Goel, (Miss) Sneh

Joshi, I. M.

Kalsh, S. C.

Lakhanpal, M. L.

English

NOPR

Indian Journal of ChemistryVol. 21A, February 1982, pp. 101-104Phase Transition in Adsorbates: Part VIII-Anomalous Phase Tran-sition Behaviour of Adsorbed Dioxane & BenzeneM. L. LAKHANPAL*, I. M. JOSHI, (Miss) SNEH GOEL & S. C. KALSHDepartment of Chemistry, Panjab University, Chandigarh 160014Received 21 October 1980; revised and accepted 10 August 1981Two different kinds of phase transitions have been observed for benzene adsorbed on TiO. and for dioxaneadsorbed on TiO. and AI.O.. The higher temperature transition occurs at temperatures just below the normal freezingpoint of the bulk adsorbate material. The lower temperature transitions are similar to those observed for systems exhi-biting single phase transitious. The occurrence of higher temperature transition in these systems has been cousideredto result from freezing of adsorbate which migrates to the outer surface of the adsorbent due to sudden variation intemperature of the systems. The lower temperature transitions have been attributed to gradual shift in triple point of anadsorbate-adsorbent system.INtwo earlier communicationsv", benzene anddioxane adsorbed on silica gel were reportedto exhibit two distinct inflection points in theplots of log p versus lIT. Considering these inflec-tions to be analogous to liquid-solid phase transi-tions, the first transition occurs at temperaturesclose to the normal freezing point of the adsorbatein its bulk state and the other occurs at a lowertemperature - the extent of depression in freezingpoint being strongly dependent on the relative vapourpressure (p/Po). Out of about twenty different adsor-bate-adsorbent systems studied by us so far, thebehaviour of adsorbed benzene and dioxane is ex-ceptional since in all other cases, only one phasetransition is observed.Two distinct phase transformations in adsorbedwater were reported by Antoniou", Litvan+, Ploosterand Gitlin", Haly" and othersv". Using highresolution NMR, Derouane" identified two differentphase transitions in adsorbates. Recently, Chungand Dash-" from heat capacity measurements carriedout on adsorbed nitrogen, observed two phase transi-tions - one near the normal freezing point of bulknitrogen and the other at a lower temperature.The inescapable conclusion from the above ob-servations is that the two transitions must correspondto two different states of the adsorbate. One of thesestates must be similar to the bulk state of the adsor-bate and responsible for higher temperature transitionand the other to the adsorbed state responsible forlower temperature transition.Since all the above mentioned observations pertainto the use of porous adsorbents, it was tempting toundertake studies on the freezing behaviour of ben-zene and dioxane on a non-porous adsorbent besidesaccumulating further information on their freezingbehaviour on other porous adsorbents. The phasetransition studies on benzene and dioxane adsorbedon a non-porous adsorbent (Ti02) and on dioxaneadsorbed on a porous adsorbent (Al203) form thesubject matter of the present investigation.Materials and MethodsSpec pure benzene was used as such. Dioxane(AR) was first distilled over P20S and then redistilledover sodium. The fraction distilling at 101.5°Cwas collected and carefully stored for use. NeutralAl203 and Ti02 were dried in an oven at 110°C beforeevacuation.Adsorption isotherms of dioxane adsorbed onTi02 and Al203 were studied at 35°C making use ofthe McBain balance. Vapour pressures of dioxaneadsorbed on Ti02 and Al203 and of benzene adsorbedon Ti02 at different temperatures were measuredwith the help of an oil-mercury manometer'. Boththe