R J Sengwa*, Vinita Khatri & Sonu Sankhla

Transcription

1 Indian Journal of Chemistry Vol. 48A, April 9, pp Structure and hydrogen bonding in binary mixtures of N,N-dimethylformamide with some dipolar aprotic and protic solvents by dielectric characterization R J Sengwa*, Vinita Khatri & Sonu Sankhla Dielectric Research Laboratory, Department of Physics, JNV University, Jodhpur 34 5, India mail: rjsengwa@rediffmail.com Received 8 July 8; revised and accepted 4 February 9 The low frequency limit static dielectric constant, optical frequency dielectric constant, excess dielectric constant, and the Kirkwood correlation factor of the binary mixtures of N,N-dimethylformamide with water, ethyl alcohol, ethylene glycol, glycerol, dimethyl sulphoxide, acetone and 1,4-dioxane over the entire composition range are investigated at 3 o C. The values of these functions emphasize strong N,N-dimethylformamide water hydrogen bond interaction due to breaking of water tetrahedral structure, whereas the N,N-dimethylformamide alcohols complexations show a strong hydroxyl group number dependence with enhancement in the structural ordering of alcohols. The dielectric functions of N,N-dimethylformamide water and N,N-dimethylformamide 1,4-dioxane mixtures reveal the similarity in the complexes, although the N,N-dimethylformamide water interactions are stronger. The excess functions confirm the weak hydrogen bond interactions of N,N-dimethylformamide with dimethyl sulphoxide and acetone. The pronounced deviation in the excess functions of the studied mixtures confirms the formation of a stable adduct of stoichiometric ratio 1:1 in these systems. Keywords: Hydrogen bonds, Solvent effects, Aprotic solvents, Protic solvents, Dielectric functions, xcess properties IPC Code: Int. Cl. 8 G1N31/ The binary mixtures of dipolar aprotic and protic solvents have current interest in biological, chemical, pharmaceutical, technological, and laboratory applications because mixed solvents manifest different physicochemical properties as compared to those of the pure constituents of the mixture 1-6. Amongst the physicochemical properties, dielectric constant of the mixed solvents, which is sensitive to molecular interactions specially H-bond formation, enhances or controls most of the applications, and hence precise dielectric characterization is important. The dipolar solvent-solvent interactions are more complex in mixed solvents than in their pure form, and the characterization of these interactions is challenging in the liquid systems in which the molecules of mixture constituents have a large number of possible atom-atom interactions. Among the different solvents, the H-bonded structures of amides and their mixtures, specially with water and aliphatic alcohols, have an important role in biological systems, because the hydrogen bonds are important as a force governing the structure and dynamics of chemical and biological systems. In the case of the amides, the dipolar aprotic solvent, N,N-dimethylformamide (DMF) is unable to engage in N H O=C hydrogen bonding with dialkyl substitution at nitrogen. However, with water and alcohols, DMF forms the C H O and C=O H type H-bond interactions, which result in the complex H-bonded network structures of these systems 7,8. DMF is also able to form the H-bonded network structures with dipolar aprotic solvents. The dielectric measurements have great potential for studying such H-bonded network structures with dipolar ordering, the strength of complexations and the stoichiometric ratio of stable adduct, and also their dynamics in the mixtures. Several investigators have studied the structural properties of pure DMF and DMF mixed dipolar solvents by dielectric measurements at fixed frequency or over a broadband microwave frequency region More precise dielectric studies on DMF mixed solvents over the entire range of mole fractions are needed to enhance their applications. In continuation of the dielectric studies on amide solvent mixtures 8 18, we report herein the dielectric constants of DMF mixed with water, and glass forming liquids, i.e., alcohols, and some common polar aprotic bio-solvents over the entire

