Substrate and Solution Effects on the Long-Range “Hydrophobic

9704

J. Phys. Chem. B 2000, 104, 9704-9712

Substrate and Solution Effects on the Long-Range “Hydrophobic” Interactions between Hydrophobized Gold Surfaces Thomas Ederth* Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden ReceiVed: June 7, 2000; In Final Form: July 31, 2000

Hydrophobic surfaces prepared by adsorption of hexadecanethiol, 1,10-dithiodecane, octadecanoic acid, and hexadecanol onto gold have been used to study long-range “hydrophobic” interactions, and direct-force measurements were performed in water, aqueous NaCl electrolytes, and water/ethanol mixtures, using a bimorph surface force instrument. Results confirm that for very stable hydrophobic surfaces the contact angle is sufficient to predict the presence of an attraction in excess of van der Waals forces, in which case the attraction is caused by the coalesence of microscopic bubbles on the surfaces. For the less-stable hydrophobic films, the properties of the adsorbed layer are important for the qualitative nature of the interaction. For such surfaces differentsand as yet unknownsmechanisms cause the attraction.

1. Introduction Direct measurements of forces between solid surfaces have provided important contributions to the current understanding of phenomena such as colloidal stability, adhesion, rheology of thin films, tribology, and various aspects of adsorption phenomena. However, despite intensive research during the last 20 years, the long-range attractive interactions observed in surface force measurements between hydrophobic surfaces remains a puzzle. The original observations by Pashley and Israelachvili indicated that an attractive component in excess of van der Waals forces was present in the interaction between mica surfaces with adsorbed cationic surfactants.1 Much to the wonder of many scientists, ensuing experiments with different types of hydrophobized surfaces generated not only quantitatively different results, but also interactions that were qualitatively different. By now, the nature of this interaction has been studied using a wide range of surfaces, predominantly mica surfaces modified by surfactant adsorption or LangmuirBlodgett deposition, silanized mica, silica or glass, and polymer particles. Further, the nature of the attraction has been investigated for dependency on many experimental parameters, among them temperature, electrolyte, contact angle, and dissolved gas. Despite a general lack of coherence in the result of this massive experimental effort, it seems possible at least to make a subdivision into a few main groups. In a recent review of the experiments in the area, Christenson and Claesson suggest that the results can be ordered in the following three major categories (of which at least the last might require further subdivision to allow for adequate theoretical treatment):2 (1) strongly attractive forces between stable surfaces (of range similar to van der Waals forces), (2) attractions of varying strength and range caused by bridging of bubbles, and (3) longrange attractive forces with exponential decay. Previous experiments in which gold surfaces have been hydrophobized with alkanethiols,3 mixtures of hydrophilically * Present address: Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, United Kingdom. Phone, +44 1865 275400; fax, +44 1865 275410; e-mail, [email protected].

(-OH) and hydrophobically (-CH3) terminated thiols,4 and semifluorinated thiols5 have shown that the interactions in these cases all are of type (2) above, and that the presence of the excess attraction depends on the contact angle being larger than 90°. However, many compounds other than thiols can form monolayers on gold surfaces, in particular alkyl chains terminally functionalized with a polar group, although the strength of the interaction with the gold surface is much lower than for the thiol in most cases.6 To supplement the studies made with thiolated surfaces, in this report two compounds similar to hexadecanethiol were chosen: hexadecanol and octadecanoic acid. Both compounds are only sparsely soluble in water and form monolayers on gold surfaces, although the density and thicknesses of the layers are less than those for thiolates of similar lengths. Further, experiments using 1,10-dithiodecane, which to some extent is expected to adsorb with both ends at the gold substrate, are reported; these surfaces were all investigated in water and NaCl electrolyte solutions. Finally, measurements were performed on hexadecanethiol modified surfaces, where the solid/liquid contact angle was varied by changing the amount of ethanol in the intervening water/ethanol mixture. 2. Materials and Methods The gold substrates were prepared as follows (further details regarding preparation and characterization can be found elsewhere3): for surface force measurements, borosilicate glass rods (r ) 1 mm) were melted in a butane-oxygen burner to make the end spherical, with r ≈ 2 mm. These sphere-ended rods were mounted in an ultrahigh vacuum evaporation system, along with 12.5 × 20 mm2 glass slides (thickness 0.15 mm) for contact angle measurements or 40 × 20 mm2 oxidized silicon wafers for infrared spectroscopy measurements. A 1-nm Ti adhesion layer and a 10-nm Au layer were evaporated onto the substrates at a rate of 0.5 nm/s and at a pressure normally below 3 × 10-8 Torr. The glass slides used for contact-angle measurements were evaporated on both sides. After being removed from the evaporator, the substrates were immersed in various adsorbate