cooling and warming runs were carried out foreach of the adsorbate-adsorbent systems underinvestigation.Results and DiscussionAs expected for a non-porous adsorbent'>, adsorp-tion of benzene on the surface of Ti02 is limited tothe formation of multilayers. The adsorption iso-therms of dioxane adsorbed on Ti02 and Al203 areof type III (showing no hysteresis) and type IV(showing pronounced hysteresis), respectively. Thisfurther strengthens our belief that adsorption onTi02 takes place in multilayers while adsorptionon Al203 is accompanied by capillary condensationof adsorbates.The vapour pressure-temperature data has beenanalysed by plotting log p versus lIT. The observedtransition temperatures corresponding to the inflec-tions in such plots for various adsorbate-adsorbentsystems are reported in Table 1. Some typicalplots of log p versus lIT for dioxane adsorbed onTi02 are shown in Fig. 1. The data reported inTable 1 reveal two phase transitions in benzene-Ti02, dioxane- Ti02 and dioxane=Al.O, systemsat p/po lower than 0.8,0.9 and 0.9 respectively. Abovethese relative vapour pressures, these systems exhibitsingle transitions like other adsorbate-adsorbentsystems.101//INDIAN J. CHEM., VOL. 21A, FEBRUARY 1982It is interesting to observe that irrespective of thenature of the adsorbent the lower temperature transi-tion (T}) observed for various adsorbate-adsorbentsystems bear the same functional dependence onpjpo' For instance, it may be seen that points corres-ponding to lower temperature transitions observedfor dioxane- Ti02 and dioJi.ane-AI203 systems ofwidely varying values of p/po lie on the same linearplot of log p versus liT (Fig. 2). This should besufficient to dispel the belief that phase transitionbehaviour is particularly associated with capillarycondensation or the existence of concave menisci.Whatever be the nature and mechanism of thesephase transitions, it has to be common to systemsexhibiting capillary condensation and those in whichmultilayer adsorption occurs. The mechanism ofgradual shift in triple point, proposed by Lakhanpalet al.12 is in fact followed by dioxane- Ti02, dio-xane-A1203 and benzene- Ti02 systems. Thevalues of l':,H* = 2.303 Rm (calculated from theslope m of straight line joining the points corres-ponding to lower temperature transitions on a plotof log p versus liT) are, however, 12.6, 12.6 and14.9 kcal/rnol for dioxane=-Tit.i., dioxane=Al.O,and benzene=-Ti'O, systems, respectively. The ob-served low value of l':,H* (12.6 kcaljmol) as com-pared with the average value-s of 15.3± 1.0 kcal/molfor other adsorbate-adsorbent systems does not,however, violate the mechanism of gradual shift intriple point.As expected from the proposed mechanism forfreezing of an adsorbate the plot of log p versus 1jTafter the commencement of freezing should followthe straight line joining the apparent freezing points.This would be the case when the slope m equals theslope (m') of the actual linear plot (of any adsorbate-adsorbent system) after commencement of freezingand l':,H* equals l':,H*' (l':,H*' = 2.303 Rm'). Inother words, the ratio l':,H*' Il':,H* is expected to beunity. On examining the present experimental data0·2 P/po(~)=0.907nl n.(~):0·9'20·10·0- 0,'35 36 370·2c-o·,'"20·0- 0'10·0-0,'.0·2'"E.- 0·1-o·~·0'.53~r>/o(_)~0·877PI ;.( -6-0-6-). O· 83537P/~(--)·0.66&PI;' (-.. ):0·66537Fig. 1 - Typical plots of log p versus liT for dioxane adsorbedon Ti02 for different values of »t», at T}.Pure p_dio~ane EIAdsorb ent Cooling Warm in9TiOZ 0 I!:.AlZ03• ,.0·30"0·0 Fig. 2 - Plot of log pversus liT passing throughthe points corresponding tothe lower temperature tran-sitions for dioxane adsorbedon TiO. and A1203D.