Effect of Hydrotropes on Solubility and Mass Transfer Coefficient of p-Nitrobenzoic Acid
D. Gnana Prakash,
S. Thenesh Kumar
N. Nagendra Gandhi
The effect of various hydrotropes such as sodium benzoate, sodium salicylate and nicotinamide on the solubility and mass transfer coefficient of p-nitrobenzoic acid was investigated. The solubility studies were carried out under a wide range of hydrotrope concentrations 0 to 3.0 kmol m-3 and different system temperatures (303-333 K). It has been observed that the solubility and mass transfer coefficient of p-nitrobenzoic acid increases with increase in hydrotorpe concentration and also with system temperature. The maximum enhancement factor, which is the ratio of the value in the presence and absence of a hydrotrope, has been determined for all sets of experimentations. The effectiveness of hydrotropes was measured in terms of Setschenow constant Ks and reported for all hydrotropes used in this study.
Hydrotropy is a unique and unprecedented solubilization technique in which
certain chemical compounds termed as hydrotropes can be used to effect a several
fold increase in the solubility of sparingly soluble solutes under normal conditions
(Neuberg, 1916). This increase in solubility in water
is probably due to the formation of organized assemblies of hydrotrope molecules
at critical concentrations (Badwan et al., 1982;
Balasubramanian et al., 1989).
Hydrotropes are water-soluble and surface-active compounds, which can significantly
enhance the solubility of organic solutes such as esters, acids, alcohols, aldehydes,
ketones, hydrocarbons and fats (Friberg and Brancewicz,
1994; Raynaud Lacroze and Tavare, 1993; Laxman
and Sharma, 1990; Maheswari et al., 2006;
Gaikar and Phatak, 1993; Colonia
et al., 1998). The solubility enhancement in the organic compounds
could be due to the formation of molecular structures in the form of complexes
(Hodgdon and Kaler, 2007; Ooya et
al., 2005; Rovetto et al., 2006). Easy
recovery of dissolved solute and possible reuse of hydrotrope solutions makes
this most attractive particularly at industrial levels.
Besides the advantage of certain properties such as the solvent character being
independent of pH, non-flammability, easy availability of hydrotropes, inexpensive
aqueous phase makes this method superior to other solubilization methods (Agarwal
and Gaikar, 1992a, b; Gaikar
and Sharma, 1986; Mahapatra et al., 1988;
Friberg et al., 1996; Gaikar
and Phatak, 1999).
Hydrotropy is a process which goes beyond other conventional solubilization
methods such as miscibility, co-solvency, salting-in etc., since it offers high
selectivity and unprecedented increase in solubility and mass transfer coefficient.
The problem of emulsification, which is normally encountered with surfactant
solution, is not found with hydrotrope solution (Hefa and
David, 2007). The effect of hydrotropes on the solubility and mass transfer
coefficient for a series of organic esters such as butyl acetate, ethyl benzoate,
amyl acetate, methyl salicylate and benzyl acetate was studied in our earlier
publications (Nagendra Gandhi et al., 1998a,
b; Nagendra Gandhi and Dharmendira
Kumar, 2000a, b; Meyyappan and
Nagendra Gandhi, 2004, 2005).
It has been observed that, in many two-phase reaction systems involving a sparingly
soluble organic compound like p-nitrobenzoic acid, the mass transfer
coefficient was found to be very low solely due to the poor solubility of p-nitrobenzoic
acid in the aqueous phase. Since, p-nitrobenzoic acid serves as raw material/intermediate
for a wide variety of chemicals and allied products and the separation of p-nitrobenzoic
acid from any liquid mixture seems to be difficult, this hydrotropic technique
can be adapted to increase the solubility as well as to separate such mixtures
effectively (Agrawal et al., 2004; Dandekar
et al., 2008). Data on various aspects of hydrotropic study on the
solubility and mass transfer coefficient for p-nitrobenzoic acid-water
system are reported for the first time.
