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Application of a Trisiloxane Surfactant for Removal of Oils from Hydrophobic Surfaces N. V. Churaev,† A. P. Ershov,† N. E. Esipova,† R. M. Hill,*,‡ V. D. Sobolev,† and Z. M. Zorin† Institute of Physical Chemistry of Russian Academy of Sciences, Leninsky Prospect 31, Moscow, 117915, Russia, and Dow Corning Corporation, 2200 West Salzburg Road, Midland, Michigan 48626-0994 Received June 20, 2000. In Final Form: December 13, 2000 Rates of advancing and receding menisci motion, v, under applied pressure difference ∆P in thin methylated quartz capillaries were investigated for trisiloxane solution-gas and trisiloxane solution-silicone oil systems. Dynamic values of the tension of wetting were calculated from the v(∆P) dependence. Both dynamic surface and interfacial tensions and contact angles were assessed. Using solutions of the trisiloxane surfactant, M(D′E8OH)M, nearly complete displacement of silicone oils from hydrophobed capillaries becomes possible. Removal of silicone oil films of various viscosities from the methylated surface of thin (about 5 µm in radius) quartz capillaries using M(D′E8OH)M was investigated. In the case of spontaneous displacement, the oil film converts into a small oil column in front of the moving meniscus. The rate of oil film detachment follows diffusion kinetics as a result of penetration of surfactant molecules between the solid surface and the oil. Comparison of the volume of a smeared off and detached volume of the oil shows that silicone oils are displaced nearly completely. Using a video camera, transformation of a flat silicone oil film with base diameter 0.6 mm on a methylated glass surface into a floating spherical oil droplet was studied. Detachment of the droplet with diameter 0.19 mm occurs very rapidly during 14 s. Removal of an oil droplet sitting on an inclined methylated glass plate occurs as a result of oil emulsification in the course of interaction with a thick climbing trisiloxane film flowing around the droplet. The results obtained show that trisiloxane surfactants may be used not only as superspreaders but also as cleaning agents.
Introduction Aqueous solutions of trisiloxane surfactants spread rapidly over hydrophobic surfaces such as Parafilm or polyethylene. This ability to spread on highly hydrophobic surfaces is called superwetting or superspreading. They are widely used as adjuvants to promote spreading of water-soluble herbicides on waxy weed leaf surfaces. The trisiloxane BE8 surfactant (Dow Corning Corp.) with a molecular structure M(D′E8OH)M, where M ) (CH3)3SiO, E ) CH3O(OCH2CH2)8, and D′ ) Si(CH3)C3H6, is an example of an effective superwetter.1,2 Hydrophobic methyl groups attached to a flexible backbone, which is shorter and wider and occupies a larger volume than, for instance, C12H25 groups, gives aqueous surface tensions of about 20 mN/m, much lower than that for other surfactants.2 In the range of concentrations C0 g 0.007 wt % BE8 aqueous solutions contain vesicles. The rate of droplet spreading over hydrophobic surface grows with solution concentration and reaches a maximum at C0 ) 0.16 wt %.1 In the present paper the effects of BE8 aqueous solutions on displacement of silicone oils from hydrophobic capillaries and on removal of oil droplets and films from hydrophobic solid surfaces are investigated. Surfactant solutions are widely used for this purpose.3-8 The goal of †
Institute of Physical Chemistry of Russian Academy of Sciences. ‡ Dow Corning Corporation. * To whom correspondence should be addressed. E-mail: r.hill@ dowcorning.com. (1) Zhu, X.; Miller, W.; Scriven, L.; Davis, H. Colloids Surf. A 1994, 90, 63. (2) Hill, R. M. Curr. Opin. Colloid Interface Sci. 1998, 3, 247. (3) Mahe, M.; Vignes-Adler, M.; Rousseau, A.; Jacquin, C. G.; Adler, P. M. J. Colloid Interface Sci. 1988, 126, 314. (4) Kao, R. L.; Wasan, D. T.; Nikolov, A.; Edwards, D. A. Colloids Surf. 1989, 34, 389.
