Langmuir 1994,10, 3765-3773
3765
The Structural Microscopic Hydrophilicity of Talc L. J. Michot,*pt F. VilliBras,? M. Franqois,? J. Yvon,t R. Le Dred,* and J. M. Cases? Laboratoire Environnement et Minkralurgie, URA 235 d u CNRS, B.P. 40, 54501 Vandoeuvre Cedex, France, and Laboratoire de M a t t r i a w Mintraux, URA 428 du CNRS, 3, rue Alfred Woerner, 68093 Mulhouse Cedex, France Received March 24, 1994. In Final Form: June 6, 1994@ Talc, Mg3S&010(OH)~is a trioctahedral phyllosilicate with no layer charge. It is macroscopically hydrophobic as it floats naturally. However, water immersion calorimetry measurements reveal that, once outgassed at medium temperatures (100-400"C),it exhibitsa strong affinity towards water molecules. Controlled rate thermal analysis measurements coupled with mass spectrometric analyses show that, upon outgassing, different surface species (water, nitrogen, carbon dioxide, and organic molecules) are released from the talc surface. The behavior of the talc surface toward nitrogen, argon, and water as a function of the outgassing temperature was then studied in detail. The surface presents highly energetic sites for nitrogen and water molecules. These sites are assigned to the hydroxyl groups pointing toward the basal surface of talc through the hexagonal cavity formed by the silica tetrahedra. The adsorption of water and nitrogen on a synthetic fluorinated talc confirms this assignment as the substitution of OH groups by fluorine atoms suppresses the specific interactions. On outgassed talc, the presence of highly hydrophilic sites isolated on a hydrophobic surface controls the adsorption of water which seems to occur through the growth of hydrogen-bonded clusters anchored on the OH groups.
Introduction Talc is a trioctahedral clay mineral with no layer charge which is widely used in industry as a filler. In some of these industrial applications, such as paper coating, the use of talc slurries is limited because of their poor rheological properties. These problems are considered to be due to the natural hydrophobicity of talc. On the other hand, the hydrophobicity of talc is extensively used in the recovery of this mineral, as it allows for the use of natural flotation techniques, i.e. flotation without collectors. Therefore, in its natural state, talc definitely appears as a hydrophobic mineral. As far back as 1940, Fowkes and Harkinsl measured a high contact angle on a polished talc surface. Similar measurements have been reproduced using either the technique of the sessile drop2 or the wicking t e ~ h n i q u e . ~ All - ~ contact angle values were around 80". Structural reasons have been claimed2v6to explain this natural hydrophobicity of talc. The basal faces of talc, being siloxane-like, are hydrophobic whereas the edges are hydrophilic. However, a certain number of experimental facts are rather troubling if talc is a truly hydrophobic material: (i) Different varieties of hydrated talc, the kerolitepimelite series, can be found in nature. These minerals contain between 0.8 and 1.2 water molecule per half formula unit of Mg3Si4010(OH)2. They can easily be synthesized at low temperature (around 150-200 "CI7 whereas talc can be synthesized a t higher temperature
* Author to whom correspondence should be addressed. t
Laboratoire Environnement et Mineralurgie.
* Laboratoire de Mat6riaux Mineraux.
* Abstract published inAdvance ACSAbstracts, August 15,1994.
(1)Fowkes, F.M.; Harkins, W. D. J.Am. Chem. SOC.1940,62,3377. ( 2 ) Schrader, M. E.; Yariv, S. J. Colloid Interface Sei. 1990,136,1, 85-94. (3)Giese, R. F.;Van Oss, C. J.; Norris, J.; Costanzo, P. M. Proc. Znt. Clay Conf., Strasbourg 1990,ZZ, 33-42. (4)Giese, R. F.; Costanzo, P. M.; Van Oss, C. J. Phys. Chem.Miner. 1991,17,611-616. ( 5 )Norris, J.;Giese, R. F.; Costanzo, P. M.; Van Oss, C. J. Clay Miner. 1993.28. 1-11. (6jChander, $.; Wie, J. M.; Fuerstenau, D. W. AZChE Symp. Ser. 1975,71 (150), 183-188. (7)Decarreau, A.;Mondesir, H.; Besson, G. C. R. Acad. Sci. Paris, Sdrie ZZ 1989,308, 301-306.
