J . Phys. Chem. 1988, 92, 1235-1239 A strong interaction between the vanadium species in solution and the alumina surface occurs during the impregnation stage. At pH 10, the major adsorbed species is a distorted tetrahedral vanadium entity. At pH 4, the major adsorbed species is a polyoxovanadate species in which vanadium belongs to a distorted octahedral environment but the presence of monomeric species is not ruled out.
1235
Hydration-dehydration sequences of the supported oxovanadium species are evidenced, similarly to those described for M003-Y-Al203 and W03-y-A1@3 systems. Experiments are now underway to characterize the hydration-dehydration process by N M R . Registry
No. V,05, 1314-62-1; A1,03, 1344-28-1.
Molecular Layering in Thin Aqueous Films P. M. McGuiggant and R. M. Pashley** Department of Applied Mathematics, Research School of Physical Sciences, and Department of Chemistry, The Faculties, Australian National University, Canberra, Australia (Received: May 6, 1987; In Final Form: September 7 , 1987)
Direct measurements of the total force as a function of separation between molecularly smooth mica sheets immersed in concentrated NaCl and urea solutions are reported. As previously found in KC1 solutions, the repulsive forces measured are composed of stepped forces superimposed upon a strong monotonically repulsive hydration force. The width of these steps was found to be 0.30 0.05 nm, which is slightly larger than those measured in KCl solutions, and appears to be related to the diameter of the water molecule. These steps in the repulsive force curve have been identified as oscillatory forces. The longer range (> 1.5 nm) monotonic contribution to the total repulsive force curve could not be fitted by the Gouy-Chapman theory of the diffuse double layer for the more concentrated solution, whereas in 0.1 M NaCI, reasonable agreement was obtained. Addition of 5 M urea was found to have little effect on oscillatory forces in 0.1 M NaC1, but the urea may adsorb at the mica surface, introducing a Stern layer.
Introduction The effect of solute species on the structure of the surrounding solvent has attracted the attention of both experimentalists and theoreticians.*,2 Various experimental techniques have been employed to study the structure of aqueous solutions in particular, including transport3 and thermodynamic properties,” NMR,5 clay swelling! and spectroscopic7measurements. Unfortunately, theory and experiment have not yet combined to form a definitive picture of even the liquid state and the theory of solute effects is still further away. The interaction between planar macroscopic solids immersed in a solvent is of fundamental importance in colloid and surface science and, in addition, is directly related to solutesolvent effects. The crystal muscovite mica has recently been successfully used to investigate macroscopic solvation. It has been found that such effects are magnified for the macroscopic cases and even subtle structural effects which would otherwise go unobserved can be quantified. In this paper we report further work on the structural properties of water adjacent to molecularly smooth mica surfaces. Direct measurement of the total interaction (force vs distance) between cleaved mica sheets immersed in concentrated aqueous electrolyte solutions show that the DLVO theory is generally accurate at separations greater than about 5 nm but at short range non-DLVO “hydration” effects can arise.9 These hydration or more generally “solvation” effects can be further classified as monotonically repulsive, oscillatory, and monotonically attractive. Oscillatory forces arise when two mica surfaces are forced to within a few (generally 6-10) solvent molecular diameters of contact. The force of interaction alternates between maxima and minima and corresponds to packing restrictions of the finite-sized molecules between the two mica surfaces, the minima corresponding to favorable configuration of the remaining layers. At separations greater than about 1.5 nm but typically less than about 5 nm, monotonically repulsive hydration forces have been measured for ‘Department of Applied Mathematics. On leave from the Department of Chemical Engineering and Materials Science, University of Minnesota, MN. *Department of Chemistry.
