Indian Journal of ChemistryVol. 25A, April 1986, pp. 319-321

Apparent Molal Volume & Compressibility of n-Hexane InMethanol & Urea-Methanol Solutions

K J PATIL* & G R MEHTADepartment of Chemistry, Institute of Science, Nagpur 440001

Received 19 June 1985: reoised 25 Nooember 1985; accepted 30 December 1985

The density (d)and sound velocity (u) data have been obtained for n-hexan.: in methanol in the concentration range of 0.05to 0.6 m at 25°C. Similar data have also been obtained for n-hexane in 3 m urea-methanol as a solvent. The adiabaticcompressibility (P •.). apparent molal volume (11'.) and apparent molal compressibility (tpoJ of n-hexane at differentconcentrations have also been calculated. The results of <Pv and <PK of n-hexan.: in its solutions in methanol are explained on thebasis of solute-solvent interactions. while the appearance of inflections in u. P.d and <;J, profiles as a function of concentrationof n-hexane in 3 III urea-methanol solvent points to probable structural transitions. In ternary solution formation of clathrate-channel like structure is indicated.

It is well known that urea forms channel-like adductswith hydrocarbons and long chain fatty acids in solidstate". Fatterly, from thermodynamic studies basedupon the solubility data, has proposed a pseudo-complex, "swarm" or "husk" like state of host andguest molecules existing in aqueous solutions". Inrecent years many studies of mixed aqueous ureasolutions have been undertaken in order tounderstand the denaturation of polypeptides orbiopolyrners by urea, and also to throw more lightupon the mechanism of micelle formation+'. Themixed aqueous solutions of urea with hydrocarbons orfatty acids are difficult to study because of the solubilityproblems. Ihe clathrate-like lattices have beenproposed by Vandcrwaals and Plaueeuw to exist insaturated solutions of f$-hydroquinone in II-propanolwith methanol as a solute". Recently, Ram Gopal andcoworkers" found evidence for channel-like structureformation in mixed mcthanolic solutions of urea withn-hexane or II-heptane from their volumetric studies.These authors reported a large volume loss by /1-

hexane or II-heptane in methanol. an observationwhich seems to be quite unusual. In this paper, theresults of volumetric and compressibility studies on 11-

hexane in methanol and in 3111 urea-methanol solventat 25 (. are reported. Fhe concentration dependencesof apparent molal volume and compressibility of /1-hexane have been examined in terms of solute-soluteand solute-solvent interactions. Fhc limiting volumeand compressibility parameters obtained for ternarysolutions are compared with similar data in binarysolutions.

Vbtcrbls and \1cthodsIl-i lcxanc (BDit) was purified as described by

Neissberger" and finally distilled over sodium wire;

b.p. 68°; d =0.66372 g/cm ' at 25"C (lit. d =0.6548 g/em:'). This discrepancy in density values was due to thepresence of isomers of hexane. Since the isomers aredifficult to remove, n-hexane was used as such.Methanol (A R) was found to have density = 0.78644 g/em:' at 2S"C, which is slightly lower than the literaturevalue (0. 76675 g/cm:') and it was used as such. The urea(AR) was used as such.

The densities of pure liquids and solutions weremeasured with a specific density bottle (capacity=2S.14Scm3 at 25"C) having an arrangement tomaintain the temperature with an accuracy of10.02"C. All the weighings were made with a vlettlerbalance and corrected for buoyancy. The accuracy inthe measurements of densities is of the order of + 5x l C .5g/cm3. Thedensities(d)andsoundvclocities(u)were measured for the binary solutions of n-hexaneand methanol in the composition range ofO.OS to 0.6111

at 2SC. The details about sound velocitymeasurements have been described previously'. Theaccuracy in u is of the order of .t I m/s, The d and uparameters were also determined for the ternarysolutions of n-hexane (0.1 tn to 0.6 m) in 3111 urea-methanol solvent. All the solutions were preparedafresh on molality basis.

ResultsApparent molal volume (cpy) and apparent molal

compressibility (cf>.J of n-hexane were calculated usingthe expressions given earlier".

