Adhesion between Hydrocarbon Particles and Silica Surfaces with

Department of Chemical Technology, Gdansk University of Technology, ... between polyethylene particles and silica surfaces with different degrees of h...
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Langmuir 2003, 19, 5311-5317

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Adhesion between Hydrocarbon Particles and Silica Surfaces with Different Degrees of Hydration As Determined by the AFM Colloidal Probe Technique Jakub Nalaskowski,*,† Jarosław Drelich,‡ Jan Hupka,§ and Jan D. Miller† Department of Metallurgical Engineering, University of Utah, 135 S 1460 E, Room 412, Salt Lake City, Utah 84112-0114, Department of Materials Science and Engineering, Michigan Technological University, M&M Bldg 506, Houghton, Michigan 49931, and Department of Chemical Technology, Gdansk University of Technology, Narutowicza 11/12, 80-952 Gdansk, Poland Received November 26, 2002. In Final Form: April 7, 2003 Adhesion between oily contaminants and soil particles is a key factor controlling oil release during soil decontamination using wet separation processes. Previous research reports showed that hydration of the soil components, mainly silica particles, governs the oil release efficiency. In this work, the adhesion force between polyethylene particles and silica surfaces with different degrees of hydration obtained by thermal treatment and silanation was measured in an aqueous environment using the atomic force microscopy colloidal probe technique. The adhesion force increases significantly with dehydration of the silica surface and the subsequent increase in hydrophobicity. It was found that experimental adhesion forces measured in the polyethylene-water-silica system are in good agreement with the values calculated theoretically using the Lifshitz/van der Waals-Lewis acid/base interaction theory for highly hydrophobic silica substrates. This agreement is less satisfactory for hydrophilic and slightly hydrophobized silica. It is hypothesized that a thin film of water is strongly adsorbed to the silanol groups on the silica surface and protects the silica surface against its “dry contact” with the polyethylene particle during the atomic force microscopy surface force measurements.

Introduction A strong demand for oil products has created significant environmental problems. For many years, discharge of various oil-bearing wastes and accidental spills of oil on land and in water streams have contaminated billions of tons of soil and sediment.1 This situation has contributed to ecological degradation and economic losses, and it poses a threat to human health. The cleanup of oil-contaminated sites costs billions of dollars and puts tremendous pressure on limited financial resources.2 Cleanup of oil-contaminated soil and sediment involves conditioning with surfactants,3-5 and subsequent froth flotation.6,7 By securing proper solution chemistry, oil is released from the contaminated soil or sediment and is * Corresponding author. E-mail: [email protected]. Fax: (801)581-4937. † University of Utah. ‡ Michigan Technological University. § Gdansk University of Technology. (1) Kostecki, P. T.; Calabrese, E. J. Petroleum Contaminated Soils, Vol. I: Remediation Techniques, Environmental Fate, and Risk Assessment; 1989. (2) Staps, J. J. M. International evaluation of in-situ biorestoration of contaminated soil and groundwater; Office of Research and Development, U.S. EPA: 1990. (3) Gannon, O. K.; Bibring, P.; Raney, K.; Ward, J. A.; Wilson, D. J.; Underwood, J. L.; Debelak, K. A. Sep. Sci. Technol. 1989, 24, 10731094. (4) Walraevens, K.; De Breuck, W. Int. J. Environ. Pollut. 1997, 7, 285. (5) Hupka, J.; Wawrzacz, B. Fizykochem. Probl. Mineralurgii 1996, 30, 177-186. (6) Gannon, O. K. Vanderbilt University: Nashville, TN, 1988; p 177. (7) Beach, E.; Gosiewska, A.; Fang, C.; Dudeck, K.; Drelich, J. In Environmental Technology for Oil Pollution, U.S. United Engineering Foundation Conference and 2nd International Conference Analysis and Utilization of Oily Wastes AUZO′99, Jurata, Poland, 1999; Vol. 1, pp 89-93.

separated from the suspension by gravity and flotation due to an affinity for nonpolar dispersed gas bubbles and the resulting buoyancy of the bubble/oil droplet aggregates. Hydrophilic soil particles remain in suspension and are discharged as the cleaned soil product while oil reports to the froth phase and can be skimmed from the surface of the suspension. The surface properties of both oil and soil in the suspension/emulsion determine the effectiveness of oil release and determine the subsequent potential for the deoiling separation. Nevertheless, understanding of interactions in the oil-soil-water systems is rather incomplete. For example, strong physicochemical interaction forces are believed to exist between the oil and soil particle. These forces include molecular interactions as well as capillary and viscous forces.8,9 The extent to which these forces contribute to the strength of oil/soil aggregation depends on the composition of the oil, surface characteristics of the soil particle, and conditions of contact between the oil and soil particle (temperature, time, moisture, etc.). Release of the oil from the soil surface frequently becomes difficult because of the strong oil/soil interaction. Silica is usually a major component of contaminated soil, and the adhesion force between oil and silica plays a key role in oil release from soil. Thermal treatment of silica sand prior to deoiling can greatly impede the oil release from the sand and, therefore, render separation processes inefficient.10 This behavior is usually explained by the thermal desorption of the adsorbed multilayer water film at the silica surface, whose thickness can vary from one monolayer to above 10 nm11,12 (8) Huang, J. S.; Varadaraj, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 535-539. (9) Kao, R. L.; Wasan, D. T.; Nikolov, A.; Edwards, D. A. Colloids Surf. 1989, 34, 389-398. (10) Hupka, J.; Budzich, M.; Miller, J. D. In 1991 Eastern Oil Shale Symposium, Lexington, Kentucky, 1991; pp 202-207. (11) Nutting, P. G. J. Phys. Chem. 1927, 31, 531-534.

