Joshi, I. M.

Lakhanpal, M. L.

Sharma, B. B.

Sharma, S. C.

English

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Indian Journal of ChemistryVol. 14A, September 1976, pp. 649-6UPhase Transitions in Adsorbates: Part III - Vapour PressureStudies on Formic Acid, rerz-Butyl Alcohol & CyclohexaneAdsorbed on CharcoalM. L. LAKHANPAL, 1. M. JOSHI, B. B. SHARMA & S. C. SHARMADepartment of Chemistry, Panjab University, ChandigarhReceived 15 October 1975; accepted 16 January 1976The vapour pressure studies on formic acid. t-butyl alcohol and cyclohexane adsorbedon charcoal have been reported. The observed behaviour regarding the depression in freezingpoint and its variation with relative vapour pressure is found to be similar to that for adsorbate-silica gel systems [Indian J. Chern •• 11 (1973). 796, 922]. The disagreement between the values ofAHt and AHf support the earlier inference that freezing in an adsorbate-adsorbent system isnot a case of first order transition. The ratio AHt/AHf has been found to decrease withdecrease in p/po.VAPOUR pressure studies on phase transitionsin a number of substances adsorbed on silicagel have been reported--s earlier. It wasvery interesting to observe that the heats of phasetransition (IlHt) obtained from the plots of log pversus l/T were generally found to be higher (in somecases four to five times higher) than the heats of fusion(t:..H!) of the adsorbates in the pure states. It wasof interest to study whether this ratio is in someway dependent on the nature of the adsorbent. Inthis paper vapour pressure studies of formic acid,t-butyl alcohol and cyclohexane adsorbed on charcoalhave been reported at temperatures above and belowtheir normal freezing points.Materials and MethodsCharcoal used was prepared by the carbonizationof sucrose with con. sulphuric acid. The samplewas freed of the sulphate ions by repeatedly washingit with hot distilled water. The charcoal thus ob-tained was activated with super-heated steam driedand stored for use.Formic acid, t-butyl alcohol and cyclohexanewere purified as reported- earlier.Oil-mercury manometer was used for determiningthe vapour pressure of an adsorbate in the adsorbate-adsorbent system.Results and DiscussionAs reported earlier--s the plots of log p versusl/T for these adsorbate-adsorbent systems consistof two linear portions. The point of intersectioncorresponds to the temperature of inception of freez-ing of the adsorbate. Some typical plots areillustrated in Fig. 1. The depressions in freezingpoint (t:..T) for the various adsorbate-adsorbentsystems, reported in Table 1. show that t:..T increasesregularly as the relative vapour pressure (P /PO),decreases.Theoretical calculation of depression in freezingpoint - The theoretical values of t:..T were calcu-Iated as in earlier communicationst.s by Batchelorand Foster's method'' according to whichsr = ~ . 2M ['!!: _ "!:].sn, r PI Ps TAssuming the applicability of Kelvin equation,I pOfp = 2MIn, rP RT••• (1)... (2)to both liquid and solid states of the adsorbateEq. (1) was obtained by substituting In (pr/tz) andIn (POs/Ps) as 2M .II/rpl RT and 2Mls/rps RT, respec-tively in Eq. (3)3In (PlJPI) - In (P~/Ps) = - !:i(1/1) !:iff! ... (3)where the symbols have their usual meaning". Whilethe value of II!Pl in Eq. (1) is estimated fromliterature data for the pure liquid at the experimentalfreezing point, the value of Is/ps is obtained usingthe relation (4)In (PI/PI) i- (P~/Ps) = ~:~~ ...