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a wide range of cations adsorbed on mica. The only exception so far found is the hydronium ion, H 3 0 + , which apparently is different because of proton penetration into the mica surface. Water is the only solvent known to give rise to both monotonic and oscillatory solvation forces, whereas other hydrogen-bonded liquids, such as ethylene glycol’o and methanol,” and nonpolar liquids including cyclohexane12 only show oscillations. Besides the repulsive monotonic and oscillatory solvation forces, a strongly attractive solvation interaction between hydrophobic surfaces can occur in aqueous solutions which is known as the hydrophobic intera~tion.’~ This attraction is of long range (- 10 nm) and is apparently caused by the expulsion of entropically unfavourable water molecules next to the surface. A more detailed explanation of hydration forces is given in ref 14. Theories of the effects of additives on aqueous solution structure are separated into the effect of electrolytes and nonelectrolytes. For example, Frank and co-workers describe a hydrated ion and its periphery in terms of three regions:I5 (1) the inner region in which the first layer of surrounding water molecules is tightly
(1) Franks, F. In Water: A Comprehensive Treatise; Franks, F., Ed.; Wiley: New York, 1973; Vol. 2, p 1. (2) Chan, D. Y. C.; Mitchell, D. J.; Ninham, B. W. J . Chem. Phys. 1979, 70, 2946. (3) Kay, R. L.; Evans, D. F. J . Phys. Chem. 1966, 70, 2325. (4) Stokes, R. H. Ausr. J . Chem. 1967, 20, 2087. (5) Finer, E. G.; Franks, F.; Tait, M. J. J. Am. Chem. SOC.1972, 94,4424. (6) Pashley, R. M.; Quirk, J. P. Colloids Surf. 1984, 9, 1. (7) Murkerjee, P.; Ray, A. J . Phys. Chem. 1963, 67, 190. (8) Pashley, R. M. Adu. Colloid Interface Sci. 1982, 16, 57. (9) Pashley, R. M. J . Colloid Interjace Sci. 1981, 83, 531. (10) Christenson, H. K.; Horn, R. G. J. Colloid Interface Sci. 1985, 103,
50. (1 1) Christenson, H. K. J . Chem. SOC.,Faraday Trans. I 1984,80, 1933. (12) Christenson, H. K.; Horn, R. G.; Israelachvili, J. N. J . Colloid Interface Sci. 1982, 88, 79. (13) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088. (14) Israelachvili, J. N. Chem. Scr. 1985, 25, 7. (15) Frank, H. S.; Wen, Wen-Yang, Discuss. Faraday SOC.1957,24, 133.
0 1988 American Chemical Society
1236 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
bound and polarized by the strong Coulombic field of the ion, (2) the outer region which contains molecules possessing the same properties as bulk water, and (3) a transition region in which the tetrahedral structure of the bulk water and the highly oriented radial electric field of the first region intermix. Electrolytes are then classified as either “structure makers” or “structure breakers” depending upon their ability to reinforce the outer or inner regions or by making the intermediate (transition) region the most important property, respectively. In addition to considering the number of bonds (or geometry) per solvent molecule in the vicinity of the solute we can also examine the radial distribution in solvent density around the solute molecule. This latter effect, due entirely to the hard-sphere nature of the molecules, will manifest itself as an oscillatory force between macroscopic surfaces, whereas the former is the analogue of the overall monotonic attractive or repulsive force. Theories of aqueous solutions of nonelectrolytes generally divide water into distinguishable molecular species in equilibrium;I6 one is extensively hydrogen bonded and has tetrahedral coordination and thus a low density, and another is a so-called “dense water” state where there is little hydrogen bonding and the 0-0 bond distance is much shorter than previously permitted. Nonelectrolytes shift the equilibrium between the two states. Urea, for example, is thought to be a structure breaker since it is believed to disrupt the long ranged “bulk” water structure which, because of the geometry of the urea molecule, is not replaced by extended short-range order. Physically, urea has been found to denature proteins,” inhibit surfactant aggregation,Is and enhance hydrocarbon ~olubility.’~ Previous measurements of hydration forces between macroscopic mica surfaces in aqueous KCl solutions suggest that oscillations may always be present in thin aqueous films, but that the relative importance of this force will depend critically on the strength of the background monotonic attractive or repulsive contribution on which it is superimposed.20 However, the earlier measurements were only conducted in KCI solutions and so the aim of this work was to test the generality of the oscillatory behavior and to determine the dependence on the hydrated nature of the solid surface, viz., the type of hydrated cation adsorbed on the mica surface. In addition, the effect of a high concentration of a nonelectrolyte, urea, on the range and magnitude of the hydration forces was also investigated.
