Langmuir 1994,10,4237-4243
4237
Electrolyte Species Dependent Hydration Forces between Silica Surfaces J.-P. Chapel? Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received May 13, 1994. In Final Form: August 1, 1994@ Force measurements between two pyrogenic silica sheets immersed in a series of monovalent electrolytes (CsCl, KC1, NaC1, LiC1) were performed using a surface force apparatus (SFA). For each species, a shortrange repulsive hydration force prevented any adhesion of the surfaces. The data were fitted using a charge regulation model ofthe double-layerrepulsion by solvingthe nonlinear Poisson-Boltzmann equation numerically, together with the full Lifshitz calculation of the van der Waals attraction. The hydration force was obtained by subtracting these calculated forces from the data. The results showed that the strength and the range of the hydration force decrease with increasing the degree of hydration of the counterion. This is opposite to the behavior of mica for which adsorbed counterions have been reported to generate a hydration repulsion. The effects of counterionson hydration forces, weakening for silica and enhancing for mica, show that the origin of the short-range interaction is not unique.
Introduction In colloid science, the forces acting between surfaces are usually classified according to their range. The wellknown Derjaguin-Landau-Venvey-Overbeek theory1,2 (DLVO), for example, explains colloidal stability by a simple combination of two interactions: the repulsive electrostatic double-layer force and the attractive van der Waals force. These two long-ranged forces are easily calculable because they can be explained in terms of macroscopic or “bulk” quantities such as the dielectric constant. However, this is a continuum approach that necessarily breaks down at shorter distances since it does not take into account either the discreteness of the solvent or that of the interacting surfaces. In the last decade, several new forces have been invoked to explain a variety of short-ranged phenomena. In particular, the so-called “hydration force”, the solvation force in aqueous solution, remains a real puzzling phenomenon. It name reflects the proposition that this shortrange force is caused by the structure of water between the surfaces. This explanation, however, is by no means universally accepted, and some authors attribute most or all of the large, short range interaction to the molecular motion of the surface. Theories which use molecular models of the solvent and those which use molecular models of the surface are all capable of producing a repulsion of the observed form: exponential decay with a characteristic length of a few tenths of nanometers. Nevertheless, the occurrence of such force between surfaces as different as lipids bilayers, aluminosilicates platelets, and silica sols raises the possibility of answering the question experimentally: does the force that prevents silica dispersion from coagulation3 have the same origin as the one that dominates the short range interactions between lipid bilayer^?^ t Present address: Laboratoire de Physique Statistique, Ecole Normale Superieure, 24,rue Lhomond, 75005 Paris, France. Abstract published inAduance ACSAbstracts, October 1,1994. (1)Derjaguin, B.V.; Landau, L. D. Acta Physicochim. URSS 1941, 14,633-52. (2)Verwey, E.J.; Overbeek, W. J. Th., The Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, The Netherlands, 1948. (3)Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (4)LeNeveu, D. M.; Rand, R. P.; Parsegian, V. A. Biochim. Biophys. Acta 1989,988,351. @
Pashley and Israelachvili5-’ have shown that the hydration repulsion occurring between two mica surfaces across a monovalent electrolyte is intimately related to the hydration number of the cation, i.e. the average numbers ofwater molecules in the first shell.8 The results showed clearly that the strength and the range of the force increased with the series Cs+ > Rb+ > K+ > Na+ > Li+. In this study, we aimed to test the generality of this mechanism by using a different surface. Several others studies have shown the presence of a hydration repulsion between nonsmooth silica s u r f a ~ e s . ~ - lFollowing l the pioneering work done by Horn et a1.12J3on “smooth” silica surfaces separated by NaCl electrolyte, we conducted experiments to determine the influence of ion size on hydration forces. We used a surface force apparatus (SFA),’*flamed silica surfaces, and a series of monovalent cations with different hydrated size. Experiments of four different kinds were performed. Following Pahley’s work, the first investigated the dependence of the hydration force on the electrolyte. Furthermore, the behavior of silica surfaces in water is stillvery confusing. Silica sols for example can ~ o a g u l a t e , ~ but there has been no report until now showing adhesion between silica sheets well-established surface force techniques. The second and the third series of experiments tried to understand these two contrasting behaviors. Silica is well-known to dissolve in water although this effect is complicated and not well understood. The contact of a glass or a silica surface with aqueous solutions produces silicic acid aggregates. The fourth series of experiments aimed to determine the role ofthat silicic acid in the shortrange repulsion by using water free from silicic acid. ( 5 ) Pashley, R. M. J . Colloid InteTface Sci. 1981,83,531-546.
