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evolutions when the sulfidation temperature of the HR346 catalyst increases. The preceding classification of the nickel species allows identification of the nickel species in the NiMo/y-Alz03 catalyst. Strikingly, the Ni-D does not appear at the lowest sulfiding temperature we used, but rather Ni-B is formed (Figure 5a). This is consistent with observations obtained by laser Raman spectroscopy on the NiMo/y-Alz03 system for which bulk Ni sulfide was also identified at low sulfiding temperature.26 This result is likely from the easier sulfidation of the supported nickel oxide species whereas MoSz and WSz are not available yet for being decorated to form the typical NiMo(W)S phase. This phase appears only after a sulfiding temperature higher than 470 K. For a temperature of sulfidation higher than 770 K, the Ni-D species disappears as a result of segregation of the bulk nickel species. The XPS intensity ratio of the Ni 2p peaks then slowly decreases. The evolution of the XPS intensity ratio Mo 3d/A1 2p with increasing the sulfiding temperature reveals the formation of MoSz, starting at about 470 K in a form less dispersed than the oxomolybdate precursor. The small tendancy of molybdenum sulfide redispersion in the third domain (Figure 5c) can be compared to the similar behavior observed in the CoMoS system (between 975 and 1075 K) where the molybdenum sulfide crystallites become larger and better ordered.27 This phenomenon is explained by a growth of MoSz crystallites parallel to the basal plane. Subsequently, cobalt located at the MoSz edges in a CoMoS phase
will tend to segregate from this phase and form bulk C09S8.7*28 The constant Ni 2~312binding energy in domain I1 of Figure 5a indicates the optimal sulfiding temperature range for NiMoS phase formation, i.e., 473-773 K for the sulfiding condition used. The evolution of the sulfidation extent shows a maximum sulfidation of the two metals at about 673 K (Figure 6). It is interesting to note that hydrotreating catalysts are usually sulfided in such conditions, leading to the presence of the “NiMoS” phase with nickel in decoration with MoSz particles. Conclusion Nickel is a XPS-sensitive ion for the identification of the promoter state in both alumina-supported and -unsupported sulfided Ni-Mo catalysts. This NiMo interaction leads to a particular phase Ni,Mo,,,,S, with a = 0.2-0.3, which can be found at the surface of the bulk NiMo sulfides and at the optimum composition of a supported catalyst. In such a phase, the Ni 2~312 core level is modified by a different structural environment and by an electron transfer to the MoSz or WSz slab. This behavior has been found similar to the CoMo system. The formation of the NiMoS phase on the surface of the alumina support has been found dependent on the temperature of sulfidation. At low temperature, no NiMoS phase is formed until the MoS2 particles are formed, and at a temperature higher than 773 K, the Ni in decoration tends to segregate into nickel sulfide particles. Registry No. Ni, 7440-02-0; MoS2, 1317-33-5. (28) Boudart, M.; Sanchez Arrieta, J.; Dalla Betta, R. J . Am. Chem. SOC.
(26) Payen, E.; Kasztelan, S.; Grimblot, J.; Bonnelle, J. P. J . Mol. Sirucr. 1988, 174, 71. (27) Candia, R.; Sorensen, 0.;Villadsen, J.; Topsee, N. Y . ;Clausen, B. S.; Topsae, H . Bull. SOC.Chim. Belg. 1984, 93, 163.
1983, 105, 6501.
(29) Fuggle, J.; Martensson, N. J. Electron Specrrosc. Relat. Phenom. 1980, 21, 275. ( 3 0 ) Bellaoui, A. Thesis, Lyon, France, 1987.
