J\

Indian Journal of ChemistryVol. 18A, July 1979, pp. 1-6

Thermodynamics of Autoionization of Glycerol + Water Mixtures& Structuredness of Solvents

INDRA N. BASUMULL CK & KIRON K.' KUNDU·Department of Chemistry, Jadavpur Univrsity, Calcutta 700032

Received 16 November 1978; accepted 25 January 1979

The autoionizatlon constants (Ks) of glycerol + water mixtures (SH) containing 10, 30, 50 and 70 wt %• 0

glycerol have been determmed from e.m.f. measurements of the cell : Pt, HI (g, 1 atm)/KOH (m1), KBr (m.), solvent/AgBr-Ag at seven equidistant temperatures between 10° and 40°. The standard free energies, entropies and ethalpiesof autoionization of the solvents have been evaluated from these data. The free energy data combined with the recentlydetermined free energies of transfer of H+, 6.Gi (H+), from water to the corresponding solvent mixtures yield increa-singly negative values of 6. Gi (S-) of the solvent anion (S-), suggesting that glycerol is more acidic than water. Analy-sis of the entropic contributions to the ionization of water in the water-rich solutions of glycerol, ethylene glycol and2-propanol suggests that the structure promoting ability of 2-propanol due to its hydrophobic -CHa groups getincreasingly reduced possibly due to 'specific site hydration' of water through the increased number of hydrophilicOH groups in glycol and glycerol.

THE mixtures of glycerol with water constitutean important biochemical fluid in view of itsability to protect proteins from denaturation-".

Just as the well known denaturing effect" of urea iswidely believed to stem from its effect on the structureof water''", the protein-stabilizing ability of glycerolis believed to result from structural changes of waterby glycerol. Some recent findings, of course, attri-bute specific interactions between glycerol and pro-teins as the genesis of the important biochemicalactivity of glycerol + water mixtures-". All theseillustrate the importance of further studies in thissolvent system.

As part of our comprehensive studies on acid-baseproperties, ion-solvent interactions and solvent struc-tures in glycerol + water mixtures we have recently re-ported? the transfer energies of hydrogen halides andinformations on the basicities as well as the structuralaspects of the solvents. As shown in the case ofurea + water mixtures", since the auto ionizationconstant (K.) of aquo-organic solvents at differenttemperatures and the related thermodynamic para-meters are likely to reflect not only the relative acidityof the solvents but also their structural features itshould be of particular interest to extend such studiesin this solvent system as well.

Earlier Hepler and coworkers" reported the appa-rent ionization constants of water in the aqueoussolutions of glycerol from e.m.f. measurements at25° using glass electrode. So far as we know thereis also no information on the thermodynamics ofautoionization of these solvent mixtures. We havetherefore determined the autoionization constant ofglycerol + water mixtures containing 10, 30, 50 and70 wt % glycerol at seven equidistant temperaturesbetween 10° and 40° by e.m.f. measurements of thecell (A) and evaluated the related thermodynamicquantities. These are also utilized to help derive

/

(

information on the relative acidity as well as thestructural aspects of the mixed solvents.Pt, HI (g, 1 atm)/KOH (ml), KBr (m.), solvent/Agfir-Ag (A)

Since acidic nature of glycerol (HZ) is well recog-nized-9-11, the auto ionization of these mixed solventsshould involve the following ionization processes(a-d)

H20 + H20 ~ H.o+ + OH- (a)H20 + HZ ~ H.Z+ + OH- (b)HZ + H.o ~ H.O+ + Z- (C)HZ + HZ ~ H.Z+ + Z- . (d)

While reactions (a) and (b) are due to the acid ioniza-tion of water, reaction (c) and (d) are due to that ofglycerol. Conveniently, all the processes (a)-(d) maybe represented by the general expression (e)

SH + SH ¢ SRt + S- .. (e)

where SH is a solvent molecule in the mixed solvents,being either water or glycerol and SH! and S- arethe lyonium and lyate ions respectively.