Method The surface forces apparatus has been described extensively elsewhere.2’ Briefly, mica was cleaved to produce molecularly smooth layers and silvered on the reverse side which was then glued onto curved transparent silica supporting disks (of mean radius Rx 1 cm). The lower disk was attached to a stiff doublecantilever variable spring which minimized lateral displacement of the surface as the spring deformed.22 The surface separation was measured by an interferometric technique which produces FECO fringes. Under the high forces exerted upon bringing the mica surfaces together in the presence of a hydration force, the glue between the silver layer and silica support disks is compressed. This was observed as a flattening of the usually parabolic tips allowing greater distance accuracy than usually obtained. In general, the measured force, F, is scaled by the mean undeformed radius of curvature of the disks, Rx, which according to the Derjaguin approximation is equivalent to 2aE where E is the
-
Franks, F. In Water: A Comprehensbe Treatise; Franks, F., Ed.; New York, 1973; Vol. 2, p 8. Dubin, P.; Strauss, U. P. J . Phys. Chem. 1973, 77, 1427. Schick, M. J. J . Phys. Chem. 1964, 68, 3585. (19) Wetlaufer. D. B.; Malik, S. K.; Stoller, L.; Coffin,R. L.J . Am. Chem. Soc. 1964,86, 508. (20) Pashley, R. M.; Israelachvili, J. N. J. Colloid Interface Sci. 1984, 101, (16) Wiley: (17) (18)
1 nm). In this region, no significant flattening was observed and thus at least this part of the curve may be compared with Gouy-Chapman theory. The dashed line in Figure 4 is calculated from this theory assuming a constant charge interaction with a 200-mV potential J. This high potential and Hamaker constant, A, of 2.2 X was necessary to give a large enough repulsion, since a more reasonable estimate of the surface potential (of less than 50 mV) predicts overall attraction due to the high degree of screening and strength of the van der Waals contribution. Even the repulsive contribution (A = 0) of the interaction forces assuming a 50-mV potential (as shown by the solid line) is substantially weaker than the measured forces. Clearly, the results do not agree with theory and hence the forces measured in 1.0 M NaCl at all distances are apparently due to the hydration of the adsorbed Na+ ions on the mica surface. Figure 5 shows that the forces measured in 0.1 M NaCl can be explained by using a potential of 90 mV and Debye length of 1.1 nm at separations beyond about 1.5 nm of mica contact. The
1238 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 I
l
l
0.1 M NaCl
2.0
‘ I
40
Dinm Figure 6. Forces measured between mica surfaces in 0.1 M NaCl with 5 M urea. FIRXis the total force scaled by the undeformed radius Rx. The dashed line is the theoretical force law computed for ‘Po- = 90 mV, A = 2.2 X J, and K-’ = 1.1 nm (Le., similar values as used for 0.1 M NaCI). Beyond 1 nm, the force curve can be fitted to the experimental curve only if a Stern layer of 0.4-0.5 nm per surface is introduced. Below 1 nm, the repulsion increases more steeply and structural forces were still present.
dashed and solid lines represent Hamaker constants of 2.2 X J and zero, respectively. The surface potential value appears slightly high since a value of less than 50 mV has been predicted from simple ion-exchange modeL9 At separations less than 1.5 nm, the measured forces could not be fitted even when only the repulsive double-layer contribution ( A = 0) was considered. Hydration forces in this case are therefore present from at least 1.5 nm separation and probably extend further out, increasing the magnitude of the double-layer forces. Obviously, treating water as a structureless medium may not be valid at these small separations, and so too the DLVO theory at these distances. However, in the 1.0 M NaCl case there is such a large discrepancy between theory and experiment that hydration effects certainly appear to be present. Also, the calculated Debye length at this concentration is 0.3 nm as opposed to 1.O nm for 0.1 M NaCl and perhaps the overlapping potentials in the latter case reduces the ordered layering of water molecules because of the presence of a significant concentration of counterions in the interlayer. The results of the measured forces in 0.1 M NaCl with 5 M urea added are shown in Figure 6 . The observed Debye length was still about 1.O nm and the structure near the mica surface is still present and quite similar to that without added urea. Clearly, the urea did not “break” up the oscillatory structure, although it may have been slightly weakened or altered since the steps appeared to be less steep. However, the background monotonic repulsive forces were displaced by about 1.0 nm in the presence of urea. As was previously observed for quaternary ammonium ions,29it is possible that urea adsorbed as a monolayer to each mica surface, which would have the effect of introducing a Stern layer of about 0.4-0.5 nm from each mica surface.
