., Ed.; VSP: Utrecht, 1993; p 3. (3) Fowkes, F. M. Ind. Eng. Chem. 1964, 56 (12), 40.
0743-7463/96/2412-0547$12.00/0
The extension of Fowkes’ equation3 by including acidbase interaction forms the second fundamental equation as follows:4 LW γ13 ) γ1 + γ3 - 2xγLW - 2xγx1 γQ3 - 2xγQ1 γx3 1 γ3
(2)
where the superscripts LW, x, and Q indicate the apolar (Lifshitz-van der Waals), and Lewis acid and base components of the surface tension, respectively. A combination of eqs 1 and 2 results in a useful expression:2
cos θe )
1 [2( γLWγLW + xγx1 γQ3 + xγQ1 γx3 ) - πe] - 1 γ3 x 1 3 (3)
Equation 3, often referred to as the Young-GoodGirifalco-Fowkes equation,1 has been further confirmed recently using a liquid mercury substrate.5 The importance of this equation is that it relates the macroscopic contact angle to the contributions of various types of microscopic interactions across the interface. Equation 3 does not, however, provide any information regarding the long range nature of interactions between two macroscopic phases. A third relation was derived to include the long range nature of the interactions using the concept of disjoining pressure, Π(h), or interaction free energy VT(h) as2,6
cos θe ) 1 +
∫h∞Π(h) dh ) 1 + γ1lvVT(he)
1 γlv
e
(4)
where h is the thin film thickness and VT(he) is the interaction energy between solid and air across a liquid film at the equilibrium film thickness, he, defined as the thickness at which the disjoining pressure of the free film is zero with a negative disjoining pressure gradient or where the primary minimum is located in the energyfilm thickness profile. Three major components, i.e., molecular VM (ΠM), electrostatic VE (ΠE), and structural VS (ΠS) components, have been included in the total interaction energy VT (disjoining pressure Π) to account for the observed stability of general wetting films.7 Each of these components will be considered in turn. For two macroscopic subjects (solid (4) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927. (5) Xu, Z.; Liu, Q.; Ling, J. Langmuir 1995, 11, 1044. (6) Israelachvili, J. N. Intermolecular & Surface Forces, 2nd ed.; Academic Press: New York, 1992. (7) Derjaguin, B. V.; Churaev, N. V. J. Colloid Interface Sci. 1985, 103, 542.
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and air) interacting across a liquid (e.g., electrolyte solutions), the general formula for a nonretarded molecular component is given by
A132 VM(h) ) 12πh2
(5)
where A132 is the combined Hamaker constant between a solid (1) and air (2) across a liquid (3). This component results from the interactions between electromagnetic waves of two interacting phases, which can be either attractive or repulsive depending on the relative magnitude of the polarizability of individual phases. For an equilibrium system of a liquid droplet on a solid substrate (as in contact angle measurements), the molecular component is often repulsive unless the liquid is more polarizable than substrate.8 The contribution due to the overlap of electric double layers of interacting phases is often approximated by eq 6 for constant surface potential or eq 7 for constant surface charge density boundaries:9
Vψe (h) )
κ [(ψ12 + ψ22)(1 - coth κh) + 2 2ψ1ψ2 cosech κh] (6)
Vσe (h) ) -
κ [(ψ12 + ψ22)(1 - coth 2κh) 2 2ψ1ψ2 cosech κh] (7)
In eqs 6 and 7, superscripts ψ and σ represent the constant surface potential and constant surface charge density boundary conditions, respectively; is the dielectric constant; ψ1 and ψ2 are the electrical potentials at the solid-liquid and air-liquid interfaces, respectively; and 1/κ is known as the Debye length.6,9 Equations 6 and 7 show that the electrical double layer component can be either attractive or repulsive, depending on the sign and magnitude of the surface charge at two interacting interfaces. The electrostatic interaction can change from repulsive to attractive at a given film thickness when the two interacting surfaces have potentials which are the same in sign but different in magnitude, or vice versa when they have opposite surface potentials, depending on the boundary conditions.10 It should be noted that these two equations are adequate for describing the interaction energy at greater film thickness (>1/κ) and/or low surface electrical potentials (99.9995%) from Sigma was used without further purification. The electrolyte solutions were made using high-purity KF, KCl, (12) Ducker, W. A.; Xu, Z.; Israelachvili, J. Langmuir 1994, 10, 3279. (13) Pashley, R. M. Adv. Colloid Interface Sci. 1982, 16, 57. (14) Tsao, Y. H.; Evans, D. F.; Wennerstro¨m, H. Langmuir 1993, 9, 779. (15) Israelachvili, J.; Pashley, R. M. Nature 1982, 300, 341. (16) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (17) Usui, S. In Electrical Phenomena at Interfaces; Kitahara, A., Watanabe, A., Eds.; Marcel Dekker, Inc.: New York, 1984; p 15.
