Green and Cost-Effective Spectrophotometric Analysis of Diclofenac Sodium Using Mixed Hydrotropy

Abstract

Diclofenac sodium is a widely used non-steroidal anti-inflammatory drug with potent analgesic, anti-inflammatory, and antipyretic properties. However, its limited aqueous solubility poses challenges in pharmaceutical formulations and analytical quantification. To address this issue, various solubility enhancement techniques have been explored to improve its bioavailability and facilitate precise spectrophotometric analysis. Hydrotropic solubilization has been widely studied as an effective technique to improve the aqueous solubility of poorly water-soluble drugs. This study aimed to investigate an eco-friendly and efficient approach to enhance the solubility of diclofenac sodium, focusing on a mixed hydrotropic approach, ensuring reliable and accurate spectrophotometric estimation without the need for organic solvents. In this work, sodium citrate and N,N-dimethylurea were explored as hydrotropic agents. The selection was based on their complementary solubilization mechanisms. Sodium citrate enhances solubility through ionic interactions, whereas N,N-dimethylurea disrupts the water structure and forms hydrogen bonds with the drug. The results showed a synergistic effect of the combined use of these agents with a solubility of 63 mg/mL, which was significantly higher than that of individual use. Besides, the Fourier transform infrared (FTIR) spectroscopy confirmed hydrogen bonding when diclofenac sodium was dissolved in the hydrotropic blend, and X-ray diffraction analysis demonstrated a decrease in the crystallinity of the drug, indicating a transition to a more soluble amorphous state. The solubility studies, molecular docking, and recovery experiments confirmed the high accuracy and precision of the proposed method with percentage recoveries near 100%. In conclusion, this green and cost-effective approach eliminates the need for toxic organic solvents, making it a sustainable and scalable solution for the analysis and formulation of poorly water-soluble drugs.

Keywords

diclofenac sodium - drug solubility - mixed hydrotropy - hydrotropic agents - spectrophotometer

Introduction

Diclofenac sodium is a widely used non-steroidal anti-inflammatory drug (NSAID) that is effective in managing pain, inflammation, and fever. Despite its therapeutic benefits, its poor aqueous solubility presents significant challenges in drug formulation and analysis.[1] Enhancing its solubility is critical to improving its bioavailability and facilitating accurate analytical quantification, particularly in spectrophotometric methods. Traditional approaches for solubility enhancement often involve organic solvents, which are costly, toxic, and environmentally unsustainable.[2]

The concept of mixed hydrotropy has emerged as an innovative, eco-friendly, and economical technique for addressing solubility challenges associated with poorly water-soluble drugs.[3] Hydrotropy involves the use of hydrotropic agents—compounds that improve the solubility of poorly soluble substances in water through non-micellar mechanisms such as hydrogen bonding and π–π interactions. Mixed hydrotropy, in particular, leverages the synergistic effect of combining two or more hydrotropic agents, resulting in a significant enhancement of solubility beyond the capacity of individual agents.[4] [5] [6] Several studies have shown that combining multiple hydrotropic agents enhances solubility more effectively than individual hydrotropes, likely due to cooperative interactions such as hydrogen bonding and micellar-like aggregation.

This study focused on the application of mixed hydrotropy to enhance the aqueous solubility of diclofenac sodium and eliminate the need for organic solvents in the spectrophotometric analysis of the drug. By employing an appropriate combination of hydrotropic agents, this approach aims to achieve (1) improved solubility and stability of diclofenac sodium in aqueous media and (2) the development of a cost-effective and sustainable analytical method for poorly water-soluble drugs. In this study, a carefully selected hydrotropic blend was used, considering its ability to enhance diclofenac sodium solubility while maintaining chemical stability and avoiding precipitation upon dilution.[7] [8] [9] [10] [11] [12] The present work highlights the potential of mixed hydrotropy as a robust and sustainable tool for pharmaceutical analysis, particularly in overcoming solubility-related challenges in drugs like diclofenac sodium.[13] [14] [15] This approach not only enhances analytical accuracy but also supports environmentally responsible research practices.[16] The current research intends to improve diclofenac sodium solubility through the use of a mixed hydrotropy technique.

