Langmuir 1994,10, 1903-1909
1903
Use of Atomic Force Microscope for the Measurements of Hydrophobic Forces between Silanated Silica Plate and Glass Sphere Ya. I. Rabinovich and R.-H. Yoon* Center for Coal and Minerals Processing, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Received November 29,1993. I n Final Form: March 17, 1994" An atomic force microscope (AFM) was used to measure the short- and long-range hydrophobic forces between a silanated glass sphere and silicaplate. The hydrophobicitywas controlled by depositingdifferent amounts of octadecyltrichlorosilane (ODTCS) and trimethylchlorosilane(TMCS). The forces measured with TMCS-coated surfaces using the AFM are comparable to those obtained previously using other techniques. The force us distance curves can be fitted by both exponential and power laws. The measured hydrophobic forces increase with contact angle, perhaps the most convenient measure of hydrophobicity. Adsorption of ODTCS results in the formation of molecular clusters (or domains), whose size remains relatively constant in the range of advancing contact angles (95-115') studied in the present work. The charge or the dipole moment associated with those domains may be reponsible for the observed long-range hydrophobic forces.
Introduction Forces acting between hydrophobic macroscopic bodies in water have been measured for various systems over the last 10 years.l-13 The attractive hydrophobic forces are larger than the van der Waals forces that can be calculated based on continuum theory. Despite numerous studies,lk% no theory can explain much of the experimental observations. While awaiting for reliable theories to emerge, many experimentalists use a double-exponential function of the form3
* To whom correspondence should be directed. published in Advance ACS Abstracts, May 1, 1994 (1) Derjaguin, B. V.; Kusakov, M. M. Acta Physicochim. URSS 1939, 10, 25. (2) Israelachvili, J. N.; Pashley, R. M. Nature 1982,300,341;J.Colloid Interface Sci. 1984, 98, 500. (3) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985,229, 1088. (4) Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986.114. 234. (5),Rabindvich, Ya. I.; Derjadn, B. V. Colloids Surf. 1988,302,243; Collord J. USSR 1987,49, 682. (6) Claesson, P. M.; Christenson, H. K. J.Phys. Chem. 1988,92,1650. (7) Herder, P. C. J. Colloid Interface Sci. 1990, 134, 336. (8) Parker, J. L.; Claesson, P. M.; Cho, D. L.; Ahlberg, A.; Blomberg, E. J. Colloid Interface Sci. 1990. 134.449. (9) Herder, P. C. J. Colloid Irherface Sci. 1991,143,573. (10) Tsao, Y.; Yang, S. X.; Evans, D. F.; Wennerstrom, H. Langmuir 1991, 7, 3154. (11) Rabinovich, Ya. I.; Guzonas, D.; Yoon, R.-H. Submitted for e Abstract
publication in J. Colloid Interface Sci. (12) Xu, Z.; Yoon, R.-H. J. Colloid Interface Sei. 1990, 134, 427. (13) Ravishankar, S. A.; Yoon, R.-H. Submitted for publication in J. Colloid Interface Sci. (14) Derjaguin, B. V.; Churaev, N. V. J. Colloid Interface Sci. 1974, 49, 249. (15) Marcelia, S.; Radich, N. Chem. Phys. Lett. 1976, 42, 129. (16) Luzar, A.; Svetina, S.; Zeks, B. Chem. Phys. Lett. 1983, W,485. (17) Yushchenko, V. S.; Yaminsky, V. V.; Shchukin, E. D. J. Colloid Interface Sci. 1983, 96,307. (18) Podgornik, R. J. Chem. Phys. 1989, 91, 5840. (19) Ruckenstein, E.; Churaev, N. V. J. Colloid Interface Sci. 1991, 147, 535. (20) Rabinovich, Ya. I.; Guzonas, D.; Yoon, R.-H. Langmuir 1993,9, 1168. (21) Tsao, Y.-H.; Evans, D. F.; Wennerstrom, H. Langmuir 1993,9, 779. .
(22) Eriksson, J. C.; Ljunggren, S.; Claesson, P. M. J. Chem. SOC., Faraday Trans. 2 1989,85, 163. (23) Attard, P. J. Phys. Chem. 1989,93, 6441. (24) Podgornik, R.; Parsegian, V. A. Chem. Phys. 1991, 154, 477.
