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01 2023
:17;
105436
doi:
10.1016/j.arabjc.2023.105436

Surfactants interaction with sulfathiazole: Spectroscopic, conductometric, and thermodynamic approach

, , , , ,
 Department of Chemistry, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia

⁎Corresponding author. zoya.zaheer@gmail.com (Zoya Zaheer) zzkhan@kau.edu.sa (Zoya Zaheer)

Received: , Accepted: ,
© The Author(s) 2023
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University. Production and hosting by Elsevier.

Abstract

Taking into the account the advantages of surfactant micelles as drug carrier, the interaction of sulfathiazole (STZ) with three types of surfactants, namely, cationic cetyltrimethylammonium bromide (CTAB), anionic sodium dodecyl sulfate (SDS), and non-ionic t-octylphenoxypolyethoxyethanol (TX-100) has been studied by using UV–visible spectroscope and conductivity measurements. The critical micelle concentration (CMC), degree of ionization (α), counter ion binding constant (β), free energy change of micellization (ΔG0m), relative solubility (St/S0), STZ-surfactant interaction constant (θ), approximate number (n) of STZ molecules incorporated per micelle, heat capacity (ΔC0m), and transfer of thermodynamic parameters were calculated and discussed for both the systems. The CMC values of CTAB and SDS were found to decrease in presence of increasing STZ concentrations. The values of counter ions dissociation were also found to be dependent of temperature. The ΔG0m values are obtained to be negative of both systems. A comparison of the binding constants (KC) calculated from the Kawamura equation indicated that the solubilization tendency of STZ with CTAB is much higher than that with SDS micelles. However, this interaction was weak since no significant change in the micellization phenomenon was observed with TX-100 under our experimental conditions.

Keywords

Sulfathiazole
Micellization
Heat capacity
Thermodynamic parameters
1

1 Introduction

Physiological interactions between drugs and surfactants can be visualized as an approximation of drug-membrane interactions (Lawernce, 1994; Torchilin, 2001). Knowledge of drug-surfactant surfactant interaction should be of great value in understanding the chemical equilibrium, mechanism, and kinetics of drug loading and /or drug release. Sulfathiazole (heterocyclic sulfa drug derived from sulfanilamide and thiazole) has been used especially in the treatment of pneumonia, staphylococcal, and urinary tract infections due to presence of aromatic ring systems, benzene and thiazole with SO2NH functional group (Suganya and Kabilan, 2004). Micellar enhancement ultrafiltration has been used for the removal of trace water contaminants such as 4-tetra- butylphenol (Dunn et al., 1985), eosin (Purkait et al., 2004), phenol (Zeng et al., 2008), and sulfonamide antibiotic drugs in wastewater (Exall et al., 2013). The amphiphilic nature of surfactants, reduces the surface tension of water, and formed various aggregates, allows one to study the affinity of polar and non-polar drugs to the biological membranes. The term micelles has been used for an entity of colloidal dimensions, in dynamic equilibrium with the monomer (Zana, 1996).

Solubilization of organic molecules in aqueous solutions of surfactants, monocomponent micelles, multicomponent micelles, vesicles, and block co-polymer micelles has been studied and reviewed on several occasions (Nagarajan,1996; Kwon, 2003; Posa, 2018). The thermodynamic parameters, and coefficient of interaction in the micellar co-solubilization of the single, and binary mixture of hydrophobic molecules have been calculated (Nagadome et al., 2001). Hossain et al. examined the cefixime trihydrate interactions with tetradecyltrimehylammonium bromide in presence of different hydrotpoes and suggested that the interaction forces between the drug and surfactant were electrostatic and hydrophobic interactions in an aqueous solution (Hossain et al., 2023). The micellization of levofloxacin hemihydrate into the cetylpyridinium bromide investigated with a conductivity method in absence and presence of various additives. The CMC, β, ΔG0m ΔH0m, and ΔG0m-ΔH0m values were calculated for the micellization of drug into the cationic micelles at different temperatures (Ali et al., 2023). Bovine serum albumin-cetyltrimethylammonium chloride interactions were studied in presence of various hydrotropes (Progga e al., 2023).

The interactions of sulfathiazole (antibacterial drug and exists in four polymorphic forms in solid state) with surfactants (ionic and non-ionic) can promote to an exhaustive understanding of transport and receptor binding of sulfathiazole at the molecular level. Sarker et al. reported the solubilization of several sulfonamide drugs in the CTAB surfactant using nuclear magnetic spectroscopy (Sarker et al., 2016a), and semi-equilibrium dialysis (Sarker et al., 2016b). Ciprofloxacin hydrochloride interactions with SDS and CTAB were determined using conductometric and spectroscopic techniques (UV–visible and proton NMR) (Banipal et al., 2018). Deva et al. used different molar ratio of SDS, polymer and sulfathiazole to improve the solubility of used model drug in an aqueous solution (Dave et al., 2013). Boyle et al. reported a safer and simple method to the synthesis of sulfathiazole for the under graduate organic classes (Boyle et al., 2012). Chen et al. reported the extraction of highly polar sulfonamides drugs by using a mixed systems of anionic SDS and cationic dodecyltrimethyl ammonium bromide. The hydrophobic and polar interactions such as electrostatic, hydrogen-bond, and π-cation responsible to the solubilization of sulfonamides drugs into coacervate phase (Chen et al., 2014). The cationic surfactant was used for the preparation of surfactant-sulfonamide conjugates, which exhibited excellent antimicrobial activities against human pathogens (Tantawy et al., 2021).