this paper consists of an analysis of the extent to which the special properties of the trisiloxane surfactant influence kinetics and efficiency of the above-mentioned processes. Materials and Methods Thin capillaries were prepared using a method of high-speed stretch of a melted quartz tube 0.5 mm i.d. The rotating thickwalled tube was heated locally using acetylene-oxygen burners.9 Quartz capillaries were drawn from high-purity quartz tubes (more than 99.99% SiO2) previously treated with 20% fluoric acid to remove the surface layers of tubes enriched by ionic admixtures. After that, the tubes are washed with triply distilled water up to neutral reaction. The freshly drawn thin quartz capillary (up to 5 m long) was cut into pieces up to 15 mm in length that are stored with sealed ends. The trisiloxane surfactant BE8 was obtained from Dow Corning Corp. Silicone oil was obtained from Rhone Poulenc. Triply distilled water with electrical conductivity 10-6 Ω-1 cm-1, pH 6.5, was used in the experiments. The experimental device developed earlier10 is shown in Figure 1. A capillary with the length L of about 15 cm is glued with epoxy resin into the walls of two high-pressure chambers. Sealed ends of the capillary were broken just before experiments. The capillary ends are placed into the vessels containing the fluids under investigation. The left vessel may be shifted back or forth (using a siphon) to bring the capillary end in contact with a (5) Ershov, A. P.; Zorin, Z. M.; Svitova, T. F.; Churaev, N. V. Kolloidn. Zh. 1993, 55, 39. (6) Churaev, N. V.; Ershov, A. P.; Esipova, N. E.; Iskandarjan, G. A.; Madjarova, E. A.; Sergeeva, I. P.; Sobolev, V. D.; Svitova, T. F.; Zakharova, M. A.; et al. Colloids Surf., A 1994, 91, 97. (7) Churaev, N. V.; Ershov, A. P.; Zorin, Z. M. J. Colloid Interface Sci. 1996, 177, 589. (8) Kabin, J. A.; Tolstedt, S. L.; Saez, A. E.; Grant, C. S.; Carbonell, R. G. J. Colloid Interface Sci. 1998, 206, 102. (9) Sobolev, V. D. A method of manufacturing quartz capillaries, USSR Licence N 833588, Bulletin of Invention N 20(1981). (10) Zorin, Z. M.; Churaev, N. V. Adv. Colloid Interface Sci. 1992, 40, 85.
10.1021/la000864y CCC: $20.00 © 2001 American Chemical Society Published on Web 02/07/2001
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Figure 1. Schematic representation of an experimental device for experiments with thin quartz capillaries. Table 1. Determination of a Mean Capillary Radiusa x (mm) Pc (mmHg) r (µm)
3.03 239 4.57
43.3 239 4.57
63.3 242 4.51
81.3 243 4.49
96.3 243 4.49
a Mean value of the radius r ) 4.52 ( 0.05 µm, conicity 10-6 rad. Gradual changes of the capillary pressure Pc values along the capillary length are caused by a small conicity of about 10-6 rad.
solution. The vessels contain two nonmiscible liquids or a liquid and a gas. In the latter case the left vessel was empty or shifted back. The cell containing the capillary is covered with transparent glass plates for observing microscopically the position of the meniscus and measuring the rate of its motion. The pressure difference in the chambers, ∆P ) P1 - P2, is measured using a mercury manometer. The nitrogen pressures P1 and P2 in the chambers could be maintained and regulated separately. The experiments were performed in a thermostated room. Quartz capillaries with radii r of about 5 µm were used in the experiments. The radius of a capillary was assessed preliminary using an optical microscope (from the side view in immersion) and after that determined more precisely by measuring capillary pressure of a completely wetting liquid in different points of the capillary. It was first shown that triply distilled water may be used for this purpose because no hysteresis was observed when shifting the meniscus back and forth along the capillary length. As an example, some results are shown in Table 1, where capillary pressures Pc measured at different distances, x, from the capillary end and calculated capillary radii r ) 2γ/Pc are shown, where γ is the surface tension. The second way to determine the radius consists of measuring the rate of meniscus movement in a capillary partly filled with a liquid under an applied pressure difference ∆P. In this case for description of the rate of meniscus movement v, cm/s, the Washburn equation11 was used 2
