EXPERIMENTAL STUDIES OF FINE-GRAINED MICAS

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1 Clay Minerals (1966) 6, 211. EXPERIMENTAL STUDIES OF FINE-GRAINED MICAS I. ORGANIC CONTAMINATION ON THE SURFACE OF WET-GROUND MUSCOVITE P. G. ROUXHET* AND G. W. BRINDLEY Materials Research Laboratory, and Department of Geochemistry and Mineralogy, Pennsylvania State University, University Park, Pennsylvania, U.S.A. (Received 28 February 1966) ABSTRACT: The surface properties of wet-ground muscovite were studied by thermodynamics of water adsorption. The surface appears to be covered by contaminating material, probably organic, which is strongly held; it was not removed at 350 ~ C and under a vacuum of 10 -~ tort. Attention is directed to the difficulty of maintaining freshly made silicate surfaces without contamination and to the importance of surface conditions in the interpretation of silicate properties which involve surface phenomena. The present studies have arisen from previous work aimed at obtaining a better understanding of naturally occurring fine-grained micas and mica-like minerals (Brindley, Takeshi & Wentworth, 1963; Wentworth & Brindley, 1965). By using finely powdered materials obtained from macrocrystalline micas, it was considered that important advantages might accrue from knowing the initial condition of the mica before its comminution. This paper describes a thermodynamic study of water vapour adsorption on the surfaces of fine-grained muscovite. Previously Gaines (1958, 1961) and Rouxhet & Fripiat (1966)have studied gas adsorption on mica surfaces. Water adsorption on montmorillonite, silica gel, and ground glass has been studied by Fripiat et al. (1965); the heat of adsorption and its variation as a function of the degree of coverage can indicate the presence of cations and whether the surface is composed of oxygens or hydroxyl groups. Thermodynamics of water adsorption by different *Present address: Laboratoire de Physico-Chimie Min6rale, Institut Agronomique de l'universit6 Catholique de Louvain, Heverlee-Louvain (Belgium).

212 P. G. Rouxhet and G. W. Brindley silicates has also been studied by Barshad (1960), Every, Wade & Hackerman (196l), Martin (1960), and by Mooney, Keenan & Wood (1952).

2 212 P. G. Rouxhet and G. W. Brindley silicates has also been studied by Barshad (1960), Every, Wade & Hackerman (196l), Martin (1960), and by Mooney, Keenan & Wood (1952). Hill (1949) has, however, proposed a more rigorous method for the calculation of adsorption heats than those used by these authors. EXPERIMENTAL A muscovite from Spruce Pine, North Carolina, was reduced to small particles in distilled water by treatment in a Waring Blendor. Excessive heating was avoided by turning the machine off after running for min. When it cooled down, it was restarted and so on until a total grinding time of 40 hr was reached. The less than 2 t~ equivalent spherical diameter fraction was separated by centrifugation in distilled water and methanol. The sample was dried at room temperature by pumping on it with a mechanical pump; it was thert stored in normal laboratory conditions. The X-ray pattern of the less than 2 /~ fraction of muscovite still showed clearly the diffraction characteristics of a 2M mica (Bradley & Grim, 1961). The adsorption apparatus was of the usual type; a McLeod gauge, thermostated at 70 ~ C to avoid water condensation, measured pressures as low as 10 -~ torr with a fairly good precision. About 1"3 g of the less than 2 p, muscovite was introduced in the cell and outgassed for one night at about 23 ~ C under a vacuum of about 10-6 torr. The sample was pretreated for 2 hr at 50~ under a vacuum better than 10-4 torr and for 2 hr at ~ C under a relative humidity of ; it was then outgassed again for one night at the same temperature with a liquid nitrogen trap near the cell and the adsorption isotherm was recorded at ~ C. After that, the powder was reheated at ~ C under 10-4 torr, kept for 2 hr under a relative humidity of 0"2-0-3, outgassed again for one night, and the adsorption isotherm recorded at 50"55 ~ C. Subsequently the sample was heated for 2 hr at 358 ~ C under about 10-4 torr, and isotherms were recorded at 25"25 ~ C and 51.5 ~ C after the pre-adsorption procedure described above. These precautions were taken to make sure that the recorded isotherms only referred to physical adsorption. The cell temperatures were regulated by a water thermostating bath. THEORETICAL CONSIDERATIONS From reversible adsorption isotherms at absolute temperatures T1 and T2, the heat of adsorption at the intermediate T given by 1 _ 1 += (1) T 2 can be calculated (Hill, 1949). By applying a Clapeyron-Clausius type relation, the isosteric heat qst is obtained by the following expression: where 0 is the degree of coverage. qst = --R \8~-/0 (2)