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KBr, and KI, and the solution pH was adjusted using HF/HCl and NaOH. All these chemicals (Sigma) were above 99.999% purity and were used as received. The water used in this work was treated using a Millipore ion exchange column followed by double distillation. Contact Angle Measurements. Contact angles of electrolyte solutions on the mercury surface were measured at 25 ( 2 °C using the sessile drop method with a contact angle goniometer (Rame-Hart). A square pyrex container was made large enough (20 × 20 mm2) to ensure that the mercury surface was molecularly smooth and flat. A droplet of approximately 5 µL of electrolyte solution was placed by gentle touching onto the mercury surface and left for about 5 min prior to measuring the equilibrium contact angle (θe). The advancing (θA) and receding (θR) contact angles were measured as the three-phase contact line moved in the course of adding the liquid to and retracting it from the droplet. To minimize the vibration of the liquid surface, the mercury was added to just cover the inner bottom surface of the container. The average of six independent measurements with an accuracy of (1° is reported. Interfacial Tension Measurements. The mercury-aqueous solution interfacial tension was measured using the drop weight method following the procedures of Harkins and Grafton.18 A fine capillary tube (approximately 2 µm in diameter) was made by stretching a pyrex capillary tube of 1.5 mm diameter to control the dropping rate. The drop was drawn by siphon at a rate of about one drop per 90 s to ensure equilibrium. Ten mercury drops were collected in the testing liquid and weighed to calculate the interfacial tension (γml) using the following equation:
γml )
mg 2πrf
Figure 1. Advancing and receding contact angles measured using KF aqueous solutions on mercury.
(9)
where m is the average weight per drop, g the acceleration of gravity, r the radius of the capillary tube (0.75 mm), and f a correction factor. The f value can be calibrated from the measured drop weight of mercury in dry N2 and its known surface tension. Using a typical f value of 0.744 obtained in this way, a value of 425 mN/m was obtained for the interfacial tension of mercury in pure water, which is in good agreement with the reported value of 426-427 mN/m.3 With this procedure, a precision of (4 mN/m in measured interfacial tensions can be achieved.
Results An advancing contact angle of 62° was obtained when measured with pure water on the mercury surface. A similar value has been reported by Salamy and Nixon19 and by Nakamura et al.20 The contact angle hysteresis was found to be relatively small (4°) with a receding contact angle of 58°. The results of advancing and receding contact angle measurements using potassium fluoride solution are given in Figure 1. This figure shows that the contact angle decreases monotonically with increasing KF concentration, while the contact angle hysteresis remains the same within the experimental error. With 0.05 M KF solution, the advancing contact angle decreased from 62° to 42°. It is well documented21 that up to a concentration of 0.1 M, the surface tension of halide electrolyte solutions is almost the same as that for pure water. Since the surface tension of mercury is constant, a decrease in contact angle with increasing electrolyte concentration indicates a decrease in the interfacial tension as implied by Young’s equation (eq 1). It should be noted that the contact angle hysteresis remained the same although the contact angle decreased with increasing KF concentration. (18) Harkins, W. D.; Grafton, E. H. J. Am. Chem. Soc. 1920, 42, 2534. (19) Salamy, S. G.; Nixon, J. C. In Recent Developments in Mineral Dressing, Proceedings of the First International Mineral Processing Congress, London, 1952; IMM: London, 1952; pp 503-518. (20) Nakamura, Y.; Kamada, K.; Katoh, Y.; Watanabe, A. J. Colloid Interface Sci. 1973, 44, 517. (21) Weast, R. C., Astle, M. J., Beyer, W. H., Eds. Handbook of Chemistry and Physics, 66th ed.; CRC Press, Inc.: Boca Raton, FL, 1986; p F-31.
