Molecular interaction study of two aliphatic alcohols with cyclohexane

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 46, December 2008, pp Molecular interaction study of two aliphatic alcohols with cyclohexane R Thiyagarajan & L Palaniappan* Department of Physics, SRM ngineering College, Chennai, Tamil Nadu *School of Physics, Universiti Sains Malaysia, Pulau Penang 11800, Malaysia -mail: lp_dde_au@yahoo.com Received 11 July 2007, revised 24 September 2008; accepted 8 October 2008 Sound velocity, density and viscosity values have been measured at 303 K in the two binary systems of cyclohexane + ethanol, 1-propanol. From these data, acoustical parameters such as adiabatic compressibility, free length, free volume and internal pressure have been estimated using the standard relations. The results are interpreted in terms of molecular interaction between the components of the mixtures. Observed excess value in all the mixture indicates that the molecular symmetry existing in the system is highly disturbed by the polar alcohol molecules and dispersive type interactions are existing in the systems. Keywords: Ultrasonic velocity, Acoustic parameters, Dipolar and dispersive interactions 1 Introduction In many industrial applications liquid mixtures, rather than single component liquid system, are used in processing and product formulations 1,2. Liquid mixtures consisting of polar and non-polar components are of considerable importance in industries such as petrochemical, pharmaceutical and dye. The formation and destruction of azeotropes in petrochemical industries 3, the biological activity of drug molecules 4 and the activation energy of the metabolic process 5 basically depend on the type and strength of the intermolecular interactions. Various methods are available to identify these interactions and ultrasonic study is one such more reliable and commonly used study. Thermodynamic and transport properties of liquid mixtures have been extensively used 6,7 to study the departure of a real liquid mixture behaviour from ideality. Further, these properties have been widely used to study the intermolecular interactions between the various species present in the mixture 8,9. As alcohols are highly polar, they can easily form azeotropes of binary complexes. thanol is reported 10 to form azeotropic mixture with cyclohexane at K whereas 1-propanol at K and the separation is a major task in petrochemical industries. In the present work, the measurement of ultrasonic velocity, density and viscosity and computation of related parameters at 303 K in two non-ideal binary mixtures of cyclohexane+ethanol and cyclohexane+1- propanol have been studied. 2 xperimental Details The mixtures of various concentrations in mole fraction were prepared by taking purified AR grade samples at 303 K. The ultrasonic velocity (U) in liquid mixtures have been measured using an ultrasonic interferometer (Mittal type, Model F-81) working at 2 MHz frequency with an accuracy of ± 0.1 ms 1. The density (ρ) and viscosity (η) are measured using a pycknometer and an Ostwald s viscometer respectively with an accuracy of 3 parts in 10 5 for density and Nsm 2 for viscosity. Using the measured data, the acoustical parameters such as adiabatic compressibility (β), free length (L f ), free volume (V f ) and internal pressure (π i ) and their excess parameters have been calculated using the following standard expressions 11 : β U = ( 2 ρ) 1 (1) ½ L f = K T β (2) V f π i Meff U = ηk kη 2 ρ 3 = brt U 7 6 M eff (3) (4)

2 THIYAGARAJAN & PALANIAPPAN: MOLCULAR INTRACTION STUDY OF ALIPHATIC ALCOHOLS 853 A Aexp A id = (5) A (6) id = xi Ai where, K T is the temperature dependent constant having a value in MKS system, k is a constant equal to in MKS system, independent of temperature for all liquids, b is the cubical packing fraction taken as 2 for all the liquids, R is the universal gas constant, T is the experimental temperature, Meff = ximi where, x is the mole fraction and m is the molecular weight of i th component and A stands for excess property of any given parameter, A exp is the experimental value and A id is the ideal value. 3 Results and Discussion The measured values of density (ρ), viscosity (η) and sound velocity (U) for the two binary systems of cyclohexane + ethanol and cyclohexane + 1-propanol at 303 K are presented in Table 1. The perusal of this Table shows that the density and viscosity shows decreasing trend in both systems with the increase in mole fraction of cyclohexane. As regards sound velocity, ethanol shows a continuous increase, whereas in 1-propanol system, it initially decreases, exhibit a minimum at 0.2 mole fraction and then continually increasing with increase in mole fraction of cyclohexane. As per dward Peters 12, a higher density or viscosity of a component molecule is a reflection of higher intramolecular interactions. So, among the three components taken here, 1-propanol is having higher intra interactions. A molecule having higher intra interaction is also expected to show high degree of intermolecular interactions. Thus among the two binaries, in any given mole fraction, 1-propanol records more velocity that reflects the existence of more intermolecular interactions. Comparing the two alcohols, ethanol is a good solvent that can dissolve both the polar and nonpolar components. The hydrophilic OH group of ethanol can dissolve the polar whereas the short hydrophobic hydrocarbon group can dissolve the nonpolar 13. But 1-propanol is not a good solvent as ethanol. As regards cyclohexane, it exhibits conformation and exists either in rigid chair form (minimum potential energy) or in boat form (maximum potential energy) and also in skew-boat form 14, all can undergo rapid interconversion at room temperature 15. Due to the presence of bowspirit-flagpole interaction in the boat cyclohexane, breaking up of hydrogen bonds are highly favoured in it 16. In the ethanol mixture, the cyclohexane is completely dissolved and so no chances of hydrogen bond ruptures and only the interaction with the cyclohexane ring and the active groups of ethanol, which are mostly dispersive in nature. The increase in mole fraction of cyclohexane increases the net dispersive interactions and hence the velocity continuously increases as observed. The case of 1-propanol mixture is different due to the less salvation tendency of 1-propanol. It is to be remembered that the methyl group becomes more nonpolar 17 with increase in chain length. Further the OH group can dissolve preferably the polar Mole fraction of cyclohexane Table 1 Values of density (ρ), viscosity (η) and ultrasonic velocity (U) at 303 K ρ /kgm 3 η 10 3 /Nsm 2 U /ms 1 ethanol 1-propanol ethanol 1-propanol ethanol 1-propanol