10.1021/jp002052g CCC: $19.00 © 2000 American Chemical Society Published on Web 09/23/2000

Interactions between Hydrophobized Gold Surfaces solutions, except surfaces intended for measurements without adsorbates, which were stored in air. All adsorbates were adsorbed from 1-mM solutions in ethanol and incubated for at least 15 h before use. Thiolate surfaces were sonicated in ethanol before use to remove physisorbed thiols from the surfaces, whereas hexadecanol and octadecanoic acid surfaces were only rinsed quickly with ethanol before being blown dry and mounted in the surface-force instrument or the tensiometer. (Sonication of hexadecanol and octadecanoic acid surfaces in ethanol was found to effectively remove the adsorbates.) Before the gold surfaces without adsorbate were used, they were cleaned in a mixture of 5/7 H2O, 1/7 30% H2O2, and 1/7 25% NH3 at 80 °C for 10 min, rinsed in pure water, and mounted in the surface-force instrument. After being immersed in the cleaning solution, the surfaces were never exposed to air, to minimize contamination of the gold layer. 1,10-Dithiodecane (courtesy of Prof. B. Liedberg, Linko¨ping University), thiohexadecane (Fluka, >95%), hexadecanol (Fluka, >99%), octadecanoic acid (Larodan, Malmo¨, >99%), and thiohexadecanol (Prof. B. Liedberg), used for reference measurements, were used as received. All ethanol was 99.5% (Kemetyl, Stockholm). Water was purified using a Milli-Q system (Millipore) and degassed with a water jet pump for 2 h before experiments started. Contact angles and surface tensions were measured using the Wilhelmy plate method (Kru¨ss 12 Tensiometer). The immersion rate was 2 mm/min, and samples were immersed and withdrawn twice in each measurement, with deviations e 2° for all surfaces. The Fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS) experiments were carried out in a grazing angle reflection setup, with the angle of incidence 83° from the surface normal. The resolution of the spectrometer (Bruker IFS113v) was 2 cm-1, and 1000 scans were collected for both the sample and the reference surface, the latter being identical to the sample, save the organic monolayer. Surface force measurements were performed using a bimorph surface force apparatus (MASIF, Australian Scientific Instruments) which is briefly described in the following; for details, see ref 7. One of the surfaces is moved relative to the other through a two-stage positioning mechanism consisting of a motorized translation stage and a piezoelectric tube transducer. The second surface is attached to a piezoelectric bimorph deflection sensor acting as a single cantilever force-measuring spring. To acquire a force-distance profile, the surfaces are moved toward each other at a constant rate, using the piezotube, until they contact each other (and the motion of the translating surface is directly transmitted to the bimorph spring). During the process, the extension of the piezotube is monitored by use of a linear position sensor, whose output is used to adjust for the nonlinear expansion of the piezotube and to determine the sensitivity of the deflection sensor. The distance resolution of the device was never better than 0.2 nm in these experiments, and all distances are relative to the “hard wall” contact of the surfaces. The normalized force resolution (F/R, where R ) R1R2/ (R1 + R2); R1 and R2 are the radii of the two spheres) was 10 µN/m at best. The stiffness of the measuring spring was measured by adding weights (0.1-0.5 g) to the spring and measuring the deflection with a microscope. The radii of the spherical surfaces were determined using a micrometer screw after the experiment. The electrostatic double-layer interactions were calculated using nonlinear Poisson-Boltzmann (PB) theory, as described by Chan.8 For the van der Waals contribution to the DerjaguinLandau-Verwey-Overbeek (DLVO) forces, a Hamaker con-

J. Phys. Chem. B, Vol. 104, No. 41, 2000 9705 TABLE 1: Contact Angles with Water ((2°) CH3(CH2)15OH CH3(CH2)16COOH HS(CH2)10SH CH3(CH2)15SH CF3(CF2)9(CH2)2SHa a

θa

θr

∆ cos θ

93 94 89 110 120

68 64 68 104 109

0.43 0.51 0.36 0.10 0.17

From ref 5.

