Me a S U Rement of the Specific Surface Area of Clays

-The specific surface area, S, of a clay is commonly measured by the adsorption of ethylene glycol monoethyl ether (EGME); however, this method can be tedious and time consuming, especially if the clay is saturated with a monovalent, highly hydrated cation. An alternative method for measuring S was developed involving infrared internal reflectance spectroscopy. This method is based on the discovery that the ratio of R1, the reflectance of a clay-HOD mixture at the frequency of O-D stretching, to R2, the reflectance of the mixture at the frequency of H O D bending, is linearly related to S. The correlation coefficient between R~/R2 and S, as measured by the adsorption of EGME, was 0.995. Consequently, a calibration curve of R1/R2 versus S was constructed, and the measured values of R~/R2 for any clay-HOD mixture were referred to it for the determination of S. Results were obtained in triplicate in about an hour; hence, this method of determining S is more rapid and convenient than that involving the adsorption of EGME. Moreover, it applies to clays in a natural condition, i.e., swollen in water. Key Words--Ethylene glycol monoethyl ether, Infrared spectroscopy, Internal reflectance spectroscopy, Molar absorptivity, Montmorillonite, Surface area. I N T R O D U C T I O N The large specific surface area, S, of clays is responsible for many of their unique properties; hence considerable effort has been expended in its measurement. Many methods have been developed, including: the method of S. Brunauer, P. H. Emmett , and E. Teller (1938), commonly called the B.E.T. method, which relies on the adsorption of a vapor; the methods of Dyal and Hendricks (1950), Diamond and Kinter (1958), and Carter et al. (1965) which rely on the adsorption of an organic solvent, namely, ethylene glycol, glycerol, and ethylene glycol monoethyl ether (EGME), respectively; and the method of Schofield (1949) which relies on the negative adsorpt ion of anions. In recent years, the EGME method has gained precedence because it is relatively quick and does not suffer from some of the disadvantages associated with the other methods (see, e.g., Mort land and Kemper, 1965). Even this method, however, can be very t ime consuming i f the clay is saturated with Na + and i f a high degree of accuracy is required (Low, 1980). A quicker method is therefore desirable, especially one in which the measurement is made while the clay layers are separated by water, as they usually are in nature. During an investigation of the spectroscopic properties of water near the surfaces of clay particles, Mulla and Low (1983) found that the molar absorptivity, e, of H O D in a clay-water system at the frequency of O D stretching is given by Journal paper number 10,117. 2 Present address: Department of Agronomy and Soils, Washington State University, Pullman, Washington 99164. e = e~ k'sm~/mw, (1) where e ~ is the molar absorptivi ty of pure bulk water, S is the specific surface area of the clay, mJmw is the mass ratio of clay to water, and k, is a constant equal to 1.368 x 10 -7 g/cm 2. Hence, a spectroscopic measurement o f e and a gravimetric measurement of m J mw should allow the determinat ion of S while the clay is mixed with water (a dilute solution of D20 in H20 ). Unfortunately, however, to determine e the sample thickness must be known precisely and, because infrared radiat ion is strongly absorbed at the frequency of O D stretching, the thickness cannot exceed -0 .0025 cm. Although these requirements can be met i f great care is exercised, they preclude the routine measurement of S by this method. Mulla and Low (1983) indicated the feasibility of measuring S quickly and conveniently by infrared spectroscopy provided the problem of sample thickness could be resolved. Internal reflectance spectroscopy (Harrick, 1967) may provide a solution to this problem inasmuch as one of its advantages is that it does not require a knowledge of the thickness of the sample. Other advantages of an internal reflectance spectroscopic method are the ease of sample preparation, the absence of interference fringes (Harrick, 1967), the ease of analyzing samples having large absorption coefficients and, hence, requiring short path lengths (Katlafsky and Keller, 1963), and the lack of scattering in the spectra of mineral powders (Harrick and Riederman, 1965). The present paper reports the results of our investigation of the measurement of S by infrared internal reflectance