Langmuir 1993,9, 779-785
779
Long-Range Attraction between a Hydrophobic Surface and a Polar Surface Is Stronger Than That between Two Hydrophobic Surfaces Yi-Hua Tsao and D. Fennel1 Evans* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
HAkan Wennerstrom Division of Physical Chemistry 1, The Chemical Center, P.O.Box 124, S-22100 Lund, Sweden Received June 19,1992. In Final Form: October 13,1992 Long-range attractive forces have been measured in the surface forces apparatus (SFA), for both a symmetrical situation with two mica surfaces made hydrophobic by surfactant adsorption and an unsymmetrical situation with one hydrophobic surface. The magnitude of the attractive force decreases as electrolyte is added to the aqueous medium and the 2:2 electrolyteMgS04 has a stronger effect than a 1:l electrolyte like NaC1. The force is also decreased by adding octanol in low concentrations (510-4 M). We find a qualitativelyvery similar behavior of the force in the symmetrical and the unsymmetrical situation although we find larger forces in the latter case. In conflict with a previous interpretation we conclude that the force in the two cases is due to the same molecular mechanism. We argue furthermore that the force has an electrostatic origin. We suggest that the electrical fields emanating from the hydrophobic surfaceare due to a lateral inhomogeneityon a nanometer to micrometer scale at the surface.
Introduction Of all the interaction forces between surfaces in simple liquids the most intriguing and confusing is the very longrange, strongly attractive force that has been observed between hydrophobic surfaces in ~ a t e r . l -That ~ a strong attractive interaction should exist follows from high surface free energy, but the range and the magnitude of the force are remarkable. Attractive forces have been measured regularly up to over 50 nmM separations and there are reports of measurable attractive forces a t 300 nm.7 These forces are 1-4 orders of magnitude larger than those predicted from continuum theory. This long-range"hydrophobic" attractive force has been observed by several groups in different laboratories using a range of different surface preparation methods and showingall the signs of a genuine phenomenon. However, there is no consensus on the mechanistic explanation of the force although several detailed explanationshave been proposed.Sl0 Similarly, there is a debate on surface preparation procedures and also on the role of surface charges and their contribution to the total force.ll To us it seems an urgent, but difficult, task to clarify the basic mechanism of this long-range force. As it stands now one can use this "hydrophobic" force to rationalize virtually any unusual observation involving colloidal interparticle forces. (1) Israelachvili, J. N.; Pashley, R. M. Nature 1982, 300, 341. (2) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985,229, 1088.
(3) Christensson,H. K.;Fang, J.; Ninham, B. W.; Parker, J. L.J.Phys. Chem. 1990,94,8004, and references therein. (4) Claeeeon,P. M.; Christensson,H. K. J. Phys. Chem. 1988,92,1650. (5) Christensson, H. K.; Claesson, P. M.; Berg, J.; Herder, P. C. J. Phys. Chem. 1989,93,1472. (6) Tsao, Y.-H.;Yang, S. X.; Evans, D. F.;Wennerstrdm, H. Langmuir 1991, 7, 3154.
(7) Kurihara, K.; Kato, S.; Kunitake, T. Chem. Lett. 1990, 1555. (8) Eriksson, J. C.; Ljunggren, S.; Claesson, P. M. J. Chem. SOC., Faraday Trans. 2 1989,85, 163. (9) Podgornik, R. J. Chem. Phys. 1989,91, 5840. (10) Attard, P. J. Phys. Chem. 1989,93, 6441. (11) Christensson,H. K.; Claesson,P. M.; Parker, J. L.J.Phys. Chem. 1992,96,6725.
