CHARACTERIZATION OF MICROPORES OF IMOGOLITE BY MEASURING RETENTION OF QUATERNARY AMMONIUM CHLORIDES AND WATER

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1 Clay Science 4, (1972) CHARACTERIZATION OF MICROPORES OF IMOGOLITE BY MEASURING RETENTION OF QUATERNARY AMMONIUM CHLORIDES AND WATER K. WADA AND T. HENMI Faculty of Agriculture, Kyushu University, Fukuoka (Received April 28, 1972) ABSTRACT The micropore status of air-dry and preheated imogolites has been examined by measuring retention of (CH3)4NCl, (C2H5)4NCl, (C4H9)4NCl, (CH3)2(C6H5)(C6H5CH2)NCl and water. Interactions between the salt and imogolite were followed by observing the moist or dry appearance of the clay-salt mixture when the salt is deliquescent and by observing the changes of the x-ray pattern of imogolite. The adsorption of water vapor at various relative humidities was determined by gravimetry. No stoichiometrical relationship was found between the maximum amounts of different salts retained by imogolite. The volume ratios of the salt and water at their retention maxima to the solid phase of imogolite were calculated from the retention data. They were found in ranges 1.3 to 1.6 and 0.5 to 1.1 for the air-dry and preheated imogolites, respectively. The difference between them gave a measure of the void ratio of the inter-structure-unit pores. The collapse of these pores by heating and its inhibition by the salt uptake were indicated by x-ray analysis. The volume ratios obtained with the preheated imogolite increased with the increasing size of the cation up to (C4H9)4N+, but the largest cation (CH3)2(C6H5)(C6H5CH2)N+ gave the lowest value. On this basis, the presence of the intra-structure-unit pores of about 10 A in diameter was inferred. INTRODUCTION Micron-length threads of imogolite with a diameter of 100 to 300A A were found to consist of fiber units with separation of the order of A by Yoshinaga et al. (1968). The arrangement and internal structure of the fiber units were then discussed by Russell et al. (1969) and by Wada and Yoshinaga (1969) taking the diffraction, infrared and other relevant data into consideration. They agreed on that imogolite has a unique chain structure with a repeat unit of 8.4 A parallel to the fiber axis, but there remained a difference of opinion regarding to the other aspects of the structure. The porosity in an air-dry imogolite was estimated to amount to about 55 % from the difference between its "true" and "bulk" densities and from its adsorption of water (Wada and Yoshinaga, 1969). They then equated this porosity with that between the structure units in each thread of imogolite,

2 128 K. Wada and T. Henmi and used as a basis for reasoning of a paracrystalline arrangement of the structure units. However, a high resolution electron microscopy has indicated that the structure unit is a hollow tube with the outside and inner diameters, A and 7-10 A, respectively (Wada et al., 1970). The results of these studies imply that there are at least two different types of micropores in imogolite, that is, the inter- and the intra-structure-unit pores. The purpose of the present study is to provide additional data on these micropores both in air-dry and preheated imogolites. The data have basic importance to assessment of the chemical reactivity of imogolite and to deduction of the arrangement of its structure unit. Electron microscopy and diffraction also provide information about the morphology and arrangement of the structure units. The specimen in the electron microscope is, however, subjected to high vacuum and bombardment of the electron beam. The observed features could therefore considerably be different from those in air-dry state. On the other hand, limited number of reflections of imogolite and their broadness present difficulties in the x-ray analysis. In the present study, the pore size and porosity were estimated from measured retention of quaternary ammonium chlorides and water as "fillers" of the micropores. MATERIALS The gel films separated from the Kitakami pumice bed (Miyauchi and Aomine, 1966 ; Wada and Yoshinaga, 1969) were used as a sample of imogolite. Those from the Kanuma pumice bed (Miyauchi and Aomine, 1966) were also used for comparison. They were treated successively with 30 % 11202, Na2S2O4-NaHCO3-Na citrate and 2 % Na2CO3 (Jackson, 1956) for removal of organic matter and extractable oxides. After washing with water, the gel films were dispersed in an acid medium (ph 4.0) by sonic wave treatment (20 KC), and the <2ƒÊm fractions were collected and freeze-dried. METHODS Aliquots of the suspension containing 4.7 mg of imogolite were spread as uniformly as possible on glass slides on a level and air-dried. The salt solutions of appropriate concentrations were prepared from reagent grade NH4C1 and quaternary ammonium chlorides. Exactly 0.1 ml of these salt solutions were added gently to the prepared clay films and air-dried again. The maximum uptake of the salt was estimated from the minimum rates of salt addition required to note the following phenomena: 1) The levelling off of the change in the x-ray pattern of imogolite due to salt addition, the detail of which will be shown later, 2) moist appearance of the clay film due to presence of the excess salt when the salt is deliquescent and 3) production of x-ray diffraction of the salt due to its excess, when the air-dried salt-clay mixture was heated. The second reaction was more sensitive and reliable in detection of the excess salt than the third.

