MICROPOROSITY IN MONTMORILLONITE FROM NITROGEN AND CARBON DIOXIDE SORPTION

Transcription

1 Clays ad Clay Merals, Vol. 25, pp Pergamo Press Prted Great Brta MICROPOROSITY IN MONTMORILLONITE FROM NITROGEN AND CARBON DIOXIDE SORPTION L. A. G. AYLMOI~ Departmet of Sol Scece ad Plat Nutrto, Isttute of Agrculture, Uversty of Wester Australa, Nedlads, W.A (Receved 15 August 1976) Abstract--Ntroge adsorpto at 78~ ad carbo doxde sorpto at 195~ o homooc lthum, sodum, caesum, calcum, lathaum ad hexae dammoum saturated motmorllotes have bee examed by meas of V- plots. I the case of carbo doxde, sorpto o the lthum saturated clay was used as a stadard for comparso of the other samples. The troge plots dcate that most of the surface area les super-mcropores of approxmately 10 A equvalet plate separato. Varatos betwee catos are attrbuted to dffereces the structure of the porous matrx formed o dryg rather tha dffereces the degree of etry to quas-crystalle regos. Whle the tal sorpto of carbo doxde clearly s flueced by the solvato propertes of the catos, the subsequet reversblty of the sotherms ad learty of the V- plots dcates that for all but the largest catos the same sorpto process s occurrg o surfaces exteral to the quas=crystalle regos INTRODUCTION Gas sorpto studes o the layer lattce alumoslcate clay merals have provded valuable formato o the surface propertes of these materals ad o the mechasms of sorpto processes mcroporous systems. The motmorllote clays are of partcular terest sce polar water molecules are able, certa crcumstaces, to peetrate betwee ad separate the dvdual alumo-slcate sheets. The extet of ths swellg process vares wth the ature of the exchageable cato ragg from ear complete dsperso for the sodum saturated clay to the restrcted sorpto of oe or two layers of water molecules for clays saturated wth polyvalet catos. Whle the ature of the exchageable cato has relatvely lttle fluece o the specfc surface area obtaed for fxed lattce clays such as llte ad kaolte, usg stadard gas sorpto procedures (Aylmore, 1974a), wde varatos the area measured are obtaed for the same motmorllote saturated wth dfferet Catos. Aylmore ad Qurk (1967, 1971) attrbuted ths varato area of the dry clay accessble to gas sorpto to chages the orderlesss of packg of the motmorllote lameltae cosequet to a large extet o the degree of dsperso of the lameuae suspeso. O the other had, Thomas ad Bohor (1968) beleved that essetally the same matrx structure s formed o dryg regardless of the cato preset but that carbo doxde ad, to a lesser extet, troge, peetrate the terlamellar space to a extet determed prmarly by the sze ad charge of the terlayer cato. Subsequetly, Aylmore et al. (1970) have show that the troge ad carbo doxde areas obtaed for motmorllote deped markedly o the outgassg procedure used ad suggested 148 that resdual water molecules facltatg the etry of sorbet molecules could expla the greater peetrato observed by Thomas ad Bohor (1968). More recetly, Frpat et al. (1974) have preseted X-ray powder dffracto ad.r. evdece supportg the terlamellar peetrato of carbo doxde homooc smecttes although, oce aga, usg a relatvely mld outgassg procedure. Kudso ad McAtee (1974) by classfyg troge sorpto sotherms o homooc motmorllote to Type I or Type II plots of the Bruauer classfcato, cocluded that o sgfcat terlamellar sorpto occurred the case of the sodum, barum ad caesum clays. Sgfcat terlametlar sorpto of troge occurred oly for a arrow sze rage of larger exchageable catos such as the dmethylammoum ad Co(e)~ + os. I a prevous paper, the author (Aylmore, 1974a) used V- plots to exame the sorpto of troge, argo ad carbo doxde o llte ad kaolte clays ad sesquoxdes. I these "reduced" sotherms the volume sorbed s plotted agast the umber of statstcal layers whch would be sorbed o a o-porous materal at the same relatve vapor pressure, thus allowg ay varato from ormal multlayer adsorpto to be observed. A smlar approach s used here to aalyse sorpto sotherms for troge ad carbo doxde o homooc motmorllotes. EXPERIMENTAL Sorpto sotherms were determed o homooc samples prepared from the followg atural motmorllote deposts: Wyomg betote-motmorllote from the Joh C. Lae Tract, Upto, Wyomg. Stadard clay