F in which F is the attractive hydrophobic force, R the curvature of the mica surfaces, H the closest distance separating two interacting surfaces, C1 and Cz are parameters characterizing the magnitudes of the short- and long-range hydrophobic forces, respectively, and D1 and DZare the decay lengths. Here the term "short-range" refers to the attractive hydrophobic forces observed at H C 10 nm with decay length of 1-2 nm rather than the true interatomic short-range forces. As will be discussed later, a third exponential term may be necessary to fit the experimental data. The hydrophobic force may also be described by a power law25 F
K
R=-S where K is the only fitting parameter. Equation 2 is in the form of the van der Waals equation; however, K is generally much larger than the Hamaker constant. Experimental results are often best-fit by an exponent other than 2,but it affords us the advantage of having only one parameter to fit. Many theories have been proposed regarding the origin of the hydrophobic force. These include entropy increase due to configurational rearrangement of water molecules when two hydrophobic surfaces approach each other,lk16 capillary force due to cavitation in the vicinity of hydrophobic surfaces,17 charge correlation,lS hydrodynamic correlation between fluctuating liquid-solid interfaces,lg and correlation of dipoles associated with molecular domains.ZOJ!l Most of the direct force measurements were conducted using curved mica surfaces due to limitations of the surface force apparatus (SFA) of Israelachvili and Tabor.26>m Mica (25) Rabinovich, Ya. I.; Derjaguin, B. V. In Proceedings of the 5th Hungarian conf. on ColloidChem.,Hungary,Balatonfured, Lorand Eotvoa Univ., Budapest, 1988. (26) hraelachvili, J. N.; Adams, G. E. J. Chem. SOC.,Faraday Trans. 1 1978, 74, 975-1001. (27) Israelachvili, J. N.; Tabor, D. Nature 1972,236, 106.
0743-7463/94/2410-1903$04.50/00 1994 American Chemical Society
1904 Langmuir, Vol. 10, No. 6, 1994
surfaces are hydrophobized either by self-assembled amines from aqueous solution" or by depositing insoluble, double-chain amines using the Langmuir-Blodgett technique.6J1*20Both the short- and long-range hydrophobic forces are observed with the latter, while former showed only the short-range hydrophobic forces. It has recently been shown, however, that soluble amines deposited on mica can also exhibit long-range hydrophobic forces,when neutral surfactants, such as octanol, are coadsorbed in the monolayer to increase the hydrocarbon chain ordering.13 The mica surfaces were also hydrophobized by treating them in water plasma, followed by methylation.8 The force curves obtained using the "methylated" mica are similar to those obtained with methylated silica, but it is not yet clear whether the two methylated surfaces are the same. Rabinovich and Derjaguin5*Bused a different type of SFA to measure hydrophobic forces between crossed silica fibers methylated with dimethyldichlorosilane(DMDCS). In the present communication, we used an atomic force microscope (AFM) to measure the hydrophobic forces between silica surfaces methylated with trimethylchlorosilane (TMCS) and octadecyltrichlorosilane(ODTCS). Because of the longer hydrocarbon chains, the latter produces stronger hydrophobic surfaces, allowing a wide range of hydrophobic surfaces to be studied.
