Langmuir 1997, 13, 1251-1255
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Monitoring, by Inverse Gas Chromatography, of the Variation of the Surface Energetic Heterogeneity of Ground Muscovite Samples† H. Balard,* O. Aouadj, and E. Papirer Institut de Chimie des Surfaces et Interfaces, CNRS, BP 2488, F-68057 Mulhouse, France Received December 11, 1995. In Final Form: October 29, 1996X A layered mineral, like muscovite, is a good model of a heterogeneous solid because it presents two types of crystalline surfaces: basal and lateral surfaces. A convenient method for changing its degree of surface heterogeneity is to submit muscovite to a grinding process, creating new surfaces: either basal surfaces if the delamination of the crystal is the dominant process or lateral surfaces if comminution is prevalent. The aim of the present work is to demonstrate how inverse gas chromatography, combined with an original method of calculation of the adsorption energy distribution functions, is a very sensitive method to monitor the evolution of the surface heterogeneity of muscovite ground in the presence of different grinding additives such as glutaric acid (0.5% in aqueous solution) or potassium chloride (1 M in aqueous solution). It is shown that the latter favors the delamination of the muscovite crystal whereas the former induces the comminution of the crystal leading to an increase of lateral surfaces.
I. Introduction The surface heterogeneity of a solid originates from the variability of the surface chemical composition and structure. Layered minerals, like phyllosilicates, are good models of heterogeneous solids because they present two types of solid surfaces: the basal surfaces, parallel to the cleavage plane and the lateral surfaces perpendicular to the latter. A possible method for changing the surface heterogeneity is to submit the solid to a grinding process, creating new surfaces. In the case of a phyllosilicate, two processes are possible: the delamination by cleavage of the crystal leading to new basal surfaces or the comminution, perpendicularly to the cleavage planes, giving rise to new lateral surfaces. The aim of the present work is to demonstrate how the determination of the adsorption isotherms by inverse gas chromatography (IGC) combined with a new method of calculation of the adsorption energy distribution functions is a very appropriate method for the monitoring of the evolution of the surface heterogeneity of muscovite ground in the presence of different grinding additives such as poly(acrylic acid) (PAA) and glutaric acid (as model of the PAA diad) or molar potassium chloride aqueous solutions. II. Experimental Section Mica. In these experiments, muscovite of high purity, from Bihar (India), was coarsely broken, using a rotating knife. Its density, measured at 25 °C, is equal to 2.5 g/cm3 and its isoelectric point, determined by electrophoresis, is situated at pH ) 2.5, a value in agreement with the literature. Its elemental composition is indicated in Table 1. The chemical composition of our sample is very close to that of pure muscovite. Its specific surface area is inferior to 2 m2/g. Before grinding, muscovite platelets have been selected by sieving, and had a size between 50 and 125 µm. Grinding Additives. According to previous studies,1,2 four grinding media were selected: pure water, a 1 M aqueous solution * Author to whom correspondence should be sent: tel, 33 03 89 60 88 00; fax, 33 03 89 60 87 99; e-mail,
[email protected]. † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakia, September 4-10, 1995. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Lowrison, G. C. In Crushing and Grinding; Butterworths: London, 1979. (2) Papirer, E.; Roland, P. Clays Clay Miner. 1981, 29 (3), 161.
