journal of MOLECULAR LIQUIDS

ELSEVIER Journal of Molecular Liquids 79 (1999) 89-99

S T U D I E S O N M O L E C U L A R I N T E R A C T I O N S I N B I N A R Y L I Q U I D

M I X T U R E S B Y V I S C O S I T Y A N D U L T R A S O N I C V E L O C I T Y

M E A S U R E M E N T S A T 3 0 3 . 1 5 K

A. Aft*, S. Hyder and A. K. Nain

Department of Chemistry, Jamia Millia lslamia, New Delhi - 110 025, India Received 11 September 1997; accepted 5 August 1998

ABSTRACT

The densities, viscosities, and uhrasonic velocities of pure ethanol, 1-hexanol, 1-octanol, acetonitrile, N,N-dimethylformamide, and of the binary mixtures of ethanol with 1-hexanol and 1-octanol, and those of acetonitrile with N,N-dimethylformamide were determined at 303.15 IC The excess adiabatic compressibility, excess intermolecular f ee length, excess volume, excess viscosity, excess acoustic impedance, and the molecular association have been calculated from the experimental data. These parameters are used to discuss the nature and the extent ofintermolecular interactions in the mixtures.

© 1999 Elsevier Science B.V. All fights reserved.

Key words : Viscosities, Ultrasunic velocities, Excess functions, Binary mixtures, Molecular interactions.

INTRODUCTION

The study of excess thermodynamic functions such as excess adiabatic compressibility, excess intermolecular f e e length, excess volume, excess viscosity, and excess acoustic impedance are of considerable interest in understanding the nature of intermolecular interactions in binary liquid mixtures [1-4]. An attempt has been made to study the intermolecuLiar interactions in binary mixtures of ethanol with 1-hexanol and 1-octanol and those of acetonitrile (ACN) with N,N-dimethylformamide (DMF) covering the whole composition range. Alkanols, ACN, and DMF are versatile compounds, especially in their wide range of applicability as solvents in chemical and technological processes [5, 6]. Alkanols are serf-associated through hydrogen bond and this association decreases with increase in molar mass of alkanols [5], whereas in ACN molecules the dipoles are oriented somewhat antiparallel to each other, and the dipoles of DMF are randomly oriented [7]. Moreover, to the best of our knowledge, no viscosity or ultrasonic study has been reported so far on mixtures of a lower alkanol mixed with the higher alkanols. Some authors [8, 9] have studied the behaviour of mixtures composed of two different alkanols, especially, the lower alkanols. Literature survey indicates that there has been practically

O167-7322/99/$ - see front matter © 1999 Elsevier Science B.V. All fights reserved. PII S0167-7322(98) 00105-6

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no study on ACN + DMF from the point o f view of their viscosity as well as ultrasonic behaviours, except for the work of Tamura and co-workers [10]. These authors studied only ultrasonic behaviour o f this system at 298.15 K.

We report here the excess adiabatic compressibility, 13 E, excess intermolecular free length, Lf E, excess volume, V E, excess viscosity, 11 ~, and excess acoustic impedance, Z E from the measurements of density, p, viscosity, lq, and ultrasonic velocity, u o f the aforementioned binary mixtures at 303.15 K covering the whole composition range. The excess functions are used to explain intermolecular interactions in these binary mixtures.

EXPERIMENTAL

Ethanol and DMF were the same as in our previous study [31. Acetonitrile, 1-hexanol, and 1- octanol (s.d. fine, India) of AnalaR grade were purified by the methods descn'bed in the literature [11, 12]. The binary mixtures of ethanol with 1-hexanol and 1-octanol, and those o f acetonitrile with DMF were prepared by weighing an appropriate volume of each liquid component and were kept in special airtight bottles. All solutions were prepared in a dry bog

The weighings were done on an electric balance with a precision o f 0.1 rag. The necessary buoancy corrections were applied. The probable error in mole fraction was estimated to be less than 10 -4. The densities of pure liquids and binary mixtures were measured using a single stem pycnometer (made of Pyrex glass) of bulb capacity 8 x 10 -3 dm 3 with a graduated stem of 5.0 x 10 -~ dm -3 divisions. The marks on the stems were cah'brated with triple-distilled water. Viscosities were determined using Cannon-Ubbelhode viscometer [13] calibrated with triple-distilled water. The viscometer containing test liquids was allowed to stand for about 20 minutes in thermostatic water bath so that the thermal fluctuation in the viscometer was minimi~d. The overall experimental uncertainty was estimated to be _+ 1.5 x 10 -3. The ultrasonic velocities through pure liquids and their mixtures were measured using a single crystal variable path ultrasonic intefferometer operating at 3 MI-Iz by the method described by Subrahmanyam and Murthy [14] with an accuracy of + 0.05%. The values o f densities and viscosities at 303.15 K were found to be accurate upto + 0.01 kg m -3 and __. 3 x 10 -6 1~[ m -2 s, respectively. The temperature o f the test liquids and thehr binary mixtures was maintained to an accuracy o f+ 0.02 K in a thermostatic water bath.

