Langmuir 1997, 13, 1337-1341
1337
Characterization of Quartz Particle Surfaces by Immersion Calorimetry† H. Malandrini, R. Sarraf, B. Faucompre´, S. Partyka, and J. M. Douillard* LAMMI, URA 79 CNRS, Universite´ Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France Received November 7, 1995. In Final Form: March 22, 1996X A quartz sample has been studied by adsorption and immersion methods after a classical characterization of its properties. The combination of adsorption isotherms and of immersion measurements allows the calculation of enthalpies and entropies of adhesion. The studied quartz is slightly hydrophobic but the immersion enthalpy into water is the most energetic one of all the solvents studied. These results are explained by the influence of intermolecular forces such as acid-base interactions in the interfacial layer.
Introduction In petroleum well engineering, the importance of wettability has long been recognized.1 It is a key parameter governing residual oil saturation in oil recovery. When linked to the porosity of the rocks, wettability controls the distribution of fluids. Consequently, it affects measurements used for the assessment of oil reserves and for the prediction of oil production. In this view the actual problem is the poor scientific knowledge of the interactions between crude oil and reservoir rocks. Despite numerous studies, accurate values of surface interactions between liquids and solids, of such types present in reservoir wells are still required. Moreover one of the major thermodynamic parameters linked to the physical wetting problem, i.e., the surface Gibbs free energy of solids, is quite impossible to measure in actual conditions. It has been determined in only a few cases for solids.2 Even its estimation by empirical methods is difficult. Quartz is one of most important subsurface minerals, and it has been described in detail.3-5 In this paper, the interfacial properties between quartz and water or organic solvents have been studied by vapor adsorption isotherms and immersion microcalorimetry. These results allow the enthalpy, entropy, and free enthalpy of adsorption, immersion, and adhesion of some quartz/liquid pairs to be calculated. Moreover it is possible to use all these thermodynamic values to predict the surface properties of a quartz sample. We have interpreted our results using * To whom correspondence should be addressed: fax, 67 14 33 04; e-mail,
[email protected]. † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakis, September 4-10, 1995. X Abstract published in Advance ACS Abstracts, September 15, 1996. (1) Cuiec, L. Evaluation of reservoir wettability and Its Effect on Oil Recovery. In Interfacial Phenomena in Oil Recovery; Morrow, N. R., Ed.; Marcel Dekker: New York, 1990; p 319 (2) Schrader, M. E. In Modern Approaches of Wettability: Theory & Applications; Schrader, M. E., Loeb, G., Eds.; Plenum Press: New York, 1992. (3) Dove, P. M.; Rimstidt, J. D. Silica-Water Interactions. In Reviews of Mineralogy; Heaney, P. J., Prewitt, C. T., Gibbs, G. V., Eds.; Mineralogical Society of America: Washington, DC, 1994; Vol. 29, p 259. (4) Parks, G. A. Surface Energy and Adsorption at Mineral/Water Interfaces: an Introduction. In Reviews of Mineralogy; Hochella, M. F., White, A. F., Eds.; Mineralogical Society of America: Washington, DC, 1990; Vol. 23, p 133. (5) Iler, R. K. The Chemistry of Silica; John Wiley and Sons: New York, 1979. (6) Lee, L. H. In Contact Angle, Wettability and Adhesion; Mittal, K. L., Ed.; VSP: Utrecht, 1993; p 45.
