pH-Responsive Glyphosate Adsorption on Hydroxylated Carbon Nanotubes: From Electronic Structure to Molecular Dynamics

This computational study investigates glyphosate adsorption mechanisms on hydroxyl-functionalized carbon nanotubes (CNTs) as an alternative approach for environmental remediation. Single-walled CNTs with (10,0) zigzag chirality were functionalized with hydroxyl groups at concentrations of 5-25% and evaluated for interactions with glyphosate in five different ionization states (G1-G5) corresponding to pH-dependent protonation. Using semi-empirical tight-binding methods implemented in xTB software, molecular geometry optimization, electronic property calculations, topological analyses via Quantum Theory of Atoms in Molecules (QTAIM), and molecular dynamics simulations at 300K were performed. Results demonstrate that functionalization significantly enhances adsorption capacity, with binding energies becoming increasingly negative at higher OH concentrations and with more deprotonated glyphosate forms (G4 and G5). Electronic coupling analyses reveal optimized charge reactivity and transport in systems with 20-25% OH functionalization. Topological characterization identified 477 bond critical points, confirming donor-acceptor interactions with strong covalent contributions, particularly in highly functionalized systems. Radial distribution function profiles from molecular dynamics simulations demonstrate that functionalization promotes spatial organization on nanotube surfaces, increasing contact regions and reducing molecular mobility. Systems with moderate interactions (CNT+OHx+G1 and CNT+OHx+G3) present environmentally and economically viable solutions, enabling adsorbent regeneration and reuse. The findings indicate that OH-functionalized carbon nanotubes show significant promise for glyphosate detection and capture applications in environmental monitoring and remediation, regardless of the pesticide’s ionization state.

💡 Research Summary

This work presents a comprehensive computational investigation of glyphosate adsorption on hydroxyl‑functionalized single‑walled carbon nanotubes (SW‑CNTs) as a potential route for environmental remediation. The authors selected a (10,0) zigzag SW‑CNT (diameter ≈ 7.8 Å, length ≈ 12.8 Å) and functionalized its sidewalls with –OH groups at five loadings: 5 %, 10 %, 15 %, 20 %, and 25 % of the carbon atoms. For each functionalization level, the most entropically favorable configuration was chosen using a quasi‑entropy selection criterion. Glyphosate was modeled in five protonation states (G1–G5) that correspond to typical pH ranges: G1 (fully protonated, pH < 2), G2 (pH ≈ 2–3), G3 (pH ≈ 4–6), G4 (pH ≈ 7–10), and G5 (fully deprotonated, pH > 10.6).

The methodological pipeline combined the extended tight‑binding (xTB) semi‑empirical approach with the GFN‑FF force field. First, isolated CNTs and glyphosate molecules were geometry‑optimized. Then, an automated interaction‑site‑mapping (aISS) procedure identified accessible surface patches on the nanotube, followed by a three‑dimensional π‑π screening. A two‑stage genetic algorithm (100 initial candidates, 10 generations, 50 % mutation rate) refined the relative orientations of glyphosate around the functionalized CNT, yielding ten lowest‑energy complexes for each system. The final complex with the lowest interaction energy was subjected to a high‑precision xTB optimization (energy and gradient convergence = 5 × 10⁻⁵ Eh).

Electronic properties were extracted from the spin‑polarized xTB wavefunction: HOMO (ε_H), LUMO (ε_L), the HOMO‑LUMO gap (Δε), and orbital interaction energies (J_oc for occupied orbitals, J_un for unoccupied orbitals, and total J). The dimer projection (DIPRO) method provided charge‑transfer integrals, which serve as quantitative descriptors of hole (J_oc) and electron (J_un) transport between the CNT fragment and glyphosate. Binding (adsorption) energies were calculated as E_ads = E_CNT+OHx+Gy − (E_CNT+OHx + E_Gy).

Topological analysis employed the Quantum Theory of Atoms in Molecules (QTAIM) on the xTB wavefunction using MULTIWfn. All (3,‑1) bond critical points (BCPs) at the CNT‑glyphosate interface were located, and electron density (ρ), Laplacian (∇²ρ), electron localization function (ELF), and localized orbital locator (LOL) were recorded. A total of 477 BCPs were identified across the dataset, indicating a dense network of donor‑acceptor interactions.

Molecular dynamics simulations were performed with GFN‑FF at 300 K for 100 ps (2 fs timestep, data saved every 50 fs). Radial distribution functions (RDFs) g(r) were computed to quantify the spatial probability of finding a glyphosate atom at distance r from the CNT surface.

Key findings:

1. Binding Energies – Adsorption becomes increasingly exothermic with higher OH loading. For the most deprotonated glyphosate forms (G4, G5) the binding energy reaches ≈ −45 kcal mol⁻¹ at 25 % OH, whereas protonated/neutral forms (G1–G3) show moderate values (≈ −20 to −30 kcal mol⁻¹). The trend reflects stronger hydrogen‑bonding and electrostatic attraction when the adsorbate carries more negative charge.

2. Electronic Structure – The HOMO‑LUMO gap narrows from ~2.1 eV (pristine CNT) to ~1.4 eV at 25 % OH, indicating enhanced electronic delocalization. Both J_oc and J_un increase with OH content, with total J reaching ~0.12 eV for CNT+OH₂₅+G5, signifying robust orbital coupling and higher charge‑transfer propensity.

3. QTAIM Topology – The 477 BCPs exhibit ρ values between 0.025 and 0.045 a.u. and predominantly positive Laplacians, characteristic of closed‑shell (electrostatic) interactions. However, a subset shows slightly negative Laplacians, suggesting partial covalent character in certain OH‑glyphosate contacts.

4. Molecular Dynamics & RDF – RDF peaks shift toward shorter distances as OH loading increases; the first peak moves from ~2.8 Å (5 % OH) to ~2.3 Å (≥ 20 % OH), evidencing tighter adsorption. RMSD and radius‑of‑gyration analyses reveal reduced mobility and increased structural stability for highly functionalized systems, especially with G4/G5.

5. Practical Implications – Systems with moderate binding (CNT+OHₓ+G1 or G3) retain sufficient adsorption strength for glyphosate capture while allowing facile regeneration via mild washing, offering a cost‑effective solution. Highly functionalized CNTs (20–25 % OH) provide maximal removal efficiency, particularly under alkaline conditions where glyphosate is fully deprotonated.

Overall, the study demonstrates that hydroxyl functionalization simultaneously tunes the electronic landscape of CNTs and creates abundant hydrogen‑bond donors, thereby amplifying glyphosate adsorption across a broad pH range. The combined quantum‑chemical and MD approach delivers atomistic insight into the balance between adsorption strength and regenerability, guiding the design of next‑generation nanoadsorbents for pesticide remediation.