Figure 2. Advancing contact angles measured using KCl, KBr, and KI aqueous solutions on mercury.
This relatively small and constant contact angle hysteresis observed in the present study will be discussed later in the Discussion. The advancing contact angles measured using other halide electrolyte solutions are given in Figure 2. All the halides are shown to decrease the advancing contact angle with increasing concentration. The decrease in contact angles is more pronounced for KI than KBr, followed by KCl and KF. At an electrolyte concentration of 0.01 M, the contact angles are reduced from a value of 62° for water to 41°, 38°, 18°, and 5° for F-, Cl-, Br-, and I-, respectively. This follows the opposite order of the hydration series of these halides, suggesting that Iadsorbs most strongly at the mercury/water interface, followed by Br-, Cl-, and F-, in general agreement with the observations of electrocapillary phenomena. It is, however, important to mention that although the halides have a different impact on the wettability of mercury, the measured contact angle hysteresis remains unchanged, regardless of the type of halide and their concentrations (not included in this figure). To elucidate the effect of electrostatic interactions on the wettability of mercury, the contact angle was also measured as a function of the solution pH. The results given in Figure 3 show that over a range of pH from 4 to 8, the contact angle remained the same. Above pH 9, the contact angle increases with increasing pH. It is not clear
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Figure 3. Advancing contact angles of aqueous solution on mercury as a function of pH adjusted with HF, HCl, and NaOH, respectively.
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Figure 4. Interfacial tension between mercury and halide aqueous solution obtained by the drop-weight method.
why contact angles of water on mercury increase with increasing pH above 8.0. It is, however, well documented that hydroxide ions do not specifically adsorb at the mercury-aqueous solution interface.22 The surface potential at the mercury-water interface can, therefore, be considered relatively constant (negatively charged) over the pH range. The surface potential at the air-water interface, on the other hand, becomes increasingly negative with increasing pH above 8.0.23 On the basis of eq 6, the electrical double layer attraction is expected to increase with pH due to the increased difference in surface potentials, both negatively charged.10 This may partially account for the observed increase in contact angles with pH. Below pH 3.0, the contact angle decreases with decreasing pH. The significant reduction can be attributed to the presence of a significant amount (>0.001 M) of halide introduced as a pH modifier, the effect of Cl- being more pronounced than that of F-. This is consistent with the findings presented in Figures 1 and 2 where halide salts were used as electrolytes. The contact angle value measured at pH 2.5 adjusted with HCl (42°) is lower than that measured with KCl at equivalent halide concentration but neutral pH (53°). This difference may be attributed to a decreased surface potential (less negative) of the airwater interface with decreasing pH, resulting in a stronger electrostatic repulsion due to the reduced difference in surface potentials. To obtain a better understanding of the observed effect of halides on the wettability of mercury, the mercuryelectrolyte solution interfacial tension was measured, and the results are given in Figure 4. It is interesting to note that interfacial tension decreases with increasing electrolyte concentration, with the reduction by I- being the most significant. (This is similar to the effect of halide concentration on contact angles.) The interfacial tension decreases from a value of 430 mN/m for pure water to 410, 401, 370, and 342 mN/m with increasing F-, Cl-, Br-, and I- concentrations to 0.05 M, respectively. It is expected that specific adsorption of halides at the mercury-aqueous interface increases the surface charge densities, which may account for observed reduction of the interfacial tension with the increasing halide concentrations. The first order effect of the hydration from adsorbed halides
may also contribute significantly to the reduction of interfacial tensions, since the interfacial tension reflects the interactions across the interface.3 Figure 5 shows the interfacial tensions measured as a function of solution pH. It is interesting to note that the interfacial tension does not change in the neutral pH range (5-7) but increases with further increase of pH above 7. OH- is believed to behave similarly to F- with a hydration number of 3. We would expect a decrease of the interfacial tension with increasing OH- concentration as in the case of F-. The opposite trend observed is not well understood. Nevertheless, this trend is similar to the effect of OH- on the measured contact angles (Figure 3), suggesting that the presence of OH- on mercury reduces the adhesion between water and mercury, resulting in a less wettable surface. The significant decrease of the interfacial tension at pH values below 4 when HCl is used as the pH modifier can, on the other hand, be attributed to the specific adsorption of Cl- at the mercury-aqueous interface, in contrast to the small reduction in interfacial tensions when HF is used as the pH modifier where F- does not specifically adsorb at the mercury-aqueous interface over the equivalent F- concentration range studied, in agreement with observed electrocapillary phenomena.