Effects of Miscible and Immiscible Polar Displacers on the Forces between Adsorbed Polystyrene Layers in Nonpolar Solvents Johan Marrat and Hugo K. Christenson* Department of Applied Mathematics. Research School of Physical Sciences, Australian National University, Canberra, Australia 2601 (Received: January 11, 1989; In Final Form: May 5, 1989)
Results are presented of direct force measurements between polystyrene (PS,MW = 207000) covered mica surfaces in miscible acetone-heptane mixtures and acetone-cyclohexane mixtures and in partly miscible water-cyclohexane mixtures. Due to their polar nature both acetone and water act as displacers of nonpolar PS from the polar mica surface. Displacement results from a lowering of the differential segment-surface adsorption affinity. At a constant adsorption level, it expands the adsorbed layers (as found in acetone-heptane mixtures at low acetone levels); however, any desorption will have the opposite effect. In acetone-cyclohexane mixtures, for example, the net result is that the distance range of the surface forces remains unchanged at acetone levels up to 3% in spite of a considerable desorption. Polymer displacement (with limited desorption) in poor solvents invariably gives rise to stronger adhesive forces. Complete PS desorption can always be achieved at sufficiently high acetone levels. In contrast, due to its small saturation concentration, water is unable to effectively desorb PS from mica in cyclohexane and the force profiles are little affected up to a water activity aw = 0.9. Above a, = 0.9, the long-range forces remain controlled by polymeric interactions whereas capillary condensation dominates the short-range forces. Due to the finite thickness of the PS layers, the onset of capillary condensation occurs at higher water levels than between bare mica surfaces. Only at saturation (a, = 1) do both long-range and short-rangeforces become controlled by capillary condensation.
Introduction The technological importance of polymeric (de)stabilization in colloidal dispersions has motivated a strong experimentall-l and theoretica11z-16interest in the basic principles governing polymer adsorption to interfaces and polymeric interactions between interfaces. Recent work*-” on the direct measurement of
’
Present address: Philips Research Laboratories, PO Box 80.000, 5600 JA Eindhoven, The Netherlands.
0022-3654/89/2093-7 180$01.50/0
surface forces between adsorbed polystyrene (PS) layers on mica in solvent mixtures has enabled the first investigation of the (1) Lyklema, J.; Van Wet, T. Faruday Discuss. Chem. Soc. 1978.65,25. (2) Cain, F. W.; Ottewill, R. H.; Smitham, J. B. Faraday Discuss. Chem. SOC.1978, 65, 33. (3) Klein, J.; Luckham, P. F. Macromolecules 1984, 17, 99. (4) Klein, J.; Luckham, P. F. Macromolecules 1984, 17, 1041. (5) Israelachvili, J. N.; Tirrell, M.; Klein, J.; Almog, Y. Macromolecules 1984, 17, 204.
0 1989 American Chemical Society
Adsorbed Polystyrene Layers in Nonpolar Solvents influence of both the thermodynamic solvent quality* and the differential segment-surface adsorption affinityg*"on the adsorbed amount and the interaction profiles. In tolueneheptane mixtures, for example, one can vary widely the effective solvent quality toward PS by changing the volume fraction of toluene (a good solvent) with respect to heptane (a nonsolvent). Heptane, toluene, and PS are all apolar whereas mica has a polar nature. It is not anticipated that the effective segment-surface adsorption affinity, mainly controlled by van der Waals dispersion forces, is substantially affected by heptane-toluene mixing ratio. The major effect on PS adsorption is therefore thought to be the change of the solvent quality. Generally speaking, a decreasing solvent quality causes the amount adsorbed from solution to increase, the adsorbed polymer layers to become more compact, the long-range attractive forces to become of relatively shorter range, and the adhesion force to increase in magnitude. When a small volume of acetone was added to heptane: direct force measurements indicated that initially collapsed adsorbed PS layers became much more expanded and eventually could be completely desorbed in spite of the fact that acetone is also a poor solvent for PS. This was accompanied by a marked change in the surface forces. Such behavior was rationalized by the higher adsorption affinity of the polar acetone molecule for the polar mica surface compared to nonpolar molecules. It thereby lowers the differential segment-surface adsorption affinity and displaces PS segments from the surface whereas the solvent quality in the bulk remains largely unchanged. The present paper aims to extend these investigations by observing the effect on the surface forces of the addition of acetone and water to a PS solution in cyclohexane. At 22 O C cyclohexane is a worse than 8 solvent for PS but is still a relatively better solvent than heptane. A comparison will be given with some of our previously published resultsg in acetone-heptane mixtures. Our previous work on interactions between adsorbed polymer layers in nonpolar media was invariably camed out in dry solvents. In the work of Klein et al.," the water activity was not controlled. Any possible displacement effects of water as a function of its concentration remain to be examined. The profound effects of small quantities of water on the forces in polymer-free nonpolar liquids are well recogni~ed.'