MATERIALS AND METHODS
This study was carried out from November 2008 to February 2009 in Department of Chemical Engineering, St. Peters Engineering College, Chennai-600 054, Tamil Nadu, India. All the chemicals used in this study were procured from S.D. Fine-Chem Ltd., Mumbai with a manufacturers stated purity of 99%.
The experimental setup for the determination of solubility values consists of a thermostatic bath and a separating funnel. For each solubility test, an excess amount of powdered solid was taken in a separating funnel and 100 mL hydrotrope solution of known concentration was added. The separating funnel was immersed in a constant-temperature bath fitted with a temperature controller which could control the temperature within +0.1°C. The setup was kept overnight for equilibration. After equilibrium was attained, the solution was filtered from the remaining solid. The concentration of the dissolved organic acid in aqueous hydrotrope solutions was analyzed by titrating against standardized sodium hydroxide solution using 0.2 mL phenolphthalein solution as an indicator. Blank titration was carried out and necessary correction was done to calculate the dissolved organic acid in aqueous hydrotrope solutions. All the solubility experiments were conducted in duplicate to check the reproducibility. The observed error in the reproducibility was <2%.
The experimental setup for the determination of the mass transfer coefficient
consisted of a vessel provided with baffles and a turbine impeller run by a
motor to agitate the mixture. The vessel used for mass transfer studies is of
height 40 cm and inner diameter 15 cm. The turbine impeller diameter is 5 cm,
the width is 1 cm and the length is 1.2 cm. It has four blades. The baffle is
40 cm height with a diameter of 1.5 cm. There are about 4 baffles that rotate
at a speed of 600 rpm (Meyyappan and Nagendra Gandhi, 2004,
For each run, to measure the mass transfer coefficient, an excess amount of powdered solid was added to the aqueous solution of the hydrotrope of known concentration. The sample was then agitated for a known time of 600, 1200, 1800 and 2400 sec. After the end of fixed time t, the entire mixture was transferred to a separating funnel. After allowing the mixture to stand for some time, the solution was filtered from the remaining solid. The concentration of the solubilized organic acid in aqueous hydrotrope solutions at time t was analyzed in the same way as done for solubility determinations. A plot of -log (1-Cb/C*) versus t is drawn, where Cb is the concentration of p-nitrobenzoic acid at time t and C* is the equilibrium solubility of p-nitrobenzoic acid at the same hydrotrope concentration. The slope of the graph gives KLa/2.303, from which KLa, the mass transfer coefficient was determined. Duplicate runs were made to check the reproducibility. The observed error was <2%.
Solubility: Experimental data representing the effect of hydrotropes,
i.e., sodium benzoate, sodium salicylate and nicotinamide on the solubility
of p-nitrobenzoic acid are presented in Table 1-3.
The solubility of p-nitrobenzoic acid in water at 303 K in the absence
of any hydrotrope is 2.09×10-3 kmol m-3 (Table
1). The solubilization effect varies with concentration of hydrotropes (Table
1). From Table 1-3 it is evident that
hydrotropy was operational above the certain significant hydrotrope concentration
called Minimum Hydrotrope Concentration (MHC) of sodium benzoate (0.30 kmol
m-3), sodium salicylate (0.4 k mol m-3) and nicotinamide
(0.5 kmol m-3) irrespective of system temperature.