RESULTS AND DISCUSSION

The excess functions were evaluated using the following relation :

y S = y _ [(1-x)V, + xV2] (1)

where Y denotes ]3, L~ V, ~, and Z, respectively; x is the mole fraction o f ethanol or ACN in the mixture. Subscript 1 refers to 1-hexanol, 1-octanol, and DMF; subscript 2 refers to ethanol

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or ACN, whereas the absence of subscript refers to the mixtures. The experimental values o f 9, TI, and u (Table 1) were used to calculate the values of adiabatic compress~'bility, [3, intermolecular free length, Lf, molar volume, V, and acoustic impedance, Z with the help of standard r e h t i o v ~ s given in the literature [3, 15]. The calculated values o f [~E, LfB, V e, ~E, and Z E as a function of composition of the mixtures at 303.15 K are given in Table 2 and graphically presented in Figs. 1-5.

Table 1 Densities, 9, viscosities, 11, and ultrasonic velocities, u of binary mixtures at 303.15 K

x p "q u Ethanol/ACN (kg m -3 ) (10 -3 N m -2 s) (ms -1 )

Ethanol + 1-Hexanol 0.0000 807.6 3.8951 1281.7 0.1433 805.6 3.3639 1270.6 0.2814 802.9 2.8872 1257.9 0.4870 798.6 2.2414 1245.9 0.6448 794.8 1.8073 1217.3 0.7691 791.2 1.5088 1194.0 0.8692 787.7 1.2795 1177.3 0.9524 785.7 1.0933 1145.3 1.0000 783.9 1.0090 1133.3

Ethanol + 1-Octanol 0.0000 817.2 6.4931 1327.5 0.1301 815.9 5.4708 1316.6 0.2430 814.6 4.6068 1307.6 0.3339 813.4 4.0015 1298.6 0.5477 808.6 2.9218 1264.0 0.6979 801.2 2.1685 1239.7 0.8094 794.1 1.6888 1200.4 0.8948 788.8 1.3623 1167.8 0.9602 785.6 1.1211 1146.8 1.0000 783.9 1.0090 1133.3

ACN + DMF 0.0000 941.9 0.7566 1446.4 0.1176 929.5 0.6994 1422.1 0.2399 914.2 0.6372 1396.5 0.4348 886.5 0.5495 1374.0 0.5933 861.4 0.4816 1344.8 0.7275 837.2 0.4283 1317.4 0.8419 813.9 0.3842 1296.0 0.9418 789.9 0.3496 1275.4 1.0000 773.3 0.3501 1263.4

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Table 2 Excess functions of binary mixtures at 303.15 K

x 13 ~ I~ E v E ~ z ~ Ethanol/ACN (10 -1° m 2 N -1 ) (10 -u m) (10 -6 m s mol -l ) (10 -3 N m -2 s) (10 s kg m -2 s -1 )

Ethanol + 1-Hexanol 0.1433 -0.1926 -0.0642 0.0421 -0.1176 0.0957 0.2814 -0.3409 -0.1127 0.1380 -0.1958 0.1621 0.4870 -0.6377 -0.2140 0.2043 -0.2482 0.3136 0.6448 -0.5908 -0.1938 0.2057 -0.2268 0.2699 0.7691 -0.5140 -0.1667 0.1936 -0.1666 0.2240 0.8692 -0.4609 -0.1498 0.1727 -0.1070 0.1982 0.9524 -0.1163 -0.0362 0.0421 -0.0531 0.0451

