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Ultrasonic and IR study of intermolecular association through hydrogen bonding in ternary liquid mixtures
Ultrasonics 41 (2003) 477–486 www.elsevier.com/locate/ultras
Ultrasonic and IR study of intermolecular association through hydrogen bonding in ternary liquid mixtures Aashees Awasthi, J.P. Shukla
*
Department of Physics, University of Lucknow, Lucknow 226 007, India Received 30 September 2002; received in revised form 19 February 2003; accepted 19 February 2003
Abstract Complex formation in ternary liquid mixtures of dimethylsulfoxide (DMSO) with phenol and o-cresol in carbontetrachloride has been studied by measuring ultrasonic velocity at 2 MHz, in the concentration range of 0.019–0.162 (in mole fraction of DMSO) at varying temperatures of 20, 30 and 40 °C. Using measured values of ultrasonic velocity, other parameters such as adiabatic compressibility, intermolecular free length, molar sound velocity, molar compressibility, specific acoustic impedance and molar volume have been evaluated. These parameters have been utilized to study the solute–solute interactions in these systems. The ultrasonic velocity shows a maxima and adiabatic compressibility a corresponding minima as a function of concentration for these mixtures. The results indicate the occurrence of complex formation between unlike molecules through intermolecular hydrogen bonding between oxygen atom of DMSO molecule and hydrogen atom of phenol and o-cresol molecules. The excess values of adiabatic compressibility and intermolecular free length have also been evaluated. The variation of both these parameters with concentration also indicates the possibility of the complex formation in these systems. Further, to investigate the presence of O–HO bond complexes and the strength of molecular association with concentrations, the infrared spectra of both the systems, DMSO– phenol and DMSO–o-cresol, have been recorded for various concentrations at room temperature (20 °C). The results obtained using infrared spectroscopy for both the systems also support the occurrence of complex formation through intermolecular hydrogen bonding in these ternary liquid mixtures. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Ultrasonic velocity; Excess function; Infrared spectrum; Liquid mixture
1. Introduction The study of molecular interactions in the liquid mixtures is of considerable importance in the elucidation of the structural properties of the molecules. The intermolecular interactions influence the structural arrangement alongwith the shape of molecules. Dielectric relaxation studies of polar molecules in non-polar solvent using microwave absorption method have been widely used to study the molecular structures including the molecular interactions in liquid mixtures [1–3]. Lagemann and Dunbar [4] were the first to point out the sound velocity approach for qualitative determination of the degree of association in liquids. Recent developments have made it possible to use ultrasonic energy in medicine, engineering, agriculture and other *
Corresponding author. E-mail address: aasheesawasthi@rediffmail.com (A. Awasthi).
0041-624X/03/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0041-624X(03)00127-6
industrial applications [5,6]. Ozawa and Minamisawa [7] have observed concentration of ultrasonic velocity invariant with respect to temperature in alcohol–water mixtures. H€anel [8] has measured sound velocity and thickness of thin samples by time-resolved acoustic microscopy. Bae and Yun [9] have studied the ultrasonic velocity in binary solutions of silicon dioxide and water. Ultrasonic waves with low amplitude have been used by many scientists to investigate the nature of molecular interactions and physico-chemical behaviour of pure, binary, ternary and quaternary liquid mixtures [10–16]. It has been reported by several workers [17–22] that the complex formation occurs if the observed values of excess parameters (e.g. excess adiabatic compressibility, excess intermolecular free length and excess volume etc.) are negative. This suggests the occurrence of discrete groups of molecules arranged into specific geometric structures. These structural arrangements are influenced not only by the shape of the molecules but also by their
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mutual interactions. Our research group has also investigated complex formation in ternary liquid mixtures of amides with ethanol in benzene using ultrasonic method [23]. Singh et al. [24] have studied the association process of dimethylsulfoxide and p-tolylsulfoxide with proton donors (phenol and o-cresol) in an inert solvent CCl4 using dielectric relaxation measurements. The complex formation has been interpreted in terms of the association equilibrium constant and thermodynamic parameters. Since acoustic parameters provide a better insight into molecular environments in liquid mixtures, it seemed important to study molecular interactions in ternary liquid mixtures of DMSO with phenol and ocresol in a non-polar solvent CCl4 using ultrasonic technique. Apart from measuring ultrasonic velocity and other acoustical parameters, the excess values of adiabatic compressibility and intermolecular free length have also been evaluated for these ternary systems at temperatures 20, 30 and 40 °C respectively. The infrared spectra for both the ternary systems have also been recorded at room temperature (20 °C).
other components (phenol, o-cresol and CCl4 ) decreases as the mole fraction of a substance in the mixture is number of moles of that substance to the total number of moles in the mixture. Since, CCl4 is a non-polar solvent which is not taking part in the reaction so the mole fraction of CCl4 does not influence the mixture. The chemicals used were of AR grade from BDH. All the chemicals were purified by standard procedures discussed by Perrin and Armarego [25] before use. The infrared spectra for DMSO–phenol and DMSO– o-cresol systems have been recorded using FT-IR spectrophotometer (Model-8201) supplied by Perkin-Elmer, at Central Drug Research Institute, Lucknow.
3. Theory Various physical parameters [13] were calculated from the measured values of ultrasonic velocity ðU Þ and density ðqÞ. Adiabatic compressibility ðbÞ ¼
1 U 2q
Intermolecular free length ðLf Þ ¼ Kb1=2 2. Experimental details
where K values for different temperatures were taken from the work of Jacobson [26].
Ultrasonic velocity for the mixtures was measured using the ultrasonic interferometer (Model M81) supplied by Mittal Enterprises, New Delhi, that has a reproducibility of 0.4 m/s at 25 °C. The temperature was maintained constant by circulating water from a thermostatically controlled water bath (accuracy 0.1 °C). The temperature of the cell as measured using a thermocouple was found to be accurate to 0.25 °C. The density of various complexed species has been measured using a sensitive pyknometer with an accuracy of 0.5 kg/ m3 . The dilute solution study minimizes the effects of viscosity, internal field etc., therefore, for dilution apolar solvent like carbontetrachloride (CCl4 ), benzene (C6 H6 ), cyclohexane (C6 H12 ) etc., are used. Apolar solvent provide the medium and dilution for the mixture which, in turn, also minimizes the requirement of pure liquids in large quantity. Twenty millilitre carbontetrachloride was used for preparing mixtures in both the systems. Fixed amounts of phenol and o-cresol (0.30 gm each) were used in the DMSO–phenol and DMSO–o-cresol systems respectively while weight of DMSO was varied in preparing mixtures of varying concentrations. In the present study, the amounts (number of moles) of phenol, o-cresol and CCl4 are taken constant while amount (number of moles) of DMSO is increased in the mixtures which, in turn, increases the mole fraction of DMSO in the mixture and consequently mole fraction of
Molar sound velocity ðRÞ ¼ U 1=3 V Molar compressibility ðBÞ ¼ ðM=qÞb1=7 where V and M are the molar volume and molecular weight of the mixtures respectively. Specific acoustic impedance ðZÞ ¼ qU The excess adiabatic compressibility ðbE Þ and excess intermolecular free length ðLEf Þ were evaluated using the expressions: bE ¼ bexp bideal and LEf ¼ Lf exp Lf ideal where bideal and Lf ideal and their excess in mixture were defined under volume additive rule [18].