~-'~ Short-range oscillatory solvation forces present in dry nonpolar liquids disappear gradually as the water activity is raised and are almost completely absent at activities above 0.5. At higher water contents capillary condensation of water occurs around the contact region between the two surfaces. This is caused by the preferential wetting of the mica surfaces by water which phase separates from the solution at concentrations below saturation. Because of a negative Laplace pressure across the newly created aqueous/nonaqueous interface, capillary condensation substantially increases the adhesion force. We will here investigate the effect of possible capillary condensation of water between polymer-coated surfaces on the surface forces. The results should contribute to a better understanding of interparticle forces between homopolymer-covered polar colloids in nonpolar media. In ordinary environments these systems will normally have a finite water content with an activity comparable to the ambient humidity. It is of interest how the moisture level could affect the surface forces and how the presence of adsorbed polymer can alter phenomena such as the rate of coagulation
(6) Luckham, P. F.; Klein, J. Macromolecules 1985, 18, 72. (7) Hadziioannou, G.;Patel, S.; Granick, S.; Tinell, M. J. J . Am. Chem. SOC.1986, 108, 2869. (8) Marra, J.; Hair, M. L. Macromolecules 1988, 21, 2349. (9) Marra, J.; Hair, M. L. Macromolecules 1988, 21, 2356. (10) Marra, J.; Hair, M. L. J . Colloid Interface Sci. 1988, 125, 552. (11) Marra, J.; Hair, M. L. J . Phys. Chem. 1988, 92, 6044. (12) Dolan, A. K.; Edwards, S. F. Proc. R. Soc. 1975, ,4343, 427. (13) DeGennes, P. G.Macromolecules 1982, 15, 492. (14) Scheutjens, J. M. H. M.; Fleer, G.J. J . Phys. Chem. 1979,83,1619. (15) Scheutjens, J. M. H. M.; Fleer, G.J. J . Phys. Chem. 1980,84, 170. (16) Scheutjens, J. M. H. M.; Fleer, G . J. Macromolecules 1985,18, 1882. (17) Christenson, H . K. J . Colloid Interface Sci. 1985, 104, 234. (18) Christenson, H. K.; Blom, C. E. J . Chem. Phys. 1987, 86, 419.
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and/or the ease of redispersion.
Experimental Section Polystyrene (MW = 207000, Mw/MN = 1.04) was obtained from Pressure Chem. Co. and purified by consecutive nonsolvent precipitations from THF into methanol and from toluene into heptane after which it was dried under vacuum. All solvents used in the present study were Reagent Grade. They were initially dried for several days over molecular sieve type 4A and distilled just prior to use. Direct force measurements were made using an apparatus originally developed by I s r a e l a ~ h v i l i . ~This ~ device permits an accurate determination of repulsive, attractive, and adhesive surface forces F(D) as a function of the surface separation D. For these experiments molecularly smooth surfaces are essential. To this end molecularly smooth mica sheets were cleaved from the crystal, silvered on one side with a 480 A thick silver layer, and then glued (using a Shell Epon 1004 resin), silvered sides down, on two cylindrically curved glass disks. The disks were mounted in the apparatus in a crossed cylinder configuration. Surface forces are determined by measuring the deflection of a nontilting double cantilever spring on which one of the surfaces is mounted. Surface separations are measured simultaneously with a resolution of 0.2 nm by using an optical interferometry technique which allows fringes of equal chromatic order (FECOs) to be observed in a spectrometer.20 This optical technique also allows an observation of the shape of the two opposing surfaces and their possible deformation/flattening in adhesive contact. Details of the above procedures have been more extensively described e l s e ~ h e r e . ~ ~ ~ ~ ~ At the beginning of each experiment, the mica sheets were brought into contact in an atmosphere of dry nitrogen and this contact position, as measured by the interferometry technique, was then defined as D = 0. After the surfaces were separated, they were immersed in cyclohexane and it was checked whether an oscillatory solvation force similar to what was reported in ref 21 could be observed. Subsequently, a concentrated PS solution in warm cyclohexane was injected into the apparatus until a final polymer concentration of about 3 pg/mL was reached. Although cyclohexane at room temperature is a worse-than-8 solvent for PS, the low turbidity of the solution did not indicate an extensive presence of large polymer aggregates at 3 Fg/mL, which could have given rise to the formation of inhomogeneous adsorbed layers. The latter was