Indian Journal of ChemistryVol. 28A, October 1989, pp. 836-839

Thermodynamic studies of solutions of n-heptane in methanol and urea-methanol at 25°C -

K J Patil* &. G R MehtaDepartment of Chemistry, Institute of Science, Nagpur 440 001

Received 30 September 1988; revised 6 December 1988; accepted 2 February 1989

The density (d) and sound velocity (u) data have been obtained for binary system of n-heptane-methanolin the concentration range of 0.1 to 0.7 m of n-heptane at 25°C.Similar data have also been obtained for ter-nary system 3 m urea-methanol-a-heptane. Adiabatic compressibility (Bad)' apparent molal volume (~v)and apparent molal compressibility (~K) of n-heptane at different concentrations have been calculated. Theresults of ~~ and ~~ and their concentration dependence and the corresponding transfer functions (~~~ and~~~) for n-heptane are explained on the basis of formation of urea-channel adducts with heptane in solution.The structural interaction in solutions seems to be accompanied by large decrease in partial volume andcompressibility of n-heptane in methanolic urea solutions. ,

Urea can form crystalline addition compounds with avariety of aliphatic, straight chain hydrocarbons orderivatives containing six or more carbon atoms I.

Fetterly? on the basis of solubility data, proposed apseudocomplex ("swarm" or "husk") crystalline stateof host and guest molecules in aqueous solutions.Urea in aqueous solutions, not only imparts solubilityto hydrocarbons, but also inhibits micellar aggrega-tion of surfactants and affects the conformationalproperties of a wide range of water soluble polymers.In particular, there is a large and still rapidly expand-ing literature on denaturation by urea in aqueous so-lutions ">. However, information about properties ofhydrocarbons in ternary solutions involving urea, fat-ty acids or surfactants or bio-polymers and a non-aqueous solvent like methanol or in any other similarsolvent is very much limited.

As an extension of our earlier work from our labor-atory, on the thermodynamic studies of the solutionsinvolving n-hexane, urea and methanol at 25°e, wereport here the sound velocity and density data for bi-nary solutions of n-heptane-methanol and ternary so-lutions of n-heptane-urea-methanol (3m) at 25°C.The data were further processed to calculate adiabat-ic compressibility (~ad)' apparent molal volume (~v)and apparent molal compressibility (~K) of n-heptaneat different concentrations at 25°C. Using the limitingvalues of volume and compressibility of n-heptane inbinary and ternary solutions, the correspondingtransfer functions D.~~and D.~~for n-heptane frommethanol to 3m urea-methanol solutions were calcu-lated and compared with those obtained for n-hexane'.

Materials and Methodsn- Heptane, methanol and urea (all AR, BDH) were

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used as such. The densities of n-heptane and metha-nol at 25°e were found to be 0.67891 and 0.78644g.cm - 3 respectively, in fair agreement with the best li-terature values". The accuracy in the measurement ofdensities was of the order of ± 5 x 10 - S g.cm - 3. Thedensities (d) and sound velocities (u) were measuredfor the binary and ternary solutions of n-heptane inmethanol and in 3m urea-methanol respectively inthe concentration range of 0.1 mto 0.7 mat 25°C. Allthe solutions were prepared afresh on molality basis.

Sound velocities were measured at 25°e ± 0.05°eusing an ultrasonic interferometer. The details aboutthe instrument, method and accuracy have been de-scribed elsewhere". The velocity values were reprod-ucible within ± 0.5 ms - !.The reliability of the mea-surements was checked by obtaining velocity data foraqueous sodium chloride solutions at 25°e and com-pared with literature data".

ResultsThe adiabatic compressibility (~ad) were calculated

using sound velocity (u) and density (d) data in La-place equation. Apparent molal volume (~v )and ap-parent molal compressibility (~K) of n-heptane werecalculated using the expressions given earlier", Thevariations of u, ~ad' ~v and ~K for methanol- n-heptanesystem as a function of molality are shown in Fig. 1.The variations of the said parameters for the ternarysystem (methanol + urea (3M) + n-heptane) as afunction of alcomolality' of n-heptane are shown inFigs 2 and 3. The accuracy in ~v and ~K at lower con-

t Alcomolality is defined as the number of mole of solute per31.21 mole of mixed solvent. [his definition is analogous toaquamolality.

PATIL et al: THERMODYNAMICS OF SOLUTIONS OF n-HEPTANE IN METHANOL & UREA-METHANOL

'" 1198

188I

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v

-cr","--; ",1190

8ZiE.