Ethanol + 1-Octanol

0.1301 -0.2624 -0.0899 -0.0774 -0.3087 0.1495 0.2430 -0.4907 -0.1685 -0.1517 -0.5537 0.2809 0.3339 -0.6519 -0.2233 -0.2145 -0.9104 0.3706 0.5477 -0.8404 -0.2819 -0.2157 -0.5678 0.4484 0.6979 -0.9083 -0.3025 0.0764 -0.4972 0.4550 0.8094 -0.6237 -0.2012 0.2729 -0.3654 0.2741 0.8948 -0.3219 -0.1002 0.2771 -0.2236 0.1210 0.9602 -0.1345 -0.0413 0.1331 -0.1061 0.0471

ACN + DMF

0.1176 -0.1636 -0.0571 -0.1386 -0.0094 0.1140 0.2399 -0.1683 -0.0453 -0.1783 -0.0219 0.0410 0.4348 -0.4162 -0.1380 -0.2086 -0.0304 0.2331 0.5933 -0.4514 -0.1480 -0.2736 -0.0338 0.2470 C.7275 -0.3946 -0.1270 -0.3045 -0.C325 0.2095 0.8419 -0.3085 -0.0995 -0.3011 -0.0302 0.1694 0.9418 -0.1428 -0.0457 -0.1824 -0.0242 0.0804

The excess functions, yE ([3 E or Lf E or V B [16] type equation, by least-squares fitting :

5

yE = x(1 - x) ~ Ai (1 - 2x) i'l i=l

The polynomial coefficients, Ai and the standard deviations, o (yE), calculated as

c 0 f E) = ~ Y o ~ E - Y~E)2/(m -- n)] 1/2

or ~1E or Z E) were fitted with Redlich-Kister

(2)

(3)

93

where m is the number of experimental data and n is the number of Ai coefficients considered (n=5 in the present case), have been presented in Table 3.

Table 3 Coefficients Ai o f Eq. (2) and standard deviations o (yE) of binary mixtures at 303.15 K

yd A~ A2 A3 A4 A~ o (Y~)

Ethanol + 1-Hexanol

[3 d (10 -~° m2N -~ ) -2.6191 1.8833 1.8877 -2.0528 -1.9761 0.0078 Lf d (10 -n m) -0.8783 0.6217 0.7349 -0.7348 -0.7561 0.0022 V d (10 -6 m 3 tool -1 ) 0.4836 -0.4679 3.2991 -0.6977 -4.7498 0.0051 1] E (10 -3 N m -2 s) -0.9954 -0.0556 0.3092 0.1634 -0.5415 0.0017 Z E (105 kg m -2 s -1 ) 1.2819 -0.8402 -1.3974 1.1876 1.4574 0.0013

Ethanol + 1-Octanol

[3 E (10 -~° m2N q ) -3.2327 2.0666 -2.6473 -2.0599 5.6720 0.0262 Lf d (10 -H m) -1.0946 0.6579 -0.8455 -0.7375 1.9564 0.0098 V E (10-6 m 3 mol q ) -1.0486 -1.5381 5.5235 -1.2140 -3.4838 0.0076 .qE 00-3 N m -2 s) -2.4370 -1.2701 -1.4491 2.1867 1.9325 0.0149 Z d (105 kg m -2 s -1 ) 1.8288 -0.2739 -0.0993 -0.0478 -0.8599 0.0163

ACN + DMF

]3 E (10 -1° m2N "-! ) -1.6666 1.2325 0.4694 -0.6042 -1.0329 0.0019 Lr d (I0 -H m) -0.5903 0.1897 0.3294 0.1405 -0.3894 0.0007 V E (10-6 m 3 moY ~ ) -0.9483 0.7797 -0.8228 0.1004 -1.6145 0.0044 11E (10 -3 N m -2 s) -0.1524 -0.0581 0.1788 0.3250 -0.4213 0.0006 Z d (105 kg m -2 s -I ) 0.9809 -0.3004 -0.0726 -0.4707 -0.0873 0.0038

The curves in Figs. I, 2 and 4 show that fld, LrE, and rl d are negative for all the three binary mixtures; V E values (Fig. 3) are entirely positive for ethanol + 1-hexanol, change from negative (with minimum at x ~ 0.45) tO large positive at higher mole fraction of ethanol (showing maximum at x ~ 0.82) for ethanol + 1-octanol, becoming entirely negative for ACN + DMF binary mixtures over the whole composition range. All the binary systems studied exhibit positive Z E values (Fig. 5) over the complete mole fraction range. Positive or negative deviations in these functions from rectilinear dependence on composition o f the mixtures indicate the extent of dissociation or association between unlike molecules.