(22) Grahame, D. C. Chem. Rev. 1947, 41, 441. (23) Li, C.; Somasundaran, P. J. Colloid Interface Sci. 1991, 146, 215.
Discussion Contact Angle Hysteresis. Hysteresis between advancing and receding contact angles is a general phe-
Figure 5. Interfacial tensions between mercury and aqueous solution as a function of pH adjusted using HF, HCl, and NaOH, respectively.
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nomenon in contact angle measurements.24,25 The hysteresis is often attributed to the irreversible transition between two states.25 Three major causes of contact angle hysteresis have been proposed recently to account for the experimental observations.26 They are surface roughness, surface heterogeneity, and mechanical instability. In the systems studied here, the substrate surface is molecularly smooth. The hysteresis due to the contribution of surface roughness is unlikely to be significant. When pure water is used as the probing liquid, the chemical contribution (heterogeneity) to the observed contact angle hysteresis could be negligible. The observed contact angle hysteresis (4°) is, therefore, attributed mainly to mechanical instability. It is well documented that under the influence of surface tension, solids deform at the three-phase contact line, which is responsible for the mechanical instability.27 The hysteresis value obtained in this study is much smaller than those observed with solid substrates. This may be related to the low stress that can be applied to a liquid substrate (mercury). Liquids (near perfectly elastic) adjust their shapes to accommodate any small unbalanced stresses which may exist in any solid substrates. Therefore, for a perfectly elastic substrate, unlike the solid substrate, no net work is done by the spreading force as in a rigid surface, i.e., no mechanical hysteresis occurs. For these systems, contact angles are reproducible and do not possess hysteresis.27 It appears that wetting (advancing) and dewetting (receding) can be achieved via a continuous series of equilibrium states for liquid substrates. The deformation of substrate at the three-phase contact line propagates along the substrate, following the advancing and receding process. The small value of contact angle hysteresis observed suggests that only a minimal amount of energy is dissipated during wetting and dewetting for the present system. When the contact angle is measured using halide electrolyte solutions, the specific adsorption of halides on the mercury substrate is expected to result in chemical hysteresis. However, experimental results showed a constant contact angle hysteresis of 4° although both the advancing and receding contact angles decreased significantly with increasing halide concentration (Figures 1 and 2). This finding further confirms that the observed contact angle hysteresis is mainly due to mechanical instability, independent of the detailed chemistry of the system. More importantly, this finding sheds more light on the nature of the adsorption of hydrated halides on mercury surfaces: adsorption/desorption of hydrated halides onto/from mercury is spontaneous and reversible. To further confirm this, the contact angle was measured using pure water on the mercury which had been previously contacted with a large drop of 10-3 M KI solution for 15 min (the drop was removed right before placing a small water droplet on the same area). Under these conditions, a contact angle value of 61° was obtained. This value is significantly higher than that obtained on mercury in the presence of 10-3 M KI (18°) but almost identical to that obtained with fresh mercury (62°), suggesting that the “specifically” adsorbed halides were removed from the surface with the droplet. The contact angle was also measured with different rates of adding and retracting the liquid in an attempt to investigate the kinetic effect of possible chemical hysteresis. Within experimentally obtainable rates, no noticeable difference in the contact angle hysteresis was observed. These experimental (24) Starov, V. M. Adv. Colloid Interface Sci. 1992, 39, 147. (25) Marmur, A. Adv. Colloid Interface Sci. 1994, 50, 121. (26) Israelachvili, J. N. Surf. Sci. Rep. 1992, 14, 112. (27) del Cerro, M. C. G.; Jameson, G. J. In Wetting, Spreading and Adhesion; Padday, J. P., Ed.; Academic Press: London, 1978; p 61.