Experiment A standard AFM can be used to measure the interaction forces between a flat smooth surface and a pyramidal tip (usually of silicon nitride). However, the resulting force us distance curves cannot be analyzed theoretically due to the ill-defined geometry, which does not allow normalization of the measured forces with respect to the radii of the objects involved. Only when the force measurements are conducted between two crossed cylinderstwo spheres,3l or a sphere and ~ l a t e can 3 ~ the data be analyzed theoretically and converted to energy or disjoiningpressure using the Derjaguin approximation.= Each of these geometries has been used with other SFA's but not with AFM. Only over the past few years, AFM's were used with correct geometries. The sphere-plate geometry was first used by Ducker et al.," Butt,% and Meagher.= These investigatorsused microspheres of radius 1-2 pm in place of the pyramidal tip. Li et al.,m on the other hand, used two small spheres. Those who used the correct geometries- limited their AFM measurements to the repulsive forces between hydrophilic silica surfaces. It is difficultto measure hydrophobicforcesusing AFM, because the surfacesstick to each other during the measurements due to the large adhesion forces. In AFM measurements, it is necessary for the two surfaces to be separated from each other in order to determine the zero-separation distance and to have reproducible results. We have solved this problem byusingstiffer, rectangular siliconcantilevers (DigitalInstruments,type ESTM) instead of the standard triangular silicon nitride cantilever. The stiffness of the cantilevers used in the present work was in the range of 30-120 N/m, while those of the standard triangular cantileversare in the 0.06-0.6 N/m range. Of course, an increase in cantilever stiffness results in a decrease in sensitivity of the ~
~~~~
~~
(28) Rabmovich, Ya.I.; Derjaguin, B. V.; Churaev, N. V. Adu. Colloid Interface Sei. 1982,16,63. (29) Derjaguin, B. V.; Rabinovich, Ya I.; Churaev,N. V. Nature 1977, 265,520; 1978,272,313. (30) Rabinovich, Ya. I.; Rihikov, M. B.; Sobolev, V. D.; Churaev, N. V. Colloid J. USSR 1982,44,977. (31) Derjaguin, B. V.; Abrikossova,I. I.; Lifshitz,E. M. Usp.Fiz. Nauk 1958,64,493. (32) Blokland, P. van; Overbeek, J. Th.G. J. Chem. Soc., Faradcry Trans. 1 1978. 74,2637. (33) Derjaghn, B. V. Kolloid 2.1934,69, 155. (34) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991,353, 239-241. (35) Butt, H.4. Biophys. J. 1991, SO,777. (36) Meagher, L. J. Colloid Interface Sci. 1992,152, 293. (37) Li,Y.Q.;Tao,N.J.;Pan,J.;Garcia,A.A.;Lindsay,S. M.Langmuir 1993,9, 637.
Rabinovich and Yoon
I
Figure 1. A glass bead glued to a rectangular cantilever. force measurement, but it enables the measurement by allowing the two surfaces separated after initial contact. In the present work, force measurementswere conducted using a polished Herasil-3 fused silica plate and a glass microbead, both of which were hydrophobized with TMCS and ODTCS. The silica plates were obtained from Herasil Amersil, Inc., and cleaned in boiling concentrated nitric acid prior to the silanation. The smoothness of surfaces was checked using a standard AFM (Nanoscope 111, Digital Instruments Co.) with a silicon nitride tip. The glass beads of radius 10-30pm in diameterwere obtained from Duke Scientific. For each measurement, a single bead was mounted on the cantilever tip using an epoxy resin (Epon R Resin 1004F, Shell Chemicals Co.) by means of a micromanip ulator. Figure 1 shows an electronmicrograph of a glass bead mounted on a rectangular cantilever. The beads used in the present work were near-spherical and showed a mean roughness of less than 1 nm.
ThesilicaplatesandtheglassbeadsweresilanatedusingTMCS and ODTCS. Both reagentswere obtained from Aldrich Chemical Co. and used as received. Prior to silanation with TMCS, the silica plates were heated to 100 "C and exposed to water vapor for 4 h to ensure water adsorption on the silica surface. The silica plate was placed in a glass cell, in which a drop of TMCS was placed, and heated to 50 "C for half an hour. The sample was then heated to 60 "C in the presence of water for another half an hour. Hydrophobizationwith ODTCSwas achievedin a cyclohexane solution at room temperature using the method described elsewhere.3 The silica plates and glass beads were immersed in the solution for different periods of time to vary the surface hydrophobicity. After the hydrophobization, the beads were filtered from solution and dried in a low vacuum. The excess reagents were removed with ethanol before the samples were dried under a nitrogen gas stream at room temperature. As a measure of surface hydrophobicity, we determined advancing (e), and receding (6,) water contact angles on the silanated silica plates using the tilting plate technique. A Ram& Hart goniometer was used to measure the angles. In a given force measurement, we used the same immersion times for silica plates and glass beads, so that their hydrophobicities were approximatelythe same. To make sure this was the case, contact angles of the beads were measured using the "swimming" method, in which a microbead was placed at an airwater interface and the depth (db)of immersion measured. The contact angle (&,) was then calculated from de as described in ref 39. Use of AFM for surface force measurements has several advantages. The surfaces of opaque material can be studied, (38) Flinn, D. S.;Guzonas,D.A.; Yoon,R.-H. Submittedforpublication in Langmuir. (39) Wei, D.; Chander, S.; Hogg, R. Coal. Prep. 1992,20,37.
