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Effects of Dissolved Gas on the Hydrophobic Attraction between Surfactant-Coated Surfaces Emily E. Meyer,†,§ Qi Lin,‡,§ and Jacob N. Israelachvili*,‡ Departments of Physics and Chemical Engineering, University of California at Santa Barbara, California 93106 Received July 6, 2004. In Final Form: September 24, 2004 The effect of dissolved gas on the hydrophobic attraction between double-chained surfactant monolayers physisorbed on mica has been studied using a surface forces apparatus (SFA). Distance vs time data were obtained over the full distance regime from D ≈ 1000 Å down to contact using the dynamic SFA method. Removal of dissolved gas was seen to reduce the range of the attraction while the short-range attraction (under ∼250 Å) remained unchanged. The implications for the possibility of two distinct force regimes in the interactions between hydrophobic surfaces are discussed.
Introduction The hydrophobic interaction is among the mostimportant nonspecific interactions in biological systems. The term covers both the low solubility of nonpolar compounds in water1 and the related force between them as a function of their separation. The significant role of the hydrophobic interaction has led to a great deal of study, and yet, over 20 years since the first direct measurement of the long-range attraction between two hydrophobic surfaces,2,3 no single theory is able to account for all observed experimental behavior, and even experimental data on seemingly similar systems is often contradictory, so that understanding of the origin of this interaction remains elusive. A great deal of the confusion associated with experimental measurements of hydrophobic interactions is the apparent existence of two different force regimes. Several authors4-6 have suggested that the interaction observed in many experiments is actually a combination of a longrange attraction unrelated to the hydrophobicity of the surfaces (which may originate by a mechanism such as bridging bubbles or correlated electrostatics charges or dipoles), depending on the surfaces, and a short-range, truly hydrophobic, interaction. The short-range attractions that is, the attraction at separation distances less than 100-200 Åsseems to be the only attraction that is present between all hydrophobic surfaces, while the long-range attraction seems to depend on factors such as the method of surface preparation. Christenson and Yaminsky6 observed an apparent correlation between contact-angle hysteresis and the existence of a long-range attractive force between hydrophobic surfaces, while stable, very hydrophobic surfaces exhibited no long-range attraction.7 * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Department of Physics. ‡ Department of Chemical Engineering. § Both authors contributed equally to this work (1) Kauzmann, W. Adv. Protein Chem. 1959, 14, 1-63. (2) Israelachvili, J. N.; Pashley, R. Nature 1982, 300, 341-342. (3) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98, 500-514. (4) Hato, M. J. Phys. Chem. 1996, 100, 18350-18358. (5) Ederth, T.; Liedberg, B. Langmuir 2000, 16, 2177-2184. (6) Christenson, H. K.; Yaminsky, V. V. Colloids Surf., A 1997, 130, 67-74. (7) Parker, J. L.; Claesson, P. M.; Wang, J. H.; Yasuda, H. K. Langmuir 1994, 10, 2766-2773.