(6)Pashley, R. M. Adv. Colloid Interface Sci. 1982,16, 57. (7)Pashley, R. M.; Israelachvili, J. N. J . Colloid Interface Sei. 1984, 97,446. (8)Saluja, P. P. S. Environment ofIons in Aqueous Solutions;Physical Chemistry; Butterworths: London, 1976;Vol. 6, Chapter I. (9)Peschel, G.;Belouschek, P.; Muller, M. M.; Miiller, M. R.; Konig, R. Colloid Polym. Sci. 1982,260,444-451. (10)Rabinovich,Ya. I.; Dejaguin, B. V.; Churaev, N. V.Adu.Colloid Interface Sci. 1982,16, 63-78. (11)Ducker, W. A.; Senden, T. J.;Pashley, R. P. Nature 1991,353, 239-241. (12)Horn, R. G.; Smith, D. T.; Haller, W. Chem. Phys. Lett. 1989, 162,404-408. (13)Horn, R. G.;Grabbe, A. J . Colloid Interface Sci. 1993,157,375383. ...
(14)Israelachvili, J. N.; Adams, G. E. J.Chem. Soc.,Furaday Trans. 1 1978,74,975-1001.
0743-7463/94/2410-4237$04.50/0 0 1994 American Chemical Society
4238 Langmuir, Vol. 10, No. 11, 1994 Experimental Section The SFA technique allows measurement of the force laws between two molecularly smooth surfaces separated by a fluid. The distance resolution is about 1-3 A, which is achieved using multiple beam interferometry. The surfaces are glued to cylindricallenses of glasswith the cylindricalaxes at right angles. This geometry is locally equivalentto a sphere near a flat surface. The force F between the surfaces of geometric mean radius R (-2 cm) is obtained as a function of their separationD from the deflectionof a spring. The measured force is given in FIR units which can be related to the correspondinginteraction free energy E per unit area between two flat surfaces using the Derjaguin appr0ximation:~5E(D) = F(D)/2nR. Following the "bubblemethod",12 silica surfaces were prepared by meltingand closing the end of a silica tube (Heraeus,Suprasil F100) using an oxy-hydrogen torch (ensuring a carbon-&ee surface) and rapidly blowing it out into a large bubble with a thickness on the order of 1pm. These surfaces had near-atomic smoothness: the root mean square surface roughness was less than 0.5nm over a 30-pm Talystep trace.12 The thinner areas, identified by colored interferences fringes, were then flattened by exposure to the radiant heat of the torch to obtain the geometry necessaryfor use in the SFA. Freshly cleaved mica patches were placed on the surfaces to protect them from any contamination and the samples were then stored in a desiccator until required for an experiment. We note that when the patches are removed, some ofthe potassium ions present on the mica will be transferred to the silica. We calculate that these give rise to at most a M when immersed in solution. concentration of only e 2 x In a dry atmosphere the adhesion force of two such surfaces is comparable with the values obtained with two mica surfaces (-400-500 mN/m). Since silica sheets obtained from the "bubble method" have different thicknesses, the optical cavity is asymmetric and the computationof the distance between the surfaces is then somewhat differentthan for the studies using mica sheets. A full description may be found in Horn and Smith.lG All water used was purified by passing it through an activated carbon bed, ion-exchange, and filtration stages and finally by distillation from a quartz sub-boiling still (Quartz et Silice, France). Before weighing, each salt (ACS reagent grade) was baked for 24 h at a temperature of 300 "C to remove organic contaminants. All the solutions were prepared and stored in Pyrex glassware (no more than 3 days)that has been previously cleaned with boiling nitric acid for a few days and then rinsed with pure water and ethanol and finally dried out with nitrogen. In each experiment, the apparatus was filled with solution through an aluminamembranetype.filter (Anotec),0.02pm pore diameter, under pressure applied by Nz. The pH was measured each time the apparatus was filled or drained. Usually there was an increase of 0.2 in pH from an average value of 5.5. We attribute the value of 5.5 to the presence of dissolved C02 and the increase to the presence of the nitrogen atmosphere above the solution.