Figure 1. Forces between bare gold surfaces in water and in 1 mM NaCl. DLVO theory was fitted to the data under constant charge and constant potential assumptions (for each data set the upper and lower dashed line, respectively), with parameters as follows: for water, ψ ) -63 mV, κ-1 ) 560 Å; for 1 mM NaCl, ψ ) -65 mV, κ-1 ) 95 Å. The Hamaker constant was set to 4 × 10-20 J.

stant of 4 × 10-20 J was used. This number was obtained by fitting a nonretarded van der Waals force to the interaction between thiohexadecanol modified gold surfaces in water. Surface-charge densities were calculated with the Grahame equation, using the parameters for surface charge and Debye length obtained from the DLVO fits. To determine the sign of the surface charge, the electrostatic interaction with a negatively charged bare glass surface was measured. This method was applied to all investigated surfaces, and the surface charge was found to be negative in all cases. 3. Results 3.1. Contact Angle Measurements. The results from contact angle measurements are summarized in Table 1, where contact angles from hydrophobic surfaces prepared from semifluorinated surfaces have been included as well, because these will be touched upon in the discussion. Bain measured both contact angles and ellipsometric thicknesses for layers similar to the first two in Table 1 (see ref 6, p 280 and ref 9). For heptadecanol, the advancing contact angle with water was 95° and the thickness 9 Å (to be compared with a calculated thickness of 21-23 Å for a close-packed monolayer of the same compound); the corresponding data for octadecanoic acid were 92° and 7 Å. 3.2. Bare Gold Surfaces. The interactions between clean gold surfaces are of minor interest as such, but they will be helpful for the interpretation of the interactions described in the subsequent sections, where weakly adsorbed molecules are studied. Force profiles were obtained in pure water and in 1 mM NaCl. The interaction is dominated by an electrostatic double-layer interaction at large separations, with the attractive van der Waals force apparent at short separations (see Figure 1). The results in pure water are somewhat difficult to quantify with precision because of the very long decay length of the interaction, but fitting the data to DLVO theory (in the range

9706 J. Phys. Chem. B, Vol. 104, No. 41, 2000

Ederth

TABLE 2: Electrostatic Properties of the Surfaces in Water and 1 mM NaCl medium gold CH3(CH2)16COOH CH3(CH2)15OH HS(CH2)10SH CF3(CF2)9(CH2)2SHc CH3(CH2)15SH

water NaCl water NaCl water NaCl waterb water water NaCl water

potential (mV) -63 -65 -62 -59 -19 -41 -50 -75 -60 not discernible

Debye length (Å)

area per charge (nm2)

560 95 770 96 (310)a 96 790 630 800 96

160 26 230 31 114 400 250 177 30

a This is not the Debye length, but the decay length of the exponentially increasing attraction. b Obtained after replacing 1 mM NaCl with pure water. c From ref 5.