Materials and MethodsGlycerol (GR Merck) and water used were puri-

fied as described". Preparation of solutions andother experimental procedures were similar to thosedescribed earlier>, Equilibrium was attained within3-4 hr when e.m.f. values remained constant within± 0.1 mV for 1 hr. The cells were maintained ateach temperature within ± 0.05°.

Vapour pressures, dielectric constants and densitiesof the solvent mixtures have been obtained as des-cribed in the earlier paper".

Results and DiscussionThe corrected e.m.f. data E (corresponding to

PH2 = 1 atm) at different temperatures and the

-,

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INDIAN J. CHEM .• VOL. 18A, JULY 1979

TABLE1- EXPTRIMENTALe.m.f. (V) DATA CORRESPONDINGTO PH. = 1 atm ATVARIOUSTEMPERATURESFORDIFFERENTCELLSOLUTIONS

mKOH mKBr e.m.f, (V) at temp. (0C)(kg/mol) (kg/mol)

10 15 20 25 30 35 40

10 wt. % GLYCEROL0.0099 0.0086 0.8941 0.8767 0.8795 0.8823 0.8854 0.8878 0.88950.0108 0.0117 0.8093 0.8718 0.8743 0.8968 0.8795 0.8820 0.88440.0191 0.0167 0.8737 0.8760 0.8792 0.8819 0.8849 0.8874 0.89030.0275 0.0241 0.8748 0.8774 0.8795 0.8825 0.8850 0.8879 0.88990.0288 0.0313 0.8692 0.8711 0.8740 0.8764 0.8800 0.8821 0.88300.0379 0.0331 0.8742 0.8769 0.8797 0.8823 0.8854 0.8877 0.89010.0367 0.0399 0.8682 0.8710 0.8736 0.8762 0.8797 0.8820 0.8865

30 wt % GLYCEROL

0.0135 0.0155 0.8395 0.8438 0.8469 0.8495 0.8527 0.8563 0.85900.0182 0.0212 0.8890 0.8435 0.8472 0.8509 0.8540 0.8572 0.85960.0195 0.0224 0.8398 0.8440 0.8478 08506 0.8531 0.8577 0.85950.0281 0.0325 0.8391 0.8434 0.8473 0.8507 0.8538 0.8570 0.85930.0292 0.0340 0.8385 0.8427 0.8465 0.8503 0.8531 0.8564 0.85910.0296 0.0341 0.8395 0.8447 0.8478 0.8508 0.8544 0.8572 0.85960.0357 0.0448 0.8407 0.8458 0.8489 0.8518 0.8549 0.8579 0.8602

50 wt % GLYCEROL

0.0093 0.0089 0.8260 0.8288 0.8322 0.8351 0.8383 0.8405 0.84410.0134 0.0149 0.8231 0.8263 0.8292 0.8318 0.8347 0.8376 0.84040.0189 0.0181 0.8284 0.8315 0.8352 0.8376 0.8411 0.8445 0.84770.0203 0.0226 0.8223 0.8251 0.8279 0.8307 0.8337 0.8366 0.83910.0209 0.0236 0.8222 0.8250 0.8280 0.8306 0.8328 0.8361 0.83780.0263 0.0253 0.8301 0.8336 0.8368 0.8394 0.8424 0.8444 0.84700.0353 0.0339 0.8276 0.8310 0.8341 0.8369 0.8404 0.8431 0.84640.0369 0.0410 0.8245 0.8262 0.8300 0.8326 0.8358 0.8383 0.8414

70 wt % GLYCEROL

0.0083 0.0086 0.8077 0.8106 0.8139 0.8165 0.8196 0.8219 0.82390.0091 0.Ql13 0.8051 0.8073 0.8097 0.8122 0.8147 0.8171 0.81910.0163 0.0170 0.8097 0.8119 0.8144 0.8170 0.8197 0.8221 0.82130.0201 0.0250 0.8059 0.8083 0.8109 0.8134 0.8155 0.8181 0.82040.0225 0.0235 0.8099 0.8125 0.8150 0.8179 0.8205 0.8230 0.82530.0304 0.0318 0.8036 0.8080 0.8121 0.8163 0.8204 0.8233 0.8267