10.1021/la026911z CCC: $25.00 © 2003 American Chemical Society Published on Web 05/24/2003

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and which is even characterized as a gel structure by some authors.13 It is expected that the presence of an adsorbed water film and hydrophilic silanol groups strongly decreases the adhesion of hydrocarbon oil to the silica by modification of the interaction forces between the hydrocarbon phase and the silica surface and thus promotes easy release of oil from the contaminated surface.14 However, in the absence of the water film and the presence of hydrophobic siloxane groups at the silica surface, the affinity of water for the surface is much weaker and the affinity of the silica surface toward hydrocarbon molecules is stronger, which results in the much more difficult release of oil from sand during the digestion in an aqueous environment. Silica surfaces can also be dehydrated and rendered hydrophobic by silanation. In this case, chlorosilanes adsorb at the silica surface and create covalent bonds with silanol groups, and thus, siloxane groups are formed.15,16 Not only does the surface of the silica become dehydrated because of the depletion of the silanol groups during the reaction, but additionally, the silica surface becomes covered with the hydrocarbon tails of the silanes. This treatment is frequently used for obtaining model hydrophobic surfaces for fundamental studies of hydrophobic interactions.17 It can be concluded that adhesion of hydrocarbons to silica surfaces should be dependent on the extent of surface hydration. This topic should be of considerable interest to environmental chemistry and oil waste processing with respect to fundamental surface chemistry and soil remediation. Although adhesion of hydrocarbons to mineral surfaces has been of interest for some time,18-20 the direct measurement of the adhesion force has not been reported. The development of the atomic force microscope (AFM)21 and, more recently, the colloidal probe technique22 has made it possible to study interaction and adhesion forces between a single particle of choice and a selected surface. Using this method, a single particle with diameter from 1 to 100 µm is attached to the AFM cantilever and the surface is moved toward the particle using a piezoelectric transducer. During this movement the deflection of the cantilever is recorded by means of a reflected laser beam which serves as an optical lever. Using these data, the profile of interaction force as a function of distance between the particle and the surface is obtained. During retraction of the cantilever from the surface, after contact, the adhesion force between particle and surface can be measured. The colloidal probe setup has been successfully used in the study of interaction forces between particles and solid surfaces, particularly in systems closely related to mineral processing23-28 and systems related to the decontamination (12) Ershova, G. F.; Zorin, Z. M.; Churaev, N. V. Kolloidn. Zh. 1975, 37, 208-210. (13) Yaminsky, V. V.; Ninham, B. W.; Pashley, R. M. Langmuir 1998, 14, 3223-3235. (14) 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. Colloids Surf., A 1994, 91, 97-112. (15) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647-1651. (16) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367-4373. (17) Parker, J. L.; Claesson, P. M. Langmuir 1994, 10, 635-639. (18) Gaudin, A. M. Eng. Min. J. 1940, 141, 43-44. (19) Nozawa, M.; Tohji, K.; Matsuoka, I. Shigen to Sozai 1993, 109, 95-100. (20) Mackenzie, J. M. W. Trans. AIME 1970, 247, 202-208. (21) Binnig, G.; Quate, C.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930933. (22) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239-241.