confirmed experimentally from the force vs distance profiles (see also ref 9). The mica surfaces were then separated a large distance and allowed to incubate in this solution overnight. Solvent compositions were changed by mixing small volumes of acetone with the cyclohexane. Where appropriate, the water concentration was raised by adding increasing volumes of water-saturated cyclohexane to the dry cyclohexane in the apparatus. Sufficient time (at least 12 h) was always allowed for the PS adsorption to come to equilibrium. All measurements were carried out at 22 OC. Heptane is too poor a solvent to avoid extensive formation of phase-separated PS clusters in solution at any finite concentration (as is obvious from a clearly increased turbidity). Adsorbed PS layers in heptane were therefore obtained by first adsorbing PS from toluene-heptane mixtures (see ref 9) and then exchanging these mixtures for polymer-free heptane. The measured forces F(D) are normalized by the mean radius of curvature R of the cylindrically curved surfaces. According to the Deryaguin the ratio F(D)/2*R equals the interaction free energy E ( D ) per unit area between two flat surfaces at a separation D. This equality is subject to the condition that the radius of curvature R is much larger than the distance range across which interactions are measured. Since the typical (19) Israelachvili, J. N.; Adams, G. E. J . Chem. SOC.,Faraday Trans. 1 1978, 74, 975. (20) Israelachvili, J. N. J . Colloid Interface Sci. 1973, 44, 259. (21) Christenson, H. K.; Horn, R. G.; Israelachvili, J. N. J . Colloid Interface Sci. 1982, 88, 79. (22) Deryaguin, B. V. Kolloid-Z. 1934, 69, 155.
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Marra and Christenson
1
I
f 1
'0
I
i i
Figure 1. Forces F ( D ) / R as a function of the distance D between two molecularly smooth mica surfaces bearing adsorbed PS layers in equilibrium with a PS/cyclohexane solution (curve A) or heptane (curve B). Ji denotes the positions from where a spontaneous slow inward jump occurred: J , denotes an outward jump (basically from the adhesive minima, outward arrows). The dashed parts of the curves are experimentally inaccessible. Part of the dashed regions could be accessed by using a much stiffer spring; however, this would significantly reduce the accuracy.
range of the surface forces was always less than about 30 nm and R 2 1.5 cm this condition is more than satisfied.
Results and Discussion A . Forces between PS Layers in Cyclohexane and Heptane. Curve A in Figure 1 shows the force vs distance profile at 22 OC between mica surfaces in adsorption equilibrium with a 3 rg/mL PS (MW = 207000) solution in cyclohexane. Refractive index measurements with similar results as those in ref 8 and 9 indicated that the adsorbed amount was approximately 1 f 0.2 mg/m2. An attractive force was first experienced at a distance D = 28 f 2 nm. Even though here the absolute magnitude of the attractive force is still very small (mN/m), it is at this distance that the gradient of the attractive force with respect to distance exceeds the spring constant K (K = (1.5-1.8) X lo2 N/m) of the spring on which one of the surfaces is mounted. The resulting mechanical instability causes the surfaces to spontaneously jump inwards until they come to rest at a separation where repulsive forces just balance the attractive forces. Due to the repulsive osmotic and excluded volume interactions at shorter range there is an adhesive minimum at D = 10 nm with a depth (FIR)*& r 1 mN/m. Since cyclohexane is a worse-than-0 solvent, the observed attractive and adhesive force must at least partly be the result of the negative free energy of mixing between the two adsorbed layers. However, a bridging of PS chains between two surfaces may also contribute to the attraction, especially at long range. As discussed in ref 3, 5, 8, and 16 bridging attraction is responsible for the frequently encountered attractive forces in better-than4 solvents under conditions where polymer adsorption is below saturation levels. It is worthwhile to compare curve A with the forces measured between PS surfaces in cyclohexane by Israelachvili et aLs Their measured attraction was of much longer range and their adsorbed amounts were several times higher than in curve A. However, they used molecular weights of 600000 and 900000. It is well-known, both t h e ~ r e t i c a l l y ' ~and , ' ~ e ~ p e r i m e n t a l l y that , ~ ~ in marginal ( - 6 ) solvents the adsorbed amount increases with the molecular weight. On the other hand Israelachvili et al.5 measured an adhesion force (FIRAdh= 0.6-0.8 mN/m which is only slightly smaller than the adhesive force measured in curve A of Figure 1. Hence we conclude that only the long-range attraction clearly increases with the adsorbed amount. The latter result was also found in toluene-heptane* and h e p t a n e a c e t ~ n esolvent ~ mixtures. A somewhat larger adhesive force between our thinner adsorbed (23) Vincent, B. Adv. Colloid Interface Sci. 1974, 4, 193.
