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pH-RESPONSIVE SWELLING OF A POLY(ACRYLIC ACID) ON TiO₂ NANOPARTICLES

Mykyta V. Prud¹, Marharyta M. Blazhynska^1,*, Alexander Kyrychenko¹, Oleg N. Kalugin¹

¹ - Education and Research Institute of Chemistry, V. N. Karazin Kharkiv National University, Kharkiv 61022, Ukraine

* Present address - Qubit Pharmaceuticals, Paris, France

Abstract

pH-Sensitive poly(acrylic acid) (PAA) is a promising coating for novel functional nanomaterials; however, elucidating the underlying interfacial mechanisms requires detailed atomistic modeling. In this work, we investigate the structural behavior of a bare amorphous titanium dioxide (TiO₂) nanoparticle in water and its functionalization with PAA₁₅₄₀ using molecular dynamics (MD) simulations. We found that at the degree of ionization α = 0, the uncharged PAA chain spontaneously self-assembles into a highly compact, dense encapsulation layer. In contrast, in its fully ionized form (α = 1), intra-chain Coulombic repulsion drives the expansion of the deprotonated polymer into a loose, swollen coating with extended loops and tails. This conformational switch underscores the critical role of pH-responsive electrostatic interactions in governing the thickness and density of the protective organic shell.

Models of TiO₂ Nanoparticle and Poly(acrylic acid)

Figure 1. (a) Amorphous (TiO₂)₇₂₀ nanoparticle (d ≈ 3.9 nm) and (b) chemical structure of PAA.

pH-Driven Adsorption of PAA onto TiO₂ Nanoparticle

Figure 2. MD snapshots of PAA₁₅₄₀ adsorption onto TiO₂ at α = 0 (0, 600 and 1000 ns): collapse into a dense, compact corona.

Figure 3. MD snapshots of PAA₁₅₄₀ adsorption onto TiO₂ at α = 1 (0, 600 and 1000 ns): loose, swollen corona with loops and tails.

Acid-Base Equilibrium in PAA

PAA is a weak polyacid: its degree of ionization α depends on pH and is set by the ratio of –COOH and –COO^– groups (Fig. 4, Eq. 1).

(1)

where N_–COO^– is the number of ionized groups and N_total is the total number of monomer units.

Figure 4. Ionization of PAA: α = 0 at pH ≤ 2; α = 1 at pH ≥ 10.

MD Simulation Setup

TiO₂-NP was described by the Matsui–Akaogi force field [1] refitted to the 12-6 Lennard-Jones potential (q_Ti = +2.196e, q_O = –1.098e); PAA was modelled with the united-atom GROMOS 53a6-based force field [2]. pH was mimicked by fixed protonation states, α = 0 and 1 [3]. Systems were solvated in explicit SPC water (Na⁺ counterions at α = 1) and simulated for 1000 ns (NPT, 298.15 K, 1 atm) in GROMACS 2019.5.

Interfacial Hydrogen Bonding and Hydration

Table 1. Number of H-bonds and contacts within 0.35 nm.

References

[1] Matsui M., Akaogi M. Mol. Simul. 1991, 6, 239–244.

[2] Kyrychenko A., Blazhynska M. M., Slavgorodska M. V., Kalugin O. N. J. Mol. Liq. 2019, 276, 243–254.

[3] Prud M. V., Kyrychenko A., Kalugin O. N. J. Phys. Chem. C 2023, 127, 11748–11759.

pH-Responsive Swelling of PAA Corona

Figure 5. (a) Compact (α = 0) vs. swollen (α = 1) PAA corona; radial profiles of (b) PAA and (c) water from the TiO₂ surface.

Conclusions

Our 1000-ns MD simulations show that PAA₁₅₄₀ spontaneously adsorbs onto the amorphous TiO₂ nanoparticle regardless of its protonation state, but the coating conformations differ drastically. Neutral PAA (α = 0), anchored by H-bonds and carbonyl–Ti coordination, forms a dense, compact corona that displaces interfacial water. Upon full ionization (α = 1), intra-chain Coulombic repulsion swells the corona into the bulk solvent (the PAA density peak shifts from 1.03 to 1.65 nm), while carboxylate–Ti coordination and Na⁺ condensation keep it anchored. Thus, the degree of ionization governs the thickness and compaction of the PAA corona, guiding the rational design of pH-responsive hybrid organic/inorganic nanomaterials.

Acknowledgements. MD simulations were performed at the Center of Collective Use of Scientific Equipment "Laboratory of Micro- and Nanosystems, Advanced Materials and Technologies" of V. N. Karazin Kharkiv National University. M.V.P. acknowledges Grant 0126U003820 of the Ministry of Education and Science of Ukraine.