Measurements of Hydrophobic Forces
Langmuir, Vol. 10, No. 6, 1994 1905
Figure 2. Illustration of the method of calibrating an AFM cantilever: 1,glass filament with known stiffness (kf); 2,glue; 3, metallic disk; 4,piezo crystal; 5, AFM cantilever with unknown stiffness (k,); 6, cantilever holder. Table 1. Results of the Cantilevers Calibration cantilever stiffness (N/m) cantilever no. from eq 3 from eq 7 1 119 112 2 84.3 90.2 Figure 3. AFM image of a clean, polished silica plate.
demand for the smoothness of surfaces is limited to a smaller area than the case of the Israelachvili and Tabor’s SFA, and geometries other than crossed cylinders can be used. On the other hand, in the AFM force measurements it is difficult to determine the stiffness (k), of the cantilever. Althoughthe Digital Instruments Co. provides the k values for the triangular cantilevers, they are only approximate. For surface force measurements,it is, therefore,necessarytodetermine the k values. One methoda suggests measuring the resonance frequency of a cantilever with various weights on the end. Unfortunately, the Nanoscope I11 does not allow the resonance frequency to be measured. For this reason, we calculated the k values from the dimensionsof the cantilever and the Young’s modulus (Y) of the material that makes up the cantilever. The cantilever dimensions were determined from the electronmicrograph. For the rectangular cantilevers used in the present work, one can calculated the k values from the following expression41
k, = wdSY/4ls
(3)
where w, d, and 1are width,thickness, and length of the cantilever, respectively. An appropriatecorrection was made for cantilevers whose cross-sectional area was not exactly rectangular; many cantileversshowed trapezoidal c r m section. Sincethe cantilevers were made of silicone, a value of Y = 1.1 X 1011N/m242 was used. The k values calculated as such were in the range of 50-120 N/m. The largest sourceof error in estimatingthe k values was probably associated with the microscopic measurements of the cantilever dimensions,particularly the thickness d. The error in measuring d was about h0.25 nm for the average value d = 5 nm, in which case the relative error in estimating k using eq 3 should be approximately 115%. There may also be an error associated with using a Y value from a handbook, because it could change dependingon the purity, crystallinity,and method of preparation. In order to minimizethe errors associated with the determination of the k values, a method of directly calibrating a cantilever was developed as illustrated in Figure 2. In this method a glass filament (1)of approximately 200 pm in diameter and 8 mm in length was glued (2)to a metallic disk (3), and its stiffness (kf) determined by microscopically measuring its deflection under a known weight placed at the end the filament. The assembly was then placed on the piezo crystal (4) in the AFM, where the cantilever (5)of unknown stiffness (k,)was brought into contact with the filament. When the piezo crystal moves by a distance AH,the following relationship holds:
LW=AD,+AD,
(4)
in which ADf and ADc are the changes in the deflections of the filament and cantilever, respectively. (40)Cleveland, J. P.; Manne, S.;Bocek, D.; Hansma, P. K. Reo. Sci. Instrum. 1993,64,403. (41)Ebert, H.Physikalisches Taschenbuch; Friedr.vieweg & s o b
Braunschweig, 1957.
(42) CRC Handbook of Tablesfor Applied EngineeringScience;Bolz,
R.E., Tuve, G. L., Eds.; CRC Press: Boca Raton, FL, 1973; p 331.
Figure 4. AFM image of a silica plate covered by ODTCS with contact angle 8, = 115’.
From the definition of k, one can see that AF = k,ADc = k W f
(5)
where AFis the change in the force applied at the point of contact between the filament and the cantilever. Equation 5 yields
From eqs 4 and 6, we obtain an expression for k,
k, = kf
(3,1)
(7)
which is a function of known parameters kf and LW/AD,. The latter can be measured directly by AFM. Table 1 compares the calibration results obtained by using eqs 3 and 7 for selected cantilevers. There is a reasonable agreement between the two calibration methods; however, use of eq 7 is more reliable although it is more time-consuming.