2 Mcroporosty motmorllote 149 meral No. 25b of the Amerca Petroleum Isttute Research Project No. 49. Redhll motmorllote--from Redhll, Surrey, Eglad: supple d by Fullers Earth Compay. Homooc samples of the < 2 Itm fractos were prepared as prevously descrbed (Aylmore, 1974a) ad compressed to 200 mg cores to facltate expermetal hadlg. Gas sorpto sotherms were determed by the usual doser techque after outgassg the samples overght, usually at 250~ ad 10-6ram mercury pressure. Prevous studes (Aylmore, 1960) have show that ths procedure has o sgfcat effect o the swellg behavor of the motmorllotes. The sample saturated wth hexae dammoum os was outgassed tally for 2 days at 50~ to prevet the destructo of the orgac cato ad the at creasg temperatures to 250~ causg the gradual destructo of the large orgac catos. Sorpto sotherms were determed wth the sample bulb mmersed a lqud troge bath for troge sorpto ad a getly strred sold carbo doxde-ethaol slush for carbo doxde sorpto. Ntroge SOrlto RESULTS AND DISCUSSION V- plots for troge adsorpto o lthum, sodum, caesum, calcum ad lathaum saturated Wyomg betote ad O sodum, calcum ad caesum saturated Redhll motmorllote are show Fgures 1 ad 2, respectvely. Values of were calculated usg a stadard sotherm costructed from a combato of the data provded by Shull (1948) for ~ ~ AO ~176 v 30 j o~ d o I I oj _x-.-"- a x..x ~'" A I I., a v ~ ~:"2~ ~v I I I 7'~_...~ I Il rp~o ~" Fgure 1. V plots for troge adsorpto o homooc Wyomg betote: x sodum, I2] lthum, 9 caesum, A calcum, V lathaum A ,,s l fz~ p X o~ _o~ o x1 Fgure 2. V- plots for troge adsorpto o homooc Redhll motmorllote: x sodum, 9 caesum, A calcum. troge adsorpto o crystalle materals of large crystal sze ad smlar measuremets obtaed these laboratores. For systems such as these clay merals, cotag essetally slt-shaped pores, lear regos V- plots dcate that wth these regos the shape of the sotherm s essetally the same as that obtaed o the o-porous referece solds ad that urestrcted physcal adsorpto s occurrg o the fat surfaces of the clay plates. A egatve curvature occurs whe the area avalable for sorpto s reduced as a result of the fllg of smaller pores by the mergg of sorbate layers. Coversely, ehaced sorpto as a result of capllary codesato wll result a postve curvature of the F- plot. I each case, the plots extrapolate readly through the org ad the specfc surface areas calculated from the slope of the tal lear regos agree well wth, although geerally slghtly hgher tha, those obtaed by applcato of the BET theory (Table 1). Followg ths tal lear rego, there s a gradual reducto slope commecg each case at approxmately = 1.2. Thus at = 1 all the surface area accessble to troge should be covered wth a Complete layer of troge molecules. Assumg a value of some 4 A for the thckess of a sgle layer of adsorbed molecules, a egatve devato slope at would, a system cotag essetally oly slt-shaped pores, suggest that the smallest pores preset were of some 10 A plate separato. Comparatve plots of the data the lear forms of the Bruauer Type I (or Lagmur) ad Type II (or BET) equatos [e.g. Fgure 4(a) for Cs + Wyomg betote] support the cosstet agreemet of the sorpto process for troge o these homooc motmorllotes wth the multlayer theory, as observed by Kudso ad McAtee (1974), rather tha

3 150 L.A.G. AYLMORE Table 1. Specfc surface areas obtaed by applcato of BET theory ad from slope of V- plots for gas sorpto o homooc motmorllotes. Surface area (m2g) N 2 C02 Clay BET (V) BET (V-) Lthum Wyomg betote Sodum Wyomg betote Caesum Wyomg betote Calcum Wyomg betote Lathaum Wyomg betote Sodum Redhll motmorllote Caesum Redhll motmorllote Calcum Redhll motmorllote Hexae dammoum Redhll motmorllote (50~ * Hexae dammoum Redhll motmorllote (200~ Hexae dammoum Redhll motmorllote (250~ * Outgassg temperature. wth the moolayer equato. I each case, the BET plots provde good straght les whlst the Lagmur plots show marked egatve curvature, dcatg that cosderably more sorpto s occurrg tha correspods to moolayer formato. Such plots are ot partcularly sestve to small varatos but should be adequate to judge the predomat process. Kudso ad McAtee (1974) reported that for motmorllote saturated wth large dmethylammoum or Co(e) ] + os, comparatve plots of the BET ad Lagmur fuctos dcated that sgfcat terlamellar sorpto of troge dd occur. I Fgure 3, V plots for troge sorpto o Redhll motmorllote saturated wth hexae dammoum os ad outgassed at temperatures of 50~ 100~ ad 250~ respectvely, are gve. The hgh surface area estmated for the clay outgassed at 50~ clearly arses largely from adsorpto mcropores. Whether ths