S0743-7463(95)01528-9 CCC: $14.00
Table 1. Elemental Analysis of the Muscovite of Bihar elements
measured value
theoretical value
SiO2 Al2O3 K2O FeO Na2O CaO MgO
45.4 37.0 11.8 1.5 0.5 0.3 0.1
45.4 38.5 11.8
of potassium chloride, 0.5% solutions of glutaric acid (as the simplest model molecule of the PAA diad), and low molecular weight PAA synthesized by radical telomerization of acrylic acid using azobis(cyano)isovaleric acid as initiator (Mn ) 2550 g/mol). Grinding Conditions. Particles were submitted to grinding in an attritor (Figure 1) made of a stirrer, rotating at 290 rpm, filled with about 1000 stainless steel balls, 10 g of mica, and 200 cm3 of a solution of the grinding additive in water. After filtration and washing, the ground mica was dried overnight in an oven at 50 °C. Specific Surface Area and Particle Size. The specific surface areas and the particle sizes of the ground solids were determined using nitrogen (or argon) adsorption volumetry and calculated from the BET method and the Laser Coulter sizer LS 130. Surface properties were studied by inverse gas chromatography (IGC) at finite concentration and at infinite dilution conditions. Inverse Gas Chromatography. Chromatograms were acquired using stainless steel short columns, having a length of 20 cm and a 2 mm inner diameter. Helium was used as the carrier gas. Chromatographic peaks were evaluated with a Shimadzu CR4A integrator. The distribution functions (DF) were further computed, according to our method combining the extended method of Rudzinski-Jagiełło3 and the multiple derivation calling on discrete Fourier transforms. In the present work, we used the Rudzinski-Jagiełło approximation of order 4 (DFRJ4)). They were computed using a Macintosh computer and an home-made program described elsewhere.4 Other Characterization Methods. The determination of the shape factor of lamellar solids such as muscovite is a difficult yet an important task since this factor will largely influence the performance of clay-filled polymer composites. Some methods may be applied such as image analysis and various adsorption techniques. For instance, Cases et al.5 studied a series of kaolinites. Low-temperature adsorption calorimetry using argon (3) Rudzinski, W.; Jagiełło, J.; Grillet, Y. J. Colloid Interface Sci. 1982, 87 (2), 478. (4) Balard, H. Langmuir, in press. (5) Cases, J. M.; Cunin, P.; Grillet, Y.; Poinsignon, C.; Yvon, J. Clay Miner. 1986, 2, 55.
© 1997 American Chemical Society
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Figure 1. Schematic representation of the attritor used for the grinding of the muscovite samples.
Figure 2. Variation of the specific surface area of ground muscovite particles with grinding duration, in the presence of glutaric acid, 1 M KCl, pure water, and poly(acrylic acid) (PAA).
Figure 3. Variation of the mean diameter of ground muscovite particles with grinding duration, in the presence of glutaric acid, 1 M KCl, pure water and poly(acrylic acid) (PAA). and the determination of alkyldodecylammonium ion adsorption isotherms gave similar and satisfactory results. More recently Michot et al.,6 comparing the adsorption of nitrogen and argon on talc samples and applying their derivative isotherm summation method, were able to evaluate their basal and lateral surface area ratio. For this study, the surface properties of the ground muscovites have also been examined by microcalorimetric measurement of the variation of the derivative adsorption enthalpy of argon, with the coverage ratio, in quasi-equilibrium conditions according to the method described in the literature.7
III. Results and Discussion Grinding Kinetics. As expected, the main result of grinding of muscovite is a significant decrease of the particle diameter (Figure 2) and, consequently, an increase of the specific surface area as shown in Figure 3. Looking first at the evolutions of the particle size, it appears clearly that the chemical nature of the additive influences significantly the grinding kinetics of muscovite. The most efficient agent is glutaric acid, especially from (6) Michot, L. J.; Villieras, F.; Franc¸ ois, M.; Yvon, J.; Le Dred, R.; Cases, J. M. Langmuir 1994, 10, 3765. (7) Grillet, Y.; Rouquerol, F.; Rouquerol, J. J. Chim. Phys. 1987 (78), 778.
Balard et al.