Materials and Methods

Diclofenac sodium was obtained from M/S Arrow Pharmaceutical, Indore, Madhya Pradesh, India. Diclofenac sodium tablets were obtained from two separate firms: Lupin Pharmaceutical Industries Ltd. (Indore, Madhya Pradesh, India) and Sun Pharmaceutical Industries Ltd. (Mumbai, Maharashtra, India). Sodium citrate and N,N-dimethylurea were purchased from Gujarat Enterprise (Ahmedabad, Gujarat, India) and Loba Chemie Pvt. Ltd. (Mumbai, Maharashtra, India). Analytical-grade chemicals were utilized. A UV–Visible spectrophotometer (Shimadzu, Model 1800) was used for spectrophotometric analysis.

Preliminary Solubility Studies

A hydrotropic solution was prepared by dissolving a blend of hydrotropic agents in distilled water. The composition of the hydrotropic blend solution included 20% N,N-dimethylurea (w/v), and 20% sodium citrate (w/v), which acted as solubilizing agents to enhance the solubility of diclofenac sodium. To assess the drug's solubility in various solutions, an ample quantity of the drug was introduced into a 25-mL vial filled with distilled water, a hydrotropic solution, as well as individual hydrotropes (20% w/v sodium citrate in distilled water or 20% w/v N,N-dimethylurea in distilled water) at room temperature. After the vial cap was secured and the aluminium seal was applied, for 12 hours at room temperature, the vial was subjected to mechanical shaking within an orbital flask shaker. The solution was given 24 hours to reach equilibrium without any disturbance. Afterward, filtration was performed by Whatman filter paper 41. To calculate the absorbance at 277 nm against reagent blanks, the filtrate was correctly diluted with distilled water.

Preparation of Calibration Curve of Diclofenac Sodium

For the preparation of the calibration curve, diclofenac sodium (50 mg) and hydrotropic solution (8 mL) were accurately weighed and transferred into a volumetric flask with a capacity of 10 mL. After accurate dissolution, by shaking the flask, the drug was completely dissolved, and an additional blend was introduced to bring the volume up to 10 mL. Standard solutions at different concentrations (10, 20, 30, 40, 50, and 60 μg/mL) were prepared from this stock solution through appropriate dilution using distilled water. At 277 nm against the respective reagent blank, the absorbance for each solution was noted.

Proposed Method for Analysis

The tablet (I) powder equal to 50 mg of diclofenac sodium and 8 mL of a hydrotropic solution were introduced into a volumetric flask with a capacity of 10 mL. The vial was immediately agitated for 15 minutes, and a hydrotropic solution was introduced to reach a final volume of 10 mL. The solution was filtered with Whatman filter paper 41 to remove the tablet excipients. A total of 0.6 mL of the filtrate was diluted to 100 mL with distilled water. The absorbance at 277 nm compared to the reagent blank was recorded. A similar procedure was applied to the tablet (II). The findings were recorded in [Table 1].

Table 1 Analysis of diclofenac sodium tablet with statistical evaluation (n = 3)

Tablet	Claimed amount of drug mg/tablet	Percent drug estimated (mean ± standard deviation)	Percent coefficient of variation	Standard error
I (Lupin)	100	98.94 ± 0.68	0.69	0.40
II (Sun Pharma)	100	98.90 ± 0.29	0.30	0.17

Recovery Studies

To conduct the recovery investigations, standard diclofenac sodium drug (20 mg and 40 mg, respectively) was introduced to the preassessed tablet powder corresponding to 100 mg diclofenac sodium. By the suggested method, the drug concentration was assessed. [Table 2] summarizes the findings of the investigation and provides statistical analysis.

