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Surface characteristics of aluminas in relation with polymer adsorption
Surface Characteristics of Aluminas in Relation with Polymer Adsorption EUGI~NE PAPIRER *'l JEAN-MARC PERRIN,* BERNARD SIFFERT,* AND GI~RARD PHILIPPONNEAUt *Centre de Recherche sur la Physico-Chimie des Surfaces Solides, C.N.R.S., 24, Avenue du Prrsident Kennedy, 68200 Mulhouse, France, and tAluminium-Pdchiney, BP 54, 13541 Gardanne, France Received July 12, 1990; accepted November 20, 1990 Inverse gas chromatography (IGC) was applied to the characterization of the surface properties (dispersive component (3,D) of surface energy, and acid/base interaction parameter (Isp)) of a series of aluminas differing not only in specific surface area, but also in content of impurities (Si, Ca, Na, Mg). It is shown that both ~s° and Isp are very sensitive in the presence of impurities. The adsorption of poly (vinyl chloride) and of poly( methyl methacrylate ) from a butanone solution on the different samples was investigated. The amount of polymer which adsorbs on the powder depends on the acid/base characteristics of both polymers and A1203 samples. © 1991AcademicPress,Inc. INTRODUCTION
An important step in the production of technical ceramic (electronic substrates and multilayer capacitors) is the preparation of green tapes ( 1). These green tapes are obtained by tape casting and drying a slurry which is composed of a ceramic powder (alumina or barium titanate . . . . ), a solvent, a plasticizer, a dispersant, and a binder. The goals of this study are first to identify and to understand the physicochemical phenomena which are at the base of the preparation of the slurry, and second to master the elaboration of ceramics. The slurry is difficult to study because of its complex composition and the lack of precise knowledge of the properties of the powder and of the other numerous organic compounds. To yield good properties, after sintering, the powder should be well dispersed in a stable slurry. The stability of the slurry is governed by the nature and the intensity of the interactions between the constitutents. Hence, in the first step of this work, an attempt is made to characterize the surface of some o~-aluminas used for ceramics in terms of interaction potential. On the one hand, an adequate method To whom correspondence should be addressed.
is developed, with a given alumina sample, to measure its surface energy and acid/base interaction potential, which are defined later. On the other hand, the method is then applied to a collection of aluminas of various granularities and chemical compositions. The method used is inverse gas chromatography (IGC), which has been already applied with success to various powder or fiber surfaces (2). Finally, the quantities measured by IGC are related to the polymer adsorption capacities of the aluminas. I. EXPERIMENTAL
L1. Aluminas Several aluminas (from Aluminium-Prchiney (A, C, C', D, E) and one from Alcoa (B)) were studied. In addition to commercially available aluminas P 152 S B and P 172 S B, Pechiney provided several special production aluminas with impurities controlled at various levels to verify if the nature and the amount of impurities have a significant influence on the surface energy and the specific interaction parameters of these aluminas. Table I shows the specific surface area values and the impurity contents of the various samples. The
263
Journalof ColloidandInterfaceSci,~nce,Vol. 144,No. I, June 1991
0021-9797/91 $3.00 Copyright© 1991 by AcademicPress,Inc. All rightsof reproductionin any formreserved.
264
PAPIRER
L3. Polymer Adsorption
TABLEI Main Charac~fi~csofAluminas BET (m2/g)
A B C C' D E
10.6 6 9.3 11.7 10.8 2.4
CaO/AI203 SIO2/A1203 Na20/A1203 MgO/AI203 (ppm) (pprn) (ppm) (ppm)
2 130 580 600 210 610
45 290 630 990 620 1900
ET AL.
<100 790 550 <150 710 150
<10 330 730 890 800 <10
surface areas vary from 2.4 to 11.7 m2/g. Impurity contents, essentially Ca, Si, Na, and Mg, are also very different. For the moment, the exact nature of oxides or of possible combinations of these elements remains unknown. One of the samples (C') has been washed with water in order to decrease its Na content.
L2. Inverse Gas Chromatography For this study, a gas chromatograph (Intersmat, Model IGC 120 DFL) with a highly sensitive flame ionization detector was used. Stainless steel columns, 30 cm long and 3 mm in diameter, were filled with about 1.5 g of alumina. Alumina granules of the recommended size (250-400 urn) were obtained by crushing and sieving alumina pellets obtained by compression in an IR die. Helium was used as carrier gas, at a flow rate of about 20 ml/ min. The solutes, purchased as puriss, grades from Aldrich, were used without any further purification. The amounts injected are very small ( 10 tsl of vapor) in order to meet the infinite dilution conditions of GC. The net retention volume (Vn) was calculated taking the retention time (tr) of methane, which is practically not retained on the support, as the reference. For symmetrical peaks, tr corresponds to the peak maximum whereas for nonsymmetrical peaks, t~ is calculated from the firstorder moment of the peak. Before all measurements, alumina samples were conditioned at 423 K, for 16 h, under a He flow. Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991
A total of 0.5 g of A1203, dried during 24 h at 373 K, was dispersed in 25 cm 3 of a polymer solution. The mixture was then stirred in a thermostated water bath (+0.1 K) for 48 h. Subsequently, the A1203 particles were allowed to decant and the supernatant solution was analyzed by gel permeation chromatography (GPC) after proper calibration. Two polymers were investigated: poly(vinyl chloride) (PVC) which is representative of an "acidic" polymer, and poly(methyl methacrylate) (PMMA) which has "amphoteric" properties. PVC was purchased from Ugilor and has a molecular weight, determined by GPC, equal to 100.000. PMMA was synthesized in suspension (3) and has about the same molecular weight. The common solvent for the two polymers is butanone (SDS). This solvent, mixed with ethanol, is currently used for the preparation of tapes.