The accuracy in If>v at the lower concentration is ofthe order of .t 1.00 em:' /mol. The variation in If>v of 11-hexane in methanol against molality is plotted inFig. I. The variations of sound velocity (u) andadiabatic compressibility Wad) parameters for the saidbinary system as a function of molality are shown in

319

INDIAN J. CHEM., VOL. 25A, APRIL 1986

138

136.'0E..;E 134u

<-132

130

~..,Eu

o

(b)

alcomolality

Q

(a)

0 ()Ol 0·2 0'3 ()o4 O·S 0·6molality

Fig. I --(a) Plot of apparent molal volume (!{)y) versus concentrationof n-hexane in methanol at 25T.

(b) Plot of apparent molal volume (!{)y) of n-hexane as a function ofalcomolality of n-hexane at 25C

Fig. 2a, while the vanation of <PKof n-hexane withmolality is plotted in Fig.2b. Similar variations of uand fJad for the ternary solutions of n-hexane in 3 murea-methanol as a function of alcomolality+ of /1-

hexane are shown in Fig. 3a. The variations of <Pvand<PKof n-hexane in 3 m urea-methanol versus molalityare illustrated in Fig. 1b and Fig. 3b respectively.

DiscussionIt has been observed that density decreases more or

less smoothly with increase in concentration of n-hexane in methanol. The plot of <Pvversus molality ofn-hexane in binary solution shows a slight positiveslope at higher concentrations indicating solute-solvent interactions. The (Pv values reported by RamGopal and coworkers" appear to be too low ascompared to our values. Since, these authors did notdescribe the details of their experiments andcalculations, we are unable to trace the reasons for thelow <Pv,values reported by them. Our values of <Pvappear to be reasonable as the molal volume of n-hexane (V~) is 129.2 cm 'ymol at 25'C. Theextrapolated value of limiting partial molal volume(V~) at 25°C obtained in the present work is 132.6 t 1.0em 3/mol (at infinite dilution, <P~ = V~)which is slightlyhigher than V~. It is observed that the excess limitingpartial molal volume (V~ = V~ - V~) is positive which

tAlcomolality is defined by us, as the number of mol of solute per31.21 mol of mixed solvent. This definition is analogous toaquomolality.

320

1092 102o 0-1 0·2 0·3 0'4 O·S 0·6

molality

Fig. 2 --(a) Variation of sound velocity (u, 0- 0),and adiabaticcompressibility (Pad' ~ -~) as a function of molality of n-hexane in

methanol at 2YC

(b) Variation of apparent molal compressibility (!{)KaJ of n-hexanewith n-hexane concentration at 25''C

1192 88

1196

1188

1184-'

al,~N-g

)(

82'8<!l:.

••<,E:> 1180

(a)

1176

o ()o1 ()o2 0·3 0'4alcomolality

O'S 0·6

Fig. 3 -Sound velocity (u, 0 - 0), adiabatic compressibility (Pad' ~-~) behaviour (a) and apparent molal compressibility of n-hexane

(b) in ternary solutions as a function of alcomolality of n-hexane at25'C

must be due to the dissociation of associated methanolmolecules by n-hexane (alcohols are known to possesstwo-dimensional associative hydrogen bonded struc-tures). This is in contrast to aqueous non-electrolyticsolutions". The cleavage of hydrogen bonds inmethanol by n-hexane is supported by the smallpositive slope of the <Pv versus molality curve inFig.la.

PATIL & MEHTA: APPARENT MOLAL VOLUME OF n-HEXANE IN METHANOL & UREA + METHANOL

The variations of u and Pad in the binary systemappear quite smooth (Fig. 2a). The u decreases and Padincreases with concentration. At higher concentrationsof n-hexane (> 0.6 m) a phase separation occurs.Similar studies on ternary solutions of n-hexane in3.6 fI1 (nearly saturated) urea in methanol in higherconcentration range were unsuccessful as urea saltedout. /II e believe that n-hexane in methanol shows phaseseparation at concentration greater than 0.6 tn. Thevariation in <PK of n-hexane with molality (Fig.2b)appears to be quite normal and <P~ is estimated to be ofthe order of 195t2:<. 10 -10 em=dyne -Imol-I whichis quite high as compared to <PK of pure hexane (134)( 10 10 em'dyne -Imol-I).