3 854 INDIAN J PUR & APPL PHYS, VOL 46, DCMBR 2008 component. So, in the lower mole fraction ranges the added cyclohexane has practically no interaction with the methyl or hydroxyl group of 1-propanol, thus sound velocity seems to decrease. However, as the mole fraction of cyclohexane increases, the hydrogen bond rupture of the boat form is of considerable extent and they leads to additional dipole type interactions. This is quite in line with the observation as the sound velocity at any given mole fraction is more for 1-propanol system than in ethanol system. Cyclohexane being non-polar the predominant dispersive type interactions with temporary dipolar type are existing as a net result of intermolecular forces 16 in both systems, but more in 1-propanol system. The observed non-linear change in all these measured parameters indicates the existence of specific interactions 18. A reduction in density and viscosity with increase in mole fraction of cyclohexane suggests that the existing intermolecular interactions are weakening in magnitude 19. However, the increasing sound velocity with increasing mole fraction of cyclohexane leads to a notion that the system is getting more and more compact, which is not true as the interactions due to cyclohexane are dispersive in nature. This is indicated by the existence of minimum sound velocity in 1- propanol system. However, as cyclohexane mole fraction is increased, sound velocity exhibits increasing trend in 1-propanol system. This increasing velocity on increasing mole fraction of cyclohexane with decreasing density is somewhat a peculiar nature and is mainly due to the conformation property of cyclohexane. It is also to be noted that the density of mixture falls below the pure component value. Conversely, the density values observed at higher mole fractions are found to be lower than the pure cyclohexane. This supports the rupture of hydrogen bonds; thereby the structure of cyclohexane is collapsed leading to a density lower than that of pure component. The calculated parameters of adiabatic compressibility (β), free length (L f ), free volume (V f ) and internal pressure (π i ) are listed in Table 2. This table suggests that for both systems β and L f are of similar nature and in decreasing trend whereas, V f and π i are of opposite nature to each other. In general, a continuously decreasing trend of β with increasing mole fraction of cyclohexane in these systems lends support to the idea that the systems are in a more compressed state. The ruptured hydrogen bonds create many new dipoles in the medium and thereby increase the compactness. The reduction in L f indicates that the components are much more closer 20. This confirms the existence of specific interactions between the components. The perusal of 1-propanol system shows that β and L f are almost unaltered in intermediate mole fraction range, i.e., the addition of cyclohexane in the mole fraction range of 0.2 to 0.8 has very small influence on compressibility and free length. This is clear evidence that interactions of weak nature, that too in very small magnitude, are existing in the system. As the dipole moment value of cyclohexane is zero, it can offer only dispersive type interactions, whereas 1- propanol, of dipole moment 1.58D 10, can exhibit dipoles and can support induced dipoles. But, this inducement of dipoles in cyclohexane seems to be highly restricted, as β and L f almost remain unaltered. This may be attributed to the fact the dipole inducement will be arrested by bowspirit-flagpole interaction 16. At higher mole fraction of cyclohexane, because of its inherent compact structure, β and L f are decreasing. Cyclohexane is a ring type molecule, whereas alcohol is a linear molecule. The reduction of alcohol reduces the straight chain type and the system is more and more occupied by ring-structured molecules, which occupies comparatively less space. In the two components, one is non-polar and the other is strong polar, attractive type interactions are feeble and so molecules are not in a closed shield or cage-like formation 21. ach component still maintains their identity and at the same time, the collective molecular influence is reduced. Thus, free volume between the components is in an increasing trend as the cyclohexane mole fraction is raised, as is seen from Table 2. The increase in free space between the components offer a support to stabilize their independent nature and hence mutual force (attractive/repulsive) retains their original value, i.e., the interactive forces decrease. To ascertain the exact type of interaction existing between the components, respective excess parameters have been calculated and for the present systems, the perusal of Figs 1 and 2 indicates that β and L f are positive over the entire mole fraction range. The positive β clearly suggests that weak interactions are existing between the components of the mixture. The trend of Figs 1 and 2 shows that β and L f are increasingly positive as cyclohexane mole fraction is increased. Thus, the strong dipolar interactions are restricted more and more whereas the

