Enhanced Solubility and Separation of m/p – Aminonitrobenze Using Different Hydrotropic Solution

The aqueous solubilit ies of m/p – amino nitrobenzene in d ifferent concentrations (0-3.0 mol/L) of hydrotropes such as sodium benzoate, Sodium saccharin, dimethyl benzamide at different system temperatures (303K to 333K) were studied. The percentage extraction (%E) o f mamino nitrobenzene from m/p – amino nitrobenzene mixture increases with an increase in hydrotrope concentration. A Minimum Hydrotrope Concentration (MHC) in the aqueous phase was required to initiate the significance of the %E of mamino nitrobenzene. Percentage extraction (%E) is the ratio of moles of m – amino nitrobenzene extracted in presence and absence of a hydrotrope. The sensitivity and feasibility of the proposed process are examined by carrying out solubilization and equilibrium precipitation experiments with the mixtures of various compositions. The effectiveness of hydrotropes was measured in terms of Setschenow constant Ks and reported for all hydrotropes used in this study and determination of aggregation behavior of all hydrotropes were also studied. Cost effective and eco friendly techniques of removal of less soluble compoments.


Introduction
A range of industrial mixtures having a close boiling point isomeric or non isomeric co mponents present a challenging separation problem, as in most cases conventional separation methods cannot be successfully applied. These components usually have similar chemical propert ies and molecular sizes and comparable volatilities. A simp le technique is employed, which involves either solubilization and precip itation, i.e., the solubilizat ion of the mixture in a hydrotrope solution and subsequent selective precipitation of a desired component by controlled dilut ion with water.
Hydrotropy is a unique and unprecedented solubilizat ion technique in wh ich specific chemical co mponents termed as hydrotropes can be used to affect a nu mber of fo ld increases in the solubility of sparingly soluble solutes under standard conditions [1][2][3][4].
Hydrotropic substances are a class of chemical co mbinations that are freely soluble in water. Hydrotropes are much effective at high concentrations which in turn enhance the aqueous solubility of organ ic co mpound, because of the opportunity of molecular solution structures probably in the form o f stack-type aggregates. The solubilized solute will therefore precipitate out on dilution with water fro m most hydrotropic solutions. This process may be used to recover the solute in a pure fo rm, and the remaining mother liquor may be used to concentrate the hydrotrope for recycle [5].
Even so, in modern age, we have established the aggregation behavior of co mmon hydrotropes by several techniques [6][7]. The self-aggregation of the hydrotropes has been considered to be a pre-requisite for a number of applications in various meadows such as drug solubilization [8][9][10], and boswellic acids fro m Boswellia serrata resins [13].
The current wo rk was co mmenced for the fundamental study of the global character of hydrotropes in the selective separation of a component from mixtures via solubilization and precipitation techniques. With particular attention on both the theoretical understanding of the mechanistic action and the experimental studies which demonstrate the utility of hydrotropes in the separation of commercially pro minent mixtu res [11][12][13][14][15][16][17]. The system m/p -amino nitrobenzene (molecular weight M = 138.13) was selected, for enhancing its solubility using several commercially available hydrotropes. Since m-amino n itrobenzene serves as a raw material/intermed iate for organic synthesis; p-phenylenediamine, azo dyes, antioxidants, fuel additives, corrosion inhibitors, pesticides, antiseptic agents, medicines for poultry and pharmaceutical synthesis and this makes its separation fro m any liquid mixture, which has been difficult, until now.
The separation of m/p -amino nitrobenzene through solubilization and selective precipitation is important as both these isomers have been not only close boiling points but also close melting points. The melting points of m/p -amino nitrobenzene are 114℃ and 149℃ , while the boiling points are 307℃and 332 ℃, respectively. All hydrotropes are nonreactive, non-to xic and do not produce any change in temperature effect when dissolved in water. The cheapness and easy availability are other factors considered in the selection of hydrotropes.

Materials
All the chemicals used in this work were manufactured by the Loba Chemie Pvt. Ltd., Mu mbai. With manufacturers stated purity of 99.9 %. The hydrotropes used in this work viz., sodium benzoate, Sodiu m saccharin. d imethyl benzamide are of analar grade. Double d istilled water was used for the preparation of hydrotropic solutions.

