Dielectric behaviour and dipole moment of binary mixtures containing certain amides in benzene

Certain polar amides(v) are dissolved in benzene, a nonpolar solvent(u) to prepare binary mixtures with different composition of the amides like acetanilide, N-N-dimethyl acetamide, N-methyl acetamide, acetamide and formamide. Static relative permittivity ɛ0uv , high frequency permittivity ɛ ∞uv for different weight fraction of polar solute at 30°C are determined. The dielectric behaviour of binary mixtures are investigated under electric field of low frequency by using the Debye model for polar liquids. Static dipole moment µs , dipole moment µv from conductivity measurements and µ t from bond angle and bond moment measurements are computed. The dipole moments are consequence of attractive interaction between the adjacent molecules in binary mixture due to variation of electron densities. Hydrogen bonding, associated charge distribution among the constituent molecules, changes in bond angle values are some of the factors that give rise to dipole moment of binary mixtures. Nearly equal and close values of µs , µv and µt proves the correctness of dielectric behaviour studies of amides- benzene binary mixtures.


Introduction
Amides are nitrogenous compounds mostly having important applications in biological, medical, and pharmaceutical sectors.In human anatomy the enzymes and protein molecules comprise of amide like structures and the interactions with fluids within the body determine their stability [1,2] .Amides are characterised by large dielectric constant mainly due to its own dipole and partly due to interactions with surrounding molecules [2][3][4].Interactions between the constituent molecules and response of binary mixtures to static low frequency electric field can be estimated from the magnitude of dielectric constant of mixture [3,[5][6][7][8].Most of the knowledge pertaining to kinematics, the molecular interactions and industrial applications of liquid mixtures can be assessed through information about dielectric properties of binary liquid mixtures [9][10][11][12][13][14][15][16][17].The Static dipole moment   in low frequency electric field helps to study molecular interactions between the solute and solvents in liquid mixtures [5,7].The dipole moment values of amides dissolved in nonpolar solvents provides vital information about intermolecular interactions, hydrogen bonding and dipolar orientation among molecules in binary mixtures [6,[14][15][16] .The knowledge of dipole moment   under high frequency electric field has been used to investigate dielectric response of binary mixture of amides dissolved in aromatic liquids [5,7,9,[15][16][17].Studies involving static relative permittivity ℇ 0 and high frequency permittivity ℇ ∞ measurements of amides in aromatic solvents have been carried out previously [3,5,10].However, very few investigations have been done related to dielectric response of binary mixtures of amides in nonpolar solvents under static low frequency electric field.Some previous studies have measured the bond angle and value of dipole moment   based on bond angle [6,11,12].This work deals with the dielectric behaviour of binary mixture obtained by dissolving amides as solute in nonpolar solvent benzene [C6H6] under static low frequency electric field.Knowledge of ℇ 0 and ℇ ∞ for several weight fraction   of amides at temperature 30 C is used to calculate the dipole moment of liquid mixtures.Benzene is taken in the study as it is widely used and readily available solvent with many industrial applications.The static dipole moment   was estimated in terms of slope  1 of graph between static experimental parameter   against weight fraction   on basis of Debye model for polar-nonpolar liquid mixture.A comparison is drawn between values of   ,   and   to assess the effect of low frequency static electric field on dipole moment of binary mixture.

Theory
The static dipole moment of polar solute(v) dissolved in nonpolar solvent(u) at T Kelvin according to Debye model [10,14,15] Where = absolute permittivity of free space = 8.854x10 Fm -1 , other symbols are in SI units and have their usual meanings.The molar concentration   is expressed as is binary mixture density and   solute weight fraction.Binary mixture density   is [17,18] and   are weight fractions of the solvent and solute given as , and 1) can be rewritten as =   , the static experimental parameter, can be expressed as polynomial equation in terms of   and written as here  1 is slope of   −   curve for very small   as shown in figure1.Higher powers of   can be neglected in accordance with Debye model which is valid for very small solute concentrations.Mv is solute molecular weight, KB is the Boltzmann Constant (1.38x10 -23 Jk -1 ), T is the temperature, N is Avogadro's number and   and   the density of nonpolar solvent and solute respectively.

Result and Discussion
Values of ℇ 0 and ℇ ∞ for different weight fraction   of the solute and static experimental parameters   are given in Table .1.Extremely low concentration of amides (solute) in the binary mixture is maintained in accordance with Debye model of polar liquids.Figure .1 show the graphs between static experimental parameter   against   weight fraction of the solute which are nearly parabolic in nature.Dipole moment µ  directly varies as square root of slope  1 of   −   curves.formamide and N-Methyl acetamide have higher values of   which is an indication of solute-solute and solute-solvent molecular interaction through hydrogen bonding in benzene, as the concentration increases.The values of slope  1 of   −   curves are higher for formamide and N-Methyl acetamide due to which the   values are higher for them.The dipole moment and dielectric properties are found to be higher as the polar amide molecules align favourably to enhance the polarization as result of hydrogen bonding.[15,16] and   the theoretical dipole moment from measurement of bond angle [10,17] show excellent agreement between them.. Small deviations in value of   ,   ,   can be attributed to applied low frequency electric field.
The hydrogen bonding in amides-benzene binary mixture exhibit solute-solute interaction.Dipole moment of the adjacent polar molecule and formation of bonds is responsible for charge distribution among the constituent molecules, which can be due to various effects like electromeric, mesomeric and inductive effect.This intramolecular charge distribution is responsible for bond moments or dipole moment.

Conclusion
The dielectric behaviour of binary mixture containing amides in benzene can be explained using the Debye model for polar liquids.The dipole moments calculated on the basis of Debye theory is found to be in agreement to practically measured values only when low concentrations of amidebenzene binary liquid mixtures are taken, which can be concluded from excellent agreement between   ,   ,   values.Hydrogen bonding, associated charge distribution among the constituent molecules, changes in bond angle values are some of the factors that give rise to dipole moment of binary mixtures which results due to electromeric, mesomeric and inductive effects .The theory can be applied for dielectric measurements of other binary mixtures and can be used for various industrial and technological applications such as manufacture of charge storing devices and organic super capacitors.

Figure 1 .
Figure 1.Graph between the static experimental parameter   against weight fraction   for binary mixture of amides in benzene.

Table 1 .
Static relative permittivity ℇ 0 , high frequency relative permittivity ℇ ∞ , static experimental parameter   of certain amides dissolved in Benzene at different weight fractions of solute.

Table 2 .
coefficient  1 from   −   curve, reported dipole moment   from conductivity measurement in high frequency fields, theoretical dipole moment   from bond angle, calculated static dipole moment   for some amides in benzene.shows that the graphs obtained for different binary mixtures are nearly same in nature indicating nearly same slope.The almost same value of slope is an indication of nearly same polarity of solutes.Static dipole moment   calculated in present work, dipole moment   on basis of conductivity measurement