Table 2 Statistically analyzed results of recovery experiments (n = 3)

Tablet	Drug content in preliminarily investigated tablet powder (mg)	Quantity of standard drug added (mg) (spiked)	Percent drug estimated (mean ± standard deviation)	Percent coefficient of variation	Standard error
I (Lupin)	100	20	98.12 ± 0.25	0.26	0.15
I (Lupin)	100	40	98.40 ± 0.18	0.19	0.11
II (Sun Pharma)	100	20	98.45 ± 0.58	0.59	0.34
II (Sun Pharma)	100	40	98.76 ± 0.27	0.27	0.16

Fourier Transform Infrared Spectroscopy Analysis

Fourier transform infrared (FTIR) spectroscopy analysis was carried out using a Bruker Alpha II FTIR spectrometer (Bruker, Germany). The samples were prepared by mixing with potassium bromide (KBr) and compressed into pellets. The spectra were recorded in the range of 4,000 to 400 cm⁻¹ with a resolution of 4 cm⁻¹, averaging 32 scans per sample.

X-Ray Diffraction Analysis

X-ray diffraction (XRD) analysis was performed using a PANalytical X'Pert PRO X-ray diffractometer (Malvern Panalytical, Netherlands) with Cu-Kα radiation (λ = 1.5406 Å). The diffraction patterns were collected in the 2θ range of 5 to 80 degrees, with a step size of 0.02 degree and a scan speed of 1 degree/min. The analysis was conducted at room temperature under standard conditions.

Molecular Docking

Molecular docking studies were performed using AutoDock Vina (The Scripps Research Institute) to predict the interaction between diclofenac sodium and hydrotropic agents. The 3D structure of diclofenac sodium was obtained from the PubChem database (CID: 3033), while the hydrotropic agents' structures were drawn using ChemSketch. The docking calculations were performed using the Lamarckian Genetic Algorithm with a grid box size of 40 × 40 × 40 Å and an exhaustiveness value of 8. The binding energy and interaction profiles were analyzed using PyMOL and Discovery Studio Visualizer.

Data Statistics

All solubility experiments were conducted in triplicate (n = 3) and expressed as mean ± standard deviation. The results were statistically analyzed using one-way analysis of variance (ANOVA), yielding a significant difference (F = 409.46, p < 0.05).

Results and Discussion

The calibration curve of diclofenac sodium in a hydrotropic solution is shown in [Fig. 1]. The aqueous solubility of diclofenac sodium at room temperature is established at 11.5 mg/mL, while in a blend solution, this solubility is notably higher at 63 mg/mL. The combination of sodium citrate and N,N-dimethylurea demonstrated a synergistic effect, significantly improving solubility (63.00 ± 1.00 mg/mL) compared to individual hydrotropes (22.33 ± 2.52 mg/mL for sodium citrate, 17.67 ± 2.52 mg/mL for N,N-dimethylurea).

Fig. 1 Calibration curve of diclofenac sodium.

Employing a spectrophotometric analysis through the mixed hydrotropy technique for diclofenac sodium tablets, the mean percent estimations range from 98.90 to 98.94 ([Table 1]), closely approximating the theoretical value of 100, thereby attesting to the precision of the proposed methodology. The method is further validated by consistently low values for standard deviation (ranging from 0.29 to 0.68), percent coefficient of variation (ranging from 0.30 to 0.69), and standard error (ranging from 0.17 to 0.40). Additionally, [Table 2] illustrates that the mean percent recoveries, as determined by the proposed method, fall within the range of 98.12 to 98.76, reinforcing the accuracy of the analytical approach. The validation of this method is substantiated by consistently low statistical parameters, including standard deviation (ranging from 0.18 to 0.58), percent coefficient of variation (ranging from 0.19 to 0.59), and standard error (ranging from 0.11 to 0.34).