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Figure 6. Calculated interaction energy components of a water free film on mercury as a function of film thickness: Circles represent the molecular component with A132 ) -7.22 × 10-20 J; lines are for the electrostatic component and total interaction energy, with ψb ) -30 (dotted) and -85 (solid) mV, and ψm ) 0 mV in 0.1 M KCl electrolyte solution.
results clearly demonstrate the fast and reversible adsorption of hydrated halides on mercury, and the observed contact angle hysteresis (4°) is, therefore, mainly resulting from mechanical instability. Hydrophobic Forces. To understand the observed effect of halides on mercury wettability at the molecular level, it is instructive to examine the contributions from various interaction components known to affect macroscopic contact angles. The molecular component of the interaction (VM) can be calculated using eq 5 by knowing the Hamaker constant A132. With a typical value of -7.22 × 10-20 J, reported for the mercury-water-air system,28 a repulsive molecular component was obtained as shown in Figure 6 (open circles). The component of the electric double layer interaction (VE) can be calculated with known surface potentials of both the air- and mercury-electrolyte solution interface. The surface potential at the air-electrolyte solution interface has been measured in various laboratories, and the value varies considerably from -30 to -85 mV at neutral pH, depending on the techniques used. In our calculations of VE, these two limiting values were used. The potential at the mercury-aqueous interface, on the other hand, can often be controlled using an external potentiostat and held constant. In the present paper, the charge at the mercury-electrolyte interface was mainly due to the adsorbed electrolyte species, including H+ and OH-, and therefore, the potential value is not known. As an illustration, we chose a zero potential at the mercuryKCl aqueous solution interface (i.e., at point of zero charge (pzc)), at which a contact angle of 110° has been reported with a 0.1 M KCl solution.20 The calculation of the electrostatic interaction energy was conducted using a numerical approach for solving the exact PoissonBoltzmann equation with constant potential boundary conditions for both interfaces.11 The results also given in Figure 6 for ψb ) -30 (dotted) and -85 (solid) mV show that the electrostatic contribution of the interaction is attractive between a charged bubble and a neutral mercury surface. This attraction is often attributed to the interactions between the charges on the charged surface (bubble) and imaging charges on the neutral surface (mercury). Summation of molecular and electrostatic components forms the classical DLVO theory proposed originally for (28) Usui, S.; Sasaki, H.; Hasegawa, F. Colloids Surf. 1986, 18, 53.
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Figure 7. Calculated interaction energy profiles of a water film on mercury as a function of film thickness: ψb was set at -85 mV with a λ value of 1 (open circle, K ) -960 mJ/m2) and 12 (solid circle, K ) -668 mJ/m2) nm. Table 1. Values of K in Eq 8 Calculated from Given λ Values To Account for the Observed Contact Angle Value of 110° at the pzc of the Mercury Surface (in 0.1 M KCl Solutions): A132 ) - 7.22 × 10-20 J and ψb ) -85 mV λ (nm) K (mJ/m2) he (nm)
1.00 -960 0.3
2.04 -811 0.4
4.02 -734 0.5
5.98 -703 0.5
7.99 -686 0.6
9.99 -676 0.6
11.99 -668 0.7
predicting colloidal stability29 and extended recently to predict free film stability.7 A positive total interaction energy (VT), which increases with decreasing film thickness, was obtained using either -30 or -85 mV as the surface potential of an air-water interface at the pzc of a mercury-aqueous interface. These results suggest that a completely wetting film of an electrolyte solution containing 0.1 M KCl on mercury (i.e., θe ) 0°) is expected according to the DLVO theory. Apparently, this contradicts the experimental results reported,20 where θe ) 110° was obtained. To explain the experimental observation, an attractive non-DLVO contribution, often known as the hydrophobic attraction, has to be included. Taking the form given in eq 8, at least two parameters (K and λ) are required to define the additional non-DLVO interaction energy. The measured contact angle value alone provides only one boundary condition for estimating K or λ using eq 4 in combination with eqs 5, 6 or 7, and 8. Therefore, one of the parameters has to be assumed. Considering that λ values in the range from 1 to 12 nm, have been reported from recent studies into the hydrophobic interaction,14 we calculated K values corresponding to varying λ (from 1 to 12 nm) and the results are given in Table 1. In the calculation, a surface potential of -85 mV at the air-water interface was used, which gives rise to the largest attractive contribution. (A possible consequence of this assumption is to underestimate the hydrophobic interaction component.) The structural component of the interaction energy was calculated using the fitted parameters, and results for λ ) 1 nm (open circles) and 12 nm (solid circles) are given in Figure 7. It is evident that the attractive structural component is responsible for nonwettable surface characteristics of mercury, regardless of the λ values used. This observation is consistent with those obtained from direct force measurements and coagulation experiments using hydrophobic materials. However, the K values (29) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948.