occurs by volume fllg wth regos where the lamellae are already suffcetly separated ro 50 V z.o I f ~ && Ax A [ zxtx I o ~o'''~ - I 9"..ooo.o.1o j 3 o j L 5 Fgure 3. V- plots for troge adsorpto o hexae dammoum Redhll motmorllote after outgassg at x 50~ A 100~ ad 9176 respectvely. to allow the sorpto of oe or more layers of troge molecules or by troge expadg the already partly expaded quas-crystals, s ot clear. Although the plot has bee extrapolated to the org to provde a estmate of specfc surface area, the V- plot does ot allow a dstcto betwee the mechasms to be made. Because of the complex terleavg of lamellae evdet the motmorllote matrx t s probable that may regos there s o sharp trasto betwee tra-crystalle ad ter-crystalle surfaces. From the comparatve plots Fgure 4(b) t s clear that after outgassg at 50~ the sorpto data do ot coform satsfactorly to ether the BET or Lagmur equato. The crease sorpto wth creasg pressure s greater tha that correspodg to Lagmur-type sorpto, causg a sgfcat dowward curvature the plot of the Lagmur fucto. O the other had, the ehaced sorpto s adequate to correspod to predomatly multlayer sorpto causg a slght upward curvature the BET plot. I these crcumstaces, the actual accessble surface area could be cosderably hgher tha that calculated by assumg the BET Vm, or that calculated from the slope of the V- plot, to represet a complete layer o separate surfaces. A extreme case of such tra-crystalle sorpto s llustrated by the BET aalyss of water vapor sorpto o motmorllote gve by Mooey et al. (1952). As poted out by Sg (1967), good agreemet betwee the values of V,, calculated by the two methods does ot tself cofrm the valdty of ether. Wth the gradual destructo of the large hexae dammoum os by progressve heatg to hgher temperatures, there s clearly a collapse the teral volume of the clay matrx resultg a marked reducto surface area accessble to troge sorpto wth the formato of larger quas-crystalle regos. After heatg to 250~ the comparatve plots [-Fgure 4(c)] suggest that subsequet troge sorpto s predomatly multlayer le wth that observed for the orgac catos.

4 Mcroporosty motmorllote x 8 ~-6 x X x ) & PP= o'2,o I I I :1 0.2 PIPo Fgure 4. Comparatve plots of Type I ad Type II fuctos of Bruauer classfcato for troge adsorpto o (a) caesum Wyomg betote, (b) hexae dammoum Redhll motmorllote outgassed at 50~ ad (c) hexae dammoum Redhll motmorllote outgassed at 250~ Carbo doxde sorpto The terpretato of carbo doxde sorpto sotherms at 195~ o clay meral systems s beset by several problems whch are of o real sgfcace troge sorpto. The assumpto that carbo doxde adsorbed at 195~ behaves lke a supercooled lqud wth the correspodg saturato vapor pressure (approx. 1.86atm), has bee made by most workers dealg wth coal ad charcoal systems (Aderso et al., 1965; Walker ad K, 1965). However, calculatg surface areas from BET plots, molecular areas much larger tha that calculated from the lqud desty (approx. 17 It) frequetly have had to be assumed to gve values cosstet wth troge areas. I addto, there s clearly a marked depedece of the packg desty o the crystalle ature of the substrate wth values for molecular area ragg from 16.3 h to 22h. I dealg wth clay meral systems the problem s further complcated by the sgfcat depedece of carbo doxde sorpto o the ature of the exchageable cato. Aylmore (1974a) showed that for homooc kaolte ad llte clays the sorpto of carbo doxde creased the order La 3 + < Ca 2 + < Na + < Cs +. The prevous factors make t dffcult to costruct a sutable calbrato curve for carbo doxde sorpto agast whch comparso by meas of V- plots ca be made for other sorbets. However, the preset study, the same crystalle surface s volved ad varatos sorpto ca be attrbuted ether to the ature of the exchageable cato or varatos porous structure. By the sutable choce of oe sotherm as stadard, several terestg comparsos ca be made. A umber of workers have demostrated that, o heatg, the small sze of the dehydrated lthum o eables t to mgrate to the crystal structure of clays wth cosequet eutralzato of the charge orgatg from somorphous replacemet. Keea et al e~ oe---'-e.o's~ -~ 00_~- ~-'01 40 o o' 30 o 20 o'd'" olr Vo ~.~-~-v--,,, o ~ " - 0 I 3O 20 +~+x ~' x+ ~ex - +x-"--" ~ J Fgure 5. P- plots for carbo doxde adsorpto o homooc Wyomg betote: sodum O, x ads., O, + des. ; lthum [] ads., HI des. ; caesum O des.; calcum A ads., 9 des.; lathaum V ads., 9 des.