TABLE2 - p Ks VALVES(MOLALSCALE)FORTHEAUTOIONIZATIONOFGLYCEROL+ WATER MIXTURESATDIFFERENTTEMPERATURES

Wt % Glycerol• 'pKs at temp. ("C)

10 15 20 25 30 35 40

10 14.220 14.055 13.900 13.750 (13.72) 13.160 13.475 13.34530 13.945 13.805 13.670 13.540 (13.56) 13.415 13.295 13.18050 13.820 13.685 13.560 13.440 (13.40) 13.330 13.220 13.11570 13.980 13.850 13.725 13.605 (13.45) 12.490 13.380 13.280

*Average uncertainties in p'Ks co ± 0.005 units. Values within parantheses are Helper's data (molal scale) obtained by glasseleetrode methode.

corresponding molalities of KOH and KBr of cell (A)for each of the solvent mixtures are given in Table 1.

The p K, values (molal scale) of these solventsat different temperatures, recorded in Table 2, wereobtained by the extrapolation procedure adoptedearlier12,13. As before12,13, the standard state is sochosen that each solvent mixture at its standard statehas unit activity and ions in their respective standardstates in each solvent mixture have unit activity co-efficient. Notably, pK, values at 25° of solvents oflow glycerol contents agree fairly well with the corres-ponding apparent ionization constants reported byHepler and coworkers". But for solvents of high gly-cerol contents, these differ to some extent and theobserved divergences are possibly due to the untoward

behaviour of glass electrode. In both the cases,however, the p K, values are found to decrease withincrease in glycerol content of the solvents and passthrough a minimum around 60 to 70 wt % glyceroland are indicative of the effect of acidic ionization ofglycerol like that of ethylene glycol and some poly-hydroxy compounds", but unlike that of some mono-hydroxy alcohols'v" which are fairly less acidic? thanwater.

The p K, values at different temperatures for eachof the solvent mixtures were fitted to Harned-Robinsontype expression" (1)

pKs =:: B + CT ... (1)

2

,(

BASUMULLICK & KUNDU : STRUCTUREDNESS OF SOLVBNTS

TABLE 3 - COEFFICIENTSOF Eq. (1) AND t::,.G·, t::,.H· AND t::,.S·'VALUES(MOLAL SCALE)AT 250 FOR THE AUTOIONIZATIONOF

GLYCEROL+ WATTR,MlXTURES

Wt % A B lOlC so: t::,.H· ss:'Glycerol kcal kcal cal deg"?

mol-I mol'? mol"?

O· 4471.33 6.085 1.705 19.09 13.51 -18.710 3709.13 -2.476 1.270 18.76 11.81 -23.330 2643.71 3.391 0.430 18.47 10.36 -271:250 3072.43 -0.215 1.124 18.34 9.48 -29.770 2966.09 0.663 1.004 18.56 9.50 -30.4

(a) Ref. 16. Maximum uncertainties in the values of t::,.Go,t::,.H· and t::,.so at 25° are ± 0.007 kcal mol'<, ± 0.10 kcalmol"! and ± 0.3 cal deg-I mol"! respectively.

TABLE4 - VALUES OF 1I(t::,.GN), 1I(t::,.SN) t::,.Gi (H+) ANDt::,.GHS-) FORGLYCEROL+ WATERMIXTURES AT 25°C

Wt. % 1I(t::,.G·)Glycerol 1cal mol-I

~(/:::,.So) t::,. Gt (H+) t::,. Gr ; S-)cal deg" mol-1 k cal mol"? k cal mol-I

-0.43-0.93-1.36-1.52

0.140.320.550.94

-0.67-1.25-1.91-2.46

10305070

- 7.6-12.2-15.8-17.7

by the method of least squares. The values of thecoefficients A, Band C for the solvents are given inTable 3. Computed p'K, values by the use of Eq. (1)are found to be reproducible within ± 0.005 unit.

The thermodynamic quantities such as b:.GO,l::,SOand l::,Ho (molal scale) for the ionization of 1 moleof each of the solvent mixtures in their respectivestandard states were evaluated using standard rela-tions16a comprising the constants of Eq. (1) and arealso presented in Table 3.

