Langmuir 1991, 7, 1088-1090
1088
Surface Transition on Silica and Adsorbed Layer Thickness Divergence near the Coexistence Curve of the Silica-Water-2,5-Lutidine System R. Bennes,* M. Privat, E. Tronel-Peyroz, and M. Amara CNRS, URA 330, B.P. 5051, 34033 Montpellier Cedex, France Received September 10, 1990. In Final Form: November 26, 1990 The adsorption on silica of the water-2,5-lutidine binary system was studied in the water-rich side of the coexistence curve in order to find a surfacetransition near the coexistencecurve as expected theoretically. The scaling laws describing the divergence of the adsorbed layer thickness, 1, and the relative adsorption, r, are given and discussed. The nature of the transition is analyzed.
Introduction A few years ago, we studied the adsorption on silica relative to the water-2,6-lutidine (2,6-dimethylpyridine) binary system, near the coexistence curve.1*2We tried to observe a surface transition similar to that predicted theoretically by Cahn? the so-called prewetting transition. This was studied in detail by several physicists (see refs 4 and 5 for review), while only a small amount of experimental data are available.6 We observed a transition by adsorption and ellipsometry on silica; at first sight, it was not of first order as predicted for van der Waals fluids for the prewetting transition. Thus, the observed transition was not the Cahn prewetting transition stricto sensu. In this report, we present another system, silica-water2,5-lutidine, which can be considered to be very similar to the previous one. However, the critical temperature of the mixture is lowered from 34 to 13.1"C, which indicates a strong modification of the molecule-molecule interactions; the 2,5-lutidine appears as more "hydrophobic" than 2,6-lutidine when dissolved in water, and in this sense the molecular interactions are probably of a more dispersive nature. We examined more systematically the adsorption on silica on the water-rich phase side of the coexistence curve (but significantly lutidine-rich phase on silica). The results are analyzed in order to give some experimental scalingrelations when getting closeto the coexistence curve and hence when the adsorbed layer thickens. Experimental Section
t
"C 50
40
30
20
0
.01
.02
x
L
Figure 1. Water-richside of the coexistence curve. The wetting transitionis W.T. = 48 O C . The surface transitionis the line S.T. The critical point (not shown) is TL = 0.0585 and t , = 13.1 O C .
The adsorption on silica (I'zl, relative surface excess of lutidine (2) with respect to water (1))was measured from 15 to 40 O C in monophasic water-rich phase. The coexistence curve is known from the literature' (Figure l),which was checked and verified several times during preliminary observations to determine the equilibrium conditions of the studied system. The silica powder (RhGne-Poulenc)had a specific area of 40 m2/g, checked by BET of nitrogen. The silica was >99% pure and constituted an agglomerate of nonporous sheres having a diameter of 0.13 pm (measured by electron microscopy).
Some runs were made with another silica from RhBne Poulenc, with a specific surface of 540 m2/g and size of 0.25 mm, spherical and highly microporous. Obviously, capillary condensation occurred for the lutidineconcentrations situated near the coexistence curve. The consequence was a large dispersion of results and therefore this material was given up. The samples contain 20 cm3 of solution and an average of 3.5 g of silica.
(1) Privat, M.; Tenebre, L.;Bennes, R.; Tronel-Peyroz, E.; Douillard, J.-M.; Ghaicha, H. Langmuir 1988,4, 1151. (2) Ghaicha, L.;Privat, M.; Tenebre, L.;Bennes, R.; Tronel-Peyroz,
Results and Discussion The relative surface excess r21 (=I' in the following) diverges at all the studied temperatures when approaching the coexistence curve (Figure 2), which is the result of the divergence of the adsorbed layer thickness (seeDiscussion). No jump can be detected on the I' = XL curves; thus, a
E. Douillard, J.-M. Langmuir 1988, 4, 1326. (3) Cahn, J. W. J. Chem. Phys. 1977, 66 (8),3667. (4) Sullivan, D. E.; Telo da Gama, M. M. In Fluid Interfacial Phenomena; Croxton, C. A,, Ed.; John Wiley & Sons, Ltd.: New York,
1986; p 48. (6) Rowlinson, J. S.; Widom, B. In Molecular Theory of Capillarity; Clarendon Press: Oxford, 1984; p 224. (6) Beysens, D.; Esteve, D. Phys. Reu. Lett. 1985,54 US), 2123. (7) Andon, R. L. J.; Cox, J. D. J. Chem. SOC.1952, 4601.
0743-7463/91/2407-1088$02.50/0
After the sample was shaken at least 1 day at the working
temperature, the supernatant was analyzed by UV absorption spectroscopy. The relative surface excess was calculated by a conventional method.*
(8) Schay, G. In Surface and Colloid Science;Matijevic,E., Ed.; WileyInterscience: New York, 1969.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 6, 1991 1089
Adsorption of Water-2,5-Lutidine on Silica
I
t
.oc
bo
x IO
-2 moloxm
30
31.3.C
-
20 : -1y /
Figure 4. Variations of the exponent y ( t ) versus the temper-
ature.
Figure 2. Surface excess divergence at ZL are calculated for each measurement.