4. Results The mole fractions of DMSO, phenol, o-cresol and carbontetrachloride for both DMSO–phenol and DMSO–o-cresol systems have been given in Table 1. Ultrasonic velocity, molar sound velocity, molar compressibility, specific acoustic impedance and molar volume for DMSO–phenol and DMSO–o-cresol systems have been listed in Tables 2 and 3. Representative
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Table 1 Mole fractions of DMSO, Phenol, o-cresol and carbontetrachloride in both DMSO–phenol and DMSO–o-cresol systems DMSO–phenol system
DMSO–o-cresol system
DMSO
Phenol
Carbontetrachloride
DMSO
o-Cresol
Carbontetrachloride
0.019 0.029 0.038 0.048 0.057 0.066 0.075 0.084 0.093 0.111 0.128 0.145 0.162
0.016 0.015 0.015 0.015 0.015 0.015 0.015 0.014 0.014 0.014 0.014 0.013 0.013
0.965 0.956 0.946 0.937 0.928 0.919 0.911 0.902 0.892 0.875 0.858 0.841 0.825
0.020 0.039 0.048 0.057 0.066 0.076 0.085 0.094 0.102 0.111 0.129 0.146 0.162
0.013 0.013 0.013 0.013 0.013 0.012 0.012 0.012 0.012 0.012 0.012 0.012 0.011
0.967 0.949 0.939 0.930 0.921 0.912 0.903 0.894 0.886 0.877 0.860 0.843 0.826
Table 2 Ultrasonic velocity and related parameters for DMSO–phenol in carbontetrachloride t (°C)
X (mole fraction of DMSO)
q (gm/cm3 )
U (104 ) (cm/s)
R (103 ) cm3 /mol (cm/s)1=3
B (103 ) cm3 /mol (cm2 /Dyne)1=7
Z (104 ) (gm/cm2 s)
V (cm3 /mol)
20
0.019 0.029 0.038 0.048 0.057 0.066 0.075 0.084 0.093 0.111 0.128 0.145 0.162
1.548 1.552 1.556 1.560 1.566 1.568 1.571 1.567 1.562 1.561 1.556 1.544 1.533
9.480 9.495 9.508 9.520 9.524 9.544 9.555 9.526 9.485 9.433 9.390 9.355 9.304
4.460 4.431 4.400 4.370 4.334 4.312 4.286 4.272 4.246 4.224 4.185 4.175 4.161
2.751 2.734 2.715 2.698 2.677 2.663 2.648 2.639 2.622 2.604 2.585 2.576 2.565
14.675 14.736 14.794 14.851 14.915 14.965 15.011 14.927 14.816 14.725 14.611 14.444 14.263
97.816 97.124 96.404 95.708 94.901 94.351 93.748 93.546 93.101 92.619 92.079 91.976 91.817
30
0.019 0.029 0.038 0.048 0.057 0.066 0.075 0.084 0.093 0.111 0.128 0.145 0.162
1.535 1.541 1.546 1.548 1.549 1.551 1.553 1.548 1.544 1.542 1.539 1.530 1.516
9.311 9.320 9.327 9.340 9.370 9.334 9.295 9.240 9.212 9.139 9.103 9.062 9.023
4.417 4.435 4.401 4.376 4.358 4.327 4.296 4.281 4.267 4.223 4.188 4.169 4.164
2.757 2.736 2.716 2.701 2.689 2.671 2.654 2.644 2.635 2.608 2.586 2.573 2.567
14.292 14.362 14.420 14.458 14.514 14.477 14.435 14.304 14.223 14.092 14.010 13.865 13.679
98.644 97.818 97.028 96.450 95.942 95.385 94.835 94.694 94.488 93.761 93.095 92.818 91.846
40
0.019 0.029 0.038 0.048 0.057 0.066 0.075 0.084 0.093 0.111 0.128 0.145 0.162
1.519 1.524 1.530 1.531 1.533 1.535 1.537 1.532 1.528 1.525 1.523 1.516 1.504
9.226 9.240 9.260 9.140 9.109 9.069 9.020 8.978 8.931 8.845 8.782 8.697 8.648
4.504 4.472 4.436 4.393 4.362 4.330 4.297 4.284 4.268 4.224 4.182 4.150 4.139
2.774 2.755 2.734 2.710 2.692 2.673 2.654 2.645 2.635 2.608 2.582 2.563 2.553
14.014 14.082 14.168 13.993 13.964 13.921 13.864 13.754 13.647 13.489 13.375 13.185 13.007
99.683 98.909 98.042 97.521 96.943 96.379 95.822 95.683 95.477 94.806 94.074 93.675 93.587
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Table 3 Ultrasonic velocity and related parameters for DMSO–o-cresol in carbontetrachloride t (°C)
X (mole fraction of DMSO)
q (gm/cm3 )
U (104 ) (cm/s)
R (103 ) cm3 /mol (cm/s)1=3
B (103 ) cm3 /mol (cm2 /Dyne)1=7
Z (104 ) (gm/cm2 s)
V (cm3 /mol)
20
0.020 0.039 0.048 0.057 0.066 0.076 0.085 0.094 0.102 0.111 0.129 0.146 0.162
1.560 1.563 1.565 1.567 1.569 1.571 1.573 1.575 1.573 1.570 1.567 1.563 1.558
9.394 9.427 9.444 9.460 9.485 9.503 9.523 9.541 9.510 9.484 9.457 9.448 9.402
4.422 4.377 4.380 4.356 4.308 4.286 4.289 4.241 4.223 4.208 4.174 4.146 4.116
2.731 2.704 2.690 2.676 2.662 2.649 2.635 2.621 2.610 2.600 2.579 2.561 2.542
14.655 14.734 14.780 14.824 14.882 14.929 14.980 15.027 14.959 14.890 14.819 14.767 14.648
97.272 96.173 95.600 95.033 94.470 93.914 93.361 92.814 92.508 92.265 91.610 91.026 90.508
30
0.020 0.039 0.048 0.057 0.066 0.076 0.085 0.094 0.102 0.111 0.129 0.146 0.162
1.545 1.549 1.551 1.552 1.553 1.555 1.556 1.557 1.556 1.555 1.551 1.548 1.545
9.205 9.230 9.245 9.267 9.288 9.310 9.275 9.254 9.235 9.223 9.160 9.150 9.117
4.435 4.386 4.362 4.342 4.322 4.300 4.272 4.247 4.227 4.209 4.172 4.142 4.108
2.738 2.709 2.694 2.682 2.670 2.656 2.640 2.624 2.612 2.601 2.578 2.559 2.538
14.222 14.297 14.339 14.382 14.424 14.477 14.432 14.408 14.370 14.342 14.207 14.164 14.086
98.217 97.043 96.463 95.952 95.443 94.880 94.381 93.887 93.519 93.155 92.555 91.908 91.269
40
0.020 0.039 0.048 0.057 0.066 0.076 0.085 0.094 0.102 0.111 0.129 0.146 0.162
1.532 1.535 1.536 1.537 1.538 1.539 1.540 1.540 1.539 1.538 1.536 1.534 1.531
8.995 9.036 9.057 9.075 9.054 9.010 8.990 8.977 8.970 8.955 8.939 8.896 8.877
4.438 4.394 4.374 4.354 4.328 4.298 4.273 4.250 4.233 4.214 4.179 4.140 4.109
2.740 2.713 2.701 2.688 2.673 2.655 2.640 2.626 2.615 2.604 2.582 2.558 2.538
13.780 13.870 13.912 13.948 13.925 13.866 13.840 13.825 13.805 13.773 13.730 13.646 13.591
99.050 97.928 97.405 96.888 96.374 95.867 95.393 94.923 94.552 94.185 93.459 92.746 92.104
graphs of U , b, Lf , bE and LEf as a function of concentration have been presented in Figs. 1–5 respectively. The measured values of standard deviation of velocities (at the peak) have been found to be lesser than 0.11 m/s for the mixtures studied at various temperatures for 20 measurements. The infrared spectra for DMSO–phenol and DMSO– o-cresol systems have been presented in Figs. 6 and 7 respectively.