The drugs can associate with micelle through multiple interactions such as cation–anion binding, ion–dipole interactions, and hydrophobic effect. It has been established that the surfactants at lower and higher concentration affect the electronic absorptions of many amphiphilic drugs (Al-Ghamadi et al., 2020; Zaheer et al., 2021; Zaheer, 2022). Various literature reports have commented on the micellization of dye (Zaghbani et al., 2007) and drugs (Ali et al., 2023; Hossain et al., 2023; Progga e al., 2023; Dey et al., 2018; Sarker et al., 2016a; Mahajan and Mahajan, 2013; Faris et al., 2009; Mall et al.,1996;) into the cationic or ionic micelles; but the use of UV–visible spectroscope and conductivity technique have been neglected to study the solubilization of STZ into the ionic micelles. Hence, it is essential to establish the effect of surfactant solutions on the spectra of sulfathiazole before its analysis.

This paper describes the interaction of STZ with anionic SDS, cationic CTAB, and nonionic TX-100 surfactants in solution as characterized by recording UV–visible spectra of STZ-surfactant complexes. UV–visible data were utilized to determine the water-micelle distribution coefficient (KC), relative solubility (St/S0), and free energy of solubilization (ΔG0P) associated with the micellization of STZ into the aggregates of micellar-pseudo phase of CTAB and SDS surfactants for the first time. The results were also used to evaluate the CMC of both surfactants under different conditions. The orientation and locus of STZ in the micelle structure have also been proposed for the first time. The structures of CTAB, SDS, TX-100, and sulfathiazole are given in Scheme 1.

Structures of CTAB, SDS, TX-100, and sulfathiazole.
Scheme 1
Structures of CTAB, SDS, TX-100, and sulfathiazole.

2

2 Experimental

2.1

2.1 Materials and CMC of surfactants

Sulfathiozol (C9H9N3O2S2, molecular mass = 255.32, purity = 98 %), cetyltrimethylammonium bromide (C19H42NBr, molecular mass = 364.45, purity ≥ 99 %), sodium dodecylsulfate (C12H25SO4Na, molecular mass = 288.38, purity ≥ 98.5 %), TX-100, and silver nitrate (AgNO3, molecular mass = 169.87, purity ≥ 99 %) were used as received from Sigma-Aldrich of highest purity, and stored in vacuum desiccator over anhydrous calcium chloride at 22 0C. Double distilled water was used as solvent for the preparation of all reagents solutions as well as for dilution. The CMC of CTAB and SDS were determined by recording the specific conductivity of both surfactants solutions with increasing concentrations at a constant temperature under different conditions (surfactant + water, and surfactant + STZ) on a digital conductivity meter (Jenway 4510 UK, along with a conductivity cell having 0.5 % precision and cell constant of 0.97 cm−1) with a temperature compensative cell. The standard 0.1 M potassium chloride solution was used to the calibration of conductivity meter prior to use. The conductivity values were measured for each dilution after complete mixing and temperature equilibration. The concentration of surfactant at the break point of two straight lines gives the value of CMC of each surfactant (Chatterjee et al., 2001). The degree of ionization (α) was evaluated from the ratio of the slopes of the straight lines in the post-micellar region to the sub-micellar region.

2.2

2.2 Interaction of STZ with surfactants

The UV–visible spectrum of STZ exhibits two absorption bands at 260 and 285 nm in an aqueous solution. Therefore, absorption spectra of STZ were recorded in presence of increasing surfactant concentrations on a UV–visible spectrophotometer (UV-1800 Shimadzu) using 1 cm length of quartz cuvettes. The CMC values of SDS and CTAB were ca. 8.0 × 10-3 and 8.0 × 104 mol/L at 25 0C. Therefore, different concentrations of SDS and CTAB were used to determine the STZ solubilization parameters into the anionic and cationic micelles (Manabe et al., 1987). The concentration of CTAB was varied from 0.0 to 2.0 mM, SDS from 0.0 to 25.0 mM, and that of TX-100 from 0.0 to 2.0 mM at different concentrations of STZ. The solubilization constant was calculated with the following relation established by Kawamura et al. for the micellization phenylalkanols into the anionic SDS micelles (Kawamura et al., 1989; Zaheer, 2022).

(1)
1 Δ A = 1 K STZ Δ A α ( [ S T Z ] + ( [ s u r f a c tan t ] T - C M C ) ) + 1 Δ A α In Eq. (1), ΔA represents a difference between the absorbance of STZ-surfactant complex and pure STZ. KSTZ is the STZ-surfactant binding constant. ΔAα is the absorbance difference at very high surfactant concentrations. The values of KSTZ and ΔAα were calculated from the intercepts and slopes of the double reciprocal plots of ΔA versus ([STZ] + ([surfactant]T – CMC)) for different drug concentrations at 22 °C. The values of KX (dimensionless partition coefficient) were calculated with Eq. (2), where 55.5 is the number of moles of water per liter.
(2)
K X = K STZ × 55.5 The relative solubility of STZ was evaluated by using Eq. (3) (Krishna and Flanagan, 1989).
(3)
S t S 0 = 1 + K X ν M where St/S0 represents the relative solubility of drug. KX = micelle-water partition. The M and ν are the molar concentration and partial molar volume of surfactant, respectively.