L 4. Small Angle X-ray Scattering The amount of polymer which will adsorb on the alumina surface is dependent on the respective affinities of the polymer for the solvent and for the solid, but is obviously also related to the size of the polymer coils in butanone. Since PVC and PMMA have quite different chemical structures, but similar molecular weights, their sizes in butanone first must be measured before definite conclusions on their adsorption capacity on A1203 can be drawn. This was done by performing small angle X-ray scattering experiments on polymer solutions at concentrations equal to 2, 0.4, and 0.08 g/liter for PMMA and concentrations equal to 0.2, 0.04, and 0.008 g/liter for PVC. A Huber 701 X-ray diffusion system (Austria) was used. The apparatus was calibrated using silica (Aerosil 130 from Degussa, Germany). The mean diameter of PMMA was found to be equal to 87 + 7 ~,, whereas that of PVC is equal to 73 + 7 A. It is seen that both polymer coils have comparable sizes in butanone. Hence, eventual differences in adsorption capacities are only accounted for by different
SURFACE CHARACTERISTICS OF ALUMINAS polymer-solvent and polymer-surface affinities. II. SURFACE PROPERTIES OF ALUMINAS Before describing the experimental results, it might be useful to briefly recall some concepts of surface energy.
alkanes (also the &G2 ) vary linearly with the number of carbon atoms of the n-alkanes (6). Therefore, it is possible to calculate an incremental value, i.e., the free energy of adsorption of a CH2 group, which does not depend on the choice of the reference state, AGcH2 =
II.1. Dispersive Component of the Surface Energy There are two types of interactions of the injected probe with the chromatographic support: - - T h e interactions which are due to London or dispersive forces (instantaneous dipoles). These interactions always occur whatever partners are present. They are called dispersive, nonspecific, or universal interactions. - - T h e interactions which arise from all other types of forces. These forces include polar, acid/base, and H-bonding interactions which depend on the partners. They are therefore termed specific interactions. Consequently, the surface free energy (ms) is considered as a sum of two components: the dispersive component (m~) and the specific component (rasP), ms =
+
According to Fowkes (4) the energy of interaction between the chromatographic support (AlzO3) and an alkane which is able to exchange only dispersive interactions is equal to 2(m~msD) ~/z, where m~ is the dispersive component of the surface energy of the alkane. The affinity of an alkane for alumina is related to Vn: the higher affinity corresponding obviously to a higher Vn. Thermodynamically, affinity is described by &G. For an alkane probe, it is known (5) that - A G a = R T l n Vn + C, where -zXG~ is the variation of standard free energy of adsorption of the probe and C is a constant depending on the arbitrary choice of the reference state of the adsorbed molecule. It is a c o m m o n observation, in gas/solid chromatography, that the logarithms of Vn of
265
Vn+l
R T In - -v . '
where Vn+l and Vn are the net retention volumes ofn-alkanes having (n + 1 ) and n atoms of carbon. Dorris and Gray (7) compared AGcH2 with the interaction energy calculated using Fowkes' equation [ \1/2 AGcH2 _ 2/mcuamsD/ N . aCH2 J
m
In this equation, the quantity N . aCH2 converts free energy units (mJ mo1-1) into free surface energy units ( m J . m -2). N is Avogadro's number and acn2 the cross-sectional area (6 ~2) of an adsorbed -CH2-group. This value was tested and adopted by Dorris and Gray (7). INCH2is the surface energy of a solid made up only of-CH2-groups, i.e., poly ( ethylene ). For PE, msD = 35 m J / m 2 at room temperature. Hence, everything is known or measurable in the previous equation except the quantity of interest, i.e., msD. As an example, Fig. 1 displays the variation of the free energy of adsorption versus the number of carbon atoms of the alkanes used to probe the surface properties of
18 14
RT.in Vn (kJ / mole)
C7f/,~ C
10 6 2 -2
rl c
I 4
I 5
I 6
I 7
I 8
F/G. 1. Variationof the net retentionvolumeof alkanes, on alumina A, measured at 383 K, with their number of carbon atoms. Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991
266
PAPIRER
alumina A. A linear relationship is recorded from which the value of 7 D s is computed. The corresponding value of T~ is equal to 104 _+4 m J / m 2. This demonstrates the very high sensitivity of IGC for 7 sD measurements of powders.
II.2. Specific Interaction Parameter (I~p) Several attempts to evaluate the specific interaction capacities of aluminas were made. For instance, Healy et aL (8) made IR spectroscopy and microcalorimetric investigations using basic probe molecules (acetonitrile, pyridine, 2-6 lutidine, and n-butylamine) to identify the acidic sites on various alumina samples. Major differences were attributed to the H-bond formation capacity of A1203 samples. Yet the proposed method does not consider basic surface sites. The same is true for the approach taken by Michels and Dorsey (9). They also titrate the surface acidity of aluminas by adsorption of pyridinium betaine and measurement of diffuse reflectance spectra, the behavior of the betaine being very sensitive to environment and showing a pronounced solvatochromic effect. With this method, the influence of thermal activation and water deactivation could be demonstrated. Boudreau and Cooper (10) used gas-solid chromatography to obtain adsorption isotherms and subsequently to evaluate them in terms of energetic heterogeneity. Several probes were tested: chloroform (proton donor), pyridine (proton acceptor), and dichloromethane (for dipole interactions). However, they did not consider the contribution of London-type interactions in the adsorption phenomena as is done in the present paper. The specific component of the surface energy can, in principle, be obtained by IGC at infinite dilution when using polar probes instead of alkanes. Often, specific interactions are ascribed essentially to acid/base interactions (11 ). Indeed it is known that the acid/base interaction energy greatly exceeds that of pure Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991
ET AL.