Hato4 suggested that the range of the “true” hydrophobic interaction is less than 200 Å, and Ederth and Liedberg5 arrived at a similar conclusion after observing a longrange interaction that was apparently the result of bridging nanobubbles that were unrelated to the “hydrophobicity” of the surfaces, as defined in terms of macroscopic contact angle. Proposed models for the hydrophobic interaction invoke such mechanisms as entropic effects due to molecular rearrangement of water near hydrophobic surfaces,8-11 electrostatic effects,12,13 correlated charge fluctuations14,15 or correlated dipole intereactions,16 bridging submicroscopic bubbles,17-24 and cavitation due to the metastability of the intervening fluid.25-34 (8) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98, 500-514. (9) Pratt, L. R.; Chandler, D. J. Chem. Phys. 1977, 67, 3683-3704. (10) Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986, 114, 234-242. (11) Eriksson, J. C.; Ljunggren, S.; Claesson, P. M. J. Chem. Soc., Faraday Trans. 1989, 85, 163-176. (12) Attard, P. J. Phys. Chem. 1989, 93, 6441-6444. (13) Miklavic, S. J.; Chan, D. Y. C.; White, L. R.; Healy, T. W. J. Phys. Chem. 1994, 98, 9022-9032. (14) Podgornik, R. J. Chem. Phys. 1989, 91, 5840-5849. (15) Podgornik, R.; Parsegian, V. A. Chem. Phys. 1991, 154, 477483. (16) Tsao, Y. H.; Evans, D. F.; Wennerstrom, H. Langmuir 1993, 9, 779-785. (17) Attard, P. Langmuir 1996, 12, 1693-1695. (18) Attard, P.; Moody, M. P.; Tyrrell, J. W. G. Physica A 2002, 314, 696-705. (19) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468-8480. (20) Nguyen, A. V.; Nalaskowski, J.; Miller, J. D.; Butt, H. J. Int. J. Miner. Process. 2003, 72, 215-225. (21) Ishida, N. S., M.; Miyahara, M.; Higashitani, K. Langmuir 2000, 16, 5681-5687. (22) Tyrrell, J. W. G.; Attard, P. Langmuir 2002, 18, 160-167. (23) Tyrrell, J. W. G.; Attard, P. Phys. Rev. Lett. 2001, 87, 176104. (24) Considine, R. F.; Drummond, C. J. Langmuir 2000, 16, 631635. (25) Rabinovich, Y. I.; Derjaguin, B. V.; Churaev, N. V. Adv. Colloid Interface Sci. 1982, 16, 63-78. (26) Yaminksy, V. V.; Yushchenko, V.; Amelina, E. A.; Shchukin, E. D. J. Colloid Interface Sci. 1983, 96, 301-306. (27) Yaminksy, V. V.; Ninham, B. W. Langmuir 1993, 9, 3618-3624. (28) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390392. (29) Claesson, P. M.; Christenson, H. K. J. Phys. Chem. 1988, 92, 1650-1655. (30) Bernard, D. J. Chem. Phys. 1993, 98, 7236-7244. (31) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. J. Phys. Chem. 1993, 97, 10192-10197.
10.1021/la048318i CCC: $30.25 © 2005 American Chemical Society Published on Web 11/25/2004
Effects of Dissolved Gas on Hydrophobic Attraction
Narrowing the field of potential models is complicated by difficulties in identifying all of the relevant parameters. The dependence of the interaction on electrolytes19,35-40 and temperature41,42 has yet to be determined. Removal of dissolved gas, however, has been shown to decrease the range of the attraction, as well as its magnitude at long range,32,39,43,44 and has been shown to increase the stability of colloids45 and emulsions31,46-48 and to affect optical cavitation.34 One might expect the presence of dissolved gas to play a significant role in models based on the presence of nanobubbles or the metastability of the liquid film, but even this is not clear. Craig et al.32 noted that, from the data currently available, one cannot distinguish whether the effect of removing dissolved gas is a bulk effect, a surface effect, or both, making it difficult to accurately determine the true role of dissolved gas. It has been shown that mica surfaces coated by a Langmuir-Blodgett (LB)-deposited hydrophobic monolayer of dimethyldioctadecylammonium bromide (DODAB) exhibit a long-range attraction.10,28,29,38,40,49,50 The effect of dissolved gas on this system, however, has not been explored. The aim of this paper is to explore the effect of dissolved gas on this interaction over the whole distance regime from D ≈ 1000 Å down to contact using the Surface Forces Apparatus (SFA). Materials and Methods DODA+Br-