5 152 L.A.G. AYLMORE 1,0 o..<yoo V I I,~O,lb,3 t,a ~" A 2(], I ^.O.o'O',~., z ds ~.'o ~; Fgure 6. V- plots for carbo doxde sorpto o homooc Redhll motmorllote: sodum O ads., 9 des. ; caesum 9 ads., des. ; calcum A ads. ; 9 des. (1931), Jurak (1961) ad others have cocluded that after dehydrato of lthum saturated kaolte, the adsorbed lthum os are stearcally hdered fromhydratg ad have o apparet effect o subsequet water adsorpto. Smlarly, the water sorpto ad cato exchage capacty of lthum motmorllote s markedly reduced by heatg (Qurk ad Theg, 1960). Sce the outgassg procedure employed here volves heatg the clay to 250~ uder vacuum t seems reasoable to adopt the sorpto sotherm for carbo doxde o the lthum saturated clay as a stadard wth whch to compare the sorpto o the other homooc samples. Whle t may ot follow that carbo doxde sorpto o ths clay s completely uflueced by the presece of the L + os, the experece wth water sorpto suggests that t s lkely to be the least affected. The V- plots obtaed ths maer for carbo doxde sorpto o sodum, caesum, calcum ad lathaum saturated Wyomg betote are show Fgure 5 together wth the calbrato plot calculated for lthum Wyomg betote. Plots for carbo doxde sorpto o sodum, calcum ad hexae dammoum saturated Redhll motmorllote are show Fgure 6; Both adsorpto ad desorpto pots have bee plotted, sce t s sgfcat that for the lthum, sodum, calcum ad lathaum catos the complete adsorpto-desorpto sotherms to codesato at PPo ~ proved effectvely reversble. I each of these cases, good straght le plots are obtaed. Whle the calbrato plot for lthum saturated Wyomg betote aturally passes through the org, those for the sodum, calcum ad lathaum catos exhbt postve tercepts o the volume axs. Although t could be argued that these tercepts result from varyg degree of tercalato of carbo doxde to the motmorllote quas-crystals, the reversblty of the sotherms suggest that the most logcal explaato les ehaced sorpto resultg from teracto of the carbo doxde wth the surface catos, as observed by Aylmore (1974a) for carbo doxde sorpto o homooc llte ad kaolte clays. The order of ehacemet dcated by the tercepts o the volume axs s the same,.e. La 3+ < Ca 2+ <Na + as observed o the kaolte ad llte clays. Varatos mcroporosty wth dfferet packg of the lamellae the presece of dfferet catos also may have a small effect o the magtude of these tercepts. The dffereces eergy of teracto of the tal sorpto are reflected the C values obtaed. Prevous argumets wth respect to the effects of adequate outgassg are hghlghted by the two plots show for sodum Wyomg betote Fgure 5, both obtaed o the same sample. Ths sample was tally advertetly otjtgassed at 100~ stead of the customary 250~ The frst sotherm obtaed, show by the crosses, tally follows a slope correspodg to a specfc surface area of some 44 m2g. Over the rage ~ 0.35~).45, a sudde crease the volume adsorbed occurs, followed by a straght le plot correspodg to a ehaced specfc surface area of 50 m2g. O desorpt0, the adsorpto sotherm s retraced to the pot of ths step after whch there s a substatal hysteress correspodg to the volume of carbo doxde tercalated. O subsequet outgassg at 250~ a completely reversble sotherm correspodg to a slope of 44 m2g was obtaed. Almost certaly, the step the frst sorpto sotherm ca be attrbuted to the sudde etry of carbo doxde to terlamellar regos propped apart by resdual water molecules. Sgfcatly, the adsorpto sotherm for carbo doxde o Cs + saturated Wyomg betote s characterzed by a umber of smlar steps followed by a marked hysteress the desorpto process. The author has prevously dscussed (Aylmore et al., 1970) the fact that for the caesum cato the d (001) spacg the dry clay s approxmately l l.sa, leavg