Results and Discussion The AFM image of the silica plate given in Figure 3 shows that the surface is composed of grains of approximately 200 nm in size. Each grain shows a smooth surface with roughness of less than 1nm, while the grain boundaries show depth profiles of about 10 nm. When the silicaplate is silanated by ODTCSsothat the advancing contact angle (0,) becomes larger than 90°, the grain boundaries are not clearly visible as shown in Figure 4. Note also that the silanated surface shows the formation of molecular clusters (or domains), whose dimensions are approximately 20 X 40 nm. The clusters are elliptical in
Rabinovich and Yoon
1906 Langmuir, Vol. 10, No. 6,1994
Figure 5. A cantilever deflection us Z-position of piezo crystal as obtained during an AFM force measurement for silica plate and glass sphere coated with ODTCS (8, = 115O). Table 2. Water Contact Angles on Silica Plates and Glass Beads in TMCS and ODTCS Solutions contact angles (deg) surfactant advancing receding 88 62 TMCS 95 80 ODTCS 105 86 ODTCS 115 88 ODTCS 116 89 ODTCS
Figure 6. Force (F)us distance (H) curve obtained from Figure 5 for ODTCS-coated silica plate and glass sphere (8, = 115"). The arrow represents thejump position. Curves 1and 2represent the results obtained while the two surfaces are approaching to
and separating from each other.
H but becomes more or less constant a t the point marked bead 91 95
shape and show heights of approximately 2 nm. A series of AFM images taken at different immersion times (and, hence, different surface coveragesand contact angles)show that the size of clusters do not change significantly a t 8,'s in the 95-115O range. We can suggest that forming the clusters is related to ordering of the hydrocarbon chains.43 Table 2 showsthe water contact angles of the silica plates and glass beads treated in TMCS and ODTCS solutions at different immersion times. The contact angles of the ODTCS-coated silica plate are larger than those of the TMCS-coated surface, which can be attributed to the longer hydrocarbon chains. With ODTCS, both 0, and Br increase with increasing immersion time. The contact angle hysteresis, which increases with increasing contact angle, indicates that the surface becomes more heterogeneous with increasing coverage. This may be explained by the formation of the clusters. Table 2 also shows that ob lies between the 8, and Or, suggesting that the hydrophobicity of the beads is comparable to those of the plates. Figure 5 shows the raw data for the AFM force measurements conducted with ODTCS-coated silica plate and glass bead. The curve was obtained by moving the piezo crystal, on which the silica plate was mounted, along the 2-direction toward the sphere at the end of the cantilever, while monitoring the cantilever deflection. The initial nonlinear portion of the curve represents the changes in attractive force with the distance. When the plate touches the sphere, the curve reverses its direction and becomes linear. The distance (H) between the plate and the sphere is calculated by subtracting the 2-position where the deflection becomes zero on the linear portion of the curve (point A) and the cantilever deflection at the point of interest. The interaction force (F)was calculated using eq 5, i.e., by multiplying the deflection with the cantilever stiffness k. Curve 1in Figure 6 shows the FIR us H curve obtained as such from the data shown in Figure 5. Note that the slope (dF/dH)of the force curve increases with decreasing (43) Rabinovich,Ya. L;Guzonas,D. A.;Yoon, R.-H.J. Colloidlnterface Sci. 1993, 155, 221.