the point of view of the particle size diminution. The less active is PAA which is even less efficient than is pure water. From the point of view of the specific surface area increase, KCl is the most efficient grinding agent. The change of the grinding kinetics observed in pure water and PAA solution is attributed to the progressive release of potassium ions into the grinding medium, accelerating thus the grinding process as observed when grinding using a KCl solution.2 Probably, K+ ions act like a “wedge”, inserting into the lateral surfaces, helping to separate muscovite layers and thus accelerating the grinding process. Such a mechanism has been proposed for the natural degradation of muscovite: K+ being the most efficient ion due to its small hydration sphere. An alternative explanation may be proposed. In a KCl solution, the surface charge on a basal plane is saturated by K+ ions. Inversely, in a solution of poly(acrylic acid) or its oligomers, the basal plane is not saturated. In this condition, the diacid molecules, which are complexing agents of aluminum ions located in replacement of silicium ions in the tetrahedral layer, can solubilize the tetrahedral aluminum and then initialize cracks by corrosion on the (a,b) plane. Electron microscopy highlights the difference of the grinding processes using mineral (KCl) or organic additives and confirms that glutaric acid is also the best additive for preserving the lamellarity of the ground solid and that KCl has the opposite effect.8 Surface Heterogeneity of Ground Micas by IGC at Finite Concentration. The energetic surface heterogeneities of the ground samples were studied using the method described elsewhere.4 The derivatives of the adsorption isotherms were directly calculated from the shape of the chromatographic peaks. Octane and isooctane (2,2,4-trimethylpentane) were chosen because the former is a very flexible linear hydrocarbon and the latter has a bulky shape in order to probe the surface nanomorphology of ground samples. On the contrary, benzene is a rather flat probe molecule being also a weak polar molecule capable for detecting particular features on the ground muscovite sample surface. Influence of Grinding Additives. Figure 4 shows the adsorption energies distribution functions of n-octane for mica ground in pure water, in the presence of glutaric acid and aqueous KCl solutions. If we except the case of the initial muscovite and sample ground in the presence of the PAA additive, exhibiting the lowest specific surface areas, leading to a monomodal DFRJ4, all other DFRJ4 are bimodal. It appears also that the areas (total area under the DF) and the ratio of the two peaks areas (numbers in brackets) are dependent on the nature of the grinding agent. Apparently, samples prepared in the presence of PAA behave differently, since the DFRJ4 is significantly shifted toward the higher values on the adsorption energy scale. Probably, some PAA remains attached on the muscovite surface, thus modifying its adsorption characteristics. It was also of interest to examine the influence, on DF, of other parametes such as grinding duration and chemical nature of the IGC probes. This study was done using the most efficient grinding additives, namely, the molar potassium chloride and the glutaric acid solutions. Figure 5 shows the adsorption energy distribution functions of n-octane for mica ground in the presence of glutaric acid and 1 M KCl. (8) Balard, H.; Aouadj, O.; Papirer, E. Mines Carrie`res: Tech. 1995, 77, 84.
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infinite dilution delivers a distorted image of the capacity of interaction of the solid’s surface, because the retention time of an isolated probe molecule is equal to the sum of its residence times (τ) on each site visited by the molecule during its progress in the column. Taking into account that the residence time increases exponentially to the site energy (eq 1), it follows that the measured global retention time tr will be dependent mainly on the contribution of the sites having the highest adsorption energy.
τ ) τ0 exp(/RT)
Figure 4. Adsorption energy distribution functions of an octane probe measured at 30 °C, on muscovite ground 20 h, in the presence of glutaric acid, 1 M KCl, pure water, and poly(acrylic acid) (PAA). Numbers in parentheses give the ratio in percent of peak areas of the higher energy peak at 28 kJ/mol and the peak at 22 kJ/mol.