3 Micropores of Imogolite 129 The adsorption of water vapor at various relative humidities was determined by gravimetry. The sample was equilibrated after evacuation with an atmosphere at 25 C over the saturated solution of an appropriate salt such as ZnC12, CH3COOK, CaC12, K2CO3, Ca(NO3)2, NH4C1 and KNO3 or BaC12. Water or 5% H2SO4 and P2O5 were used in place of the salt solution for equilibration with the highest and lowest relative humidities, respectively. Uptake of Quaternary RESULTS AND DISCUSSION Ammonium Chlorides by Air-Dry Imogolites Fig. 1 shows the x-ray patterns of imogolite to which different amounts of (C2H5)4NC1 were added. X-ray scanning was made both at room temperature and at 140 C by applying a simple heating device on the back of the glass slide. The latter heating results in an enhancement of the diffraction intensity at A relative to that at A in the first broad band of imogolite. No much difference in the x-ray pattern was observed between the reference sample and the sample to which 95 m. mols of the salt was added per 100 g clay, but the relative enhancement of the diffraction intensity FIG. 1. X-ray diffraction patterns of imogolite samples to which different amountsof (C2H5)4NC1 were added. œand œ œ denote the rates of salt addition at which moistening and x-ray diffraction due to the excess salt were at first detected, respectively.

4 130 K. Wada and T. Henmi FIG. 2. X-ray diffraction intensity ratios (19A/12. 6A) measured for imogolite samples to which different amounts of NH4C1 or quaternary ammonium chloride were added. A horizontal bar indicates the range in which the excess salt was at first detected. at A is more marked for the latter (Fig. 2). With the increasing salt addition, this enhancement was reduced, and the first band became increasingly more symmetrical and showed no observable change upon heating. This effect of salt addition on the x-ray pattern of imogolite was levelled off by addition of the salt more than 320 m. mols per 100 g clay. Moistening of the clay film due to deliquescence of the excess salt and its x-ray diffraction peaks at first appeared by addtion of 320 and 415 m.mols of the salt per 100 g clay, respectively. The maximum uptake of (C2H5)4NC1 was then estimated to be m.mols per 100 g clay. Similar determinations were carried out with NH4C1, (CH3)4NC1 and (C4H9)4NC1. The limit of detection of the excess salt expressed in m.mols per 100 g clay decreased with the size of the cation: NH4C1, <635 ; (CH3)4NC1, ; (C2H5)4NC1, ; (C4H9)4NC1, Evidently no stoichiometrical relationship holds in the reaction. In Fig. 2, the ratios of the diffraction intensity at 19.0 A to that at 12.6 A found both at room temperature and at 140 C are plotted against the rate of salt addition expressed in ml per ml clay. In the latter calculation, known densities of the respective salts and imogolite were used, except for (C4H9)4NC1, the density of which was assumed to be 1.0. The ranges of detection of the excess salts are also shown as horizontal bars. The maximum-uptake values of all the quaternary ammonium chlorides are found to be in a range from 1.3 to 1.6 ml per ml clay, suggesting that the reaction is a simple filling of the micropores in imogolite with the salt molecules. The retention of (CH3)4NC1 was also determined with imogolite from the Kanuma pumice bed, but practically no difference was found between the Kitakami and Kanuma samples. The salt retntion expressed