(4)This method of introducing the value of I./ps inEq. (1) is artificial since replacement of Is/Ps inEq. (1) by In (P~/ps)/ln (PI/PI) yields Eq. (3). AsPs = PI at the freezing point, Eq. (3) is reallyIn (pfIP~) = -!:i (1/T) • ~I ... (5)It is obvious that Eq. (5) is independent of thereal system under examination. It may be clearthat selection o.f In (P~/Pl) and In (P~/Ps) from experi-mental data IS nothing but determination ofIn (P~/Ps) at a particular temperature. The agree-ment shownl-3 between the so-called theoreticalvalues of tlT with the experimental values is thusmeaningless. Batchelor and Foster's methods for649INDIAN J. CHEM., VOL. 141\, SEPTEMBER 1976\02jI0'34 0-35 0·36 0-33 0-34 O-~-b'34 0'35 0-36t X r::/- t X 102 f x 102plot, of log p versus l/T for formic acid, I-butyl alcohol and cyclouexane, adsorbed[Circles (-0-0-) and triangles (-l\.-l\.-) represent cooling and warming runs]fORMIC ACIDt>/t>o.a. 7660'3()o2.A.r0.10'0-0,1-o.~'330>6Fig. 1 - TypicalTABLE 1 - FREEZING POINT AND HEAT OF PHASETRANSITION DATA FOR VARIOUS ADSORBATES ADSORBEDON CHARCOALFreezing PWat Expl. su, tlHt/point freezing freezing (caljrnole) tlHf(T) point point(OK) depression(tlT)(0C)FORMIC ACID (tlHf = 3038 c?.l/mole)280·9 0·879 0·8 2870 0·94280·3 0·873 1·4 3754 1·23278·6 0'796 3·1 2650 0·87277·6 0·750 4-1 2502 0·82275·3 0·711 6·4 2355 0·78274'0 0·668 7·7 2282 0·75I-BUTYL ALCOHOL (t>Hf = 1629 caljrnole]29'1,2 0·977 1·5 4406 2·70297·0 0·933 1·7 3680 2-26296·3 0·865 H 3036 1·86292·4 0·832 6·3 2944 1·81287·1 0·832 11·6 3404 2·08284·2 0·764 14·5 2484 1·52CYCLOHEXANE (t>Hf = 627 caljmole)278·9 0·942 0·8 4160 6·63276·3 0·869 3·4 3220 5·13274·0 0·738 5·7 3220 5·13270·3 0·601 9·4 2075 3·31650tcrt -eurv L ALCOHOl.t>/t-°=O'96SCYcLOHEXANE1'/"'·:0·669006. 0Z76·3k0·5 \0'40·3~\0·36on charcoalcalculation of depression in freezing point has hencebeen abandoned. At the same time, in view of thecharacteristic behaviour observed for the adsorbate-adsorbent systems after the commencement offreezing, there can really be no way for theoreticalcalculation of AT unless the nature of phase transi-tion is better understood.Nature of phas(' transition - The heats of phasetransition (AHt) determined from experimentalcurves of log p versus liT and the ratio AHt/Wffor adsorbate-charcoal systems are given in Table 1.It is worth mentioning that AHf values used for thecalculation of AHt/AHf are those reported inLiteratures and have not been determined fromvapour pressure-temperature data, as was doneearlier". Since the values of AHf determined fromthese two sources differ considerably from eachother in some cases, it has been felt that calorimetricvalues of AHf should be more reliable. Keepingthis in view, the values of AHtfAHf for adsorbate-silica gel systems, wherever necessary, have beenrevised and are given in Table 2.It is interesting to note that the values of Wtdiffer considerably from the corresponding valuesof AHf for almost all the adsorbates whetheradsorbed on charcoal or silica gel (Tables 1 and 2).The values of AHt/AHf generally vary from 2 to 7,the notable exception being formic acid for whichthe flHt/flH! values are nearly unity or even less.A careful examination of flHt/AHf values for all theadsorbate-adsorbent systems (Tables land 2) makesit abundantly clear that the ratio is dependent onthe value of AHf. At present, we do not see anyreason for the observed dependence and an explana-tion for this behaviour may not be possible unlessLAKHANPAL et al.: VAPOUR PRESSURE STUDIES ON PHASE TRANSITIONST ..