2 SNGA et al.: DILCTRIC CHARACTRIZATION OF H-BONDS IN BINARY MIXTURS 513 mole fraction range at 3 o C. These investigations provide information on dipolar ordering, strength of H-bond complexation and stoichiometric ratio corresponding to stable adducts in the DMF-dipolar solvent mixtures with a view to gain deeper insight into the mixed solvents H-bonded network structures. The effect of the number of hydroxyl groups of the cosolvent 19, and the type of aprotic solvent for the formation of complexes in the DMF mixtures have also been studied on the basis of the comparative dielectric parameters of the mixed solvents. Materials and Methods Grade reagent N,N-dimethylformamide (DMF) was purchased from Loba Chemie, ethyl alcohol (A) and glycerol () were purchased from S D Fine Chemicals, ethylene glycol (G) and 1,4-dioxane () were purchased from Merck, and dimethyl sulphoxide () and acetone () were purchased from Qualigens Fine Chemicals of India. Doubly distilled deionized water () was also used as one of the solvents. Binary mixtures of DMF with other solvents were prepared at 16 volume concentration over the entire composition range at room temperature. Simultaneously, the mole fractions of the mixture constituents were determined by weight measurements. The values of static dielectric constant,, of pure solvents and m, of the DMF-cosolvents binary mixtures were determined by using capacitive measurement method with a short compensation at 1 MHz. Agilent 484A precision LCR meter and a four terminal cell Agilent 1654A liquid dielectric test fixture were used for the capacitance measurement of the cell without and with sample. The measurement accuracy of the cell was ±.3%, estimated by calibration of the cell with the standard liquids by using their accurate dielectric constant values from the literature. The high frequency limit dielectric constant,, was taken as the square of the refractive index n D, which was measured with an Abbe refractometer at the wavelength of sodium-d light. The maximum measurement error in values was ±.%. All measurements were made at 3 o C and the temperature was controlled thermostatically using the microprocessor based Thermo Haake DC1 controller. The evaluated values of pure liquids alongwith literature values 6,1,17,1 7 are recorded in Table 1, and the m values of the mixtures against DMF mole fraction x DMF are recorded in Table. Data analysis The excess static dielectric constant mixture, is defined as 9 m m 1 1 x1 x, for binary =( ) [( ) + ( ) ] (1) where x is the mole fraction and subscripts m, 1 and represent the binary mixture and components 1 and of the binary mixture, respectively. is the deviation evaluated after subtracting the effect of high frequency limit dielectric constant from values. The evaluated values along with m and m values of the binary mixtures versus DMF mole fraction, x DMF are plotted in Figs 1 and. Table 1 xperimental and literature values of static dielectric constant ( ), dipole moment (µ), and evaluated values of the Kirkwood correlation factor (g) of pure solvents at 3 o C. [The x DMF values are the mole fraction of N,N-dimethylformamide at maximum value of excess dielectric constant for the binary mixtures of N,N-dimethylformamide with the corresponding solvent listed in first column] Solvent xpt Lit. µ (D) g x DMF N,N-dimethylformamide a, b (5 o C) 4.9 l 1.1 ater c, 76.6 d 1.8 m thyl alcohol e, 3.87 f 1.8 m thylene glycol g, 4.3 h (5 o C).38 n cerol i (5 o C).56 i Dimethylsulphoxide e, j (5 o C) 4.34 l etone.5.7 e (5 o C).93 o ,4-Dioxane.6. k, 3.19 g.45 p a Ref 1, b Ref 17, c Ref 1, d Ref, e Ref 6, f Ref 3, g Ref 4, h Ref 5, i Ref 6, j Ref 1, k Ref 7, l Ref 15, m Ref 8, n Ref 9, o Ref 3, p Ref 31

