Alkali Lignin and Lignosulfonate Tune Strength, Extensibility, and Hydration in Multifunctional Deep Eutectic Gel Strain Sensors

Abstract

Integrating strength, large deformation, adhesion, self-repair, antimicrobial activity, and stable ionic sensing within a biomass-containing soft material remains difficult because these properties depend on competing network requirements. Here, alkali lignin (AL) and lignosulfonate (LS) were incorporated into an acrylic acid/choline chloride/betaine polymerizable deep eutectic matrix to regulate these trade-offs. Spectroscopic, diffraction, microscopic, thermal, and electrokinetic measurements showed that lignin chemistry altered the local chemical environment, morphology, water uptake, and degradation behavior of the resulting deep eutectic gels. AL shifted the mechanical response toward reinforcement: tensile strength increased from 0.90 MPa for AL0.5-DG to 5.75 MPa for AL2.5-DG, while strain at break decreased from approximately 960% to 435%. LS retained a deformation-tolerant window, with LS1.5-DG reaching approximately 1010% strain at break and a final swelling ratio of approximately 249%. Density functional calculations and a ChCl-centered molecular dynamics analysis identified localized lignin contacts with betaine, choline/chloride, acrylic species, and polymer-associated sites. The gels also adhered to dissimilar substrates, recovered 0.67 MPa tensile strength after 8 h of interfacial healing, reduced bacterial survival at higher lignin loading, and produced repeatable resistance responses over 10–150% strain, 3600 s cycling, and subzero deformation. The results establish lignin chemistry as a practical control variable for balancing load bearing, hydration, recovery, and ionic sensing in multifunctional deep eutectic gels.