(around 250 "C). By retromorphosis talc can be transformed into ker01it.e.~Furthermore, a 10A hydrated phase of talc, with one water molecule per unit cell, has been synthesized and described.s (ii)Attempts made to correlate natural flotation results to the electrokinetic properties of talc have not been suc~essful.~ (iii) Immersion calorimetry measurementslo-l2 showed that outgassed talc could not be considered as truly hydrophobic. The aim of this paper is then to study in detail the adsorption of different gases (nitrogen, argon, and water vapor) at the surface of talc outgassed at different temperatures in order to understand the hydrophobic/ hydrophilic behavior of talc.
Experimental Details Materials. The talc sample used in this study was supplied by Talcs de Luzenac SA. It is referred to as T6123 and is a very pure sample obtained by hand sampling from the open-pit mine of Trimouns (Arihge,France). This sample is composed of talc (98%), chlorite (1.5%),and a few other accessory minerals such
as pyrite and r~ti1e.l~ This talc was ground for 80 s in a vibrating Mill Aurec T100. A synthetic fluorinated talc was also used in this study. It was synthesized from a hydrogel with a molar composition of 1.00 SiOz, 0.75 MgO, 0.15 HF, 18 HzO. This hydrogel was prepared by adding under agitation fluorhydric acid (40% solution, Prolabo), magnesium acetate (Mg(CH&00)2, 4 H20, Prolabo), and silica (Aerosil 130, Degussa) to water. The pH is adjusted to 5 using nitric acid. The hydrogel is aged at room temperature for 4 h. It is then autoclaved in a stainless steel reactor covered with polytetrafluorethylene at 220 "Cunder selfgenerated pressure for 10 days. After being cooled to room temperature,the solid is filtered, washed with distilled water, (8) Yamamoto, K.; Akimoto, S. I. Am. J. Sci., 1977,277,288-312. (9)Steenberg, E.;Harris, R. J. Mintek Report no. M 209,1984,18
pages.
(10)Yvon, J. Elements s u r les PropriBtAs Cristallochimiques, Morphologiques et Superficiellesdes MinBraux Constitutifs des Gisements de Talc. These de Doctorat es Sciences Physiques, INPL, Nancy, 1984, 303 pages. (11)Michot, L. J.;Yvon,J.; Cases, J. M.; Zimmermann, J. L.; Baeza, R. C. R. Acad. Sci. Paris, Sdrie ZZ 1990,310,1063-1068. (12)Michot, L. J.;Yvon, J.;Cases, J. M. In Advances in Measurement and Control of Colloidal Processes; De Jaeger, N., Williams, R. A., Eds.; Butterworth-Heinemann: London, 1991;pp 233-246, 1991. (13)Yvon, J.;Mercier, R.; Cases,J. M. Compte Rendu de la convention INPrSTalcs de Luzenac 904-650-290, 1982,32 pages.
0743-7463/94/2410-3765$04.50/00 1994 American Chemical Society
Michot et al.
3766 Langmuir, Vol. 10, No. 10, 1994 and dried at 60 "C for 12 h. The mineral obtained is a fluorinated stevensite which is transformed into a fluorinated talc by calcination at 250 "C. Methods. Thermal analysis of the talc sample was carried out using a conventional thermogravimetric procedure (UgineEyraud B70 balance, Setaram). A 135 mg sample was heated in a platinum crucible from room temperature to 1030 "C, using a heating ramp of 5 "C/min. The controlled rate thermal analysis procedure (CRTA)14J5coupled with a quadrupole mass spectrometerle was also used. In this method the rate of sample outgassing is kept constant over the entire temperature range by means of an appropriate heating loop which results in an a priori unknown temperature program. The flow of gas evolved from the sample submitted to a dynamicvacuum is used to control the heating of the furnace and is directly analyzed by mass spectrometry. A sample mass of 200 mg and a residual pressure of 1.2 Pa were used. The mass spectra were obtained on a Balzers QMG 420 C mass spectrometer. Nitrogen adsorption-desorption isotherms were carried out at 77 K on classical step by step all-glass volumetric equipment. Prior to each experiment zz 1 g samples were outgassed under a residual pressure of 0.1 Pa for 18 h at 25, 150, 250, and 400 "C. Surface areas were determined using the BET treatment. The presence of micropores in the samples was assessed by using the t-plot meth0d.1~The mesopore distribution was determined using the parallel pore model proposed by Delon and Dellyesl* for the case of phyllosilicates. Low-pressure isotherms of argon and nitrogen were recorded on a lab-built automatic quasiequilibrium volumetric setup. 