3 514 INDIAN J CHM, SC A, APRIL 9 Table Static dielectric constant ( m ) of the binary mixtures of DMF-cosolvent at various mole fractions of DMF (x DMF ) at 3 o C x DMF m x DMF m x DMF m x DMF m DMF- DMF-A DMF-G DMF DMF- DMF- DMF The Kirkwood correlation factor, g, of the pure liquid is determined by the following expression 8, 4π N d ( )( + ) g µ = 9kT M ( + ) () where µ is the dipole moment, d the density of liquid at temperature T, M the molecular weight, k the Boltzmann constant and N the Avogadro s number. The µ values 15,6,8 31, used in q. (), along with the evaluated g values of the pure solvents, are listed in Table 1. In the present study we have used = n D, and the literature µ values of the dipolar solvents (gas-phase or in non-polar solvent) for the evaluation of pure polar solvent g values. In binary mixtures of polar solvents, the effective averaged angular Kirkwood correlation factor, g eff, of unlike molecules is evaluated from the modified Kirkwood equation 3,33 based on volume fraction mixture law 4π N µ 1 d1 µ d eff φ 1 + φ g 9k T M1 M (m m )(m + m ) = ( + ) m m (3) where φ 1 and φ are the volume fractions of liquids 1 and, respectively, and m and m are the measured values of static dielectric constant and high frequency limiting dielectric constant of the binary mixture. For

4 SNGA et al.: DILCTRIC CHARACTRIZATION OF H-BONDS IN BINARY MIXTURS 515 m m 75 (a) (b) x DMF A G A G (c) A G m m x DMF (a) (b) (c) Fig. 1 Plots of (a) m, (b) m, and, (c) against mole fraction of DMF (x DMF ) for DMF-cosolvents. [( ) DMF-; ( ) DMF-A; ( ) DMF-G; and ( ) DMF- binary mixtures at 3 o C. In (a) and (b), lines are smooth joining through the data points, whereas in (c) lines are non-linear fits. For clarity, error bars are not indicated. rror bars are smaller than the size of the symbols. Dotted horizontal line in (c) is to see the deviation]. φ 1 = 1 and φ =, q. (3) reduces to q. () with the g value of pure liquid 1, and vice versa. The evaluated g eff values of the DMF cosolvent binary mixtures are plotted against the volume fraction of DMF, φ DMF, in Figs 3 and 4. Assuming that the Kirkwood correlation factor for the molecules of liquids 1 and are g 1 and g in the mixture, and contribute to g eff values proportional to their pure liquid g values, then the Kirkwood correlation factor q. (), for the binary mixture on the basis of simple volume mixture law, can be written as 11 : 4π N µ 1 d1 g1 µ d g φ 1 + φ gf 9k T M1 M (m m )(m + m ) = ( + ) m m (4) Fig. Plots of (a) m, (b) m, and, (c) against mole fraction of DMF (x DMF ) for DMF-cosolvents at 3 o C. [( ) DMF-; ( ) DMF-; and ( ) DMF-]. where g f is the corrective Kirkwood correlation factor (also called deviation in Kirkwood correlation parameter) for a binary mixture. The evaluated g f values of the DMF-cosolvent binary mixtures, at different volume fractions of DMF, φ DMF, are plotted in Figs 3 and 4. Results and Discussion Table 1 shows that values of the pure polar solvents are in good agreement with the reported literature values. The plots of m and m values against DMF mole fraction, x DMF (Figs 1 and ) are more or less non-linear, and confirm the molecular interactions between the DMF and cosolvent molecules of the binary mixtures in the present study. The non-linear behaviour indicates that the use of approximated dielectric constant values by simple molar additive equations will lead to erroneous results in the dielectric constant dependent applications of the mixed solvents.