was purchased from Kodak. All water used in cleaning and the LB trough subphase was purified by a Milli-Q water purification system consisting of an Elix-10 and a Milli-Q Gradient A10. Mica surfaces were rendered hydrophobic by the LB deposition of a monolayer of the double-chained cationic surfactant moiety DODA+ to the negatively charged surfaces of mica. The deposition pressure and deposited molecular area were 25 ( 0.5 mN m-1 and approximately 50 Å2, respectively, and all depositions and SFA measurements were carried out at 25 °C. Deaeration of Milli-Q water was carried out using a 500 mL filtering flask attached to an oil-less vacuum pump. Several small pieces of Teflon tubing, as well as a Teflon-coated stir bar, were placed in the bottom of the flask, which was then placed offcenter on a stir plate. As the vacuum pumped out the air, the stir bar was set to turn at 500 rpm. Bubbles nucleating on the Teflon pieces were released after collisions with the stir bar and rose to the air-water interface. Although bubbles were no longer (32) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1999, 15, 1562-1569. (33) Vinogradova, O. I.; Bunkin, N. F.; Churaev, N. V.; Kiseleva, O. A.; Lobeyev, A. V.; Ninham, B. W. J. Colloid Interface Sci. 1995, 173, 443-447. (34) Bunkin, N. F.; Kiseleva, O. A.; Lobeyev, A. V.; Movchan, T. G.; Ninham, B. W.; Vinogradova, O. I. Langmuir 1997, 13, 3024-3028. (35) Kekicheff, P.; Spalla, O. Phys. Rev. Lett. 1995, 75, 1851-1854. (36) Christenson, H. K.; Claesson, P. M.; Berg, J.; Herder, P. C. J. Phys. Chem. 1989, 93, 1472-1478. (37) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1998, 14, 3326-3332. (38) Christenson, H. K.; Claesson, P. M.; Parker, J. L. J. Phys. Chem. 1992, 96, 6725-6728. (39) Meagher, L.; Craig, C. S. J. Langmuir 1994, 10, 2736-2742. (40) Christenson, H. K.; Fang, J. F.; Ninham, B. W.; Parker, J. L. J. Phys. Chem. 1990, 94, 8004-8006. (41) Christenson, H. K.; Parker, J. L.; Yaminksy, V. V. Langmuir 1992, 8, 2080. (42) Tsao, Y. H.; Yang, S. X.; Evans, D. F.; Wennerstrom, H. Langmuir 1991, 7, 3154-3159. (43) Considine, R. F.; Hayes, R. A.; Horn, R. G. Langmuir 1999, 15, 1657-1659. (44) Mahnke, J.; Stearnes, J.; Hayes, R. A.; Fornasiero, D.; Ralston, J. Phys. Chem. Chem. Phys. 1999, 1, 2793-2798. (45) Snoswell, D. R. E.; Duan, J. M.; Fornasiero, D.; Ralston, J. J. Phys. Chem. B 2003, 107, 2986-2994. (46) Pashley, R. M. J. Phys. Chem. B 2003, 107, 1714-1720. (47) Karaman, M. E.; Ninham, B. W.; Pahsley, R. M. J. Phys. Chem. 1996, 100, 15503-15507. (48) Wennerstrom, H. J. Phys. Chem. B 2003, 107, 13772-13773.
Langmuir, Vol. 21, No. 1, 2005 257 visible after 30 min, the process was carried out for a total of 2.5 h before the start of each experiment. Dissolved oxygen measurements using an Orion 830A dissolved oxygen sensor from Thermo Electron Corp. find that approximately 90% of dissolved oxygen is removed using this procedure. Forces were measured using an SFA Mark III as previously described51 with particular attention given to the cleanliness of the chamber and solutions used. The SFA technique is unique in being able to directly and unambiguously measure the surfaces’ radii of curvature, R, the interfacial energy, γi, surface deformation, refractive index changes, determine if impurities are present, andsmost importantlysdetermine the absolute distance between the two interacting surfaces, i.e., our experimentally defined D ) 0. The dynamic method of force measurements, as described by Chan et al.,52 was used so that distances could be measured as a function of time, even at small separations, and the force was calculated from the equation of motion of the moving surfaces:
mx¨ + 6πR2ηD˙ /D + Kx + F(D) ) 0
(1)
