Influence of surfactants and surfactant-coated IONs on the rate of alkaline hydrolysis of procaine in the presence of PEG

2.1 Reagents

Procaine HCl (98%), obtained from TCI, Tokyo, Japan. Polyethylene glycol with molecular weight 1500, 4000, 6000, and 8000, Ferrous chloride dihydrate (99%), CTABr (99%), SDS (99%) and Ferric chloride (97%) were acquired from CDH, New Delhi, India. 25% Ammonium hydroxide solution with 99% purity was obtained from Thermo Fisher Scientific, Mumbai, India and sodium hydroxide (97%) was procured from Merck, Mumbai, India to carry out the studies.

These reagents were of analytical grade and used without further purification. The stock solution of procaine hydrochloride was prepared in 99.9% ethanol, while all other solutions were prepared in double distilled water.

2.2 Synthesis of Surfactant-Coated iron oxide nanocomposites (Surf-IONs)

Surfactant coated magnetite nanocomposites were synthesised by mixing the surfactants (CTABr and SDS) in an appropriate amount during the co-precipitation of Fe³⁺ and Fe²⁺ salts. The ferric and ferrous salts were taken in 2:1 M ratio (Fe³⁺; 4.0 × 10⁻¹ mol dm⁻³ and Fe²⁺; 2.0 × 10⁻¹ mol dm⁻³) and dissolved in 300 mL of 2.0 × 10⁻¹ mol dm⁻³ surfactant solution taken into a 1000 mL capacity conical flask. The mixture containing FeCl₃, FeCl₂·2H₂O and the surfactant (SDS or CTABr) was then purged with N₂ gas to remove any oxygen and stirred strenuously for an hour to ensure absolute mixing. Co-precipitation was achieved by adding 200 mL of 25% ammonium hydroxide solution (liquor ammonia) and a few mL of 2.0 mol dm⁻³ NaOH dropwise in the reaction mixture. Then the temperature of the mixture was elevated to 60 °C while continuously shaking and purging Nitrogen gas for 4 h. The precipitated magnetite nanoparticles were separated using a magnet and washed with ethanol and de-ionised water until the pH comes to neutral. The freshly prepared nanoparticles were then dispersed in the surfactant solution (CTABr or SDS), and the solution was sonicated at room temperature in an ultrasonic bath (LABMAN scientific instruments) for 30 min to ensure the maximum yield of surface-modified nanoparticles. The surfactant-coated nanocomposites were separated using a magnet and washed with acetone and de-ionised water to remove any unbound surfactant molecules and then dried for 3–4 h under vacuum and used for kinetic studies and characterisations.

The synthesised surf-IONs were characterised for FT-IR spectra using Nicolet iS50 FT-IR Spectrometer (Thermo Fisher Scientific, Madison, USA). The XRD patterns of the nanocomposites were recorded by MiniFlex II, X-ray diffractometer (Rigaku, Japan) provided with a Cu Kα radiation source (λ = 1.5406 nm). The Thermogravimetric analyses (TGA) of the nanocomposites were done by using Pyris 1 Thermogravimetric analyser (Perkin Elmer). Vibrating sample magnetometer (VSM), MicroMag-3900 (Princeton, USA) was used to determine the magnetic character of the nanocomposites.

2.3 Kinetic measurements

The kinetic measurements were performed under the pseudo-first-order reaction conditions (i.e. [NaOH]≫ [Procaine]), using GENESYS 10S UV/VIS spectrophotometer (Thermo Fisher Scientific, Madison, USA). A fall in the absorbance for the hydrolysis of procaine was measured at a wavelength of 291 nm with respect to time. The absorbance of the samples was measured in a 3.0 mL quartz cuvette having a path length of 1 cm. A calculated amount of reactants, surfactants and/or surf-IONs were mixed in a 100 mL three-necked round bottom flask and held in a water bath (Ferrotek Equipments, Ghaziabad, India) to keep the temperature constant at 37.0 ± 0.2 °C for 1 h to equilibrize the mixture. The reaction initiated immediately after the introduction of NaOH into the reaction vessel. The kinetic measurements were set in the spectrophotometer at a time gap of 300 s, and the reaction was tracked until the completion of 3 half-life periods. The rate constants were determined by the slope of the plot of ln (absorbance) versus time. Multiple kinetic runs were repeated for every set of reaction to minimize random errors, and the values of rate constants were observed to be consistent under the error limit of ±5%.