5 516 INDIAN J CHM, SC A, APRIL 9.8 (a) 1.1 (a) g eff A G g eff (b) A G 1.5 (b) g f g f φ DMF φ DMF Fig. 3 Plots of (a) g eff, and, (b) g f against volume fractions of DMF, (φ DMF ) for DMF-cosolvents at 3 o C. [( ) DMF-; ( ) DMF-A; ( ) DMF-G; ( ) DMF-]. Figure 1 shows that the concentration dependent m values of the DMF-dipolar protic solvents deviate from ideality, which confirms the formation of complexes between DMF and dipolar protic solvent molecules mainly through H-bonds. The non-linear behaviour of the m values also confirms the change in the electronic polarization caused by unlike molecular interactions in these solvents 9,3. The nonzero values of the excess dielectric constant of the mixed solvents is the experimental evidence of the formation of hydrogen bond complexes and the strength of H-bond connectivity between the unlike molecules in the mixed solvents 9,11,17,18. The values of DMF mixed with, A, G and were found to be non-zero over the entire concentration range and these values range from the large negative for DMF- to positive for DMF-alcohols. The change in structures of the mixtures constituents due to variation in dipolar alignments and subsequent H-bond formation is responsible for the divergent Fig. 4 Plots of (a) g eff, and, (b) g f against volume fractions of DMF (φ DMF ) for DMF-cosolvents at 3 o C. [( ) DMF-; ( ) DMF-; ( ) DMF-]. signs for the observed values of these systems 9,18. The correlation of the molar concentration of DMF, x DMF to pronounced maximum positive or negative values is evaluated by fitting the (x DMF ) data to the polynomial equation using Origin non-linear curve fitting tool. The x DMF values alongwith the maximum values are recorded in Table 1. The magnitude of concentration dependent negative values of DMF- mixtures and its maximum value position on the mole fraction scale of DMF i.e., x DMF ~.46 (Table 1) is in good agreement with the results reported earlier 9. The concentration of maximum deviation in values corresponds to the stoichiometric ratio of stable adduct formation with comparatively higher range dipolar ordering 9,17,18,4. The maximum deviation in values of DMF- at x DMF ~.46 is attributed to the formation of 1:1 H-bonded complexes between DMF and mole-

6 SNGA et al.: DILCTRIC CHARACTRIZATION OF H-BONDS IN BINARY MIXTURS 517 cules, probably through C=O H, which also supports the conclusion drawn from the thermodynamical properties 7, FTIR 34,35 measurements, and molecular simulations study 8,36,37 on DMF- mixtures. The negative values of DMF- mixtures suggest that the DMF molecules coalesce and break the tetrahedral structure of water, resulting in decrease in effective number of dipoles contributing to the polarization of DMF- mixtures. The positive values of DMF-alcohols mixtures over the entire concentration range are in the order: DMF- > DMF G > DMF-A as shown in Fig. 1 and Table 1, which is according to the descending number of OH groups of the aliphatic alcohol molecules. The positive values of DMF alcohols suggest that the DMF molecules are built-in via H-bonding into the H-bonded structures of pure alcohols, which results in the net increase in effective number of dipoles contributing to the mixtures dielectric polarization. The A molecules have H-bonded polymeric chain-like structures with a switch over type of molecular reorientation throughout the chain 38. The comparative small positive values of DMF-A mixtures suggest that the C H O hydrogen bonds are formed due to switching of H-bonded A molecules. G molecules have OH groups at both ends with a gauche conformation, which is a result of intramolecular hydrogen bonding, and probably because only one OH group of G molecule can interact with neighbouring molecules through H-bond 9. The comparative increase in values of DMF-G mixture as compared to the DMF-A mixture suggests that the DMF molecules built-in via H-bonding into G structure results in the breakup of G intramolecular H-bond association. This promotes the parallel dipolar ordering and net increase in effective number of dipoles contributing to the polarization of the DMF-G mixture. The higher positive values of DMF- confirm the comparative large increase in the net effective number of dipoles due to DMF complexation with. The DMF molar concentrations corresponding to a pronounced maximum value of excess dielectric constant (max) is nearly.5 (Table 1) indicating the formation of a stable adducts in the stoichiometric ratio 1:1 in all the investigated mixtures. The non-linear behaviour of m values of DMFpolar protic solvent mixtures (Fig. 1) indicates the change in electronic polarization of the mixtures constituents, which is caused by H-bond complexation 8,9. The magnitude of non-linearity of the m values of DMF-polar protic solvents mixtures has the same order as m values. This suggests that the change in electronic polarization due to unlike molecules H-bond interactions is governed by the strength of H-bond complexations in these mixtures. Figure shows the m and m values of DMF mixtures with polar aprotic solvents i.e.,, and. In the case of DMF- mixtures, the deviation in m from ideality is very small and does not appear in the m (x DMF ) plots of DMF- and DMF- mixtures. The values of DMF- mixtures are comparatively large and negative over the entire concentration range, whereas these values are positive but comparatively small in the case of DMF- and DMF- mixtures. From comparative results, it is inferred that in the DMF- mixture, the H-bonded interactions between unlike molecules are comparative strong whereas DMF- and DMF- have very