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of oily wastes.7,29,30 Many important findings pertaining to the role of hydrophobic interactions, system stability, coagulation, influence of surfactants and flocculants, and so forth have been established. The colloidal probe technique has also been proven valuable for quantification of adhesive force, and measurements of adhesion between the colloidal probe and a flat surface in air31-36 and in aqueous solutions37-40 were reported in the literature. In the present paper, the colloidal probe technique allowed for the examination of the adhesion force in water between hydrocarbon, polyethylene (PE) particles, and silica surfaces of different degrees of hydration as obtained by thermal treatment and/or silanation. The experimental results show the effect of the silica surface state on the adhesion to the hydrocarbon particle in water and may be helpful in further understanding of interfacial processes responsible for the oil attachment and release during soil decontamination. Materials and Methods Chemicals. Fused silica (optical grade) plates were obtained from Harrick, Inc. Epitaxial silicon wafers (111) were obtained from MEMC, Inc. Octadecyltrichlorosilane (OTS, 95% purity) was obtained from Aldrich. Cyclohexane (spectrophotometric grade, Mallinckrodt, Inc.) was dried over freshly activated 3 Å molecular sieves (Mallinckrodt, Inc). The polyethylene (PE) powder was a low-density polyethylene (Scientific Polymer Product, Inc.) with a molecular weight MW ) 1800 and melting point mp ) 117 °C. Other reagents included chloroform (spectrophotometric grade, J. T. Baker), glycerol (certified ACS grade from Fisher Scientific), ammonium hydroxide (reagent grade, Fisher Scientific), hydrogen peroxide (reagent grade, Fisher Scientific), hydrofluoric acid (reagent grade, Fisher Scientific), nitrogen (reagent grade N2, Mountain Airgas), and deionized water (18 MΩ‚cm) obtained using a Milli-Q System (Millipore). Silica Surface Preparation. Silica plates were thoroughly cleaned by sonication in acetone, then in methanol, and subsequently in low-temperature argon plasma. Subsequently, they were immersed in RCA SC-1 cleaning solution composed of 5 vol H2O, 1 vol 29% NH3aq, and 1 vol 30% H2O2 at 80 °C,41 rinsed with deionized water, and dried in a nitrogen stream. Such a procedure (23) Pazhianur, R.; Yoon, R. H. Processing of Complex Ores: Mineral Processing and the Environment, Proceedings of the UBC-McGill BiAnnual International Symposium on Fundamentals of Mineral Processing, 2nd, Sudbury, Ont., Aug. 17-19, 1997; pp 247-256. (24) Rabinovich, Y. I.; Yoon, R. H. Langmuir 1994, 10, 1903-1909. (25) Yoon, R.-H.; Flinn, D. H.; Rabinovich, Y. I. J. Colloid Interface Sci. 1997, 185, 363-370. (26) Yoon, R.-H.; Pazhianur, R. Colloids Surf., A 1998, 144, 59-69. (27) Toikka, G.; Hayes, R. A.; Ralston, J. Colloids Surf., A 1998, 141, 3-8. (28) Biggs, S.; Proud, A. D. Langmuir 1997, 13, 7202-7210. (29) Nalaskowski, J.; Hupka, J.; Miller, J. D. Environmental Technology for Oil Pollution, Proceedings of U.S. United Engineering Foundation Conference and 2nd International Conference Analysis and Utilization of Oily Wastes AUZO’99, Jurata, Poland, Aug. 29-Sept. 3, 1999; Vol. 1, pp 76-83. (30) Hartley, P. G.; Grieser, F.; Mulvaney, P.; Stevens, G. W. Langmuir 1999, 15, 7282-7289. (31) Schaefer, D. M.; Gomez, J. J. Adhes. 2000, 74, 341-359. (32) Heim, L.-O.; Blum, J.; Preuss, M.; Butt, H.-J. Phys. Rev. Lett. 1999, 83, 3328-3331. (33) Fuji, M.; Machida, K.; Takei, T.; Watanabe, T.; Chikazawa, M. J. Phys. Chem. B 1998, 102, 8782-8787. (34) Segeren, L. H. G. J.; Siebum, B.; Karssenberg, F. G.; Van Den Berg, J. W. A.; Vancso, G. J. J. Adhes. Sci. Technol. 2002, 16, 793-828. (35) Biggs, S.; Spinks, G. J. Adhes. Sci. Technol. 1998, 12, 461-478. (36) Biggs, S.; Cain, R. G. Proc. Annu. Meet. Adhes. Soc. 2001, 24th, 453-455. (37) Freitas, A. M.; Sharma, M. M. J. Colloid Interface Sci. 2001, 233, 73-82. (38) Toikka, G.; Hayes, R. A.; Ralston, J. J. Colloid Interface Sci. 1996, 180, 329-338. (39) Bowen, W. R.; Hilal, N.; Lovitt, R. W.; Wright, C. J. Colloids Surf., A 1999, 157, 117-125. (40) Vakarelski, I. U.; Ishimura, K.; Higashitani, K. J. Colloid Interface Sci. 2000, 227, 111-118. (41) Kern, W.; Puotiene, D. A. RCA Rev. 1970, 31, 187.

