Oct., 1961
THERMODYNAMICS OF WATERADSOEPT~ON BY KAOLINITE
1853
CATION HYDltATION EFFECTS ON THE THERMODYNAMICS OF WATER ADSORPTION BY KAOLINITE' BYJ. J. JURINAKAND D. H. VOLMAN Department of Soils and Plant Nutrition and Department of Chemistry, University of California, Davis, California Received April 1 1961 ~
The isosteric aH and differential AS values are calculated for the adsorption of water on Ca- and Ba-kaolinite and compared with similar functions for water adsorption by Li-kaolinite, which is regarded free from cation hydration effects. In the coverage range studied water interaction with Ca- and Ba-kaolinite was thermodynamically similar. The effect of hydration was noted throughout the monolayer region. The apparent heat of cation hydration suggests that forces emanating from the kaolinite surface do not extend much beyond the monolayer.
Introduction Previous studies in this Laboratory have indicated that water adsorbed on Li-kaolinite a t low coverages does not react with sorbed lithium ions. This lack of reactivity permits Li-kaolinite to be regarded as a kaolinite surface free from exchangeable cation hydration effects.* Thermodynamic data3 of water adsorbed on Li-kaolinite have indicated that adsorbed water molecules attract each other and form. a structural surface network which reduces the entropy of the adsorbed water molecules to a value that is similar to the entropy of ice at 0". Prior to adsorbate interaction, water molecules appear to be mobile, although localized adsorption with a high degree of vibration ca.nnot be pre~ludecl.~The entropy of adsorbed water (So298 - ASds),with no configurational correction, is less than that of liquid water between W/W, values of about 0.30 and 0.70. No data were obtained for W/W, less than 0.15. These differential thermodynamic data are in fair agreement with those reported by Martin.5 He concluded that sorbed water on Li-kaolinite and Na-kaolinite was more random than liquid water because adsorbate interaction only occurred over a relatively small PiPo range and integral ent'ropy calculations of sorbed water resulted in values that exceeded those of liquid water. Since water adsorption on Li-kaolinite is considered free from cation hydration, the study reported liere is an attempt to ascertain the effect of cation hydration on the adsorption of water by kaolinite. Experimental ]Methods.-The kao!init,e used was from Dry Branch, Geo:rgia, and donated by the Georgia Kaolinite Co. The exchange capacity was determined as 1.5 meq./ 100 g. with regard t o lithium and sodium ions and 2.0 meq./100 g. with regard to calcium and barium ions. The kaolinite w:ts made monoionic by washing the clay several times with a normal chloride solution of a given cation t h m washing free of chloride with 9570 alcohol. The clay was air-dried, crushed, and the 0.5-1.0 mm. aggregate fraction retained as the adsorbent. Prior to adsorption all samples were degassed a t 68-70' for 24 hours, a t 1 X 10mm. pressure. Water was found to be reversibly adsorbed on Li-kaolinite when outgsssed at this temperature.2 In regards to Ca-lkaolinite the choice of this outgassing temperature is rather arbitrary. Previous studies suggested that a t io* adsorption of water is at an apparent maximum, but because of the complicating factors of particle coalescence and possible dehpdroxylation of the mineral a t higher tem(1) Support of this work by a grant (NSF-G10228)from the National Science Founclation is gratefully acknowledged. (2) J. J. Jurinak. J . Phya. Chem.. 66, 62 (1961). ( 3 ) J. J. Jurinak and D. €1. Volrnan. (bid.. 65, 150 (1961). (4) J. W. Ross and R. J. Good, ;hid., bo. 1167 (1956). (5) R. T. Martin. Paper presented at 8th National Clay Conference, Norman, Oklahoma. 1959.
peratures the exact hydrated state of the calcium ion cannot be fully defined. For purposes of comparison, the outgassing temperature of Ba-kaolinite also was chosen a t 50 . The adsorption apparatus and technique have been previously described .z Isotherms were obtained a t 29.45 f 0.05" and 15.30 i -15". The adsorption datfa for a given monoionic sample at each temperature represent the composite of data for 3 or 4 independent adsorption cycles.