·o·,·0·43735102ILAKHANPAL et al. ANOMALOUS PHASE TRANSITIONS IN ADSORBED BENZENETABLE 1 - VALUES OF TEMPERATURES OF FREEZING (Tf" AND TF) AT DIFFERENT VALUES OF plpoBeozene-c-TtO, (To = 278.7 K) Dioxane-Ti02 (To =285.0 K) Dioxane-A120. (To = 285.0 K)p/Po Tl plpo Tn piPo 1 plpo Tn plpo Tl· »l», Tnf f Tr f f(at T}) (OK) (at Tp) (OK) (at TP CK) (at TF) eK) (at T)} (OK) (at TF) (OK)0.961 ct 278.0 0.972 283.7 0.986 284.60.920 W 277.0 0.964 282.9 0.966 284.30.877 C 275.8 0.907 281.3 0.980 283.40.857 W 275.1 0.912 281.1 0.981 283.70.802 C 272.9 0.680 278.1 0.917 280.0 0.944 281.60.780 W 272.9 0.597 278.7 0.896 280.0 0.921 281.70.718 C 272.2 0.596 277.5 0.877 279.6 0.873 284.5 0.919 279.9 0.878 285.00.706 W 272.1 0.564 278.7 0.835 279.3 0.846 285.0 0.871 281.4 0.874 285.00.703 C 270.6 0.606 276.5 0.827 276.6 0.746 285.0 0.848 279.10.695 W 270.4 0.564 277.8 0.812 276.2 0.741 284.0 0.859 278.40.667 C 269.5 0.537 276.2 0.799 275.0 0.746 282.6 0.867 278.3 0.841 285.00.658 W 269.1 0.525 276.5 0.783 274.6 0.712 283.1 0.842 277.5 0.849 283.40.653 C 768.1 0.484 275.9 0.780 273.4 0.667 283.8 0.803 276.1 0.767 284.70.643 W 267.8 0.509 274.7 0.729 271.80.603 C 268.0 0.471 274.4 0.731 271.1 0.633 283.5 0.768 274.9 0.745 283.30.604 W 267.7 0.469 274.4 0.731 271.4 0.617 282.7 0.750 276.2 0.714 283.40.596 C 266.0 0.405 275.2 0.668 270.1 0.556 280.1 0.695 271.2 0.662 282.20.576 W 266.5 0.403 275.6 0.665 270.4 0.566 281.1 0.712 273.4 0.69 282.80.530 C 265.1 0.334 274.4 0.634 268.3 0.694 269.90.528 W 263.5 0.331 274.6 0.631 268.2 0.543 280.3 0.687 270.2t C and W indicate cooling and warming runs.TABLE 2 - RATIO (£-,H*'I[:),.H*) CORRESPONDING TO DIFFERENT VALUES OF plpo FOR VARIOUS ADSORBATE-ADSORBENT SYSTEMSp/po [:),.H*'I[:),.H* plpo [:),.H*'I[:),.H* plpo [:),.H*'H* p/Po [:)"H*'I[:),.H* p/Po [:),.H*'I[:),.H* plpo [:),.H*'I[:)"H*(at T/) (at TP (at TP(at Tb (at T}) (at Tl)BENZENE- Ti02 DIOXANE- Ti02 DIOXANE-AI203 t-BUTYL ALCOHOL-AI20" CYCLOHEXANE-AI203 ACETIC ACID-Al.Os0.961 ct 0.70 0.972 0.95 0.986 1.05 0.966 C 0.84 0.959 0.520.920 W 0.66 0.964 0.97 0.966 1.01 0.931 W 0.83 0.916 0.47 0.979 1.020.877 C 0.60 0.907 0.98 0.980 1.00 0.891 C 0.72 0.929 0.49 0.949 0.970.857 W 0.57 0.912 0.99 0.981 0.99 0.860 W 0.73 0.918 0.48 0.919 0.940.802 C 0.44 0.917 1.00 0.944 0.99 0.851 C 0.69 0.883 0.47 0.844 0.970.780 W 0.49 0.896 0.94 0.921 0.94 0.806 W 0.70 0.871 0.46 0.834 0.980.718 C 0.35 0.877 0.94 0.919 1.04 0.827 C 0.67 0.893 0.47 0.825 0.970.706 W 0.33 0.835 0.84 0.871 0.85 0.791 W 0.64 0.861 0.43 0.796 0.920.703 C 0.38 0.827 1.05 0.848 1.00 0.781 C 0.73 0.821 1.020.695 W 0.38 0.812 0.89 0.859 1.03 0.763 W 0.64 0.847 0.45 0.809 0.970.667 C 0.38 0.799 0.94 0.867 0.95 0.715 C 0.71 0.819 0.43 0.732 0.940.658 W 0.38 0.783 0.93 0.842 0.97 0.699 W 0.67 0.769 0.39 0.738 0.940.653 C 0.34 0.780 1.03 0.803 0.99 0.664 C 0.64 0.728 0.39 0.684 0.970.643 W 0.32 0.729 1.00 0.639 W 0.54 0.713 0.35 0.684 0.970.603 C 0.28 0.731 0.97 0.768 0.85 0.601 C 0.76 0.631 0.42 0.569 1.100.604 W 0.27 0.731 . 1.08 0.750 0.82 0.582 W 0.64 0.622 0.420.596 C 0.33 0.668 0.92 0.695 1.40 0.588 C 0.74 0.624 0.36 0.563 0.710.576 W 0.32 0.665 0.98 0.712 0.97 0.572 W 0.63 0.619 0.35 0.564 0.920.530 C 0.32 0.634 0.96 0.694 1.02 0.560 C 0.67 0.562 0.37 0.480 0.800.528 W 0.33 0.631 0.95 0.687 0.95 0.541 W 0.65 0.560 0.36 0.472 0.99tC and W indicate cooling and warming runs.along with the data on other adsorbate-adsorbentsystems reported earlier1h13, it is seen that the valuesof the ratio tc,.H*'f tc,.H* (Table 2) lower than unityobserved for some adsorbates (benzene, r-butyl alco-hol and cyclohexane) can be attributed to the possi-bility of condensation of vapour with the decreasein temperature which becomes significant in the caseof