5 Micropores of Imogolite 131 FIG. 3. X-ray diffraction patterns of imogolite samples to which NH4C1, (CH3)4NC1 and (C4H9)4NC1 were added. For legends, see FIG. 1. in the volume ratio may be taken as a measure of the void ratio for air-dry imogolite, which corresponds to the porosity 57 to 62 %. Most of the changes in the diffraction intensity ratios (Fig. 2) could be interpreted in terms of the penetration of the salt in the inter-structureunit pores of imogolite. This seems to support a previous interpretation of the x-ray pattern of imogolite, that is, the enhancement of the diffraction intensity at A by heating relates to a change in the arrangement of the structure units upon removal of interstitial water (Wada and Yoshinaga 1969). The presence of the "salt molecules" between the structure units likely prevents their rearrangement. The detail of the change in the x-ray pattern is, however, different from one salt to another. Addition of NH4C1 resulted in two splitted maxima at 19.2 and 12.3 A when the air-dried salt-clay mixture was heated at 140 Ž They persisted 'even after the diffraction lines due to NH4C1 appeared (Fig 3). The ratios of the diffraction intensity at 19.0 to that at 12.6 A determined at room temperature and at 140 Ž thus never have the same value with the increasing addition of NH4C1 (Fig. 2). This probably associates with an incomplete filling of the pores with NH4C1 as suggested from its much smaller maximum uptake. The cation size is evidently not a cause of this incomplete reaction. The lack of deliquesent nature of NH4C1 may deserve attention since some properties associated with deliquescence may also have an important role in intercalation of some compounds such as CH3COOK and hydrazine in kaolinite. The former compound was found to be also a very effective complexing reagent with imogolite (Inoue and Wada, 1971). Addition of (CH3)4NC1 produced a very similar effect on the x-ray pattern as found with (C2115)4NC1 (Fig. 3). Imogolite saturated with these salts show a symmetrical diffraction band with maximum at A, both at room temperature and at 140 Ž. This temperature indifference may be interpreted in terms of the geometrical fitness between the fillers and the pores, although there are slight yet unaccountable differences in the location,..

6 132 K. Wada and T. Henmi of the second diffraction bands (Figs. 1 and 3). Saturation of (C4H9)4NCl also resulted in a diffraction band with a single maximum, but its location at A upon heating at 140 Ž is different from that at room temperature (Fig. 3). The ratio of the diffraction intensity at 19.0 to that at 12.6A therefore never has the same value at room temperature and at 140 Ž (Fig. 2). The difference observed may signify a strain brought about by the presence of the larger cation or by unfavorable packing of the constituent ions in the inter-structure-unit pores. Uptake of Quaternary Ammonium Chlorides by Preheated Imogolite Measurements of retention of quaternary ammonium chlorides were carried out with imogolites preheated at 250 or 300 Ž. Heating at 250 Ž ensures nearly complete removal of interstitial water from imogolite, while that at 300 Ž results in considerable dehydroxylation (Wada and Yoshinaga, 1969). The maximum uptake of (CH3)4NCl expressed in ml per ml clay is estimated to be (Fig. 4), which is about half of that obtained of the air-dry sample. There is no significant difference in the maximum uptake between the samples preheated at 250 Ž and 300 Ž. In line with the foregoing interpretation, the reduction in the salt uptake can be interpreted in terms of the collapse of the inter-structure-unit pores by preheating. If the first diffarction band of imogolite arises solely from interference between the structure units, the addition of (CH3)4NCl to the preheated sample should have no effect on the x-ray pattern of imogolite. As shown in Fig. 4, however, it resulted in a decrease of the diffraction intensity ratio at 19.0 A to that at 12.6 A measured at room temperature. It is suggested FIG. 4. X-ray diffraction intensity ratios (19A/12.6A) measured with preheated imogolite samples to which different amounts of (CH3)4NCl were added. For legends, see FIG. 2.