S0743-7463(95)01014-6 CCC: $14.00
“component of surface tension” theory6 in order to predict the interfacial behaviour of quartz interacting with any liquid. Experimental Methods 1. Materials. Solid Sample. The C800 sample is a natural quartz and has been supplied by Sifraco (France). Before use, the solid was washed with hydrochloric acid (2 N) at boiling temperature, for 2 h. It was then washed two times with distilled water. The average particle diameter as determined by scanning electron microscopy (with a Cambridge Serial 100 microscope) is below 15 µm. One observes rough surfaced conchoidally fractured grains. The specific surface area (Asp) is estimated by the volumetric determination of nitrogen adsorption (Figure 1) at 77 K and application of BET method. The selected area σi occupied per molecule at monolayer coverage taken for the nitrogen molecule is 16.2 Å2. The best values obtained by linearization are 5.2 m2‚g-1 for Asp and 146 for the CBET constant.7 The experiment is performed with an Analsorb 9011 device. Before measurement, the sample is kept under vacuum (better than 10-3 Torr) at 150 °C for 10 h. Liquids. Organic solvents (Merck, France) were high-purity grade (better than 99.5%). Prior to the experiments, they were dried with molecular sieves in order to eliminate water. Water was distilled after deionization. 2. Apparatus. Adsorption. The adsorption isotherm of water vapor was performed on the C800 quartz with a homemade multigas adsorption device, built around a M.K.S. gauge and a Sartorius SD3V balance ((0.1 µg). The solid sample was maintained before the experiment at a temperature of 150 °C, under a vacuum better than 10-5 Torr, for 5 h. The precise experimental conditions and results are presented elsewhere.8 Microcalorimetry. The experimental device was a Tian-Calvet differential calorimeter described in detail elsewhere.9 The accuracy of the calorimeter is better than 3%. The solid sample is put in a glass bulb, closed by a brittle tail, and heated at 150 °C under vacuum. The glass bulb is then placed in a calorimetric cell. The cell is filled with the immersion liquid. The insulation of the cell is maintained by a ring seal. When thermal equilibrium is obtained, the brittle tail is broken, and liquid enters the bulb. The experiments have been repeated three times. The values reported below are mean values. All the experiments were performed at 300 K. The solid sample can be submitted, before the immersion experiment, to a partial pressure of the studied liquid. The partial pressure is driven by a temperature gradient. The temperature of the glass bulb which contains the solid sample during the coverage process is maintained at the temperature of the calorimetric experiment. The vapor pressure in the bulb is maintained by a glass solder before the immersion experiment. (7) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area & Porosity; Academic Press: London, 1982. (8) Sarraf, R. Thesis, Montpellier Sciences University, 1994. (9) Partyka, S.; Rouquerol, F.; Rouquerol, J. J. Colloid Interface Sci. 1979, 68, 30.
© 1997 American Chemical Society
1338 Langmuir, Vol. 13, No. 5, 1997
Malandrini et al.
Figure 1. Adsorption isotherm of nitrogen on quartz at 77 K.
Thermodynamics 1. Surface Pressure. The adsorption of vapor on a solid surface is accompanied by a decrease of the surface free enthalpy. This decrease is the film pressure πS, given by the following equation at constant temperature and pressure:
πS ) γS° - γSV
Figure 2. Adsorption isotherms of water (black triangles) and benzene (black circles) on quartz at 25 °C and desorption isotherms (water, open triangles; benzene, open circles) in the same conditions.