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if

1182

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'",106

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~150

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"fu

$I ~a~100

'"-s,

0+ ' , I >, , I I154

148.f; , , , I I 'I1104

;~1100

I II I II 0.4ll986 ' 0·2 molality

Fig. I-Variation of sound velocity (u), adiabatic compressibility(f3ad), apparent molal volume (<\>v) and apparent molal compres-sibility (<\>K)with concentration (m) of n-heptane in methanol-a-

heptane mixtures at 25°C

centrations is estimated to be of the order of ± 1,0cm3.mol-1 and ± 2 x 1O-4cm3.bar-l.mol-1 respect-ively. The values of ~oz,and ~'kobtained on the basis ofsmooth extrapolation were used for calculating thetransfer functions (~~oz, and~~'k)for n-heptanefrommethanol to 3murea-methanol. The ~oz"~'k,~~oz, and~~'k values for n-heptane are collected in Table 1. Si-milar data for n-hexane in identical solvent systemsare also included in Table 1 for comparison.

DiscussionAddition of n-heptane to methanol causes a dec-

rease in u and increase in ~ad (Fig. 1).This is in contrastto expectation, if one compares sound velocities inpure liquids. It has been reported that in a system likeC2H50H - CCI4, the velocity of sound decreaseswith concentration of C2H50H in dilute region al-though an increase is expected 10. The probable causefor an extrema in this system is given in terms of theconversion of monomeric to dimeric (or higher aggre-gates) due to If-bonding of the alcoholic - OHgroups. The velocity behaviour observed in the pres-ent work provides support to the hypothesis of break

1174 I , I I I' I Jo 0·2 0·4 (}6 0·7ctcornoloti ty

Fig. 2- Variation of sound velocity (u) and adiabatic compressibil-ity ( f3ad) as a function of alcomolality of n-heptane in 3m urea-

methanol mixtures at 25°C

Ii.

0,

148

'-E142

"'E•...•..

~136I

0

130' I I I I I , I I

a w ~ ~ ~alcomolality

Fig. 3-Apparent molal volume (<I>v)and compressibility (<!>K)be-haviour in ternary solutions as a function of alcomolality of n-

heptane at 25°C

down ofH -bonded species of methanol by n-heptanemolecules.

A perusal of Fig. 2 shows that in solution thoughsound velocity (u) decreases with increase in n-heptane concentration, it shows an inflection in theregion of 0.3 to 0.4 alcomolality. Similar inference canbe drawn by examining the variation of ~ad' Consider-

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INDIAN J CHEM, SEe. A, OCTOBER 1989