Discussion Although only a stepped repulsive force curve was observed in concentrated sodium chloride solutions, it is quite clear that this force is actually oscillatory but because of the nature of the spring technique the complete curve cannot be measured, as was also noted in previous studies on concentrated potassium chloride.23 However, in the earlier case the salt concentration was lowered to a level (M) where the background repulsive hydration force was significantly reduced (by H+ replacement of K+) and the minima in the force curve were then both adhesive and easily observed.20 By comparison, reduction to 0.1 M NaCl did not reduce the hydration forces sufficiently to enable observation of the oscillatory nature of the force. The amplitudes of the oscillations in NaCl solution can be estimated from the pressure curve in Figure 2 and are roughly
McGuiggan and Pashley in the region of 3-10 atm. Although this range is substantially lower than that observed in KC1 (of about 30-50 atm) it is possible that in the NaCl case the D = 0 value was not in fact the mica-mica contact distance but still contained a significant layer of bound water (e.g., of 0.5-1.0 nm thickness). This possibility is supported by the lower overall pressures observed in NaCl compared with KC1. An alternative possibility is that the more hydrated Na+ ions were to some extent forced out of the interlayer between the mica surfaces on forcing them together to be replaced (for overall electroneutrality) by H 3 0 +ions, which can penetrate the mica lattice as protons. Such a process reduces the overall strength of the hydration force. That this exchange process occurs on forcing mica surfaces together has been proven in earlier work26 and is more likely for Na+ than K+ ions. In order to prevent this exchange process and hence measure the maximum amplitude in the oscillatory force it would be necessary to increase the NaCl concentration above 1 M. The other main difference in the short-range forces in concentrated KC1 and NaCl solution is the periodicity of the steps or oscillations, which were 0.25 f 0.03 nm and 0.30 & 0.05 nm, respectively. Although both of these values are close to the size of a water molecule, they suggest that the type of cation in the interlayer region may influence the periodicity. The cation must be present in the interlayer region at relatively high concentrations in order to maintain electroneutrality. Thus, when the mica surfaces are 1.O nm apart the ratio of cations to water molecules would be about 1:8, which is high enough to expect some perturbation on the density distribution of the water layers. A further study using a cation (e.g., Ca2+)which should have a greater effect on the interlayer structure will perhaps throw more light on this important effect. Reviews on the crystalline swelling of ~ l a y s ~ ’ , * ~ also suggest that the cation has a slight influence on the periodicity of the oscillations. The results obtained in this study show quite clearly that even at high concentrations urea has no significant effect on the short-range (< 1.O nm) swelling forces of mica. It is important to point out, however, that this result does not imply that urea has no effect on the structure of bulk water. Thus, it is quite possible that urea alters the structure of water both in the bulk phase and in the interlayer region between the mica surfaces, in which case no change in the interaction forces would be expected. Alternatively, it is possible that the strong oscillatory structure in the interlayer cannot be significantly affected by the relatively weak perturbation of urea. These observations might also apply to other structure-breaker molecules and, if so, it would appear that only inorganic cations and surfactants have any significant influence on the crystalline swelling of clays. The effect of urea on the longer range double-layer forces could be quantitatively explained by introducing a Stern layer which has the effect of displacing the plane from which the diffuse double-layer originates. A similar model has already been successfully applied to explain the effect of a series of symmetrical quaternary ammonium ions adsorbed onto mica.29 If this is also the correct explanation for urea, measurements at lower concentrations of electrolyte and urea should clearly demonstrate the effect.