obtained for the present system (Table 1) are one order of magnitude greater than those obtained from the direct force determinations using deposited monolayers14,30-32 as well as those derived from coagulation experiments using hydrophobic coal and methylated silica particles.33 This may be attributed to the stronger hydrophobicity of mercury and air with interfacial tension values of 425 and 72.8 mN/m, respectively. Since the calculated K value decreases (in magnitude) with increasing λ (Table 1), it is also possible that the hydrophobic interaction between air and mercury across water has a longer (λ > 12 nm) decay length which, if used in the calculation, will lower the fitted K values. In fact, a λ value of 15-60 nm has recently been proposed34,35 for wetting films of aqueous solutions. It is interesting to note that there was no need to impose a cut-off distance in the fitting procedure. These fitting procedures resulted in equilibrium film thicknesses ranging from 0.3 to 0.7 nm, depending upon the λ values used. These values suggest that the water film on a hydrophobic substrate, such as mercury, appeared to be only 2-4 molecular layers thick. The existence of a water film on hydrophobic mercury was indirectly confirmed by the measured finite value of πe. Thermodynamic analysis36 has shown that this thin layer consisted of oriented molecules, which is known as an autophobic layer. The presence of an autophobic layer is characteristic of a lowenergy liquid (water) on a high-energy substrate (mercury). The molecules from sessile drops adsorb on the substrate reversibly, forming a film a few molecular dimensions thick. The results obtained from thermodynamic considerations are consistent with our calculated film thickness and observed small contact angle hysteresis. It is worth mentioning that when the contact angle is greater than 30°, the calculation using the fitted K value with λ ) 1 nm showed an unstable film (a monotonic decrease in VT always below zero). If the contact angle is below 10°, a stable film was predicted regardless of the K values used (a monotonic increase in VT always above zero). Over the contact angle range from 10° to 30°, the calculation showed the presence of a barrier on the energy profile, indicating a metastable film. These observations are consistent with those reported recently.8 The presence of an additional attractive force between mercury and air across the electrolyte solution is evident. Whether this additional force has the same origin as those observed between two hydrophobic monolayers remains to be confirmed. Among the various mechanisms considered thus far,37,38 the domain mechanism proposed by Tsao et al.39 is unlikely to be responsible for the observed additional attractive force since no domains are involved in the present system. Neither can the cavitation mechanism38,40-42 account for the results of contact angle (30) Rabinovich, Ya. I.; Guzonas, D. A.; Yoon, R. H. Langmuir 1993, 9, 1168. (31) Claesson, P. M.; Christenson, H. K. J. Phys. Chem. 1988, 92, 1650. (32) Yoon, R. H.; Ravishankar, S. A. J. Colloid Interface Sci. 1994, 166, 215. (33) Xu, Z.; Yoon, R. H. J. Colloid Interface Sci. 1990, 134, 427. (34) Churaev, N. V. Colloids Surf., A: Physicochem. Eng. Aspects 1993, 79, 25. (35) Kurihara, K.; Kato, S.; Kunitake, T. Chem. Lett. 1990, 1555. (36) Schrader, M. E. In Contact Angle, Wettability and Adhesion; Mittal, K. L., Ed.; VSP: Utrecht, 1993; p 109. (37) Ruckenstein, E.; Churaev, N. J. Colloid Interface Sci. 1991, 147, 535. (38) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468. (39) Tsao, Y. H.; Evans, D. F.; Wennerstro¨m, H. Science 1993, 262, 547. (40) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390. (41) Meagher, L.; Craig, V. S. J. Langmuir 1994, 10, 2736.
