• Acidic SiO2 nanofluids significantly enhance coal wettability, reducing the contact angle by up to 43.86% at pH 2.
• SiO2 nanoparticles act as bridging agents, increasing the adsorption layer thickness from 15.44 Å to 20.51 Å in high-mass fraction systems.
• Mild acidity (pH 5) improves coal nanomechanical properties, with elastic modulus and hardness increasing by ~18%.
• Strong acidity causes corrosion that outweighs nanoparticle support, leading to mechanical degradation.