polar interactions. Hence, when acid/base interactions are occurring between a solute and alumina, the possible polar contribution may be neglected. This raises the problem of the definition of acid/base scales of organic molecules. For this study, the semiempirical scale ofGutmann (12) was adopted for two reasons: the amphoteric character (possibility for a given molecule to exchange both acidic and basic interactions) of most polar molecules is taken into account, and enough data on the most common chromatographic solutes are available. According to Gutmann, an acid is defined by its ability to attract electrons (acceptor number: AN) and a base by its ability to release electrons (donor number: DN). For example, due to the strong electronegative properties of chlorine atoms, CHC13 can be considered as an acid since the C atom becomes electron-depleted. Diethyl ether, on the contrary, possesses an O atom rich in electrons which is willing to release its electron density: diethyl ether is a base. When a polar solute is injected in the GC column containing alumina, both dispersive and specific interactions will take place. Yet, only one retention time or retention volume will be recorded. The problem is now to extract, from this sole observation, two parameters: one corresponding to nonspecific and the other to specific interactions. When the net retention volume (or AG °) versus the vapor pressure of the injected solutes is plotted, all points corresponding to alkane probes fall on a straight line whereas the corresponding points of polar probes, interacting with a polar chromatographic support, are located well above the alkane line. By definition (13), the specific interaction parameter (Isp) of a given probe is given by the deviation of the experimental point from the alkane line as schematically shown in Fig. 2. Figure 2 gives the example of alumina A from Prchiney. The Isp is equal to 4.2 kJ/mol in the case of CH2C12 adsorption. The I~p of a base probe (ether) is much higher, a result which points out the rather acidic character of the alumina surface.
SURFACE CHARACTERISTICSOF ALUMINAS
22 . 18 . 14.
RT.In Vn (kJ / mole)
C8
TABLE II
ACETONE
TOLUENE ".q" ~ ~ _ "
Dispersive Component of the Surface Energy (-ysD, mJ/m2) of Alumina Samples, Measured at 373 K
C2H5-O-C2H5
~.~,.~.
BENZENE
267
,~ Experiment number
10
C7
6
~ C6
2 2.5
E 2.75
I 3
Mean value of
CH2CI2 I
I 3.25
~ " ~ l o g C5 ~ I I 3.5 3.75
Sample
1
2
3
4
A
104 86 67 96 86 161
100 88 65 99 90 169
102 86 65 96 88 158
96 89 62 95 86 162
(Po) 10 I 4
FIG. 2. Variation of the net retention volumesof solutes on sample A, measured at 383 K, with the logarithm of the saturated vapor pressure: definition of Isp.
Further, Isp values may be used to calculate KA (acidity) and KD (basicity) of alumina according to the following relationship (14), assuming that the alumina surfaces exhibit amphoteric properties: /so = KA(DN) + KD(AN). Such a procedure has already been applied to the study of glass fibers (14), silica ( 15 ), and other mineral oxides. One plots/so/AN versus D N / A N , where the D N and A N values of the probes are known from G u t m a n n ' s measurements.
H.3. Acid~Base Index (~2) A more qualitative description of acid/base characteristics of solid surfaces is possible through the measurement of f~ (16). [2 is the ratio of net retention volumes of a base molecule (ether) and of an acidic molecule (dichloromethane). It allows the detection of variations of acid / base properties of aluminas using just two probe injections. Suppose that the A1203 sample has, for instance, a strong acid interaction potential; then the value of ~2 will be rather high since obviously the interaction (or V~) of the basic probe (ether) will be important. On the contrary, ~2will take low values for base-like surfaces. ~2 can thus become a sensitive indicator of the variation of the surface properties of the alumina samples
B
C C' D E
100 ± 4 87 ± 2 65 ± 3 96 ± 3 88 ± 2 162 ± 7
according to their origin, chemical composition, and possible modification.
II.4. Influence of Impurities Table II exhibits the 7 ~ values (expressed in m J / m 2) for the various alumina samples, measured four times, at 373 K and the mean value observed at that temperature. The experimental error does not exceed 7 m J / m 2. Indeed important variations of 3'D are observed. It appears thus, as expected, that the impurities have an influence on the value of q/s°, a point which so far has never been analyzed quantitatively. Table 111 shows the KA and KD values, defined before, and the quantity ~2which is equal to V~/C2Hs- O-C2H5
f~--
V~H2CI 2
TABLE III KA, KD and ft Values for the Various Aluminas Sample
KA
KD
£
A
5.6 7.5 9.5 9.9 8.6 9.9
2.1 2.3 2.6 2.6 2.2 2.7
6 20 57 83 44 51
B
C C' D E
Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991
268 10
PAPIRER ET AL. KA
C
• •
10o "l"
80
5 4
C'
E
].
C
~
E
A i 250
i 500
i 750
i 1000
i 1250
i 1500
SiO2 ppm i 1750 2000
FIG. 3. Variation of KA with the amount of SiO2in the alumina samples. It is seen from the values of KD that there is no major change in basicity of the samples according to their content of impurities. But for the acidity and for ~2, differences are significant. The change in acidity may be related to the SiOz content since it is known that silica and silicoaluminate surfaces behave rather acidically (17 ). Figure 3 shows the variation of KA versus the amount of SiO2 in the alumina samples. The experimental points fall on a curve, suggesting that KA reaches a constant value for a SiO2 content of about 1000 ppm. Whereas KA describes the acidic character of A1203, the value of ~2gives more global information on the acid/base properties of the samples. The acid/base parameter f~ (Fig. 4) first increases with increasing amounts of SiO2 and then decreases, indicating that the presence of the other impurities cannot be ignored and may become prevalent. Further studies are necessary to come to a better understanding of the most important effect of impurities which definitively will determine the polymer adsorption capacity of ceramic powders as will be seen later.