5. Discussion It is seen from the Fig. 1 that at 20 °C ultrasonic velocity ðU Þ increases with increasing concentration, attains a maximum value at 0.075 and 0.094 mole
fractions of DMSO for DMSO–phenol and DMSO–ocresol systems respectively. On further increasing the concentration, the velocity decreases. As the temperature is increased from 20 to 30 and 40 °C, the velocity maxima shifts to 0.057 and 0.038 mole fractions of DMSO for DMSO–phenol system and to 0.076 and 0.057 mole fractions of DMSO for DMSO–o-cresol system respectively. The non-linear variation of ultrasonic velocity with concentration indicates occurrence of complex formation between unlike molecules [27]. The molecular association becomes maximum at those concentrations where velocity maxima occurs. This may be interpreted due to the formation of strong hydrogen bonding resulting into complex formation producing displacement of electrons and nuclei. The chemical interaction may involve the association due to hydrogen
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Fig. 1. Ultrasonic velocity (U ) versus mole fraction of DMSO(X) at 20 °C ðÞ, 30 °C ( ) and 40 °C ( ).
Fig. 2. Adiabatic compressibility ðbÞ versus mole fraction of DMSO(X) at 20 °C (), 30 °C ( ) and 40 °C ( ).
bonding, due to dipole-dipole interaction or due to the formation of charge-transfer complexes. All these processes may lead to strong interaction of forces (Fort and Moore, 1965). The occurrence of maximum velocity at 0.075 and 0.094 mole fractions of DMSO for DMSO–phenol and
DMSO–o-cresol systems respectively and subsequent decrease of velocity with further increase in mole fraction of DMSO may be explained as under: At 0.075 and 0.094 mole fractions of DMSO for respective systems it is probable that the maximum associated DMSO molecules are broken into their
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Fig. 4. Excess compressibility ðbE Þ versus mole fraction of DMSO(X) at 20 °C (), 30 °C ( ) and 40 °C ( ). Fig. 3. Intermolecular free length ðLf Þ versus mole fraction of DMSO(X) at 20 °C (), 30 °C ( ) and 40 °C ( ).
monomers and the hydrogen bonds are formed between the hydrogen atom of respective phenol and o-cresol molecules and the oxygen atom of DMSO monomers. It is likely that the DMSO molecules in excess of these concentrations may stay in associated form. These associated molecules are fairly larger in size as compared
to phenol or o-cresol molecules and have to be accommodated in the system and this may cause some structural changes resulting in the weakening of the intermolecular forces. This may probably would be the reason for the decrease in ultrasonic velocity above 0.075 and 0.094 mole fractions of DMSO for respective systems. There is a probability that the DMSO exists in the associated form in the solution above these concentrations.
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Fig. 5. Excess intermolecular free length ðLEf Þ versus mole fraction of DMSO(X) at 20 °C (), 30 °C ( ) and 40 °C ( ).
Further, it may be seen from Fig. 1 that in DMSO– phenol system, the velocity peak obtained at a specific temperature exists at a lower concentration compared to the DMSO–o-cresol system. This may arise due to the fact that phenol is more acidic in character than o-cresol. The attached methyl group (–CH3 ) in o-cresol has + I effect as well as hyperconjugative effect. Hence, due to presence of methyl group, the removal of proton in o-cresol is not facilitated as quickly as in phenol. Thus the phenol molecules may form hydrogen bonds with DMSO molecules more readily, giving rise to velocity peak at a lower concentration than with o-cresol. Singh et al. [24] have reported that the association equilibrium constant for the DMSO–phenol system has been found to be greater than for DMSO–o-cresol system. The higher values of the association equilibrium constant show that the phenol has greater proton donating ability than o-cresol i.e. the stability of the phenoxy ion decreases on the addition of –CH3 group at ortho position in the ring. Thus the molecular associa-
483
tion takes place between DMSO and phenol molecules through hydrogen bonding in DMSO–phenol system earlier than DMSO–o-cresol system. Adiabatic compressibility ðbÞ shows an inverse behaviour as compared to the ultrasonic velocity. b decreases with increasing concentration, shows a minimum at the corresponding concentrations in each system (Fig. 2). It is primarily the compressibility that changes with the structure. This leads to a change in ultrasonic velocity. In case of liquid mixtures showing minimum in b, there is a definite contraction on mixing and the variation is due to complex formation. The occurrence of minimum compressibility at 0.075 and 0.094 mole fractions of DMSO for DMSO–phenol and DMSO–o-cresol systems respectively, shows that the complexation becomes maximum at these concentrations and decrease with increase in concentration of the solute. The occurrence of U maxima and b minima at the same concentrations indicates that there is a significant interaction between the two solute molecules [14,21]. Intermolecular free length ðLf Þ shows a similar behaviour as reflected by b. The decreased compressibility brings the molecules to a closer packing resulting into a decrease of intermolecular free length as shown in Fig. 3. Lf is a predominant factor in determining the variation of U in solutions. As Lf decreases U increases and vice versa, showing an inverse behaviour. The interdependence of Lf and U has been evolved from a model for sound propagation proposed by Eyring and Kincaid [28]. The decrease in the values of b and Lf with increase in ultrasonic velocity further strengthens the process of complex formation between the two solute molecules through hydrogen bonding due to which structural arrangement is considerably affected [14]. The variation in molar sound velocity ðRÞ, molar compressibility ðBÞ and specific acoustic impedance ðZÞ is non-linear with concentration at various temperatures for both the systems [Tables 2 and 3]. When an acoustic wave travels in a medium, there is a variation of pressure from particle to particle. The ratio of the instantaneous pressure excess at any particle of the medium to the instantaneous velocity of that particle is known as ‘‘specific acoustic impedance’’ of the medium. This factor is governed by the inertial and elastic properties of the medium. It is important to examine specific acoustic impedance in relation to concentration and temperature. When a plane ultrasonic wave is set up in a liquid, the pressure and hence density and refractive index of the liquid show a periodic variation with distance from the source along the direction of propagation. If there is stationary ultrasonic wave pattern in the liquid, the density will be greater in the nodal planes than in any other plane. It has been observed that like other parameters e.g. U , b and Lf ; Z also exhibits a non-linear variation with concentration giving a peak at a particular concentration where ultrasonic velocity maximum
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Fig. 6. Observed O–H stretching bands in infrared spectrum of DMSO–Phenol system at various mole fractions of DMSO(X).