Adhesion between Hydrocarbon Particles and Silica Surfaces results in a completely hydrated, hydrophilic (fully wettable) silicon oxide surface with no contamination detected using optical and atomic force microscopy. After cleaning, the silica plates were placed in an electric furnace and subjected to temperatures of 400, 600, and 900 °C for 2 h. They were then placed in a desiccator and after cooling to ambient temperature were used for adhesion force measurements. OTS solution preparation and the complete silanation process were conducted in a glovebox under a slight nitrogen pressure in order to avoid the presence of atmospheric moisture in the system. An OTS solution (1.01 × 10-3 M) was prepared in dried cyclohexane and placed in a glass beaker (the beaker had been exposed to the OTS solution before the experiment in order to saturate the glass surface with OTS molecules and eliminate any reaction with the walls during silanation experiments). The beaker was placed in an ultrasonic bath filled with hexadecane (to avoid humidity effects). The ultrasonic bath was cooled during silanation; a constant temperature of 24 °C was maintained.15,16 The silica plates were placed on a Teflon rack and immersed in the OTS solution for a specified time. Then, the rack with silanated plates was removed from the OTS solution and immersed in dried cyclohexane and subsequently in a dried chloroform bath to remove excess OTS from the surface. Each silica plate was silanated using fresh OTS solution. After preliminary rinsing, the plates were removed from the glovebox, rinsed with copious amounts of chloroform, dried in a nitrogen stream, and used for adhesion force measurements. Contact Angle Measurement. Contact angles for water were measured on differently treated silica surfaces using a Rame´Hart goniometer and the sessile-drop technique. The water drop was placed on the surface and remained in contact with the needle of the microsyringe. The drop volume was increased and decreased until the three-phase boundary moved over the silica surface. Both advancing and receding contact angles were measured as described in a previous contribution.42 Reported values are the average values from measurements for drop diameters between 3 and 7 mm. Colloidal Probe Preparation. Spherical polyethylene (PE) particles were obtained using a procedure, which involves suspending a powder of polymeric thermoplastic materials, such as PE, in glycerol, heating the suspension above the melting point of the polymer, and then solidification of the dispersed polymeric droplets at a reduced temperature. After appropriate filtration and drying, this procedure was found not to change the surface properties of PE particles, which retained a high degree of hydrophobicity. These particles had a relatively smooth surface and were particularly useful for investigating interaction forces using the AFM colloidal probe technique.43 The prepared spherical particles were glued to the AFM cantilever with a small amount of epoxy resin using a procedure described elsewhere.22 Tipless rectangular silicon cantilevers (Digital Instruments, Inc.) were used. The cantilever was placed under the CCD camera fitted with a long distance lens giving a 500× magnification. A small amount of epoxy resin was transferred on the tip of the cantilever using a 50 µm diameter tungsten wire attached to a micromanipulator. Subsequently, a selected sphere was picked up using a new tungsten wire and carefully placed on the cantilever using the micromanipulator. It has been established that there is no contamination of spherical particles with epoxy resin during gluing. Using this procedure, particles in diameter from 1 to 200 µm can be precisely glued to the AFM cantilever. Cantilevers were used for measurements after at least 24 h of drying. An example of the cantilever with attached PE particle is shown in Figure 1. Adhesion Force Measurement. A Nanoscope IIIa atomic force microscope (Digital Instruments, Inc.) equipped with a fluid cell was used for the measurements. During a measurement the laser beam is focused on the back of the cantilever to detect the cantilever’s deflection as it interacts with the surface beneath it. The reflected light is directed onto a split photodiode detector, which produces a voltage signal proportional to the cantilever (42) Drelich, J.; Miller, J. D.; Good, R. J. J. Colloid Interface Sci. 1996, 179, 37-50. (43) Nalaskowski, J.; Drelich, J.; Hupka, J.; Miller, J. D. J. Adhes. Sci. Technol. 1999, 13, 1-17.

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Figure 1. Scanning electron micrograph of an 18 µm PE particle at the end of an AFM cantilever (2000× mag.). The probe was used in the pull-off force measurements before this picture was taken. Note that the sphericity of the used particle is intact. deflection. If the spring constant is known, the deflection of the spring can be converted to force using Hooke’s law. Using such a setup, the force acting on the cantilever can be determined with sensitivity > 0.1 nN. The sample beneath the cantilever is moved using a piezoelectric transducer. In the force measurements, motion in the x and y directions is disabled, the piezoelectric tube is used to move the surface in the z direction, and the cantilever deflection is continuously measured. The surface is first moved toward the cantilever until the particle contacts the surface and then is retracted from the cantilever until the particle snaps off from the surface. The adhesion force was obtained from measuring the deflection of the cantilever at the point where the particle snaps off from the surface after contact. In this research, we followed the recommendations posed by Biggs and Spinks35 and used stiff cantilevers and moderate loads during pull-off force measurements. These strict experimental conditions are needed to comply with the JKR contact mechanics model in the analysis of adhesion forces. The maximum applied load during contact was 6.5 ( 1.9 µN, and the scan rate was kept constant at 1 Hz (speed 1 µm/s). No significant trend in pull-off force for subsequent measurements was observed and no permanent deformation of the particle was found (Figure 1). After the force measurement the particle size, as well as the cantilever’s dimensions, was measured using scanning electron microscopy. The spring constant of the cantilever (k) was calculated from the dimensions of the cantilever according to the formula44

k)

Eh3(a + w) 12L3

(1)

where a and w are the widths of the parallel sides of the trapezoid, h is the height of the trapezoid, L is the length of the cantilever measured from the base to the center of the glued particle, and E is Young’s modulus for the cantilever (1.5 × 1011 N/m2). The spring constant of the cantilevers used in this study was found to be from 27 to 30 N/m. At least 10 consecutive adhesion force measurements were taken (values of the cantilever’s deflection converted to force, normalized with respect to the PE sphere radius) and averaged.