19,20 In this method a slow, constant, and continuous flow of adsorbate was introduced into the adsorption cell. From the recording of quasiequilibrium pressures vs time, the adsorption isotherms were derived. The experimental conditions were a sample mass of %1g, outgassing at 0.001 Pa, and temperatures of 25, 150, 250, and 400 "C. The data were then treated using the derivative isotherm summation (DIS) procedure designed by Villikras et a1.20 to examine the surface heterogeneity of the samples. In this procedure, the experimental derivative adsorption isotherm on a heterogeneous surface is simulated by the sum of local theoretical derivative adsorption isotherms on homogeneous surfaces. The theoretical isotherms used20 are either one layer limited isotherms (Langmuir or Bragg-Williams-Temkins, depending on the lateral interactions between adsorbed molecules) or multilayer isotherms (BET or Hill, depending on the lateral interactions between adsorbed molecules). Each local derivative isotherm exhibits a maximum which is used, together with the full width at half-maximum, to derive three parameters: (i) the normal adsorbate-adsorbent interaction, (ii)the intensity of lateral interactions, and (iii) the amount of gas adsorbed on the homogeneous domain. The simulated derivative adsorption isotherm is obtained by summation of each local model. It is then compared to the experimental isotherm and adjusted by the operator until a satisfactory fit is obtained. Water adsorption-desorption isotherms were recorded at 303 K on an automatic gravimetric apparatus based on a quasiequilibrium adsorption procedure2l built around a Setaram MTB 10-8 symmetrical microbalance. Prior to each experiment 300 mg samples were outgassed under a residual pressure of 0.1 Pa for 18 h at 25, 150,250, and 400 "C. Water was supplied from a source kept at 41 "C through a Granville-Philips leak valve t o ensure quasiequilibrium conditions at all times. The adsorption isotherms (Le. mass adsorbed at 303 K vs quasiequilibrium pressure) were directly recorded on a simple X-Y recorder. (14)Rouquerol, J. J . Thermal Anal. 1970,2,123-140. (15)Rouquerol, J. Thermochim. Acta 1989,144,209-224. (16)Rouquerol, J.;BordBre, S.;Rouquerol, F. Thermochim.Acta 1992, 203, 193-202. (17)De Boer, J.H.; Lippens, B. C.; Linsen, B. G.; Broekhoff, J. C. P.; Van Den Heuvel, A.; Osinga, Th. J . Colloid Interface Sci. 1966,21, 405-414. . .. -. .
(18)Delon, J. F.; Dellyes, R., C. R. Acad. Sci. Paris, Sdrie D 1967, 265, 1161-1164. (19)Michot, L. J.;Franqois, M.; Cases, J. M. Langmuir 1990,6,677681.
(20)Villibras, F.; Cases, J. M.; Franqois, M.; Michot, L. J.; Thomas, F. Langmuir 1992,8, 1789-1796. (21)Poirier, J. E.; Franqois, M.; Cases, J. M. In Fundamentals of Adsorption; Liapis, A. T., Ed.; AIChE: New York, 1987;pp 473-482.
-30 -40
0
200
400
600
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Figure 1. Conventional thermogravimetric analysis of talc. Water immersion microcalorimetry experiments were carried out in order to analyze the wettability ofthe sample as a function of outgassing temperature and water precoverage. Furthermore, such a technique yields the value of the external surface area of the sample through the use of a modified Harkins and Jura procedure.22~23The sample was sealed in a glass cell with a brittle end. After being outgassed, the sample was equilibrated with water vapor at increasing relative pressures and subsequently placed in a cell containing water that was previously equilibrated with the solid. This assembly was introduced into a Calvet differential calorimeter (Setaram M70). The brittle end was then broken, the water wetted the solid, and the corresponding calorimetric signal was recorded. The value of the immersion enthalpy was then obtained.
