Methods and compositions for forming durable porous low refractive index coatings on substrates are provided. In one embodiment, a method of forming a porous coating on a substrate is provided. The method comprises coating a substrate with a sol-formulation comprising a silane-based binder, silica-based nanoparticles, and an inter-particle interaction modifier for regulating interactions between the silica-based nanoparticles and annealing the coated substrate. Porous coatings formed according to the embodiments described herein demonstrate good optical properties (e.g., a low refractive index) while maintaining good mechanical durability due to the presence of the inter-particle interaction modifier. The inter-particle interaction modifier increases the strength of the particle network countering capillary forces produced during drying to maintain the porosity structure.