(1)
in which γS° is the surface tension of the solid in equilibrium with its own vapor and γsv is the solid/vapor surface tension. If the variation versus the pressure of the adsorbed vapor quantity on a solid surface is known, one obtains the film pressure, by the use of the Gibbs adsorption equation
πS ) RT
P Γ ln P ∫P)0
(2)
where Γ is the Gibbs surface excess of the adsorbate. It is interesting to compute two other quantities. Namely the work of adhesion, as defined by Hiementz10
WSL ) γLV(cos θ + 1) + πe
(3)
where θ is the contact angle of the drop liquid on the solid surface and πe is the equilibrium film pressure. The word “equilibrium” in this designation refers explicitly to the fact that the adsorbed molecules are in equilibrium with a sessile drop of bulk liquid. The second one is the Gibbs free energy of immersion in the pure liquid11
-∆immG ) πe + γLV cos θ
(4)
We notice that, for high-energy solids (i.e., when the liquid spreads on the solid), the term cos θ is equal to unity).12 2. Immersion Enthalpy-Adhesion Enthalpy. The immersion calorimetric experiment can be performed under three different thermodynamic conditions: the powder particles are partially, fully, or not at all covered by vapor molecules of the immersion liquid. First, when the solid particles are immersed in the liquid from the vacuum, carrying no film with them, the relationship by unit area is
(
-∆immH ) γS° - γSL - T
)
∂(γS° - γSL) ∂T
P
(5)
where ∆immH is the enthalpy of immersion.11 (10) Hiementz, P. C. Principles of Colloid & Surface Chemistry; Marcel Dekker: New York, 1986. (11) Douillard, J. M.; Elwafir, M.; Partyka, S. J. Colloid Interface Sci. 1994, 164, 238. (12) Adamson, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1976. (13) Jura, G.; Hill, T. L. J. Am. Chem. Soc. 1952, 74, 15987.
Figure 3. Adsorption isotherms of water (black triangles), benzene (black circles), and n-heptane (black squares) on quartz at 35 °C and desorption isotherms (water, open triangles; benzene, open circles) in the same conditions.
Second, the powder particles are partially covered by vapor molecules of the immersion liquid. The difference between the enthalpy involved (∆WH, the immersional wetting enthalpy14) and the immersion enthalpy of the outgassed solid (∆immH) is related to the adsorption enthalpy of the vapor molecules (∆adsH) of the immersional liquid during the coverage process
∆adsH ) ∆immH - ∆WH
(6)
When the vapor pressure is large, the Young equation (i.e., γLV cos θ ) γSV - γSL) is verified. In the case of perfect wetting one obtains
-∆WH ) γSV - γSL - T
( )
) γLV - T
∂γLV ∂T
(
)
∂(γSV - γSL) ∂T
P
) HL
P
(7) (8)
where HL is the surface enthalpy of the liquid.7
By definition,11 one has
-∆adhH ) -∆immH + HL
(9)
where ∆adhH is the enthalpy of adhesion. Experimentally, when the heat involved during the immersion is measured for a solid covered with a film in the conditions of eq 8, the following equation, due to Jura and Harkins15 is verified
-(∆WH)exp ) AspHL
(10)
where Asp is the specific surface area of the solid and (14) Dekany I.; Nagy L. G. J. Colloid Interface Sci. 1991, 147, 119.
Surface Interactions of Quartz Particles
Langmuir, Vol. 13, No. 5, 1997 1339 Table 2. Enthalpy of Immersion of the Quartz in Various Solvents, T ) 300 K
Figure 4. Number of adsorbed layers vs the partial pressure for water (black circles), benzene (open triangles), and n-heptane (cross) on quartz at 35 °C. Table 1. Parameters Obtained from a BET Analysis of the Adsorption Isotherms at 25 and 35 °C T ) 25 °C liquid
T ) 35 °C
benzene water n-heptane benzene water
Γm (µmol‚m-2) CBET σi (Å2‚molecule-1) πe (mJ‚m-2)
3.23 12 51.4 36
9.16 26 18.1 120
1.91 5 87 19
3.10 14 54 35.5
9.47 25 17.5 128
(∆WH)exp is the experimental (exothermic) enthalpy. Hence the immersion calorimetry appears as an alternate method to determining the specific surface areas. Results and Discussion 1. Adsorption. The vapor adsorption isotherms of water (at 25 and 35 °C), benzene (at 25 and 35 °C), and n-heptane (at 35 °C) are reported in Figures 2 and 3. For the low vapor pressure range (0.05 < P/P0 < 0.3), it is possible to apply the BET method7 in order to compute the CBET solid-liquid interaction constant and the adsorbed quantities at monolayer coverage (Γm). These results are reported in Table 1. The CBET constant describes the affinity between the vapor and the outgassed solid surface. One observes