0
250
500
750
1000
1250
1500
1750
2000
FlG. 4. Variation of f~ with the amount of SiO2in the aluminas.
that surface properties and polymer adsorption will also be related. In fact, polymer adsorption from a solution has received much attention since this phenomenon is determinant for numerous applications (18). The nature of the polymer, the molecular weight, and the molecular weight distribution, and also of the solvent and, obviously, the solid surface, strongly influences the adsorption process. The competition between the polymer and the solvent for the adsorption on solid surfaces has been explained by their difference in acid/base interaction potential (19): strong solvent-solid interaction will hinder the polymer adsorption. In order to verify the adsorption capacity of the various alumina samples, experiments were carried out either with an acidic (PVC) or an amphoteric ( P M M A ) polymer. The solvent used was butanone. Butanone, like PMMA, is an amphoteric molecule. It ob-
o.5
q (rng/m2)
u
0.4
~
PMMA
0.3
III. RELATION BETWEEN SURFACE PROPERTIES OF ALUMINAS AND POLYMER ADSORPTION
0.2
c (rng/I)
0
The results obtained so far clearly demonstrate the contribution of impurities to the adsorption potential of aluminas. It is expected Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991
PVC
0.1 I
I
i
i
100
200
3oo
400
5oo
FIG. 5. Adsorption isotherms of PMMA and PVC on alumina A.
SURFACE CHARACTERISTICS OF ALUMINAS viously interacts with the polymer since it dissolves it, but butanone will also interact with the surface of alumina: a competition will take place. Figure 5 compares the adsorption isotherms of PVC and P M M A on alumina A: the adsorbed a m o u n t is expressed per unit surface area. It can be seen that, even though the polymers have similar molecular weights and dimensions, they lead to different adsorption isotherms: the PVC adsorption is significantly lower than the P M M A adsorption. This result suggests that butanone adsorbs more strongly on A1203 A than PVC does. This observation is consistent with the I G C observation, demonstrating the very acidic nature of this alumina. As we would also expect, the shape of the isotherm depends on the alumina sample. For instance, Fig. 6 compares the adsorption isotherms of P M M A on aluminas A and C which have different surface properties. According to the IGC results (cf. Table III), samples A and C have comparable KD values, but the latter sample is more acidic: it has a higher KA value. This is also expressed by the f~ values. Higher acidity means higher affinity for the basic PMMA. Qualitatively the polym e r adsorption behavior of aluminas is in line with their surface properties. Some further examples are given in Table IV. It is seen that P M M A adsorption follows the variation of 9 (the more acidic surfaces) corresponding to the higher P M M A uptake.
q (rng/m2)
1.4 1.2 1 0.8 0.6 0.4 02 0
r
a
{
200
g
I 400
/
i
~
600
)
I
I
800
| 000
1200
FIG. 6. Adsorption isotherms of PMMA on aluminas A and C.
269
TABLE IV Maximum Amount of Polymer Adsorbed
Sample A C
PMMA(mg/m:)
PVC(mg/m2)
0.6 1.3
0.14 --
6 57
CONCLUSION The surface energy, i.e., the potential of alumina samples to exchange reversible physical interactions, has been measured using inverse gas chromatography ( I G C ) , which appears to be a very appropriate method for the surface characterization of powders. Aluminas are high surface energy oxides, able to exchange dispersive (3, D ) but also acid/base interactions (I~p). The presence of mineral impurities significantly influences the surface properties. Finally, the surface energy characteristics determined by I G C allow one, at least on a qualitative level, to understand the polymer adsorption ability of the different aluminas. ACKNOWLEDGMENTS One of us (J.M.P.) received a grant for the preparation of a Ph.D. degree and thanks particularly AluminiumPrchiney. Further, this work was supportedby the C.N.R.S. and the French Ministry of Research and Technology. REFERENCES 1. Boch, P., and Chartier, T., in "Ceramic Developments" (C. C. Sorrell and B. Ben-Nissan, Eds.), Mater. Sci. Forum, Vols. 34-36, p. 813, 1988. 2. Papirer, E., Balard, H., and Vidal, A., Eur. Polym. J. 25(8), 707 (1988). 3. Riess, G., ENSC Mulhouse; personal communication. 4. Fowkes, F. M., J. Phys. Chem. 66, 382 (1962). 5. Conder, J. R., and Young, C. L., "Physicochemical Measurement by Gas Chromatography." Wiley, New York, 1979. 6. Kiselev,A. V., "Gas Chromatography."Butterworths, London, 1962. 7. Dorris, C. M., and Gray, D. G., J. Colloid Interface Sci. 77(2), 353 (1980). Journal of Colloid and Interface Science,
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8. Healy, M. H., Wieserman, L, F., Arnett, E. M., and Wefers, K., Langmuir 5, 114 (1989). 9. Michels, J. J., and Dorsey, J. G., Langmuir 6, 414 (1990). 10. Boudreau, S. P., and Cooper, W. T., Anal. Chem. 59, 353 (1987). I 1. Fowkes, F. M., J. Adhes. Sci. Tech. 1(9), 7 (1987). 12. Gutmann, V., "The Donor-Acceptor Approach to Molecular Interactions." Plenum, New York, 1978. 13. Saint Flour, C., and Papirer, E., Ind. Eng. Chem. Prod. Res. Dev. 21(4), 666 (1982). 14. Saint Flour, C., and Papirer, E., J. Colloid Interface Sci. 91(1), 69 (1983).