Free energy changes - The standard free energy.changes 8(6GX,) accompanying the auto ionizationof the solvents relative to that of water have been

-computed on mole fraction scale (N) using Eq. (2),II (t::,.G'N )=st::,.G'N -wt::,.G'N =2.303 RT[p (sKs)N-P (wKs)N] •

= 2.303 RT f [ p(sK.) + 2 log :0]_ [p(wKw)

1000 ]2+ 2 log Mw 5 ...(2)

where M•.•and M. are the molar and mean molarmass of water and the solvent mixtures respectivelyand the subscript w refers to water and s to the mixedsolvents.

Since 8(6. GN) values (Table 4) and their variationwith solvent composition (Fig. I) are guided by the·combined effects of acidity and basicity of the mixedsolvents besides the effect of decreased dielectric

·constant, the individual contributions of H+ and lyate(S-) to 8(.6 GN) values should be more informativein understanding such a combined effect. If the·auto ionization of water in water as the standardstate (w) and that of any of the mixed solvents (SH)in the respective mixed solvent as the standard state(s)be expressed in a form like (e), then 8(6GN) is thefree energy change due to the transformation (f)

[HsO+(w) + SH (s) + SH (s) + OH-(w) ;= H.O(w) +SHi (s) + S-(s) + H20 (w). . . (f)

,(

--------£_~------ llG'(H')_-€)-'-----<r------ •o

'Lo~~-2.5oil

-5.0

o 10 20 30

Mol Z Glycerol

Flg.t- !:e;~~~YnamiCSor the ionisotion of glyeerol+wGter mixtures

This is equivalent to the sum of the processes (g) and(h)

H30+ (w) + SH (s) = HsO (w) + SH~ (s) .. (g)

SH (s) + OR- (w) = S- (s) + HaO (w) .. (h)

If we denote free energy change due to (g) as .6. G'/(S-) and that due to (h) as b:.at (H+) it appears from(f) that

a(t::,. G'N) = t::,.G1 (H+) + t::,.Gt (S-) (i)

hence l::,G~ (H+) gives a measure of the 'basicity'of the mixed solvents relative to that of water andb:.G'i (S-) reflects their relative acidity".

Till recently two sets17,18 of l::,G~ (H+) values arereported in the literature. While Wells' values"are increasingly positive, those of Khoo", which arebased on extrapolation procedure", are increasinglynegative. In view of the controversy arising from therelative uncertainties of the involved extrathermo-dynamic assumptions we have recently determined"l::,G~(i) values for individual ions by the use of refe-rence electrolyte (Ph4AsBPht) (Ph = phenyl) assump-tion :

l::,G? (Ph.tAs+)= .6.G~(Ph4B-)=t l::,G~ (Ph4AsBPh4)... (3)

which was recently been demonstrated by Kim21bothfrom theoretical and experimental evidences to bemore sound and useful than others. As the data inTable 3 indicate. l::,G~ (H+) values though differ inmagnitude from those of Wells, are also increasinglypositive. So, l::,G~ (S-) values as obtained by uti-lizing these values of l::,G~ (H+) are found to be in-creasingly negative(Table 3). These values of l::, G~(H+)and l::,G~ (S-) suggest that increasing proportion ofglycerol makes the mixed solvents less basic and atthe same time more acidic compared to pure water.This is also in conformity with what is expected from

3

INDIAN J. CHEM., VOL. 18A, JULY 1979

the 'acid-base' nature of glycerol. This is becausejust as the electron donating -~H3 group in methanolmolecule induces larger negative charge density III

O-atom in -OH group compared to that in isolatedwater molecule", so also the R-group

(CH2 - CH- CH2- )

= I IOH OH

in glycerol containing two strongly electron with-drawing -OH groups will diminish the negativecharged density on the attached O-atom of the -OHgroup compared to that in water molecule. As aresult, glycerol molecule is likely to behave as a weaker'base' and at the same time stronger 'acid' than water.Wells' recent observation'? that K. ( = [ROH"'2]/[ROH] [H~q]) values are too low to measure in theglycerol + water mixtures and that acid ionizationconstant (pKa) values for glycerol in aqueous solutionsare reported to be 14.07 (ref. 9) and 14.4 (ref. 11) bydifferent authors compared to 15.74 for water",also substantiate the implications of the presentresults.