3 Contamination on ground muscovite 213 A different expression for the heat of adsorption is q, = --R \-~-]~ (3) where ~r is the spreading pressure in dyes/cm. At absolute temperature T and pressure P, ~r is given by the Bangham formula C" de 7r A./o p where k is the Boltzmann constant, A the surface area, and n the total number of molecules adsorbed at the pressure P. The calculation of 7r is made by graphical integration and this requires the adsorption isotherms to be known with precision at pressures as low as 10 -~ torr. Relations (2) and (3) are evaluated from a plot of 0 or ~r as a function of log P at two temperatures. The quantities q~ and q,t have the following significance: q~ is equal to the integral enthalpy of adsorption and allows one to calculate the change of entropy during the adsorption process: q~ = H---g -- H-'~ = A/~ ---- TAS. (5) The isosteric heat, qst, is a differential function and is related to q~ as follows: q,t ~ H~ -- H~ + T,r(3rr/ST) o = q,, + T~r(8~r/ST),, (6) where (r, the surface available to one adsorbed molecule, is given by,~ =,~olo = AIn (7) and ~0 is the packing area of an adsorbed water molecule and is equal to 10"8 A 2 in the range of temperature of the experiments (Livingston, 1949). RESULTS Adsorption isotherms of less than 2 ~ muscovite are given in Fig. 1. Each curve has been measured by at least two series of experimental points. The corresponding surface areas are 22-2 and 17-7 m2/g respectively for the samples outgassed at 50 ~ C and at 358 ~ C. Fig. 2 shows that the state equation of the adsorbed film is represented fairly well by the equation: rrg = CkT. (8) However, it must be pointed out that this is an approximation in that the observed

4 214 P. G. Rouxhet and G. W. Brindley fines do not exactly intercept the origin. Replacing 7r by its value in relation (4) leads to a Freundlich adsorption isotherm of the form 0 = b pl/o (9) where b and C are constants with C > 1. The observed values of C are given in Table 1. 2O0 (b) 1~ % 200 / IOOF -g ~ 100 E.<[ 5O t 0 tog P A -3 tog P FIG. 1. Water adsorption isotherms of the < 2/z muscovite. (a) General view of the curves. 1, Isotherm recorded at ~ C after pretreatmerit at 50 ~ C; 2, isotherm recorded at 50"55 ~ C after pretreatment at 50 ~ C; 3, isotherm recorded at ~ C after pretreatment at 358 ~ C; 4, isotherm recorded at 51 "5 ~ C after pretreatment at 358 ~ C. (b) Parts of isotherms with experimental points. Data for the sample pretreated at 50 ~ C. O, Isotherm recorded at ~ C; O, isotherm recorded at ~ C. J TABLE 1. Values of C in the state equation of water adsorbed on muscovite Treatment temperature (~ Isotherm temperature (~ C Fig. 3 gives the experimental values of q,t and q= as a function of the degree of surface coverage. It is noteworthy that the slope of qst above 0=0.3 is similar to the slope of q. Moreover, the difference (qst--q~,) is much larger for the sample outgassed at 50~ than for the one heated at 358 ~ C. This last observation is