v ) r (∆P + Pc)/8ηl
(2)
where l is the length of the liquid column, Pc ) 2γ/r is the capillary pressure of a completely wetting liquid with surface tension γ, and η is the viscosity. The rate of meniscus motion is determined from v ) dl/dτ, where τ is the time. Substituting this expression into eq 2 and integrating this equation from some initial value l0 to a given value of l, we arrive at an expression that gives the dependence of l on τ:
l2 ) (r2/4π)[(∆P + (2γ/r)]τ
(3)
From this equation it follows that at constant r and ∆P values the dependence l2(τ) must be linear. Experimental data obtained for the same capillary as in Table 1 are shown in Figure 2. The slope of the graphs allows the radius of the capillary to be
Figure 2. Results of calibration of a quartz capillary. In accordance with eq 3, the l2(τ) dependence is linear. The applied pressure drop ∆P was equal to 306.5 mmHg (graph 1) and 344 mmHg (graph 2). calculated from a quadratic equation which follows from eq 3:
r2 + (2γ/∆P)r - (4η/∆P)K ) 0
(4)
where K ) (l2/τ). The position l of the meniscus was determined with an accuracy of 1 µm using a horizontal comparator IZA-2. The device shown in Figure 1 was placed on the movable table of the comparator. Figure 2 shows that in accordance with eq 3 the l2(τ) dependencies are linear. Curve 1 was obtained at an applied pressure ∆P ) 306.5 mmHg and curve 2 at a larger pressure difference of 344 mmHg. The slope of the graphs corresponds to K ) 0.459 cm2/s in the first case and to 0.71 cm2/s in the second. Solution of eq 4 for these two cases gives nearly the same values of radii: r ) 4.55 and 4.54 µm. They do not differ much from the results given in Table 1. The accuracy of the capillary radii determination was usually better than 1%. For experiments with BE8 solutions, hydrophobed quartz capillaries with a methylated surface were prepared. The method consisted in pumping a 5% trimethylchlorosilane solution in benzene through a capillary for several hours and followed by washing of the capillary with pure benzene and drying. Advancing contact angles of water in the hydrophobed capillaries fell in the range from 101° to 105° and receding contact angles from 71° to 78°. For hydrophobed glass surfaces prepared by other agents, contact angle hysteresis ranges from 40° for chlorosilane12 to 22° for paraffin13 and 16-20° for a methylated quartz plate.14 The differences arise mainly due to substrate heterogeneity. BE8 solutions at a concentration of C0 ) 0.16 wt % were sonicated before experiments to destroy aggregates of vesicles. The equilibrium surface tension of the BE8 solution is equal to γ0 ) 21 mN/m.
Trisiloxane Solution-Gas System Figure 3 shows the v(∆P) dependence obtained for the solution-nitrogen system in a methylated quartz capillary, r ) 5.25 µm. The rate of meniscus movement v was determined by measuring the time of travel between two marks on the microscope scale of the comparator IZA-2. The position of the meniscus was determined with an accuracy of about 1 µm. The rates v of meniscus motion were measured within a small part of the capillary, the length of which (e1 mm) was much smaller than the length l of the liquid column in the capillary. In this case, the l value in eq 2 may be considered as constant. (11) Washburn, E. W. Phys. Rev. 1921, 17, 273. (12) Fadeev, A. Y.; Eroshenko, V. A. J. Colloid Interface Sci. 1997, 187, 275. (13) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic: New York, 1963. (14) Crawford, R.; Koopal, L. K.; Ralston, J. Colloids Surf. 1987, 27, 57.
Trisiloxane Surfactants
Figure 3. Rate of meniscus forward (curve 1, v > 0) and backward (curve 2, v < 0) motion in dependence on pressure gradient ∆P for a BE8 solution-gas system in a methylated quartz capillary (r ) 5.25 µm, L ) 12.3 cm). The length of the solution column l ) 7.12 cm. The result obtained for the same capillary previously equilibrated with BE8 solution is shown by curve 3 (l ) 6.5 cm).