Entropy chgnges - Figure I illustrates the variationof298 x '8L,So ('8L,SN = sL,S~-wL,S<;') with mole

N .fraction of glycerol, the values of f:::"S<;,beingobtained using the relation (4)

1000f:::" s~ = f:::"so-2R ln~

The resulting profile though monotonic like that of3(L,G~) is not that simple to interpret. This isbecause unlike 8(L, G<;,)the corresponding enthalpyand entropy changes contain apart from other effectsthe relative effects of the structural changes accom-panying the involved auto ionization of the solvents.Moreover, the individual contribution of thesequantities in a manner similar to the case of '8(L,GN)is also not feasible at this stage. Hence, the under-standing of these quantities, particularly T 'tJ(L,SN)which is simpler and more informative regardingsolvent structure than '8(L,HN) (ref. 23), is also an

.involved problem.However, as in a previous cases, it seems worthwhile

to examine the results in the light of Hepler's treat-merit". But since glycerol is relatively more acidic butless basic compared to pure water, the overall auto-ionization of these mixed solvents is likely to be dictatedby the combined effects of reactions (a) and (c).Consequently, Hepler's original treatment-s which wasessentially based on the implicit assumption thatwater being much more acidic than the other co-solvents is the effective ionizing species, is not appli-cable in this case. But Hepler's modified treatment"for evaluating the effect of ionization of water in thepresence of acidic cosolvents such as this, should berewarding.

Accordingly, the experimental autoionization cons-tant K. (molar scale), henceforth designated as(Ka/l) exp- can be written as

(Ka{l) exp = KHZ. CHZ + (Kall)corr

where KHZ and (Kafl) corr- are the ionization cons-

4

.. (4)

tants of the cosolvent HZ and water respectively inthe mixed solvents and CHZ is the molar concentra-tion of HZ in the respective solvents. In order tohave an estimate of the desired quantity (Ka{l}corr,determination of KHZ is in order.

Applying Hepler's procedure for water-rich co~-positions KHZ values of glycerol were determined inthe following manner. Assuming that KHZ =(KHZ) cnem X (KHZ) etec- (ref. 24) and that KHZin pure water differs from that at 5 wt % cosolvent(say) (obtained from the interpolated data of (Ka{l)expand taking (Karl) corr as equal to that of purewaterl6b) by electrostatic contribution (KHz)elee.only, KHZ values at different temperature in purewater were first determined by computing (KHZ )elecwith the help of the appropriate form of Born expres-sion'", The required value of rH+ was taken as0.276 nm26 and that of rz+ as 0.237 nm (half of theterminal 0-0 distance of glycerol molecules") and thedielectric constant data from Akerlof's paper".

To compute the KHZ values for 10, 30 and 50 wt%glycerol the following points have been taken intoconsideration. When increasing amounts of glycerolare added to dilute aqueous solutions of HZ, theionization of HZ is likely to change due to (a) changeof dielectric constant of the medium affecting (KHZ )elec.and (b) the basic reaction of HZ as in reaction (d)and/or the change of basicity of H20 that takes partin ionization reaction (c), thus affecting (KHZ )chem.As indicated earlier, since the basicity of HZ is lessthan that of H20 and as the basicity of H20 moleculeis likely to remain virtually unchanged, at least in thelower percentages of this acidic cosolvents, we mayassume that KHZ values get altered through (KHZ )elec.only. With these assumptions and the fact that ionicradii of H+ and Zr remain effectively constant withinthe temperature range studied, KHZ values at differenttemperatures were computed. Taking these values ofKHZ, (Ko/l)corr values for the ionization of water inthe mixed solvents have been evaluated at differenttemperatures with the help of the relation (5) and are

, presented in Table 5. These values in the respectivesolvents were then fitted as before to Harned-Robinsontype expression (2) by the method of least squares.The entropy of ionization (L,S~rl)corr in the mixed