DLVO,Hydration, and Measured Forces DLVO theory remains the reference for any starting study in colloid science. Information on short-ranged forces is obtained by calculating and extrapolating the long-ranged DLVO component to short distances, and subtractingthis from the experimental data. This method has its own limitations but it allows the description of the strong short-range forces that are not predicted by DLVO theory: the forces that prevent surfaces from coming into contact. This procedure has often been used in the literature5J3J5and it permits then direct comparison with previous results. The repulsive electrostatic force is often fitted using one of two approximations: either the electric potential or the surface charge density is assumed to be independent ofthe separation. Usually the data lie between the curves calculated using these two limiting cases. This suggests that the degree of dissociation of surfaces groups, and (15) Dejaguin, B.V. Kolloid. Zh.1934,69,155. (16) Horn, R. G.;Smith, D. T. Appl. Opt. 1991,30,59.
Chapel Table 1. Charge Regulation Parameters Derived by Fitting the Double-Layer Force at Long Range for Different Electrolyte and Ionic Strengtha
cs+ 6.46 2.97 0.43
pK(H+) pK(M+)
K+ 6.30 3.2 0.48
Na+ 6.35 3.25
Li+
6.21 3.46 0.46
site area (nm2) 0.50 0.57 0.70 cation area bm2) 0.51 0.53 a To minimize the number of parameters,the effective site area occupied by an adsorbed ion is fixed at the value given by Pashley for mica experiments. The values for NaCl agree well with those reported by Horn and Grabbe.13
thus the surface charge density and potential, are dependent upon the proximity of the other surface. In this study we attempt to include this effect by fitting the data using a charge regulation modell' of the double-layer repulsion and by solving the nonlinear Poisson-Boltzmann equation numerically,'* together with the full Lifshitz calculation of the van der Waals attraction including retardation effects.19 We followed along the line of the original formulation of the charge regulation model of Pashley5 which has been reanalyzed recently by Miklavic and Ninham17 to include explicitly the size of hydration cations. The main idea of the model is that a n adsorbed cation may occupy a larger area AMthan that, As, of a negative surface site (N E 1IAs being the total number of sites per unit area). Taking & = AJAs as the ratio of occupied area to site area by species SiO-, H+,and M+ and 8i = nilN as the fractional coverage of surface by the combined species SiOH and SiOM, it has been shown by Miklavic and Ninham that the surface could be characterized by two coupled equilibria and their corresponding dissociation-association constantsKHand KM as given SiO-
+ H+
SiO- M+
-
-
SiO-H
SiO-M
where [H+Land [M+], are the concentrations of hydrogen and the hydrated metal cation directly near the surface. They are related to the bulk concentration by the Boltzmann distribution:
being the surface potential. To minimize the number of disposable parameters,13 & = AH/& can be set to 1 and AMfured to the values established by Pashley5 for binding of metal cations to mica (see Table 1). Then, apart from the known parameters of pH and ion concentration, this model reduces to three disposable parameters which describe some of the physical-chemical properties of the silica surfaces. (17) Miklavic, S. J.; Ninham, B. W. J. Colloid Interface Sci. 1990, 134, 305. (18) Chan, D.Y.C.; Pashley, R. M.; White, L. R. J.Colloid Interface Sci. 1989,77,283. (19) Mahanty, J.;Ninham, B. W. DispersionF0rces;AcademicPress: London, 1976.