3

3 Results and discussion

3.1

3.1 General consideration

It is well known that sulfathiazole exists in many polymeric forms in the solid state (Apperley et al., 1999). The amino and imine tautomer forms exists in the equilibrium. Out of these, imine form is the major existing species (Scheme 2). The proton exists in the nitrogen of thiazole ring system (Eq. (4)) (Kruger and Gafner, 1971).

Tautomer forms of sulfathiazole.
Scheme 2
Tautomer forms of sulfathiazole.

The solubility of STZ depends on the nature of the solvent. The solubility are 0.1 and 0.2 g per 100 g, respectively, in water and n-propanol at 30 0C (Khoshkhoo and Anwar, 1993). Therefore, 0.001 M stock STZ solution is prepared in deionized water. UV–visible spectra of STZ are recorded as a function of time to establish the stability of STZ at reaction temperature. In an aqueous solution, STZ shows two strong absorption peaks at 260 and 285 nm, both assigned to the π → π* of aromatic benzene and thiazole rings. In order to determine the molar extinction coefficient of STZ in water, the spectra of a series of STZ aqueous solutions containing the different concentrations of drug were recorded. The absorbance at 285 nm was chosen to the construction of the calibration curve. Fig. 1A illustrate that the calibration plot (Beer-Lambert curve between absorbance at 285 nm versus STZ concentration) is linear over the entire range of STZ concentrations with zero intercept. The molar extinction coefficient is 15385.71 L. mol−1. cm−1 calculated from the slope of the plot. No deviation from the Beer-Lambert law was observed at higher STZ concentration under our experimental conditions. We did not observed any significant change in the shape and position of STZ wavelength over the entire storage observation period (ca. 2 weeks).

Beer-Lambert plot of STZ at 22 0C (A), and effects of CTAB on the UV–visible spectra of STZ (B). Reaction conditions: [STZ] = 0.05 mM, pH = 5.0.
Fig. 1
Beer-Lambert plot of STZ at 22 0C (A), and effects of CTAB on the UV–visible spectra of STZ (B). Reaction conditions: [STZ] = 0.05 mM, pH = 5.0.

3.2

3.2 Conductivity measurements

CMC is a mandatory data for the calculation and discussion of micelles interactions with additives and/or micelles assisted reaction kinetics. In order to determine the CMC of both CTAB and SDS surfactants, the conductivity of surfactants aqueous solution were recorded with and without added different concentrations of STZ. The conductivity has been linearly related to the surfactant concentration in sub-, and post-micellar region. The CMC values are calculated from the break point of two straight lines from the conductivity versus surfactant concentration plots as shown in Fig. 2A and 3A, respectively, for CTAB and SDS (Cifuentes et al., 1997). The 0.82 mM and 8.0 mM CMC are calculated for CTAB and SDS are in close to previously reported values, and compared with literature values (Banipal et al., 2017; Shirahama and Kashiwabara, 1971). As it is clear from Table 1 (Fig. 2B and 3B), STZ decreases the CMC values of CTAB and SDS. These results indicate the formation of STZ bound micelles, which might be attributed to the various types of STZ interactions (electrostatic attraction, hydrogen bonding, and hydrophobic) with positive and negative head groups of both surfactants. The values of α have also been found to increase with STZ concentrations (Table 2) due to the incorporation of STZ into the micelles, which preponed the process of micellization (Rosen, 1978; Sarker et al., 2016). The decrease in the degree of hydrophobic hydration causes a reduction in the hydrophobic interaction between the non-polar parts of surfactant and STZ, which hampers the micelles formation. As a result, CMC increases with rising the temperature (Ali et al., 2023; Progga et al., 2023).

CMC determination of CTAB by conductivity (A), and decay of CMC with STZ (B). Reaction conditions: pH = 5.0, temperature = 22 0C.
Fig. 2
CMC determination of CTAB by conductivity (A), and decay of CMC with STZ (B). Reaction conditions: pH = 5.0, temperature = 22 0C.
Table 1 Effects of CTAB, and SDS on the position of λmax, differential absorbance (ΔA), and CMC at different STZ concentrations.
[CTAB] (mM) λmax (nm) ΔA [STZ] (mM) 104 [CMC] (mol/L)
0.0 260 and 280 0.00 0.0 8.2
0.4 260 and 285 0.97 0.04 7.9
0.8 260 and 285 1.03 0.08 7.6
1.2 260 and 285 1.36 0.12 7.3
1.6 260 and 285 1.90 0.16 7.1
2.0 260 and 285 1.91 0.20 6.9
[SDS] (mM) λmax (nm) ΔA 105[STZ] (mol/L) 103[CMC] (mol/L)
0.0 260 and 280 0.00 0.0 8.0
1.25 260 and 285 0.00 1.25 7.9
2.5 260 and 285 0.01 2.5 7.3
5.0 260 and 285 0.015 3.5 7.0
7.5 260 and 285 0.02 4.2 6.7
10.0 260 and 285 0.025 5.0 6.4
15.0 260 and 285 0.03 5.5 6.2
20.0 260 and 285 0.035 6.0 6.0
25.0 260 and 285 0.037 7.2 5.7
CMC determination of SDS by conductivity (A), and decay of CMC with STZ (B). Reaction conditions: pH = 5.0, temperature = 22 0C.
Fig. 3
CMC determination of SDS by conductivity (A), and decay of CMC with STZ (B). Reaction conditions: pH = 5.0, temperature = 22 0C.
Table 2 Values of α, β, and θ for the micellization of STZ into CTAB and SDS micelles.
CTAB SDS
[STZ] (mM) α β θ 105[STZ] (mol/L) α β θ
0.05 0.27 0.73 0.93 1.25 0.25 0.75 0.86
0.08 0.29 0.71 0.78 2.5 0.27 0.73 0.77
0.10 0.33 0.67 0.67 5.0 0.30 0.70 0.51