Journal of Colloid and Interface Science, Vol, 144, No. 1, June 1991
15. Papirer, E., in "Composite Interfaces: Proceedings of the International Conference on Composite Interfaces, 1st, (ICCI- 1), Cleveland, Ohio, May, 1986" (H. Ishida and J. L. Koenig, Eds.), Vol. I, p. 203. Elsevier, New York, 1986. 16. Schreiber, H. P., Wertheimer, M. R., and Lambla, M., J. Appl. Polym. Sci. 27, 2269 (1982). 17. Ligner, G., Vidal, A., Balard, H., and Papirer, E., J. Colloid Interface Sci. 134(2), 486 (1990). 18. Sato, T., in "Stabilization of Colloidal Dispersions by Polymer Adsorption" (T. Sato and R. Ruch, Eds.), Vol. 9. Dekker, New York, 1980. 19. Fowkes, F. M., and Mostafa, M. A., Ind. Eng. Chem. 17, 3 (1978).
An important step in the production of technical ceramic (electronic substrates and multilayer capacitors) is the preparation of green tapes ( 1). These green tapes are obtained by tape casting and drying a slurry which is composed of a ceramic powder (alumina or barium titanate . . . . ), a solvent, a plasticizer, a dispersant, and a binder. The goals of this study are first to identify and to understand the physicochemical phenomena which are at the base of the preparation of the slurry, and second to master the elaboration of ceramics. The slurry is difficult to study because of its complex composition and the lack of precise knowledge of the properties of the powder and of the other numerous organic compounds. To yield good properties, after sintering, the powder should be well dispersed in a stable slurry. The stability of the slurry is governed by the nature and the intensity of the interactions between the constitutents. Hence, in the first step of this work, an attempt is made to characterize the surface of some o~-aluminas used for ceramics in terms of interaction potential. On the one hand, an adequate method To whom correspondence should be addressed.
is developed, with a given alumina sample, to measure its surface energy and acid/base interaction potential, which are defined later. On the other hand, the method is then applied to a collection of aluminas of various granularities and chemical compositions. The method used is inverse gas chromatography (IGC), which has been already applied with success to various powder or fiber surfaces (2). Finally, the quantities measured by IGC are related to the polymer adsorption capacities of the aluminas. I. EXPERIMENTAL
L1. Aluminas Several aluminas (from Aluminium-Prchiney (A, C, C', D, E) and one from Alcoa (B)) were studied. In addition to commercially available aluminas P 152 S B and P 172 S B, Pechiney provided several special production aluminas with impurities controlled at various levels to verify if the nature and the amount of impurities have a significant influence on the surface energy and the specific interaction parameters of these aluminas. Table I shows the specific surface area values and the impurity contents of the various samples. The
263
Journalof ColloidandInterfaceSci,~nce,Vol. 144,No. I, June 1991
0021-9797/91 $3.00 Copyright© 1991 by AcademicPress,Inc. All rightsof reproductionin any formreserved.
264
PAPIRER
L3. Polymer Adsorption
TABLEI Main Charac~fi~csofAluminas BET (m2/g)
A B C C' D E
10.6 6 9.3 11.7 10.8 2.4
CaO/AI203 SIO2/A1203 Na20/A1203 MgO/AI203 (ppm) (pprn) (ppm) (ppm)
2 130 580 600 210 610
45 290 630 990 620 1900
ET AL.
<100 790 550 <150 710 150
<10 330 730 890 800 <10
surface areas vary from 2.4 to 11.7 m2/g. Impurity contents, essentially Ca, Si, Na, and Mg, are also very different. For the moment, the exact nature of oxides or of possible combinations of these elements remains unknown. One of the samples (C') has been washed with water in order to decrease its Na content.
L2. Inverse Gas Chromatography For this study, a gas chromatograph (Intersmat, Model IGC 120 DFL) with a highly sensitive flame ionization detector was used. Stainless steel columns, 30 cm long and 3 mm in diameter, were filled with about 1.5 g of alumina. Alumina granules of the recommended size (250-400 urn) were obtained by crushing and sieving alumina pellets obtained by compression in an IR die. Helium was used as carrier gas, at a flow rate of about 20 ml/ min. The solutes, purchased as puriss, grades from Aldrich, were used without any further purification. The amounts injected are very small ( 10 tsl of vapor) in order to meet the infinite dilution conditions of GC. The net retention volume (Vn) was calculated taking the retention time (tr) of methane, which is practically not retained on the support, as the reference. For symmetrical peaks, tr corresponds to the peak maximum whereas for nonsymmetrical peaks, t~ is calculated from the firstorder moment of the peak. Before all measurements, alumina samples were conditioned at 423 K, for 16 h, under a He flow. Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991
A total of 0.5 g of A1203, dried during 24 h at 373 K, was dispersed in 25 cm 3 of a polymer solution. The mixture was then stirred in a thermostated water bath (+0.1 K) for 48 h. Subsequently, the A1203 particles were allowed to decant and the supernatant solution was analyzed by gel permeation chromatography (GPC) after proper calibration. Two polymers were investigated: poly(vinyl chloride) (PVC) which is representative of an "acidic" polymer, and poly(methyl methacrylate) (PMMA) which has "amphoteric" properties. PVC was purchased from Ugilor and has a molecular weight, determined by GPC, equal to 100.000. PMMA was synthesized in suspension (3) and has about the same molecular weight. The common solvent for the two polymers is butanone (SDS). This solvent, mixed with ethanol, is currently used for the preparation of tapes.