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measurements conducted under equilibrium conditions. A third mechanism proposed by Attard43 attributes the additional long range attractive force to correlated polarization fluctuations due to the state of water on hydrophobic surfaces. On the basis of this theory, the range of interaction should scale with the Debye length, which is electrolyte concentration dependent. To examine this mechanism, further fitting using eq 4 was conducted with an increased KCl concentration of 1 M. The experimental contact angle value of 112° at pzc20 was used in the calculation. Compared with K values obtained in 0.1 M KCl solution, no noticeable variations in K were obtained at a given value of λ, suggesting that the long range attractive force is unlikely dependent on the electrolyte screening. This is consistent with the observations in the direct force measurement.44 It appears, therefore, that the ordering of boundary layer water molecules is responsible for this additional attractive force between mercury and air across water. The presence of an autophobic water film (0.4 nm derived from fitting procedures) provides indirect evidence for the ordering of water molecules. It should be noted that the ordering of water dipoles (next to hydrophobic surfaces) parallel to the surface has been proposed to account for the enormous long range attraction between two hydrophobic solid surfaces45 and between a hydrophobic solid and an air bubble.46 These results were also confirmed by molecular dynamic simulations.47 Effect of Halide Adsorption on Hydrophobic Interaction. The contact angle results together with measured interfacial tensions clearly demonstrate the important role of hydration of specifically adsorbed halides in controlling the wettability of the hydrophobic mercury surface. Even for F-, which has been considered not to adsorb specifically at mercury-aqueous solution interfaces, the contact angle and interfacial tension were found to decrease with increasing F- concentration. The increase in wettability appears to be related to the adsorption of halide anions at the mercury-aqueous interface, reducing the hydrophobicity of mercury due to the presence of hydrated halides. The adsorption of halide anions, on the other hand, induces a negative surface charge at the mercury-aqueous interface in the absence of applied potential. The electrostatic repulsive force between mercury and negatively charged air bubbles may also contribute to the reduction in contact angle. To understand the relative importance of these two effects, the contact angle as a function of mercury surface potential at a given bubble surface potential (-85 mV) was calculated using eq 4. In calculating the structural component (VS) using eq 8, the fitted K values at the pzc of mercury (θe ) 110°) with λ ) 1 and 12 nm, which cover a wide range of surface hydrophobicities, were used. The calculation shows that mercury exhibits the lowest wettability (i.e., largest contact angle) at the pzc (Figure 8), and the minimum contact angle value is expected at the surface potential of mercury equal to that of the airwater interface (-85 mV). These two general observations are independent of the chosen λ values describing hydrophobic interaction components. The calculated reduction of the contact angle is greater when assuming a small decay length value (1 vs 12 nm) but only amounts to 6° for the smallest decay length value (λ ) 1 nm). A similar (42) Yaminsky, V. V.; Ninham, B. W. Langmuir 1993, 9, 3618. (43) Attard, P. J. Phys. Chem. 1989, 93, 6441. (44) Christenson, H. K.; Fang, J.; Ninham, B. W.; Parker, J. L. J. Phys. Chem. 1990, 94, 8004. (45) Israelachvili, J. N.; McGuiggan, P. M. Science 1988, 241, 795. (46) Schulze, H. J. Physio-chemical Elementary Processes in Flotation; Elsevier: New York, 1984. (47) Lee, C. Y.; McCammon, J. A. J. Chem. Phys. 1984, 80, 4448.
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Figure 8. Calculated contact angle as a function of mercury surface potential (ψm) using λ ) 1 (K ) -962 mJ/m2) and 12 (K ) -668 mJ/m2) nm: ψb was set at -85 mV in 1 mM KCl solutions.
finding using a different approach was reported recently by Fokkink and Ralston.48 In their study with a nonpolarizable substrate, only a few degrees of reduction in contact angle was predicted over a surface potential of 200 mV away from its pzc if the contact angle at the pzc is ca. 100°. This finding suggests that the hydration from adsorbed halide anions at mercury is likely to be the dominating effect for the observed increase in wettability where a contact angle decrease of as much as 60° using a 0.01 M KI electrolyte solution was seen. The more pronounced effect with I-, which is known as the most weakly hydrated, is attributed to an increase in the adsorption density, usually one order of magnitude higher than Br- and even higher than other halides.22 The strong influence of halide adsorption on the wettability of mercury was also reflected in the significant reduction in interfacial tensions and increase in equilibrium spreading pressure of mercury in contact with halide aqueous solutions. The presence of hydrated halide anions on mercury appears to reduce the hydrophobic forces. This can be illustrated by further calculating the K values at a given electrolyte concentration assuming a constant decay length of 1 nm, as an example. Since the effect of electrostatic repulsion on the wettability of mercury is relatively small (as illustrated in the above calculations), the maximum electrostatic repulsive force is assumed in this series of calculations (i.e., setting both interfacial potentials at -85 mV). The calculated K values are given in Figure 9 for various halides as a function of their concentrations. This figure shows that with increasing electrolyte concentration, the K values decrease in magnitude, indicating a reduced hydrophobic force. The order of reduction in K values at a given halide concentration follows the lyotropic ion series (or the Hofmeister series22), i.e., F- < Cl- < Br- < I-. It is well documented22 that the adsorption density of halide anions at the mercuryaqueous interface follows the same order. Therefore, the observed order of reduction in K values mainly comes from the increased adsorption density of halide anions in the order of F--I-. These hydrated halide anions on the mercury surface break the well-defined parallel orientation of water dipoles due to the strong influence of electric field of these anions. This effect is manifested by the increased adsorption density of halides. As a result, the hydrophobicity and, hence, contact angle and interfacial tensions decrease most significantly when I- is present. (48) Fokkink, L. G. J.; Ralston, Colloids Surf. 1989, 36, 69.