It is well known that the CMC of surfactants may be depressed or increased by the addition of additives due to the various reasons. Therefore, it is necessary to determine the rate of decrease CMC versus the concentration of STZ (dCMC/dSTZ) at limiting dilution to the present system. The rates of CMC decrease were calculated from the slopes (- dCMC/dSTZ = k) of Fig. 2B and 3B. The following relation between the limiting slope (- dCMC/d(additive)) and partition coefficient (KX) of the additive between bulk water and pseudo-micellar phases at the low concentration of additives was used (Eq. (5)).

(5)
- dCMC d ( a d d i t i v e ) = CMC × θ × K X n w where θ = constant of additive-surfactant interaction, nw = 55.5 mol of water in solution, additive = STZ in the present study. These investigators also suggests that the θ remain unchanged with an increase in the hydrophobic character of additive (alkanol) in SDS solutions. For CTAB-STZ, the KX = 71650.5 was calculated by using slope (0.692), β = 0.62, and CMC = 8.0 × 10-4 mol/L. For SDS-STZ, KX = 36474.6 was calculated by using slope (3.6), β = 0.70, and CMC = 8.0 × 10-3 mol/L. The values of θ are calculated for different values of KX (determined by differential absorbance), and are given in Table 2 for both CTAB and SDS. The counter ion binding constant, β, are also calculated by subtracting the values of α from unity with Eq. (6) (Kuiper et al., 2001).
(6)
β = 1 - α Table 2 shows that the values of α increases with increasing the drug concentrations for both surfactants. Fig. 4 shows the linear variation of β as a function of temperature at fixed concentration of additive. Our finding is consistent with the solubilization of additives into the ionic micelles (Abu-Hamdiyyah, 1986; Ali et al., 2023; Progga et al., 2023). The polar portion of STZ molecule located in the surface region of micelles, which releases counter ions from the micelles because of the dilution of surface charge density (Manabe et al., 1987; Hossain et al., 2023). The other STZ molecules dissolved monomerically in the bulk water depresses the intermicellar concentration of CTAB and/or SDS. The θ values (Table 2) also decreases with the concentrations of STZ present in the micellar systems. The θ is a physically complex value, which depends on the activity coefficients of all species participate in the micellization processes (Abu-Hamdiyyah, 1986).
Temperature dependence of degree of ionization (β).
Fig. 4
Temperature dependence of degree of ionization (β).

3.3

3.3 Interaction of STZ with surfactants

In order to determine the partitioning and/or binding constant between STZ and surfactants (CTAB, SDS, and TX-100), the electronic spectra of STZ with different concentrations of surfactants were recorded at different time intervals. On the addition of CTAB, hyperchromic shift (increase in absorbance) has been observed, which demonstrates the complex formation between STZ and CTAB aggregates. Table 1 shows that the differential absorbance increases and position of absorption peak remains constant with CTAB concentrations, respectively. Fig. 1B shows that the absorbance of STZ peaks increases with CTAB concentrations at fixed STZ concentration (0.05 mM). Fig. 5A shows the differential absorbance (ΔA = absorbance of STZ with CTAB – absorbance of pure STZ at same wavelength, i.e., 285 nm) of STZ solutions in presence of different CTAB concentrations. It is evident from the plots that, differential absorbance increases as the CTAB concentration increases from pre-micellar to post-micellar region. The differential plots are used to determine the various binding constants. According to Eq. (1), the plot of 1/ ΔA versus ([STZ] + (CTAB]T –CMC)) should be linear with positive intercept for different concentrations of CTAB (Fig. 5B) (Shah et al., 1999). The CMC values of surfactants obtained in the presence of the STZ were used for the determination of KSTZ. The values of ΔAα and KSTZ are calculated from the intercept and slopes of Fig. 5B. KX values are calculated from KSTZ with Eq. (2), and are given in Table 2, which were founds to increase with STZ concentrations.

Plots of ACTAB – ASTZ versus CTAB concentrations to the micellization STZ (A), and their reciprocal plots (B). Reaction conditions: pH = 5.0, temperature = 22 0C.
Fig. 5
Plots of ACTAB – ASTZ versus CTAB concentrations to the micellization STZ (A), and their reciprocal plots (B). Reaction conditions: pH = 5.0, temperature = 22 0C.