Results Thermal Analyses. Figure 1 presents the thermogravimetric analysis of the talc sample. The weight loss is very low up to 850 "C, where dehydroxylation ofthe talc sheets begins as classically ~ b s e r v e d . ~ ~ , ~ ~ The weight loss before dehydroxylation is mainly due to the outgassing of surface species which have been analyzed by controlled rate thermal analysis coupled with mass spectrometric analyses (Figure 2). Between 25 and 500 "C, two phenomena can be observed: for temperatures < 100 "C, the main products outgassed from the talc surface are physically adsorbed water and carbon dioxide; at higher temperatures, organic molecules, revealed by the masses 15 and 43 and smaller amounts of carbon dioxide and nitrogen (masses 28 and 14), are released from the talc surface. This second phenomenon is centered around 250 "C and is completed around 350 "C. These experiments (Figure 2) and other mass spectrometric analyses carried out on talc samples26show that the surface species include molecular nitrogen, carbon dioxide, and organic molecules which seem to be long-chain amines and nitriles. ImmersionMicrocalorimetry. Figure 3 presents the immersion enthalpy of talc without water precoverage measured as a function of the outgassing temperature. The following behaviors can be observed. After the solid is outgassed at 25 "C, the value ofthe immersion enthalpy is around 5 J*g-l. It increases up to 100 "C to a value of approximately 9 J-g-l, remains nearly constant up to 250 "C, and increases again up to 400 "C to reach a value around 16 J-g-'. This evolution shows that the initial wettability of talc increases strongly with outgas(22)Partyka, S.;Rouquerol, F.; Rouquerol, J. J . Colloid Interface Sci. 1979,68, 21-25. (23)Cases, J. M.; FranGois, M. Agronomic 1982,2,10, 931-938. (24)De Souza Santos, H.; Yada, K. Clays Clay Miner. 1988,36,4, 289-297. (25)Villibras, F.Etude des Modifications des Propribtes du Talc et de la Chlorite par Traitement Thermique. These de Docteur INPL, Nancy, 1993,568 pages. (26)Michot, L. J.Etude de Quelques PropriBt6s Physico-chimiques et Superficielles du Talc et de la Chlorite. T h h e de Docteur INPL, Nancy, 1990,440 pages.
Structural Microscopic Hydrophilicity of Talc
Langmuir, Vol. 10,No.10,1994 3767 Table 1. Parameters Derived from the BET Treatment of the Adsorption Isotherms of Water Vapor at 303 K on Talc Outgassed at Different Temperatures
1.40E-11
outgassing temp ("C) Qm (mmo1.g-l) 1.20E-11
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Temperature ("C)
Figure 2. Mass spectrometric analyses of the products outgassed from the talc surface during a controlled rate thermal analysis. 20
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0
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100 150 200 250
300 350 400 450 500
OutgassingTemperature ("C) Figure 3. Evolution of the immersion enthalpy of talc samples with the outgassing temperature (precoverage pressure = 0).
sing temperature as the immersion enthalpy value is more than tripled between outgassing temperatures of 25 and 400 "C. On the basis of these results, four different outgassing temperatures were chosen for studying evolution of the immersion enthalpy with water precoverage: 25,150,250, and 400 "C. The curves are presented in Figure 4. Differences in
25 150 250 400
0.207 0.266 0.279 0.225
CBET SBET (m2*g-l) 63 58 53 50
18.4 23.8 24.9 20.1
immersion enthalpy are extremely marked at zero coverage but start fading for higher values. For high outgassing temperatures the sharp decrease of immersion enthalpy at low water coverage reveals the presence of high-energy sites which are screened by the first adsorbed molecules. From 0.4, all the curves are superimposed and reach a plateau at a value of 0.5 J0g-l corresponding to a Harkins and Jura surface area of 4 m2*g-l. Water Vapor Adsorption-Desorption. Figure 5 presents the adsorption-desorption isotherms of water vapor on talc T6123 80s outgassed at different temperatures. The shape of the four isotherms suggests a hydrophilic behavior. The outgassing temperature has a strong influence on these isotherms. Table 1 presents the values of the adsorbed quantities at the monolayer and of the C energetic constant derived from the BET treatment. The energetic constant is always high ( 250). The monolayer quantity increases from 0.207 to 0.279 mmo1.g-l for outgassing temperatures of 25 and 250 "C, respectively. For an outgassing temperature of 400 "C, the monolayer quantity decreases to a value of 0.225 mmol-g-l . The desorption branch of the isotherms is also affected by the outgassing temperature. In the isotherms obtained after the sample is outgassed at 25 and 150 "C, the desorption branch exhibits a hysteresis loop, and all the adsorbed water is desorbed when the relative pressure reaches values close to zero. Samples outgassed at 250 and 400 "C exhibit a different behavior, as