3.1 Characterization of Surfactant-coated iron oxide nanocomposites

3.1.1

3.1.1 X-ray diffraction (XRD)

Fig. 1 represents the XRD patterns of the synthesised surf–IONs, which confirm the crystalline nature of the nanocomposites. The diffraction peaks are visible at 2θ = 30.26° (2 2 0), 35.5° (3 1 1), 43.12° (4 0 0), 53.74° (4 2 2), 57.10° (5 1 1) and 62.92° (4 4 0). The sharp peaks signify the ultrafine nature and high crystallinity of the prepared surf–IONs having the cubic structure (Mahadevan et al., 2007).

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Fig. 1

XRD patterns of the synthesised SDS-IONs (a), and CTABr-IONs (b).

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3.1.2

3.1.2 Fourier transform infrared spectroscopy (FTIR)

Fig. 2 (a) and (b) show the FTIR spectra of SDS–IONs and CTABr–IONs, respectively. The peaks arising at 3431 cm⁻¹ in (a) and 3430 cm⁻¹ in (b) are due to O–H stretching vibration arising by the presence of moistures on the surfaces of Surf–IONs. The peaks at 2922 cm⁻¹ and 2854 cm⁻¹ in (a) and at 2920 cm⁻¹ and 2850 cm⁻¹ in (b) are assigned to CH band vibrations of the –CH₂ group of SDS and CTABr. The H-O–H bending of H₂O appears at 1635 cm⁻¹ and 1631 cm⁻¹ in (a) and (b) respectively (Saranya et al., 2015). The peaks at 1460 cm⁻¹ in (a) and 1465 cm⁻¹ in (b) represents the –CH₂ group bending vibration in SDS and CTABr. The S = O stretching vibrations peak for SDS–IONs can be seen at 1220 cm⁻¹. The peak at 964 cm⁻¹ in (a), corresponds to the out-of-plane bending vibration of C–H bond. The peaks at 547 cm⁻¹ and 474 cm⁻¹ in (a) and 566 cm⁻¹ and 475 cm⁻¹ in (b) are attributed to the Fe-O bond of Fe₃O₄ and the 2 peaks confirms the spinal structure of Fe₃O₄ (Aliramaji et al., 2015). The differentiation in the Fe-O bond length in magnetite molecule is the reason for 2 peaks for Fe–O bonds.

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Fig. 2

FTIR spectra of the synthesised SDS-IONs (a), and CTABr-IONs (b).

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3.1.3

3.1.3 Vibrating sample magnetometer (VSM)

The magnetic character of the Surf–IONs was studied using VSM and the hysteresis curve of SDS–IONs and CTABr–IONs are shown in Fig. 3. The saturation magnetisation for SDS–IONs was observed at 25.28 emu/g and for CTABr–IONs at 55.94 emu/g (Lu et al., 2002).

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Fig. 3

Hysteresis curves at room temperature.

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3.1.4

3.1.4 Thermogravimetric analysis (TGA)

The TGA curves (Fig. 4) provide the quantitative proofs for the successful coating of surfactants on the magnetite nanoparticles. The Surf–IONs were heated to 800 °C and a significant weight loss occurred on increasing the temperature upto 800 °C. The weight loss is attributed to the loss of organic material (surfactants) coated on the nanoparticles (El-kharrag et al., 2011; Kavas et al., 2013).

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Fig. 4

TGA curves of synthesised Surf-IONs.

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3.2 Influence of PEG-Surfactant aggregates on the rate of “hydrolysis of procaine”

The kinetics of the hydrolysis of procaine was performed in the copresence of 0.5% (w/v) PEG and at different concentrations of CTABr and SDS, ranging between 1.0 × 10⁻² mol dm⁻³ to 5.0 × 10⁻² mol dm⁻³. The rate of hydrolysis decreased in the presence of PEG on increasing the concentrations of surfactants. The rate constant values for the hydrolysis of procaine in the absence and presence of PEGs at different [surfactants] are compared in Tables 1 and 2. The rate of hydrolysis is found to be higher when the PEG is present the at respective surfactant concentration (Figs. 5 and 6). The enhancement in the rate of hydrolysis is due to the exclusion of procaine from the PEG-surfactant phase to the aqueous phase. The micellar aggregates formed by surfactant-polymer interaction is much smaller in size than the respective surfactant micelles (Shaban Ansari et al., 2018). The Necklace model suggests that the spherical structure of surfactant-polymer aggregates is compact, and polymer strands are bound with surfactant molecules (Nikas and Blankschtein, 1994). The surfactant-polymer aggregates provide a less binding opportunity for the substrate. The increase in [surfactant] in the presence of PEG excludes more and more substrates from the micellar aggregates to the aqueous phase. The increase in the chain size of PEG makes the PEG-surfactant structure more compact and results in a decrease in the binding ability of surfactants with procaine (Mészáros et al., 2005). It is evident from the data presented in Tables 3 and 4. As the PEG and surfactant are being present in the solution, the reaction media can be divided into three distinct but homogeneous microscopic phases, namely; PEG-surfactant, micellar and aqueous pseudophases. The hydrolysis is assumed to occur in all these pseudophases. The mechanism of hydrolysis of procaine is presented in Scheme 1.