||Effect of sodium benzoate concentration (C) on the solubility
(S) of p-nitrobenzoic acid in water
||Effect of sodium salicylate concentration (C) on the solubility
(S) of p-nitrobenzoic acid in water
||Effect of nicotinamide concentration (C) on the solubility
(S) of p-nitrobenzoic acid in water
This MHC value assumes greater significance in the context of recovery of
hydrotrope solutions. This increasing trend is maintained only up to a certain
concentration of hydrotrope in the aqueous solution, beyond which there is no
appreciable increase in the solubility of p-nitrobenzoic acid. This concentration
of hydrotrope in the aqueous solution is referred to as the maximum (Table
4), which seem to depend on the hydrophilicity of a hydrotrope. As can be
seen from Table 4, Cmax values of sodium benzoate,
sodium salicylate and nicotinamide are 2.50, 2.00 and 2.00 kmol m-3,
|| MHC and Cmax values for Hydrotropes
||Maximum solubilization enhancement factor (φs)
of p-nitrobenzoic acid
The maximum solubilization enhancement factor (φs) which is
the ratio of solubility values in the presence and absence of hydrotrope, ranges
between 5.12 and 21.08. The low value of φs 5.12 was observed
in the presence of nicotinamide at a system temperature of 303 K. The highest
value of φs 21.08 has been observed in the case of sodium benzoate
at a system temperature of 333 K (Table 5).
Mass transfer coefficient: The mass-transfer coefficient of p-nitrobenzoic
acid+water system in the absence of any hydrotrope was determined as 0.96×10-3
s-1 at 303 K (Table 6). The reported mass transfer
coefficient enhancement (φmtc) of p-nitrobenzoic acid
of hydrotrope solutions are 15.20 for sodium benzoate, 10.10 for sodium salicylate
and 5.43 for nicotinamide at their maximum hydrotrope concentrations. The highest
value of φmtc (15.20) has been observed in the presence of sodium
benzoate as hydrotrope at Cmax of 2.50 kmol m-3.
Effectiveness of hydrotropes: The effectiveness factor of each hydrotrope with respect to p-nitrobenzoic acid at different system temperatures has been determined by analyzing the experimental solubility data for each case applying the model suggested by Setschenow (1951) and later modified by Pathak and Gaikar (1992), as given by the equation:
where, S and Sm are the solubility of p-nitrobenzoic acid at any hydrotrope concentration Cs and the minimum hydrotrope concentration Cm, respectively.
The Setschenow constant values of hydrotropes namely sodium benzoate, sodium salicylate and nicotinamide for p-nitrobenzoic acid+water system at different system temperatures are listed in Table 7.
||Effect of hydrotrope concentration (C) on the mass transfer
coefficient (kLa) of p-nitrobenzoic acid
||Setschenow constant (Ks) of hydrotropes with respect
to p-nitrobenzoic acid
The solubility of p-nitrobenzoic acid in water is 2.09x10-3 kmol
m-3 at 303 K, compared to 1.197×10-3 kmol m-3
at 288 K. Thus, the solubility values in water are in excellent agreement with
the earlier reported values (John, 1987; Perry,
1997). Sodium benzoate is one of the hydrotropes used in this study. It
was observed that the solubility of p-nitrobenzoic acid did not show
any appreciable increase up to addition of 0.30 kmol m-3 of sodium
benzoate. On subsequent increase in the concentration of sodium benzoate above
0.30 kmol m-3, the solubility of p-nitrobenzoic acid in water
was found to increase significantly. This concentration of sodium benzoate in
the aqueous phase is termed as Minimum Hydrotrope Concentration (MHC), which
is the minimum required sodium benzoate concentration in the aqueous phase to
effect significant increase in the solubility of p-nitrobenzoic acid
in water (Miguel et al., 2007). It has been observed
that the MHC of sodium benzoate in the aqueous phase does not vary even at increased
temperatures. A similar trend in MHC requirement in the aqueous phase has also
been observed for other hydrotropes. In the present case, a clear increasing
trend in the solubility of p-nitrobenzoic acid was observed above the
MHC of sodium benzoate. This increase in solubility of p-nitorbenzoic
acid is maintained up to 2.50kmol m-3 of sodium benzoate in the aqueous
phase, beyond which there is no appreciable increase in the solubility of p-nitrobenzoic
acid. This concentration of sodium benzoate in aqueous phase is referred to
as the maximum hydrotrope concentration (Cmax). From the analysis
of the experimental data, it is observed that further increase in hydrotrope
concentration beyond Cmax does not bring any appreciable increase
in the solubility of p-nitrobenzoic acid even up to 3.00 kmol m-3
of sodium benzoate in the aqueous solution. Similar to the MHC values, Cmax
values of hydrotropes also remained unaltered at increased system temperatures.
