pH- and Thermo-Responsive Polymeric Nanoparticles for Divalent Cation Complexation: Potential Applications in Carbonate Reservoir Treatment

Introduction.

Acid stimulation in carbonate reservoirs traditionally depends on hydrochloric acid (HCl); however, operational challenges necessitate the examination of alternative chemical agents. pH-thermo-responsive polymeric nanomaterials bearing carboxylate functional groups are considered promising candidates due to their capability to complex Ca²⁺ and Mg²⁺ ions. This study explores the synthesis and fundamental physicochemical responses of functionalized polymeric nanoparticles upon interaction with divalent electrolytes under temperature conditions pertinent to reservoir environments.

Methods.  

In this research, a series of polymeric nanoparticles functionalized with carboxylic and amide groups was synthesized by emulsion polymerization. A core-concentration-gradient morphology was achieved using a power-feed process. Polymers were characterized before and after interaction with electrolyte solutions (CaCl₂ or MgCl₂) using dynamic light scattering (DLS), electrophoresis (zeta potential, ζ), scanning electron microscopy (SEM), FT-IR spectroscopy, and X-ray diffraction (XRD). All measurements were performed in triplicate over a temperature range of 20 °C to 60 °C.

Results.

DLS analysis showed variations in particle diameter over the temperature range attributed to the phase-transition temperature of (-COOH). Negative ζ-potential values (-50 ≤ ζ/mV ≤ -20) were observed, resulting from the deprotonation of carboxyl groups (-COO^-). As temperature increased, ζ-potential values approached zero, suggesting decreased stability. SEM images revealed homogeneous particle morphologies prior to electrolyte titration, whilst interaction with the electrolytes induced temperature-dependent morphological changes, indicating the incorporation of divalent ions into the polymeric chain, which is corroborated by XRD and FT-IR analyses.

Conclusions.

The results confirm that the synthesized materials exhibit pH- and temperature-responsive behavior and effective complexation with divalent cations (Ca²⁺ and Mg²⁺), inducing temperature-dependent conformational changes via Hofmeister effects. These findings establish the fundamental physicochemical basis of these nanoparticles, highlighting their potential relevance for future application in fluids for carbonate reservoir treatments.