exists. The peak shifts with change in temperature as in the case of U . The non-linearity in R, B and Z further supports the possibility of molecular interactions due to the H-bonding as well as due to the formation of charge-transfer complexes between solute molecules [15,16,20,21]. As the temperature is raised, the velocity maxima shifts towards lower concentration in each system (Fig. 1). This is due to the fact that as the temperature is increased, thermal energy facilitates the breaking of bonds between the associated molecules of DMSO. Thus the DMSO molecules are broken into their monomers. The hydrogen bonds are then formed between the oxygen atom of DMSO monomers and the hydrogen atom of respective phenol and o-cresol molecules. This may probably be the reason for the shift in velocity maxima (b and Lf minima) towards lower concentrations [13]. The increase of thermal energy weakens the molecular forces and hence decrease in the velocity is expected [13]. The molar volumes ðV Þ evaluated using density data also support the complex formation of solute molecules in dilute solutions. It is seen from Tables 2 and 3 that the
molar volumes of both the systems are in decreasing order. This is because of the fact that molecular weights of mixtures are decreasing as concentration of solute (DMSO) increases in the mixture. The molecular weights of DMSO, phenol, o-cresol and CCl4 are 78.13, 94.11, 108.14 and 153.82, respectively. As concentration of the mixture increases, amount of DMSO, having lowest molecular weight in the solution increases while others (phenol, o-cresol and CCl4 ) are fixed which gives lowering in molecular weight of the mixture. Since molecular weight is directly proportional to molar volume, therefore, molar volume also decreases. Decrease in molar volume signifies more compact structure i.e. lesser space will be available for reaction to take place. On increasing concentration, more molecules interact together and more complexes are formed, but it will increase upto a certain limit of mole fraction of solute. Once the number of moles of DMSO are increased upto this saturation value, the molecules of DMSO will find less space for association with phenol and o-cresol molecules hence reaction will become slow i.e. the process of complex formation slows down (19).
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Fig. 7. Observed O–H stretching bands in infrared spectrum of DMSO–o-cresol system at various mole fractions of DMSO(X).
Molar volume increases with increasing temperature in both the complexes considered here. This may probably would be caused from the fact that thermal energy facilitates an increase in the molecular separation in liquid mixtures and hence an increase in the molar volume. Further, in order to examine the nature and strength of molecular association in these systems, excess adiabatic compressibility ðbE Þ and excess intermolecular free length (LEf ) have also been evaluated. The dispersion forces should make positive contributions to excess values while dipole–dipole, dipole-induced dipole, charge-transfer interaction and hydrogen bonding between unlike components should make negative contributions [17]. For both the systems, the excess adiabatic compressibility and excess intermolecular free length show minima at the same concentrations where ultrasonic velocity maxima (b and Lf minima) occur. This further supports the molecular association by complex formation. In the case of DMSO–phenol system the negative values of bE and LEf become more negative and in the case of DMSO–o-cresol system the positive values of bE
and LEf tend to become negative at 20 °C while at other temperatures (30 and 40 °C), the excess parameters bE and LEf both become more negative as the complex formation reaches to a saturation value at certain concentration. This clearly indicates the presence of strong hydrogen bonding which becomes maximum where the minima of bE and LEf occur. This is in confirmity with the earlier results of Fort and Moore [17] and Gour et al. [19] on the study of complex formation in mixtures. The strength of the interaction between the components increases when excess values tend to become increasingly negative. This may be qualitatively interpreted in terms of closer approach of unlike molecules leading to reductions in compressibility and volume [17]. The interaction strength in DMSO–phenol system is greater than that in DMSO–o-cresol system. The excess properties are found to decrease with increasing temperature [19]. The above discussion clearly indicates the occurrence of complex formation between unlike molecules, which is maximum at the concentration indicated by the maxima or the minima corresponding to the respective parameters for the systems.
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6. Infrared spectrum In order to investigate the presence of O–HO bond complexes and the strength of molecular association at specific concentrations (at and near the ultrasonic velocity peaks) in these systems further, infrared spectrum of both DMSO–phenol and DMSO–o-cresol systems are recorded for various concentrations at room temperature (20 °C). It is seen from the infrared spectrum that, for DMSO–phenol system a broad band appears at frequencies 3457.2, 3417.6 and 3448.9 cm1 for 0.066, 0.075 and 0.084 mole fractions of DMSO respectively and for DMSO–o-cresol system a broad band appears at frequencies 3421.4, 3388.7 and 3428.3 cm1 for 0.085, 0.094 and 0.102 mole fractions of DMSO respectively [Figs. 6 and 7]. It is well established that for less extensive hydrogen bonding a sharper and less intense band is observed at a higher frequency but due to extensive hydrogen bonding a broad band appears at a lower frequency. The O–H stretching due to the free O– H group appears as a sharp band at higher frequency range of 3650–3590 cm1 but the O–H stretching due to intermolecular hydrogen bonding which broadens the band and shifts its position to lower frequency appears in the range 3550–3200 cm1 [29]. In DMSO–phenol system the O–H band appears at frequency 3417.6 cm1 for 0.075 mole fraction of DMSO and in DMSO–o-cresol system the O–H band appears at frequency 3388.7 cm1 for 0.094 mole fraction of DMSO but with further increase or decrease in concentration in both the systems the O–H band shifts towards higher frequency which indicates the weakening of molecular association through intermolecular hydrogen bonding. Thus, the study of infrared spectra shows that the complex formation becomes maximum at 0.075 mole fraction of DMSO for DMSO–phenol system and at 0.094 mole fraction of DMSO for DMSO–ocresol system, which is also indicated through peaks of ultrasonic velocities as discussed above. Thus, the pattern, position and intensity of the O–H band as per infrared data strongly supports the conclusions drawn from the ultrasonic data that molecular association through hydrogen bonding is maximum at those concentrations where ultrasonic velocity maxima occurs [23]. It may be concluded that ultrasonic studies provide for a comprehensive investigation of molecular associ-
ation between DMSO and phenol and DMSO and ocresol molecules arising from the hydrogen bonding between the oxygen atom of DMSO molecule and the hydrogen atom of phenol and o-cresol molecules. The variation of acoustic parameters with temperatures and also the IR data strongly support the molecular association in these ternary liquid mixtures.