Conclusions In concentrated sodium chloride solutions, an oscillatory force between mica sheets has been measured which is similar to that observed earlier in potassium chloride solutions. The oscillatory force appeared to be superimposed upon a strong repulsive hydration force. In more dilute NaCl solution (0.1 M) fewer oscillations were observed and at separations beyond about 1.5 nm the forces could be fitted to the Gouy-Chapman theory of the ( 2 6 ) Claesson, P. M.; Herder, P.; Stenius, P.; Eriksson, J . C.; Pashley, R. M. J. Colloid Interface Sci.1986,109,31. (27) Del Pennino, U.; Mazzega, I.; Valeri, S.; Aliette, A.; Brigatti, M. F.; Poppe, L. J . Colloid Interface Sei.1981, 84, 301. (28) Quirk, J. P. Isr. J . Chem. 1968, 6, 213. (29) Claesson, P. M.; Horn, R. G.; Pashley, R. M . J . Colloid Interface Sci. 1984, 100,250.
J . Phys. Chem. 1988, 92, 1239-1244 diffuse double layer. Addition of 5 M urea was found to have little effect on the oscillatory forces in 0.1 M NaCl, but the urea may adsorb at the mica surface, introducing a Stern layer.
Acknowledgment. We thank Dr. S. Marcelja and Professor B. W. Ninham for interest and helpful discussions during this
1239
work. Support was provided under the National Energy Research, Development and Demonstration Program, which is administered by the Commonwealth Department of National Development and Energy. Registry No. NaCl, 7647-14-5;urea, 57-13-6.
Surface Enhanced Raman Scattering of Derivatives of Adenine. The Importance of the External Amino Group in Adenine for Surface Binding C. Otto,* F. F. M. de Mul, A. Huizinga, and J. Greve Department of Applied Physics, University of Twente, P.O. Box 21 7 , 7500 AE Enschede, The Netherlands (Received: May 19, 1987; In Final Form: August 12, 1987)
In this article various proposals for the orientation of the adenine molecule and derivatives of adenine molecules with respect to an electrochemically roughened silver surface will be discussed. The SERS spectra of adenine, 2’-deoxyadenosine 5’-phosphate (dAMP), poly(rA), adenylyl-3’,5’-cytidine(ApC), and denatured DNA are presented and compared with the SERS spectra of 6-methylpurine, 1-methyladenine, 6-(methylamino)purine, and 6-(dimethy1amino)purine. The SERS spectra of adenine-containing compounds are independent of the potential of the working electrode. The spectra of methylated derivatives are dependent on the potential. The position of the totally symmetric ring breathing mode at a potential of -0.7 V is characteristic for each of these compounds. The spectra of those derivatives that contain an external amino group all show a new vibration at 732 cm-’ if the surface is positively charged. This is not the case for 6-methylpurine. The position of the vibration at 732 cm-’ is identical with the position of the totally Symmetric ring breathing mode in adenine. Therefore, it is concluded that at -0.2 V the external amino group of the adenine derivatives are oriented toward the surface. Adenine does not change its orientation upon a decrease of the potential. The methylated derivativesare parallel to the gurface at -0.7 V. At a potential between -0.2 and -0.7 V the methylated adenine derivatives occur in two different orientations at the surface. The fractional occupation is determined by the potential of the working electrode.
