Study on the mechanism of zwitterionic modification of PVDF/TA-PEI/PS membranes for efficient separation of traditional Chinese medicine volatile oil emulsions

Persistent emulsification of TCM volatile oil distillates seriously restricts oil–water separation and causes severe fouling of pristine PVDF ultrafiltration membranes. To address this bottleneck, a two-step modification strategy combining tannic acid–polyethyleneimine (TA-PEI) co-deposition and covalent grafting of 1,3-propanesultone (PS) was developed to fabricate a zwitterionic PVDF/TA-PEI/PS composite membrane. Multiple characterizations confirmed the successful construction of a stable zwitterionic coating anchored via hydrogen bonding and covalent grafting. The modified membrane exhibited significantly enhanced hydrophilicity, with the water contact angle decreasing from 91.5° to 31.7° and the underwater oil contact angle reaching 155.72. Under optimized conditions, the membrane achieved a pure water flux of 2500 L·m −2 ·h −1 and a turmeric volatile oil enrichment efficiency of > 92%, with a flux recovery ratio of 86.52% after five fouling-cleaning cycles. This superior performance arises from the synergistic effect of the zwitterionic hydration barrier and the near-neutral surface charge (pH 4–12). The membrane also demonstrated excellent environmental tolerance in 60–100% ethanol, pH 4–12 buffers and 50–90 °C. Molecular dynamics simulations revealed that zwitterionic grafting increased the interfacial adhesion work from 76.9 to 80.2 kJ·mol −1 ·nm −2 , elucidating the origin of coating stability. Long-term tests in corrosive and non-corrosive TCM volatile oil systems confirmed its operational durability. This work provides a scalable route for constructing durable zwitterionic PVDF membranes, offering both mechanistic understanding and a practical platform for the industrial-scale enrichment of TCM volatile oils.