In a previous paper6 we argued that the detailed molecular properties of the surface have a strong influence on the force. In particular we could show, by comparing surfaces covered with dialkyl cationic surfactants with three different chain lengths, that the force is most longranged when the alkyl chains are in a frozen state. When the hydrophobic monolayer becomes disordered at higher temperatures, the attractive force decreased to the extent that it could not, with certainty, be distinguished from a van der Waals force. The state of the alkyl chains was inferred from a combination of direct observation using atomic force microscopy and a comparison with transition temperatures in bulk phase. In the present paper we continue our study of the longrange force by focusing on the role of the electrostatic interactions. It is well established315J1J2that the longrange force is sensitive to the addition of electrolytes even in low concentrations. This shows that there is some electrostatic contribution, direct or indirect, to the force. One obvious effect is that the surfaces might acquire an electrical charge either at the preparation stage or by adsorption of ions in the solution. In some studies this is clearly one contribution, but it is not sufficient for explaining the effect of electrolyte in general. We here report measurements on the force between two mica surfaces, covered with a hydrophobic monolayer, varying the salt concentration and the electrolyte. We alsoshow the effect of addingodanol in low concentrations. In addition similar experiments are reported for an unsymmetrical situation, where we use one bare mica surface and one hydrophobic surface.
Materials and Methods The dihexadecyldimethylammoniumacetate (DHDAA) and dioctadecyldimethylammonium acetate (DODAA) salts were prepared by eluting correspondent bromide salts (DHDABrand DODABr, Sogo Pharmaceutical Co., Ltd.) in methanol on ion(12) Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986,114, 1.
0743-746319312~09-O779$O4.OO/O 0 1993 American Chemical Society
Tsao et al.
780 Langmuir, Vol. 9, No. 3, 1993 exchange resin (Fisher REXYN 201) in acetate form. The salts were then recrystallizedin ether.13 Dieicosyldimethyiammonium bromide (DEDABr) and (16,16’-dihydroxydihexadecyl)dimethylammonium bromide ((HOC&N2ClBr) were gifts from Professor Robert Moss and Dr. John Trend, respectively, and were used as received. Cyclohexane (Mallinckrodt) of spectrophotometric grade and analytical grade sodium chloride (Mallinckrodt), sodium hydroxide (J. T. Baker, Inc.), potassium chloride (J. T. Baker, Inc.), magnesium sulfate (J. T. Baker, Inc.), and 1-octanol (Aldrich) were used without further purification. Millipore water was processed using a Water Prodigy (Labconco, Corp.) and showed high resistivity (-18 Q cm). The water was heated with gentle stirring under low pressure for 3-4 h to remove dissolved gases. The monolayers were prepared by dipping mica glued onto cylindrical lenses, radius of curvature R = 1-2 cm, into cyclohexane solutions containing 2 X lo4 M DHDAA, DODAA, DEDABr, or (HOCl&N2C1Br. The surfactant covered surfaces were rinsed in warm cyclohexane to remove excess surfactant and then dried. (HOCl&N2C1Br-covered surfaces were rinsed with warm water after cyclohexane was dried. Monolayers thus formed are uniform and continuous over micrometers as observed by atomic force microscopy (AFM). The surface forces apparatus (SFA) used to measure the interaction forces is a modified version14of the one developed by Israelachvili and Adams.15 Two molecularlysmooth mica sheets of identical thickness were mounted in a cross-cylindrical configuration in the SFA. The surface separation ( f lA) was measured with an interferometric technique by observingfringes of equal chromatic order (FECO). A piezo crystal was installed in the upper mount to control the surface separation precisely. A variable double-leaf spring was used in our SFA to carry out measurements.16 The spring constant K was calibrated by placing weightson the lower lens and measuringthe vertical displacement using a traveling microscope. The surfaces jumped into contact when the slope of the force curve equaled or slightly exceeded the spring constant of the variable spring under attractive forces. The spring constant divided by the radius of the curvature of the glass cylinder KIR, assumed to equal d(FIR)ldD, was plotted versus surface separation D where the surfaces began to jump. Measurements of monolayer surfaces immersed in different concentrations of 1-octanol (CaOH) or salt were achieved by successively injecting aliquots of aqueous C8OH or salt solutions into the SFA. Monolayers in low concentrations of salt (- 1 X 1o-L M)or C8OH ( 1 X M) solutions remained stable for at least 24 h by checkingthe reproducibility of the force curves. For measurements at higher concentrations, the monolayer is less stable (-8 h), so the measurements were performed starting with freshly prepared monolayer surfaces with subsequent injection of salts or C80H to the desired concentration. Measurements were carried out 2 h after the injection was made. Each curve represents at least two independent experiments.