that n-heptane has the less energetic interaction with the quartz sample at monolayer coverage and that water shows the largest one. This is confirmed by the behavior of the isotherms. We notice that CBET increases with the adsorbate polarity. It is possible to use the specific surface area (Asp) of the quartz sample, determined by the classical BET method applied to nitrogen vapor adsorption isotherms, to estimate the cross sectional areas (σi for the i molecule) of the adsorbed molecules in the monolayer state, according to the relationship σ ) Asp/Γm. The equilibrium film pressures (πe) for the three liquids studied on the quartz surface are integrated between 0 and P0 using the vapor adsorption isotherms, according to eq 2, and they are also reported in Table 1. The πe values show an affinity gradation for the different liquids vapors versus the solid surface. It is also possible, starting from the adsorption isotherms, to compute the number of adsorbed layers, n, dividing the total adsorbed amount by the monolayer value Γm defined above (Figure 4). Surprisingly, the number of layers follows the same trend for the three liquids. In fact, the total thickness of the film is not the same because the three molecules do not have the same molar volumes, but the trends observed show that the Gibbs free energy of the layer (i.e., πs ) depends on the liquid, whereas the film structure does not depend on the liquid. (15) Jura, G.; Harkins, W. D. J. Am. Chem. Soc. 1944, 66, 919. (16) Douillard, J. M.; Malandrini, H.; Zoungrana, T.; Clauss, F.; Partyka, S. J. Therm. Anal. 1994, 41, 1325.
liquid
-∆immH, mJ‚g-1
-∆immH, mJ‚m-2
squalane cyclohexane decane n-heptane hexane cyclohexene benzene pyridine water
445 459 518 541 549 784 817 1649 1993
85 88 99 104 105 151 157 317 383
Table 3. Surface Enthalpy of the Water, Benzene, and n-Heptane, Free Enthalpies, Enthalpies, Entropies of Immersion, Adsorption and Adhesion Data at 300 K, Expressed in mJ‚m-2 n-heptane
benzene
water
HL -∆immG -∆immH -∆immS
50 39 104 65
70 65 157 92
118 202 383 181
-∆adsG -∆adsH -∆adsS
19 54 35
36 87 51
130 264 134
-∆adhG -∆adhH -∆adhS
60 154 94
94 227 133
275 502 227
2. Immersion. In Table 2 the results of the immersion of quartz C800 in various solvents are reported. The value, in m2, is obtained by dividing the original experimental values by the BET specific surface area. These results follow three standard trends:11 the enthalpies of immersion vary according to the molar volume of the liquids; the shape of the molecule has an influence; and the enthalpies show a strong dependence on the polarizability of the liquids. Consequently, the cycloalkanes give relatively low enthalpy values. 3. Free Enthalpies. In the case of the three solvents water, n-heptane, and benzene, we have performed both adsorption isotherms (vapor phase) and enthalpy of immersion experiments. We have reported in Table 3 the computed values obtained for the enthalpies of adsorption, adhesion, and also the corresponding free enthalpies and entropies as defined by eqs 4, 6, and 9. The HL have been obtained from thermodynamic tables. There are some discrepancies between the value of the term (∂πS/∂T), obtained alternatively from direct measurement of adsorption isotherms and from comparison of enthalpies and free enthalpies. For water, the first value is 0.8 and the second is 0.47. From our point of view, the less precise value is the one deduced from the isotherms, the difference between the two temperatures being too low. These results show the same trend as the immersion enthalpies. It must be noticed that the Gibbs free energies have the same order of magnitude as the entropies. This is a confirmation of a hypothesis we have made before, allowing the calculation of the solid surface tensions, starting from immersion microcalorimetry.16-18 The main point here is the fact that it is too simple to define solid surfaces as “hydrophobic” or “hydrophilic”. The results obtained with benzene show that a so-called “hydrophilic” solid such as quartz can develop large surface interactions with some hydrocarbons. It is better to define the interactions between solids and liquids starting from more precise variables, such as intermolecular forces. (17) Zoungrana, T.; Douillard, J. M.; Partyka, S. J. Therm. Anal. 1994, 41, 1287. (18) Douillard, J. M.; Zoungrana, T.; Partyka, S. J. Pet. Sci. Eng. 1995, 14, 51.
