5-Fluorouracil forms molecular complexes with anionic SDS and cationic TBAB micelles: TBAB equilibrates faster and more stably while SDS shows relaxation processes

In potassium phosphate buffer at pH 7.4, 37 °C and 0.1 mM, 5-FU (λ = 266 nm, A = 0.673) shifted its absorption peak to 268 nm with SDS and 267 nm with TBAB, indicating formation of transient molecular complexes. The author notes that although 5-FU–SDS micelle interactions have been reported, a detailed kinetic equilibrium report was not found, so the same spectral data are treated both as a simple first-order interaction and as a reversible first-order equilibrium. Based on UV-Vis spectra (Shimadzu 1800PC, 1 cm quartz cells) with micelle solutions as blank; all spectral measurements were repeated three times without change in absorbance, and rate constants came from ln(At) versus time over 10–120 min, with R² = 0.9734 for SDS and 0.9871 for TBAB.

TBAB (cationic) micelles interacted with 5-FU more regularly and faster than SDS: TBAB gave k* = 17 × 10⁻³ min⁻¹ and t1/2 = 40.76 min, versus k* = 7.60 × 10⁻³ min⁻¹ and t1/2 = 91.20 min for SDS. The author attributes this difference to the net negative charge on 5-FU (linked to the fluorine atom in its structure), which favors binding to cationic micelles, and concludes that 5-FU can be regarded as a polar, negatively charged molecule. Derived from the absorbance–time series and linear fits in Table 1; the correlation coefficient for TBAB (0.9871) is higher than for SDS (0.9734), which the author uses to judge the TBAB system as more regular and stable.

When reprocessed as a reversible first-order equilibrium, TBAB retained simple reversible behavior (k1 = 17.04 × 10⁻³, k−1 = 0.96 × 10⁻³ min⁻¹, Keq = 17.73), while SDS yielded k1 = 15.18 × 10⁻³, k−1 = 1.22 × 10⁻³ min⁻¹ and Keq = 12.39 and was interpreted as involving relaxation processes. Using the 360 min absorbance as the equilibrium value Ae and 1440 min as A∞, the author plotted ln(Ae − Ax) versus time (R² = 0.9426 for SDS, 0.9703 for TBAB), upgrading the same data from a simple first-order to a reversible first-order equilibrium treatment. Based on the linear regressions in Tables 2 and 3; the author highlights that SDS gave t1/2 = 91.20 min under the simple treatment versus (t1/2)total = 42.26 min under the reversible equilibrium treatment, whereas TBAB values were close under both, and this contrast is the main basis for the relaxation-process interpretation.

Complex formation was spontaneous in both cases, with negative ΔG⁰ (SDS −6486.62 J/mol, TBAB −7409.98 J/mol) and positive chemical affinity Ah, leading the author to attribute binding to van der Waals forces or hydrogen bonding. The author converted kinetic equilibrium constants into thermodynamic quantities using ΔG⁰ = −RT ln Keq (R = 8.314 J/K·mol, T = 310 K), linking kinetic and thermodynamic behavior. Based on the ΔG⁰ and Ah values calculated from Keq in Table 5; the abstract expresses this as ΔG⁰ < 10 kJ/mol and uses it to infer the type of binding forces.

The results apply to 5-FU with SDS or TBAB micelles at 0.1 mM in potassium phosphate buffer under physiological conditions (pH 7.4, 37 °C), a concentration the author emphasizes keeps the micelles monomeric, the solutions clear and the system within Beer-Lambert linearity. For readers working on drug delivery and surfactant interactions, this set of rate constants, half-times, Keq and ΔG⁰ values can serve as reference points for comparing how differently charged micelles bind 5-FU; the author also suggests that if 5-FU is treated as a polar, negatively charged molecule, the cationic micelle TBAB may offer a more regular and faster route to complex formation.

The author's interpretation of relaxation processes in the SDS system rests mainly on the difference in t1/2 between the simple first-order and reversible equilibrium treatments (91.20 min versus 42.26 min), an explanation built on mathematical fitting rather than independent structural characterization, so readers may watch whether other spectroscopic methods are needed to corroborate it. In addition, conclusions are based on a single concentration, a single temperature and two surfactants, so extension to other micellar systems, other pH values or in vivo environments remains an open question; the author also notes instability in the SDS system between 360 and 1440 min, a time window whose behavior warrants further observation.