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Results Figure 1shows how the force between two hydrophobic DODA monolayer surfaces depends on the NaCl concentration in the range 10-5 to 5 X 10-4 M. There is a strong dependence on the salt concentration, but the shape of the force curves is largely unchanged. As in the previous paper we have fitted the data to a double exponential, with adjustable amplitudes and decay lengths. The fitting parameters for this and subsequent systems are summarized in Table I. We consider the double exponential form to be a convenient representation of the data, but there is at present no physical motivation for this functional form. (13) Brady, J.; Evans, D. F.; Ninham, B. W.; Kachar, B. J. Am. Chem. SOC.1984, l&, 4279. (14) Tsao, Y.-H.i Yang,S. X.; Evans, D. F. Langmuir 1992,8, 1188. (15)Israelachvili, J. N.;Adam, G. E. J. Chem. Soc., Faraday Trans. 1978, 74, 975. (16) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98,500.
[NaCIl=OM o [NaCl] = I x M a [NaCl] = I x 1 0 4 M A [NaCIl = 2 x M e [NaCI] = 5 x 10.4 M
too
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Figure 1. Interactions between DODA monolayers in water with addition of NaCl obtained from surface forces measurements. The monolayer contact position was set to be D = 0. Each curve represents at least two seta of independent experiments. All experiments were carried out at room temperature (=25 “C). Table I. Biexponential Fit for Interactions between Two DODA Monolayers with Addition of NaCl and NaOH at 25 d(F/R)/dD= A exp(-D/Cl) + B exp(-D/Cz) concentration(M) W A 104B C1 0 16 5.0 21 NaCl 1x104 5.0 4.0 20 2x104 1.5 1.5 18 5X10-4 0.6 1.0 16 NaOH 5 x 10-5 10 4.5 18 1x10-4 8.0 4.2 15 MgSO4 1 x 10-5 10 15 20 1x104 2 20 18 1 x 10-3 0.5 20 18 d(F/R)/dD,A, and B in N/m2; D, C1, and CZin A.
salt
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iNaOHl= 0 M = I x 10.5 M [NaOH] = 5 x 10.5 M [NaOH] = I x 10.4 M
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’” 0
100
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Figure 2. Interactions between DODA monolayers in water with addition of NaOH.
As seen in Figure 1 there is only a small difference between the data for ‘pure” water and for the 10-5 M solution. It should be remembered that although the water has been carefully purified, it is in contact with the COa of the air and there is always some leakage of ions both monovalent and divalent from the surfaces in the measuring cell. This could conceivably lead to an ionicstrength of order 10-5 M. When the salt concentration is increased, the attractive force becomes progressively weaker. It can be noted that the attractive force is measurable up to a separation of the same order as the Debye screeninglength in the solution, while the interaction force as such does not seem to decay with a characteristic length equal to either the screening length or half this value. Figure 2 shows a similar series as in Figure 1but with the anion changed to OH-. Qualitatively the resulta are the same as in Figure 1, but the force decreases more in amplitude in NaOH than in NaCl at the same concentrations.
Langmuir, Vol. 9, No. 3, 1993 781
Long-Range Attractiue Forces [MgS041= 0 M [MgSO4] = 1 X IWJ M [MgS041= I X I O 4 hi n [M~SOII= 1 x M 0
lo5
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io0
'
io0 D
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Figure 3. Interactions between DODA monolayersin water with addition of MgS04.