L 4. Small Angle X-ray Scattering The amount of polymer which will adsorb on the alumina surface is dependent on the respective affinities of the polymer for the solvent and for the solid, but is obviously also related to the size of the polymer coils in butanone. Since PVC and PMMA have quite different chemical structures, but similar molecular weights, their sizes in butanone first must be measured before definite conclusions on their adsorption capacity on A1203 can be drawn. This was done by performing small angle X-ray scattering experiments on polymer solutions at concentrations equal to 2, 0.4, and 0.08 g/liter for PMMA and concentrations equal to 0.2, 0.04, and 0.008 g/liter for PVC. A Huber 701 X-ray diffusion system (Austria) was used. The apparatus was calibrated using silica (Aerosil 130 from Degussa, Germany). The mean diameter of PMMA was found to be equal to 87 + 7 ~,, whereas that of PVC is equal to 73 + 7 A. It is seen that both polymer coils have comparable sizes in butanone. Hence, eventual differences in adsorption capacities are only accounted for by different
SURFACE CHARACTERISTICS OF ALUMINAS polymer-solvent and polymer-surface affinities. II. SURFACE PROPERTIES OF ALUMINAS Before describing the experimental results, it might be useful to briefly recall some concepts of surface energy.
alkanes (also the &G2 ) vary linearly with the number of carbon atoms of the n-alkanes (6). Therefore, it is possible to calculate an incremental value, i.e., the free energy of adsorption of a CH2 group, which does not depend on the choice of the reference state, AGcH2 =
II.1. Dispersive Component of the Surface Energy There are two types of interactions of the injected probe with the chromatographic support: - - T h e interactions which are due to London or dispersive forces (instantaneous dipoles). These interactions always occur whatever partners are present. They are called dispersive, nonspecific, or universal interactions. - - T h e interactions which arise from all other types of forces. These forces include polar, acid/base, and H-bonding interactions which depend on the partners. They are therefore termed specific interactions. Consequently, the surface free energy (ms) is considered as a sum of two components: the dispersive component (m~) and the specific component (rasP), ms =
+
According to Fowkes (4) the energy of interaction between the chromatographic support (AlzO3) and an alkane which is able to exchange only dispersive interactions is equal to 2(m~msD) ~/z, where m~ is the dispersive component of the surface energy of the alkane. The affinity of an alkane for alumina is related to Vn: the higher affinity corresponding obviously to a higher Vn. Thermodynamically, affinity is described by &G. For an alkane probe, it is known (5) that - A G a = R T l n Vn + C, where -zXG~ is the variation of standard free energy of adsorption of the probe and C is a constant depending on the arbitrary choice of the reference state of the adsorbed molecule. It is a c o m m o n observation, in gas/solid chromatography, that the logarithms of Vn of
265
Vn+l
R T In - -v . '
where Vn+l and Vn are the net retention volumes ofn-alkanes having (n + 1 ) and n atoms of carbon. Dorris and Gray (7) compared AGcH2 with the interaction energy calculated using Fowkes' equation [ \1/2 AGcH2 _ 2/mcuamsD/ N . aCH2 J
m
In this equation, the quantity N . aCH2 converts free energy units (mJ mo1-1) into free surface energy units ( m J . m -2). N is Avogadro's number and acn2 the cross-sectional area (6 ~2) of an adsorbed -CH2-group. This value was tested and adopted by Dorris and Gray (7). INCH2is the surface energy of a solid made up only of-CH2-groups, i.e., poly ( ethylene ). For PE, msD = 35 m J / m 2 at room temperature. Hence, everything is known or measurable in the previous equation except the quantity of interest, i.e., msD. As an example, Fig. 1 displays the variation of the free energy of adsorption versus the number of carbon atoms of the alkanes used to probe the surface properties of
18 14
RT.in Vn (kJ / mole)
C7f/,~ C
10 6 2 -2
rl c
I 4
I 5
I 6
I 7
I 8
F/G. 1. Variationof the net retentionvolumeof alkanes, on alumina A, measured at 383 K, with their number of carbon atoms. Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991
266
PAPIRER
alumina A. A linear relationship is recorded from which the value of 7 D s is computed. The corresponding value of T~ is equal to 104 _+4 m J / m 2. This demonstrates the very high sensitivity of IGC for 7 sD measurements of powders.
II.2. Specific Interaction Parameter (I~p) Several attempts to evaluate the specific interaction capacities of aluminas were made. For instance, Healy et aL (8) made IR spectroscopy and microcalorimetric investigations using basic probe molecules (acetonitrile, pyridine, 2-6 lutidine, and n-butylamine) to identify the acidic sites on various alumina samples. Major differences were attributed to the H-bond formation capacity of A1203 samples. Yet the proposed method does not consider basic surface sites. The same is true for the approach taken by Michels and Dorsey (9). They also titrate the surface acidity of aluminas by adsorption of pyridinium betaine and measurement of diffuse reflectance spectra, the behavior of the betaine being very sensitive to environment and showing a pronounced solvatochromic effect. With this method, the influence of thermal activation and water deactivation could be demonstrated. Boudreau and Cooper (10) used gas-solid chromatography to obtain adsorption isotherms and subsequently to evaluate them in terms of energetic heterogeneity. Several probes were tested: chloroform (proton donor), pyridine (proton acceptor), and dichloromethane (for dipole interactions). However, they did not consider the contribution of London-type interactions in the adsorption phenomena as is done in the present paper. The specific component of the surface energy can, in principle, be obtained by IGC at infinite dilution when using polar probes instead of alkanes. Often, specific interactions are ascribed essentially to acid/base interactions (11 ). Indeed it is known that the acid/base interaction energy greatly exceeds that of pure Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991
ET AL.