SolventTable 1- ~~,~K' 6~v and 6~K values for n-hexane and n-heptane

~v ~~x 104 6~cm3.mol-' cm+bar t l.mol " I cm3.mol-1

~~~x 1Q4

crn+bar " '.mol- I

Methanol 132.6 190 n-hexane +4.4 +103murea + methanol 137.0 200Methanol 151.0 215 n-heptane - 14.4 -553murea + methanol 136.6 160

ing the errors in u and ~ad (± 0.5 m.s.-I and± 0.07 x 106 bar- I), the inflections appear to be real.

Further the apparent molal volume (~v) of n-heptane increases with increase in n-heptane concen-tration in binary system while, ~K varies linearly withconcentration. The limiting values of ~y{~~) and~K(~~) are more in magnitude than that of liquid n-heptane at 25°C. A methanol n-hexane system stud-ied earlier shows a similar behaviour. These observ-ations of positive limiting excess volume and com-pressibility for n-hexane and n-heptane suggest thatthe hydrocarbon molecules are more fragile agairistpressure effects in methanol compared to those inpure liquid state. Alternatively, this also can be ex-plained by postulating uncoiling of the hydrocarbonmoieties in solutions of methanol. Considering the in-ertness of hydrocarbon molecules, their dissolutionin methanol is, thus governed by weak interactions ofvan-der Waals type. The increased hydrocarbon con-centration should result in coagulation of hydrocar-bon molecules or groups, i.e. the phase separation.The limited solubility of n-hexane and n-heptane sup-port this. Partial enthalpy and partial entropy of n-hexane and n-heptane in methanolic solutions wouldhave provided the support to the uncoiling of hydro-carbon molecules in dilute solutions, but such dataare lacking.

It is observed from Fig. 3, that ~v of n-heptane internary system increases initially with concentrationand then remains almost constant indicating a changein slope of ~v against mcurve at about 0.3 to 0.4 mofn-heptane. However, ~v of n-hexane in the said ter-nary system (methanol-urea, 3m) decreases initiallywith concentration and then remains constant indi-cating again a change of slope of ~v against m curve atabout 0.3 to 0.4 m: Based on the data in Figs 2 and 3and similar observations as presented in our earlierpaper! with concentration of n-hexane, specific inter-actions of n-heptane with urea gives rise to the inflec-tion in u and ~ad' The present results indicate the sub-tle effect of size of the hydrocarbon molecule. In solidurea-n-heptane channel adduct the stoichiometry be-tween the constituents is 1:72. On this basis, if suchaggregates persist in solution, one expects extremas inthe thermodynamic properties at about 0.12 to 0.13mole fraction of hydrocarbon in urea. The concentra-

838

tionof n-hexaneor n-heptane( == 0.09-0.1 mole frac-tion) in urea at which the inflection in the studied pro-perties appear is not much different from the concen-tration of stoichiometry of 1:7. Of course, one shouldnot expect the exact stoichiometry in solution as weare dealing herewith liquid state. Considering theagreement between the two concentrations, our sug-gestion regarding the formation of urea-channel ad-duct like structure may not be different from reality.

On comparing the variations of ~v against m for n-hexane and n-heptane in ternary solution, one canpostulate that the size of the urea-channel may be ap-propriate for n-heptane molecules and hence initiallythey occupy the channel totally withouthavingappre-ciable affinity for other molecules, but the so-calledchannel structure formed in the solution, may be stab-ilized by interaction of these molecules with the hoststructure, leading to observed increase in ~v of n-heptane.

The negative d~v/dm for n-hexane reported earli-er', can be viewed as solvophobic solvation of n-hexane in urea-methanol solution. The urea-channeladduct may be present in such solutions but may notbe of sufficient stability because of inadequate size ofn-hexane molecules.

The variation of ~K with concentration of n-heptane (Fig. 3) is similar to that of ~v although itsmagnitude is less. This supports the postulated inter-action of guest n-heptane molecule with the host, i.e.urea structures present in solution which are moresusceptible to pressure effects initially.

The limiting transfer functions ~~~ and ~~~ of n-heptane from methanol to urea-methanol (3m) aredefined as:

(~~~ )trans = ~~ (3murea-methanol)- ~~ (methanol)(~~~ )trans = ~~(3m urea-methanol) - ~~ (methanol).

It appea~s from ~~~ value (Table 1) that the trans-fer of n-hexane and n-heptane molecule from binarymethanolic solutions to ternary solutions occurs byexpansion in volume of the former and by large con-centration of the latter. The negative ~~~ of n-heptane is similar to the negative volume changes ob-served in aqueous hydrocarbon or rare gas solu-tions 11. On the basis of these arguments, our postul-ate, that in methanolic solution urea can form

PATIL et al.: THERMODYNAMICS OF SOLUTIONS OF n-HEPTANE IN METHANOL & UREA-METHANOL

H-bonded network having appropriate channelstructures which are further stabilized by guest n-heptane, can be fullyjustified. On this basis one can in-terpret the negative ~~'kfor n-heptane as the loss incompressibility of n-heptane in terms of the hoststructure supported by H-bonding interactionamongst urea molecules and the dispersive interac-tion with guest n-heptane molecules.

AcknowledgementWe thank ProfRB Kharat and Prof'(Mrs) Sapkalfor

facilities and encouragement during the course of thiswork.

References1 Patil K J & Mehta G R, Indian 1 Chern, 25A (1986) 319.

2 Fetterly L C, Non-stoichiometric compounds, edited by LMandelcom (Academic Press, New York and London), 1964,Chapter 8.

3 Franks F & Reid D S, Waten A comprehensive treatise, Vol 2,edited by F Franks (Plenum Press,New York), 1973, Chapter5.

4 Kresheck G C & Scheraga H A,J phys Chern, 69 (1965) 1704.5 EaglandD, Wate~A comprehensive treatise, Vol 4, editedbyF

Franks (Plenum Press, New York), 1973, Chapter 5.6 Weissberger A, Techniques of organic chemistry, Vol 7 (Inters-

cience, New York), 1955.7 Patil K & Mehta G, lchem SocFaraday Trans, 84(1988) 2297.8 Gucker F T, Chernick C L & Chowdhury P Roy, NatAcad Sci;

55 (1966) 12.9 Patil K J & A1i S I, Induml Pure App/ Phys, 19 (1981) 617.

10 Patil K J, Mehta G R & Chandewar R K, Indianl Chern, 25A(1986) 1147.

11 Klotz I M, Membranes and ion transport, edited by P Bittar(Wiley Interscience, London), 1970.

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