Figure 2. Forces F ( D ) / R as a function of distance between two adsorbed PS layers in a 0.3% acetone-cyclohexanemixture (curve A), 3% acetone-heptane mixture (curve B), and a 3% acetonecyclohexane mixture (curve C). Ji and J,, have the same meaning as in Figure 1. The forces between PS layers in cyclohexane at a water activity of 0.8 were very similar to those in curve A.
layers could easily be rationalized through an increased contribution from bridging attraction. Bridging is well-knownI6 to become more important when the surface coverage is diminished. To appreciate the effect of the solvent quality on the interaction profiles, we have reproduced in Figure 1, curve B, the forces measured in ref 9 between two adsorbed PS layers in the nonsolvent heptane. Although the adsorbed amount is similar to what it is in cyclohexane (=2 mg/m2 as judged from a measured refractive index p = 1.52 f 0.01 at D = 5.2 nm, see ref g), the attractive force only extends to D = 10 nm. In contrast the adhesive force is about 6 times stronger. Clearly, the poorer solvent quality of heptane is reflected in much more compact adsorbed layers while the negative free energy of mixing, which is exp e ~ t e d 'to ~ ,be~ mainly ~ responsible for the adhesive force, becomes much larger. B. Forces in CyclohexaneAcetone and Heptane-Acetone Mixtures. Curve A in Figure 2 was measured after 0.3%of the nonsolvent acetone was added to the cyclohexane/F?S solution and the PS adsorption was allowed to come to equilibrium. Curve C was measured after the acetone content was raised to 3%. We found that surface forces always changed instantaneously after solvent mixing was established and remained essentially the same for at least several hours thereafter. Curve B resulted when 3% acetone was added to heptane. When more than 20% acetone was added to cyclohexane, the PS became fully desorbed from the mica and a deep adhesive minimum existed at a surface separation close to D = 0 nm. The same could only be achieved in heptane when more than 50% acetone was introduced. Clearly, the addition of acetone displaces PS segments from the mica surface. As already discussed in ref 9 this must be mainly due to a reduction in the differential effective segment-surface adsorption affinity caused by the stronger adsorption of polar acetone molecules to the polar mica surface. The result is that part of those segments, originally in direct contact with the mica (train segments), become resident in the loops and tails of adsorbed chains. We anticipate that at the same time some degree of overall desorption will result. In cyclohexane, little PS desorption is apparent when 0.3% acetone is introduced but, as is obvious from the pronounced inward shift of the force profile, most of the PS has desorbed when the acetone content is adjusted to 3%. In contrast, as judged from refractive index measurements, only little PS desorption occurs on addition of 3% acetone to heptane. We conclude that PS becomes more easily desorbed when the effective solvent quality is better. (24) Napper, D. H.Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983.