Introduction Not long after the recognition by Jeanmaire and van Duyne’ that a previously unobserved enhancement effect had occurred in the Raman experiments of Fleischmann et a1.2 with pyridine, it was established that also nucleic acid base^,^-^ nucleic and other molecules of interest to the biologist and biophysicist* produced enhanced Raman scattering if these molecules were adsorbed to roughened silver surfaces. Apart from the vast experimental evidence for the existence of the SERS effect, a large body of literature has now accumulated dealing with the theoretical explanation of the various aspects of the o b s e r v a t i o n ~ . ~ JA~critical review of the experimental data from a theoretical point of view has recently been given by Efrima.” Several groups have directed their attention to the measurement of nucleic acid bases and (poly)nucleotides.3~4~6~7~1* Various proposals for the orientation of the base molecules with respect to the surface were put forward. It was assumed by Koglin et (1) Jeanmaire, D. L.; van Duyne, R. P. J . Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1. (2) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. J . Chem. SOC., Chem. Commun.1973,80. (3) Ervin, K. M.; Koglin, E.; Sequaris, J. M.; Valenta, P.; Nurnberg, H. W. J. Electroanal. Chem. Interfacial Electrochem. 1980, 114, 179. (4) Otto, C.; van den Tweel, T. J. J.; de Mul, F. F. M.; Greve, J. J . Raman Spectrosc. 1986, 17, 289. (5) Suh, J . S.;Moskovits, M. J . Am. Chem. SOC.1986, 108, 4711. (6) Koglin, E.; Sequaris, J. M.; Valenta, P.J . Mol. Struct. 1980, 60, 421. (7) Brabec, V . ;Niki, K. Collect. Czech. Chem. Commun. 1986, 51, 167. (8) (a) Nabiw, I. R.; Savchenko, V. A.; Efremov, E. S.J . Raman Spectrosc. 1983, 14, 375. (b) Cotton, T. M.; Timkovich, R.; Cork, M. S. FEES Lett. 1981, 133, 39. (c) Cotton, T. M.; van Duyne, R. P. FEES Lett. 1982, 147, 81. (d) Otto, C.; Huizinga, A.; de Mul, F. F. M.; Greve, J., manuscript in preparation. (9) Chang, R. K.; Furtak, T. E. Surface Enhanced Raman Scattering, Plenum: New York/London. 1982. (10) Moskovits, M.Rev. Mod. Phys. 1985, 57, 783. (11) Efrima, S.Mod. Aspects Electrochem. 1985, 16, 253. (12) Watanabe, T.; Kawanami.0.; Katoh, H.; Honda, K.; Nishimura, Y.; Tsuboi, M. Surf.Sci. 1985, 158, 341.
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aL6 and Brabec et al.’ that the adenine ring has an orientation parallel to the surface. Watanabe et al.Iz concluded from the presence of a strong band at 1334 cm-’ in the SERS spectrum of adenine that adsorption takes place through the nitrogen atom on the 7-position of the ring. This conclusion was based on the assumption that the most intense lines in the SERS spectrum are due to the atoms in closest proximity to the surface. Otto et al.4 used a slightly different idea, namely, that the vibrations originating in groups of the molecule closely situated near the surface are most strongly enhanced. They noted4 that several vibrations, Le., at 326,626,960, 1028, and 1194 cm-I, in which the motion of the c6 N H 2 group participates, give rise to strong contributions in the SERS spectrum. They concluded that adenine was connected to the surface with the external amino group. In order to verify the proposed importance of the external amino group for surface coordination, we made a study of several adenine derivatives. In this article, we present the results that were obtained. The following compounds were studied: purine, 6-methylpurine, 1methyladenine, 6-(methylamino)purine, 6-(dimethylamino)purine, and e-dAMP. Use has been made of the possibility which an electrochemical cell offers to change the potential of the working electrode. The potential dependence of the spectra of all compounds mentioned has been investigated. The consequences of the conclusions will be extended to the model for the adsorption of poly(rA) and single-stranded DNA to the surface.
Materials and Methods Adenine, purine, e-dAMP, 1-methyladenine, 6-(methylamino)purine, 6-(dimethylamino)purine, 6-methylpurine, poly(rA) cytosine, 2’-deoxycytidine, 5’-dCMP, 3’-dCMP, and DNA were products of Sigma. ApC was a product of PL Biochemicals. SERS measurements were carried out in a mi~rocell.’~This cell was equipped with a three-electrode system consisting of a Ag (13) Otto, C.; van Welie, A.; de Jong, E.; de Mul, F. F. M.; Mud, J.; Greve, J. J . Phys. E 1984, 17, 624.
0 1988 American Chemical Society