1340 Langmuir, Vol. 13, No. 5, 1997
Malandrini et al.
Figure 5. Variation of the immersional wetting enthalpy vs the number of layers for water and quartz at 25 °C.
4. Surface Heterogeneity. Quartz appears to be a good example of a mineral with surface heterogeneity. The surface of any silica can be described as composed of patches of interacting adjacent SiOH groups, isolated SiOH groups alternating with patches of siloxane bridges. The relative populations and reactivities of the different patches vary from one silica sample to another, and in the case of quartz depend on the position of the groups: on the edges of the particles or on the faces. Immersional wetting microcalorimetry in water appears to be a convenient tool to study the surface heterogeneity of insoluble divided solids. But to understand what is the heterogeneity described by this experiment, it is necessary to describe the free enthalpy process. The immersional wetting experiment is similar to the contact angle experiment. It is the difference between two thermodynamic states that is described, i.e., the difference from the same reference state: the solid/liquid interface tension γSL. When the solid is homogeneously covered by the layer of vapor, the solid/vapor surface tension γSV is the same all over the surface. One measures the difference between the solid vapor and the solid/liquid surface tensions and one knows that this difference is equal to the liquid vapor surface tension of the wetting liquid (this is the Young equation)
-∆WG ) γSV - γSL ) γLV
(11)
When the solid is not fully covered by the vapor layer, patches of dewetted solid remain in some places. These patches are characterized by a surface tension which is the tension of the solid in equilibrium with its own vapor (γSo). Assuming, for simplicity, that the surface is divided between area fractions θ* and (1 - θ*), we may write
-∆WG ) θ*(γS° - γSL) + (1 - θ*)(γSV - γSL)
(12)
Of course, the immersional wetting enthalpy follows the same trend, being approximately proportional to the immersional wetting free enthalpy. Therefore, the variation shape of the immersional wetting enthalpy versus the number of layers indicates wether or not the coverage is homogeneous. If it is the case, the immersional wetting enthalpy must vary linearly between the immersion enthalpy value and the surface enthalpy value. We report in Figure 5 the variation of the immersional wetting enthalpy versus the number of adsorbed layers. These variations can be described by two straight lines. The surface enthalpy value HL is obtained for about 1.2 layers. Hence the experimental results do not show an heterogeneity. It is probably due to the poor precision of our results in the low precoverage range. These results, in agreement with literature results,9 show that the immersional wetting enthalpy method needs very precise results to be used in the analysis of the heterogeneity.