roughly 2.3 A betwee the lamellae. I ths case, much of the work of expaso already has bee accomplshed ad some tercalato of carbo doxde clearly occurs. For the Redhll motm0rllote saturated wth hexae dammoum os (Fgure 6) the d (001) spacg 'the dry state was 13.8 A. The ehaced carbo doxde sorpto exhbted by ths clay exhbts much less hysteress, presumably as a result of the greater accessblty of the terlayer area. Mcroporosty motmorllote Much of the cotroversy surroudg the questo of the possble peetrato of gases to quas-crystalle regos of the motmorllote structure, arses from the somewhat urealstc tedecy to regard the structure as beg composed of eat stacks or booklets. Ths cocept leads to the assumpto that the surfaces of the lamellae are ether teral,.e. perfectly alged wth ther eghbors, or exteral ad thus able to accommodate urestrcted multlayer adsorpto. I realty, varatos the lateral dmesos of dvdual lamellae, ther juxtaposto wth respect to ther eghbors ad terleavg of lamellae

6 Mcroporosty motmorllote 153 o dryg from suspeso, wll udoubtedly lead to a much more complex structure. Ths complexty has bee emphaszed by the author a umber of prevous publcatos (Aylmore ad Qurk, 1967, 1971; Aylmore et al., 1970b; Aylmore, 1974). Almost certaly there wll be may regos whch the terleavg of dvdual lamellae, the bucklg of lamellae, frequetly observed electro mcrographs, ad dscotutes of oe lamellae wth respect to ts eghbors, eve whe perfect parallel algmet, wll lead to the formato of slt-shaped pores of correspodg dmesos. Such pores of the order of 10 A plate separato would provde a satsfactory explaato of the behavor observed the troge V plots,.e. by accommodatg lttle more tha a sgle layer of sorbate molecules o opposte surfaces before fllg of the pores occurs. Pores of these dmesos have bee desgated 'super-mcropores' (Dub, 1974) to dstgush ther sorptve behavor as termedate betwee ultra-mcropores whch surface teractos domate ad mesopores whch capllary codesato occurs. Whe thoroughly outgassed, the troge surface area for a gve sample s accurately reproducble ad shows o sgfcat crease o prologed exposure to the sorbate. The surface areas accessble to troge sorpto represet a small proporto of the total lamellar area ad, as has bee show by varous workers (Greee-Kelly, 1954; Kudso ad McAtee, 1974), ths area for sodum motmorllote ca be reduced to less tha 5 m2g by careful mapulato of the packg durg the dryg process. I these crcumstaces, the most logcal terpretato of the V- plots s that troge sorpto occurs essetally oly o surfaces exteral to the quas-crystalle regos of parallel algmet ad pores able to accommodate geerally oe complete layer o opposg surfaces. The extet of troge peetrato to wedge-shaped pores, e.g. where two lamellae dverge, wll deped o the ature of the packg ad partcular o the fluece of the exchageable cato. However, ths clearly does ot progress to allow sgfcat further expaso of the matrx for the orgac catos studed. Ntroge sorpto sotherms o clay merals geerally exhbt cosderable hysteress at relatve vapor pressures above about 0.4 whch ca be related to rreversblty the processes of capllary codesato ad evaporato depedet o the porous structure of the matrx (Aylmore ad Qurk, 1967; Alymore, 1974b). I most cases, whe thoroughly outgassed, the hysteress loops close o desorpto at PPo ~- 0.4, as would be expected the absece of terlamellar sorpto. The exceptos to ths occur whe the expadg clays such as motmorllote have bee completely outgassed (Brooks, 1955) or the presece of some large exchageable catos (Barter ad McLeod, 1955). I cotrast, carbo doxde sorpto sotherms at 195~ o clay merals are, most cases, essetally reversble up to the maxmum pressure whch ca be