7 Micropores of Imogolite 123 from comparison with the corresponding data obtained of the air-dry samples (Fig. 2) that the change of the diffraction intensity ratio denotes a partial rehydration taking place in the inter-structure-unit pores. The presence of water but not of (CH3)4NC1 can be inferred from the increase in the diffraction intensity ratio measured at 140 Ž relative to that at room temperature. This increase did not occur in the samples which were not preheated and had an ample supply of the salt (Fig. 2). The result as a whole suggests that water was introduced into the incompletely collapsed pores by assistance of nearby present (CH3)4NC1, possibly due to its deliquescent nature. Maximum uptakes of (CH3)4NC1 and (C2115)4NC1 were found at ml per ml clay, while that of (C4119)4NC1 was found at 1.1 ml per ml clay. The latter higher value associates with the greater reversion of the change in the x-ray pattern of imogolite brought about by preheating. X-ray analysis indicated that (C4119)4NC1 in addition to water penetrated into the interstructure-unit pores once collapsed by preheating. The maximum uptake of (CH3)4NC1 by preheated imogolite from the Kanuma pumice bed was found at ml per ml clay, which was lower than that found with imogolite from the Kitakami pumice bed. The difference was ascribed to the difference in the collapse of their inter-structureunit pores. X-ray analysis indicated that the collapse by heating even at 250 Ž took place in more irretrievable way in the Kanuma sample. Adsorption of Water Vapor The adsorption curves of water vapor on the air-dry and preheated imogolites are shown in Fig. 5. The amounts of water adsorbed on the respective samples at the highest relative humidity are 1.16 and 0.70 expressed in ml per ml clay. The corresponding retentions of quaternary ammonium chlorides are and ml per ml clay. The difference between the two curves was shown in broken line in Fig. 5, which may be assigned to water adsorption into the inter-structure-unit pores. It is different from the adsorption curve of the preheated imogolite. This difference indicates different mechanism of adsorption and different nature of adsorption sites between the inter-structure-unit pores and the pores left in the preheated imogolite. The adsorption of water into the inter-structure-unit pores shows a fairly steep rise at higher relative humidities, suggesting that capillary condensation is taking place. In contrast, the adsorption of water into the pores left in the preheated imogolite manifests a monolayer feature, indicating that stronger intermolecular interaction is involved. However, the initial portion of the adsorption curve shows a good agreement between the air-dry and preheated imogolites. It suggests that the micropores left in the prehated sample are present as such in the air-dry sample. Determination of water vapor adsorption on imogolites preheated at 200 Ž was also carried out by Yoshinaga (1969). From his data at about 95 % in relative humidity, the following retention values are calculated: Kitakami,

8 134 K. Wada and T. Henmi FIG. 5. Adsorption of water vapor by air-dry and preheated imogolites. A curve showing their difference is shown in broken line. 1.11; Imaichi, 0.74 and, Kanuma 0.71 ml per ml clay. The larger value of the Kitakami sample signifies an incomplete collapse of its inter-structure-unit pores at 200 Ž in comparison with the other samples. If the retention value is equated with the void ratio, the value of 0.7 may therefore be taken as that common to the dehydrated imogolite. Correlation between Morphology, Diffraction and Porosity Data As described in Introduction, imogolite threads appear in the electron microscope as assemblies of a tubular structure unit in nearly parallel alignment (Wada et al., 1970). Three different pores would then exist in imogolite, which may be termed inter-thread, inter-structure-unit and intra-structureunit pores. It has been shown that the x-ray and porosity data obtained in the present study can qualitatively be interpreted by assuming the presence of the inter- and intra-structure-unit pores, the former pores being collapsed upon removal of interstitial water by heating. The maximum uptake of either salt or water by the preheated imogolite can be calculated by assuming an ideal assembly of their tubular structure units and accommodation of the salt or water in the inter-thread and intrastructure-unit pores. In this calculation, a hexagonal close packing of the tubular structure units was assumed as a first approximation, and the uptake of the salt or water into the inter-thread pores was equated with its monolayer adsorption on the external surface of the thread. In Fig. 6, A