Again, if we except the monomodal DF of the initial mica, all other DF are bimodal and the intensity of the second peak at the higher adsorption energies scale increases with increasing grinding durations. Nevertheless, the relative area of both peaks is dependent on the nature of the grinding agent. The potassium chloride favors the increase of the peak at 21 kJ/mol whereas the peak at 28 kJ/mol is more important with the glutaric acid additive confirming our earlier results. Finally, we verified our results using two additional IGC molecular probes: a sterically hindered apolar molecule (isooctane) and a planar and slightly polar probe (benzene). The determined DF are depicted in Figures 6 and 7. The evolutions observed with the two last probes are very similar to those recorded with the n-octane probe: with increasing grinding durations, a second peak of increasing intensity appears. Moreover, the difference in the evolution of the relative intensity of both peaks of the DF is again observed. The potassium chloride grinding additive leads to a more intensive peak of lower energy whereas the glutaric acid favors the apparition of a relatively more intense and continuously increasing peak in the higher energy domain while the area of the lower energy peak becomes rapidly quite stable. A reasonable hypothesis is to attribute the peak, centered around 28 kJ/mol, to the creation of new lateral surfaces during the grinding process whereas new basal surfaces will correspond to the peak centered around 22 kJ/mol. In other words, these observations suggest that the glutaric acid favors the breaking of muscovite crystals leading to the formation of numerous lateral surfaces whereas, in the presence of 1 M KCl, preferential delamination of the muscovite crystal occurs. Furthermore, for a grinding duration of 80 h, a third peak appears located intermediately between the two preceding observed peaks. Electronic microscopy suggests that, in this medium and after a long grinding duration, this additional peak may origin from the formation of an amorphous colloid resulting from the strong degradation of the muscovite crystal.8 In order to support this hypothesis, we examined the ground muscovites by IGC at infinite dilution conditions. We also determined, by microcalorimetry, the relative importance of the lateral surface of the ground samples. Surface Properties of Ground Micas by IGC at Infinite Dilution. It is worth pointing out that IGC at
(1)
Consequently, the higher the surface heterogeneity of the solid, the higher the distortion between the information provided by IGC at infinite dilution and the one obtained from IGC measurements at finite concentration conditions. The dispersive component γsd of the suface energy was determined by applying the approach of Dorris and Gray.9 A series of n-alkanes is injected in the GC column containing the mica, and from their retention times, tr, one calculates the retention volume Vn corresponding to the volume of carrier gas (He) required to push the alkane through the column. The standard variation of the free energy of adsorption ∆Ga° and Vn are related by:
-∆Ga° ) RT ln Vn + C
(2)
where C is constant depending on the chosen bidimensionnal reference state. The plot of ∆Ga° of n-alkane probes versus their number of carbon atoms is usually linear, the slope of that line corresponds to an incremental value of ∆Ga° per methylene group, i.e,. ∆GaCH2. Then, γsd is calculated by the relation
[ ]
∆GaCH2 1 γs ) 4 γCH2 NaCH2 d
2
(3)
where γCH2 is the surface energy of a molecule made entirely of methylene groups (i.e., polyethylene), aCH2 is the surface area of one adsorbed CH2 group, and N is Avogadro’s number. As evidenced elsewhere,10 calling on size exclusion IGC, namely, measuring the relative retention times of branched and linear alkane isomers, we have shown that the highest energetic adsorption sites of n-alkanes correspond often to sites in which the probes are able to insert partially whereas bulky probes are size excluded. The importance of the size exclusion effect can be semiquantitatively determined by the nanomorphological index given by eq 4
Im )
Vnb Vnlref Vnl Vnbref
(4)
where Vnb and Vnl are respectively the retention volumes of the n-alkane probe and of a branched isomer on the solid of interest and Vnbref and Vnlref, are those measured on a reference solid, here a pyrogenic silica, flat at the molecular scale. The lower the nanomorphological index, the more important the size exclusion effect and therefore the higher the surface nanoroughness. Figure 8 shows the evolutions of these two surface parameters (γsd determined using a set of n-alkanes and Im using isooctane), versus the grinding duration, for the two grinding media. (9) Dorris, G. M.; Gray, D. G. J. Colloid Interface Sci. 1979, 71, 93. (10) Balard, H.; Papirer, E. Prog. Org. Coat. 1993, 22 (1-4), 1.
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Figure 5. Adsorption energy distribution functions of octane probe, measured at 30 °C, on muscovite ground in the presence of 1 M KCl (A) and glutaric acid (B), for different grinding durations.
Figure 6. Adsorption energy distribution functions of benzene probe, measured at 30 °C, on muscovite ground in the presence of 1 M KCl (A) and glutaric acid (B), for different grinding durations.
Figure 7. Adsorption energy distribution functions of an isooctane probe, measured at 30 °C, on muscovite ground in the presence of 1 M KCl (A) and glutaric acid (B), for different grinding durations.