Results and Discussion Thermally Dehydrated Silica. The thermal treatment of silica results in the gradual loss of the adsorbed water layer, whose thickness can vary from one monolayer to above 10 nm.11,12 In fact, the film is even characterized as a gel structure by some authors.13 (44) Drelich, J.; Nalaskowski, J.; Gosiewska, A.; Beach, E.; Miller, J. D. J. Adhes. Sci. Technol. 2000, 14, 1829-1843.

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Figure 2. Dehydration of a silica surface during thermal treatment. Fully hydrated silica covered with adsorbed multilayers of water (a), gradual desorption of water (b), and exposure of silanol groups (c), and finally complete dehydration leading to a surface covered with siloxane groups (d).

Figure 4. Pull-off force in water for a PE particle vs contact angle for thermally treated silica.

Figure 3. Water contact angle at a silica surface vs treatment temperature.

During dehydration of silica, not only the thickness of water film decreases but also, subsequently, silica silanol groups are transformed into the siloxane groups,45 which can be deduced from the increase in contact angle and a decrease in water adsorption density. Further, the dehydration is revealed from FTIR spectra.45,46 This process is schematically shown in Figure 2. The decrease in silanol groups at the surface leads to a significant increase of hydrophobicity of the silica surface. This increase in hydrophobicity was revealed from contact angle measurements. For example, the advancing contact angle measured using the sessile drop technique is shown as a function of treatment temperature, in Figure 3. The contact angle values increased from 0° for a completely hydrophilic silica surface to 37° after 2 h of treatment at 900 °C. These results are in good agreement with previously reported values of water contact angle on a silica surface after thermal treatment.45,46 It can be concluded that the amount of water and silanol groups on the fully hydrated silica surface is relatively constant with little variations for different specimens. Such a conclusion (45) Muster, T. H.; Prestidge, C. A.; Hayes, R. A. Colloids Surf., A 2001, 176, 253-266. (46) Lamb, R. N.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1 1982, 78, 61-73.

is also supported by measurements of concentration of hydroxyl groups at a silica surface.47 It also has to be noted that the receding contact angle for water at these treated surfaces remains low (below 5°) for the whole range of treatment temperatures. The high hysteresis can indicate significant heterogeneity of the silica surface with respect to the distribution of hydrophilic and hydrophobic sites. It has been currently established that silanol (-Si-OH) groups at the silica surface are completely hydrophilic and act as primary binding sites for the physisorption of water.45,48 A completely hydrated silica surface is covered with silanol groups with an adsorption density of approximately 4.9 groups per square nanometer.47 On the contrary, the siloxane groups (-Si-O-Si-) are hydrophobic and water is not strongly bound to them. For states other than the full hydration state, there is a distribution of the silanol and siloxane groups at the silica surface49,50 but siloxane groups usually form microscopic hydrophobic patches at the silica surface and account for the high hysteresis in the contact angle. The results of measurements of the adhesion force between a PE sphere and a silica surface after thermal treatment are shown in Figure 4. The force needed to remove the PE particle from the silica surface (pull-off force) is given as a function of the contact angle. The presence of a physisorbed water film and hydrophilic silanol groups strongly decreases the adhesion of hydrocarbons to silica by modification of the interaction forces between the hydrocarbon compound and the silica surface.14 The repulsive component of the hydrocarbon/silica interaction is mostly electrostatic in origin. In addition, the affinity of water to the silanol groups is much stronger than the affinity of hydrocarbon due to the formation of hydrogen bonding and smaller disruption to the network of hydrogen bonded water molecules.51,52 However, in the (47) Zhuravlev, L. T. Langmuir 1987, 3, 316-318. (48) Naono, H.; Fujiwara, R.; Yagi, M. J. Colloid Interface Sci. 1980, 76, 74-82. (49) Bergna, H. E. Adv. Chem. Ser. 1994, 234, 1-47. (50) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry; 1979. (51) Drost-Hansen, W. Ind. Eng. Chem. 1969, 61, 10-47. (52) Grigera, J. R.; Kalko, S. G.; Fischbarg, J. Langmuir 1996, 12, 154-158.

