Calcium Alginate/Polyaniline-PSS Core–Shell Particles: Synthesis and Multi-Mechanism Water Vapor Adsorption Behavior

Abstract Core–shell composite particles combining a calcium alginate (Alg-Ca) hydrogel core with a polyaniline/polystyrene sulfonic acid (PANi/PSS) conductive polymer shell were synthesized and evaluated for water vapor adsorption applications. The resulting particles exhibited a well-defined, phase-separated architecture, comprising a highly hydrated, porous hydrogel core encapsulated by a structurally and chemically distinct shell, consistent with successful incorporation of the sulfonated conductive polymer onto the surface of the alginate network. The core and shell regions displayed markedly different thermal decomposition behavior, confirming their compositional distinction and demonstrating that the composite retains thermal stability within the operating range typical of gas-sensing applications. Kinetic analysis of water vapor adsorption as a function of relative humidity (RH, 50–95%) and temperature (25–45 °C) revealed a systematic transition among pseudo-first-order, pseudo-second-order, and Elovich kinetic models. At moderate RH, adsorption was governed by chemically homogeneous interactions at polar functional groups, while at near-saturation RH, the mechanism shifted toward multilayer adsorption and capillary condensation. Temperature-dependent measurements at fixed RH further revealed an unusual diffusion–reaction–diffusion transition across the studied temperature range. Across three adsorption–regeneration cycles, adsorption capacity increased progressively while the dominant kinetic model at each condition remained unchanged, indicating that regeneration preserves the fundamental water–material interaction mechanism while increasing the number of accessible active sites. These findings establish Alg-Ca@PANI–PSS as a structurally robust, reusable, and mechanistically well-characterized composite for humidity-responsive and gas-sensing applications.