5 o, Contamination on ground muscovite ~0 so I/o (1o-3,~-2) FIG. 2. Establishment of the state equation of water adsorbed on the < 2/z muscovite. 0, Sample at 25"35 ~ C, pretreated at 50 ~ C; O, sample at 50"55 ~ C, pretreated at 50 ~ C; A, sample at 25"25 ~ C, pretreated at 358 ~ C;., sample at 51 '5 ~ C, pretreated at 358 ~ C. explained by considering relations (6) and (8): (37r/aT). = nk/a and therefore (qst--q~) is about constant. At low coverages (0 < 0"3), the relative error between the experimental points and the straight lines of Fig. 2 is appreciable and (6~r/aT) is not constant. The reversibility of the isotherms has not been checked because this was impossible to do with precision at the low pressures required for accurate calculations of rr. However, the validity of the results can be reasonably assumed from the quantitative verification of relation (6) as shown in Table 2; this strongly supports the accuracy of the rr calculations and of the related deductions. TABLE 2. Quantitative check of Equation (6): data for muscovite Sample ~ ~TI between 0 = 0-3 and 0 = 1 for ~ : 108/60 Gutgassed at 50 ~ C 4"2 kcal/mole 4-3-5"4 kcal/mole Treated at 358 ~ C 1 "65 kcal/mole 1 "3-2-4 kcal/mole Relation (5) allows one to calculate the entropy of adsorption at the mean temperature at ~ K. The entropy of adsorption for 0 > 0.15 is cal/~

6 216 P. G. Rouxhet and G. W. Brindley "G -6 E v 8 o ~E 7 Z. i 0-~ t i I i. t i I l "/, 0 5 O-fi "9 I-0 Degree of coverage (~I FI~. 3. Heat of adsorption of water by the < 2/~ muscovite as a function of the degree of coverage. 9 q,t for the sample pretreated at 50 ~ C; 0, q~, for the sample pretreated at 50 ~ C;,, q,t for the sample pretreated at 358 ~ C; ~7, q~ for the sample pretreated at 358 ~ C. mole for the sample outgassed at 50~ and cal/~ mole for the sample heated at 358 ~ C. If the adsorbed molecules are assumed to have two degrees of freedom for translation and three degrees for rotation, the frequency of vibration with respect to the surface is found to be of the order of magnitude of sec -1 or 0"1 cm -1. DISCUSSION It must not be emphasized that the enthalpy of adsorption, q~, (see Fig. 3) is smaller than the heat of condensation of water, 10.3 kcal/mole, since this comparison has no clear physical significance. However, it indicates why the enthalpy of adsorption increases with the completion of the monolayer. A more valuable observation is that q~ is smaller than the values, kcal/mole, observed by Fripiat et al. (1965) for hydroxyl surfaces of glass and silica gel or oxygen--cation surfaces of montmorillonite. These are comparable, respectively, with disordered surfaces and well-preserved surfaces of mica when dean. As the muscovite powder is less hydrophylic than expected, it is concluded that its surface is contaminated. Moreover, contrary to what happens for silica gel,

Contam&ation on ground muscovite 217 glass, and montmorillonite, the adsorption enthalpy is much lower at the beginning of the adsorption than at degrees of coverage from 0"2 to 1; even the first