Adsorbed Polystyrene Layers in Nonpolar Solvents
In both cyclohexane and heptane the adhesion increases with the acetone concentration. Given that the effective bulk solvent quality at these small acetone levels cannot have significantly altered, we may explain the increase in the adhesion through a more expanded adsorbed layer, which enhances the degree of mutual interpenetration of the adsorbed layers and thus the magnitude of the negative free energy of mixing. Here we see the influence of both the solvent quality atld the segment-surface adsorption energy on the strength of the adhesion. In curve C, an adhesive minimum is found at D = 2 nm. This small surface separation indicates that here a van der Waals attraction between the two mica surfaces will also contribute to the measured adhesive = -1 1 mN/m. It was noted that the latter adforce hesion was only found when the surfaces were separated immediately after they jumped into adhesive contact. When the adsorbed layers were first compressed to FIR = 6 mN/m and then separated, the adhesive force became an order of magnitude stronger, as indicated by the very deep minimum in Figure 2. We attribute this hysteresis to a strong increase in the van der Waals attraction when the surfaces are first compressed to a smaller separation. Desorption and/or polymer diffusion away from the contact zone is likely to occur simultaneously. Similar strong adhesive forces have also been found2*at short separations between bare surfaces in pure solvents or solvent mixtures. An important observation is that in both heptane-acetone and cyclohexane-acetone mixtures an attractive force is first experienced at D = 28-30 nm. From here an inward jump occurs which takes about 10 s to be completed. (The sluggishness of the inward jump is likely to be related to the hindered drainage of solvent through the adsorbed layer.3) We see that the inward jump occurs from virtually the same position as was found in pure cyclohexane (curve A, Figure 1) and therefore seems to be rather insensitive to the adsorbed amount (Figure 2, curve A vs curve C). The latter observation contrasts with our previous results concerning the force profiles between PS surfaces in cyclohexane at different adsorption levels. The distance range of the surface forces then clearly increases with the adsorbed amount. Apparently, in Figure 2 part of the adsorbed PS layers in cyclohexane desorbs and the remainder expands when more acetone is added. The net result at these acetone levels is that the distance range of the forces remains more or less unchanged. On the other hand, when 3% acetone is added to heptane the attraction moves outward from 10 to about 30 nm (Figure 2, curve B). We propose that, although adsorbed polymer chains always tend to displace themselves away from trains on the surface into loops and tails when the adsorption affinity is lowered, a net expansion of the force profile is only encountered when the degree of desorption remains small. Higher acetone levels in heptane also cause significant desorption to occur and the force profiles do not expand any further. Full details of the surface forces under these conditions have been given in ref 9. C. Effects of Water on the Forces between Adsorbed PS Layers in Cyclohexane. At room temperature, water is soluble in cyclohexane up to a concentration of about 0.006%. Up to a water concentration of 80%of saturation, no dramatic changes occurred in the force profiles. The distance range of the attractive forces remained at approximately 30 nm throughout. At 80%of saturation the measured force curve appears rather similar to curve A in Figure 2. Apparently the effect of 0.005%water is about the same as the effect of 0.3% acetone. It may therefore be concluded that water is potentially a quite effective polymer displacer and in cyclohexane apparently has a stronger adsorption affinity for mica than has acetone. Above a water level of 90% of saturation, the surfaces still jumped into contact from about D = 28 nm. However, once they reached contact a t D = 5.8 nm, it was observed that the two crossed cylindrically curved mica surfaces slowly began to flatten at the contact region. This flattening process took about 20 s after which a strong adhesive contact of 650 f 50 nN/m had to be overcome in order to separate the surfaces. From the shape of the FECO fringes it could often be observed that after flattening had occurred the perimeter of the contact region tended to slowly
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7183
curve 2-3 nm inward. The result was that a lens of polymeric material, displaced from the perimeter, accumulated at the center of the contact region. Such observations can only be explained if a capillary condensation of water around the contact region takes place once the surfaces come close together. A description of capillary condensation from nonpolar liquids between bare mica surfaces has been given in ref 17. The Young-Laplace equation AP = y / r
(1)
gives the pressure difference AP across an interface in terms of the interfacial tension y and the radius of curvature r of the interface. Equation 1 predicts a reduced pressure on the convex side of any interface which explains our strong adhesive forces. From eq 1 one can derive the Kelvin equation which for the present system can be written asi7 -RT In aw = Vwy/r (2) Here Vw is the molar volume of water and aw is the water activity in cyclohexane. Water is monomeric in nonpolar liquids and obeys Henry’s law.25 Therefore, its activity should simply be proportional to the mole fraction for the low concentrations (