References [1] P.J. Singh, K.S. Sharma, Pramana 46 (1996) 259. [2] M. Jorg, A. Ghoneim, G. Turky, M. Stockhausen, Phys. Chem. Liq. (UK) 29 (1995) 263. [3] S.L. Abd-EL-Messieh, Indian J. Phys. 70B (2) (1996) 119. [4] R.J. Lagemann, W.S. Dunbar, J. Phys. Chem. 49 (1945) 420. [5] S.K. Chauhan, V.R. Singh, Indian J. Pure Appl. Phys. 31 (1993) 635. [6] R.L. Blokhra, A. Nag, Indian J. Pure Appl. Phys. 29 (1991) 756. [7] A. Ozawa, A. Minamisawa, Jpn. J. Appl. Phys. 37 (1998) 2799. [8] V. H€anel, J. Appl. Phys. 84 (1998) 668. [9] J.R. Bae, S. Yun, Jpn. J. Appl. Phys. 37 (1998) 2801. [10] V.K. Syal, G. Lal, P. Bisht, S. Chauhan, J. Mol. Liq. (Netherlands) 63 (1995) 317. [11] K. Mohankrishnan, K. Rambabu, D. Ramachandran, G.K. Raman, Phys. Chem. Liq. (UK) 29 (1995) 257. [12] J.D. Pandey, R.D. Rai, R.K. Shukla, A.K. Shukla, N. Mishra, Indian J. Pure Appl. Phys. 31 (1993) 84. [13] S. Velmourougane, T.K. Nambinarayanan, A.S. Rao, B. Krishnan, Indian J. Phys. 61B (1987) 105. [14] V.K. Syal, U. Kumari, S. Chauhan, M.S. Chauhan, Indian J. Pure Appl. Phys. 30 (1992) 719. [15] K.M. Swamy, Acustica (Germany) 29 (1973) 179. [16] D. Ambanathan, Indian J. Pure Appl. Phys. 16 (1978) 713. [17] R.J. Fort, W.R. Moore, Trans. Faraday Soc. 61 (1965) 2102. [18] S.B. Khasare, B.A. Patki, Indian J. Pure Appl. Phys. 25 (1987) 180. [19] S.N. Gour, J.S. Tomar, R.P. Varma, Indian J. Pure Appl. Phys. 24 (1986) 602. [20] A.N. Kannappan, V. Rajendran, Indian J. Pure Appl. Phys. 30 (1992) 240. [21] M. Gupta, J.P. Shukla, Indian J. Pure Appl. Phys. 34 (1996) 769. [22] A.N. Kannappan, R. Palani, Indian J. Phys. 70B (1) (1996) 59. [23] A. Awasthi, M. Rastogi, M. Gupta, J.P. Shukla, J. Mol. Liq. (Netherlands) 80 (1) (1999) 77. [24] A. Singh, R. Mishra, J.P. Shukla, M.C. Saxena, J. Mol. Liq. (Netherlands) 26 (1983) 29. [25] D.D. Perrin, W.L.F. Armarego, Purification of Lab. Chem., third ed., Pergamon Press, Oxford, 1988. [26] B. Jacobson, J. Chem. Phys. 6 (1952) 927. [27] O. Nomoto, J. Phys. Soc. Japan 8 (1953) 553. [28] H. Eyring, J.F. Kincaid, J. Chem. Phys. (USA) 6 (1938) 620. [29] J.R. Dyer, Applications of Absorption Spectroscopy of Organic Compounds, fourth ed., Prentice-Hall of India, New Delhi, 1978.
Ultrasonic and IR study of intermolecular association through hydrogen bonding in ternary liquid mixtures Aashees Awasthi, J.P. Shukla
*
Department of Physics, University of Lucknow, Lucknow 226 007, India Received 30 September 2002; received in revised form 19 February 2003; accepted 19 February 2003
Abstract Complex formation in ternary liquid mixtures of dimethylsulfoxide (DMSO) with phenol and o-cresol in carbontetrachloride has been studied by measuring ultrasonic velocity at 2 MHz, in the concentration range of 0.019–0.162 (in mole fraction of DMSO) at varying temperatures of 20, 30 and 40 °C. Using measured values of ultrasonic velocity, other parameters such as adiabatic compressibility, intermolecular free length, molar sound velocity, molar compressibility, specific acoustic impedance and molar volume have been evaluated. These parameters have been utilized to study the solute–solute interactions in these systems. The ultrasonic velocity shows a maxima and adiabatic compressibility a corresponding minima as a function of concentration for these mixtures. The results indicate the occurrence of complex formation between unlike molecules through intermolecular hydrogen bonding between oxygen atom of DMSO molecule and hydrogen atom of phenol and o-cresol molecules. The excess values of adiabatic compressibility and intermolecular free length have also been evaluated. The variation of both these parameters with concentration also indicates the possibility of the complex formation in these systems. Further, to investigate the presence of O–HO bond complexes and the strength of molecular association with concentrations, the infrared spectra of both the systems, DMSO– phenol and DMSO–o-cresol, have been recorded for various concentrations at room temperature (20 °C). The results obtained using infrared spectroscopy for both the systems also support the occurrence of complex formation through intermolecular hydrogen bonding in these ternary liquid mixtures. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Ultrasonic velocity; Excess function; Infrared spectrum; Liquid mixture
1. Introduction The study of molecular interactions in the liquid mixtures is of considerable importance in the elucidation of the structural properties of the molecules. The intermolecular interactions influence the structural arrangement alongwith the shape of molecules. Dielectric relaxation studies of polar molecules in non-polar solvent using microwave absorption method have been widely used to study the molecular structures including the molecular interactions in liquid mixtures [1–3]. Lagemann and Dunbar [4] were the first to point out the sound velocity approach for qualitative determination of the degree of association in liquids. Recent developments have made it possible to use ultrasonic energy in medicine, engineering, agriculture and other *
Corresponding author. E-mail address: aasheesawasthi@rediffmail.com (A. Awasthi).
0041-624X/03/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0041-624X(03)00127-6
industrial applications [5,6]. Ozawa and Minamisawa [7] have observed concentration of ultrasonic velocity invariant with respect to temperature in alcohol–water mixtures. H€anel [8] has measured sound velocity and thickness of thin samples by time-resolved acoustic microscopy. Bae and Yun [9] have studied the ultrasonic velocity in binary solutions of silicon dioxide and water. Ultrasonic waves with low amplitude have been used by many scientists to investigate the nature of molecular interactions and physico-chemical behaviour of pure, binary, ternary and quaternary liquid mixtures [10–16]. It has been reported by several workers [17–22] that the complex formation occurs if the observed values of excess parameters (e.g. excess adiabatic compressibility, excess intermolecular free length and excess volume etc.) are negative. This suggests the occurrence of discrete groups of molecules arranged into specific geometric structures. These structural arrangements are influenced not only by the shape of the molecules but also by their
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mutual interactions. Our research group has also investigated complex formation in ternary liquid mixtures of amides with ethanol in benzene using ultrasonic method [23]. Singh et al. [24] have studied the association process of dimethylsulfoxide and p-tolylsulfoxide with proton donors (phenol and o-cresol) in an inert solvent CCl4 using dielectric relaxation measurements. The complex formation has been interpreted in terms of the association equilibrium constant and thermodynamic parameters. Since acoustic parameters provide a better insight into molecular environments in liquid mixtures, it seemed important to study molecular interactions in ternary liquid mixtures of DMSO with phenol and ocresol in a non-polar solvent CCl4 using ultrasonic technique. Apart from measuring ultrasonic velocity and other acoustical parameters, the excess values of adiabatic compressibility and intermolecular free length have also been evaluated for these ternary systems at temperatures 20, 30 and 40 °C respectively. The infrared spectra for both the ternary systems have also been recorded at room temperature (20 °C).
other components (phenol, o-cresol and CCl4 ) decreases as the mole fraction of a substance in the mixture is number of moles of that substance to the total number of moles in the mixture. Since, CCl4 is a non-polar solvent which is not taking part in the reaction so the mole fraction of CCl4 does not influence the mixture. The chemicals used were of AR grade from BDH. All the chemicals were purified by standard procedures discussed by Perrin and Armarego [25] before use. The infrared spectra for DMSO–phenol and DMSO– o-cresol systems have been recorded using FT-IR spectrophotometer (Model-8201) supplied by Perkin-Elmer, at Central Drug Research Institute, Lucknow.
3. Theory Various physical parameters [13] were calculated from the measured values of ultrasonic velocity ðU Þ and density ðqÞ. Adiabatic compressibility ðbÞ ¼
1 U 2q
Intermolecular free length ðLf Þ ¼ Kb1=2 2. Experimental details
where K values for different temperatures were taken from the work of Jacobson [26].