SDS is an anionic surfactant and bearing a negative head group (-OSO3-). To establish the STZ solubilization into the anionic micelles, a series of experiments are performed at fixed three different concentrations of STZ with varying SDS concentrations from 0.0 to 25.0 mM at 22 0C. Fig. 6A indicates that the differential absorbance was found to increase with increasing SDS. The plots of 1/ ΔA against ([STZ] + (SDS]T –CMC)) were used to the evaluation of binding parameters (Fig. 6B). The drug micellization parameters are summarized in Table 3. The values of KX obtained from the differential absorbance method were used to calculate the relative solubility (St/S0) of STZ by using Eq. (3) for both CTAB and SDS. The partial molar volumes of CTAB and SDS are reported to be 0.3654 and 0.193 mol−1 dm3, respectively (Corkill et al., 1967). The values of relative solubility are summarized in Table 3 for CTAB and SDS, respectively. The St/S0 of STZ was found to be higher for SDS than that of CTAB, which can be ascribed to the lower local pH value in the vicinity of micellar head group region. The standard free energy change to the transfer of STZ from bulk aqueous phase to the micellar phase (ΔG0m) were calculated with Eq. (7).

(7)
Δ G m 0 = - ( 8.314 ) T ln K X Using the KX obtained from Eq. (2), the ΔG0m values are calculated at 22 0C (absolute temperature = 273 + 22 = 295 K). The negative values of ΔG0m at all concentrations of STZ indicate that the solubilization process is spontaneous thermodynamically for both CTAB and SDS (Table 3). The values of KC was higher for CTAB micellization than that of SDS, which might be due to the strong binding of STZ with cationic micellar head group through ionic interactions.
Plots of ASDS – ASTZ versus SDS concentrations to the micellization of STZ (A), and their reciprocal plots (B). Reaction conditions: pH = 5.0, temperature = 22 0C.
Fig. 6
Plots of ASDS – ASTZ versus SDS concentrations to the micellization of STZ (A), and their reciprocal plots (B). Reaction conditions: pH = 5.0, temperature = 22 0C.
Table 3 Values of KC, R2, KX, St/S0, ΔG0m, and Cm for the STZ interactions with CTAB and SDS.
CTAB
[STZ] (mM) KC (mol-1dm3) R2 KX St/S0 ΔG0m (kJ/mol) Cm (mol/L)
0.05 923.9 0.991 51277.0 15.98 − 26.6 5.2 × 10-6
0.08 1195.7 0.994 66322.5 20.38 − 27.2 8.8 × 10-6
0.10 1283.9 0.995 71257.1 21.82 − 27.4 11.8 × 10-6
SDS
105STZ] (mol/L) KC (mol−1 dm3) R2 KX St/S0 ΔG0m (kJ/mol) Cm (mol/L)
1.25 529.1 0.958 29365.1 46.33 − 25.2 1.4 × 10-7
2.5 586.7 0.959 32564.3 51.27 − 25.4 1.9 × 10-7
5.0 894.3 0.999 49636.3 77.63 − 26.5 2.2 × 10-7

The concentration of micellized STZ (Cm) is estimated by using Eq. (8) (Reeves et al., 1973). C m = A 0 - A ε 0 - ε where A0 is the absorbance of pure STZ in absence of surfactant, and A is the absorbance of drug with surfactant at the same wavelength. The ε0 and ε, respectively, are the molar extinction coefficient of STZ (Fig. 1A) with water and in surfactant at above the CMC where the absorbance of STZ-surfactant solutions becomes almost constant. For CTAB, the Cm values were calculated by using ε0 = 15.38 × 103 mol−1 dm3 cm−1. The ε = 21.02, 18.75, and 23.00 × 103 mol−1 dm3 cm−1, respectively, were calculated for 0.1, 0.08, and 0.05 mM STZ with CTAB = 30.0 × 10-4 mol/L by using Beer-Lambert law relationship. The Cm values are summarized in Table 2.

The approximate number of STZ molecules (n) incorporated into a single micelle of CTAB was calculated with the following relations (Eqs. (9) and (10) (Rosen, 1978; Miyashita and Hayano, 1981; Wang and Verrall, 1994; Shah et al.,1999).

(9)
n = C m M
(10)
M = [ C T A B ] T - C M C N where M = micellar concentration, Cm = micellized STZ concentration and N = mean aggregation number of CTAB surfactant (=80) at the CMC (Leibner and Jacobus, 1977). The approximate number of STZ was calculated by substituting the values of M (=2.5 × 10-6) at [CTAB] = 10.0 × 10-4 mol/L, and CMC = 8.0 × 10-4 mol/L, in Eq. (9), and was found to be ca. 2.0, 3.0, and 4.0, respectively, for 0.05, 0.08, and 0.1 mM STZ. Miyashita and Hayano reported the interaction of CTAB with anthraquinoid acidic dye, and explains the impact of n value (number of dye molecules penetrate into a single micelle) on the dye penetration into the cationic micelles of CTAB. The significant of n values, which depends on the CTAB concentrations in the lower range. When the n is 1.0, indicates that a single dye molecule reorient into a single micelle without any interaction with the other dye molecules (Miyashita and Hayano, 1981). The n values are more than unity, as in the present study, indicates that STZ molecule on the surface of a single micelle reorient into its core slowly, because the STZ molecules are able to interact each other directly and/or indirectly through the micelles forming surfactants due to its polymorphism nature.

TX-100 is a non-ionic surfactant, and widely used to lyse cells to extract protein (Koley and Bard, 2010)). It also exhibits absorption peak at 290 nm in the visible region due to the presence of benzene ring in their structure. UV–visible spectra of pure TX-100 and STZ + TX-100 were recorded to determine the micellar binding constant. The TX-100-STZ absorbance complex were determined by subtracting the absorbance of pure TX-100 from the absorbance of surfactant-drug complex. Fig. 7A demonstrates the effect of TX-100 concentration on the STZ interaction. No significant effect of TX-100 concentrations were observed on the micellization of STZ into the non-ionic micelles (Fig. 7B). The absorbance remains constant at higher TX-100 (≥1.0 mM). Thus, we may safely state that the electrostatic interactions are mainly responsible to the solubilization of STZ drug into the ionic micelles. However, the role of hydrophobic interaction cannot be ruled out completely in the drug micellization.