polar interactions. Hence, when acid/base interactions are occurring between a solute and alumina, the possible polar contribution may be neglected. This raises the problem of the definition of acid/base scales of organic molecules. For this study, the semiempirical scale ofGutmann (12) was adopted for two reasons: the amphoteric character (possibility for a given molecule to exchange both acidic and basic interactions) of most polar molecules is taken into account, and enough data on the most common chromatographic solutes are available. According to Gutmann, an acid is defined by its ability to attract electrons (acceptor number: AN) and a base by its ability to release electrons (donor number: DN). For example, due to the strong electronegative properties of chlorine atoms, CHC13 can be considered as an acid since the C atom becomes electron-depleted. Diethyl ether, on the contrary, possesses an O atom rich in electrons which is willing to release its electron density: diethyl ether is a base. When a polar solute is injected in the GC column containing alumina, both dispersive and specific interactions will take place. Yet, only one retention time or retention volume will be recorded. The problem is now to extract, from this sole observation, two parameters: one corresponding to nonspecific and the other to specific interactions. When the net retention volume (or AG °) versus the vapor pressure of the injected solutes is plotted, all points corresponding to alkane probes fall on a straight line whereas the corresponding points of polar probes, interacting with a polar chromatographic support, are located well above the alkane line. By definition (13), the specific interaction parameter (Isp) of a given probe is given by the deviation of the experimental point from the alkane line as schematically shown in Fig. 2. Figure 2 gives the example of alumina A from Prchiney. The Isp is equal to 4.2 kJ/mol in the case of CH2C12 adsorption. The I~p of a base probe (ether) is much higher, a result which points out the rather acidic character of the alumina surface.
SURFACE CHARACTERISTICSOF ALUMINAS
22 . 18 . 14.
RT.In Vn (kJ / mole)
C8
TABLE II
ACETONE
TOLUENE ".q" ~ ~ _ "
Dispersive Component of the Surface Energy (-ysD, mJ/m2) of Alumina Samples, Measured at 373 K
C2H5-O-C2H5
~.~,.~.
BENZENE
267
,~ Experiment number
10
C7
6
~ C6
2 2.5
E 2.75
I 3
Mean value of
CH2CI2 I
I 3.25
~ " ~ l o g C5 ~ I I 3.5 3.75
Sample
1
2
3
4
A
104 86 67 96 86 161
100 88 65 99 90 169
102 86 65 96 88 158
96 89 62 95 86 162
(Po) 10 I 4
FIG. 2. Variation of the net retention volumesof solutes on sample A, measured at 383 K, with the logarithm of the saturated vapor pressure: definition of Isp.
Further, Isp values may be used to calculate KA (acidity) and KD (basicity) of alumina according to the following relationship (14), assuming that the alumina surfaces exhibit amphoteric properties: /so = KA(DN) + KD(AN). Such a procedure has already been applied to the study of glass fibers (14), silica ( 15 ), and other mineral oxides. One plots/so/AN versus D N / A N , where the D N and A N values of the probes are known from G u t m a n n ' s measurements.
H.3. Acid~Base Index (~2) A more qualitative description of acid/base characteristics of solid surfaces is possible through the measurement of f~ (16). [2 is the ratio of net retention volumes of a base molecule (ether) and of an acidic molecule (dichloromethane). It allows the detection of variations of acid / base properties of aluminas using just two probe injections. Suppose that the A1203 sample has, for instance, a strong acid interaction potential; then the value of ~2 will be rather high since obviously the interaction (or V~) of the basic probe (ether) will be important. On the contrary, ~2will take low values for base-like surfaces. ~2 can thus become a sensitive indicator of the variation of the surface properties of the alumina samples
B
C C' D E
100 ± 4 87 ± 2 65 ± 3 96 ± 3 88 ± 2 162 ± 7
according to their origin, chemical composition, and possible modification.
II.4. Influence of Impurities Table II exhibits the 7 ~ values (expressed in m J / m 2) for the various alumina samples, measured four times, at 373 K and the mean value observed at that temperature. The experimental error does not exceed 7 m J / m 2. Indeed important variations of 3'D are observed. It appears thus, as expected, that the impurities have an influence on the value of q/s°, a point which so far has never been analyzed quantitatively. Table 111 shows the KA and KD values, defined before, and the quantity ~2which is equal to V~/C2Hs- O-C2H5
f~--
V~H2CI 2
TABLE III KA, KD and ft Values for the Various Aluminas Sample
KA
KD
£
A
5.6 7.5 9.5 9.9 8.6 9.9
2.1 2.3 2.6 2.6 2.2 2.7
6 20 57 83 44 51
B
C C' D E
Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991
268 10
PAPIRER ET AL. KA
C
• •
10o "l"
80
5 4
C'
E
].
C
~
E
A i 250
i 500
i 750
i 1000
i 1250
i 1500
SiO2 ppm i 1750 2000
FIG. 3. Variation of KA with the amount of SiO2in the alumina samples. It is seen from the values of KD that there is no major change in basicity of the samples according to their content of impurities. But for the acidity and for ~2, differences are significant. The change in acidity may be related to the SiOz content since it is known that silica and silicoaluminate surfaces behave rather acidically (17 ). Figure 3 shows the variation of KA versus the amount of SiO2 in the alumina samples. The experimental points fall on a curve, suggesting that KA reaches a constant value for a SiO2 content of about 1000 ppm. Whereas KA describes the acidic character of A1203, the value of ~2gives more global information on the acid/base properties of the samples. The acid/base parameter f~ (Fig. 4) first increases with increasing amounts of SiO2 and then decreases, indicating that the presence of the other impurities cannot be ignored and may become prevalent. Further studies are necessary to come to a better understanding of the most important effect of impurities which definitively will determine the polymer adsorption capacity of ceramic powders as will be seen later.