Positive values of flow rates, v > 0, characterize advancing motion of the meniscus, and negative ones relate to its receding motion. Results for a capillary that was not preequilibrated with the BE8 solution are shown by the open points (curves 1 and 2). The intersection of the continuation of the linear part of graph 1 with the pressure axis gives the dynamic tension of wetting γA cos θA ) 8.75 mN/m. Because the capillary was not preequilibrated, some loss of surfactant due to adsorption was possible, and the surface tension of the meniscus may be higher than γ0. Calculated dynamic advancing contact angles of water θA lie in the region from 115° (assuming γA ) γ0) to 87°, assuming that the meniscus surface was completely free from surfactant, and γA ) γ ) 72 mN/m for water. In the case of a retreating meniscus, intersection of graph 2 with the pressure axis gives a tension of wetting value equal to γR cos θR ) 17.5 mN/m. Because the surface tension cannot be lower than γ0, values of receding dynamic contact angles of water θR lie in the region between 34° (assuming γR ) γ0) and 74° (assuming γR ) γ). From the slope of the linear graphs 1 and 2, the viscosity η of the solution was calculated using eq 2. The value calculated from graph 2, η ) 0.96 cP, coincides with the value for the bulk solution. However, the slope of graph 1 is larger and corresponds formally to η ) 1.7 cP. This discrepancy may be caused by additional viscous resistance in the zone near the meniscus. Transfer of surfactant molecules from the meniscus to the capillary wall by adsorption results in formation a gradient of surface tension along the meniscus profile, which causes Marangoni flux directed toward the wall. Circulation may arise in the liquid resulting in a supplementary viscous loss. When a capillary was preequilibrated with the solution, the v(∆P) dependence obtained is shown in Figure 3 by curve 3. The slopes of the linear parts of the graph are identical and correspond to the viscosity of the bulk solution. The points of intersection of the extensions of the linear parts with the pressure axis give tensions of wetting equal to γA cos θA ) 18 mN/m and γR cos θR ) 24 mN/m. The intersection of curve 3 with the pressure axis, which corresponds to v ) 0, gives the static value of surface tension γ ) 21 mN/m, which is close to the bulk surface tension of the solution γ0.
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Figure 4. Rate of meniscus forward (v > 0, curve 1) and backward (v < 0, curve 2) motion in dependence on pressure gradient ∆P for a BE8 solution-silicone oil system in a methylated quartz capillary (r ) 3.63 µm, L ) 12.95 cm). The length of the solution column l2 ) 8.39 cm, and of the oil column l1 ) 4.56 cm (see Figure 1).
Because the capillary surface was equilibrated with the solution, it may be supposed that the surface tension of an advancing meniscus remains constant and equal to γ0. This results in the dynamic advancing contact angle of water θA ) 23°. The dynamic surface tension of a retreating meniscus γR exceeds the equilibrium value due to transfer of some surfactant molecules from the meniscus to the wetting film left behind.5-7 Assuming that for water θR g 0, we arrive at γR g 24 mN/m. The procedure of equilibration consisted in pumping of the BE8 solution through a capillary for 3-4 h. Pretreatment of a hydrophobed capillary with the BE8 solution made the surface hydrophilic, and this secures spontaneous suction of the solution into the capillary. An untreated capillary remains hydrophobic, and advancing contact angles do not differ much from contact angles for pure water. A diffusion mechanism of very slow penetration may take place in this case in which the meniscus follows the surface diffusion front of surfactant molecules along the dry part of the capillary.15 Trisiloxane Solution-Silicone Oil System Figure 4 shows the results obtained for the BE8 solution-silicone oil system in a methylated quartz capillary, r ) 3.63 µm. Interfacial tension of silicone oil (molecular mass 2400, viscosity η2 ) 21 cP) in contact with bulk BE8 solution is equal to γ0 ) 2.5 mN/m.16 Three methylated quartz capillaries were first filled with the silicone oil. After that, the oil meniscus was shifted a distance of 83.9 mm from the capillary entrance. At this position the meniscus was shifted back and forth within a small part of the capillary. The v(∆P) dependencies are obtained in the same way as for the solution-air system. Equation 2 in the following form was used for describing the kinetics of mutual displacement:11
v ) r2(∆P + Pc)/8(ηmlm + ηnln)
(5)
where ηm, ηn and lm, ln are the viscosity and length of displaced and displacing liquids, respectively. As seen from Figure 4, when silicone oil displaces the BE8 solution (v < 0, graph 2), the intersection of graph 2 with the pressure axis gives a very low tension of wetting γ cos θA ≈ 2.8 mN/m that does not differ much from γ0. (15) Churaev, N. V.; Zorin, Z. M. Colloids Surf., A 1995, 100, 131. (16) Svitova, T. F., personal communication.
