(164b) Permeability Study of Urea and Alkylated Derivatives through Different Polymeric Membranes

Efficient molecular separation of small, uncharged solutes such as urea and its derivatives is central to improving separation technologies in pharmaceutical processing, nutrient recovery, and water purification. While urea transport across membranes has been studied, the role of hydrogen bonding and steric effects in alkyl-substituted derivatives remains poorly quantified. In this work, we systematically investigated the permeability of urea, N,N-dimethylurea, N,N,N′-trimethylurea, N,N,N′,N′-tetramethylurea, and N,N-diethylurea through three structurally distinct membranes: cellulose triacetate (CTA), poly(m-phenylene isophthalamide) (PMIA), and sulfonated poly(styrene-isobutylene-styrene) (SIBS). Membranes were fabricated via non-solvent-induced phase separation (NIPS), with controlled casting parameters yielding anisotropic, finger-like structures, membrane thickness ≈ 150 – 200 μm. Transport experiments were conducted using a side-by-side diffusion cell under isothermal conditions (25 ± 0.3 °C), with time-lag analysis providing both diffusivity and permeability values. Complementary measurements—including water uptake, contact angle, SEM, and SAXS were used to evaluate membrane hydrophilicity, and nanoscale morphology.

Quantitative analysis revealed a systematic decline in permeability with increasing methyl substitution: urea (9.1 × 10⁻⁶ cm²/s) > dimethylurea > trimethylurea > tetramethylurea (6.4 × 10⁻⁶ cm²/s).> diethylurea (7.1 × 10⁻⁸ cm²/s, SIBS membrane). This trend supports a reduction in hydrogen bonding capacity with alkyl substitution. Notably, despite a lower degree of substitution, diethylurea exhibited the lowest permeability across all membranes tested. SAXS data suggest that steric bulk and chain flexibility contribute to increased entropic barriers to diffusion, indicating that molecular geometry can outweigh hydrogen bonding in governing transport. Water uptake and contact angle trends further support a diffusion mechanism sensitive to both solute–polymer interactions and membrane polarity.

These results define key structure–permeability relationships for alkylated ureas and highlight the combined roles of hydrogen bonding capacity, molecular size, and geometry in regulating solute diffusion through polymeric membranes. This understanding provides a foundation for the rational selection and design of membrane materials in applications such as urea removal in portable dialysis devices, nutrient capture in water reuse systems, and the selective separation of uncharged small molecules in advanced filtration processes.