where m is the mass of the moving surface, x is the deflection of the force-measuring spring supporting the moving surface, K is the spring stiffness, D is the surface separation, F(D) is the molecular surface force between the surfaces, R is the mean local radius of curvature of the surfaces, and η is the viscosity of the liquid medium. This equation assumes a no-slip boundary condition, which may not necessarily be valid in the case of water next to a hydrophobic surface. The implications of such an assumption have been previously discussed.53,54 However, inclusion of a finite slip length simply results in the multiplication of the viscous drag term in eq 1 by a factor, f*,54 which affects only the shape of force curves, meaning that the effect will be the same for both systems (with and without deaeration) and the inclusion of a slip length would have no impact on the comparisons being made. For this reason, a no-slip boundary condition is used in all calculations of F(D). During a force run at constant driving velocity, V, fringes of equal chromatic order (FECO) were recorded at all stages of the approach to give the surface shape and separation as a function of time, D(t). The fringes were later analyzed to determine R, Fad (the adhesion force), D, D˙ , x¨ , and x at any time, t. The first three terms of eq 1 could then be directly calculated to give the instantaneous force between the two surfaces, F(D), at any separation, D, both before and during a “jump” into contact, i.e., the instability regime. The inertial term mx¨ was found to be negligible at all separations for all the approach speeds used. A spring constant of K ) 160 N m-1 was used in all of experiments presented here. The adhesion force, Fad, was determined from the distance, DJ, that the surfaces jumped out from adhesive contact according to Fad ) KDJ, which in turn gave the interfacial energy, γi, of the hydrocarbon-water interface using the equation based on the Johnson-Kendall-Roberts (JKR) theory:55
γi ) Fad/3πR
(2)
Results Figure 1a shows representative distance vs time data for two DODAB monolayers brought together in water at a constant driving velocity (in this case V ) 18 Å s-1). Closed circles represent data taken in water containing dissolved gas, while open circles represent data taken in a different experiment using water that had been deaerated for 2.5 h. DODAB surfaces in the presence of dissolved gas are seen to accelerate away from the theoretical F(D) ) 0 curve at a distance of ∼450 Å, as has been shown in (49) Wood, J.; Sharma, R. Langmuir 1995, 11, 4797-4802. (50) Hato, M. J. Phys. Chem. 1996, 100, 18530-18538. (51) Israelachvili, J. N.; McGuiggan, P. M. J. Mater. Res. 1990, 5, 2223-2231. (52) Chan, D. Y. C.; Horn, R. G. J. Chem. Phys. 1985, 83, 5311-5324. (53) Lin, Q.; Meyer, E. E.; Tadmor, M.; Israelachvili, J. N.; Kuhl, T. 2004, xxxx. (54) Vinogradova, O. I. Langmuir 1995, 11, 2213-2220.
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Figure 2. Force vs. distance curves are shown for the data shown in Figure 1b. Again, closed circles represent surfaces in water containing dissolved gas and open circles represent water that has been deaerated for 2.5 h. The solid line represents the calculated van der Waals force. The inset shows the same data on a log-linear scale in the regime after the force sets in for the partially deaerated system. The solid lines represent fits of the data in this regime using the exponential force function of eq 3. Decay lengths in the absence and in the presence of dissolved gas are λ ) 55 Å and λ ) 100 Å, respectively.
Figure 1. Distance-time data for two DODAB monolayercoated surfaces in pure water containing dissolved gas (closed circles) and water that has been deaerated for 2.5 h (open circles) taken in (a) different experiments and (b) the same experiment in which water containing dissolved gas was injected after measurements were made in deaerated water. The solid curves represent the theoretical paths the surfaces would take in the absence of any surfaces force. The insets show the result of shifting the time axis of the deaerated data so that the jump-in regimes are aligned.