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It may be supposed that in this case nearly bulk interfacial tension is maintained at the meniscus due to desorption of surfactant molecules from the hydrophobic wall in front of the advancing meniscus. Formally, the dynamic advancing contact angle of the oil is about 110°. Positive values of flow rates, v > 0, correspond to displacement of silicone oil by BE8 solution. As seen from Figure 4 (graph 1), to displace the oil, one needs to overcome a dynamic capillary counter pressure of the oil, Pc ) 2γ12 cos θR/r. The intersection point of the linear part of the graph gives γ12 cos θR ) 10 mN/m. For two other capillaries (r ) 5.2 and 5.5 µm) close values, 10 and 12.4 mN/m, were obtained. Because of adsorption of surfactant on the hydrophobic surface newly exposed when the oil is displaced, the interfacial tension becomes higher than γ0. The calculated dynamic retreating angle of silicone oil ranges from θR ) 0, assuming γ12 ≈ 10 mN/m, to θR ) 75°, when the meniscus surface becomes free from surfactant and γ12 equals 41 mN/m, as for the bulk water-oil interface. The capillary counter pressure, which is proportional to the tension of wetting γ12 cos θR ≈ 10 mN/m, is about 3 times smaller than in the case when silicone oil is displaced by pure water. The value of the tension of wetting measured in hydrophobed capillaries in the latter case equals up to 30 mN/m. Therefore, use of the BE8 trisiloxane surfactant may facilitate displacement of oils from hydrophobic pores. Besides, when pure water or electrolyte solution displaces silicone oil, a dynamic wetting film of oil remains on the capillary surface. Because the thickness of the remaining dynamic film grows, according to ref 17, with flow rate, only low rates of displacement are possible in these cases. In the case of trisiloxane surfactant, complete removal of oil films from a hydrophobed surface becomes possible. This was shown in special experiments with methylated quartz capillaries covered inside with a relatively thick wetting film of a silicone oil. Removal of Oil Films from a Hydrophobed Capillary Surface An oil film was formed by smearing off a small oil column over a capillary surface. First, the oil column with length l1 of about 2-3 mm was formed near the open end of a capillary (Figure 5a). The column was shifted a distance L and after that returned to its initial position (Figure 5b). The column length l2 was measured at the same place to avoid the effect of conicity of the capillary (≈10-6). The measured difference ∆l ) l1 - l2 was used for calculation of the thickness of the film. From the equality of the change in the column volume πr2∆l and the volume of the smeared off film 2πrLh, it follows that the mean film thickness is equal to
h ) ∆lr/2L
(6)
The calculated thickness h was compared with that predicted using Derjaguin’s equation:17,18
hd ) (Ar/6πγ)1/3 + 1.32r(vη/γ)2/3
(7)
where the first term takes into account the equilibrium film thickness h0 which corresponds to v ) 0, and the second gives the dynamic thickness of the flowing film. (17) Derjaguin, B. V. C. R. Acad. Sci. USSR, 1943, 39, 13. Derjaguin, B. V. Acta Phys. Chim. 1945, 20, 349. (18) Derjaguin, B. V.; Zheleznyi, B. V.; Tkachev, A. P. Dokl. Akad. Nauk SSSR 1972, 206, 1146 [Phys Chem].
Figure 5. Schematic representation of the smearing off method that was used for formation of oil films on a capillary wall and for measuring their thickness: (a, b) film formation; (c) equilibration of the left empty part of the capillary with BE8 solution; (d) the BE8 solution is brought in contact with the film edge; (e) spontaneous displacement of an oil film by the BE8 solution and conversion of the film into an oil column in front of the moving meniscus. Table 2. Comparison of the Smeared-Off h and Calculated hd Film Thickness of Silicone Oils on Methylated Surface of Quartz Capillaries N
r (µm)
η (cP)
l1 (mm)
l2 (mm)
∆l (mm)
L (mm)
h (nm)
hd (nm)
1 2 3 4 5
4.33 6.7 6.8 6.7 6.4
20 20 105 340 340
6
7.5
500
3.094 2.820 2.258 5.457 3.226 2.869 3.235
2.686 2.580 1.416 3.510 2.502 2.189 2.309
0.408 0.240 0.842 1.95 0.746 0.480 0.926
54.6 46.4 61.0 74.0 71.1 46.3 72.0
16.2 17.3 47.0 88.3 34.3 33.2 48.2
17.1 17.9 47.9 90.9 33.1 33.1 48.6
In the case of nonpolar silicone oil, wetting film stability is governed by dispersion forces only. Therefore, it was possible to use the known equation for disjoining pressure Π(h) ) γ/r ) A/6πh3, where A is the Hamaker constant. The value of A ) 5 × 10-14 erg, determined experimentally for tetradecane films on glass,19 was used in eq 7. Use of the surface tension of silicone oils, γ ) 21 mN/m, gives the thickness of an equilibrium film from 4.1 to 4.4 nm, depending on capillary radius. The thickness of the residual film is also influenced by the rate of smearing v. The second term in eq 7, which gives the dynamic thickness of the film, depends on the surface tension of the oil, its viscosity η, and the capillary radius r. Regulating gas pressure applied to the oil column (Figure 5a), a nearly constant rate v was maintained, of about 10-2 cm/s. In Table 2 are collected the results obtained for six capillaries with radii from 4.3 to 7.5 µm and three silicone oils with viscosities from 20 to 500 cP. The results in Table 2 show that values of h and hd are quite consistent. The process of oil film displacement was investigated in the following way. After an oil film was formed, the (19) Shishin, V. A.; Zorin, Z. M.; Churaev, N. V. Kolloidn. Zh. 1977, 39, 47. Derjaguin, B. V.; Zorin, Z. M.; Churaev, N. V.; In Wetting, Spreading and Adhesion; Academic Press: New York, 1977; p 201.