Ultrasonic velocity for the mixtures was measured using the ultrasonic interferometer (Model M81) supplied by Mittal Enterprises, New Delhi, that has a reproducibility of 0.4 m/s at 25 °C. The temperature was maintained constant by circulating water from a thermostatically controlled water bath (accuracy 0.1 °C). The temperature of the cell as measured using a thermocouple was found to be accurate to 0.25 °C. The density of various complexed species has been measured using a sensitive pyknometer with an accuracy of 0.5 kg/ m3 . The dilute solution study minimizes the effects of viscosity, internal field etc., therefore, for dilution apolar solvent like carbontetrachloride (CCl4 ), benzene (C6 H6 ), cyclohexane (C6 H12 ) etc., are used. Apolar solvent provide the medium and dilution for the mixture which, in turn, also minimizes the requirement of pure liquids in large quantity. Twenty millilitre carbontetrachloride was used for preparing mixtures in both the systems. Fixed amounts of phenol and o-cresol (0.30 gm each) were used in the DMSO–phenol and DMSO–o-cresol systems respectively while weight of DMSO was varied in preparing mixtures of varying concentrations. In the present study, the amounts (number of moles) of phenol, o-cresol and CCl4 are taken constant while amount (number of moles) of DMSO is increased in the mixtures which, in turn, increases the mole fraction of DMSO in the mixture and consequently mole fraction of
Molar sound velocity ðRÞ ¼ U 1=3 V Molar compressibility ðBÞ ¼ ðM=qÞb1=7 where V and M are the molar volume and molecular weight of the mixtures respectively. Specific acoustic impedance ðZÞ ¼ qU The excess adiabatic compressibility ðbE Þ and excess intermolecular free length ðLEf Þ were evaluated using the expressions: bE ¼ bexp bideal and LEf ¼ Lf exp Lf ideal where bideal and Lf ideal and their excess in mixture were defined under volume additive rule [18].
4. Results The mole fractions of DMSO, phenol, o-cresol and carbontetrachloride for both DMSO–phenol and DMSO–o-cresol systems have been given in Table 1. Ultrasonic velocity, molar sound velocity, molar compressibility, specific acoustic impedance and molar volume for DMSO–phenol and DMSO–o-cresol systems have been listed in Tables 2 and 3. Representative
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479
Table 1 Mole fractions of DMSO, Phenol, o-cresol and carbontetrachloride in both DMSO–phenol and DMSO–o-cresol systems DMSO–phenol system
DMSO–o-cresol system
DMSO
Phenol
Carbontetrachloride
DMSO
o-Cresol
Carbontetrachloride
0.019 0.029 0.038 0.048 0.057 0.066 0.075 0.084 0.093 0.111 0.128 0.145 0.162
0.016 0.015 0.015 0.015 0.015 0.015 0.015 0.014 0.014 0.014 0.014 0.013 0.013
0.965 0.956 0.946 0.937 0.928 0.919 0.911 0.902 0.892 0.875 0.858 0.841 0.825
0.020 0.039 0.048 0.057 0.066 0.076 0.085 0.094 0.102 0.111 0.129 0.146 0.162
0.013 0.013 0.013 0.013 0.013 0.012 0.012 0.012 0.012 0.012 0.012 0.012 0.011
0.967 0.949 0.939 0.930 0.921 0.912 0.903 0.894 0.886 0.877 0.860 0.843 0.826
Table 2 Ultrasonic velocity and related parameters for DMSO–phenol in carbontetrachloride t (°C)
X (mole fraction of DMSO)
q (gm/cm3 )
U (104 ) (cm/s)
R (103 ) cm3 /mol (cm/s)1=3
B (103 ) cm3 /mol (cm2 /Dyne)1=7
Z (104 ) (gm/cm2 s)
V (cm3 /mol)
20
0.019 0.029 0.038 0.048 0.057 0.066 0.075 0.084 0.093 0.111 0.128 0.145 0.162
1.548 1.552 1.556 1.560 1.566 1.568 1.571 1.567 1.562 1.561 1.556 1.544 1.533
9.480 9.495 9.508 9.520 9.524 9.544 9.555 9.526 9.485 9.433 9.390 9.355 9.304
4.460 4.431 4.400 4.370 4.334 4.312 4.286 4.272 4.246 4.224 4.185 4.175 4.161
2.751 2.734 2.715 2.698 2.677 2.663 2.648 2.639 2.622 2.604 2.585 2.576 2.565
14.675 14.736 14.794 14.851 14.915 14.965 15.011 14.927 14.816 14.725 14.611 14.444 14.263
97.816 97.124 96.404 95.708 94.901 94.351 93.748 93.546 93.101 92.619 92.079 91.976 91.817
30
0.019 0.029 0.038 0.048 0.057 0.066 0.075 0.084 0.093 0.111 0.128 0.145 0.162
1.535 1.541 1.546 1.548 1.549 1.551 1.553 1.548 1.544 1.542 1.539 1.530 1.516
9.311 9.320 9.327 9.340 9.370 9.334 9.295 9.240 9.212 9.139 9.103 9.062 9.023
4.417 4.435 4.401 4.376 4.358 4.327 4.296 4.281 4.267 4.223 4.188 4.169 4.164
2.757 2.736 2.716 2.701 2.689 2.671 2.654 2.644 2.635 2.608 2.586 2.573 2.567
14.292 14.362 14.420 14.458 14.514 14.477 14.435 14.304 14.223 14.092 14.010 13.865 13.679
98.644 97.818 97.028 96.450 95.942 95.385 94.835 94.694 94.488 93.761 93.095 92.818 91.846
40
0.019 0.029 0.038 0.048 0.057 0.066 0.075 0.084 0.093 0.111 0.128 0.145 0.162
1.519 1.524 1.530 1.531 1.533 1.535 1.537 1.532 1.528 1.525 1.523 1.516 1.504
9.226 9.240 9.260 9.140 9.109 9.069 9.020 8.978 8.931 8.845 8.782 8.697 8.648
4.504 4.472 4.436 4.393 4.362 4.330 4.297 4.284 4.268 4.224 4.182 4.150 4.139
2.774 2.755 2.734 2.710 2.692 2.673 2.654 2.645 2.635 2.608 2.582 2.563 2.553
14.014 14.082 14.168 13.993 13.964 13.921 13.864 13.754 13.647 13.489 13.375 13.185 13.007
99.683 98.909 98.042 97.521 96.943 96.379 95.822 95.683 95.477 94.806 94.074 93.675 93.587
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Table 3 Ultrasonic velocity and related parameters for DMSO–o-cresol in carbontetrachloride t (°C)
X (mole fraction of DMSO)
q (gm/cm3 )
U (104 ) (cm/s)
R (103 ) cm3 /mol (cm/s)1=3
B (103 ) cm3 /mol (cm2 /Dyne)1=7
Z (104 ) (gm/cm2 s)
V (cm3 /mol)
20
0.020 0.039 0.048 0.057 0.066 0.076 0.085 0.094 0.102 0.111 0.129 0.146 0.162
1.560 1.563 1.565 1.567 1.569 1.571 1.573 1.575 1.573 1.570 1.567 1.563 1.558
9.394 9.427 9.444 9.460 9.485 9.503 9.523 9.541 9.510 9.484 9.457 9.448 9.402
4.422 4.377 4.380 4.356 4.308 4.286 4.289 4.241 4.223 4.208 4.174 4.146 4.116
2.731 2.704 2.690 2.676 2.662 2.649 2.635 2.621 2.610 2.600 2.579 2.561 2.542
14.655 14.734 14.780 14.824 14.882 14.929 14.980 15.027 14.959 14.890 14.819 14.767 14.648
97.272 96.173 95.600 95.033 94.470 93.914 93.361 92.814 92.508 92.265 91.610 91.026 90.508
30
0.020 0.039 0.048 0.057 0.066 0.076 0.085 0.094 0.102 0.111 0.129 0.146 0.162
1.545 1.549 1.551 1.552 1.553 1.555 1.556 1.557 1.556 1.555 1.551 1.548 1.545
9.205 9.230 9.245 9.267 9.288 9.310 9.275 9.254 9.235 9.223 9.160 9.150 9.117
4.435 4.386 4.362 4.342 4.322 4.300 4.272 4.247 4.227 4.209 4.172 4.142 4.108
2.738 2.709 2.694 2.682 2.670 2.656 2.640 2.624 2.612 2.601 2.578 2.559 2.538
14.222 14.297 14.339 14.382 14.424 14.477 14.432 14.408 14.370 14.342 14.207 14.164 14.086
98.217 97.043 96.463 95.952 95.443 94.880 94.381 93.887 93.519 93.155 92.555 91.908 91.269
40
0.020 0.039 0.048 0.057 0.066 0.076 0.085 0.094 0.102 0.111 0.129 0.146 0.162
1.532 1.535 1.536 1.537 1.538 1.539 1.540 1.540 1.539 1.538 1.536 1.534 1.531
8.995 9.036 9.057 9.075 9.054 9.010 8.990 8.977 8.970 8.955 8.939 8.896 8.877
4.438 4.394 4.374 4.354 4.328 4.298 4.273 4.250 4.233 4.214 4.179 4.140 4.109
2.740 2.713 2.701 2.688 2.673 2.655 2.640 2.626 2.615 2.604 2.582 2.558 2.538
13.780 13.870 13.912 13.948 13.925 13.866 13.840 13.825 13.805 13.773 13.730 13.646 13.591
99.050 97.928 97.405 96.888 96.374 95.867 95.393 94.923 94.552 94.185 93.459 92.746 92.104
graphs of U , b, Lf , bE and LEf as a function of concentration have been presented in Figs. 1–5 respectively. The measured values of standard deviation of velocities (at the peak) have been found to be lesser than 0.11 m/s for the mixtures studied at various temperatures for 20 measurements. The infrared spectra for DMSO–phenol and DMSO– o-cresol systems have been presented in Figs. 6 and 7 respectively.