Langmuir, Vol. 10,No. 11, 1994 4239
Hydration Forces between Silica Surfaces
NaCl
0
LiCl
~ o - ~ M
106,
-mvo
'"t' Io5 n
0
1 0 3 ~
+
10'M
A
10'M
0
1 0 . 4 ~
-DLVO 0
10.3~
0
10'M
+
~ o - ~ M
lo4 103
1' 0
1 o2 10'
1 0
'
I
I
20
40
\
I
I
60
80
100
D (nm) Figure 1. Typical force us distancecurve between two pyrogenic silica sheets immersed in a series of increasing NaCl solutions. The force F has been normalized by the surface radius of curvature R to be proportional to the energy E per unit area between two equivalentflat surfaces,accordingto the Derjaguin approximation FIR = 2nE. The solid line is a sum of a full Lifshitz calculation of the attractive van der Waals force and a repulsive electrical double-layer force computed by numerically solving the Poisson-Boltzmann equation. The double layer has been analyzed in terms of a charge regulation model which, given the pH, concentration,dissociationconstants,and area per site, computes the surface potential 1/1 at all separations. Those values are pK(H+)= 6.35,pK(Na+)= 3.25,Area per site = 50 A2 and for each molar concentration: 1.3 x M, pH = 5.79,V O= -90 mV; 1.5 x M, pH = 5.67,1/10 = -83 mV; 0.8x M, pH = 5.28,1/10 = -63 mV 1 x 10-1M,pH = 5.55, 1/10 = -45 mV (1/10 being the potential at infinity). These potentials are higher than those reported by Hornet al. because of the difference in pH.
The three parameters were computed as the best fit to the data. It should be emphasized that, for any given electrolyte, the same set of parameters is used for every concentration. The fitting procedure therefore needed many iterations for each electrolyte. Once the best fit was obtained for the long-range force data, this was subtracted as described above. The difference, which we attribute to the short range force, was then fitted using three different empirical expressions. The first ofthese is a double exponential function, as used by P a ~ h l e y .Of ~ course, this representation should not be taken as the right theoretical form of the short-range repulsion, since within the scatter of the data, a simple power law works better. We used the power law as the second representation. The third representation was then a mixture of the former two, i.e. the sum of a n exponential function and a power law following ref 20.
Results In the first series of experiments we measured the force between pairs of silica surfaces immersed in solutions in which the concentration was increased in steps between measurements. We show in Figures 1-4 the force as a function of distance between the silica surfaces for a range of different electrolyte species and concentrations. For the four electrolytes studied, the behaviors were qualitatively similar. The results agree well with DLVO theory except a t separations of a few nanometers where the predicted van der Waals attraction is overcome by a n extra repulsive force. This force has been described in the literature a s a hydration repulsion qualitatively similar to those observed between lipids or mica s u r f a ~ e . ~ The .~ (20)Basu, S.;Sharma, M.J. Colloid Interface Sci. 1984,165,355.
40
20
0 I
60
100
80
D (nm) Figure 2. Same as Figure 1 but for LiC1, with pK(H+)= 6.21 pK(Li+)= 3.46,area per site = 45.75A2,and for each molar M, pH = 6.10,1/10 = -119 mV, 1.1 x concentration: 1.1 x M, pH = 5.78,1/10 = -104 mV; 1.1 x M, pH = 5.52, 1/10 = -96 mV; 1 x lo-' M,pH = 5.97,1/10 = -86 mV.
KCl
~ o - ~ M
0
__ DLVO 10.3~
0
h
Z
5.
lo4
v
$
io3 1 o2 10'
0
40
20
60
100
80
D (nm) Figure 3. Same as Figure 1 but for KCl, with pK(H+)= 6.35, pK(K+)= 3.3,area per site = 48 A2,and for each molar conM, pH = 5.40,1/10 = 71 mV, 1.6 x low3 centration: 1.9 x M,pH = 5.45,1/10 = -69 mV 1.4x M, pH = 5.10,V O= -51.5 mV; 1.4 x 10-1M,pH = 5.22,1/10 = -32 mV.
CSCl
1 o6
A
1
1 0 4 ~
-DLVO 0
1
M 10'M 10.' M
o4
o3 1 O2 -A I U
I
0
\
l
20
"
'
"
'
40
'
l
"
"
'
60
'
'
I
80
100
D (nm) Figure 4. Same as Figure 1but for CsC1, with pK(H+)= 6.46, pK(Cs+)= 2.97,area per site = 43.29k ,and for each molar concentration: 1.1 x low4M,pH = 5.42,1/10 = -70 mV, 1.1 x M, pH = 5.47,~0 M, pH = 5.30,1/10 = -53 mV; 1.1 x = -65 mV; 1 x 10-l M,pH = 5.71,V O= -54 mV.