by an arrow. We believe that this point represents the distance (Hj) at which the sphere jumps into contact with the silica plate.26~30~31*44 Although AFM often gave data below this point, they may represent nonequilibrium results. For this reason, the experimental data obtained below the jump distance are given as a dashed line. Figure 6 also shows the outward curve, which yields the adhesion force FaJR = 430mN/m and a jump out distance of 120 nm. The adhesion force is comparable to those reported in literature based on measurements conducted with SFA. For example, Claesson and Christensons obtained a value of 200-500 mN/m for mica surfaces coated with dioctadecyldimethylammonium (DODA) bromide (e, = 9 4 O ) . Considering that the data given in Figure 6 were obtained with ODTCS-coated silica surface with ea = 115O, the higher adhesion force obtained in the present work is reasonable. From the adhesion force (Fad) and the contact angle value, it is possible to calculate the surface free energy (rsV) of the silanated silica to be 15 mJ/m2. This value may be compared with the surface free energies of many hydrocarbon surfaces, which are in the range of 20-27 mJ/m2. The lower ysvvalue obtained for the silanated surface may be attributed to the fact we used the advancing rather than equilibrium contact angle in using the Young's equation. The correspondence between the values obtained in the present work and those in literature supports that the AFM measurement technique employed in the present work is acceptable. The measured force is largely due to attractive hydrophobic force. The dispersion force Calculated using the Hamaker constant of A = 1.2 X J for silica in water28145 is much less than the measured force. The ion-electrostatic force calculated with the usual formula& assuming 1 X lo4 mol/L of electrolyte concentration and a Stern potential of $8 = 50 mV is also negligibly small. Figures 7 and 8 show the force curves obtained with TMCS and ODTCS plotted conforming to exponential and power laws, respectively. The hydrophobicity was controlled by varying the immersion times. Curve 2 in Figure 7, representing the results for 8, = 95O, conforms to eq 1,while curves 3 and 4 (8, = 105O and 115') appear to be represented by single exponential functions. On the (44)Rabmovich,Ya I.; Derjaguin,B. V. InSurfuceforces and boundary layers of liquids (translated into English); Derjaguin, B. V., Ed.; Nauka: Moscow,1983; p 13. (45) Rabinovich, Ye. I.; Churaev, N. V. Colloid J. USSR 1981,41,468. (46) Derjaguin, B. V.; Landau, L. D. Zh.Eksp. Teor. Fiz. (translated into English) 1941, 11, 802.
Measurements of Hydrophobic Forces
Langmuir, Vol. 10, No. 6, 1994 1907 jump distance (Hj)determined as described above is used to calculate the force (more precisely disjoining pressure) parameters. Knowing that the two surface jump into contact when force gradient (dF/dH)exceeds the spring constant k, one can readily derive the following relationship:
-1'
'
0
'
'
'
'
40
'
*
'
*
80
'
'
'
120
Hbm) Figure 7. FIR us H curves obtained with TMCS- and ODTCScoated silica plate and glass sphere. Contact angles were varied by controlling immersion times. Solid lines represent eq 1 with the best fit parameters. 21
for the exponential force law. Equation 8 can be used when the jump occurs a t H below approximately 10 nm where the short-range hydrophobic force predominates. If the jump occurs above approximately 20-30 nm where the long-range force becomes predominant, the following equation may be used
1
t
I
lS*
For the power law, the following expression can be used for calculating the force parameter kHj" Kj=3-
R
-a.b..
'
0:s' '
' g g
"
"
'
I
.
2.0 ' .
" I
Hlh5m)
'
2.5
"
Figure 8. FIR us H curves obtained with TMCS- and ODTCScoated silica plate and glass sphere. Contact angles were varied by controlling immersion times. Solid lines represent eq 2 with the best-fit parameters.
Table 3. Force Parameters of Equations 3 and 4,
88 95 105 115 116
24.2
44.1
1.42 4.10 160 98.4 320 255 385 175
2.4
16.4 20.5 16.6 18.8 21.7
0.89 8.2 83 300 276
0.44 11.2 54 280 351
a Subscripts 1 and 2 refer to short- and long-range hydrophobic forces. Subscript j refers to those obtained by jump method.