UV–visible spectra of TX-100- STZ as a function of TX-100 (A), and plot of absorbance at 285 nm versus TX-100 concentrations (B). Reaction conditions: [STZ] = 0.05 mM, pH = 5.0, temperature = 22 0C.
Fig. 7
UV–visible spectra of TX-100- STZ as a function of TX-100 (A), and plot of absorbance at 285 nm versus TX-100 concentrations (B). Reaction conditions: [STZ] = 0.05 mM, pH = 5.0, temperature = 22 0C.

3.4

3.4 Mechanism of micellization

The pH of the working solution plays a significant role in the micellization of STZ into the ionic micelles. STZ has two pH sensitive groups such as one basic amino group (pK1 = 2.0), and one acidic amide linkage (pK2 = 7.1) (Sanli et al., 2009). At a specific pH conditions, –NH2 group accepts a proton, while the amide nitrogen atom (–NH-) donates a proton (Polster and Lachmann, 1989). Thus, a positive and a negative charges exists on the STZ molecule at a certain pH (Scheme 3, Eqs. (11) and (12)).

Ionization of STZ in an aqueous solution.
Scheme 3
Ionization of STZ in an aqueous solution.

The control of pH in micellar media is not as straightforward as in the aqueous system. Tondre and his coworkers discussed that the pK’s in the micellar systems are quite different most of the time from pK’s determined in water (Tondre and Boumezioud, 1989). The microscopic pH in the ionic micelles is different from the macroscopic pH of the bulk solutions due to the incorporations of various ionic species into the Stern-layer of micelles. The difference between the bulk pH and the local one in the vicinity of micelles was shifted by plus two and minus two units, respectively, for anionic and cationic micelles (Hebrant and Tondre, 1992). For CTAB cationic micelles, the pH of the working solution was increases by two units (from 5.0 to 7.0) in the vicinity of micelles, and A- form of STZ exists as a major species, which interacts with the positive head group of the micelles. Finally, negative charge bearing STZ species was completely solubilized into the micelles of CTAB. The STZ molecule is located at the junction of Stern-palisade interface. Palisade layer is relatively less hydrophobic than that of micellar interior. Electrostatic and hydrophobic interactions operates simultaneously in the micellization process of STZ (Scheme 4).

Proposed locus and orientation of STZ in cationic micelles.
Scheme 4
Proposed locus and orientation of STZ in cationic micelles.

For anionic SDS micelles, the pH was shifted to plus two units (pH decrease from 5.0 to 3.0) due to the incorporation of all cationic species near the micelles interface. The protonated STZ species (H2A) interacts with the negative head group of SDS micelles. As a result, the equilibrium shifts (Eq. (11) toward the left hand side, and STZ was micellized into the anionic micelles (Scheme 5).

Proposed locus and orientation of STZ in anionic micelles.
Scheme 5
Proposed locus and orientation of STZ in anionic micelles.

Sarker et al. reported that the some protons of STZ were located either in the Stern-layer or just below in the Palisade-layer of the cationic micelles on the basis of 1H NMR spectroscopy (Sarker et al., 2016a). The our values (log KC = 2.96 to 3.10) of binding constant for CTAB-STZ interactions are in good agreement to the binding constants values, log 3.56, and log 3.96, calculated by using1H NMR (Sarker et al., 2016a), and semi-equilibrium dialysis methods, respectively (Sarker et al., 2016b). These results indicate that the UV–visible method is able to calculate the drug-micelle binding constant values and other parameters with reasonable efficacy.

3.5

3.5 Micellization thermodynamic parameters

The values of CMC were also determined by recording the conductivity at four different temperatures. The magnitude of CMC (STZ + CTAB), and STZ + SDS) systems were found to increase with rising in temperature for both surfactants (Table 4). The effects of temperature on the CMC of surfactant molecule might be due to the fact that the degree of hydrophilic hydration decreases with increasing temperature. As a result, the micellization process was delayed. It has been assumed that the STZ forms a 1: 1 complex with CTAB in the bulk of aqueous phase prior to penetration into the pseudo-micellar phase (Miyashita and Hayano, 1981). Therefore, stoichiometric equation was applied to the STZ micellizaion in the ionic micelles. The standard free energy of solubilization, for the ionic surfactants of the 1:1 electrolyte, was calculated with Eq. (13) (Zana, 1996; Mukherjee et al.,1994).