50-200 nm) resulted in a surface potential of -63 mV, with a Debye length of 560 Å. The Grahame equation yields an area of 160 nm2 per charged site on the surface. Similar results for interaction in 1 mM NaCl produce a surface potential of -65 mV with a Debye length of 95 Å; the data are summarized in Table 2. The charge density on the surface is considerably increased when water is replaced by 1 mM NaCl, and now the available area per charge is only 26 nm2. The interaction in 1 mM NaCl follows the constant charge solution of the PoissonBoltzmann equation very closely over the whole interaction range, with the limited stiffness of the spring causing the deviation at separations < 2 nm (possibly in combination with a hydration-type repulsion). A high affinity of Cl- ions to gold surfaces has been observed earlier,10,11 although the quantitative data differ considerably from these results; one charge per 27 nm2 corresponds to 6 mC/m2, whereas, e.g., the ellipsometric study by Paik et al. produced charge densities of the order of 100 mC/m2.10 No results for the adhesion forces are available, because the very high surface energy of the clean gold surfaces causes instant cold-welding when they are contacted. To separate the surfaces, negative loads in the range 5-10 N/m had to be applied, and it was never possible to separate the surfaces without destroying the metal layer at the point of contact. Thus, two successive force profiles could not be acquired at the same position, but repeat profiles obtained at different contact positions show good agreement. 3.3. Hexadecanol. Two gold surfaces covered with a layer of hexadecanol attract each other in pure water (see Figure 2), and the interaction increases exponentially down to separations where the van der Waals force can be resolved; at even smaller separations (90° for the steps to be present (and in combination with studies using other hydrophobic surfaces and liquids, the phenomenon seems to be rather universal for stable solvophobic surfaces) but the contact angle for the dithiol surfaces is a mere 89°. The lack of steps appears to be unrelated to the surface charge: semifluorinated surfaces are charged in pure water,5 but still have the steplike force onsets. Further, the charging of the dithiol surfacesif it is not caused by dangling thiols on the surfacesis also consistent with the study of mixed monolayers. In that study, it was found that mixed monolayers exposing methyl- and hydroxyl groups were charged in pure water whenever the contact angle was less than 90°, but were uncharged at higher contact angles (however, the mechanism causing the charging of the surfaces is unknown).4 Parker et al. used glass surfaces modified with fluorocarbon silanes, and they observed steps of the kind discussed here.15 The addition of ethanol in their system likewise eliminated the steps, and only small amounts of ethanol (90°. The situation is not as simple for weakly adsorbed layers. The principal difference between hexadecanol and octadecanoic acid is that the latter has an ionizable polar group, which gives the surface a net charge in pure water; the length difference surely should not matter in this respect, and the contact angles are very similar, too. Comparing the results, one observes that the octadecanoic acid modified surfaces interact with a DLVO-type force profile, whereas hexadecanol modified surfaces show a long-range exponentially decaying attraction. Thus, it appears that the lack of charge on the hexadecanol modified surfaces is essential for the excess attraction at large separations in pure water. Adding 1 mM of NaCl to the hexadecanol system irreversibly changes the behavior (rinsing with water does not recover the original attraction, and the adhesion is not affected significantly by the procedure). This finding shows that the attraction is sensitive to the presence of charges on the surfaces, and that the charges themselves are not the cause of the attraction; rather, it appears that they eliminate it. The formation of cavities between surfaces in contact is expected on thermodynamic grounds at contact angles >90°,29 and the fact that the effect becomes more pronounced during the course of an experiment only reflects the increased amount of dissolved gases in the (originally degassed) water. With reference to the previous discussion on differences between the first and subsequent approaches, it is perhaps appropriate to emphasize that for the octadecanoic acid and hexadecanol films, the first approach in each experiment was identical to subsequent approaches, and also independent of observed differences regarding cavities upon separation. The long-range exponentially decaying attraction between hexadecanol films is of the same kind as those referred to as type (3) interactions by Christenson and Claesson,2 and are frequently observed in measurements using LB films or adsorbed surfactants. These interactions usually fit a doubleexponential function:

F/R ) C1e-D/λ1 + C2e-D/λ2

(2)