weak interactions. From these results, it can be concluded that in the DMF- and DMF- mixtures, the mixture constituents mix almost ideally. However, the significant negative values of DMF- mixture indicate that this is always not true for mixtures of all polar aprotic solvents. Further, in the case of DMF-, the x DMF corresponds to a pronounced maximum value at ~.46, suggesting the formation of a stable adduct of stoichiometric ratio 1:1, which is exactly same as in the case of DMF- mixture. The magnitude of of DMF- mixture is about half of that of of DMF- mixtures, which suggests that the DMF- H-bond complexes are about two times stronger than the H-bond complexes formed between DMF and molecules in the DMF- mixture. The H-bond interaction strength of the DMF with molecules is very weak over the entire molar concentration range and has almost the same values over a wide middle concentration range (Fig. ). The almost linear behaviour of m (x DMF ) values of the DMF-polar aprotic solvent mixtures (Fig. b) confirm that the H-bond complexation

7 518 INDIAN J CHM, SC A, APRIL 9 between these aprotic solvents does not affect or has a very small effect on their electronic polarization. Table 1 shows that the g values of the pure liquids A, G, and are much higher than unity and confirm the existence of H-bonded multimers with parallel dipolar alignments 8. The order of g values of the alcohols is: A > > G, and these values are lower than the g value of the pure water molecules. The higher magnitude of g value confirms the stronger strength of H-bonded structures with large range dipolar ordering of these pure liquid alcohols 39. The unity g value of DMF confirms its non selfassociative molecular behaviour in the pure liquid state. Monohydric alcohol (A) has a comparative higher g value and the dihydric alcohol (G) has lower g values. The order of g values of A, G and are not according to the number of OH groups in these molecules, and may be because of the nearly equal values of G and (Table 1). However, the values of DMF-alcohols increase with the increase of the number of OH groups, which suggests that the H-bonded molecular structures of the DMF with alcohols, are governed by the number of OH groups of the alcohol molecule. Figure 3 shows that the g eff values of the mixed solvents vary in the range of g values of the mixtures constituents. However, the non-linear behaviour of g eff values of DMF- and DMF-alcohols mixtures confirms the significant change in dipolar ordering in these mixed solvents due to hetero molecules H bond interactions. Further, it is found that DMF- mixture has positive deviation in g eff values, whereas all the DMF-alcohols mixtures have negative deviations, which also supports the change in sign of the deviation of values of DMF- and DMF-alcohols mixtures from their ideality. In mixtures the deviation of g f values from unity also confirms that the structures of the mixture constituents change significantly in the investigated mixtures. The unity g f values of the DMF-A indicate that the H-bond interaction between DMF and A molecules changes their pure liquid structures in such a way that the net structural variation compensates the dipolar ordering in the mixture. Similar to DMF, the pure polar aprotic solvents and also have g values nearly unity (Table 1), which indicates the absence of H-bond molecular interactions in their pure liquid state. The g value of is slightly higher than unity, which indicates the presence of very weak H-bonded interactions between the molecules in their pure liquid state. The non-linear behaviour of g eff values and the deviation of g f values from unity also support the formation of H-bonded complexes between DMF and the polar aprotic solvents used in the present study. Further, the magnitude of g f values shows that the DMF- has stronger H-bonded structures as compared to the DMF- and DMF- complexations. Conclusions The precisely measured concentration dependent values of dielectric parameters of DMF mixed with different polar solvents have been reported. The dielectric study attempts to explain the hydrogen bond formation and the variation in pure solvent structures over the entire concentration range in DMF-polar protic and DMF-polar aprotic solvents. The number of OH groups of aliphatic alcohols as well as water molecules has an influence on the H-bond complexes properties and dipolar ordering of DMF molecules. The dielectric parameters values of DMF with aprotic polar solvents show that the DMF molecules form weak hydrogen bonds with and, but the strength of the H-bonds formation of DMF with is comparatively strong. Further, the H-bond complexes between DMF and molecules are similar to the DMF and water molecules complexes, but the strength of H-bond in the DMF- complexes is nearly half of that in the DMF- complexes. knowledgement The authors thank the University Grants Commission, New Delhi, for a project grant under which the work was carried out and also the Department of Science and Technology, Government of India, New Delhi, for providing experimental facilities. References 1 Grant H, Sheppard R J & South G P, Dielectric Behaviour of Biological Molecules in Solution, (Clarendon Press, Oxford, UK), Barthel J, Krienke H & Kunz, Physical Chemistry of lectrolyte Solutions, (Springer, New York), Craig D Q M, Dielectric Analysis of Pharmaceutical System, (Taylor and Francis, London, UK) Geiser L, Cherkaoui S & Veuthey J L, J Chromatogr A, 979 () Hernández-Luis F, Galleguillos-Castro H & steso M A, Fluid Phase quil, 7 (5) Cossi M & Crescenzi O, Theor Chem c, 111 (4) 16.