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Figure 6. Dependence of the length L02 of meniscus penetration of the BE8 trisiloxane solution on time τ into the part of a capillary (r ) 4.33 µm) covered with 16.2 nm thick film of an silicone oil (η ) 20 cP). Table 3. Comparison of the Smeared off h and Collected ho Film Thickness of Silicone Oils on Methylated Surface of Quartz Capillaries N
r (µm)
η (cP)
L0 (mm)
l0 (mm)
h0 (nm)
h (nm)
Ds (cm2/s)
1 2 3 4 5 6
4.33 6.7 6.8 6.7 6.4 7.5
20 20 105 340 340 500
14.6 46.4 59.0 2.15 13.1 23.5
0.117 0.240 0.830 0.055 0.196 0.290
17.3 17.3 47.8 85.7 48.0 46.8
16.2 17.3 47.0 88.3 33.7 48.2
1.25 × 10-4 1.10 × 10-5 3.0 × 10-6 2.8 × 10-5 3.3 × 10-5
remaining surface of the capillary was equilibrated with the BE8 solution by shifting the solution back and forth 10-12 times up to the position of the left end of the oil film (Figure 5c). After that the solution was brought in contact with the film edge (Figure 5d), and this was the starting time for measurements of the rate of spontaneous meniscus movement. After some time the oil film was converted into a small oil column in front of the meniscus (Figure 5e). The length of the column, l0, was measured as a function of time τ. After passing some defined distance L0 (Figure 5e), the mean thickness of the collected film h0 ) rl0/2L0 was determined from the volume of the oil column πr2l0. Now, we can compare the initial film thickness h (Table 2) with the thickness h0 calculated from the value of the collected oil volume. The results are shown in Table 3. As seen from Table 3, agreement between smeared off and collected film thickness is satisfactory. Small differences between h and h0 values are within the limits of experimental error. Capillary N 5 represents an exception. However, in this case undulations on the oil film surface were observed. This shows that trisiloxane surfactants may be used for cleaning oiled hydrophobic surfaces. The experiments have shown that an oil film was nearly completely removed from a methylated quartz surface. The remaining adsorbed layers of BE8 surfactant may be removed from hydrophobed surface be rinsing water through the capillary. One example of the dependence of the measured length L0 of meniscus penetration on time τ is shown in Figure 6. The rate of meniscus motion obeys a linear dependence between L02 and τ which is characteristic of both capillary suction and diffusion. The theory of capillary suction results in this case in a cos θA value of about 2.5 × 10-4, which corresponds to an advancing contact angle θA ≈ 89.99°, very close to 90°. A more realistic mechanism seems to be spontaneous motion of the meniscus following the
front of surface diffusion of trisiloxane molecules that render the capillary surface hydrophilic. The slope of the graph in Figure 6 gives a reasonable value for the coefficient of surface diffusion Ds ≈ 1.25 × 10-4 cm2/s. A similar effect was observed earlier for a nonionic surfactant solution penetrating a hydrophobic capillary.15 Calculated values of surface diffusion coefficients for other capillaries are given in Table 3. The rate of meniscus motion follows diffusion kinetics with coefficients of surface diffusion that range from 10-4 to 10-5 cm2/s. The much lower value, 3 × 10-6 cm2/s, calculated for capillary N 4 in Table 3, may be connected with formation of visually observed small bridges and small oil columns formed as a result of coalescence of an undulating oil film far from the meniscus. This result was confirmed in experiments when a silicone oil and the BE8 surfactant solution were stored in direct contact with each other in a hydrophobed quartz capillary with a much larger radius of about 0.6 mm. Using an optical microscope, it was observed that the interfacial contact angle decreases in the course of time from its initial value ≈90° to 10° after 3 h. The occurring changes may be explained by diffusive penetration of surfactant molecules between the oil and the hydrophobic wall of the capillary. The reality of diffusion of trisiloxane BE8 molecules over the dry methylated quartz surface in front of the meniscus was confirmed in the following way. A meniscus of the BE8 solution was held in a constant position in a methylated quartz capillary (r ) 5.75 µm) for 5 days. After that the solution was displaced from the capillary, and contact angles of water were measured along the capillary axis from the dry part up to the marked position of the meniscus. First, contact angles were measured far from the position. Here advancing and receding contact angles, θA ) 93° and θR ) 