5. Discussion. Because the immersion enthalpy thermodynamics are very similar to contact angle thermodynamics, it appears very fruitful to use the models derived in this last field. But it is important to stress the differences between the physical conditions of contact angle experiments and immersion experiments. First the classical contact angle experiment is performed with plates, whereas the immersion experiment is performed with divided solids. Hence it is only possible to compare immersion results with contact angle results obtained by using the Washburn equation, which links the capillary height through a column of powder to the contact angle.12 Second it must be emphasized that in the Young equation, it is the solid-vapor interfacial tension which is involved. In the case of the enthalpy of immersion, it is the solidvacuum tension which is involved, and in fact the contact angle appears only as a correction term in the less important parameter of the immersion equations (see eqs 4 and 5). That is why it is not possible to obtain solidvacuum tension values starting from contact angle experiments. The more powerful “nonthermodynamic” model applied to interpret contact angle results is the “surface tension component” model. The first attempt to interpret the surface tension in terms of additive components has been made by Fowkes19 and his explanation in terms of dispersive components has shown conclusive results in the case of the interface between hydrocarbons and other liquids. The discrepancies between this theory and the experimental results concerning contact angles, adsorption isotherms, and immersion calorimetry have been later interpreted in two ways. Some authors20 have considered that the summation route followed by Fowkes (i.e., the Berthelot equation) is wrong, implying some differences between theoretical and experimental results. Other authors21 have preferred to introduce in Fowkes’ theory a new parameter called Φ, taking into account the difference between dispersive results and actual results. These different choices have driven Neumann22,23 and van Oss24 in two different directions. On one hand, Neumann and co-workers, seeing that the interactions between three phases are generally led by a three-term equation, where at least two terms are unknown, i.e., the solid/liquid interfacial tension and the other solid/fluid interfacial tension, thought it was possible to find another equation that links the three terms of the seminal equation. This point has been demonstrated thermodynamically.25 Such an equation has been found and has been called the “equation of state”. Many results have been obtained by this approach, but it must be noted that the “equation of state” is not in its present form applicable to high-energy solids, and then to our immersion results. In fact, one of the assumptions of this approach implies that for perfect wetting, the solid/liquid surface tension is nil. This point is not in agreement with our experimental results. On the other hand, van Oss and co-workers have attributed the values of Φ to another component of the surface tension, due to acid-base interactions, following the Gutman approach.26 This way belongs to a general analysis of the surface tension, in terms of polar compo(19) Fowkes, F. M. Ind. Eng. Chem. 1964, 5, 40. (20) Wu, S. J. Phys. Chem., 1970, 74, 632. (21) Girifalco L. A.; Good R. J. J. Colloid Interface Sci. 1957, 61, 904. (22) Neumann, A. W.; Good, R. J.; Hope, C. J.; Sejpal, M. J. Colloid Interface Sci. 1974, 49, 291. (23) Spelt, J. K.; Absolom, D. R.; Neumann, A. W. Langmuir 1986, 2, 620. (24) van Oss, C. J.; Giese, R. F., Jr.; Good, R. J. Langmuir 1990, 6, 1711. (25) Li, D.; Gaydos, J.; Neumann, A. W. Langmuir 1989, 47, 3956.
Surface Interactions of Quartz Particles
Langmuir, Vol. 13, No. 5, 1997 1341
Table 4. Surface Tension Component Values of the Different Solvents
heptane benzene water
γLLW (mN‚m-1)
γ L(mN‚m-1)
γ L+ (mN‚m-1)
γL (mN‚m-1)
19.6 27.8 21.5
0 1.9 25
0 0 25
19.6 27.8 71.5
nents and apolar component, developed recently by many authors.27 However, the van Oss et al. method to conjugate surface tension components is an original one, and many experimental results seem understandable following this route, even those obtained using the immersion technique. The results reported here are an indirect proof of the “components of the surface tension theory”. At first approximation, the solid surface tension is the most important term in the immersion enthalpy. Then the immersion enthalpy results must trend to a constant value for the same solid, the solid/liquid interfacial tension being considered by many authors as negligible. If one computes the solid surface tension starting from the immersion in water results, one obtains a solid surface tension value of some hundreds of mN‚m-1. Yet quartz does not engage this whole tension in its interaction with alkanes where (-∆immH) is below 110 mN‚m-1. Hence the solid surface tension seems to depend on the liquid. The simplest explanation is the idea of a surface tension divided into components. These immersion calorimetry results can be explained by the existence of three components. In the van Oss hypothesis the surface tension is due to three terms: an apolar component, an acid component, and a basic component. We have tried to apply these ideas to our results. The van Oss model defines the surface tension between two phases S and L in the following manner