Adhesion between Hydrocarbon Particles and Silica Surfaces Table 1. Surface Tension Components of Water, Polyethylene, and Silica (in mJ/m2) water polyethylene silica a

γ+

γLW

liquid/material

21.8 33 41-43

25.5a 0 43-60

ref 53 own data ref 54

Assumed standard values.

presence of siloxane groups, the affinity of water to the surface is much weaker due to the entropically unfavorable disruption of water hydrogen bonding.51,52 The hydrophobic attraction between hydrocarbon and hydrophobic silica surface then becomes predominant, which results in an increase in the adhesion force between the hydrocarbon particle and the silica surface. The effect of increasing silica surface hydrophobicity on polyethylene-silica adhesion can also be analyzed through free energy considerations. The free energy of adhesion (∆GPWS) is defined as53

∆GPWS ) γPS - γPW - γSW

(2)

and using the Lifshitz-van der Waals Lewis acid-base interaction theory53

∆GPWS ) 2

[xγ

P

LW

γWLW

+ xγS

LW

γWLW

- xγP

LW

LW

γS

-

γWLW + xγW+(xγP- + xγS- - xγW-) +

xγW -(xγP+ + xγS+ - xγW+) - xγP+γS- - xγP-γS+] (3) where γ is the surface tension or surface tension component; the subscripts P, W, S, PS, PW, and SW refer to polyethylene, water, silica, polyethylene-silica, polyethylene-water, and silica-water, respectively; and the superscripts LW, +, and - refer to the Lifshitz-van der Waals component and electron-acceptor and electrondonor parameters of the Lewis acid-base component of surface tension. Table 1 shows the surface tension components for water, polyethylene, and silica. The thermal treatment of silica influences its surface polarity. Changes in the free energy of adhesion for the polyethylene-water-silica system result from a decrease in the γS- value, an effect that is caused by the reduction in the number of hydroxyl groups, electron-donor in nature, during heat treatment. As shown in Table 1, γS+ is much smaller than γS- and can be neglected. Also, small chemical changes on the surface of the substrate such as changes in the number of hydroxyl groups on the silica surface have little effect on the Lifshitz van der Waals component of the surface tension for silica (γSLW), and this effect can be ignored in our analysis as well. Taking these assumptions into account, together with the surface tension component values given in Table 1, eq 3 can be reduced to

∆GPWS ) -105.9 + 10.1xγS-

(4)

The advancing contact angles that were measured for water drops on the heat-treated silica samples can also (53) Van Oss, C. J. J. Adhes. Sci. Technol. 2002, 16, 669-677.

Langmuir, Vol. 19, No. 13, 2003 5315 Table 2. Electron-Donor Parameter of the Lewis Acid-Base Component of Surface Tension (γS-) for Fresh and Heat-Treated Silica, the Free Energy of Adhesion (∆GPWS), and the Normalized Adhesion Force (F/R) for Polyethylene on Silica in Water for Different Extents of Dehydration (Contact Angle θ) θ (deg)

γS(mJ/m2)

∆GPWS (mJ/m2)

F/R(theor) (mN/m)

F/R(meas) (mN/m)

0 9 18.5 37

71 69 65 49

-20.8 -22.0 -24.5 -35.2

98.0 103.6 115.4 165.8

20.8 ( 7 62.3 ( 22 88.3 ( 31 101.2 ( 35

be used to calculate γS-. For the substrate with γS+ , γS-, the following equation holds:53

(1 + cos θ)γW ) 2(xγSLWγWLW + xγS-γW+)

(5)

where γW is the surface tension of water (72.8 mN/m). The calculated γS- and ∆GPWS values are shown in Table 2. We also used the ∆GPWS values to calculate the normalized adhesion force (F/R) using the JKR theory:55

3 F ) - π∆GPWS R 2

(6)

The theoretical F/R values for heat-treated silica are 1.5-1.7 times larger than the values measured with the AFM colloidal probe (Table 2). As we hypothesize, this discrepancy may be a result of the inability of the polyethylene particle to displace a thin film of water, strongly adsorbed to the silanol groups on the silica surface, during the adhesion force measurements. On the other hand, it is well documented in the literature that surface roughness can also affect adhesion values obtained by pull-off force measurements.33,34,56-58 The polyethylene particle probes used in this study, although spherical in shape, had submicroscopic irregularities on the surface (Figure 1). The mean roughness of silica samples used in this study was from 0.2 to 0.8 nm for 0.25 µm2, as determined with an AFM. The contact area between such a particle and a substrate surface might be reduced as compared to the contact area established by the perfect sphere, for which eq 6 was derived. As a result, the measured adhesion for a particle with a rough surface is always smaller than eq 6 predicts. It should be recognized, however, that the use of stiff cantilevers and moderate loads during pull-off force measurements in this study causes “squeezing out” of nanoscale roughness of the probe and greatly diminishes the effects of nanoroughness on probe-substrate contact area.35,59 The value of F/R calculated for the polyethylene colloidal probe on an untreated silica surface is about 4.7 times smaller than that experimentally measured, 20.8 mN/m versus 98 mN/m (Table 2). Such a large discrepancy indicates that factors other than particle surface roughness also affect the adhesion. It is hypothesized that a mono(54) Gonzalez-Martin, M. L.; Janczuk, B.; Labajos-Broncano, L.; Bruque, J. M.; Gonzalez-Garcia, C. M. J. Colloid Interface Sci. 2001, 240, 467-472. (55) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc., Ser. A 1971, 324, 301-313. (56) Rabinovich, Y. I.; Adler, J. J.; Ata, A.; Singh, R. K.; Moudgil, B. M. J. Colloid Interface Sci. 2000, 232, 17-24. (57) Sirghi, L.; Nakagiri, N.; Sugisaki, K.; Sugimura, H.; Takai, O. Langmuir 2000, 16, 7796-7800. (58) Rimai, D. S.; Quesnel, D. J.; Reifenberger, R. J. Adhes. 2000, 74, 283-299. (59) Beach, E. R.; Drelich, J. In Functional Fillers and Nanoscale Minerals; Kellar, J. J., Herpfer, M. A., Moudgil, B. M., Eds.; SME: Littleton, CO, 2003; pp 177-193.

