Carboxymethylcellulose (CMC), also known as sodium carboxymethylcellulose, is a water-soluble anionic derivative of cellulose. It is widely employed as a versatile hydrocolloid in food, pharmaceutical, and industrial applications due to its thickening, stabilizing, and film-forming properties. 

Chemical Structure and Synthesis

CMC is derived from native cellulose, a linear β-(1→4)-D-glucopyranose polymer, through alkaline activation of hydroxyl groups (reactivity order C6 > C2 > C3), followed by etherification with monochloroacetic acid (ClCH₂COOH). This process introduces carboxymethyl (–CH₂COO⁻) substituents along the cellulose backbone, with typical degrees of substitution ranging from 0.4 to 1.5 (commercial food-grade products generally 0.65–0.95). The repeating unit, [C₆H₇O₂(OH)₃₋ₓ(CH₂COONa)ₓ]ₙ (n ≈ 500–3000; Mw ≈ 90–700 kDa), preserves the polysaccharide framework while conferring polyelectrolyte behavior.

Physicochemical Properties

Carboxymethylcellulose exhibits high water solubility (>10% w/v in both cold and hot water) and forms viscous, pseudoplastic solutions characterized by shear-thinning behavior (power-law index < 1). Its viscosity can be tailored by controlling molecular weight, degree of substitution, and counterion type, with sodium salts generally providing the highest viscosity. CMC swells irreversibly in alkaline conditions and precipitates in acidic media or ethanol (pKa ≈ 4.3). It shows good salt tolerance up to approximately 0.5 M, thermal stability up to about 120 °C, and moderate surface activity, enabling emulsion formation and stabilization. High-purity grades are manufactured with low heavy-metal content (<10 ppm) and controlled microbial levels to comply with food and pharmaceutical quality requirements.