TABLE 5 - ESTIMATED P(Ka/l)corr FOR, THE IONIZATION OFWATER (MOLAR. SCALE) IN GLYCEROL + WATER AND ETHYLENEGLYCOL + WATER MIXTURES AT DIFFERENT TEMPERATURES

Wt &G or EG

15° 40·10· 20°

o103050

Glycerol (G) + water mixtures14.35 14.17 '14.00 13.84 13 6814.27 14.10 13.94 13.79 13.6313.91 13.78 13.66 13.55 12.4913.87 13.76 13.66 13.56 13.40

13.5413.5213.2713.31

14.5714.4514.0514.00

o103050

Ethylene glycol (EG) + water mixture14.73* 14.35 14.00 13.68 13.40t14.78· 14.34 13.98 13.64 13.35t14.55* 14.30 13.96 13.62 13.32t14.42* 14.27 13.87 13.59 13.31t

... (5) ·Values at 5°tValues at 45°

BASl!MULLICK& KUNDU : STRUCTUREDNESS OF SOLVENTS

the corresponding entropy changes are given by (6)and (7) " " "

solvents were then evaluated as before using the co-efficients of that expression for the respective solvents.Figure 2 illustrates the variation of n(,6s:,c)[T. (s,6S:,c)-T(w,6S:rc)] at T = 298.15K with co-solvent composition, where b. S:/ti is related to(,6S:'I)corr by the relation f:,S:,c = (f:,S:/I)corr/C",where C» is the molar concentration of water in therespective solvent mixtures.

For the sake of comparison exactly similar com-putations were also made for ethylene glycol (EG) +water mixtures which were shown to be more acidicthan water'"!'. The required data were taken fromthe literature'" and rzr: for Born equation was takenas 0.20 nm (half of 0-0 distance, as tentatively com-puted from geometrical consideration) and rH+ as0.276 nm26• The resulting plot of 298 8(f:,S~/c)versus composition of EG is shown in Fig. 2. Inaddition, since 2-propanol (2-PrOH) is less acidicthan water". correspondi~~ curve for 298 ~(,6S~/c)ve.rsus cosolvent .compositlOn for 2-PrOH + watermixtures, as obtained by one of us earlier" using theoriginal treatment of Hepler", have also been illus-trated in Fig. 2.

It is interesting to note that while. the profiles for2-PrOH + water and EG + water mixtures exhibitsomewhat pronounced maxima at about 8 and 3 mol% cosolvents respectively, t~at for glycerol (GL) +-water mixtures show only a kink, Moreover leavingaside the ~trinsi? differences the nature of th~ profilesare essnetially similar to the corresponding profilesfor f:, Y l= 1::,.S,/ (H+) +-,6 S'lch (X-)] for the sol-vents studied by us earlier-. The implications ofthese results as are follows :

Since the ionization of water in pure water and inany of the mixed solvents can be presented by (j) and(k) respectively

H20 (w) = H+ (w) + OH- (w)

H20 (s) = H+ (s) + OH- (s)

2.5 r-------------------.

'0E~ o~ __ -------~~-~~~----.x

.s--..~o({)

~'t>J-'

-2.5

GlyeerOIHtater

-~"OO!:-------:I~O------2L.O------l30

MOlY. eo-solvent

Fig.2-Thermodynomies 0' ionisot!on 0' water In differentaquo-orgonte ,olvenll at 2S·C.

/

(

W/::,S;/c =wSfI+ + wSOH- ~ wSH~ .

s/::,S~/c = sSH+ + sSHO- --sSfhO

..(6).. (7)

where the terms have their usual significance. So;

~(/::,Soa/c v= s/::'°a/c-w/::'Soa/c=/::'SOt(H+) +/::,SOt (OH-)--/::,SOt (HaO) .. (8)

.. 0)

.. (k)

Recently Kundu and coworkers" have shown bothfrom experiment and a semi-quantitative theory thatsum of" the transfer entropies of H+ and halide(X- = Cl-, Be or 1-) ions from water to aquo-organic solvents are positive where the solvents aremore structured than water. Again, simple con-sideration of order-disorder phenomena also indicatesthat ,6S~ (H20) will be negative when the solventis more structured than water. Since transfer be-haviour of OH- is"likely to be similar to that of X-,positive magnitude of 8(f:,Sa/~) would imply greaterstructuredness of the solvents compared to water.