where the first term normally refers to interactions of compara-

tively short rangesless than, say, 20 nmsand the second term applies to interactions of the range observed here. In this case, the decay length λ2 ) 31 nm, and the preexponential factor C2 ) -0.37 mN/m. 4.3. Electrolyte Effects. The hexadecanol molecules are weakly adsorbed onto the gold surface, but the ions present in pure water are not capable of displacing the adsorbed layer to penetrate to the gold surface. This is hardly surprising, considering that the gold surfaces are covered by a low dielectric constant layer upon immersion in water, which does not not promote the penetration of ions. Adding 1 mM of NaCl increases the number of ions in the solution to a level at which some ions penetrate the layer. It has been observed that Cl- ions have higher affinity to gold surfaces than do OH- ions,11 but this is perhaps of lesser importance than the concentration increase. Once the ions are adsorbed, they tend to be stuck to the surface, and whatever the mechanism causing the long-ranged attraction in pure water is, it is eliminated. With the data at hand, it is impossible to say whether the charging of the octadecanoic acid surfaces is caused by dissociation of the adsorbed acid or by adsorption of ions from the solution. The effect of added electrolyte on the interactions between the hexadecanol surfaces is in qualitative agreement with the analysis by Christenson et al. of several experiments investigating the effect of electrolyte: the decrease in the strength of the attraction upon addition of electrolyte has more to do with an increase in the surface charge than with the presence of ions in the water layer between the surfaces.30 For the case of LB films or adsorbed surfactants on mica, this is a matter of desorption as electrolyte is added, leaving a larger area of the charged mica substrate exposed to the solution and effectively reducing the area covered by hydrophobic species. The results obtained with hexadecanol surfaces differ from many of the experiments discussed by Christenson in that the net attraction is not just diminished, but has disappeared altogether. However, the slight increase in adhesion during the experiment with hexadecanol modified surfaces also is evidence of degradation of the adsorbed layer in this case, because removal of the organic molecules increases the number of gold-gold contacts across the interface. This increase would, in turn, increase the adhesion, but that the change is small also suggests that most of the hexadecanol still remains at the surface. The important effect then seems to be the (irreversible, as it seems) adsorption of ions to the exposed gold surface. It is worth noting that in some systems of the kind where a long-range exponentially decaying “hydrophobic” attraction has been observed, the effect of adding electrolyte has been a decrease in the magnitude, with the decay length remaining the same, as the electrolyte concentration increases.31 Tsao et al. observed that when octanol was added to an experiment using surfactant-covered mica surfaces, the longrange attraction disappeared. The authors conclude, “Thus, maintaining the basic structure of the monolayer but generating an in situ polar surface appears to be sufficient to obliterate the long-range attractive force.”31 Parker and Claesson made a similar observation in a study using silanated glass surfaces (where the observed attractions were exponentially decaying); they found that a small number of charges on the hydrophobic surfaces significantly reduced the range of the attraction.32 This finding is in contrast to the situation for the rigidly attached surface films, where the interaction is unaffected by the presence of surface charges (as in the case of semifluorinated surfaces5). No steps of the kind reported here for covalently attached films were found in these authors data, and also, the stability of their surfaces was not very good: waiting overnight significantly

Interactions between Hydrophobized Gold Surfaces reduced the attraction, and it seems reasonable to classify their silanes as “weakly” adsorbed or possibly as “unstable”. The stability of the silanated surfaces used in their study is considerably less than that of alkanethiolate surfaces, which remain unaffected by 6 months of storage in 1 mM NaCl, and whose interactions do not change over the course of experiments lasting several days. 4.4. Theory. The theoretical understanding of the long-ranged exponentially decaying (type (3)) interaction is still unsatisfactory, although some progress has been made: Christenson and Yaminsky demonstrated that for surfaces hydrophobized under equilibrium conditions, such as charged surfactants adsorbed to oppositely charged surfaces, a long-range attractive force is expected if the density of hydrophobic groups can be changed by adsorption as the two surfaces approach each other.33 Forsman et al. suggested a mechanism by which local variations in the surface density of charged surfactants adsorbed to an oppositely charged (and, on average, neutralized) surface result in highly charged aggregates. These aggregates occasion an attraction of magnitude and range much larger than the van der Waals interaction.34 Both models predict that surfaces with immobile adsorbates will not yield such an interaction, and also require electrostatic mechanisms; indeed, there must be some interaction acting at a very long range for the surfaces to recognize the presence of each other, and it is difficult to imagine any other interaction acting at the very large separations involved. However, in the former case, the authors suggest that a similar mechanism is responsible also for the long-range attractions in nonequilibrium systems where only lateral mobility of the adsorbed molecules is possible, such as LB films or films adsorbed from nonaqueous solvents. In these situations, lateral diffusion produces the required changes in adsorption excess, rather than adsorption, although not in the local manner suggested by Forsman et al., but presumably on a larger scale. Further, the magnitude of the contact angle is less important than the contact-angle hysteresis, because the hysteresis is also a measure of the adsorbed molecules’ ability to rearrange at the surface. The results presented here are in partial agreement with this hypothesis: the covalently attached (immobile) films with small contact-angle hysteresis do not produce the exponentially decaying attraction, whereas the weakly adsorbed hexadecanol surfaces with larger hysteresis give an attraction. The surfaces prepared from dithiols have a large contact-angle hysteresis, and even though the surface is rather disordered, the molecules are still rigidly attached to the surface through at least one thiolate bond, and the mobility is very limited. The interpretation of the results for octadecanoic surfaces is not clear with respect to the model, however. If these surfaces are considered “unstable” in the sense that (at least) lateral mobility is possible, then the model predicts an attraction, which is not observed. However, such a long-range attraction could be weak and completely drowned by the electrostatic repulsion; or, the surface layers might be too rigid to actually qualify as type (3) surfaces; or, the model might be incorrect. It should be noted, though, that the stated agreement is merely qualitative, and no quantitative predictions of the magnitude of such an interaction have been presented. 4.5. Surface Rougness. The gold substrates onto which the hydrocarbons are adsorbed are polycrystalline, with peak-totrough roughness of about 1.5 nm and rms roughness in the 0.15-0.20 nm range (as measured over 1 × 1 µm2).3 Generally, this roughness causes scatter in the adhesion data, because the effective surface area upon contact varies over the surfaces, but it might also have implications for the “hydrophobic” forces.