••BLE 2 - REVISED VALUES OF IlHt/IlHj FOR FORMIC ACID, t-BUTYL ALCOHOL, CYCLOHEXANE AND BENZENEADSORBED ON SILICA GELP/po at sn, IlHt/t1Hj Plpo at t1HI t1Ht/IlHjfreezing point (caljmole) freezing point teal/mole)FORMIC ACID (t1Hj = 3038 caljrnole) BENZENE (t1Hj = 2375 caljrnole)0·946 2439 0·80 0·908 4850 2·040:885 4012 1·32 0·904' 4850 2·040·834 3894 1·28 0·887 4805 2·020·787 2778 0·91 0·771 4233 1·780·640 613 0·20 0·867 3318 1·390·640 2439 0·80 0·851 4956 2·080·580 1524 0·50 0·838 4461 1·870'832 6237 2·62I-BUTYL ALCOHOL (t1Hj = 1629 cat/mole] 0'828 3967 1-670'817 5188 2·180·908 4107 2·52 0·826 4730 1·990·885 3208 1·96 0·809 6022 2·530·869 2974 1·82 0·736 5185 2·170·791 3052 1·87 0·803 5111 2·150·673 2540 1·56 0·785 4118 1·730'596 2066 1·26 0·716 4942 2·080·773 7473 3-14CYCLOHEXANE (t1Hj = 627 cal/molc] 0·667 7166 3·010·724 5994 2·520·891 4732 7·55 0·705 6713 2·820·849 3510 5-60 0·689 6782 2·840·723 2468 3·93 0·637 3638 1·520·696 3299 5·26 0·612 3853 1·620·671 1904 3·030·600 458 0·730·582 1776 2-83o F'~"'IC ACIOb. tCH-8UTYL Al.COMOLo C'rI:LOHEXANEX WATER• &NZENE8·0I·,,7.0o , 0,,,,,,'0,,,,,,,,8·0 (b) ADSORBATE- SILICA Ge:L SYSTEMSo7·0o5·0 ,,,,,,o2·0'-0for various adsorbate-adsorbent systems651INDIAN J. CHEM., VOL. 14A, SEPTEMBER 1976a meaningful appreciation of the difference betweenMI, and Mil is available.As pointed out earliert, the ratio MI,/MII alsoshows some dependence on relative vapour pressure(P/PO) of the adsorbate-adsorbent systems. Thishas been illustrated in Fig. 2(a) for adsorbate-charcoal systems and in Fig. 2(b) for adsorbate-silica gel systems. Although a fair amount of scatter(which may be due to experimental difficulties) isobserved but, in general, the values of MI,/AHIseem to decrease with decrease in P/po, as indicatedby the dotted lines in Fig. 2. It is interesting toobserve that, except in the case of benzene-silicagel systems, the lines indicating the trend ofvariation of AH,IMII with P/po appear to converge,in almost all the cases, to zero value at a relativevapour pressure somewhere between 0·3 and 0·4.This range of P /po may be imagined to correspondto a few adsorbed layers which on account of theirbeing in close vicinity of the adsorbent surface, mayshow no phase transition. This inference issupported by calorimetric studies carried out byMorrison et al.II •• on multilayer adsorption of argonon rutile where the authors report that phasetransition in adsorbate is noticeable only after oneor two adsorbed layers.It is abundantly clear from the observed loweringof vapour pressure of solid adsorbates and from the652differences between the values ofM, and MI thatphase transitions in adsorbates are not cases of firstorder phase transition. It is tempting to suggesttentatively that the linear portion of the plot of logpversus lIT after the commencement of freezing(usually considered to be the curve for solid adsor-bate) may represent the gradual shift of the triplepoint where the chemical potential of the liquidadsorbate (!JoI) equals the chemical potential of thesolid adsorbate (IL.). The process of phase transitionof adsorbates is such that the solid phase is beingcontinually separated from the liquid phase as thetemperature is gradually lowered. It may only bea coincidence that all the points for plots of logp versus 1fT lie on a linear curve.References1. LAKHANPAL. M. L .• JOSHI. I. M. & SHARMA, S. c., IndianJ. Chem., 11 (1973). 796.2. LAKHANPAL. M. L., JOSHI. I. M. & SHARMA. S. C.• IndianJ. Chem.• 11 (1973). 922.3. BATCHELOR. R. W. & FOSTER. A. G., Trans. Faraday Soc.,40 (1944). 300.4. Handbook of chemistry and physics. edited by Robert C.Weast (Chemical Rubber Co.• Cleveland. Ohio). 1971-72. C-717.5. MORRISON. J. A. & DRAIN. L. E .•J. chem. Phys .• 19 (1951).1063.6. MORRISON. J. A.• DRAIN. L. E. & DUGDALE. J. S .• Can.J. Chem.• 30 (1952), 890.

Phase Transitions in Adsorbates: Part III - Vapour Pressure Studies on Formic Acid, tert-Butyl Alcohol & Cyclohexane Adsorbed on Charcoal

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Phase Transitions in Adsorbates: Part III - Vapour Pressure Studies on Formic Acid, tert-Butyl Alcohol & Cyclohexane Adsorbed on Charcoal

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