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Figure 5. Silanation of a silica surface. Adsorption of the OTS at the adsorbed water layer (a) leads to the hydrolysis of OTS molecules (b). Lateral and vertical rearrangement of OTS molecules can occur (c). Water elimination and bonding to the silica surface (d), and finally cross-linking of the OTS molecules (e).

layer, or a thin film, of water could be strongly (physically) bonded to the untreated silica surface, protecting silica against direct contact with the polyethylene particle. Also, since thermal treatment usually produces microscopic hydrophobic patches on the silica surface,49,50 it is likely that the strongly adsorbed water remains on the surface of heated silica samples, anchored by hydrophilic patches, and is responsible for experimental F/R values smaller than the F/R values calculated theoretically (Table 2). Silanated Silica. The silanation reaction leading to the formation of a hydrophobic, dehydrated silica surface is rather complex and is frequently carried out in nonpolar solvent where only traces of water are present. Molecules of alkyltrichlorosilane are initially adsorbed at the silica interface. The presence of the adsorbed water layer seems to be crucial for the process, since in the presence of a reactive, external layer of adsorbed water, and/or traces of water in the nonpolar solvent, the silane molecules hydrolyze.60 Adsorbed, hydrolyzed silane molecules can rearrange due to the van der Waals forces and by lateral movement create a monolayer with alkyl chains closely packed together. Additionally, vertical movement is also possible where the adsorbed silane travels through the adsorbed water layer to the silica surface.15 Finally, the elimination of water occurs and silane molecules are being covalently bonded to the surface through the siloxane groups. Additionally, further elimination of water leads to the lateral cross-linking of the silane molecules. This results in the creation of extremely robust hydrophobic layers on the silica surface. This process is schematically shown in Figure 5. It has to be emphasized that some aspects of the silanation reaction are not completely understood. Traces of water15 and elevated temperature61 affect the morphology of the silane layers. It is also unclear to what extent silane molecules bind directly to the silica surface, as only binding to the adsorbed water layer has been suggested.60 Nevertheless, the adsorption of silanes leads to the significant hydrophobization of the silica surface. The contact angle of the treated surface is related to the surface coverage by the OTS molecules and can be varied by time (60) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120-1126. (61) Brzoska, J. B.; Shahidzadeh, N.; Rondelez, F. Nature 1992, 360, 719-721.

Figure 6. Water contact angle at the silica surface vs silanation time.

Figure 7. Pull-off force of PE particle in water vs contact angle for the silanated silica surface.

of silanation.62 The variation of the water contact angle at the silica surface with silanation time is shown in Figure 6. The advancing contact angle increases rapidly with silanation time and reaches a maximum for near-monolayer coverage. The contact angle hysteresis (not studied in detail in this research) is significant and reaches a maximum for medium surface coverage, which is related to the heterogeneous, patchy adsorption of OTS.62,63 Adhesion between the PE sphere and the silanated silica surface in water increases rapidly with increasing hydrophobicity of the silica surface expressed in terms of the advancing contact angle (Figure 7). The measured (62) Nalaskowski, J.; Veeramasuneni, S.; Hupka, J.; Miller, J. D. J. Adhes. Sci. Technol. 1999, 13, 1519-1533. (63) Flinn, D. H.; Guzonas, D. A.; Yoon, R. H. Colloids Surf., A 1994, 87, 163-176.

Adhesion between Hydrocarbon Particles and Silica Surfaces Table 3. Fractional Surface Coverage of Silica by Silane (f), Free Energy of Adhesion (∆GPWS), and Normalized Adhesion Force (F/R) for Polyethylene on Silane-Treated Silica in Water for Various Levels of Fractional Coverage by OTS θ (deg)

f

∆GPWS (mJ/m2)

F/R(theor) (mN/m)

F/R(meas) (mN/m)