other hand, curve 1 (ea = 88') requires three exponential terms. Since most literature results have been given in double exponential functions, we may also consider curve 1represented by a double exponential function. In this case, the midrange may be regarded as a transition zone between the short- and long-range hydrophobic forces. For this reason, we used the data obtained in the shortdistance range to calculate C1 and D1 and those in the long-distance range to calculate CZand D z . As such, the data obtained in the transition zone have been ignored. Likewise, the data in curve 2 obtained at H > 32 nm are used for calculating the long-range hydrophobic forces, while the data obtained below this distance are considered to belong to a transition zone. The short-range hydrophobic forces were not measurable in this case. Curves 3 and 4 should be considered as representing only the longrange hydrophobic forces. Table 3 gives the best-fit parameters of the exponential and power laws for the data shown in Figures 7 and 8. Also shown are the force parameters obtained using the socalled jump (or gradient) method.26 In this method, the
(10)
where the subscript j refers to the values obtained using the jump method. In using eqs 8 and 9, we used the D1 and D Zvalues obtained by the equilibrium method. This was necessary because we usually used only one cantilever in a given set of force measurements. We would like to note that the gradient method was used for the first time with AFM in the present work. The force parameters obtained in the present work using AFMwithTMCS (whichgave 0, = 88') andODTCS (which gave ea = 95') are comparable to those obtained by using other SFA's. Mica surfaces coated by DODA (0, = 94°)4J0 and silica surfaces coated by dimethyldichlorosilane (0, = 100°)5gave similar force parameters to those given in Table 3. The force measurements conducted at larger s6: also gaveDZvalues comparable to those obtained by using other SFA's, but the CZvalues are considerably larger as will be further discussed later. The force parameters obtained by the jump method, i.e., Clj, Czj, and Kj are comparable to those obtained by using the equilibrium method. It should be noted here that the CZvalues obtained with ODTCS at 8, = 115' are substantially larger than those reported in ref 6 for mica surfaces coated by fluorocarbon surfactants (ea = 113'). The reasons for the apparent discrepancy will be discussed later; however, it should not be considered as reflecting problems associated with the AFM measurement technique. That the adhesion force measured in the present work (Figure 6) is close to literature values proves the validity of the AFM measurements conducted in the present work. Furthermore, the CZvalues of the present work are comparable to those obtained by Claesson et al.47 with silanated glass using a new surface force apparatus. Comparison of Figures 7 and 8 shows that both eq 1and eq 2 can fit the experimental data. However, the double exponential law with four parameters fits experimental results better. On the other hand, the power law is more convenient because it has only one parameter. The force parameters obtained in the present work using the equilibrium and jump methods are plotted in Figures 9-11 as a function of 8., Also shown for comparison are (47)Claesson, P. Personal communication at the Engineering Foundation Conferenceon Surface Characterizationof Adsorption and Surfaces Reactions, Kona, HI, 1994.
Rabinovich and Yoon
1908 Langmuir, Vol. 10, No. 6,1994 400
t
10
40
60
80
100
120
8, Figure 9. C2 us 8, for the data obtained in the present work and those from literature. Filled symbols represent the AFM data of the present work and the open circles represent the literature data obtained by using SFA. The solid line is the best exponential fit of the AFM data. Preirnt data o titeraturo data
I
.I
-
20 A
E -
U
a" 10 O
O
-
0
40
80
60
I
O
100
120
8,
Equilibrium method m Jump method
'i
5-200 0 r(
loo
1 40
60
80
100
120
8, Figure 11. K us 0, for the AFM data obtained in the present work using equilibrium and jump methods. The solid curve represents the best exponential fit.
the literature values from refs 2-13. These were obtained with mica and glass surfaces hydrophobized by various surfactants at various temperatures. The solid lines represent the best fits (exponential or linear) of the data obtained in the present work. It is evident from Figures 9 and 11 that hydrophobic force, as measured by C2 and K, increases sharply with contact angle a t Be above approximately 90". On the other hand, based on the data obtained in the present work, D2 does not change greatly with ea in the range of 90-115O (see Figure 10). It is clear that the contact angle, which is a reflection
of the surface free energies involved a t a three phase contact, is an important factor in determining the hydrophobic force. However, there may be an additional factor affecting the long-range hydrophobic force. That the long-range force parameters, i.e., C2 and K, change drastically in a rather narrow range of 8, (=90-115") suggests this view. Obviously,the additional factor affects the contact angle rather weakly, but strongly influences the long-range hydrophobic force. We suggest that the additional factor is the ordering of hydrocarbon chains, which results in the formation of domains (or molecular The domains of ordered hydrocarbon chains may create charges (or dipoles), which in turn creates attractive electrical forces due to induced dipoles or charge correlation. If the hydrophobic force originates from the charge correlation, the force should be screened by electrolyte. Actually, ref 