J. Phys. Chem. B, Vol. 104, No. 41, 2000 9711 The effect of roughness on the interactions between hydrocarbon layers covalently attached to the gold has been discussed previously, with particular emphasis on the role of surface imperfections at grain boundaries as nucleation sites or traps for gas bubbles.4 A recent investigation using silanized silica substrates demonstrated that exposure of the hydrophobized surfaces to air after preparation was a necessary requirement for the observation of steps in the approach profiles,23 suggesting that surface imperfections might work more like traps for air bubbles, rather than as bubble nucleation sites. The effect of roughness on the interaction between weakly adsorbed molecules is difficult to assess, given that the mechanism for the interaction is unclear; if surface mobility is an important factor, however, grains and grain boundaries obstructing or inhibiting motion along the surface might affect both the magnitude and the qualitative behavior. 5. Summary and Conclusions Interactions between gold surfaces hydrophobized by adsorption of hexadecanethiol, hexadecanol, octadecanoic acid, and 1,10-dithiodecane have been studied in water, 1 mM aqueous NaCl solutions, and water/ethanol mixtures. Comparisons have been made with other systems using similar organic molecules adsorbed onto gold substrates, and the following conclusions regarding long-range “hydrophobic” interactions can be drawn from the observations: (1) The excess attraction observed between covalently attached (stable) hydrophobic films is sensitive to the contact angle with the liquid, and is not present for contact angles less than 90°. The attraction in this case is caused by the coalescence of bubbles on the surfaces. Although the experimental evidence for the presence of such bubbles on stable hydrophobic surfaces is considerable, as yet no satisfactory explanation of their stability has been provided. (2) The attractions observed with weakly adsorbed molecules follow an exponentially decaying force law, whereas the attractions between rigidly attached films are of a more random character, suggesting that the mechanisms causing the attractions differ depending on the details of the surface. (3) The presence of ions adsorbed onto the surfaces of weakly attached hydrophobic molecules inhibits the long-range exponentially decaying attractions observed in pure water, whereas for covalently attached molecules, the presence of electrolyte does not change the interaction. (4) The results are in partial agreement with a recently proposed model in which adjustment of the local adsorption excess due to lateral diffusion of the adsorbed molecules leads to a long-range attraction for films that are not covalently attached to the substrate. Acknowledgment. The author thanks P. Claesson, H. Christenson, and K. Tamada for discussions and valuable comments; B. Liedberg, Linko¨ping University, for generous access to the surface-preparation facilities in his laboratory; I. Engquist for assistance with the FTIR-RAS measurements; and the Swedish Natural Science Research Council (NFR) for financial support. References and Notes (1) (2) press. (3) (4)

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