The curve in Fig. 3a shows that in ternary solutionsthough 1I decreases with increase in concentration of n-hexane. it shows an inflection in the region of 0.3 to 0.4alcomolality. Similar inference can be drawn byexamining the variation of Ilad; it also increases withincrease in concentration of n-hexane. Considering theerrors in 1I and Ilad (t 1 m/s and 0.2:<.10 -12

dyne -lcm2 respectively) the inflections appear to bereal. [he <Pv behaviour of n-hexane(Fig. 1b) appears tobe somewhat similar to that of monofunctional non-electrolyte solutes in water, i.e. (Pv decreases initially(solute-solute interaction) and remains more or lessconstant in the range of 0.3 to 0.5 alcomolality. Thechange in slope of <Pv versus molality curve in aqueoussolutions of electrolytes has been ascribed to structuraltransition 10. Similar interpretation can be advancedfor the present (Pv versus alcomolality curve. Theconcentration of n-hexane in this region is appropriatefor 1:7 (i.e. 1 molecule of urea and 7 molecules ofalcohol) clathrate geometrical structural environmentand hence the structural reaction if any, occurring inthis region. may be due to the formation of urea-n-hexane adduct. This argument can probably besupported by the observation that in II-propanol +p-quinol + methanol system a property like refractiveindex shows non-variance with concentration in theregion of clathrate formation. The <PK of n-hexane(Fig. 3b) does not show significant variation withconcentration, indicating that such systems are notsusceptible to pressure effects.

The extrapolated values of <Pv (<p~) and <PK (<p~ tozero alcomolality in ternary solutions signifies theeffect of solute-solvent interactions in solution. The <P~and <P~ values are 138 .t 1.0 cm=.mol "! and £97±2x 10 -10 ern 'dyne -Imol -I, respectively. for n-hcxane.On comparison with <P~ and (P~ values in binary

solutions we find that <p~' value is more in ternarysolutions while <P~ value is almost the same. It appearsfrom <P~ I. and <P~ 1 values that the transfer of n-hexanemolecules from pure liquid state to binary and ternarysolutions is accompanied by volume and compre-ssibility gain. A comparison of cp~Eand cp~Evalues inbinary and ternary solutions probably signifies anopen but equally compressible structure in ternarysolution. The observed expansion in volume andcompressibility of n-hexane can be interpreted as dueto structural modification of solvent structure (urea-methanol) which provides support to the hypothesisthat urea forms a channel-like structure in methanolsolution, which gets further stabilized by the dispersiveinteractions between the guest hexane molecules andthe urea-methanol clusters. The details about suchinteractions can only be known from spectroscopicstudies which are lacking at present stage.

AcknowledgementThe authors are grateful to Prof. vt V Kaulgud,

Department of Chemistry. Nagpur University forsuggesting this problem. Also, the experimentalfacilities and encouragement from Prof. R B Kharat,Department of Chemistry, Nagpur and Or 0 B vlule,of our Institute, are gratefully acknowledged. Theauthors are also thankful to the referee for hisconstructive comments and help in modifying themanuscript.

ReferencesI Fatterly L C. Non-stoichiometric compounds. edited by L.

Mandelcom (Academic Press, New York), 1964. Chapter 8.2 Franks F .& Eagland D. Crit Rev Biochem, 3 (1975) 165.

3 Kresheck G, Water, a comprehensive treatise. Vol. 4. edited by F.Franks (Plenum Press. New York), 1975. Chapter 2.

4 Vanderwaals J .& Platteeuw J C. Adrances in chemical physics.Vol. II, edited by I. Prigogine (Interscience, New York).1959, Chapter I.

5 Chauhan R S. Pathak R ~ .& Gopal R, Indian J Chem, 23A(1984)145.

6 Weissberger A, Techniques of organic chemistry. Volume VII(Interscience, New York), 1955.

r Patil K J .& Raut D ~, Indian J pure appl Phys, 18 (1980) 499.8 Patil K J, Thakare 0 B .& Tirpude D D, Indian J Chem, llA

(1982) 604.

9 Franks F .& Reid D S, Water, a comprehensive treatise, Vol. 2,edited by F. Franks (Plenum Press. New York). 1975,Chapter 5.

10 Vaslow F. Water and aqueous solutions: Structure. thermodv-namics and transport properties, edited by R. Horne (Wiley-Interscience, New York), 1971, Chapter 12.

321