48 showed that the hydrophobic force between mica surfaces coated by DODA is screened by electrolyte; however, the dependence of the decay length (D2) on the electrolyte concentration (C,d is weaker than same between the Debye length ( 1 / ~and ) C,1. We may attribute it to the large dimensions of the domains (and, hence, those of the charges or dipole moments). More detailed explanation of this effect will be given in ref 49. As has already been noted, the CZvalues obtained a t higher contact angles (6, > 100') are substantially larger than those obtained by Claesson and Christenson6 with mica surfaces coated by fluorocarbon surfactant (4= 113'); the two C2 values obtained by these investigators are plotted in Figure 9. This difference may suggest that the magnitudes of the charges (or dipole moments) associated with the domains of ODTCS formed on silica are larger than those of fluorocarbon surfactants on mica. I t is also important to point out that the mica surfaces coated with fluorocarbon surfactant showed receding contact angles considerably lower than those (8, E 90") obtained in the present work with ODTCS-coated silica. Thus, the surfaces we used were considerably more hydrophobic than those used by Claesson and Christenson. According to the AFM and FTIR studies conducted by Flinn et al.,38 ODTCS adsorbs on silica initially in ellipsoidal patches with average dimensions of 20 X 40 nm. As the coverage increases, the patches become more numerous without significantly increasing their size. As the coverage reaches 0.25-0.3, as determined by a quantitative adsorption studies, the patches essentially cover the entire surface and 8, becomes approximately 90". At this coverage, each molecule occupies approximately 0.8 nm2,which gives a circular radius of about 0.5 nm. Since this value is close to the length of an 18-carbon chain, Flinn et al.= considered that ODTCS molecules lie flat on the surface. As the coverage increases beyond the 0.250.3 range, the bandwidth and band frequencies of the CH2 asymmetric and symmetric stretching vibrations gradually decrease, indicating an increase in hydrocarbon chain ordering. Based on Flinn et al.'s finding and the AFM images obtained in the present work, we may suggest the following. The decay length, D2, of the long-range hydrophobic force may be determined by the domain size. This suggestion is borne out from the observation that D2 does not change significantly with ea and that the domain size does not increase with increasing coverage. On the other hand, the (48)Tsao, Y. H.; Evans,D. F.; Wennerstrom, H. Langmuir 1993, 9, 782.
(49) Rabinovich,Y. 1.;Yoon,R.-H.Submittedforpublicationin Colloids
Surf.
Measurements of Hydrophobic Forces preexponential parameter, CZ,may be determined by the adsorption density (or the degree of hydrocarbon chain ordering) of the ODTCS molecules in patches. This view may be supported by the finding that CZdoes not increase sharply until ea becomes approximately 90°, a t which point hydrocarbon chains begin to associate with each other to form more ordered structure. Figures 9 and 10 also show the data obtained by other investigators. Considering widely different reagents and experimental conditions employed to obtain these results, the literature data might be viewed as supporting the suggestions made in the foregoing paragraph. It should be noted, however, that the adsorption mechanisms for ODTCS on silica are quite different from those for soluble and insoluble surfactants deposited on mica. For one thing, the ODTCS adsorption involves grafting polymerization between the siloxane groups, which may be a major reason for being able to clearly observe the large domains and, hence, the long-range hydrophobic force at relatively low coverages. For another, the packing density of hydrocarbon chains on mica, which has been used most widely in literature for force measurements, may be quite different from those on silica because of the differences in crystal structure. Therefore, further studies are necessary to generalize the suggestions made in the foregoing paragraph.
Conclusions 1. Adsorption of ODTCS on silica results in the formation of domains whose size remains relatively constant with increasing adsorption density.
Langmuir, Vol. 10, No. 6,1994 1909 2. AFM has been used successfully to measure the attractive hydrophobic forcesbetween silanated silica plate and glass sphere. At low coverage and small contact angles, the results are consistent with earlier reports based on other methods. 3. AFM can be used for determining the distance at which two hydrophobic surfaces jump into contact. The force parameters obtained by using this method are consistent with those obtained by using equilibrium method. 4. Both the exponential and power laws can be used to fit experimental force curves. 5. The long-range hydrophobic force parameters, Ca and K, increase sharply at advancing contact angles above 95'. This may be attributed to an increase in the adsorption density of ODTCS molecules in each domain, which results in an increase in hydrocarbon chain ordering. 6. The decay lengths of the long-range hydrophobic forces measured with silanated surfaces are relatively independent of the contact angle, which may be related to the observation that the domain size remains relatively constant in the range of contact angles studied.
Acknowledgment. The authors wish to express gratitude for the financial support from the U.S.Department of Energy (DE-AC22-91PC91164)and the helpful discussions from Dr. Richard B. Read. They are also thankful for Mr. Darrin S. Flinn for his assistance in the silanation of samples.