At the beginning of the grinding process, one observes a rapid increase of the γsd value. A maximum is reached for γsd after 30 h of grinding; the value at the maximum is lower for 1 M KCl than for glutaric acid. Correlatively, we observed a significant decrease of the nanomorphological index (Im) for both grinding media for the same grinding duration. For the 1 M KCl additive, the index of morphology goes through a clear-cut minimum whereas a plateau is reached for glutaric acid. A possible interpretation of this evolution is that, during grinding, comminution of the muscovite crystals increases their lateral/basal surface areas ratio. Whereas the basal surfaces are rather flat and regular, the peripheral lateral
surfaces are often the location of structural defects such as stacking faults or partially missing atomic planes. The remaining sheet structure constitutes molecular size cavities in which the very flexible n-alkane chains will be able to insert and experience therefore very high force fields leading to apparent high γsd values. On the contrary, their branched isomers, because of their large size, are excluded from these peculiar structures leading to lower retention times than those obtained from linear homologous alkanes. For long grinding duration, the crystal size will tend to a limit and the morphology index too as observed for glutaric acid. On the contrary, in the case of 1 M KCl, the nanomorphological index rises again to
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Figure 8. Variation of the dispersive component γsd (A) and of the nanomorphological index Im (B) with grinding duration in the presence of glutaric acid and 1 M KCl. Table 2. Comparison of the Relative Area of the Lateral Surface in % of the Ground Muscovites Measured Using Microcalorimetry and Calculated by Integration of the Peaks on the Adsorption Energy Distribution Functions (DF) KCl DF C8 DF isooctane DF benzene DF mean value microcalorimetry
Figure 9. Evolution of the differential enthalpy of adsorption of argon with the surface coverage ratio for two grinding durations, 60 and 80 h, in the presence of 1 M KCl (A) and glutaric acid (B).
100%; this phenomena probably originates from the plugging of the slotlike structures on the lateral surfaces by the deposition of a colloid evidenced by electronic microscopy,8 on these sites having the highest energy. Again, IGC at infinite dilution highlights the difference of behavior of the most efficient grinding additives, i.e., the glutaric acid and 1 M KCl solutions, and supports qualitatively the interpretation of the DF evolution with grinding durations given previously. As demonstrated by Grillet et al.,7 microcalorimetric measurements, coupled with the adsorption isotherm determination of argon, are also suitable methods for estimating the lateral/basal surface areas ratio. It was therefore interesting to analyze our sample using this technique. Microcalorimetric Measurements. The determination of the relative importance of the lateral surfaces was performed on muscovite ground during 60 and 80 h in the presence of the most efficient grinding agents: 1 M KCl and glutaric acid. Figure 9 shows the evolution of the differential enthalpy of adsorption with the surface coverage ratio.
glutaric acic
60 h
80 h
60 h
80 h
10.6 19.4 23.3 17.7 17.5
17.0 19.2 23.1 19.7 15.5
23.1 29.1 29.4 27.2 19.5
24.7 36.7 29.2 30.2 22.5
The observed break points give an estimation of the relative surface area corresponding to the lateral surfaces. These relative percentages of the lateral surface areas, measured by microcalorimetry and determined from the relative area of the peak centered around 28 kJ/mol of our distribution functions, are reported in Table 2. Taking into account the errors of determination of these relative areas, as well from the distribution functions and from microcalorimetric measurements, one may consider that the trends indicated by the two independent methods are in fairly good agreement. The microcalorimetric technique confirms therefore our hypothesis attributing the peaks of higher energy to sites located on the lateral surfaces of the ground muscovite crystal. IV. Conclusion In conclusion, IGC at infinite dilution and IGC at finite concentration conditions appear to be complementary and powerful methods to follow the changes of surface properties of a crystalline solid submitted to grinding. It demonstrates that, in the case of muscovite, delamination and breaking occur simultaneously, but not to the same extents, leading to an increase of the specific surface area of the samples, presenting hence different morphologies that might have important consequences when the mica is used, for instance, as a filler for plastics. Acknowledgment. The authors thank Dr. Yves Grillet (CNRS, Centre de Microcalorime´trie, Marseille) for the microcalorimetric measurements. LA951528Y