Calculations-The differential free energy, enthalpy and entropy of adsorption are calculated for an isothermal transfer of one mole of adsorbate from its standard state, at 760 mm., to a surface which exists a t various coverage^.^.' The ClausiusClapeyron equation was used to calculate the differential (isosteric) heat, A H ; the differential entropy, AS, of adsorption is calculated by its well known relation to AF and AH. An at,tempt to separate the AH of cation hydration from the energetics of surface adsorption was made by handling the adsorption data in a novel manner. At a given temperature and equilibrium pressure the amount of water adsorbed by lithium kaolinitme was subtracted from the water adsorbed by a hydrated system, i . e . , Ca- or Ba-kaolinite. The water adsorption difference was found a t the two adsorption temperatures and the Clausius-Clapeyron equation was applied holding the wat,er adsorption difference constant. The differential enthalpies calculated in this manner are plotted against W/W,, and can be compared with the AH of adsorption. Results Figure 1 shows the adsorption of water by the monoionic syst,ems a.t the two tempcraturcs used in the study. I t was noted that at low pressures the adsorption of water by Ca- and Ba-kaolinite was similar. Figures 2 and 3 show the AH and AS values for the adsorption of water by the monoionic kaolinites. The thermodynamics values refer to the defined process and are plotted relative to the reference Li-kaolinite surface. The apparent heats of hydration of adsorbed Ra and Ca ions arc shown in Fig. 4 and also are plotted against surface coverage relative to Li-kaolinite. Discussion Data in Fig. 1 show the cation effect, on thc adsorption of water by Ca and Ba systems. As the relative pressure reached approximat,ely 0.04 the cation effect became evident in the expected manner. Assuming that Li-kaolinite can lie rc( G ) C. Bimball and E. K. Rideal. Proc. R o y . SOC.(London). A181, 53 (1946). (7) n. II. Everett, Trans. Faraday Soc., 46, 942 (1950).
J. J. JURINAK AND D. H. VOLMAN
1854
Vol. 65
-AS
:
29.45 'C
*: 1530.C
I Fig. 1.-The adsorption of water vapor by monoionic kaolinite a t the designated temperatures. The Li-kaolinite data have been reported previ0udy.a
11,.
0' . & . & & & j i j W/wm (Li), 0 1 ,
.
Fig. X-The ASof water adsorption by Ca- and Ba-kaolinite a t T = 296"K., using Li-kaolinite as the reference surface. The Li-kaolinite data have been reported previous1y.a
\
_j
10 t
Fig. 2.-The AH of water adsorption by Ca- and Bausing Li-kaolinite as the reference kaolinite, a t T = 296"K., surface. The Li-kaolinite data have been reportedS previously.
garded as a .reference surface and that the exchangeable cai;ion does not alter the surface area8.9 during the adsorption of water, the amount of water associated with each cation was calculated by using the BET equation and subtracting the water adsorbed by Li-kaolinite. The calculations indicate that in the completion of the monolayer each calcium ion coordinated 6.5 water molecules while each barium ion coordinated 2.7 molecules. The interaction of 6.5 water molecules with each Ca ion is in fair agreement with the data of Hendricks, Xelson and Alexander'O and Meringl' who 18) A. G. Keenan. R. W. Mooney and L. A. Wood, J . Phys. Collord Chem.. S I , 1462 (1951). (9) .I .J. Jiirinak and D. H. Volrnan. ibid., 68, 1373 (1959).
I
O L L L L L - - I--/ J l i 0 0.30 0.50 0.70 0.90 1.1 1.3 W/Wnl(LI) Fig. 4.-The apparent differential enthalpy of adsorbed Ba and Ca ion hydration calculated from adsorption data as described. 1
both using different techniques estimated that each Ca ion adsorbed on montmorillonite was initially associated with 6 water molecules. Considerable disagreement is found with the data of Keenan, Mooney and Wooda who reported that a t the formation of the water monolayer on kaolinite an average of 10 water molecules were associated with each exchangeable Ca ion and 4.7 molecules associated with each Ba ion. However, if the ratio of water oriented by Ca to the water (10) S. B. Hendricks. R. A . Nelson and L. T. ilexander. J . Am. Chcm. Soc., 64, 1457 (1940). (11) J. hlering. Trans. Faradag Soc.. 4!ZB, 205 (1946).
Oct., 1961
THERMODYNAMICS O F
WATER ADS~MJTTOX BY I(.4OLINITE
oriented by Ba can be used as an index of relative attracting strength, their data result in a ratio value of 2.2, which is close to the ratio of 2.4 calculated from the data of the present study. The differential enthalpies and entropies of water adsorption by Ca- and Ba-kaolinite are compared with Li-kaolinite in Fig. 2 and 3. All functions are plotted against W/Wm where W, is based on the a,mount of adsorbate required for thc development d the monolayer on Li-kaolinite. All curves are thus plotted relative to the defined reference surface. It should be noted that if the convention were allowed to plot the AH and AS values for Ca-. and Ba-kaolinite against their respective W , values the thermodynamic values for all monoionic systems studied would be very similar over the major portion of the monolayer region. The hydration effect would be restricted mainly to the coverage region