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Fig. 5

Plots of k_ψ versus [CTABr] in the presence of PEGs. Reaction conditions: [Procaine] = 5.0 × 10⁻⁵ mol dm⁻³, [NaOH] = 5.0 × 10⁻² mol dm⁻³, [PEG] = 0.5% w/v and Temperature = 37 °C.

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Fig. 6

Plots of k_ψ versus [SDS] in the presence of PEGs. Reaction conditions: [Procaine] = 5.0 × 10⁻⁵ mol dm⁻³, [NaOH] = 5.0 × 10⁻² mol dm⁻³, [PEG] = 0.5% w/v and Temperature = 37 °C.

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Scheme 1

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In this Scheme 1, k_p is the first-order rate constant for the hydrolysis of procaine when the PEG-surfactant aggregate is being present, [S_p] is the amount of procaine attached with PEG-surfactant aggregate, OH⁻_p is the amount of OH^– in the PEG-surfactant pseudophase. Similarly, the notations with subscripts ‘w’ represent the aqueous pseudophase and ‘m’ represent the micellar pseudophases. The corresponding rate equation for Scheme 1 can be represented by Eq. (1):

The micelles and PEG-surfactant aggregates are dynamic. They are continually aggregating and disintegrating to give PEG-surfactant aggregates in equilibrium with the surfactant monomers. The surfactant molecules in the presence of polymers remain aggregated together through the cooperative binding between them through the hydrophobic interactions. It is evident from the neutron scattering studies that the surfactant molecules are located at the surface at the surface of polymer aggregates (Claesson et al., 2000; Mészáros et al., 2003). These surfactant molecules are associated with the polymers as monomers rather than in the form of micelles. Based on above theories, it is presumed that approximately the whole quantity of surfactants exists in the complexed state with PEG and an almost negligible amount of free micelles exist in the presence of PEG (Ghosh and Pandey, 1999; Rahman and Rafiquee, 2019). This observation is supported by the results obtained at different concentrations of PEG (from 0.5% to 2.5% (w/v)), in which the values of rate constant was almost in the similar range (Table 5). Thus, the Eq. (1) takes the form of Eq. (2):

In the presence of PEG and surfactants, equilibrium exists between the procaine molecules associated with PEG-surfactant (S_p) and free procaine molecules distributed in the aqueous phase (S_w). This equilibrium distribution can be represented by Eq. (3) as follows:

Here, [PD_n] is the micellized surfactant with PEG.

K_p is the equilibrium constant and given by Eq. (4).

The rate of reaction in terms of total procaine $[{\mathrm{S}}_{\mathrm{T}}]$ (= $\left[{\mathrm{S}}_{\mathrm{w}}\right]+[{\mathrm{S}}_{\mathrm{p}}]$ ) is given by Eq. (5).

Or,

For PEG-CTABr aggregates

The rate Eq. (7) in terms of K_p, k_w and k_p can be written as (Eq. (8)):

The values of ${\mathrm{k}}_{\mathrm{p}}\mathrm{a}\mathrm{n}\mathrm{d}{\mathrm{K}}_{\mathrm{p}}$ were determined by the iterations method as described in the literatures (Menger and Portnoy, 1967; Raees et al., 2019; Rodenas and Vera, 1985) $\mathrm{a}\mathrm{n}\mathrm{d}$ the obtained values are given in Table 1.

