BY C. S . BROOKS I'/it~licationiYo. 21 7 , Shc!l Dcvdopiiictit t'oitrpany, Exploration und P~odiiclioiiResearch Ihisioii, Houston, 'I'e.ras Receaoed 'Vouember 1 1 IDSO ~
The fret: onergios ol immersion for several clay ininerals-p2-rophyIlite, liaolinite and niontmorillonitc-in m-ater, ethanol and n-ht:pt:ine a t 30" :tnd for illite in water a t 20' have been calculated from vapor sorption isotherms by application of the Uibbs ittlsorpt,ion eqiiation. The thermodynamic validity of free energies of immersion calculated from vapor sorption isotherms on clay minrrals has been examined, particularly in regard to (a) hysteresis in the isotherms,.and (b) selection of a realistit. vapor-solid iiit,erfaci:tl area. The infliienre of exchangeable cation and of sample preparation (in particular, the w-wter ld't on the montmorillonite surface) on t,he accessibility of the intralattice surfares for adsorption of polar vapors has been tlt:iuonatiat,cd. The same degmsing conditions that leave calcium montmorillonit,e platelcts separated by one nioleriilar layer oi wttcr adsorbate collapse sodium montmorillonite so that rehydrat~ionby water vapor sorption a t lorn relative humidity is drasticallyoreduced. This demonstrates the effect of strong van der \Vaals interaction a t plat,elet-platelet separations less than 3 A., which is a result of the lower free energy of hydration of the sodium exchange cation. It was considered especially significant that the surface-charge densities of clay minerals as varied in morphology, rheniical composition and p:trticle size as pyrophyllite, kaolinite, illite and montmorillonite were all of the order of 20-23 X 10-8 meq. of CEC/cm.a when referred to ivater and ethanol surface areas. Comparison of free energies of immersion for clays with those for quartz and for sodium ion-exchange resins demonstrated the contribution of the cation exchange capacity; for three clays-sodium illite, calcium illite and calcium mont,morillonite-a quantihtive est,imate was obtained for the contribution of the cationexchange capacit,y t,o the total free energy of hydration. Reference of free energies of immersion calculated from vapor isotherms to the cation-exc,hange capacity, rather than to the often indeterminate vapor-solid interfacial area, Tvas shown to have considerable utility for polar adsorbates on clay minerals.
Introduction Surface teiisions and oil-water interfacial tensions are susceptible to direct measurement, but vapor-solid and liquid-solid iiiterfacial tensioiis cannot be el-duated directly. By application of the Gihhs adsorptioii equation' to vapor isotherms, the difference between the solid surface tension and the liquid- solid iiiterfacial teiisioii caii be estimated. This difference has been defined as the free energy of iminersioii of the solid in the liquid. SeI.eral studies have been made in which the free energies of immersion of mineral surfaces in liquids were d(1termined 1)y this method from vapor sorption isotherms. These investigations were made with n ater, n-heptane and nitrogen vapors on hydrophilic mineral surfaces, such as silica, calcite, mica and anatase, and 011 hydrophobic rniiieral surfaces, such as graphite. Iii this study the free energies of immersion of several clay minerals-pyrophyllite, kaolinite aiid montmorillonite-in water, ethanol a i d n-heptane a t 30" :md of illite in water at 20" were determined from vapor sorption isotherms. Of particular interest were the dependence of the free energy of adsorption a t vapor activities less thnii 1 upon the cntion-exrhange capacity a i d the exchangeahlt. (*:ition,the amount of water present on thr rlny surfaw, the vapor polarity, and the coliditioiis of :rtl.;orl)eiit prepar:itioii aiid accessibility of clay sul'facei. The free energies of immersion of these clay minerals in nater, ethanol and n-heptane are discussed in the light of similar studies made by other investigators for vapor sorption on mineral surfaces,'-5 ( 1 ) C, J
Iura and W. D Harkins, J . Am. Chem. S O P ,6 6 , 1356
(194%)
R i j r d nnii H I< Lirinrvton i b z d , 6 4 , 2383 (19-12) ( $ 1 N H.rcheiiiian and k C Hall THIJJ O L R ~ A 6L2 , 1212 (19%) (0 11 rf ~ ~ J I L I I I S(: 1 i i ~ . i n d l ' . €1 1,nrstr.J A m ( ' k u m Sol 6 8 , 554 (194b ( 5 ) I> F1 ~ J I i ; ~ 2 d l L i &lid S -Voshuir. I'roc. IC?OI{ Suc ( L o u d t i n ) ,
(2)
( 7
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particularly silica, and for miter ~ . a p o sorption r ion-exchange resim6-B
011
Experimental Apparatus and Procedure.-The sorption isotherms wero measured with hrlical spring balanc.es.10 The constriiction of the apparatus ; ~ n dthe operating procedures have been described elsewhere .I1 Adsorbents.-The pyrophyllite, kaolinite, illite and montmorillonite used in this study were portions of rlay minerals from XPI Research Project 49 on Reference Clay Ilinerals. The rhcmical analyses and physical properties of these claj-s have been reported by that projcct.12 Pyrophyllite Kaolinite S1li t.c hIontmorillonite
A P I Sample KO. 47 AI'I Sample Eo. 3 A1'I Sample No. 35 Same source as A N Sample KO. 26
Rol>I)ins,K. C. l,lacon, GR. Fithian, 111. Clay Spur, \Vyo.
Pyrophyllite and kaolinite wcrc nsed i n 1,hcirnative condition wit,h whatever exchangeable cations wcre originally prrscnt (predominantly CaZ+and Mgz+). The sodium and calcium illites and the c:dcium montmorillonite were prepared by conversion on ion-exchange resin columns . I 3 The Wyoming montmorillonite was a stock sample used for colloid investigations in this Laboratory and was supplied by the Raroid Division, National Lead Company. The native (sodium) montmorillonite was used in its untreated condition, in which approximately 90ybof the exrhangeable cation was sorlirim. The qnart'z w:is n. Iiigli-purity s:tnipIc~IIi t l i :t siiriwc arcla of 1.8 m.z/g. 1he polystyrtw? ion-csrh:mgc: rvsins hrttl vxryirig clcyws of siilfon:hon, n-liich pruvitlccl cation c~\c~li:tngc~ cal):ic.itic:s J .
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11. P. Gregor, B. R. Siindl~ciiii,I