4 THIYAGARAJAN & PALANIAPPAN: MOLCULAR INTRACTION STUDY OF ALIPHATIC ALCOHOLS 855 Table 2 Values of adiabatic compressibility (β), free length (L f ), free volume (V f ) and internal pressure (π i ) at 303 K Mole fraction of cyclohexane β /Pa 1 L f /m V f 10 7 /m 3 mol 1 π i 10 8 /Pa ethanol 1-propanol ethanol 1-propanol ethanol 1-propanol ethanol 1-propanol Fig. 3 Mole fraction versus excess free volume at 303 K Fig. 1 Mole fraction versus excess adiabatic compressibility at 303 K Fig. 4 Mole fraction versus excess internal pressure at 303 K Fig. 2 Mole fraction versus excess intermolecular free length at 303 K weak dispersive interactions are predominating and are maximum around 0.8 mole fraction. As the molecular symmetry is retained to a greater extent at 0.9 mole fraction, β and L f are found to decrease. Further, it may be noticed from the Fig 2 that both systems exhibit same magnitude of interaction at 0.7 mole fraction of cyclohexane, beyond which the L f of systems show reverse nature that reflects the existing intermolecular interactions. However, V f and π i, for the mixtures, are mostly negative (Figs 3 and 4) and at higher mole fractions of cyclohexane, they become positive in ethanol system. Such transition in sign at higher cyclohexane mole fraction indicates that the dispersive type interaction alone is existing, which is weak in nature. xistence of such weak interactions was confirmed by Palaniappan et al. 22 in some cyclohexane systems. Among the two alcohol systems π i considered, a more systematic variation in V f and is observed for ethanol system whereas it is not so for 1-propanol system. The presence of one more methyl group in 1-propanol system produces more number of

5 856 INDIAN J PUR & APPL PHYS, VOL 46, DCMBR 2008 induced dipoles in cyclohexane, provided chair form of cyclohexane is available in the system as it is much more stable than boat form 16. Such restrictions are reflected in the observed haphazard trend of V f and π i in 1-propanol systems. 4 Conclusions Weak dispersive type intermolecular interactions are confirmed in the systems. Dipole inducement is found to be more in 1-propanol system. Components maintain their individuality in the mixture. References 1 Fidkowski Z T, Doherty M F & Malone M F, AICh Journal, 39(8) (1993) Wilson R Q, Mink W H, Munger H P & Clegg J W, AICh Journal, 1(2) (1955) Robert,. Treybal, Mass transfer operations, 3 rd edn, (McGraw Hill Co., Singapore) 1981, Section 1. 4 Gareth Thomas, Chemistry for Pharmacy and the Life Sciences (Prentice Hall, London), 1996, Chapters 2, 15 & Anas M Ababneh, Large C C & Georghiou S, Biophys J, 85 (2003) Oswel S L, Oswel P & Phalak R P, J Sol Chem, 27 (1998) Rajasekhar J & Naidu P, J Chem ng Data, 41 (1996) Boch W & Miecznik P, Acoustics Lett, 10 (1986) Kalidoss M & Srinivasamoorthy R, J Pure Appl Ultrason, 19 (1997) David R Lide, CRC Handbook of Chemistry and Physics, 83 rd edn (CRC Press, London), 2003, Section Arul G & Palaniappan L, Indian J Pure Appl Phys, 43 (2005) dward I Peters, Introduction to Chemical Principles, 3 rd edn., Ch.14 (CBS College Publishing, Philadelphia) 1982, pp accessed on 3st March Sharp D W A, Miall s Dictionary of Chemistry, ssex, 5 th edn., (UK Longman) 1981, pp Jensen F R, Noyce D S, Sederholm C H & Berlin A J, J Am Chem Soc, 84 (1962) rnst L liel, Stereochemistry of Carbon Compounds, 9 th edn (Tata McGraw Hill, New Delhi), 1985, pp Piracha N K, Ito F & Nakanaga T, Chem Phys, 297 (2004) Kaatze U, Telgmann T & Miecznik P, Chem Phys Lett, 310 (1999) Palaniappan L, Indian J Pure Appl Phys, 40 (2002) Mehra Rita & Pancholi Meenakshi, Indian J Pure Appl Phys, 45 (2007) Sundharam N & Palaniappan L, Indian J Phys 79(10) (2005) Palaniappan L & Arul G, Indian J. Pure Appl Phys, 39 (2001) 561.