the hysteresis loop of the desorption branch is much larger and does not close. All the adsorbed water cannot be removed even by pumping the samples under a vacuum of 0.1 Pa at 30 "C. This means that water molecules remain on the talc surface. The amounts are equal to 0.157 and 0.159 mmol-g-l for samples outgassed at 250 and 400 "C, respectively. These results confirm the immersion enthalpy measurements, as they reveal that the affinityof water toward the talc surface is strongly affected by the outgassing conditions. It increases with the pretreatment temperature and is very high for temperatures 2250 "C. As conventional TGA shows that dehydroxylation of the structure occurs at higher temperature (Figure l), this change in surface properties cannot be due to structural modifications but rather must be due to the desorption of adsorbed molecules evidenced by mass spectrometry. Nitrogen Adsorption-Desorption. Figure 6 presents the adsorption-desorption isotherms of nitrogen at 77 K upon outgassing talc 6123 80s at different temperatures. It reveals the same type of behavior as that for water. The adsorbed quantity increases with the outgassing temperature. The hysteresis loop at the desorption is very small for outgassing temperatures of 25 and 150 "C, which shows that the sample is slightly mesoporous. The size distribution calculated according to a parallel pore modeP reveals mesopores larger than 10 nm which can be assigned to interparticular pores due to the arrangement of the powder. For an outgassing temperature of 250 "C, talc exhibits a large desorption loop which continues down to very low relative pressures and is located roughly 2 cm3*g-l above the adsorption
Michot et al.
3768 Langmuir, Vol. 10, No. 10, 1994
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curves displayed in Figure 7 show that all the changes occur in the very low pressure range, i.e., in the domain of the highest adsorption energies. First, new homogeneous domains appear at very high energy when the outgassing temperature increases up to 250 "C (Figure 7). The adsorbed amount in these domains starts decreasing when the outgassing temperature reaches 400 "C (Figure 7, Table 3).
Discussion All these experiments suggest the following hypothesis: Outgassing liberates some sites at the surface of talc. In the natural state, the surface of talc is covered with some species, preventing the access of gas molecules t o these sites. The accessibility of these superficial sites controls the affinity of talc toward nitrogen and water molecules. Indeed, outgassed talc behaves as a hydrophilic mineral, whereas it exhibits a hydrophobic behavior in its natural state. This very peculiar behavior of talc could be explained by its structure (Figure 8). Among all phyllosilicates, talc is the only mineral with no layer charge and a perfect trioctahedral nature. This results in the hydroxyl groups pointing directly toward the surface inside the cavity formed by the arrangement of the silica tetrahedra of the tetrahedral layer. These OH groups could then exhibit
Structural Microscopic Hydrophilicity of Talc 15ooc
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Langmuir, Vol. 10, No. 10, 1994 3769
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a particular reactivity toward polar molecules. Computer simulations of the adsorption of water on the basal surface of talcz7reveal one single high-energy site located just above the OH group in the silica tetrahedra. Figure 8 also presents the result of the simulation, using the MCY modelz8for a water molecule located in a specific orientation above one OH group. The binding energy of water in this site is equal to 21.6 kcaVmol whereas the binding energy is equal to ~ 1 . kcaVmo1 0 (i.e. hydrophobic) on all the other sites of the basal faces. These hydroxyl groups could also exhibit a particular affinity toward nitrogen molecules because of their quadrupolar moment. It is then possible to reexamine the nitrogen adsorption data. Table 3 displays the results obtained from the DIS procedure. The surface areas were calculated on the basis of a cross sectional area of 0.162 nmz for the nitrogen molecule. Three domains (domains 4-6) appear to be nearly unmodified by the outgassing procedure. In all cases, the cumulative surface area corresponding to the two less energetic domains (domains 5 and 6) is equal to approximately 16 mz*g-l, which corresponds exactly to the nonmicroporous surface area (27) Skipper,N. T.;Refson, K.;McConnell,J. D. C. Clay Miner. 1989, 24, 411-425. (28) Matsuoka, 0.; Clementi, E.;Yoshimine, M. J.Chem.Phys. 1976,
86, 2592-2601.
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Table 2. Parameters Derived from the BET and t-Plot Treatment of the Adsorption Isotherms of Nitrogen at 77 K on Talc Outgassed at Different Temperatures
25 150 250 400
0.4
on talc outgassed at different temperatures.
