Indian Journal of ChemistryVol. 23A, June 1984, pp. 510-511

Free Volumes of Aqueous Nonelectrolytes

N MANOHARA MURTHY*Department of Physics,

Sri Krishnadevaraya University, Anantapur515003and

G NAGABHUSHANAMDepartment of Physics,

The Adoni Arts & Science College, Adoni 518 302Received 3 November 1983; revised and accepted 26 December 1983

Free volumes (V,) have been evaluated for aqueous solutions ofp-dioxane, pyridine, N-methylformamide (NMF) and N,N-dimethylformamide (DMF) as a function of concentration usingultrasonic velocity, density and heat capacity data at 298.15 K. V, isfound to be minimum at X, ",,0.16, 0.23, 0.25 and 0.25 for p-dioxane.pyridine, NMF and DMF respectively. These results indicate theformation of 5: 1 water-dioxane and 3: 1 water-pyridine, water-NMF and water-DMF complexes.

Earlier papers from our laboratory I -3 reported onsuch properties of aqueous nonelectrolytes as heatcapacities etc., and the data were utilised to understandsolute-solvent interactions. p-Dioxane, pyridine, N-methylformamide (NMF) and N,N-dimethylformamide (DMF) are known to destabilizethe hydrogen bonded structure of water leading to theformation of complexes of varying compositions:' ·11.

To examine how far this aspect is characterized by freevolumes, we have evaluated the free volumes ofaqueous mixtures of p-dioxane, pyridine, NMF and

DMF using ultrasonic velocity, density and heatcapacity data and the results are reported in this note.

According to Kincaid and Eyring"? the soundvelocity in a liquid (u 1) is related to the free volume (Vf)by the relation (I)

(u )3V,=V ~ ... (I)

where V and ug represent the molar volume and soundvelocity in the vapour respectively. The sound velocityin the vapour can be calculated using the relation (2)

_ (YR. T)1!2ug- M ...(2)

where I', R, T and M represent the ratio of specificheats, gas constant, temperature of the liquid inabsolute scale and effective molecular weight.

The free volumes of aqueous mixtures of p-dioxane,pyridine, NMF and DMF at different concentrationshave been evaluated making use of necessaryexperimental data obtained earlier+":" and Eqs (I)and (2).The variation of V f with X 2, the mol fraction ofthe organic component in the mixture for all thesystems is shown graphically in Fig. I.

An examination of the data presented in Fig. Iindicates that Vr becomes minimum in the water-richregion for all the systems. The value of X2 at which V,becomes minimum is ~ 0.16, 0.23, 0.25 and 0.25respectively for p-dioxane, pyridine, NMF and DMF.These concentrations correspond to the formation of

Table I-Thermodynamic Property Versus Extremum Concentration. (X 2) ..vtr for Aqueous Mixtures of p-Dioxanc. Pyridine.NMF and DMF at 298.15 K

Water + p-Dioxane Water + Pyridine Water+ NMF Watcr+DMF-.---.---------.-- ._---_ ..._-----------'------ ---_._--_ .. -_.__ .._---_ ..._-------Property (X,)"", Property (XJ"m Property (X2)"-,,, Property (Xl).·.".

e" 0.18 Ii' 0.27 {it O.D liE 0.25p

C: 0.18 K" 0.27 K' 0.23 KE 0.25Vf 0.16 ef: 0.40 Cf 0.35 c/o 0.33

I' p pVI: 0.16* e~ 0.27 c~ 0.27 c/o 0.27f ,.

n': 0.32 f 0.23 7[" 0.237!

H" 0.21 HE 0.33 H': 0.23TS£ 0.27 TS' 0.22 TS" 0.22G" 0.21 G' ().3~ CE 0.21.J

VI 0.23 VI 0.25 VI 0.25V" 0.22' V" 0.23' V" 0.23*f J I

pE,KE,C:, e~:.n', HE, TSE. GE and V} represent the excess adiabatic compressibility, isothermal compressibility, molar heat capacity atconstant pressure, molar heat capacity at constant volume, internal pressure, enthalpy. entropy Gibbs' free energy and free volumerespectively.• The extremum concentration has been evaluated from VJ versus X 2 data for all the systems using Vf versus X2 data of the present work. VJis obtained using the relation V~= VI. -(X 1V'fI + X 2V ,,) where V fI and V f2 represent free volumes of pure water and the organic soluterespectively. oz,

510

030r-------,

0.28

0·26

0·2 0.4 .6 0.8 1.0

X2

Fig. I--Variation of free volume (Vr) with mol fraction (X2) ofnonelectrolytes [I, p-dioxane; 2, pyridine; 3, NMF; 4, DMF]

complexes of the type 5: I water-dioxane and 3: 1water-pyridine, water-NMF and water-DMF com-plexes. Beyond the optimum concentration, furtheraddition of nonelectrolyte breaks the structure ofwater as well as the polymeric clusters and aqueoussolution behaves as a normal liquid and that is why weobserve the value of Vf tending towards the valuecorresponding to the pure nonelectrolyte. Extrema inmany thermodynamic functions as a function ofconcentration for these systerns+":" as detailed inTable 1occur at or around the same X 2 observed in thevariation of v, with X2 in the present study.

NOTES

The results of the present study confirm theapplicability of free volume parameter as a potentialprobe to investigate the solute-solvent interactions inaqueous nonelectrolytes.

The authors express their sincere thanks toProfS V Subrahmanyam for his interest and en-couragement in this work.

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