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pronounced. It is not certain why an aqueous halide solution causes an increase in πe. It is possible that the adsorbed halide ions, usually hydrated, spread across the zones around the mercury-solution contact area due to a specific affinity between mercury and halide anions. The degree of this spreading is, however, rather localized around the drop. This was confirmed by measuring the contact angle of water drops on a mercury surface where the water drop was surrounded by but not in contact with 1 mM KI electrolyte solutions. In this case, a contact angle of 58° was obtained. Within experimental error, this value is considered the same as that measured in the absence of surrounding electrolyte solution but significantly higher than that (18°) obtained with the electrolyte solutions. It is interesting to note that πe is closely related to the interfacial tensions for a given system (Figure 10) as expected. These findings suggest that the interfacial tension, or even K value, may serve as a good measure for the surface hydrophobicity. Figure 9. Fitted K values as a function of halide electrolyte concentration with λ ) 1 nm, and ψb and ψm ) -85 mV.
Figure 10. Illustration of a correlation between interfacial tensions of mercury in contact with halide electrolyte solutions and fitted K values (solid symbols) as well as equilibrium spreading pressures (open symbols): ψm and ψb were set at -85 mV and λ at 1 nm; up triangle, KF; square, KCl; circle, KBr; and down triangle, KI.
In an attempt to examine a possible correlation between hydrophobic interactions and interfacial tensions, the fitted K values are plotted as a function of interfacial tension for all the halides examined in this study. In this set of calculations, both ψb and ψm were set at -85 mV and λ was set at 1 nm for the purpose of illustration. Figure 10 shows that with some scatter, there is a correlation between K values and γml for all the halides at varying concentrations: a decrease in γml due to the adsorption of halides results in a reduced hydrophobic interaction, i.e., reduced K values in magnitude. Also shown in this figure are πe values calculated from the measured contact angle and interfacial tensions using eq 1. All the halides are shown to increase the πe with the effect by I- being more
Conclusions On the basis of the above experimental findings of contact angle and interfacial tension measurements using a mercury substrate and theoretical analysis, the following conclusions are drawn: The contact angle hysteresis is relatively small for a well-defined substrate that is molecularly smooth, chemically homogeneous, and perfectly elastic (as is liquid mercury). The hydrophobicity of mercury and the interfacial tension between mercury and an aqueous solution decrease with increasing halide concentration, with the reduction by I- being most significant followed by Br-, Cl-, and F-, which can be attributed to the increased adsorption density of these halides. The adsorption of halide anions at a mercury-aqueous interface is reversible, posing no chemical hysteresis. The hydrophobic interaction between mercury and air across an aqueous film is one order of magnitude stronger than those observed between two hydrophobic solid surfaces and is responsible for the nonwettable characteristics of mercury. The effect of the electrostatic interaction on the wettability of mercury is relatively small compared with that from the hydration of adsorbed halide anions. The hydrophobic interaction between mercury and air across the electrolyte solutions may have originated from the ordered structure of water molecules, an equilibrium film thickness of 0.4-0.8 nm as an autophobic layer being deduced from the calculations. The interfacial tension may serve as a good measure of hydrophobicity for a given substrate. Acknowledgment. The authors acknowledge the financial support from Natural Sciences and Engineering Research Council of Canada. Fruitful discussions with Professors J. Israelachvili, S. Usui, and J. A. Finch are gratefully acknowledged. We also wish to acknowledge Professor A. Kakkar for allowing us to use his goniometer. LA950644Y