The behaviours of three systems under investigation have been qualitatively examined using the excess functions. As mentioned earlier, in pure alkanols the molecules are associated through hydrogen bond, mixing of ethanol with 1-hexanol or 1-octanol will induce the rupture of hydrogen bonds in the liquids with subsequent increase in 13E and Le d values. However, due to simultaneous formation of hydrogen bonds between OH groups of unlike molecules there

0.~ I ~4~'- Ethanol + l-I-le)~mol j

\ \ ~ . I . - i i - - Ell~Inol + 1 -Octanol I I

..O.m

-0.40 Z

= 4 ~ . ~

-0.80

94

ol .00 ~ i ,

0.00 0.20 0.40 0.60 0.80 1.00

X

Figure 1. Variation of[3 E against x (ethanol/ACN) of binary mixtures at 303.15 IC

o,oo-,. I ~:~"o=°'i i , / /

o.,o \ \ , , ~ J #

~_~ -0.20

-0.25 ~

-0.30

0.00 0.20 0.40 0.60 0.80 1.00

x

Figure 2. Variation ofLr e against x (ethanoFACN) of binary mixtures at 303.15 K.

95

is a compensating effect resulting in an overall decrease in ~E and l.e ~ values with x, as evident from Figs. 1 and 2. It has been suggested [4, 17] that the excess adiabatic compresm'bility and excess free length become increasingly negative with increasing strength of interaction between the component molecules. It is interesting to note that ~E and Lf E become more negative for ethanol + 1-octanol than corresponding values for ethanol + t-hexanol mixtures. This may be due to the fact that the large alkyl residue (in 1-octanol) destroys the hydrogen bonding between ethanol molecules more easily, thus releasing relatively more free ethanol molecules to form ethanol-octanol associated species that seems to be more stable than the associates present in pure ethanol or 1-octanoL ACN + DMF binary mixtures are also quite interesting as ACN and DMF both are dipolar aprotic liquids with large, but nearly equal, dipole moments [18]. Thus, mixing of DMF with ACN will induce the mutual distruction of dipolar structures of the component liquids releasing free dipoles. As a result, strong dipolar interaction between ACN and DMF molecules is expected. The observed negative values of [3 E and Lf E (Figs. 1 and 2) over the complete range of composition of ACN + DMF binary mixtures support the above view. Similar trends in [3 E and Lr E with composition have also been reported for ACN + dimethylsulphoxide [17] and ACN + methanol [19] binary mixtures.

020 1 ---'0"-- Ethanol + 1-Hexanol • - I ' - - Ethanol + l-Octano ~ ~ ~ I

0.20

0.10

C" '~ 0.00 I

%

-0.10

-0.20

-0.30

-0.40 ~ ~ = 0.00 0.20 0.40 0.60 0.80 1 .(30

x

Figure 3. Variation o f V E against x (ethanol/ACN) of binary mixtures at 303.15 IC

The factors which may influence the values of V E on mixing of ethanol with 1-hexanol or 1-octanol are : i - the mutual dissociation of hydrogen bonded structures of the components, ii - geometrical effects allowing the accommodation of species of too different size into each other's structure due to the differences in molar volumes between components,

96

and iii- the formation of(new) hydrogen bonds between component molecules. The first effect leads to increase in V s, while the remaining two effect tend to decrease V E. The observed positive values o f V E (Fig. 3) with increase in mole fraction of ethanol for ethanol + 1-hexanol mixtures may be attributed to the breaking up of hydrogen bonds in the component fiquids, which is not compensated by the decrease in V E due to the combined effects of possa~le hydrogen bonding between , , i ike molecules and the fitting of ethanol molecules into the structure of 1-hexanol molecules. The negative values o fV E upto x ~ 0.68, with minimum at x ~ 0.45, (Fig. 3) for ethanol + 1-octanol mixtures may be predominantly due to the fitting of smaller ethanol molecules (molar volume = 5.87 x 10 -5 m s moV s at 303.15 K) in the vacancies created by the bigger 1-octanol molecules (molar volume = 15.94 x 10 -5 m 3 mol -l at 303.15 K) in addition to the hydrogen bonding between unlike molecules. Beyond x ~ 0.68, the added ethanol causes more and more disruption of the associated structures of the components which seems to dominates over the combined effect of the fitting of ethanol molecules into the vacancies in 1-octanol molecules and hydrogen bond between unlike molecules, resulting an increase in V E values. Contn"outions arising from geometrical fitting of one component into the other due to the difference in molar volumes were also considered by others to explain the variation of V E values with composition of formamide [20] and pyrolidin-2-one [15] with alkanols (C1-C5). Figure 3 shows that the values o f V E are negative over the whole range of mole fraction, x of ACN for ACN + DMF binary mixtures with minimum at x ~ 0.80. Negative V E values suggest that the strong dipole-dipole interaction between ACN and DMF molecules, as both the liquids have large dipole moments, overweighs the poss~%le increase in V E due to the structure breaking effect between like molecules. There are evidences involving strong dipole-dipole interaction between ACN and hexamethylpho sphotictriamide/ dimethylsulphoxide [7, 17] molecules, all the liquids being dipolar aprotic in the pure state [18], as in our case.