8 SNGA et al.: DILCTRIC CHARACTRIZATION OF H-BONDS IN BINARY MIXTURS Garcia B, Alcalde R, Leal J M & Matos J S, J Phys Chem B, 11 (1997) Lei Y, Li H, Pan H & Han S, J Phys Chem A, 17 (3) Rohdewald P & Möldner M, J Phys Chem, 77 (1973) Kaatze U, Neumann Chr & Pottel R, J Sol Chem, 13 (1984) Chaudhari A, More N M & Mehrotra S C, Bull Korean Chem Soc, (1) Corradini F, Marchetti A, Tagliazucchi M, Tassi L & Tosi G, J Chem ng Data, 37 (199) Barthel J, Bachhuber K, Buchner R, Gill J B & Kleebauer M, Chem Phys Lett, 167 (199) Barthel J, Buchner R & urm B, J Mol Liq, () Sengwa R J, Madhvi, Abhilasha & Sankhla S, Indian J Pure Appl Phys, 44 (6) Sengwa R J, Khatri V & Sankhla S, Fluid Phase quil, 66 (8) Sengwa R J, Madhvi & Sankhla S, Phys Chem Liq, 44 (6) Sengwa R J, Khatri V & Sankhla S, J Mol Liq, 144 (9) Sengwa R J, Chaudhary R & Mehrotra S C, Mol Phys, 99 (1) 185. Sengwa R J & Sankhla S, J Non-Cryst Solids, 353 (7) Buchner R, Barthel J & Stauber J, Chem Phys Lett, 36 (1999) 57. Rived F, Rosés M & Bosch, J Chem ng Data, 4 (1995) Chaudhari A, Das A, Raju G, Chaudhari H, Khirade P, Narain N & Mehrotra S, Proc Natl Sci Council ROC (A), 5 (1) 5. 4 Corradini F, Marchetti A, Tagliazucchi M, Tassi L & Tosi G, Aust J Chem, 48 (1995) Schrödle S, Buchner R & Kunz, J Phys Chem B, 18 (4) Sengwa R J, Indian J Pure Appl Phys, 41 (3) Papanastasiou G & Zlogas I I, J Chem ng Data, 37 (199) Hill N, Vaughan, Price A H & Davies M, Dielectric Properties and Molecular Behaviour, (Van Nostrand Reinhold Co, London), Sengwa R J & Sankhla S, J Mol Liq, 13 (7) Georg H C, Coutinho K & Canuto S, Chem Phys Lett, 49 (6) McClellan A L, Tables of xperimental Dipole Moments, edited by H Freeman, (San Francisco, California), Hasted J B, Aqueous Dielectrics, (Chapman & Hall, London), Tabellout M, Lanceleur P, mergy J R, Hayward D & Peltirick R J, Chem Soc Faraday Trans, 86 (199) Sharma A, Kaur S, Mahajan C G, Tripathi S K & Saini G S S, Mol Phys, 15 (7) Xu Z, Li H, ang C, u T & Han S, Chem Phys Lett, 394 (4) Cordeiro J M M & Freitas L C G, Z Natureforsch A: Phys Sci, 54 (1999) Vargas R, Garza J, Dixon D A & Hay B P, J Am Chem Soc, 1 () Sengwa R J, Abhilasha, More N M & Mehrotra S C, J Polym Sci: Part B: Polym Phys, 43 (5) Sengwa R J, Sankhla S & Shinyashiki N, J Sol Chem, 37 (8) 137.