84°, were the same for the case of a freshly prepared methylated capillary. Changes in contact angles were detected only in a zone (5 mm in length) near the marked position. Here advancing and receding contact angles of water decrease to 76° and 70°, respectively. This may be explained as a result of surface diffusion of the BE8 molecules in front of the stopped meniscus. From the length of the part of the capillary covered with surfactant molecules (≈5 mm) and time, the value of coefficient of surface diffusion Ds was assessed to be on the order of 10-7 cm2/s. In the case of an oil-covered methylated quartz surface the Ds values (as was shown above) are higher. This means that penetration of surfactant molecules between the oil and the methylated surface occurs much faster. This may be connected with a two-layered diffusion of surfactant molecules. The first layer is formed on the solid hydrophobic surface and the second on the inner oil surface. As a result, both interfaces become hydrophilic, and water molecules can penetrate into the formed gap, securing a higher rate of diffusion. Removal of Oil Droplets from a Hydrophobed Solid Surface in Contact with Trisiloxane Surfactant Solution Figure 7 shows a sketch of the experimental setup (on the top) and the results obtained. A nearly flat round film of a silicone oil with a viscosity of 500 cP, initial diameter D ) 600 µm, and mean thickness of about 13 µm was formed on a methylated microscope cover glass (photo a at the bottom of Figure 7). The glass was placed into a vessel filled with BE8 solution, 0.16 wt %. Changes in the base diameter D as a function of time τ were recorded using a video camera (WV-CP412 Panasonic) and a
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Figure 8. Interaction of a BE8 trisiloxane solution film (3) climbing over an inclined hydrophobed plate (1) on which is resting a silicone oil droplet (2). Positions of the edge of the film are shown before (a) and after contact (b) with the oil droplet.
Figure 7. Transformation of a silicone oil film (1) formed on a methylated glass plate (2) into a floating spherical droplet in a BE8 trisiloxane solution (3). The dependence of the base diameter D of a silicone oil film/droplet on time τ is shown by the points. Arrows a and b show the diameters of the initial oil film and of the floating spherical droplet, respectively.
computer program. The results obtained are shown in Figure 7 by square points and by an approximated solid curve which gives the D(τ) dependence. The shape of the film was gradually changed (Figure 7), and after 14 s a spherical droplet (with diameter of about 19 µm) was detached from the surface (the double image above arrow b is a mirror effect). No visible oil residue was detected on the surface, as was the case for oil films displaced from capillary walls considered above. It may be supposed that the same diffusive mechanism of oil detachment due to penetration of surfactant molecules between the hydrophobed glass surface and the oil takes place in this case. The initial part of the D(τ) dependence shown in Figure 7 corresponds to a coefficient of surface diffusion Ds ≈ 2 × 10-5 cm2/s. The value is close to the data listed in Table 3. However, the last stage of transformation of a bellshaped film (Figure 7) into a spherical droplet needs further specification. Surfactant molecules, when penetrating between the oil and the hydrophobed quartz surface, may absorb on both interfaces to render them hydrophilic, because the polar group of the adsorbed molecules is oriented toward the newly forming hydrophilic gap. This allows water molecules to penetrate into the gap forming a thin water interlayer. Consequently, repulsive structural forces arise in the thin water interlayer, which widen the gap and facilitate detachment of the oil from quartz surface. It is known that turbid trisiloxane BE8 solutions spreading over hydrophobic surfaces may form wetting films with a thickness of several microns.1 This explains