5. Discussion It is seen from the Fig. 1 that at 20 °C ultrasonic velocity ðU Þ increases with increasing concentration, attains a maximum value at 0.075 and 0.094 mole
fractions of DMSO for DMSO–phenol and DMSO–ocresol systems respectively. On further increasing the concentration, the velocity decreases. As the temperature is increased from 20 to 30 and 40 °C, the velocity maxima shifts to 0.057 and 0.038 mole fractions of DMSO for DMSO–phenol system and to 0.076 and 0.057 mole fractions of DMSO for DMSO–o-cresol system respectively. The non-linear variation of ultrasonic velocity with concentration indicates occurrence of complex formation between unlike molecules [27]. The molecular association becomes maximum at those concentrations where velocity maxima occurs. This may be interpreted due to the formation of strong hydrogen bonding resulting into complex formation producing displacement of electrons and nuclei. The chemical interaction may involve the association due to hydrogen
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481
Fig. 1. Ultrasonic velocity (U ) versus mole fraction of DMSO(X) at 20 °C ðÞ, 30 °C ( ) and 40 °C ( ).
Fig. 2. Adiabatic compressibility ðbÞ versus mole fraction of DMSO(X) at 20 °C (), 30 °C ( ) and 40 °C ( ).
bonding, due to dipole-dipole interaction or due to the formation of charge-transfer complexes. All these processes may lead to strong interaction of forces (Fort and Moore, 1965). The occurrence of maximum velocity at 0.075 and 0.094 mole fractions of DMSO for DMSO–phenol and
DMSO–o-cresol systems respectively and subsequent decrease of velocity with further increase in mole fraction of DMSO may be explained as under: At 0.075 and 0.094 mole fractions of DMSO for respective systems it is probable that the maximum associated DMSO molecules are broken into their
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Fig. 4. Excess compressibility ðbE Þ versus mole fraction of DMSO(X) at 20 °C (), 30 °C ( ) and 40 °C ( ). Fig. 3. Intermolecular free length ðLf Þ versus mole fraction of DMSO(X) at 20 °C (), 30 °C ( ) and 40 °C ( ).
monomers and the hydrogen bonds are formed between the hydrogen atom of respective phenol and o-cresol molecules and the oxygen atom of DMSO monomers. It is likely that the DMSO molecules in excess of these concentrations may stay in associated form. These associated molecules are fairly larger in size as compared
to phenol or o-cresol molecules and have to be accommodated in the system and this may cause some structural changes resulting in the weakening of the intermolecular forces. This may probably would be the reason for the decrease in ultrasonic velocity above 0.075 and 0.094 mole fractions of DMSO for respective systems. There is a probability that the DMSO exists in the associated form in the solution above these concentrations.
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Fig. 5. Excess intermolecular free length ðLEf Þ versus mole fraction of DMSO(X) at 20 °C (), 30 °C ( ) and 40 °C ( ).
Further, it may be seen from Fig. 1 that in DMSO– phenol system, the velocity peak obtained at a specific temperature exists at a lower concentration compared to the DMSO–o-cresol system. This may arise due to the fact that phenol is more acidic in character than o-cresol. The attached methyl group (–CH3 ) in o-cresol has + I effect as well as hyperconjugative effect. Hence, due to presence of methyl group, the removal of proton in o-cresol is not facilitated as quickly as in phenol. Thus the phenol molecules may form hydrogen bonds with DMSO molecules more readily, giving rise to velocity peak at a lower concentration than with o-cresol. Singh et al. [24] have reported that the association equilibrium constant for the DMSO–phenol system has been found to be greater than for DMSO–o-cresol system. The higher values of the association equilibrium constant show that the phenol has greater proton donating ability than o-cresol i.e. the stability of the phenoxy ion decreases on the addition of –CH3 group at ortho position in the ring. Thus the molecular associa-
483
tion takes place between DMSO and phenol molecules through hydrogen bonding in DMSO–phenol system earlier than DMSO–o-cresol system. Adiabatic compressibility ðbÞ shows an inverse behaviour as compared to the ultrasonic velocity. b decreases with increasing concentration, shows a minimum at the corresponding concentrations in each system (Fig. 2). It is primarily the compressibility that changes with the structure. This leads to a change in ultrasonic velocity. In case of liquid mixtures showing minimum in b, there is a definite contraction on mixing and the variation is due to complex formation. The occurrence of minimum compressibility at 0.075 and 0.094 mole fractions of DMSO for DMSO–phenol and DMSO–o-cresol systems respectively, shows that the complexation becomes maximum at these concentrations and decrease with increase in concentration of the solute. The occurrence of U maxima and b minima at the same concentrations indicates that there is a significant interaction between the two solute molecules [14,21]. Intermolecular free length ðLf Þ shows a similar behaviour as reflected by b. The decreased compressibility brings the molecules to a closer packing resulting into a decrease of intermolecular free length as shown in Fig. 3. Lf is a predominant factor in determining the variation of U in solutions. As Lf decreases U increases and vice versa, showing an inverse behaviour. The interdependence of Lf and U has been evolved from a model for sound propagation proposed by Eyring and Kincaid [28]. The decrease in the values of b and Lf with increase in ultrasonic velocity further strengthens the process of complex formation between the two solute molecules through hydrogen bonding due to which structural arrangement is considerably affected [14]. The variation in molar sound velocity ðRÞ, molar compressibility ðBÞ and specific acoustic impedance ðZÞ is non-linear with concentration at various temperatures for both the systems [Tables 2 and 3]. When an acoustic wave travels in a medium, there is a variation of pressure from particle to particle. The ratio of the instantaneous pressure excess at any particle of the medium to the instantaneous velocity of that particle is known as ‘‘specific acoustic impedance’’ of the medium. This factor is governed by the inertial and elastic properties of the medium. It is important to examine specific acoustic impedance in relation to concentration and temperature. When a plane ultrasonic wave is set up in a liquid, the pressure and hence density and refractive index of the liquid show a periodic variation with distance from the source along the direction of propagation. If there is stationary ultrasonic wave pattern in the liquid, the density will be greater in the nodal planes than in any other plane. It has been observed that like other parameters e.g. U , b and Lf ; Z also exhibits a non-linear variation with concentration giving a peak at a particular concentration where ultrasonic velocity maximum
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Fig. 6. Observed O–H stretching bands in infrared spectrum of DMSO–Phenol system at various mole fractions of DMSO(X).
