Ultrasonic Studies of Copper Soap-urea Complexes Derived from Mustard and Soybean Oils

Ultrasonic velocities and densities have been measured in ternary mixtures containing copper soap-urea complexes derived from mustard and soybean oils in 20% and 40% methanol-benzene mixture to assess the effect of polarity of solvent on the micellar features of surfactant. The studies suggest that predomination of benzene (higher percentage of non-polar solvent) plays a significant role in compacting the molecular organisation of the micelles. From these values, the ultrasonic velocity (u), the specific acoustic impedance (Z), adiabatic compressibility (β), intermolecular free length (Lf ), apparent molar compressibility ( fk ), molar sound velocity (R) and primary solvation number (Sn ) have been calculated. The data clearly indicate that the values of u, Z, Sn and R increase whereas the values β and Lf decrease consistently with increase in the soap complex concentration. The critical micelle concentration (CMC) has also been determined and it has been found that CMC is dependent on the composition of the solvent mixture and composition of the edible oil. The results have been analysed in terms of Masson’s equation. The results have been explained on the basis of intermolecular interactions between solute and solvent molecules and effect of polarity and concentration were accessed on various acoustic parameters.


INtrODUCtION
Study of propagation of ultrasonic waves and their absorption forms one of the most important methods of investigation of properties of matter in all the three states. It is well known that study of absorption of ultrasonic waves in a medium provides important information about various inter and intra-molecular processes such as relaxation of the medium or the existence of isomeric states or the exchange of energy between various molecular degrees of freedom. [1][2][3][4][5][6] Derived parameters from ultrasonic velocity measurements provide qualitative information regarding the nature and strength of molecular interactions in liquid mixtures. 7,8 This has been studied for various binary and ternary mixtures with respect to variation in concentration of the liquids and temperatures. 9,10 Ultrasonic velocity measurements have been extensively used to study the molecular interactions in electrolyte solutions and to get significant information on the arrangement of molecules in aqueous electrolyte solutions. [11][12][13][14][15] In the present investigation, we have studied various thermo-acoustical parameters, such as specific acoustic impedance (Z), adiabatic compressibility (β), intermolecular free length (L f ), apparent molar compressibility, (f k ), molar sound velocity (R) and primary solvation number (S n ) to examine the ion-solvent and ion-ion interactions in the aqueous solutions of soap urea complexes made by mustard and soybean oils which have wide applications in pharmaceutical, medicinal, agricultural, environmental, textile and leather industries. [16][17][18][19][20] The present work deals with the determination of ultrasonic velocity measurements which have been used to obtain information regarding various acoustic parameters and solute-solute and solute-solvent interactions of solutions of Cu(II) soap urea complex of edible oil of mustard and soybean in pure non-polar solvents, polar and non-polar solvent mixtures (benzene and its varying compositions with methanol) to assess the effect of polarity of solvent molecules on the micellar nature of solute molecule. The studies have also been done to understand the nature of the solution and observed that the increase in the internal pressure and in lowering the compressibility of the solution. Benzene as a solvent has been used due to maximum solubility of soaps is this non-aqueous solvent.

EXPErIMENtAL
All the chemicals used were of lab reagent/analytical reagent (LR/AR) grade. Copper soap was prepared by refluxing the mustard and soybean oils (mustard oil extracted from seeds and purified, and soybean oil available in market with reputed brand) with ethyl alcohol and 2N KOH solutions for 3-4 h (direct metathesis). The neutralisation of excess KOH present was done by slow addition of 0.5N HCl. Saturated solution of copper sulphate was then added to it, for conversion of neutralised potassium soap into their corresponding copper soaps. Copper soap so obtained was then washed with warm water and 10% alcohol at 50°C and recrystallised using hot benzene. Molecular weights of copper soaps were determined from saponification values. 21 The copper soap complexes are abbreviated as copper-mustard urea (CMU) and copper-soybean urea (CSoU). Fatty acid composition of oils used for synthesis of Cu(II) soaps and their analytical and physical data are summarised in Tables 1 and 2.  The ultrasonic velocity was determined with the help of a single crystal ultrasonic interferometer model M-82 (Mittal Enterprises, New Delhi) working at a fixed frequency of 2 MHz. The temperature of water was maintained at 30°C ± 0.1°C by a thermostat through the jacket of the cell before the measurement was actually made. The measured velocities have an uncertainty of 0.5 m s −1 . Further, the operative part of the measurement involves the slow movement of micrometer till the anode current meter shows a maximum. A number of maxima readings of anode current are passed on and their number (n) is counted. The total distance (d) thus moved by the micrometer gives the value of wavelength (λ) with the help of the following relation: Once the wavelength is known, the velocity (u) in the liquid can be calculated with the help of the following relation: where f is the frequency of instrument used, i.e., 2 MHz.