0
250
500
750
1000
1250
1500
1750
2000
FlG. 4. Variation of f~ with the amount of SiO2in the aluminas.
that surface properties and polymer adsorption will also be related. In fact, polymer adsorption from a solution has received much attention since this phenomenon is determinant for numerous applications (18). The nature of the polymer, the molecular weight, and the molecular weight distribution, and also of the solvent and, obviously, the solid surface, strongly influences the adsorption process. The competition between the polymer and the solvent for the adsorption on solid surfaces has been explained by their difference in acid/base interaction potential (19): strong solvent-solid interaction will hinder the polymer adsorption. In order to verify the adsorption capacity of the various alumina samples, experiments were carried out either with an acidic (PVC) or an amphoteric ( P M M A ) polymer. The solvent used was butanone. Butanone, like PMMA, is an amphoteric molecule. It ob-
o.5
q (rng/m2)
u
0.4
~
PMMA
0.3
III. RELATION BETWEEN SURFACE PROPERTIES OF ALUMINAS AND POLYMER ADSORPTION
0.2
c (rng/I)
0
The results obtained so far clearly demonstrate the contribution of impurities to the adsorption potential of aluminas. It is expected Journal of Colloid and Interface Science, Vol. 144, No. 1, June 1991
PVC
0.1 I
I
i
i
100
200
3oo
400
5oo
FIG. 5. Adsorption isotherms of PMMA and PVC on alumina A.
SURFACE CHARACTERISTICS OF ALUMINAS viously interacts with the polymer since it dissolves it, but butanone will also interact with the surface of alumina: a competition will take place. Figure 5 compares the adsorption isotherms of PVC and P M M A on alumina A: the adsorbed a m o u n t is expressed per unit surface area. It can be seen that, even though the polymers have similar molecular weights and dimensions, they lead to different adsorption isotherms: the PVC adsorption is significantly lower than the P M M A adsorption. This result suggests that butanone adsorbs more strongly on A1203 A than PVC does. This observation is consistent with the I G C observation, demonstrating the very acidic nature of this alumina. As we would also expect, the shape of the isotherm depends on the alumina sample. For instance, Fig. 6 compares the adsorption isotherms of P M M A on aluminas A and C which have different surface properties. According to the IGC results (cf. Table III), samples A and C have comparable KD values, but the latter sample is more acidic: it has a higher KA value. This is also expressed by the f~ values. Higher acidity means higher affinity for the basic PMMA. Qualitatively the polym e r adsorption behavior of aluminas is in line with their surface properties. Some further examples are given in Table IV. It is seen that P M M A adsorption follows the variation of 9 (the more acidic surfaces) corresponding to the higher P M M A uptake.
q (rng/m2)
1.4 1.2 1 0.8 0.6 0.4 02 0
r
a
{
200
g
I 400
/
i
~
600
)
I
I
800
| 000
1200
FIG. 6. Adsorption isotherms of PMMA on aluminas A and C.
269
TABLE IV Maximum Amount of Polymer Adsorbed
Sample A C
PMMA(mg/m:)
PVC(mg/m2)
0.6 1.3
0.14 --
6 57
CONCLUSION The surface energy, i.e., the potential of alumina samples to exchange reversible physical interactions, has been measured using inverse gas chromatography ( I G C ) , which appears to be a very appropriate method for the surface characterization of powders. Aluminas are high surface energy oxides, able to exchange dispersive (3, D ) but also acid/base interactions (I~p). The presence of mineral impurities significantly influences the surface properties. Finally, the surface energy characteristics determined by I G C allow one, at least on a qualitative level, to understand the polymer adsorption ability of the different aluminas. ACKNOWLEDGMENTS One of us (J.M.P.) received a grant for the preparation of a Ph.D. degree and thanks particularly AluminiumPrchiney. Further, this work was supportedby the C.N.R.S. and the French Ministry of Research and Technology. REFERENCES 1. Boch, P., and Chartier, T., in "Ceramic Developments" (C. C. Sorrell and B. Ben-Nissan, Eds.), Mater. Sci. Forum, Vols. 34-36, p. 813, 1988. 2. Papirer, E., Balard, H., and Vidal, A., Eur. Polym. J. 25(8), 707 (1988). 3. Riess, G., ENSC Mulhouse; personal communication. 4. Fowkes, F. M., J. Phys. Chem. 66, 382 (1962). 5. Conder, J. R., and Young, C. L., "Physicochemical Measurement by Gas Chromatography." Wiley, New York, 1979. 6. Kiselev,A. V., "Gas Chromatography."Butterworths, London, 1962. 7. Dorris, C. M., and Gray, D. G., J. Colloid Interface Sci. 77(2), 353 (1980). Journal of Colloid and Interface Science,
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8. Healy, M. H., Wieserman, L, F., Arnett, E. M., and Wefers, K., Langmuir 5, 114 (1989). 9. Michels, J. J., and Dorsey, J. G., Langmuir 6, 414 (1990). 10. Boudreau, S. P., and Cooper, W. T., Anal. Chem. 59, 353 (1987). I 1. Fowkes, F. M., J. Adhes. Sci. Tech. 1(9), 7 (1987). 12. Gutmann, V., "The Donor-Acceptor Approach to Molecular Interactions." Plenum, New York, 1978. 13. Saint Flour, C., and Papirer, E., Ind. Eng. Chem. Prod. Res. Dev. 21(4), 666 (1982). 14. Saint Flour, C., and Papirer, E., J. Colloid Interface Sci. 91(1), 69 (1983).
Journal of Colloid and Interface Science, Vol, 144, No. 1, June 1991
15. Papirer, E., in "Composite Interfaces: Proceedings of the International Conference on Composite Interfaces, 1st, (ICCI- 1), Cleveland, Ohio, May, 1986" (H. Ishida and J. L. Koenig, Eds.), Vol. I, p. 203. Elsevier, New York, 1986. 16. Schreiber, H. P., Wertheimer, M. R., and Lambla, M., J. Appl. Polym. Sci. 27, 2269 (1982). 17. Ligner, G., Vidal, A., Balard, H., and Papirer, E., J. Colloid Interface Sci. 134(2), 486 (1990). 18. Sato, T., in "Stabilization of Colloidal Dispersions by Polymer Adsorption" (T. Sato and R. Ruch, Eds.), Vol. 9. Dekker, New York, 1980. 19. Fowkes, F. M., and Mostafa, M. A., Ind. Eng. Chem. 17, 3 (1978).