Methods
The experimental setup for conducting a single-stage batch wise liquid-liquid extraction consisted of a thermostatic bath and a separating funnel. Measurement of the solubility of m-amino nitrobenzene was carried out at temperatures of 303, 313, 323, and 333 K.
For each solubility test, an equal volu me (100mL) of m/p -amino nitrobenzene was comprehensively mixed to make a single-phase solution using a mechanical shaker. The hydrotrope solutions of different known concentrations were prepared by dilution with distilled water. Following to this, 100 ml of m/p -amino n itrobenzene mixtu re was taken and added to 100ml of hydrotrope solution of known concentration. The mixture was then made to mix consecutively for three hours. The mixture was then allowed to settle and was transferred to a separating funnel, which was immersed in a thermostatic bath with a temperature controller within ± 0.1 ℃. The setup was kept overnight for equilibrat ion. After equilibriu m was attained, the organic phase containing mamino nitrobenzene was carefu lly separated and analyzed to determine the concentration using a high-performance liquid chromatography (HPLC). The mobile phase as a 40% isopropanol /60% hexane isocratic, 1ml / min and silica co lu mn is used. All the solubility trials were conducted in duplicate runs to check their reproducibility. The %E has been calculated fro m these solubility data. The observed error was <2%.

Results and Discussion
Extracted m-amino nitrobenzene been shown in schemat ic co mparative HPLC chro matogram in Fig.1.
Experimental data on the effect of hydrotropes, i.e sodium benzoate, and Sodium saccharin and dimethyl benzamide on the percentage extractions (%E) of m-amino nitrobenzene is displayed in Figs     dimethyl benzamide is one of the hydrotropes used in this research. It was observed that the %E of m-amino nitrobenzene did not indicate any appreciab le increase until 0.20 mo l/ L of d imethyl ben zamide. However, upon a subsequent increase in the concentration of dimethyl benzamide, i.e., 0.30 mo l/ L, the %E of m-amino n itrobenzene was found to increase significantly. This concentration of dimethyl benzamide in the aqueous phase, i.e., 0.30 mo l/ L, is termed as the Minimu m Hydrotrope Concentration (MHC), wh ich is the minimu m required amount of dimethyl ben zamide in the aqueous phase to commence a significant increase in the percentage extraction of m-amino nitrobenzene. It was observed that the MHC of dimethyl benzamide in the aqueous phase does not modify even at increased system temperatures, i.e., 313, 323, and 333 K.
A related tendency in the MHC requirement has also been observed for other hydrotropes. Accordingly, it is manifest that hydrotropic separation is displayed only above MHC, irrespective of the system temperature. Hydrotropey does not seem to be functioning below the M HC, which may be a distinctive of a part icular hydrotrope with respect to each solute. The percentage extraction effect changes with concentration of the hydrotropes. In this case, a clear increasing trend in the percentage extraction o f m-amino nitrobenzene was observed above the MHC of dimethyl benzamide. This increase is affirmed only up to a certain concentration of dimethyl benzamide in the aqueous phase, i.e, 2.20 mo l/L further than which there is no appreciab le increase in the percentage extraction of m-amino nitrobenzene. This concentration of dimethyl benzamide in the aqueous phase is referred to as the maximu m hydrotrope concentration (Cmax). Fro m the analysis of the experimental data, it is observed that further increase in the hydrotrope concentration beyond Cmax does not cause any considerable increase in the percentage extraction even up to 3.0 mol/ L in the aqueous phase. Similar to the MHC values, the Cmax values of the hydrotropes also remained unaltered with the increase in system temperature. (table.1) The maximu m effectiveness of hydrotrope (φ) which is the ratio of the percentage extraction value in the presence and absence of a hydrotrope respectively. It was determined and the highest value of(φ) 93.95 in the case of dimethyl benzamide at a system temperature of 333 K ( Tab. 2).

Effectiveness of Hydrotrope
The effectiveness factor for each hydrotrope with respect to the percentage extraction of m-amino nitrobenzene at different system temperatures was determined by applying the model suggested by Setschenow and later modified by Phatak and Gaikar as given by the equation.: log (E/ Em) = Ks(Cs -C m) (1) Where E and Em are the %E values of m-amino nitrobenzene maximu m hydrotrope concentration Cs (same as Cmax) and the minimu m hydrotrope concentration Cm (same as MHC) respectively. The Setschenow constant (Ks) can be considered as a measure of the effectiveness of a hydrotrope at any given conditions of hydrotrope concentration and system temperature. The Setschenow constant values of hydrotropes, namely, sodium benzoate, Sodiu m saccharin and dimethyl benzamide for percentage extractions of m-amino nitrobenzene d ifferent system temperatures are listed in Table. 3. The highest value was observed as 0.59 in the case of dimethyl benzamide as the hydrotrope at temperature 333K..