force-distance profiles measured upon approach and separation of the surfaces were in each case superimposable. No adhesion was ever detected. It should be noted that there was some variance among samples of silica. Any two samples taken from the same silicon bubble
Chapel
4240 Langmuir, Vol. 10,No. 11, 1994
Hydratation force = Data
- DLVO A exp(-XiD)
-A
+ SIX"
H+
cs+
1o5
K+
1 o5
Na+
h
4
5E l o 4
L+
v
1 o2
1 o2
0
I
I
I
0.5
1
1.5
2
0.5
0
1
I
2.5
3
1
1.5
2
2.5
3
D(t" D(nm)
Figure 5. Hydration force obtained by subtractingthe DLVO force at short range from the data of Figures 1-4, Le., CsC1,
KC1, NaCl, and LiCl electrolyteat a concentration ranging from to lo-' M. The solid lines are empirical power law fits t o the data.
%
Figure 7. Same as Figure 5. The solid lines are empirical (exponential power law) fits to the data.
+
250001
20000
t
Ae
-9
1 o2
-1 0
0.5
1
I
I
1
I
1
1.5
2
2.5
3
D(nm)
Figure 6. Same as Figure 5. The solid lines are empirical double exponential fits to the data.
gave similar results, however the results obtained with silica samples from different bubbles but under otherwise identical conditions gave variations of as much as 10%in surface groups density. In the NaCl graph each electrolyte concentration comes from a different experiment but with silica samples taken from the same bubble. It should be underlined that in the first series of experiments, silica surface has been in contact with a n aqueous solution during several hours before the concentration is raised up to 0.1 M. This feature is the main difference with experiments of the second series described further down. The behavior of Li+is slightly different from that of the other cations: a small van der Waals jump-in is always observed in compression (and jump out in decomposition) followed by a weak hydration repulsion or none at all. The accuracy of the measurement (0.1-0.3nm) prevented any more detailed study of the effect. Figures 5-7 show the excess repulsive forces after subtraction of the DLVO forces from data of Figures 1to 4. Each different symbol represents a given salt and a n electrolyte concentration ranging from to 10-1 M. The magnitude of the force seems to be electrolyte dependent, but within the same electrolyte the force is independent of cation concentration (a least up to 10-1 M). The hierarchy is exactly the reverse of that found by Pashley with mica. Li+ gives the weakest hydration repulsion, Cs+the strongest one (with other ions following the lyotropic series). The size seems to be the pertinent
-DLVO
Hydration Forces between Silica Surfaces
Langmuir, Vol. 10, No. 11, 1994 4241
Table 2. Power Law Parameters Derived by Fitting the Hydration Repulsion at Short Range power law AX-” A x 105Q4N/m) n
H+
Cs+
K+
Na+
Li+
2.36 -1.57
2.16 -1.69
1.78 -2.00
1.87 -2.07
1.43 -2.24
Table 3. Same Legend as Table 2 for the Double Exponential Fit double exponential A1 exp(-XID1) A2 exp(-XIDz)
+
A1 x lo5 Q4Nlm) Di(A) A2 x lo5 Q4N/m) DdA)
H+
cs+
K+
Na+
Li+
20827 9.56 1762818 0.47
15249 7.78 1156363 0.59
10998 5.96 1031039 0.60
10555 5.63 1167155 0.57
1304 9.62 747941 0.77
Table 4. Same Legend as Table 2 for the (Power Law Exponential) Fit
+
exponential and power law A1 exp(-XID) A a - “
+
H+ A1 x lo5 Q4N/m) -105003 Di (A) 1.99 A2 x lo5 (uN/m) 282628 n -1.59
cs+ -234229 0.86 241771 -1.81
K+
Na+
Li+
5213.61 3.68 162761 -2.04
24039.5 3.88 156685 -2.13
-3908 6.45 160388 -2.08
Experiments of the third type started with silica surfaces immersed in pure water, then after a short period ( ~ 2 0 min) the NaCl concentration was raised to 0.1 M. In this case the surfaces showed no adhesion at all, but a hydration repulsion was present (though somewhat lower than that measured in experiments in the first type). Some authors propose that the strong short range force results from the compression of polysilicic acid (“silica hairs”) or silica between silica surfaces.23Horn et al. present evidence that these features are not pertinent to silica prepared by the bubble method. To address this problem, a fourth kind of experiment was conducted to investigate the effect of the silicic acid present in solution on either the short range force or the adhesion. To minimize any contact with glass (which is a source of Si(OH)4) solutions were made using water from a Labconco Purified Water System (Labconco Corp.) and stored in polypropylenebottles. All the results described above were reproduced a t least once, with the Labconco water, for NaC1. These results exclude compression of silicic acid aggregates or formation of siloxane bridges (Si-0-Si) as possible explanation of either the short range repulsion or the adhesion.