Table 3. Energetic and Monolayer Parameters of the Different Homogeneous Domains Obtained from the Application of the Derivative Isotherm Summation Procedure to the Adsorption Isotherms of Nitrogen at 77 K on Talc Outgassed at Different Temperatures
outgassing domain 1
25°C
150°C
250°C
400°C
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-13.7 0.35 1.53 -12.8 0.37 1.62 -11.9 0.51 2.23 -10.1 0.49 2.14 -7.8 0.88 3.85 -5.5 2.80 12.23 23.5
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obtained by the t-plot treatment (Table 2). It is then possible to assign these two domains as corresponding to the edge faces and basal faces, respectively. The basal faces and edge faces then correspond to surface areas of approximately 12 and 4 mzg-l, respectively. The other high-energy domains can then be tentatively assigned to the OH groups inside the cavity formed by the arrangement of silica tetrahedra. The volume adsorbed in these sites would then be equal to 0.84,1.22,1.72, and 1.28 cm3*g-' for outgassing temperatures of 25,150,250,
Michot et al.
Vol. 10,No.10, 1994
3770 Langmuir,
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Ln(P/Po) Ln(P/Po) Figure 7. Results of the DIS procedure applied to the adsorption isotherms of nitrogen at 77 K on talc outgassed at different temperatures. and 400 "C, respectively. These values agree with the Further evidence can be obtained by studying the microporous volumes determined by the t-plot treatment adsorption of argon on the talc surface at very low relative (Table 2). The maximal value of this particular volume pressures. Figure 9 presents the results of the DIS (1.72cm3g-') corresponds to 4.62 x lo1'nitrogen molecules procedure for argon adsorption isotherms carried out after per gram of talc. According to the proposed assignment the talc sample is outgassed at 25,150,250,and 400 "C, of domain 6, the surface area of the basal faces of talc is and Table 4 presents the numerical values obtained. As equal to mag-' From the CWStallograPhiCal data in the case of nitrogen and water, the surface area that each hexagon given by R a p e r and B ~ o w I Iit, ~appears ~ increases with the outgassing temperature up t o a formed by the arrangement of silica tetrahedra has an temperature of250 "c. However, the new sites appearing area Of 0*25nm2* One gram of then represents 4*8 between outgassing temperatures of 25 and 150 "C are x1019 sites. The near Perfect agreement located at medium energy and not, as in the case of between the two values corroborates the assignment of nitrogen, at high energies. the high-energy domains to the OH sites of the basal faces As in the case of nitrogen adsorption, domains 3 and 4 of talc. can be assigned to the adsorption on the edge and basal (29) Raper, J. H.; Brown, G. Clays Clay Miner.1973,21,103-114. faces of talc, respectively. Taking into account a CrOSS +
Langmuir, Vol. 10,No.10,1994 3771
Structural Microscopic Hydrophilicity of Talc
:j 50 40
c L 0 E
30
-30
I
I
I
I
I
outgassing domain 1
25°C ln(PIP0) -10.9 Vads (Cm3*g-l) 1.17 S, (mZg-9 4.3
2
ln(PIP0)
3
S,(m2*g-1) ln(PIP0) -7.1 Vads (cmag-l) 1.02 S,(m2.g-I) 3.8
4
In(PIPo)
vada
(cm3-g-')
Vads (Cm3'g-')
S,(mZg-l)
-42. 2.72 10.1 18.2
150°C 250°C -11.5 -11.9 0.94 0.70 3.5 2.6 -9.0 -9.5 0.69 0.74 2.6 2.7 -6.7 -6.9 1.01 1.1 3.7 4.1 -4.1 -4.1 2.72 2.84 10.1 10.5 19.9 19.9
400°C -12.0 0.44 1.6 -9.6 0.62 2.3 -6.8 1.15 4.3 -4.0 2.93 10.9 19.1
total sectional area of 0.138 nm2 for the argon molecule, the edge faces (Table 4, domain 3) then account for x4 m2-g-' (as in the case of nitrogen) and the basal faces (Table 4, domain 4) for ~ 1 0 .mag-'. 5 This slight difference between the surface areas of the basal faces measured for argon and nitrogen can be explained, as the value of 0.138 nm2 has been recommendedfor hydroxylated surfaces,3Owhich is not the case for the basal faces of talc. For such surfaces, a value of 0.165 nm2 for the cross sectional area of argon would certainly be more a d a ~ t e d . 3On ~ the basis of 0.165 nm2,the surface area of the basal faces equals 12.5mag-', which corresponds to the value derived from nitrogen adsorption (Table 3, domain 6). Domains 1and 2 can be assigned to the adsorption of argon molecules on the OH of the basal faces of talc. The maximal value of the adsorbed volume in sites 1and 2 is obtained upon outgassing a t 150 "C and is equal to 1.63 (30) Rouquerol, J. Contribution 1'Etude par Adsorption Gazeuse de la Texture de Solides Divisks. Application B l'Alumine, B la Glycine et h Diffkrents Gels d'Oxydes. These de Dodorat d'Etat, Pans, 1965,90 pages.