the rapid spreading of such solutions.20,21 Recently this was confirmed by Wagner et al.,22 who showed that rapid spreading takes place only in the case of turbid trisiloxane solutions. The thickness of BE8 films, up to 4 µm, climbing over a methylated glass plate inclined at 11° to horizontal was assessed using computer analysis of the interference pattern imaged by a video camera.21 Figure 8 shows an oil drop (2) sitting on an inclined hydrophobed plate (1) interacting with a climbing thick trisiloxane film (3). The rate of the film edge movement was about 10-20 µm/s. The film rises from the vessel (4) filled with bulk BE8 surfactant solution. The process of interaction was recorded using a video camera. The video images obtained are shown in Figure 9. The first of the images (Figure 9a) shows a moment when the moving front of the BE8 film approaches a silicone oil droplet (M ) 20 000; η ) 500 cP). Around the oil droplet a thin wetting film of the oil was formed as a result of oil spreading over the hydrophobic surface. When the edge of the trisiloxane film comes into contact with the oil film, emulsification in the contact zone becomes visible (Figure 9b). After that, the trisiloxane surfactant film starts to flow around the droplet (Figure 9c), and emulsification embraces a larger zone. To the end of observation, after 5 s, emulsified oil is carried away by climbing BE8 film (Figure 9d,e). The volume of the droplet diminishes as a result of removal of oil emulsion by the flowing trisiloxane film. Let us now compare the results obtained with known experimental data on oil droplet detachment from solid surfaces. Separation of a crude oil droplet from a glass surface in the presence of an anionic micellar solution (1 wt % C16 R-olefin sulfonate) in 1 wt % NaCl was investigated by Kao et al.4 Separation of a droplet occurs as a result of diffusion of surfactant molecules between the solid and the oil phases and formation a thick aqueous interlayer. The rolling-up mechanism of separation first proposed by Adam23 was suggested. As was shown in ref 3, alkane droplets may be removed from a smooth glass surface by a shear flow of water, (20) Churaev, N. V.; Starov, V. M. Colloid J. 1998, 60, 790. (21) Churaev, N. V.; Esipova, N. E.; Hill, R. M.; Sobolev, V. D.; Starov, V. M.; Zorin, Z. M. Langmuir, submitted for publication. (22) Wagner, R.; Wu, Y.; Czichocki, G.; VonBerlepsch, H.; Rexin, F.; Perepelittchenko, L. Appl. Organomet. Chem. 1999, 13, 201. (23) Adam, N. K. J. Soc. Dyers Colour 1937, 53, 121.
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Figure 9. Video images of the interaction of a silicone oil droplet with a climbing BE8 solution film as a function of time: (a) approaching film; (b) after 1 s; (c) after 2 s; (d) after 3 s; (e) after 5 s.
when stable water films were also formed between decane and glass. In the case of pristane (2,6,10,14-tetramethylpentadecane), the film ruptures, and a critical shear stress needs to be overcome. The effect of pH on the process of transformation of a bitumen film into a droplet on glass and Teflon surfaces in HCl and NaOH solution was investigated by Basu et al.24 The bitumen film on Teflon gradually thinned and ruptured at a defined film thickness. In the case of a glass surface, the bitumen film at pH 3 and 40 °C transforms after 100 s into a droplet with a contact angle of θ ) 115°. In an alkaline medium, at pH 11, a droplet with θ ) 170° was formed more slowly, requiring some 350 s. The droplets were detached from the surfaces using a shear flow. Three cleaning mechanisms, namely solubilization, shear-driving cleaning, and roll-up, were observed when (24) Basu, S.; Nandakumar, K.; Masliyah, J. H. J. Colloid Interface Sci. 1996, 182, 82. Basu, S.; Nandakumar, K.; Masliyah, J. H. J. Colloid Interface Sci. 1997, 190, 253.
abietic acid (C19H29COOH) films were removed from a rotating fiberglass disk by aqueous solutions of nonionic surfactants. The mechanism controlling the cleaning depends on the alkyl and ethoxy chain lengths.8 In contrast to these results, detachment of oil films and droplets from hydrophobic surfaces occurs spontaneously in the case of trisiloxane solutions. Detachment of an oil droplet occurs very rapidly. This may be caused by formation of much thicker aqueous interlayers between hydrophilized by surfactant adsorption on both solid and oil surfaces. The mechanism of rapid detachment may consist, in the framework of the mechanism discussed above, in the combined action of repulsive structural forces in the thin water interlayer and, probably, the presence of vesicles in the trisiloxane BE8 solution stabilizing the aqueous interlayer. This suggests the opportunity to trisiloxane surfactants not only as superspreaders but also as cleaning agents for specialty cleaning problems where difficult oil films need to be removed from hydrophobic surfaces such as plastics.
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Acknowledgment. The authors thank Dow Corning Corporation for financial support and kindly supplied samples of chemicals. This work was also supported by
Churaev et al.
Russian Foundation for Basic Investigations, Grant 98-03-32770. LA000864Y