adsorbates having high vapour pressures at theirrespective freezing points.According to Litvarr', the lower temperaturetransinon results from migration of adsorbate fromsmaller pores to larger ones. Since the hypothesisof migration of adsorbate from smaller pores tolarger ones cannot account for similar characteristicfeatures (such as the vapour pressure-temperaturedata) for all adsorbate-adsorbent systems, there arestrong reasons to believe that lower temperaturetransitions occur by the gradual shift of triple point.The high temperature transition (Tn for adsorbeddioxane or benzene-'", is close to the normal freezing103//INDIAN J. CHEM., VOL. 21A, FEBRUARY 1982point of the bulk adsorbate. The transition tem-peratures show some dependence on p/Po (thetransition Tf' is found to occur at lower temperatureswith decrease in p/Po)' From the limited data it isdifficult to account for the dependence of Tr onp/Po but for the present it is perhaps more importantto assign reasons for the occurrence of higher tem-perature transition.Litvan! attributes the high temperature transitionto freezing of adsorbate which migrates from largervoids to outer surface of the adsorbent. Mechanismfor the migration of adsorbate from larger voidsto outer surface must, however, be associated withsudden variation in temperature of the system asadmitted by Litvan himself. Since higher tempe-rature transition is also exhibited by adsorbates ad-sorbed on non-porous materials (present studies),it would be more appropriate to consider the hightemperature transition to result from the migrationof adsorbate to outer surface of the adsorbent ratherthan to migration of the adsorbate from large voidsto outer surface of the adsorbent under non-equili-brium conditions. The explanation afforded byAntoniou", Plooster and Gitlin" attributing the twodistinct phase transitions to two different structuresof adsorbate, is also inconsistent with conditions ofthermodynamic equilibrium. The absence of highertemperature transition in adsorbates other thandioxane and benzene may be attributed to the fact104//that time allowed for minute changes in temperaturein the vapour pressure-temperature studies hasbeen long enough such that the systems at any stageare not far removed from equilibrium conditions.AcknowledgementThe authors wish to thank the CSIR, New Delhifor the award of a senior research fellowship to oneof them (S. C. K.).References1. LAKHANPAL,M. L;.:,(JOSHI,I. M. & SHARMA,S. C., Indian1. Chem., 11 (19~, 796.2. LAKHANPAL,M. L., JOSHI, J. M. & SHARMA,S. c., Indian1. Chem., 11 (1 , 3), 922. . /3. ANTONIOU,A. A., 1. phys. Chem., 68 (1904), 2754.4. LITVAN,G. G., 1. colloid interface Sci., 38 (1972), 75.5. PLOOSTER,M. N. & GITLIN, S. N., 1. phys. Chem., 75(1971), 3322.6. HALY, A. R., 1. phys. Chem. Solids, 33 (1972), 129.7. KATO CHUZO & TSUNASHIMAYUTAKA, Kogyo KagakuZasshi, 69 (1966), 1690.8. DRANSFELD,K., FRISCH, H. L. & WOOD, E. A., 1. chem.Phys., 36 (1962), 1574.9. DEROUANE,E. G., Bull. Soc. Chim. Be/g., 78 (1969), 1,11.10. CHUNG, T. T. & DASH, J. G., 1. chem, Phys., 64 (1'f6),1855.11. LAKHANPAL,M. L., SHARMA,S. c., JOSHI,I. M. & KALSH,S. C., Indian J. Chem., 20A (1981), 228.12. LAKHANPAL,M. L., JOSHI, I. M., SHARMfI"B. B. &SHARMA,S. c., Indian J. Chem., 15A (19177),276.13. LAKHANPAL,M. L., SHARMA,S. c., JOSHI,I. M. & KALSH,S. c., Indian J. Chem., 19A (1980). 1142.

Phase Transition in Adsorbates: Part VIII-Anomalous Phase Transition Behaviour of Adsorbed Dioxane & Benzene

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Phase Transition in Adsorbates: Part VIII-Anomalous Phase Transition Behaviour of Adsorbed Dioxane & Benzene

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