The knowledge of MHC and Cmax values of each hydrotrope with respect
to a particular solute assumes greater significance in this study since, it
indicates the beginning and saturation of the solubilization effect of hydrotropes.
The values of MHC and Cmax of a hydrotrope with respect to p-nitrobenzoic
acid may be useful in determining the recovery of the dissolved p-nitrobenzoic
acid by simple dilution with distilled water. This is the unique advantage of
the hydrotropic solubilization technique. Since, hydrotropy appears to operate
only at significant concentrations of hydrotrope in water, most hydrotropic
solutions release the dissolved p-nitrobenzoic acid on dilution with
water below MHC. The knowledge of MHC values is necessary especially at industrial
levels, as it ensures ready recovery of the hydrotrope for reuse. It can further
be observed from results that, in order to achieve the particular solubility
of p-nitrobenzoic acid, say 16.6x10-3 kmol m-3,
the sodium benzoate concentration should be 1.00kmol m-3 at 303 K,
0.90 kmol m-3 at 313 K, 0.70 kmol m-3 at 323 K and 0.60
kmol m-3 at 333 K in the aqueous solution. Thus, it can be seen that
as the system temperature increases, the concentration of sodium benzoate required
in the aqueous phase to achieve a particular solubility of p-nitrobenzoic
acid decreases. A similar trend has been observed for other systems also. It
has also been observed that the solubilization effect of sodium benzoate was
not a linear function of the concentration of the sodium benzoate. The solubilization
effect of sodium benzoate increases with increase in hydrotrope concentration
and also with system temperature (Wagle et al., 2007).
The effect of different hydrotropes on the mass transfer coefficient of p-nitrobenzoic acid at different hydrotrope concentrations is also given in the Table 6. It can be seen that a threshold value of 0.30 kmol m-3 is required to effect significant enhancement in the mass transfer coefficient of p-nitrobenzoic acid+water system, as observed in the case of solubility determinations. The mass-transfer coefficient of p-nitrobenzoic acid+water system increases with increase in sodium benzoate concentration. Beyond a Cmax of 2.50 kmol m-3 there is no appreciable increase in the mass transfer coefficient of p-nitrobenzoic acid, as observed in the case of solubility determinations. The maximum enhancement factor for mass transfer coefficient of p-nitrobenzoic acid+water system in the presence of sodium benzoate was found to be 15.20 (Table 6). A similar trend in the mass transfer coefficient enhancement (φmtc) of p-nitrobenzoic acid has been observed for other hydrotropes also namely sodium salicylate and nicotinamide. 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 highest value has been observed as 0.531 in the case of sodium benzoate as hydrotrope at 333 K. The order of effectiveness of various hydrotropes based on Ks values is given by sodium benzoate>sodiumsalicylate> nicotinamide.
The solubility of p-nitrobenzoic acid, which is practically insoluble in water, has been increased to a maximum of 21.08 times in the presence of sodium benzoate as hydrotrope. The mass transfer coefficient was also found to increase to a maximum value of 14.59×10-3 sec-1 and with an enhancement factor of about 15.20 times in the presence of sodium benzoate as hydrotrope. This would be useful in increasing the rate of output of the desired product made from p-nitrobenzoic acid. The MHC and Cmax values of the hydrotrope with respect to p-nitrobenzoic acid can be used for the recovery of the dissolved p-nitrobenzoic acid and hydrotrope solutions at any hydrotrope concentration between the MHC and Cmax by simple dilution with distilled water. This will eliminate the huge cost and energy normally involved in the separation of the solubilized p-nitrobenzoic acid from its solution. The unprecedented increase in the solubilizing effect of hydrotropes is attributed to the formation of organized aggregates of hydrotrope molecules at a particular concentration.
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