ACOUStIC PArAMEtErS
The values of Z, β, L f , k z , R and S n are calculated by using the following relationships: 22 where b 0 , β, r 0 , ρ, n 0 , n, V 0 and V are adiabatic compressibility, density, numbers of moles and molar volume of solvent and solute, respectively. The expression for molar volume of solvent V 0 and soap solution V may be written as: In Equations 6, 8 and 9, M represent the molecular weight and X the mole fraction of the component. Here the subscripts 1, 2 and 3 refer to soap, methanol and benzene, respectively.

Acoustic parameters
After perusal of the observed data, it may be mentioned that the acoustic parameters and equations related to ultrasonic investigations are the same as has been enumerated for the previous soaps. The values of ultrasonic velocity and allied parameters for these complexes in methanol-benzene mixture are given in Tables 3 and 4.
A perusal of the data suggests that the values of u, Z, β, k z , R and S n increase with increasing complex concentration whereas β and L f decrease with the increase in the concentration. Z has a greater significance and as a characteristic property of the medium than either density or ultrasonic velocity individually. 23 Table 3: Ultrasonic velocity and other acoustic parameter of copper urea complex derived from mustard oil in 20% methanol-benzene mixture (CMU 20 ).   The values of Z follow the order: CMU > CSoU.
This indicates that lyophobic interaction between complex and solvent molecules is higher in CSoU than those in CMU. It may be suggested that as >C=O and NH 2 groups of urea are capable of making intermolecular hydrogen bonding with other CSoU molecules, their interactions strength contribution is highest. For both complexes, the values of β and L f decrease with increasing complex concentration which indicates that there are significant interactions between complex and solvent molecules. It is interesting to point out that the values of the parameters u, Z, β and L f with respect to chain length of the complex follow the order:

Micellar features
The plots of u, Z, β and L f against complex concentration (c) are characterised by an intersection of two straight lines corresponding to the CMC of the complex (Figures 1 to 4). It reveals that CMC is dependent on the composition of the solvent mixture. Like copper soaps derived from various edible oils in methanolbenzene mixture, copper soap-urea complex too also obeys Equation 10, showing a linear variation of ultrasonic velocity with complex concentration. The values of ϕ k increases with increasing complex concentration. The ϕ k values are negative for all urea complexes of copper soap. Literature survey reveals that negative value of ϕ k indicates electrostriction and hydrophobic interaction. 26 The negative values of ϕ k are also attributed to the loss of structural compressibility of solute molecules.
The plot of ϕ k vs. √c is characterised by an intersection of two straight lines corresponding to the CMC of the complex ( Figure 5). The ϕ k data has also been analysed in terms of Masson's equation. The parameters k2 0 z and S k have been evaluated both below and above CMC. The order of this parameter is as follow: From this order, it may be suggested that solute-solvent interaction is more pronounced in concentrate solution, i.e., above CMC as compared to below CMC, this observation is in good agreement with our apparent molar volume ϕ v interpretations for complexes.  Figure 1: Plots of u vs. c of copper soap urea complex derived from mustard and soybean oil in 20% and 40% methanol-benzene mixture.  Figure 2: Plots of Z vs. c of copper soap urea complex derived from mustard and soybean in 20% and 40% methanol benzene mixture.  Figure 3: Plots of β ad vs. c of copper soap urea complex derived from mustard and soybean in 20% and 40% methanol benzene mixture.  Figure 4: Plots of L f vs. c of copper soap urea complex derived from mustard and soybean oil in 20 % and 40 % methanol benzene mixture.
The CMC value follows the order:

CSoU > CMU
The results are in agreement with the fact that there is decrease in the CMC value with the increase in average molecular weight of the complex.