Association Model
The solubility values were fitted into an association model for the hydrotropic solubilization which shows the aggrega-tion behavior of hydrotrope and succeeding interaction of a solute with the hydrotrope assemblies. The model describes the hydrotrope-hydrotrope and hydrotrope-solute interactions with the mass-action law, n-1 + 1 � � Through the hypothesis that hydrotrope molecules associate in a step-by-step manner to form o ligo mers and multimers such that the association constant gets to be weaker on addition of succeeding hydrotrope mo lecules. The association constant for an n-mer of hydrotrope with a monomer is related to the dimerization constant (K2, L/ mo l), i.e., Kn = K2/n.
The concentration of a monomeric hydrotrope mo lecule,[H1], is related to the total hydrotrope concentration (CS, mo l/ L) through the following equations..
3 Moreover, the model adopts that the hydrotrope assemblies co solubilize the solute, where an n-mer is capable of take up a maximu m of "(n -1)" solute molecules and that the solutes' association with the hydrotrope assemblies gets to be weaker on addition of an extra solute molecule in the same manner as the hydrotrope aggregation method.
The total solute concentration associated with all hydrotrope aggregates is given by eq 4.
The hydrotrope-solute interaction constant (KS, L/ mo l) and hydrotrope-hydrotrope interaction constant (K2) were consequently, calculated for each pair of amino nitrobenzene and hydrotrope by fitting the experimental solubility data in eq 3 and 4. The solubility data of m-amino nitrobenzene, above the MHC of each hydrotrope, was fitted into the aforesaid modified association model. The equations are nonlinear, and a nonlinear least-squares approach had to be adopted. The free solute concentration in the solution[S1], mo l/ L) was taken equal to the solubility of m-amino nitrobenzene and p-amino nitrobenzene in water, at the corresponding temperatures. The values of KS and K2 for hydrotropes at different temperatures are given in Table 4. The Association model essentially predicts an increase in the solubility of the solute. Table 4 shows that hydrotrope-hydrotrope association constant (K2) to be much smaller than that of the hydrotrope-solute interaction constant (KS) for all hydrotropes. While the hydrotrope aggregates are shaped in aqueous solutions, their aggregation trend is much weaker than that of solute-hydrotrope co aggregation. With the increase in temperature, the interaction constants KS and K2 also increase. Probably, temperature causes a significant modification in the aggregate structures, thereby causing more solutes to be solubilized in the hydro-trope solutions. It indicates that m-amino n itrobenzenedimethylbenzamide interactions are clearly the strongest compare to other hydrotrops

Mass-Transfer Coefficient
The mass transfer coefficient of the m-amino nitrobenzene + water system in the absence of any hydrotrope is 8.4 x 10 -3 s -1 at 303 K ( Table 5). The effect of different hydrotropes on the mass transfer coefficient of m-amino nitrobenzene at different hydrotrope concentrations is also given in the same table. It can be seen that a threshold value of 0.30 mo l/ L is needed to affect significant enhancement in the mass transfer coefficient of m-amino nitrobenzene + water system, as observed in the case of solubility determinations. The mass transfer coefficient of m-amino nitrobenzene + water system increases with increase in dimethyl benzamide concentration. A similar tendency in the mass transfer coef-ficient of m-amino nitrobenzene has been observed for other hydrotropes also, namely, sodium benzoate and Sodium saccharin.

Conclusions
Selective solubilization of isomeric co mb inations of m/pamino nit robenzene was determined in aqueous solutions of a number o f hydrotropes at different hydrotrope concentrations and temperatures. The MHC and Cmax values of hydrotropes with respect to m-amino nitrobenzene can be used for the recovery of the dissolved m-amino nit robenzene and hydrotrope solutions at any hydrotrope concentration between MHC and Cmax by simp le d ilution with distilled water. It was potential to ext ract 81% of the material, and the technique was optimized with respect to concentration of hydrotrope solution. Fro m the data obtained by this research to be found that individual co mponents using a step-by-step aggregation model indicated a weaker hydrotrope aggregation process but a much stronger hydrotrope-solute association. The association model p redicted the trend in the solubility of the isomers, including selectivity in the solubilization of a particular iso mer fro m the mixture. These sigmoidal-type solubility deviations are controlled by molecular structures. The differences in solubilities with hydrotrope concentration and temperature can be employed for the separation of closely related compounds. This will eliminate the huge cost and energy normally concerned in the separation of solubilized m-amino n itrobenzene fro m its solution. Hence dimethylbenzamide is found to be the best suitable hydrotrope for the enhancement of solubility of poorly soluble m-amino n itrobenzene within the framework of the current research.