(31)Gregg, S.J.;Sing, K. S.W.Adsorptwn,Surface Area andPorosity, 2nd Ed.; Academic Press: London, 1982;303 pages.
cm3.g-l, which is very close to the value of 1.72 cm3g-' obtained in the case of nitrogen. It then seems that the behavior of the talc surface is mainly controlled by the presence and accessibility of the hydroxyl groups. In nonoutgassedtalc, most ofthese sites are hindered. Outgassing at sufficiently high temperatures (2250 "C) renders these sites accessible and completely alters the adsorption of polarizable molecules. Further evidence of the capital role played by the OH groups can be obtained by examining the adsorption of nitrogen and water at the surface of a fluorinated talc in which most of the OH groups have been replaced by fluorine. Figure 10 presents the adsorption-desorption isotherm of nitrogen at 77 K on a synthetic fluorinated talc outgassed at 250 "C. In contrast to the natural talc sample, the fluorinated talc exhibits no low pressure hysteresis. This sample has a BET surface area of 98 mzg-'. It is microporous with a microporous volume of 11.3 cm3*g-l (as obtained from the t-plot) and a nonmicroporous surface area of 52 mag-'. The large hysteresis loop reveals the presence of some mesoporosity. The size distribution calculated accordingto a parallel pore model18 reveals two types of mesopores: mesopores larger than 10 nm which can be assigned to interparticular pores and mesopores around 3 nm which can be assigned to growth irregularities on edge faces. The adsorption-desorption isotherm of water vapor at 303 Kon the fluorinated talc outgassed at 250 "C is typical of a hydrophobic material (Figure 10). Almost no water molecules adsorb at the surface at low and medium relative pressures. The adsorbed quantity increases only for a relative pressure of 0.85, which indicates capillary condensation, which is confirmed by a desorption loop that closes for a relative pressure of 0.63. These experiments on fluorinated talc confirm, as do the computer simulations,2' that the Si-0-Si basal surface oftalc is truly hydrophobic and that the structural OH groups of natural talc are the only hydrophilic sites of the basal faces. Water adsorption isotherms (Table 1) suggest that much more than one water molecule per OH site adsorb on the surface. Indeed, the BET monolayer obtained in the case of water vapor adsorption on talc outgassed at 250 "C (Table 1)is equal to 0.279 mmo1.g-l of water, whereas one water molecule per OH group represents only 0.08 mmo1.g-'. As in the case of nitrogen, and argon to a lesser degree, the first water molecules will adsorb on the discrete highly hydrophilic OH groups. As the binding energy of water to the silicon and oxygen sites is lower that the binding energy in the liquid, the following water molecules are likely to adsorb on the first water molecules through hydrogen bonding. This hypothesis can be tested by plotting, as in the DIS procedure, the first derivative of the water adsorption isotherm vs the natural logarithm of relative pressure. The result of this treatment is shown in Figure 11in the case of the adsorption of water vapor on talc outgassed at 250 "C. As in the case of nitrogen, this plot features maxima and minima which reveal a distribution of discrete energetic domains of adsorption. However, due to the design of the adsorption apparatus, derivative water adsorption isotherms are not as precise as nitrogen ones. This prevents a precise calculation of the adsorbed amounts on each domain. Therefore, the values given in Table 5 are estimates, as they are obtained from the position of the minima only. The sum of the amount of water adsorbed on each domain correspondsto the "monolayer"quantity obtained by the BET treatment (Table 1). As said previously, the most energetic domain (domain 1)can be assigned to the adsorption on the OH sites ofthe basal faces. It is maximal
Michot et al.