obtaed before codesato of the sold carbo doxde occurs. For these samples, there s o evdece of capllary codesato of carbo doxde the pores. Oce aga, the hysteress whch occurs does so ether as a result of complete outgassg of the clay or the presece of certa of the largest exchageable catos, I these cases, the hysteress creases wth desorpto ad perssts to low relatve vapour pressures, almost certaly dcatg that t results from tracrystalle sorpto. Because of the dffereces sze, polarty, etc. of the molecules of dfferet gases, the dstcto betwee teral ad exteral surfaces of a motmorllote matrx becomes largely a matter of defto terms of ther accessblty to a gve gas. I vew of the prevous cosderato, t seems reasoable to coclude that the varatos troge surface area wth exchageable cato result essetally from real dffereces matrx structure cosequet o the degree of dsperso suspeso. Whle the tal sorpto of carbo doxde clearly s flueced by the solvato propertes of the catos, the subsequet reversblty of the sotherms ad learty of the V- plots dcates that for all but the largest catos the same sorpto process s occurrg o surfaces exteral to the quas-crystalle regos. REFERENCES Aderso, R. B., Bayer, J. ad Hofer, L. J. E. (1965) Determg surface areas from CO~ osotherms: Fuel 44, 443~452. Aylmore, L. A. G. (1960) Ph.D. thess, Uversty of Adelade. Aylmore, L. A. G. (1974a) Gas sorpto clay meral systems: Clays ad Clay Merals 22, Aylmore, L. A. G. (1974b) Hysteress gas sorpto sotherms: J. Collod Iterface Sc. 46, Aylmore, L. A. G. ad Qurk, J. P. (1967) The mcropore sze dstrbutos of clay meral systems: J. Sol. Se. 18, Aylmore, L. A. G. ad Qurk, J. P. (1971). Domas ad quas-crystalle regos clay systems: Proe. Am. Sol Sc. Soc. 35, Aylmore, L. A. G., Slls, I. D. ad Qurk, J. P. (1970a). Surface area of homooc llte ad motmorllote clay merals as measured by the sorpto of troge ad carbo doxde: Clays ad Clay Merals Aylmore, L. A. G., Slls, I. D. ad Qurk, J. P. (1970b). Reply to commets of Thomas, Bohor ad Frost o Aylmore, L. A. G., Slls, I. D. ad Qurk, J. P. (1971). The surface area of homooc llte ad motmorllote clay merals as measured by the sorpto of troge ad carbo doxde: Clays ad Clay Merals 18, 4O7-4O9. Barrer, R. M. ad MacLeod, D. M. (1955) Actvato of motmorllote by o exchage ad sorpto complexes of tetra-alkylammoum motmorllote: Tras. Faraday Soc. 51, Brooks, C. S. (1955) Sol Sc. 79, 331. Dub, M. M. (1974) O physcal feasblty of Bruauer's Mcropore Aalyss Method: J. Collod Iterface Sc. 46, Frpat, J. J., Cruz, M. I., Bohor, B. F. ad Thomas, J., Jr. (1974) Iterlamellar adsorpto of carbo doxde by smecttes: Clays ad Clay Merals

7 154 L.A.G. AYLMOgE Greee-Kelly, R. (1964) The specfc surface areas of motmorllote: Clay Mer. Bull. 5, 392~400. Jurak, J. J. (1961) The effect of pretreatmet o the adsorpto ad desorpto of water vapor by lthum ad calcum kaolte: J. Phys. Chem. 65, 62. Kudso, M. I. ad McAtee, J. L., Jr. (1974) Iterlamellar ad multlayer troge sorpto by homooc motmorllotes: Clays ad Clay Merals 22, 59. Mooey, R. W., Keea, A. G. ad Wood, L. A. (1952) Adsorpto of water vapour by motmorllote II. Effect of exchageable os ad lattce swellg as measured by X-ray dffracto: J. Am. Chem. Soc. 74, Qurk, J. P. ad Theg, B. K. G. (1960) Effect of surface desty of charge o the physcal swellg of lthum ad motmorllote: Nature 187, Shull, C. O. {1948) The determato of pore sze dstrbuto from gas adsorpto data: d. Am. Chem. Soc Sg, K. S. W. (1967) Assessmet of mcroporosty: Chem. Id Thomas, J., Jr. ad Bohor, B. F. 0968) Surface area of motmorllote from the dyamc sorpto of troge ad carbo doxde: Clays ad Clay Merals 16, Walker, P. L., Jr. ad K, K. A. (1965) Measuremet of ultrafe surface area of coals: Fuel 44,