The negative values of excess viscosities, TI E, for all the three binary systems investigated (Fig. 4) over the whole composition range, suggest that the viscosities of associates (ethanobl-hexanol/-1-octanol and ACN-DMF) formed between .nlike molecules are relatively less than those of the pure components. Negative E deviations fxom rectilinear dependence on mole fIaction may also occur where dispersion forces are dominant, particularly, for the systems having different molecular size [2, 21, 22] as in the present case. As expected, the observed positive deviations in Z E (Fig. 5), where Z = up, and an opposite trend in the behaviour o f~ E (Fig. 1), where fl = 1/u2p (discussed earlier), over the entire range of mole fIaction of the systems investigated again support our view that the interactions between . , l ike molecules are quite obvious.

It has been suggested [23] that the concentrations at which the excess functions exhibit extrema (Figs. 1-5) indicate strong interactions between , , l ike molecules, leading to formation of associates. Hence, the maximum stability of the associates, thus formed, is reached in the concentration range where such extrema occur. However, the extrema in excess functions were viewed as due to the transition from component l-in-component 2 solutions to component 2-in-componentl solutions [24, 25].

O.00

-0.10

-0,20

~E -0,30

z

~ - 0 . ~

-0.50

-0.60

-0.70 ' ' = 0,00 0.20 0.40 0.60 0.80 1.00

x

Figure 4. Variation of ~E against x (ethanol/ACN) of binary mixtures at 303.15 K.

97

0,60

--IP-Ethanol+ 1,Hexanol I --.B-- Ethanol + 1-Octanol --~Ik--ACN + DMF

0.50

0.40

;o ~ 0.30

? N

0.20

0.10

0 . ~ 0.00 0.20 0.40 0.60 0.80 1.00

x

Figure 5. Variation ofZ E against x (ethanol/ACN) of binary mixtures at 303.15 K=

98

Another useful parameter, M~ the molecular association, a measure ofnon-ideality of the system, is calculated using the relation :

MA = [(U2/Ulm 2) " 1] (4)

where u~ is the ultrasonic velocity of the ideal mixture. The values of urn, were obtained using the equation proposed by Van Dael and Vangeel [26]. The values Of MA are included in Table 4. The system ethanol + 1-oetanol shows large deviations in MA followed by ethanol + 1- hexanol and then by ACN + DMF system Thus, it is concluded that the non-ideality of the systems varies in the order : (ethanol + 1-ocatnol) > (ethanol + 1-hexanol) > (ACN + DMF). This fitrther reinforces our view discussed earlier. Such behaviour in the variation of MA was also reported for pyrrolidin-2-one + n-alkanols (C1 - C5) [15] and ethylbenzene + n-alkanol (C6 - Clo) [27] binary mixtures.

Table 4 Molecular association, ]VIA of binary mixtures at 303.15 K

x MA x MA X MA Ethanol Ethanol ACN

Ethanol + 1-Hexanol Ethanol + 1-Octanol ACN + DMF 0.1433 0.1457 0.1301 0.2385 0.1176 0.0607 0.2814 0.2354 0.2430 0.3901 0.2399 0.1005 0.4870 0.3115 0.3339 0.4719 0.4348 0.1540 0.6448 0.2727 0.5477 0.5093 0.5933 0.1457 0.7691 0.2096 0.6979 0.4406 0.7275 0.1132 0.8692 0.1453 0.8094 0.2986 0.8419 0.0754 0.9524 0.0473 0.8948 0.1670 0.9418 0.0299

0.9602 0.0660

ACKNOWLEDGEMENT

Financial assistance fIom CSIR (New Delhi) to A.ICN. is gratefully acknowledged.

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