2.5 57 71 94

0 0.34 0.50 0.80

-21.3 -36.3 -44.7 -65.8

100.3 170.8 210.6 309.8

32 ( 2 122 ( 9 210 ( 8 302 ( 18

pull-off force reaches significantly higher values than those in the case of thermally treated silica. The hydrophobicity of the silane layer is greater than that for siloxane groups due to the presence of nonpolar hydrocarbon chains. We used eq 2 to calculate the free energy of adhesion between a polyethylene particle and modified silica. Because of the heterogeneous nature of the silanated silica surface, the surface tension component of silica must reflect this heterogeneity, and the corresponding equations are modified as follows:

∆GPWS ) 2

[xγ

LW

P

γWLW +

xγS-CHLWγWLW - xγPLWγS-CHLW - γWLW + xγW+(xγP- + xγS-CH- - xγW-) + xγW-(xγP+ + xγS-CH+ - xγW+) xγP+γS-CH- - xγP-γS-CH+] γS-CHLW ) fγCHLW + (1 - f)γSLW γCHLW

)

(1 + cos θCH)2γW2 4γWLW

(7) (8) (9)

γS-CH- ) (1 - f)γS-

(10)

γS-CH+ ) (1 - f)γS+

(11)

cos θ ) f cos θCH + (1 - f) cos θS

(12)

where the subscripts S-CH, S, and CH refer to silicasilane substrate, silica, and silane layer, respectively; f is the fraction of the silica surface covered by a silane layer; and θ, θS ()2.5°), and θCH ()110°) are the advancing contact angles measured on silica-silane substrate, silica, and silane monolayer. After substituting all numerical values, eq 7 is simplified to

∆GPWS ) -91.96 - 2.14x42 - 15.7f + 84.5x1 - f (13) The ∆GPWS and F/R values calculated from eqs 13 and 6, respectively, are shown in Table 3. The correlation between theoretical and experimental F/R values is similar to that for the heat-treated silica reported in the previous section. The weak adhesion measured between the polyethylene colloidal probe and the untreated silica surface is attributed to the presence of a strongly bonded water film separating polyethylene from “clean” contact with the silica surface. Theoretical and experimental adhesion forces (F/R) are in good agreement for the silica substrate covered with a

Langmuir, Vol. 19, No. 13, 2003 5317

submonolayer of OTS, f ) 0.34-0.8 (Table 3). This agreement is remarkable for f ) 0.5 and f ) 0.8 and suggests that at f < 0.5 indeed a water film strongly adsorbed to the silica surface by silanol groups causes a weakening of the silica-polyethylene adhesion. Long-range attractions often appear between hydrophobic surfaces in water and are believed to be caused by nucleation and growth of gas nanobubbles and nanobridges.64,65 Such hydrophobic interactions were only observed for silane-treated silica when the contact angle was about 94°. However, in this research, capillary forces present between separated hydrophobic surfaces appeared to have a little influence on the measured pull-off force values. This negligible effect results from the fact that the capillary forces, as estimated from the equation F ) -4πRγ cos θ (where γ is the water surface tension, R is the probe radius, θ is the contact angle), are very small, F ≈ 0, for contact angles close to 90°. Conclusions Thermal treatment of silica leads to the removal of adsorbed water layers and dehydration of silanol groups at the surface. Similarly, the silanation reaction blocks the active silanol groups and/or adsorbed water layer, covering silica with a closely packed monolayer of hydrocarbon chains. Both treatments result in an increase in hydrophobicity of silica, as was shown in this study by the contact angle measurements. The increase in the hydrophobicity of silica causes an increase in the adhesive force between a PE sphere and the silica surface in water. While this increase is much stronger for the highly hydrophobic silanated surface, the increase in adhesion is also substantial for the thermally treated silica surface. Such an increase in adhesion between a hydrocarbon and a dehydrated silica surface is expected to be one of the main causes for the decrease in the efficiency of oil removal from contaminated soil which was exposed to elevated temperature or contacted with oil for extended periods of time prior to cleanup and remediation. Experimental adhesion forces measured in the polyethylene-water-silica system with the atomic force microscopy colloidal probe technique were compared with the values calculated theoretically using the JKR theory55 and the Lifshitz-van der Waals Lewis acid-base interaction theory43 and found to be in good agreement for hydrophobic silica substrates. This agreement is less satisfactory for hydrophilic and slightly hydrophobized silica. Under these circumstances, it is hypothesized that the polyethylene particle was unable to displace a thin film of water, strongly adsorbed to the silanol groups on the silica surface, during the adhesion force measurements. Acknowledgment. Financial support provided by the Department of Energy, Basic Science Division Grant No. DE-FG-03-93ER14315, and from the U.S. Environmental Protection Agency, National Center for Environmental Research and Quality Grant No. R825306-01-0, is gratefully acknowledged. Although the research described in this article has been funded partially by these agencies, it has not been subjected to the agencies’ required peer and policy review and therefore does not necessarily reflect the views of these agencies, and no official endorsement should be inferred. J.D. would like to acknowledge the donors of the Petroleum Research Fund, administered by the ACS, for partial support of his contribution to this research. LA026911Z (64) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468-8480. (65) Attard, P. Langmuir 1996, 12, 1693-1695.