*3.2. pH-Induced Gelation*

The pH-induced gelation can be promoted changing pH value of some polysaccharide solvent; at the contact point of each droplet of polymeric solution with the acidic or alkaline bath, the gelation starts forming first a shell, and becomes later complete thanks to the diffusion of the ions through the shell (see Figure 6b). This method is often used alone or in combination with other gelation methods to prepare gel particles of chitosan, pectin and alginic acid [15]. For example, alginic acid gels are formed when pH of the solution is brought down below the disassociation constant (pKa) of the polymer [65]. As reported by Draget et al. [66] also the rate of decrease in pH can affect gel properties; in fact, a rapid decrease in pH results in precipitation of alginic molecules in the form of aggregates while a slow and steady drop in pH results in the formation of a continuous alginic acid bulk gel. Unlike ionic gels, acid gels of alginate are stabilized by intermolecular hydrogen bonds between carboxylic groups of different chains and M-blocks residues have been shown to play a part in gelation. This also applies to pectin, for which the gelation is stabilized by hydrophobic interactions of methylated groups [67,68]. By contrast, chitosan gel particles are prepared at higher pH values. Chitosan is firstly dissolved under mild acidic condition (usually realized using acetic acid) by protonating the amine functional group, and then gel particles are produced under alkaline medium (usually with NaOH solution); the pH value of the alkaline solution must be maintained above the pKa value (6.3) of -NH2 functional groups in order to deprotonate the amine groups [4]. Cellulose is instead coagulated using strong acidic solutions of H2SO4 [69], HNO3 [70] or HCl [71–73] that, acting as non-solvents, induce the formation of a gel-like structure [4].