Discussion Silica and Mica Behaviors. The results of experiments of the first series illustrate clearly the difference in behavior between silica and mica. For silica, it is the more hydrated cation that produces the weaker force. This difference is not simply compatible with the explanation that the hydration force is the derivative with respect to separation of the energy needed to dehydrate the adsorbed cations on the surface. Furthermore, it should be emphasized that this shortrange force is always present between silica surfaces in “pure” water. With mica, on the other hand, this extra repulsion appears only for a finite concentration of salt, but never in pure water. In experiments conducted with “pure” water the double-layer computation has been (22)Faraday Discuss. Chem. SOC.1978, 65, 44. (23) Tadros, T. F.; Lyklema, J. Electroanal. Chem. 1968, 17, 267.
carried out using the approximation of constant charge density which gives better fits than that of constant potential. This observation could be explained in terms of a slow kinetic of ion exchange at the surface. When the DLVO force (double-layer van der Waals) is subtracted from the data, the remaining force is larger and longerranged than that calculated with any monovalent electrolyte investigated so far. The empirical double exponential representation of the hydration force takes the following form
+
FIR = A l exp(-XID,)
+ A, exp(-XID,)
It is possible to justify the presence of two decay lengths,
D Iand Dz,in the expression by assuming that a t short distances and under a large pressure the cations lose their hydration shell, giving rise to a second regime and then to a second decay length. However, a t high pressure (-25 OOOpN/m) and short distances, the surfaces start to flatten and then the local curvature radius increases. This effect leads to overestimate the real force FIR. It is then wise to take into account only the larger decay length in the discussion. Hydration Repulsion and Silanols. The hydration force which is strongest in pure water, seems to decrease with the hydrated size of the cation. This is not selfcontradictory because Hf plays, in the silica case, the central role and then it should not be taken as a regular cation as in mica experiments. Furthermore, within the accuracy of the experiments (measurements and computation) the repulsion appears to be constant within each ion series for different ionic strengths (lo-*- 10-1M). This statement is in agreement with previous work done by Horn et al. on NaC1. If one considers the range of the hydration repulsion, a hierarchy is also present between each of the salts, although this differencemight be very small. The lithium is the only one that gives a van der Waals attraction. The fit might be here irrelevant. With a simple power law the exponent varies well with each cation type. It seems then that hydration force is associated with silanols on the surface. The key factor is the hydrogen bonding of water to undissociated silanols. Further evidence of this hypothesis is the results given by Grabbe,24 for which the interaction between two silica surfaces bearing very hydrophilic amine groups showed no shortrange repulsion a t all. It could be that here silanols are killed by the silane treatment. I propose that the hydration force comes from the creation of a hydrogen bonding network a t the silanol level. Each cation which replaces a hydrogen ion reduces the extent of network and thus diminishes the repulsion. Lithium is the most hydrated cation and so, in this model, it would be expected to have the most dramatic effect on the repulsion, as is observed. In the case of mica, it acts as a “structure-maker”, and in the silica one as a “structure-breaker”. In this study, the hydration force is approximately constant over the electrolyte range studied (10-1-10-4 M). The higher salts concentration (> 1 M) experiments were impossible to analyze completely because of the presence of impurities. However, from experiments performed with silica sols, it indeed appears that this extra repulsion is approximately independent of concentration a t values below 1M and begins to decrease a t higher concentration^.^^ The charge regulation model tells us what is the surface concentrations of cations versus hydrogens concentration (24) Grabbe, A. Langmuir 1993,9, 797. (25) Yoon, R. H. Private communication.