The choice of sodium citrate and N,N-dimethylurea was guided by their individual solubilizing properties and potential for synergistic action. Sodium citrate enhances solubility through ionic interactions with the drug, stabilizing its molecules in solution. N,N-dimethylurea disrupts the hydrogen bonding network of water, reducing cohesive forces and increasing the solvent's ability to interact with the drug. The combination of these agents leverages their complementary mechanisms, as predicted by the solubility parameter theory and Hansen solubility parameters, which identify compatible hydrotropic blends based on molecular interactions.[17]

Previous studies have demonstrated a significant 250-fold enhancement in diclofenac sodium solubility using 20% urea and 10% sodium citrate as hydrotropic agents.[16] Inspired by this, our study explores an alternative approach by replacing urea with N,N-dimethylurea. This substitution was made to optimize solubilization while potentially reducing urea-related limitations such as thermal instability and unwanted interactions. Sodium citrate was selected due to its buffering capacity and high hydrotropic efficiency. The results indicate that this novel hydrotropic combination effectively enhances solubility while maintaining chemical stability, providing a promising alternative to previously reported systems.

The enhanced solubility of diclofenac sodium in the mixed hydrotropic solution can be attributed to multiple mechanisms.

• Hydrogen bonding: FTIR spectroscopy revealed shifts in the carbonyl (–COOH) group of diclofenac sodium when dissolved in the hydrotropic blend, confirming the formation of hydrogen bonds.

• Ionic interactions: Sodium citrate stabilizes the drug by forming ionic bonds, reducing the energy required for dissolution.

• Water structuring disruption: N,N-dimethylurea disrupts water's cohesive forces, creating a less structured environment that facilitates solubilization.

• Synergistic effect: The combination of these agents achieves a solubility of 63 mg/mL, far exceeding the solubility achieved with sodium citrate (22 mg/mL) or N,N-dimethylurea (18 mg/mL) alone.

Preliminary solubility studies confirmed the superior performance of the mixed hydrotropic blend. Molecular docking simulations quantified binding affinities, showing stronger interactions between diclofenac sodium and the mixed agents compared to individual agents ([Table 3]). XRD analysis demonstrated a reduction in the crystallinity of the drug, indicating its transition to an amorphous, more soluble state ([Fig. 2]).

Table 3 Molecular docking binding affinities of diclofenac sodium with hydrotropic agents

Hydrotropic system	Binding affinity (kcal/mol)
Sodium citrate (20%)	−5.8
N,N-Dimethylurea (20%)	−6.1
Combination (20% + 20%)	−7.4

Fig. 2 Simulated XRD spectra of diclofenac sodium in pure and hydrotropic forms. XRD, X-ray diffraction.

The FTIR analysis of diclofenac sodium in the hydrotropic blend showed a shift in the C = O stretching frequency from 1,666.5 cm⁻¹ to 1,672.3 cm⁻¹, indicating possible hydrogen bonding interactions between the drug and hydrotropic molecules ([Fig. 3]). Additionally, the solubility studies demonstrated a 5.3-fold increase in drug solubility compared to water alone, confirming the effectiveness of the hydrotropic blend. These findings suggest that the primary solubilization mechanism involves hydrogen bonding and the formation of hydrotrope–drug complexes, which enhance drug dissolution.

Fig. 3 FTIR spectra of pure diclofenac sodium and hydrotropic blend. FTIR, Fourier transform infrared.

The proposed method offers significant advantages, including eco-friendliness, cost-effectiveness, and the elimination of toxic organic solvents. The validated methodology provides a scalable solution for the solubility enhancement of poorly water-soluble drugs, with potential applications in pharmaceutical analysis and formulation development.

Conclusion

The study demonstrates that the mixed hydrotropic blend of sodium citrate and N,N-dimethylurea is an effective and eco-friendly solution for enhancing the solubility of diclofenac sodium, a poorly water-soluble drug. The theoretical selection criteria, based on solubility parameter theory and Hansen solubility parameters, combined with experimental validation, underscore the practical utility and scalability of this method to enhance poorly water-soluble drugs. The findings pave the way for further applications in pharmaceutical analysis and green chemistry formulations.

Conflict of Interest

None declared.

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Publication History

Received: 27 December 2024

Accepted: 26 March 2025

Article published online:
28 April 2025