spectroscopy. Copyright 9 1985, The Clay Minerals Society 391 392 Mulla, Low, and Roth Clays and Clay Minerals Parabollc~ ~Parabolic Mirror~ N~ / ~Mlrror ZnS Heiicylinder ~ FMl~rtor~// suC/aYsion \ ~FMl~tro r Figure 1. Diagram illustrating the principle of internal reflectance spectroscopy (0 = angle of incidence). MATERIALS A N D METHODS The following clays were used: nine <2-tzm, Nasaturated montmori l loni tes (Belle Fourche, California Red Top, Czech #650, Italian, Nevada, Rio Escondido, Romanian, Texas, and Upton) prepared by Low (1980); five <2-am, Na-saturated soil clays (nos. 128, 207, 208, 210, and 217) prepared by Romkens et al. (1977); kaolinite (Hydrite 10) from the Georgia Kaolin Company; and six <2-am, homoionic samples of Upton montrnofillonite saturated with Li +, K § Cs § Rb § Mg ~§ and Ca 2§ The last samples were prepared from Nasaturated clay by washing it with chloride solutions of these ions, dialyzing out the excess electrolyte, and freeze-drying the products. The value of S for each of the clays was determined by the EGME method of Carter et al. (1965) as modified by Low (1980). A stock solution of 8% D20 in H20 was prepared by pipeting 8 ml of pure D20 into a 100-ml volumetric flask and adding distilled, deionized water to volume. This solution was mixed with the various clays to yield samples for spectroscopic analysis having accurately known values ofmJmw, the mass ratio of clay to water. Before being analyzed, the samples were allowed to equilibrate for one day in sealed weighing bottles. A grating, ratio-recording infrared spectrophotometer (Perkin-Elmer Model 180) was used for the spectroscopic analyses. It included a wire grid-AgBr, common-beam polarizer and a single pass, variable angle, internal reflectance accessory manufactured according to our specifications by Harrick Scientific Corporation, Ossining, New York. A simplified diagram of this accessory is presented in Figure 1. Radiat ion from the source of the spectrophotometer passes through the hemicylinder of ZnS (diameter = 2.54 cm) and strikes the sample. A common-beam polarizer on the spect rophotometer allows only that radiat ion properly polarized relative to the plane of the hemicylinder to pass on to the detector. Depending on its frequency and angle of incidence, different fractions of the radiat ion Table 1. Specific surface area, S, of clay minerals as estimated by the adsorption of EGME. S Mineral Source (mUg) Na-Belle Fourche Belle Fourche, South 771 Dakota Na-California Red Top Red Top, California 605 Na-Czech #650 Unknown 438 Na-Italian Ponza, Italy 696 Na-Nevada Lovelock, Nevada 487 Na-Rio Escondido Unknown 664 Na-Romanian Unknown 753 Na-Texas Sierra Blanca, Texas 568 Li-Upton Upton, Wyoming 800 Na-Upton Upton, Wyoming 800 K-Upton Upton, Wyoming 480 Rb-Upton Upton, Wyoming 470 Cs-Upton Upton, Wyoming 540 Ca-Upton Upton, Wyoming 800 Mg-Upton Upton, Wyoming 720 Soil Na-clay #128 Romkens et aL (1977) 205 Soil Na-clay #207 Romkens et al. (1977) 335 Soil Na-clay #208 Romkens et al. (1977) 340 Soil Na-clay #210 Romkens et al. (1977) 411 Soil Na-clay #217 Romkens et al. (1977) 303 Kaolinite (Hydrite 10) Georgia Kaolin 35 Company are absorbed, refracted, and reflected. Hence, the intensity o f the reflected radiat ion differs from that of the incident radiat ion and can be used to determine the spectroscopic and optical properties o f the sample. The recommended procedure follows: 1. Warm up the spectrophotometer for at least 30 rain. 2. Set the spectrophotometer for 1.5-cm -~ resolution at 2600 cm -1. 3. With the ZnS hemicylinder, support, and goniometer mounted in the sample compartment , set the goniometer at 51". 4. Set the polarizers to a setting of 90 ~ for maximal absorption by the sample. 5. Adjust the 100% readings on the recorder using a reference beam at tenuator in the reference compartment. 6. Place a drop of the H O D solution on the support that holds the sample against the flat surface of the hemicylinder, carefully bring the support and hemicylinder together, and gently tighten the connecting screws. When the solution (sample) is viewed through the hemicylinder, no air bubbles should be visible. 7. Place the assembled hemicyl inder and sample support containing the H O D solution in the reflectance assembly and adjust the goniometer until the reflectance reading on the recorder chart is 72%. (For our instrument, this reading was attained when the goniometer was set at 51~ Vol. 33, No. 5, 1985 Surface area of clays by internal reflectance spectroscopy 393