Figure 5. Interactionsbetween a DODA monolayer surface and a bare mica surface in water with addition of KCl. 106
m CI
A
\\
Ln-octanoll = 1 x 10-5 M [n-octanoll = 3.7 x 10-5 M [n-octanoll = 1.4 x 10.5 M
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~
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(A) Figure 4. Interactionsbetween DHDA monolayersin water with addition of 1-octanol. The area indicated by the arrow is the calculated nonretardedvan der Waals force for hydrocarbon and mica surfaces separated by water (d(F/R)/dD = A/3D3, where J). the Hamaker constant A = 0.8 X 10-*0 to 2.2 X lezo
lo3
Z200 OO
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The effect of changing from a 1:l to a 2 2 electrolyte like MgS04 is shown in Figure 3. In this case there is a substantial effect of going from pure water to a 1 X M solution. The force decays further in magnitude as the MgS04 concentration is increased. In the case of MgSOr, we tried for measurements also at a concentration of 0.01 M. However, in this case a monotonic double layer repulsion was found indicating that the hydrophobic monolayer had disintegrated. We were thus not able to reproduce the findings of Christensson et alS3who made measurementsfor 0.1 and 0.01 M MgSO4 solutions. This discrepancy is probably due to a difference in the preparation of the surfaces. We also chose to investigate the effect on the force of a hydrophobic solute, octanol. As seen in Figure 4 a 1X 1W M concentration of octanol has an effect only at shorter range while increasing the octanol content to 3.7 X and to 7.4 X lOi, M practically reduces the force to a magnitude expected for a van der Waals force. In a separate series of experiments we studied an unsymmetricalsituation with one bare mica surface and one covered with a hydrophobicmonolayer. Figure 5 shows the results for a DODA covered surface in "pure" water and in 1 X 10-4 and 1 X le3M KC1. We observe an attractive force with a qualitatively very similar behavior to the one found for the symmetrical situation with two hydrophobic surfaces. The quantitativedifference is that the force is stronger in the asymmetrical case. We also observe a decrease of the force on adding electrolyte. A very analogous behavior was found also for the DEDAmica system (Figure 6). The double exponential fitting parameters are summarized in Table 11. In an additional test we repeated the study of the effect of octanol for the unsymmetrical case (Figure 7). As in Figure 4 addition
BOO
600 D
D
1 100
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(A)
Figure 6. Interactions between a DEDA monolayer surface and a bare mica surface in water with addition of KCl. Table 11. Biexponential Fit for Interactions between a DODA (DEDA) Monolayer and a Bare Mica Surface with Addition of KCl at 26 O c a d(F1R)ldD = A exp(-D/Cd 0 1x10-4 1 x 10-3
20 18
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In-octanol] E 0 M In-octanol] = 3.7 x 10-5 M [n-ocunol] = 7.4 x 10.5 M [n-octanoll = 1 . 5 x 10.4 M
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:'01 -m
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Figure 7. Interactions between a DHDA monolayersurface and a bare mica surface in water with addition of 1-octanol.
of octanol reduces the attractive force substantially at long range, while at short range (-100 A) octanol seems to have a smaller effect in this case. Discussion Relation to Previous Studies. In interpreting the rather remarkable results summarized above it seems appropriate to start by comparing them with relevant previous studies. There are several investigations of the force between mica surfaces covered with a monolayer of
782 Langmuir, Vol. 9, No, 3, 1993
DHDA or DODA. Hydrophobic surfaces on mica can be generated by equilibrium adsorption from solution,2by Langmuir-Blodgett deposition from various surface press u r e ~or , ~as in the present case from adsorption in an organic solvent.6 These procedures result in measured force curves which display a long-range attractive force but with different magnitudes and dependencies upon distance. In the case of DODA, the Langmuil-Blodgett deposition method in one case12 leads to a force curve which is repulsive down to =30 nm in 3 X le5M KBr after which it rapidly becomes attractive. The resulting force curve is similar to that measured using mica surfaces prepared by the equilibrium deposition method (Figure 1). While in a later experiment: using a different surface pressure, a monotonically attractive force was observed. When the attractive force curves are fitted to a double exponential, there is some discrepancy. We obtained decay lengths of 21 and 250 A, while Claesson and Christensson4obtained 25 and 150A. This discrepancy is partly caused by fitting to FIR rather than (d(F/R)/dD), which