7 Contam&ation on ground muscovite 217 glass, and montmorillonite, the adsorption enthalpy is much lower at the beginning of the adsorption than at degrees of coverage from 0"2 to 1; even the first molecules to be adsorbed are not located at any strong adsorption sites. This indicates that the mica surface is fully contaminated and that the impurity is not in the form of small foreign particles. If the surface is heterogeneous, an inference possible from the Freundlich type adsorption isotherms (Brunauer, 1945), the heterogeneity must relate to the nature of the contaminating layer and not to its distribution. The increase of the adsorption enthalpy by heating at 358~ under vacuum indicates a modification of the contaminating material without its complete disappearance. After such a treatment the sample is grey and becomes darker as the pretreatment temperature increases. The dark colour disappears during calcination in air. Infrared spectra of the less than 2 t~ muscovite showed small bands around 2936 and 2869 cm -1, which are not found with large flakes. Similar observations were made with the less than 2 t~ fluorphlogopite described in part II; very well defined bands were found at 2970, 2936, and 2869 cm -1. According to Bellamy (i958), they may be attributed to -CH2 (2926 and 2853 cm -1) and -CH~ (2962 and 2872 cm -~) stretching frequencies. These results point to the organic nature of the contaminating material. Organic impurities may have been adsorbed during grinding the micas, but it must be recalled that they are not foreign particles resulting from any decomposition of a part of the grinding machine. Other possibilities are adsorption of organic matter from the methanol used during fractionation, or from the atmosphere, or from the vacuum apparatus. The contaminating material was very strongly held by the mica surfaces and was not desorbed at 350 ~ C and under a vacuum of 10-" torr. It appears so far that it can only be eliminated by oxidation at high temperatures. It must be emphasized that grinding silicates leaves very reactive surfaces which would strongly retain organic compounds. It is thought that the present results are not exceptional and that great care must be taken in handling ground silicates, in recording their weight loss curves, and in the interpretation of experiments in which surface properties are important. For example Rosenqvist (1963) reported a small infrared band around 2900 cm -~ in the spectrum of a powder of hydromuscovite. It was attributed to moisture adsorbed from the atmosphere, but it is not improbable that it was clue to organic contaminant. CONCLUSIONS Thermodynamic data for the adsorption of water on a fine powder of wet-ground muscovite indicate a contaminated surface. The contaminant is probably organic; it is not in the form of particles but covers all the mica surface. It is firmly held and not removed by heating at 350 ~ C in vacuo. Oxidation appears to be the only method for removal. Emphasis is placed on the extreme care necessary in studying properties of high-surface-area silicates when surface conditions affect measurements.

218 P. G. Rouxhet and G. W. Brindley ACKNOWLEDGMENTS This investigation was supported by the American Petroleum Institute, Project 55. The authors wish to thank Professor J.

8 218 P. G. Rouxhet and G. W. Brindley ACKNOWLEDGMENTS This investigation was supported by the American Petroleum Institute, Project 55. The authors wish to thank Professor J. J. Fripiat, head of the Laboratoire de Physico. Chimie Min6rale, AgronOmy Department of the University of Louvain, for the infrared data. REFERENCES BARSHAD I. (1960) Clays and Clay Minerals, Proc. 8th Conf., p. 84. Pergamon Press, Oxford. BELLAMY L.J. (1958) The In/rated Spectra of Complex Molecules, Chap. II, p. 13. Methuen, London; Wiley, New York. BRADLEY W.F. & GRIM R.E. (1961) The X-ray Identifica!ion and Crystal Structures of Clay Minerals (G. Brown, editor), Chap. V, p Mineralogical Society, London. BRINDLEY G.W., TAKESHI H. & WENTWORTH S.A. (1963) Annual Report, p. 1. American Petroleum Institute, Project 55. BRUNAUER S. (1945) The Adsorption o] Gases and Vapors, Vol. I, p. 82. Princeton University Press. EVERY R.L., WADE W.H. ~; HACKERMAN N. (1961) J. phys. Chem., Wash. 65, 25. FRIPIAT J.J., JELLI A., PONCELET G. & ANDR~ J. (1965) J. phys. Chem., Wash. 69, GAINES G.L. (1958) 1. phys. Chem., Wash. 62, GAINES G.L. (1961) Solid Surfaces and the Gas-Solid Interfaces (R. F. Gould, editor), p Advances in Chemistry Series, No. 33. American Chemical Society, Washington. HILL T.L. (1949) J. phys. Chem., Wash. 17, 520. LIVINGSTON A.K. (1949) J. Colloid Sci. 4, 447. MARTIN R.T. (1960) Clays and Clay Minerals, Proc. 8th Con[., p Pergamon Press, Oxford. MOONEY R.W., KEENAN A.G. & WOOD L.A. (1952) J. Am. chem. Soc. 74, ROSENQVIST I.TH. (1963) Clays and Clay Minerals, Proc. llth Con[., p Pergamon Press, Oxford. ROUXI-IET P. & FRIPIAT J.J. (1966) Bull Grpe ft. Argiles. 17, 81. WENTWORTH S.A. t~ BRINDLEY G.W. (1965) Annual Report, p. 1. American Petroleum Institute, Project 55.

Analysis of Clays and Soils by XRD

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