Eq. (9) relates the rate of hydrolysis of procaine occurring in the copresence of PEG-SDS system:

In Eq. (9), ${\mathrm{k}}_{\mathrm{w}}^{\text{'}}$ and ${\mathrm{k}}_{\mathrm{p}}^{\text{'}}$ are the first order rate constant for the hydrolysis of procaine in the aqueous and PEG-SDS pseudophases, respectively. [PD_n] denotes the equilibrium concentration of PEG-surfactants complex. The values of ${\mathrm{k}}_{\mathrm{p}}^{\text{'}}$ and ${\mathrm{K}}_{\mathrm{p}}$ were acquired by the plot of $\frac{1}{{\mathrm{k}}_{\mathrm{w}}^{\text{'}}-{\mathrm{k}}_{\psi }}$ versus $\frac{1}{{[\mathrm{P}\mathrm{D}}_{\mathrm{n}}]}$ , and presented in Table 2.

3.3 Influence of surfactant-coated iron oxide nanocomposites (Surf-IONs) on the “hydrolysis of procaine” in the copresence of PEGs

The introduction of Surf-IONs during the reaction of procaine with NaOH lowered the rate of hydrolysis of procaine. A fall in the rate of hydrolysis was observed more pronounced in the presence of Surf-IONs than bare Fe₃O₄. The alkaline hydrolysis of procaine in the copresence of PEGs of molecular weight 1500, 4000, 6000 and 8000 was performed at varying amount of Surf-IONs (from 0.02% to 0.40% w/v), at fixed [NaOH] (5.0 × 10⁻² mol dm⁻³) and fixed [PEGs] (0.5% w/v). Figs. 7 and 8 show that the values of rate constant decreased on increasing the [Surf-IONs]. It was observed that the rate constants were comparatively higher in the presence of PEG than in the absence of PEG at fixed [Surf-IONs], which might be due to the hydrophobic interaction between the PEG and coated surfactant molecules. It makes it difficult for procaine molecules to cross the bulky PEG molecules, and therefore, procaine largely remains in the aqueous phase. The binding and adsorption of procaine with Surf-IONs become difficult, and so the higher rate of reaction is observed when the PEG is present. Following is the mechanism (Scheme 2) that describes the reaction in the copresence of Surf-IONs and PEG:

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Fig. 7

Plots of k_obs versus [CTABr-IONs] in the presence of PEGs. Reaction conditions: [Procaine] = 5.0 × 10⁻⁵ mol dm⁻³, [NaOH] = 5.0 × 10⁻² mol dm⁻³, [PEG] = 0.5% w/v and Temperature = 37 °C.

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Fig. 8

Plots of k_obs versus [SDS-IONs] in the presence of PEGs. Reaction conditions: [Procaine] = 5.0 × 10⁻⁵ mol dm⁻³, [NaOH] = 5.0 × 10⁻² mol dm⁻³, [PEG] = 0.5% w/v and Temperature = 37 °C.

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Scheme 2

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The increase in [Surf-IONs] to the solutions containing procaine and hydroxide ions decreased the rate of reaction progressively. This observation supports the compartmentalization of reactants into two different pseudophases i.e. procaine is associated with Surf-IONs while hydrophilic OH^– ions remains in aqueous phase. Therefore, the Eq. (9) based on the pseudophase model has been used in the form of Eq. (10) to determine the equilibrium constant (K_e) for the binding between procaine and Surf-IONs.

The values of the rate constant ( ${\mathrm{k}}_{\mathrm{S}\mathrm{u}\mathrm{r}\mathrm{f}-\mathrm{I}\mathrm{O}\mathrm{N}\mathrm{s}}^{\text{'}}$ ) for the hydrolysis of procaine in the copresence of Surf-IONs and PEG were calculated from the plot of $\frac{1}{{\mathrm{k}}_{\mathrm{w}}^{\text{'}}-{\mathrm{k}}_{\mathrm{S}\mathrm{u}\mathrm{r}\mathrm{f}-\mathrm{I}\mathrm{O}\mathrm{N}\mathrm{s}}}$ versus $\frac{1}{[\mathrm{S}\mathrm{u}\mathrm{r}\mathrm{f}-\mathrm{I}\mathrm{O}\mathrm{N}\mathrm{s}]}$ . The values of equilibrium constant (K_e) were obtained from the slopes of the above plots. These values are given in Tables 6 and 7.

A close observation from the Figs. 7 and 8 displays that the increase in the molecular weight of the polyethylene glycol increases the rate of reactions for both the nanoparticles (CTABr–IONs and SDS–IONs). The values of K_e, (given in Tables 6 and 7) decreases on increasing the molecular weights of the PEG in the copresence of CTABr–IONs and SDS–IONs. The higher molecular weights PEG wraps the CTABr–IONs and SDS–IONs more compactly and favours the equilibrium concentration of procaine molecules more into the aqueous phase and therefore, the higher rate constant values are observed.