gives different weights to the long- and short-range components. The DHDA force curves obtained from measurements using the equilibrium adsorption and solution deposition methods are quite different. With the equilibrium adsorption method2 only a single exponential with a decay length of 30 A was obtained while the data shown in Figure 4 give a double exponential curve with decay lengths of 21 and 250A. In the experimentsinvolvingthe equilibrium adsorption of DHDA from solution, there is a DHDAA concentration of 1 X M in the aqueous phase. The hydrophobic surfaces are metastable and transform to bilayers after some period of time. Thus there is some ambiguity in interpreting the force curves obtained from equilibrium adsorption of DHDA onto mica. The effect of salt on the attractive force is less well studied. However, it is qualitatively well established that the observed attractive force decreases upon addition of electrolyte. Christensson et al.ll concludes that the effect of the salt is mainly to decrease the amplitude and not the decay length of the force. Our observations are by and large consistent with this conclusion. There are few studies on the effect of amphiphilic additives on the force. Herder17found that addition of 1 X M dodecylammonium chloride causes the longrange part of the attractive force to disappear. He estimated that under these conditions there was one adsorbed molecule per 60 nm2. The long-range attraction with asymmetrical surfaces where one surface is bare mica and the other possesses a hydrophobic monolayer, has been previously studied by Claesson et al.l* There is good quantitative agreement between their force curves and ours. There are not reports on the effect of other additives in this case. Surface Charges. The mica surface contains one ionizable group per 50 A2 while the area per dialkyldimethylammonium ion in a monolayer can range from 40 to 60 A2. The latter value is found in lamellar liquid crystals, while the former represents the limit of packing crystalline chains perpendicular to the surface. Thus, it is conceivable that perfect matching between the mica surface and the surfactant monolayer could lead to a neutral surface. However, no preparation technique guarantees that such a matched state is obtained. Odd sites on the mica surface could retain either K+ or H+ (17)Herder, P. C. J. Colloid Interface Sci. 1990,134, 336. (18)Claesson, P. M.; Herder, P. C.; Blom, C. E.; Ninham, B. W. J. Colloid Interface Sci. 1987,118, 68.
Tsao et al.
counterions, which could then dissociate generating a negatively charged surface in water. Alternately, small amounts of excess surfactant could be incorporated into the hydrophobic monolayer during deposition leading to a positively charged surface. Once in solution, even perfectly matched surfaces could become charged by ion adsorption from solution. If excess surfactant is present, then adsorption driven by hydrophobic interactions would generate positively charged surfaces. At aqueous polar interfacessimple anionsadsorb more readily than the corresponding cations leading to negatively charged surfaces. From surface forces measurements it is difficult to determinewhether a chargedsurfaceis positiveor negative. However, recent streaming potential measurements by Scales, Grieser, and Healylgon mica surfaces coated with dialkyldimethylammonium ions provide direct information on the sign and potential associated with these surfaces. When the surfaces are prepared using the Langmuir-Blodgett deposition, the zeta potential for DODA monolayers is { = -38 mV in 10-3 M KC1 at pH = 6.9. This value is in reasonableagreementwith the surface potential of 45 mV deduced by Claesson et del2 from fitting force curves for similarly prepared Langmuir-Blodgett deposited monolayers. With the solution deposition method used by us, the measured zeta potentials for DHDA and DODA are { = -8 to -10 mV in 10-5to 5 X le3 M NaCl or NaOAc at pH = 6.8.20 These potentials correspond to surface ionization rangingfrom0.02to0.5%. Weshouldthusexpectadouble layer repulsion contribution to the measured force, but the measured force shows no sign of such a force. An explicit calculation using the measured potential indeed reveals that the expected double layer contribution to the force derivative is at most 5% of the magnitude of the measured force derivative and typically even smaller. The fact that the zeta-potential does not change upon addition of 5 X M NaCl demonstrates that selective adsorption of ions onto the hydrophobic monolayer does not occur. This observation argues against the proposals that the decrease in the long-range attractive force upon addition of salt arises from generation of double-layer repulsion forces. Additives. Addition of very small amounts of octanol (