Chapel
4242 Langmuir, Vol.10,No.11,1994 N
B
I
C
1o
. ~
-H+
HI
-Na’
Li+
1o
.~
1 o.*
A
H’ CS’
10’’
Bulk ion concentration (mol/l)
Figure 9, Surface adsorbed ions densities Cs+,Na+,Li+,and H+on silica surfacecalculatedfrom the chargeregulation model. This graph showsvariation in surface densitiesofthe competing species H+ and M+ as a function of bulk alkali metal ion concentration. The pH is fixed to 5.5 in the computation. The behavior of K+ is very close to that of Na+ and is omitted for clarity reasons. (pH of 5.5 in average). As shown in Figure 9, the surface is mostly covered by hydrogen ions until the bulk concentration reaches 10-2-10-1 M. This is consistent with the observation that the hydration force is constant up to 10-1 M within the same electrolyte. Does the extra repulsion depend on site density? Horn and Grabbe have shown that the hydration repulsion is insensitive to the surface treatments or in other words to the silanol density (respectively in their three silica treated surfaces 1.8-2, 4.6, and 7 sites by nm2). But, for density smaller than 2 site by nm2, the hydration could well depend on the site density. The insensitivity of the extra repulsion to the density of silanol groups appears to contradict the hypothesis that the excess repulsion is due to the structuring of water by silanols. However, the exact dependence of the excess repulsion on the density is not known. Peschelet aL9studied the interaction between two highgrade polished silica surface plates separated by a thin layer of aqueous LiC1, NaC1, and KCl solution. They found that the decay length of the interaction increased in the order LiCl NaCl KCl in agreement with the present work. In order to analyze this effect, they suggested the model which assigns the magnitude of the extra repulsion to the difference of chemical potentials of the cations between the surface layer and the bulk phase. In other words, they assumed that since Li+ is a water structure maker, it should be more “soluble”in bulk water and then less adsorbed on the silica surface. This hypothesis is indeed verified by the charge regulation model used in the present work which shows that lithium is less adsorbed than the other cations on the silica surface (see Figure 9). Comparison between Sols and Blown Silica. To the author’s knowledge, the results ofthe second and third series of experiments are the first experimental evidence of a n adhesion between macroscopic silica surfaces immersed in a monovalent electrolyte. At this point of the discussion it may be worth noting one of the major differences between sols and blown silica. The latter surface has a mixture of siloxanes and hydroxyl groups (at a surface density of %2/nm2). This value is in agreement with a observed contact angle with water of 45”. On the surface of sols, impurities such as Na and K, coming from the synthesis ofthe sols in alkaline m e d i ~ m (26) Brinker, C. J.; Sherer, G. W. Sol-Gel Science; Academic Press: London, 1983.
are presumed to have taken the place of the silanol hydrogen (Si-OH us Si-ONa). The observation in this study of a n adhesion a t high NaCl concentration is then consistent with the proposition that the silica surface is mostly covered by cations, as in silica sols. Similar adhesion forces (-6.1 mN/m) have been observed by Meagher2’ between silica glass spheres (which is likely to have a high concentration of Na+ a t the surface) and a n oxidized silicon wafer immersed in a 1M divalent CaCl electrolyte. Ifwe suppose that hydration force comes from water hydrogen-bonding to silanols, a large amount of Ca2+bound to the dissociated silanols, would prevent hydrogen-bonding of water. The short-range repulsion will then be removed. Between mica surfaces, adhesion that is 1 order of magnitude higher than that reported here occurs a t low salt concentration, where the surfaces are relatively free from cations. A hydration force is observed between mica surfaces in high electrolyte concentrations, leading to the conclusion that adsorbed cations are the origin of the hydration repulsion. Here it turns out to be the contrary, i.e. cations weaken the repulsion created by hydroxyl groups and their hydrogen bonding network. The fact that the strongest repulsion is obtained between silica surfaces immersed in pure water indicates that hydroxyl groups are more efficient than cations a t structuring water adjacent to the surfaces. It seems then likely that the combined effects of a decreasing hydration repulsion due to adsorbed cations, which destroy the structuring effect of hydroxyl groups, and a reduced van der Waals attraction acting through adsorbed cation layers, might be a reasonable explanation for the attraction observed a t 0.1 M. The fact that adhesion occurs only if one starts directly at high concentration is certainly