9 Micropores of Imogolite 135 and C denote the cross sections of the intra-structure-unit pore and the monolayer of the adsorbed molecule, respectively. The approximate dimensions of the tube and thread were taken from the electron microscopy. The inter-structure-unit pore left in the preheated imogolite is indicated as B, which can accommodate a smaller water molecule but not a larger quaternary ammonium ion. In calculation the molecules accommodated and adsorbed were also assumed to occupy the full space of A, B and C. FIG. 6. Cross section of an ideal assembly of tubular structure units in preheated imogolite. FIG. 7. Comparison between maximumuptake values of water or quaternary ammonium chlorides determined and calculated for preheated imogolite. Points ; (1) water, (2) (CH3)4NC1, (3) (C2115)4NC1, (4) (C4H9)4NC1 and (5) (CH3)2(C6H5)(C6H5CH2)NC1. In Fig. 7, the maximum-uptake values measured and calculated are plotted against the monolayer thickness of the fillers. The latter thickness was calculated for quaternary ammonium chloride simply by adding the diameters of the constituent ions and then by dividing it by two. The probable range of both the measured maximum uptake and the monolayer thickness of the fillers are indicated by vertical and horizontal bars, respectively. Agreements between those measured and calculated are found for water, (CH3)4NC1 and (C2I-I5)4NC1. Since less ambiguities from the assumptions described above are expected for these smaller molecules, the observed agreement gives a support to the general picture proposed for the micropore status in the preheated imogolite. The intra-structure-unit pore is then estimated to occupy the volume of 0.35 ml per ml clay. From the values of A and C and the retention measured with the air-dry imogolite, the inter-structure-unit pore is also estimated to occupy the volume of ml per ml clay.

136 K. Wada and T. Henmi No particular incompatibility was found between the retention values measured and calculated for the quaternary ammonium chlorides up to (C4H9)4NCl (Fig. 7).

10 136 K. Wada and T. Henmi No particular incompatibility was found between the retention values measured and calculated for the quaternary ammonium chlorides up to (C4H9)4NCl (Fig. 7). The higher value for (C4H9)4NCl is expected from its penetration not only into the intra-structure-unit pores but into the interstructure-unit pores once collapsed by preheating, as indicated from x-ray analysis. The diameter of (C.119),N+ was estimated from a scale model to be 9-11 A. However, the measured retention of (CH3)2(C6H5)(C6H5CH2) NCl by the preheated imogolite was much smaller than that calculated. Griesbach (quoted by Helfferich, 1962) gave this cation a possible largest diameter of 11 A. Although the salt is not deliquescent, its maximum uptake by the air-dry imogolite of ml per ml clay is not much different from those of the other quaternary ammonium chlorides. The result indicates that the inter-structure-unit pores can accommodate, but the intra-structure-unit pores can not accommodate (CH3) 2 (C6H5)(C6H5CH2)N+. It is estimated that the opening of the latter micropore is of the order of 9-11 A, showing a good agreement with that observed in the electron micrograph. CONCLUSION Imogolite in air-dry state contains three different pores, the inter-thread, inter-structure-unit and intra-structure-unit pores. The latter two micropores can be filled with water or with quaternary ammonium chloride which contains an appropriate size of the cation and preferably is of deliquescent nature. The porosity in air-dry imogolite accounts for 55 to 60 % of the total volume. Roughly 50 and 25 % of the total porosity belongs to the inter- and intra-structure-unit pores, respectively. The openings of the former micropores are generally larger than those of the latter of 9-11 A. REFERENCES HELFFERICH, F. (1962) Ion Exchange. McGraw-Hill Book Co., Inc. New York. p INOUE, T. and WADA, K. (1971) Clay Sci. 4, JACKSON, M. L. (1956) Soil Chemical Analysis Advanced Course. Published by the author, Madison, Wisconsin. MIYAUCHI, N. and AOMINE, S. (1966) Soil Sci. Plant Nutr. (Tokyo) 12, RUSSELL, J. D., MCHARDY, W. J. and FRASER, A. R. (1969) Clay Minerals 8, WADA, K. and YOSHINAGA, N. (1969) Am. Mineralogist 54, WADA, K., YOSHINAGA, N., YOTSUMOTO, H., IBE, K. and AIDA, S. (1970) Clay Minerals 8, YOSHINAGA, N. (1968) Soil Sci. Plant Nutr. (Tokyo) 14, YOSHINAGA, N., YOTSUMOTO, H. and IBE, K. (1968) Am. Mineralogist 53,

REACTIONS BETWEEN HUMIFIED CLOVER EXTRACT AND IMOGOLITE AS A MODEL OF HUMUS-CLAY INTERACTION : PART II

Clay Science 4, 71-80 (1971) REACTIONS BETWEEN HUMIFIED CLOVER EXTRACT AND IMOGOLITE AS A MODEL OF HUMUS-CLAY INTERACTION : PART II TAKAHIRO INOUE and Koji WADA Faculty of Agriculture, Kyushu University,