our previous work.53 The behavior of the surfaces in deaerated water is distinctly different: under deaerated conditions, the surfaces are seen to follow the F(D) ) 0 curve down to a distance of just over 250 Å, at which point the “jump-in” regime begins. As can be seen in the inset of Figure 1a, where the time axis has been shifted so that this regime can be directly compared, the surfaces follow the same curve in the jump-in regime as D approaches zero regardless of the force at larger distances. Figure 1b demonstrates the same phenomenon, this time from data acquired during the same experiment to ensure that the dissolved gas effect, though reproduced in 5 different experiments at more than 15 different contact positions and driving velocities ranging from 10 to 100 Å s-1, was not an anomaly due to different surfaces being used. Indeed, the same behavior was observed. In deaerated water, the surfaces are seen to follow the theoretical F(D) ) 0 curve down to a distance of just over 250 Å, at which point the jump-in regime begins. Water containing dissolved gas was subsequently injected while the surfaces were kept at the same contact position. After 30 min, the surfaces were brought together at the same driving velocity and were seen to return to behavior consistent with
surfaces in water containing dissolved gas. The inset demonstrates that, again, the surfaces follow the same asymptotic curve in the jump-in regime. Figure 2 shows representative normalized force vs distance, F(D), curves calculated from the distance vs time data, D(t). The surfaces in water containing dissolved gas are seen to experience an attractive force from a distance of ∼450 Å from contact which becomes stronger at approximately 250 Å from contact, consistent with previous studies on LB-deposited DODAB monolayers.10,28,29,38,40,49,50,53 The surfaces in deaerated water experience no force until they reach a separation distance of approximately 250 Å. As shown in Figure 2, this force is well-fitted by the exponential function
F/R ) C exp(-D/λ)
(3)
with λ approximately equal to 50 Å. It should be noted that the two force curves converge to roughly the same force at contact and removal of dissolved gas had no discernible effect on the value of the measured adhesion forces, which corresponded to fully hydrophobic surfaces. Details of adhesion measurements are discussed in another publication.53 No change in the measured adhesion value was found with the removal of dissolved gas. This is consistent with previous experimental32 and theoretical56 reports that the effects of dissolved gas are dramatic at long range but minimal at close range or in determining the adhesion force or interfacial energy, γi. Discussion The results presented here offer significant new information about the effects of dissolved gas on the interaction between macroscopic hydrophobic surfaces across aqueous solution. It has been shown previously in a study using (55) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London A 1971, 324, 301-313. (56) Leung, K.; Luzar, A.; Bratko, D. Phys. Rev. Lett. 2003, 90, 065502.
Effects of Dissolved Gas on Hydrophobic Attraction
a light lever instrument for force evaluation (LLIFE)32 and in atomic force microscopy (AFM) studies39,43,44 that the range of this interaction is diminished with the removal of dissolved gas. The data reported in this paper represent the first SFA study on the effects of dissolved gas on the hydrophobic attraction between two surfaces. The capability of the dynamic SFA method to measure surface separation as a function of time provides new data to compliment force studies. The acceleration away from the F(D) ) 0 curve observed in the presence of dissolved gas is seen to disappear in the deaerated system. Only the jump-in regime below 250 Å is present in both systems, and this regime appears identical despite the very different behavior at larger separations. The forces measured on approach at small separations, as well as on separation from contact, are the same in both cases. The question, posed earlier, as to the origins of the both long- and shortrange attractions and why it should be affected by the presence of dissolved gas, remains. It should be noted that there was no evidence of preexisting nanobubbles in the experiments presented here, either in the case of fully aerated or partially deaerated water. The force curves exhibit none of the discontinuities or steplike behavior that is often attributed to nanobubbles,57 and the FECO exhibited no discontinuities indicative of a bridging vapor bubble prior to contact. AFM (57) Attard, P. Adv. Colloid Interface Sci. 2003, 104, 75-91.
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imaging of these surfaces carried out under water, to be presented in a future publication, also shows no evidence of nanobubbles. On the basis of this evidence, it does not appear that pre-existing nanobubbles are responsible for the long-range attraction that disappears in the absence of dissolved gas in this case. The data presented here are more consistent with a mechanism involving cavitation or metastability of the intervening fluid, as has been suggested in previous work on the effects of dissolved gas,32,39 or another mechanism such as one invoking electrostatics. Theoretical implications of the data presented here will be discussed in an upcoming publication detailing additional experiments. The absence of only the long-range component of the interaction between two physisorbed hydrophobic surfaces with the removal of dissolved gas while the short-range remained unaffected is a strong indication that two independent mechanisms are in play. These data support the idea that what has been referred to as the “hydrophobic” interaction may be the combination of a truly hydrophobic interaction at short range and a distinctly different interaction at larger separations which is not in fact related to the “hydrophobicity” of the surfaces. Acknowledgment. This paper made use of NASA Grant NRA01-OBPR-08-D. LA048318I