a n intriguing result. Why would a surface that has been transferred directly from air to a high concentration of cations display a completely different behaviour from one that was exposed to pure water first? No definite explanation is available, but it is most likely that some slow kinetics are involved. The experimental observations can be explained by the following model. First, a hydration repulsion is present, but this repulsion is removed or greatly reduced when H+ of the silanol is replaced by Na+. In the presence (in solution) ofboth hydrogen and sodium ions, silica surfaces covered with H+ are the stable state. Once H ions are present, they are not displaced by sodium ions. On the other hand, if sodium ions are present on the surface first (due to a n overwhelming preponderance of Na+ over H+ in the first solution to which pure silica is exposed, or to the presence of surface sodium in less-pure silica or other glass compositions), they can be displaced by hydrogen ions. However, this only occurs on a comparatively slow time scale: some tens of minutes even with the low surface area of silica present in the SFA experiment (-1 cm2in 10 mL of solution or a characteristic length of cm). In colloidal suspension where the characteristic lengths are micrometers, exchange may be very slow indeed. Let us verify whether this model is consistent with the behavior of silica sols (aqueasol). At high and low pH, silica sols remain stable. At a n intermediary pH, they flocculate. At high pH, their surface is highly charged, and there must be a reasonable double-layer stabilization. Between pH = 4 and pH = 7, the surfaces are weakly charged since they are covered with many ionic impurities (Na, K) and then, as explained above, the sol precipitates. At low pH (around the isoelectrical point pH = 2) many ~ , ~if ~not most of the SiONa groups will have been ion(27) Meagher, L. J. Colloid Interface Sci. 1992,152, 293.
Hydration Forces between Silica Surfaces exchanged to SiOH;the hydration repulsion appears then to stabilize the dispersion. Blown silica is hydrophobic. So, in aqueous solution, siloxane groups (Si-0-Si) will hydrolyze3 slowly to produce hydrophilic silanol groups (SiOH). This effect could then explain the time-dependenteffect reported here. A drop of water was deposited on a blown silica sheet and the contact angle measured did not vary over severalhours. The hydrolyze hypothesis is then not a relevant explanation for the adhesion observed in the second series of experiments.
Conclusion The hydration force is always present between silica surfaces in pure water with a magnitude greater than that measured with any electrolyte investigated so far. Indeed, this repulsion is reduced in the presence of any salt, and this reduction is correlated to the hydrated size of the cation. The more hydrated the ion, the weaker the force. This result is the reverse of the result for mica, for which Pashley has shown that adsorbed cations are responsible for the observed short-range repulsion. The latter is interpreted as the energy needed to dehydrate the adsorbed ions. Strength and range of the force decrease with the cation’s state of hydration. For silica, hydroxyl groups present on the surface appear to be responsible for the water structure that gives rise to the observed force;they are more efficient than anything
Langmuir, Vol. 10,No. 11, 1994 4243 else a t creating a hydrogen bonding network. Cations tend rather to destroy the edifice. This model fails to explain why lithium, which enhances the repulsion in the mica case, reduces the repulsion in the case of silica. No definite answer is available right now but it may be that it competes with the hydroxyl groups to impose an order in the surrounding water. This effect would decrease with the hydration number. Within the same electrolyte and up to 10-l M in concentration, the hydration repulsion does not appear to vary. This is another feature that distinguishes the shortrange behavior of silica and mica. I would like to bring to the reader’s attention that the time-dependent adhesion was only seen in the second series of experiments under special conditions,i.e. when the electrolyte concentration is raised up directly a t 0.1 M. In the first, third, and four series of experiments there was always a short-range repulsion preventing any adhesion of the surfaces. The different behavior of silica surfaces and mica surfaces, as revealed in this study, is not entirely surprising given that the mica surface does not H bond to water while silica does, with its silanol groups. This is an issue that clearly needs to be addressed further in a comprehensive theory of the hydration force.
Acknowledgment. The author thanks A. Grabbe, R. G. Horn, D. T. Smith, E. Perez, and J. Wolfe for many helpful discussions.