(13)
Δ G m 0 = 1 + β × 8.314 × T × ln X CMC XCMC = CMC in terms of mole fraction units, and 8.314 is the standard value of gas constant. The values ΔG0m of (STZ + CTAB) and (STZ = SDS) systems were found to be negative (Table 4), which indicates that the STZ supported CTAB and SDS micelles formation processes are thermodynamically spontaneous. The standard enthalpy change of the micellization (ΔH0m) was estimated using the modified van’t Hoff relation (Eq. (14)) (Islam and Kato, 2003; Mata et al., 2005).
(14)
Δ H m 0 = - 1 + β × 8.314 × T 2 × d ln X CMC dT where dlnXCMC /dT = slope of the straight line, estimated by plotting lnXCMC against temperature for both CTAB-STZ and SDS-STZ systems. The magnitudes of ΔH0m, and ΔG0m were utilized to determine the change in standard entropy of micellization (ΔS0m) at a given temperature, by using the Eq. (15).
(15)
Δ S m 0 = Δ H m 0 - Δ G m 0 T The ΔH0m values are negative for both systems, and these values increase with temperature (Table 4). On the other hand, ΔS0m values are positive for the solubilization of STZ into the CTAB and SDS, and the positive values increase with increase of temperature (Table 4). The large positive value ΔS0m can be ascribed to the increase complete disorder, and disturbance inside the system, which might be due to the heat adsorption, phase shifts, and spontaneous processes (Hossain et al., 2023). For both systems, the magnitude of ΔH0m becomes more negative with temperature, suggesting that the electrostatic interactions become stronger with temperature. These findings indicate that in addition to hydrophobic, electrostatic forces also play a significant role in the micellization of STZ antibiotic with CTAB and/or SDS during formation of STZ supported micelles of both surfactants. The penetration, locus, and orientation of sulfonamides drugs into the CTAB micelles depends due to the presence of negative charge on the nitrogen of sulfonamide groups. In general, the entropy-enthalpy compensation phenomenon in the micellization process can be described by using the Eq. (16) (Kumar and Parikh, 2012; Sugihara and Hisatomi, 1999).
(16)
Δ H m 0 = Δ H m 0 , * + T C Δ S m 0 The plot of ΔH0m versus ΔS0m should be linear with intercept = ΔH0,*m, characteristics of solute–solute interactions (aggregation of hydrophobic tails in to the micelle), and slope = TC, characteristics of solute–solute and solute–solvent interactions (dehydration of the hydrocarbon chain). It provides information regarding the range of temperature studied. The entropy-enthalpy compensation plots are linear for both systems (STZ + CTAB, and STZ + SDS), and such plots are given in Fig. 8. Thus, the micellization of STZ with both surfactants are entropy and enthalpy controlled. The large positive magnitude of ΔS0m indicates the transfer of hydrophobic chains of both surfactants from the aqueous pseudo phase to the STZ supported surfactant micelle core.
Table 4 Values of CMC, XCMC, α, and standard thermodynamic parameters (ΔG0m, ΔH0m and ΔS0m for the STZ interactions with CTAB and SDS at different temperatures.
CTABa
T (K) CMC (mol/L) XCMC α ΔG0m (kJ/mol) ΔH0m (kJ/mol) ΔS0m (J/K/mol)
295 8.21 × 10-4 1.47 × 10-5 0.27 − 47.22 −7.45 134.50
303 8.53 × 10-4 1.53 × 10-5 0.24 − 48.73 − 8.05 134.25
308 8.81 × 10-4 1.58 × 10-5 0.21 − 50.39 − 8.46 136.12
313 9.21 × 10-4 1.65 × 10-5 0.19 −51.86 −8.88 137.57
SDSb
T (K) CMC (mol/L) XCMC α ΔG0m (kJ/mol) ΔH0m (kJ/mol) ΔS0m (J/K/mol)
295 8.12 × 10-3 1.46 × 10-4 0.25 − 16.24 −7.63 29.18
303 8.43 × 10-3 1.51 × 10-4 0.20 − 17.28 − 8.19 30.00
308 8.71 × 10-3 1.56 × 10-4 0.17 − 18.18 − 8.60 31.11
313 9.01 × 10-3 1.62 × 10-4 0.14 −19.07 −9.03 32.07
a [STZ] = 0.05 mM, dlnXCMC/dT (=0.00603) was calculated from the slope of the plot of lnXCMC versus time, b[STZ] = 1.25 × 10-5 mol/L.
Entropy-enthalpy compensation plot for CTAB-STZ (A) and SDS-STZ (B) at different temperature.
Fig. 8
Entropy-enthalpy compensation plot for CTAB-STZ (A) and SDS-STZ (B) at different temperature.

The molar heat capacity changes (ΔC0m) of (CTAB + STZ) and SDS + STZ) systems are obtained from the slopes of the ΔH0m versus temperature according to the Eq. (17) (Vamvaca et al., 2008).

(17)
Δ C m 0 = ∂ Δ H m 0 ∂ T p For (STAB + STZ) and (SDS + STZ) systems, the ΔC0P are found to be negative, −0.077 and −0.078 kJ/K/mol, respectively (Fig. 9). The change in the heat capacity during the micellization process is directly proportional to the funeral of the molecular surface, and might be associated to the motion restriction. The positive ΔS0m and small ΔC0P suggests minor structural rearrangement of CTAB and SDS micelles during the interaction and/or incorporation of STZ with the head groups.
Plots of ΔH0m versus temperature for the micellization of STZ with CTAB and SDS.
Fig. 9
Plots of ΔH0m versus temperature for the micellization of STZ with CTAB and SDS.

The transfers of free energy (ΔG0m, tr.), enthalpy (ΔH0m, tr.), and molar heat capacity (ΔC0P. tr.) of micelles from water to the bulk aqueous phase of the investigated additive (STZ) are calculated with the following equation (Shivaji and Rakshit, 2004).