Acoustic parameters
At the very outset it may be mentioned that the acoustic parameters and related equations pertaining to ultrasonic investigations for the previously-referred system are almost the same as have been enumerated for the 40% methanol-benzene mixture and are recorded in Tables 5 and 6. The data clearly indicate that the values of u, Z, ϕ k , R and S n increase whereas the value of β and L f decreases consistently with increase in the complex concentration in varying compositions of methanolbenzene mixture. 27,28 Table 5: Ultrasonic velocity and other acoustic parameter of copper urea complex derived from mustard oil in 40% methanol-benzene mixture (CMU 40 ).  Table 6: Ultrasonic velocity and other acoustic parameter of copper urea complex derived from soybean oil in 40% methanol-benzene mixture (CSoU 40 ).

Micellar features
The plots of acoustic parameters viz. u, Z, β and L f against complex concentration are characterised by an intersection of two straight lines (Figures 1 to 4). Before CMC, there is slight curvature and after CMC, the value of acoustic parameters are in a linear manner which corresponds to the CMC of complex. It is obvious from Table 7 that the CMC is depending upon the composition of the solvent mixture. Like other copper soap and their urea complexes, these complex also obeys the equation u = u 0 + Gc. The values of G and u 0 were evaluated from the u vs. c plots and are recorded in Table 8.  This obviously suggests that the behaviour of complex in benzene-dominated environment and methanol dominated environment is quite different as they occupy different position in palisade layers of soap and complex agglomerations. 29,30 With regards to solvent mixture u, Z and k z increases with the predominance of methanol in 40% methanol-benzene solvent mixture whereas decreases with the predominance of benzene in 20% methanol-benzene mixture. The results are viceversa for β and L f for copper soap-urea complex system (Tables 3 to 6). 31,32 The values of CMC of ultrasonic parameters are higher in 40% methanol-benzene mixture as compared to 20% methanol-benzene mixture. With regards to chain length of the soap segment of the complex molecules, the CMC follows the order:

CSoU > CMU
The plots of apparent molar compressibility ϕ k against square root of complex concentration√c are characterised by an intersection of two straight lines followed by convex curve ( Figure 5) and after CMC, the value of ϕ k increases near the CMC. The values of ϕ°k are evaluated from the intercept of the plots of ϕ k vs. √c and values of S k are calculated from the slope of these plots and are recorded in (Table 8). The order of these parameters is as follows: This suggests that solute-solvent interaction is more prominent in concentrate solution, i.e., above CMC as compared to below CMC. The value of ϕ k is higher in 40% methanol-benzene mixture as compared to 20% methanol-benzene mixture. The value of ϕ k is negative for all the copper urea complexes.

CONCLUSION
The nonlinear variation of apparent molar compressibility and other thermo acoustical parameters with molar concentration of copper soap-urea complex derived from mustard and soybean oils with 20% and 40% methanol-benzene mixture shows the intermolecular interaction occurs in constituent molecules.
This provides useful information about the nature of intermolecular forces existing in the ternary mixture. The change in the behaviour below and above the CMC observed is attributed to the fact that aggregation of the soap-complex molecules is different in the ternary mixture of various concentrations which suggests that the solute-solvent interaction is more prominent in concentrate solution, i.e., above CMC as compared to below CMC. The value of ϕ k is higher in 40% methanolbenzene mixture as compared to 20% methanol-benzene mixture. The negative ϕ k values may be due to the loss of compressibility of solvent due to electrostatic forces in the vicinity of ions. 6.