γSL ) γS° + γL - 2(γS°LWγLLW)1/2 2(γS° + γL-)1/2 - 2(γS° - γL+)1/2 (13) where γiLW is the “Lifshitz-van der Waals” component of the phase i surface tension, γi+ is the “acid” component (acceptor effect) of the surface tension of the phase i, and γi- is the “basic” component (donor effect) of the surface tension of the phase i. If one puts the van Oss equation into eq 4 written in the form
∆immG ) γSL - γS°
(14)
one gets
-∆immG ) -γL + 2[(γS°LWγLLW)1/2 + (γS° + γL-)1/2 + (γS° - γL+)1/2] (15) Assuming that the variations of the different components versus the temperature are the same, it is possible to define all the parameters of the equation, using three different molecules as probes. We have chosen water, n-heptane, and benzene. We have calculated the different (26) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum Press: New York, 1978. (27) Staszczuk, P.; Janczuk, B.; Chibowski, E. Mater. Chem. Phys. 1985, 12, 469. (28) Douillard, J. M.; Berrada, A.; Zoungrana, T.; Partyka, S.; Toulhoat, H. In Physical Chemistry of Colloids and Interfaces in Oil Production; Toulhoat, H., Lecourtier, J. Eds.; Technip: Paris, 1992. (29) Riddle, F. L.; Fowkes, F. M. J. Am. Chem. Soc. 1990, 112, 3259.
Table 5. Surface Tension Components and Total Surface Tension Values for the Solid γSLW (mN‚m-1)
γS+ (mN‚m-1)
γS(mN‚m-1)
γS° (mN‚m-1)
43
90
140
267
quartz
surface component values for the quartz sample starting from polar and apolar values for pure liquids as determined by van Oss. These values are listed in Tables 4 and 5. Then it is possible to estimate the immersion enthalpy values corresponding to the immersion of quartz in other liquids. This has been done with pyridine, using the assumption discussed above.16-18 The experimental results listed above suggest a value of 25 mJ‚m-2 for the LW component and of 5 and 6.5 mJ‚m-2 for the acid and basic components. These results are in good agreement with the results obtained on silica.11,28 But there is not agreement with some arguments of Fowkes,29 because he denies in his analysis the influence of the basic character of pyridine on its surface tension. However our estimation seems in good agreement with the basic properties of pyridine. Anyway this estimation is given to indicate a general trend of reasoning. The immersion technique is better able to estimate the solid surface tension, more than it is to determine the polar character of a liquid. Conclusion The whole results show a gradation of affinity between the quartz studied and the different solvents. All the energetic terms related to the adhesion between water, n-heptane, benzene, and the quartz have been determined. The decrease in surface tension due to water vapor adsorption is around 130 mN‚m-2, a relatively low value. Even though this result shows the studied silica is weakly hydrophilic, water is the most energetic solvent adsorbed. The influence of delocalized electrons and of acid/base interactions on the immersion process has been pointed out. It has been shown that the enthalpy of immersion has the same order of magnitude as the energy of adhesion for hydrocarbons and that these two values follow the same trend. This could be important information for engineers. The organic solvents studied have been shown to be less adsorbed on silica than water, in pure liquid conditions. At this step it appears that it is possible to compute directly the solid surface tension of a solid, and consequently the other ones: solid-vapor surface tension and solid-liquid surface tension. It can be noticed that the film pressures of three different components on the same solid are sufficient to obtain all these parameters. But in fact the important value needed to obtain the surface tension is the equilibrium film pressure, and this value is approximated, by using the integration of adsorption isotherm, as is well-known in surface chemistry. To go further in the estimation of solid surface tensions requires further developments of the theory of surface components. In all cases, the first step is the measurement of the adsorption isotherms of correctly chosen probe molecules, an area in which we are continuing our work. Acknowledgment. This work has been supported by a Research Foundation CNRS/ARTEP. We thank T. Palermo, K. Trabelsi, L. Cuiec, and H. Toulhoat from the French Petroleum Institute (I.F.P.) for helpful discussions. We also thank the referees for their helpful comments. LA951014Z