(18)
Δ Z m , t r 0 = Δ Z m ( S T Z ) 0 - Δ Z m ( a q ) 0 In Eq. (18), Z represents the free energy, enthalpy, and molar heat capacity. The values of ΔG0m, tr., and ΔH0m, tr. for (CTAB + STZ) and (SDS + STZ) systems are summarized in Table 5. The ΔG0m, tr values of both systems, (CTAB + STZ), and (SDS + STZ), are found to be positive at each temperature studied, which reveals a reduction in the spontaneity of STZ assisted micellization of CTAB and/or SDS. The ΔH0m, tr. for both systems are also found to be positive, which indicates that the transfers of hydrophilic and hydrophobic groups from water to the aqueous solution of STZ, respectively, are endothermic and exothermic in nature. The negative ΔC0P. tr. = -0.74 for (CTAB + STZ), and − 0.75 for (SDS + STZ) are calculated from Eq. (18), which indicates that the hydration of micelles increased due to the increased hydrogen bonding.
Table 5 Values of ΔG0m, tr., and ΔH0m, tr. for (CTAB + STZ) and (SDS + STZ) systems at different temperatures.
CTAB + STZ SDS + STZ
T (K) ΔG0m, tr. (kJ/mol) ΔH0m, tr. (kJ/mol) ΔG0m. tr. (J/K/mol) ΔH0m, tr. (kJ/mol)
295 0.98 1.8 1.68 1.3
303 0.92 2.1 1.23 2.4
308 0.58 3.2 0.82 3.6
313 0.66 4.5 0.73 4.2

The micellization capabilities of surfactants with STZ are compared with the results available in the literature in which different drugs are employed (Table 6). The CMC of each surfactant decreases after the addition of drug. Inspection of Table 6 clearly shows that the degree of ionization, and standard free energy of micellization depends on the structure of the drug, polar head group, and hydrophobic tail of the micelles (Farias et al., 2009; Bhardwaj et al., 2014; Banipal et al., 2018; Ali et al., 2023). A series of cationic surfactant-sulfonamide conjugates were synthesized and characterized by using infra-red and nuclear magnetic resonance spectroscopy. The as-prepared conjugates show excellent antimicrobial activities against the human pathogens such as Gram-negative (Escherichia coli, and Pseudomonas aeruginosa), Gram-positive (Bacillus subtilis and Staphylococcus aureus) bacteria, and fungal models (Aspergillus niger, and Candida albicans). These investigators also reported that the electrostatic interaction between the cationic surfactant and sulfonamide alters the biocidal activity of the conjugates (Tantawy et al., 2021).

Table 6 Values of CMC, degree of ionization (α), and standard free energy of micellization (ΔG0m) for the solubilization of sulphathiazole, and other related drugs into the ionic micelles.
Drug Surfactanta CMC (mol/L) α ΔG0m (kJ/mol) Ref.
Sulfamethoxazole DDBAC 79.6 × 10-4 0.65 −29.6 Farias et al., 2009
TDBAC 21.9 × 10-4 0.55 −36.4
HDDBAC 5.6 × 10-4 0.75 −36.0
Metronidazole DDBAC 82.0 × 10-4 0.59 −30.8 Farias et al., 2009
TDBAC 19.8 × 10-4 0.48 −38.6
HDDBAC 5.3 × 10-4 0.71 −36.9
Levofloxacin
hemihydrate		 CPB 6.3 × 10-4 0.61 −48.7 Ali et al., 2023
Ciprofloxacin hydrochloride SDS 8.1 × 10-3 0.50 −31.7 Banipal et al., 2018
HTAB 8.2 × 10-4 0.30 −46.9
Levofloxacin SDS 8.1 × 10-3 0.40 −25.8 Bhardwaj et al., 2014
Sulfonamides CTAB 9.7 × 10-4 – – Exall et al., 2013
Sulfathiazole CTAB 8.2 × 10-4 0.27 −47.2 Present work
SDS 8.1 × 10-3 0.25 −16.2
a Dodecyldimethylbenzylammonium chloride (DDBAC); tetradecyldimethylbenzylammonium chloride (TDBAC); Hexadecyldimethylbenzylammonium chloride (HDDBAC); cetylpyridinium bromide (CPB); hexadecyltrimethylammonium bromide (HTAB); sodium dodecyl sulfate (SDS); cetyltrimethylammonium bromide (CTAB).

4

4 Conclusion

The interactions of sulfathiazole with ionic and anionic surfactants, namely CTAB, SDS, and TX-100 were examined using differential absorbance and conductomeric methods. STZ micellized into the anionic and cationic micelles due to various interactions. The CMC values of CTAB and SDS decreases by adding the STZ. TX-100 did not solubilized the STZ significantly. Thermodynamic parameters suggests (ΔG0m, ΔH0m and ΔS0m) the hydrophobic and exothermic interactions occurred during the STZ micellization with CTAB and SDS micelles. The negative ΔH0m, and positive ΔS0m connote that the STZ micellization process was governed by the compensation of enthalpy and entropy-subjugated. LogKC (2.96) is also to very close for the CTAB-STZ binding constants values reported by other investigators (Sarker et al., 2016a; 2016b) using1HNMR, and semi-equilibrium dialysis methods. This agreement demonstrates that the simple and cost effective methods (differential absorbance and conductance) follow the process of micellization remarkably well, and that the values of KX are independent of the method of investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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