exists. The peak shifts with change in temperature as in the case of U . The non-linearity in R, B and Z further supports the possibility of molecular interactions due to the H-bonding as well as due to the formation of charge-transfer complexes between solute molecules [15,16,20,21]. As the temperature is raised, the velocity maxima shifts towards lower concentration in each system (Fig. 1). This is due to the fact that as the temperature is increased, thermal energy facilitates the breaking of bonds between the associated molecules of DMSO. Thus the DMSO molecules are broken into their monomers. The hydrogen bonds are then formed between the oxygen atom of DMSO monomers and the hydrogen atom of respective phenol and o-cresol molecules. This may probably be the reason for the shift in velocity maxima (b and Lf minima) towards lower concentrations [13]. The increase of thermal energy weakens the molecular forces and hence decrease in the velocity is expected [13]. The molar volumes ðV Þ evaluated using density data also support the complex formation of solute molecules in dilute solutions. It is seen from Tables 2 and 3 that the
molar volumes of both the systems are in decreasing order. This is because of the fact that molecular weights of mixtures are decreasing as concentration of solute (DMSO) increases in the mixture. The molecular weights of DMSO, phenol, o-cresol and CCl4 are 78.13, 94.11, 108.14 and 153.82, respectively. As concentration of the mixture increases, amount of DMSO, having lowest molecular weight in the solution increases while others (phenol, o-cresol and CCl4 ) are fixed which gives lowering in molecular weight of the mixture. Since molecular weight is directly proportional to molar volume, therefore, molar volume also decreases. Decrease in molar volume signifies more compact structure i.e. lesser space will be available for reaction to take place. On increasing concentration, more molecules interact together and more complexes are formed, but it will increase upto a certain limit of mole fraction of solute. Once the number of moles of DMSO are increased upto this saturation value, the molecules of DMSO will find less space for association with phenol and o-cresol molecules hence reaction will become slow i.e. the process of complex formation slows down (19).
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Fig. 7. Observed O–H stretching bands in infrared spectrum of DMSO–o-cresol system at various mole fractions of DMSO(X).
Molar volume increases with increasing temperature in both the complexes considered here. This may probably would be caused from the fact that thermal energy facilitates an increase in the molecular separation in liquid mixtures and hence an increase in the molar volume. Further, in order to examine the nature and strength of molecular association in these systems, excess adiabatic compressibility ðbE Þ and excess intermolecular free length (LEf ) have also been evaluated. The dispersion forces should make positive contributions to excess values while dipole–dipole, dipole-induced dipole, charge-transfer interaction and hydrogen bonding between unlike components should make negative contributions [17]. For both the systems, the excess adiabatic compressibility and excess intermolecular free length show minima at the same concentrations where ultrasonic velocity maxima (b and Lf minima) occur. This further supports the molecular association by complex formation. In the case of DMSO–phenol system the negative values of bE and LEf become more negative and in the case of DMSO–o-cresol system the positive values of bE
and LEf tend to become negative at 20 °C while at other temperatures (30 and 40 °C), the excess parameters bE and LEf both become more negative as the complex formation reaches to a saturation value at certain concentration. This clearly indicates the presence of strong hydrogen bonding which becomes maximum where the minima of bE and LEf occur. This is in confirmity with the earlier results of Fort and Moore [17] and Gour et al. [19] on the study of complex formation in mixtures. The strength of the interaction between the components increases when excess values tend to become increasingly negative. This may be qualitatively interpreted in terms of closer approach of unlike molecules leading to reductions in compressibility and volume [17]. The interaction strength in DMSO–phenol system is greater than that in DMSO–o-cresol system. The excess properties are found to decrease with increasing temperature [19]. The above discussion clearly indicates the occurrence of complex formation between unlike molecules, which is maximum at the concentration indicated by the maxima or the minima corresponding to the respective parameters for the systems.
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6. Infrared spectrum In order to investigate the presence of O–HO bond complexes and the strength of molecular association at specific concentrations (at and near the ultrasonic velocity peaks) in these systems further, infrared spectrum of both DMSO–phenol and DMSO–o-cresol systems are recorded for various concentrations at room temperature (20 °C). It is seen from the infrared spectrum that, for DMSO–phenol system a broad band appears at frequencies 3457.2, 3417.6 and 3448.9 cm1 for 0.066, 0.075 and 0.084 mole fractions of DMSO respectively and for DMSO–o-cresol system a broad band appears at frequencies 3421.4, 3388.7 and 3428.3 cm1 for 0.085, 0.094 and 0.102 mole fractions of DMSO respectively [Figs. 6 and 7]. It is well established that for less extensive hydrogen bonding a sharper and less intense band is observed at a higher frequency but due to extensive hydrogen bonding a broad band appears at a lower frequency. The O–H stretching due to the free O– H group appears as a sharp band at higher frequency range of 3650–3590 cm1 but the O–H stretching due to intermolecular hydrogen bonding which broadens the band and shifts its position to lower frequency appears in the range 3550–3200 cm1 [29]. In DMSO–phenol system the O–H band appears at frequency 3417.6 cm1 for 0.075 mole fraction of DMSO and in DMSO–o-cresol system the O–H band appears at frequency 3388.7 cm1 for 0.094 mole fraction of DMSO but with further increase or decrease in concentration in both the systems the O–H band shifts towards higher frequency which indicates the weakening of molecular association through intermolecular hydrogen bonding. Thus, the study of infrared spectra shows that the complex formation becomes maximum at 0.075 mole fraction of DMSO for DMSO–phenol system and at 0.094 mole fraction of DMSO for DMSO–ocresol system, which is also indicated through peaks of ultrasonic velocities as discussed above. Thus, the pattern, position and intensity of the O–H band as per infrared data strongly supports the conclusions drawn from the ultrasonic data that molecular association through hydrogen bonding is maximum at those concentrations where ultrasonic velocity maxima occurs [23]. It may be concluded that ultrasonic studies provide for a comprehensive investigation of molecular associ-
ation between DMSO and phenol and DMSO and ocresol molecules arising from the hydrogen bonding between the oxygen atom of DMSO molecule and the hydrogen atom of phenol and o-cresol molecules. The variation of acoustic parameters with temperatures and also the IR data strongly support the molecular association in these ternary liquid mixtures.
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