Indian Journal of ChemistryVol. 23A, February 1984, pp. 145-146

Apparent Molal Volumes of InterstitialCompounds Forming Components in

Solution

R S CHAUHAN, R N PATHAK & RAM GOPAl·

Chemistry Department. Lucknow University. Lucknow 226007

Received 20 June 1983; revised and accepted 9 September 1983

Dependence of cp, (apparent molal volume) on C for urea andthiourea in methanol. methanol + hexane and methanol + heptanemixtures has been studied. The results indicate that theaccommodation of the guest (in this case hexane or heptane) leads toa decrease in its apparent molal volume and the forces which lead tothe formation of solid lattice of a host-guest system (urea orthiourea-hexane or heptane) appear to be already operative insolution.

One of the suggestions made to explain the abnormalnegative slope of the apparent molal volume (<Pv)versus JC curves of some electrolytes (the largertetraalkylammonium and some common salts) insolvents of high dielectric constant 1.2 is that the ionsare accommodated inside the large vacant spaces leftduring packing of the solvent molecules; thispossibility and electrostatic ion-solvent and ion-ioninteractions somehow give a negative slope in the <p,versus JC curves in these cases. This suggestionalthough theoretically unsound, is interesting and callsfor investigations in ·systems in which interstitialcompounds+" are known to be formed and thisprompted us to undertake the present investigation.

Methyl alcohol (AR, BOH) was purified by thestandard procedure: n-hexane (Indian Drugs andPharmaceuticals, Hyderabad) and n-heptane(Polypharm, Bombay) were purified by shaking themwith one quarter of their respective volumes ofsulphuric acid for several minutes; the process wasrepeated until the lower layer was colourless. Thehydrocarbon layer was then washed twice withdistilled water, dried over anhydrous calcium sulphateand distilled twice retaining the middle fraction for use:n-hexane, b.p. 68.3c (lit. 68".:F); II-heptane, b.p. 98.2°(lit. 98.4). Urea and thiourea (M Sarabhai Chemicals.Baroda) were recrystallized from conductivity water,dried and kept in a vacuum desiccator. All thesolutions were prepared on molal basis. Theexperimental procedure for determining density and<p, therefrom, was the same as given in earliercommunications 5 8.

The plots of ({),versus C lead to the following generalconclusions:

(i) The <py of urea (or thiourea) in methanol decreaseswith the increase in concentration (Fig. I), indicatingself-association among the solute molecules andconfirming the results obtained by Hamilton andStokes".

(ii) The sp ; of hexane (or heptane) in methanolincreases rapidly with the increase in its concentration(Fig. 1) indicating solute-solvent interaction.

(iii) The <py of urea (or thiourea) in methanol+ hexane (or methanol + heptane) mixtures increaseswith increase in concentration; it also increases withthe increase in the concentration of hexane or heptane(Fig. 2) in mixtures; on the other hand, ip ; of hexane (or

40r-------------,

~ 35:>

"0 30f?

25~~~~--~~0-00 ()'10 0·15 0·20 0·25

Cone.of ureo140r-----------,

70o~:>.2£;"0 50.fl

0-05 0-10 0·15 0'20 0·25 0'30Cone.of th"",rea

I60r-------------,

..C,,~130 5150

i1! z:"0120 "0 140.if .fl

tt 0().·'7:10::-:0:-':'2:-=0--:-o-~30:-::()O-':-4::-0-::o-:-:!50 ' 300~.1O;;--;0;-';;'2:;;-0-::O;;-!'3:-=0-,"""().I,()l:-::--::O:-!'50

Cone.of hexane Cone.of neptone

Fig. l--Apparent molal volume (cp,.) versus concentration curves forurea (and thiourea) and hexane (or heptane) in methanol

15% 60

10~_~::--::~~~~~0-15 0·20 0·25 0·30 0·10 0·15 0·20 0·25 0·30

Cone of urea Conc. of thiourea

40nc----------------,

35o

1! 30o><

~ 25

a

150~.0:-:0-::0~.0:-:5~0::'-.1:-;;0-:::-0.'::15:--:0;..1;·2'""0,-0~.Jg!25

Cone of urea

40

1! 35 0

~ I 0

"0 30~.if I251 1

0·00 0·05 0·10 0·15 0·20 0·30Cone of th ioureo

Fig. 2---Apparent molal volume (cp,) versus concentration curves forurea (and thiourea) in methanol + hexane mixture and '{" of hexane

versus concentration of urea (or thiourea)

INDIAN J. CHEM., VOL 23A, FEBRUARY 1984

heptane) decreases with the increase in concentrationof urea (or thiourea) (Fig. 2).

The above results point to the existence of differentmodes of molecular interactions in the presence andabsence of the host and guest molecules. The followingtentative picture of interaction may be suggested toexplain these experimental results:

The decrease ill ({Jv of urea (or thiourea) in methanol,with increase in concentration is due to preferentialdipole-dipole interaction among the solute moleculeson account of its large dipole moment. On the otherhand, due to the absence of dipole in hexane andheptane, there appears to be a net attractive interactionbetween methanol molecules and induced dipolemoment in hexane (or heptane). This interaction wouldbe maximum at infinite dilution and would decreasewith increase in concentration of solute urea (orthiourea); hexane (or heptane). Now, when to asolution of urea (or thiourea) in methanol some hexane(or heptane) is added, there appears to be a preferentialdipole-induced dipole attraction between hexane (orheptane) and urea (or thiourea). Dipole and induced-dipole interaction would be weaker between methylalcohol and hexane (or heptane) in comparison to thatbetween urea (or thiourea) and hexane (or heptane)because of large dipole moment of the urea. Thepreferential interaction between hexane (or heptane)and urea (or thiourea) breaks down the association of

146

urea (or thiourea) resulting in a net decrease in its ({Jv

with increase in concentration of hexane (or heptane).It seems that due to this interaction, apparently nobreak down of molecular association in these cases[hexane (or heptane), unlike urea (or thiourea), is notself-associated in methyl alcohol] occurs which couldcompensate for the loss in volume of hexane (orheptane) due to host-guest interaction [interstitial ion-pair formation between hexane or heptane (guest) andurea or thiourea (host)] and so ({Jv of hexane (orheptane) decreases with increase in its concentration.This picture appears to explain the results obtained sofar.

The authors' thanks are due to the UGC, NewDelhi, for financial support.

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chemistry (Prentice-Hall Chemistry Series, USA) 1970, 162-163.

5 Gopal R, Agarwal D K & Kumar R, Indian J Chern, 14A (1976)803.

6 Gopal R & Pathak R N, Indian J Chern, 14A (1976) 806.7 Gopal R & Pathak R N, Indian J Chern, 16A (1978) 150.8 Gopal R & Pathak R N, J Indian chern Sac, 55 (1978) 128.9 Hamilton D & Stokes R H. J soln Chern, 1(1972) 213.