The interconnected porous architecture of aerogels is not determined solely by the drying stage, but originates in the chemical and physical assembly of the gel network during the sol-gel process. At this early stage, how a homogeneous precursor solution self-assembles via chemical reactions and phase separation into a two-phase micro-topology—consisting of a solid skeleton and an interstitial liquid solvent—represents a topic in physical chemistry and materials thermodynamics. Understanding mass transfer kinetics and thermodynamic evolution during gelation provides the physical basis for molecular-scale control of aerogel pore structures.

The sol-gel process represents a phase separation and assembly phenomenon driven by chemical reactions. As metal alkoxides (such as TEOS or TMOS) or organic monomers undergo hydrolysis and polycondensation within solvent media (such as ethanol or water), nanometer-scale sol particles form in the liquid phase. As reactions proceed, inter-particle chemical bonding causes sol particles to aggregate, driving the thermodynamic state of the system away from the stable region of a single-phase mixture toward metastable or unstable regions, triggering phase separation.

Thermodynamically, this phase separation proceeds via two mechanisms: Nucleation and Growth, or Spinodal Decomposition. When a system undergoes phase separation via spinodal decomposition, a precursor-rich phase and a solvent-rich phase spontaneously co-form a bicontinuous interconnected network throughout the volume. Under cross-linking chemical reactions, the precursor-rich phase rapidly solidifies to build the three-dimensional aerogel skeleton, while the solvent-rich phase fills the internal pores, establishing the interconnected nanoporous network.

During this physical phase separation process, a competition exists between solvent mass transfer diffusion kinetics and chemical reaction rates. If the chemical polycondensation rate significantly exceeds the solvent diffusion rate, sol particles are locked into disordered aggregations, resulting in high bulk density or non-uniform pore size distributions. Conversely, if phase separation progresses rapidly while chemical cross-linking lags, coarse domains undergo gravitational sedimentation and Ostwald ripening, degrading nanopores into micrometer-scale voids and reducing the nanostructural traits of the aerogel.

By introducing polymeric phase-separation agents and regulating solvent ratios, pH levels, and reaction temperatures, material scientists control the onset time and thermodynamic trajectory of phase separation. This allows gelation to freeze at the moment of bicontinuous nanonetwork formation. Controlling phase separation thermodynamics and mass transport during sol-gel processing dictates the resulting specific surface area, pore size distribution, and mechanical strength of the aerogel.