The observed results in Fig. 2 therefore suggestthat as in the case of 2-PrOH small addition of EGpromotes three dimensional (3D) water structureand larger amounts break-up of the same. Buttheaddition of GL induces hardly any promotion of sucha 3D structure. If the possible uncertainties involvedin estimating f:,Salc for the cases of EG and GLcompared to that of 2-PrOH be ignored, the apparentsignificant differences in the profiles of 2-PrOH +water and GL + water mixtures suggest that theeffect of structure promoting propensity of hydropho-bic -CHa groups in 2-PrOH is appreciably reducedby -CH20H groups forming GL. As indicatedearlier", this is possibly due to the fact that the mono"meric 'dense' water molecules in equilibrium with'bulky' 3D structure of water instead of being re-pelled due to hydrophobicity of -CHa groups andinducing ice-berg formation" or hydrophobic hy-dration", are likely to induce 'specific site hydration'Pthrough the hydrophilic -OH sites of -CH20Hgroups, thus appreciably reducing the structure promo-tion. Since this effect is likely to be more extensive inthe case of GL as compared to EG, the maximum isexpected to shift to lower cosolvent composition asobserved. In fact the former apparently starts tobehave more like an effective structure breaker almostfrom zero cosolvent composition. This also con-forms to the contentions arrived at from the variationsof some physico-chemical properties, such as wateractivities", viscosities'", sound absorption's''", partialmolal volumes" and apparent molal volumes and heatcapacities of transfer of several ionic solutes fromwater to GL + water mixtures, which indirectlysuggest that there is less structural order in thesesolvents than in water. Of course, the small andbroad characteristic maxima of ,6Y-composition pro-files observed by us earlier? leads us to contend thatthe effect of specific site hydration of -OH groupsovercomes the effect of apparently small structurepromoting ability of hydrocarbon skeleton of GLmolecule at about 10 mole % GL instead of almostzero mole % as observed in the present study.

5

-,

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INDIAN J. CHEM., VOL. 18A, JULY 1979

Similar contention has also been arrived at by Khoofrom a number of studies18'37 in GL + water mixtures.

Admittedly, due to involved uncertainties in esti-mating the ionization constants of water (Kat1)corr inthese solvents, the structural information derivedfrom this study is not that conclusive. But never-theless, the pattern of the results as illustrated inFig. 2, being essentially similar to that in our previousstudy, leads us to believe that glycerol lies in theborderline of the structure making mono-ols" andstructure breaking poly-ols",

AcknowledgementThanks are due to the UGC, New Delhi for the

grant of a Teacher Fellowship to one of us (I.N.B.).

References1. DIPAOLA, G. & BELLEAU,B., Can. J. Chem. 53 (1975)

3452. and see the relevent references therein for the earlierwork.

2. VON HIPPl!R, P. H.& WONG, K. Y., J. Bioi. Chem., 250(1965), 3905.

3. SIMPSON,R. B. & KAUZMANN,W., J. Am. chem. Soc., 75(1953), 5134; FRl!NSDORFF,H. K., WATSON,M. T. &KAUZMANN, W., J. Am. chem. Soc. 75 (1953), 5157.

4. SUBRAMANIAN,S., SARMA, T. S., BALASUBRAMANTAN,D.& AHLUWALIA, J. c., J. phys. Chem., 75 (1971), 815and the relevent references therein.

, 5. MASTROIANNI,M. J., PIKAL, M. J. & LINDENBAUM,S.,J. phys. Chem., 76 (1972); 3050.

6. SCHUMBER,F. & BELLAU, B., Bio-org, Chem., 2 (1973),111. '

7. BASUMULLICK,I. N. & KUNDU, K. K., Can. J. Chem.,In press.

8. DAS, A. K. & KUNDU. K. K., J. phys. Chem., 79 (1975),2604. ' ,

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