-
x ~ . . Error bars
51
precisely to give the experimental laws concerning the thickening of the interface under consideration, the region studied is situated between this surface transition and the coexistence curve. For this, we tried to fit the results with respect to the coexistence curve composition (xmx) at each studied temperature. The imposed constraint is the finding of the transition at the same XL values as the preceeding breaks. The general equation (the simplest possible) describing the results is as follows
r = A X Y ( ~ ) In AX
(1) with hx = XL - xmx as the order parameter. y ( t ) (always negative) depends on the temperature as shown in Figure
x lo* -2 mole. c m
4.
The shape of the curves also depends on the temperature domain (Figure 5). The results can be summarized as follows
15
t < 19 "C, ly(t)l < u; y ( t )
- 0,t
t,
19 < t < 25 " C , y ( t )=-u t > 30 "C,y ( t ) = 0 where v is the critical exponent of the correlation length
10
5.
On the basis of the transition shape, another series of results can be obtained 5
t = 15 "C, no transition 15 < t
I
20
30
40 t°C
-
Figure 3. Fal versus t showing a break corresponding to the surface transition. The example given is at ZL = 7.5 X l t 3 .
priori, we are not dealing with a first-order transition. The present case is similar to the case of 2,6-lutidine. However, when I' is plotted as a function of the temperature t, at constant lutidine molar ratio XL (Figure 31, a break appears separating two adsorption regimes; these breaks correspond to a transition and the locus of these are reported on the phase diagram (Figure points (t, XL-) 1where S. T. is the surface transition). The error on each point of the transition temperature is about i0.5 "C. Previous observations showed that the setting transition is around 48 "C (unpublished result). As the aim of this paper is to describe the phenomena occurring when the coexistence curve is approached, more
< 25 "C first-order transition (?) (see below) t > 30 "C, continuous transition
The change of the y ( t ) value from -u to zero around 30 "C can be explained easily by the failing of the critical laws when too far from t,. This fact was experimentally observed by the surface tension critical laws9 measured on the coexistence curve which effectively applied only from t, to 30 "C. Now the question becomes: how is eq 1related to the known law describing the variation of the interfacial thickness? According to the literature,415for ALL 0 (approaching the coexistence curve from the monophasic domain), for T,< T < T, (between the critical point and the wetting transition tw = 48 "C in our case), the variation of the thickness of the adsorbed layer is5 1 = -E In t (2) where t is a measure of the distance from the stable j3
-
(9) To be published.
1090 Langmuir, Vol. 7, No.6, 1991
phase state (here the lutidine-rich phase) to a slightly unstable @ phase state. p is the chemical potential. This equation was established for a liquid-vapor onecomponent system but, formally, we can translate it in the system under consideration as l=-[hAx (3) By comparison with the mathematical form found for r, we have [ being assumed constant at constant temperature for t > 30 O C , 1 = -In Ax = r (4) for 19 c t c 30 OC, r = h-"I (5) I' varies more sharply than 1 for t = 15 "C where no transition was found, r = Ax4*' I ; r varies more rapidly than 1. The most striking observation lies in the shape of the curves of Figure 5. At 15 "C no transition occurs, which can mean that the surface critical point (t,'"3 is reached at some slightly higher temperature (15 C C 16.8 "C). Maybe the proximity of the surface critical point could explain the nonzero value for y(t), Le., a nonpurely logarithmic divergence of r as the thickness 1 or a new variation of the thickness. A more appropriate scaling should take into account this proximity. In other words, the chosen order parameter, x -xm. is not relevant to the description of the results in the surface critical domain. Of course we do not known the pertinent order parameter, which is certainly a combination of the previous one, Ax, and a surface order parameter. In this case, the theory is lacking. When b(t)l- Y, the transition is located in a very narrow domain of the used variable, Le., A . 4 ) In Ax; we call it first-order transition with respect to this behavior, owing to the sharpness of the jump, although some experimental points are situated on the raising part of the curve. This behavior corresponds to a jump in the surface excess at practically constant thickness and indicates a sudden enrichment in lutidine like a surface demixion.10 Is the variable used here a pertinent one, as seems to be the case? We do not known the reason why the nature of the transition changes around 30 "C. Maybe the surface correlations, if they exist up to 30 "Cand drive the surface demixion, have disappeared.
Conclusion From the present investigation, we can conclude that the lutidine surface excess on silica varies more rapidly (10) Privet, M.;Bennes, R.J. Colloid Znterfoce Sci. 1982.90 (2), 464.
Bennes et al.
dry lag A.
Figure 6. 'I versus b y ( # ) log Ax variation at 15, 19.2, and 31.3 OC. No transition is observed at 15 O C . The transitions at 19.2 and 31.3 "C are indicated by an arrow; we observe two kinds of surface transition.
than the thickness of the interface except for the higher temperature where a logarithmic law is found when the coexistence curve is approached as expected. This logarithmic behavior agrees with finite-range forces as expected at the beginning of this work. The behavior of the surface excess seems to be a little more complicated than that of the thickness, due, in our opinion, to the proximity of a surface critical point or due to a new thickness variation. The region of surface critical point is situated between 15 and 16.8 O C . We shall try to locate it more precisely in the future. Ellipsometric measurements will be undertaken to confirm the existence of a first order transition domain.