3772 Langmuir, Vol. 10, No. 10,1994
1.6 1.4
150°C
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1
i
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Ln (PIPo)
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0.8 0.6 0.4
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Figure 9. Results of the DIS procedure applied to the adsorption isotherms of argon at 77 K on talc outgassed at different temperatures. Table 5. Monolayer Parameters of the Different Homogeneous Domains Defined on the Derivative Adsorption Isotherms of Water at 303 K on Talc Outgassed at Different Temperatures
total
25 0.120 0.035 0.030 0.040 0.025 0.040 0.290
150 0.170 0.065
250 0.160 0.060
400 0.140 0.040
0.035 0.055 0.050 0.375
0.045 0.065 0.075 0.410
0.030 0.040 0.075 0.320
for outgassing temperatures of 150 and 250 "C,where it corresponds to the adsorption of two water molecules per site. The two steps cannot be distinguished due to the
low relative pressure of water vapor and to the precision of the apparatus. For an outgassing temperature of 25 "C, all the sites are not liberated, which explains the lower value. Domain 4 is nearly constant whatever the outgassing temperature. With the assumption of a cross sectional area of 0.148 nm2 for water molecules adsorbed on hydroxylated surfaces, the surface area corresponding to this domain accounts for 3.6 m2.g-'. It can then be tentatively assigned to the adsorption on the edge faces. In that case, the remaining domains (domains 2 and 3) would correspond to the adsorption of a third water molecule on the basal surfaces of talc, suggesting the growth of water "clusters" over OH groups. After the "monolayer" completion, derivative isotherms show additional large steps (Figure ll),indicating further cluster growth.
,-.
Structural Microscopic Hydrophilicity of Talc Nitrogen
0.2
0
0.4
0.6
0.8
Water Vapor
0
1
Langmuir, Vol. 10, No. 10, 1994 3773
0.2
0.6
0.4
0.8
1
P/Po
PIP0
Figure 10. Nitrogen adsorption-desorption isotherm at 77 K and Water adsorption-desorption isotherm at 303 K on fluorinated talc outgassed at 250 "C. 0.3
0'25 0.2
I
I I
0.1
4
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/
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3
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.I
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Figure 11. Derivative adsorption isotherm of the water adsorption isotherm at 303 K on talc outgassed at 250 "C.
The special behavior of the surface of talc toward water is again illustrated by the evolution of the immersion enthalpy as a function of the precoverage relative pressure.
Indeed at 250 "C, the curve exhibits two well-defined steps (Figure 4) which confirm the additional adsorption after monolayer adsorption. It is also worth noting that the immersion enthalpy decreases extremely sharply, revealing the presence of highly energetic adsorption sites which are saturated at very low relative pressures. In this case, the modified Harkins and Jura procedure,22which allows the calculation of the external surface area from the asymptotic branch of the curve immersion enthalpy vs precoverage relative pressure, does not apply. Indeed, 0 yields an external the value at the plateau ( ~ 0 . 5 Jag-') surface area of 4.2 m2.g-l. Such a peculiar behavior reveals that the organization of water molecules at the surface of talc differs from water films observed on hydrophilic surfaces. The observed difference could also be due to the fact that the Harkins and Jura formula assumes the contact angle of water to be equal to zero, which could be totally false in the case of a precovered talc surface. On the basis of the nitrogen and argon adsorption results, the external surface area of talc is equal to 16 mzg-l, which can be decomposed into 3.5 mzg-' of edge faces and 12.5 msg-l of basal faces. If one assumes the contact angle on the edge faces to be equal to 0, then their contribution to the immersion enthalpy for water precoverage pressures around 0.8 would be 3.5(0.1195) x 0.4 Jg-l. Therefore, the signal arising from the basal faces would be around 0.1 J0g-l whereas a complete wetting wouldyield avalueof12.5(0.1195)= 1.5Jog-'. The cosine of the contact angle can then be obtained by dividing the experimental value by the theoretical one. This calculation leads to a contact angle 6 x 86", which is close to the values previously ~ b t a i n e d . l - ~
Conclusion The surface behavior of talc is mainly controlled by the OH groups of the octahedral layer, which point directly toward the basal surfaces, because of the perfect trioctahedral nature of the mineral. In the natural state, organic and inorganic species are adsorbed on the talc surface and screen these highly energetic sites. Outgassing at sufficiently high temperatures, i.e. 250 "C, eliminates the superficial adsorbed molecules. Talc then behaves as a special material as very strong interactions occur between the OH groups and polarizable molecules. It can then be considered as microscopically very hydrophilic. On such a surface, water adsorbs through the growth of hydrogen bonded clusters over the OH sites. Spectroscopic studies are in progress to further characterize the state of adsorbed water on talc.
