Bio-inspired eutectogels enabled by binary natural deep eutectic solvents (NADESs): Interfacial anti-frosting, freezing-tolerance, and mechanisms

Abstract

To explore food-grade anti-freezing strategies for the frozen food industry, three bio-inspired binary natural deep eutectic solvents (NADESs) comprising proline, glucose, and sorbitol were prepared. Besides, through gelling the NADESs, two-dimensional planar film-like eutectogels (PG53, PG11, and PS11) showing sandwich-like structures were fabricated, of which basic characteristics, anti-frosting capacities and freezing-tolerance were investigated. Results indicated that the eutectogels possessing plenty of hydroxy groups within systems could show more hydrophilic surfaces (47.6°–55.7°) as compared with the control group (69.1°). Besides, PS11 showed the best anti-frosting capacity, exhibiting only 36.31% frost-covered area after 120 min under extreme conditions, probably due to its more rugged surface possessing more and spikier hill-like profiles. For freezing-tolerance, all the eutectogels could stay flexible at −40 °C and show stable mechanical strengths before (stress: 3.1–4.4 MPa, strain: 82.6%–104.7%) and after (stress: 2.5–3.9 MPa, strain: 71.9%–96.5%) extreme temperature fluctuation. Given this, an absorption-inhibition hypothesis was proposed and verified by gradient moisture absorption test and cooling-heating cyclic test, through which critical absorption relative humidity (CARH) and glass transition temperature (Tg) were obtained, which suggested that the eutectogels firstly absorbed the ambient moisture easily (CARH: 49% RH ∼ 55% RH) and subsequently inhibited the crystallization through immobilizing the moisture within the NADESs systems (Tg: −58.6 °C ∼ −50.5 °C), endowing them with great potentials for interfacial anti-frosting. It is hoped that this work could broaden the field of bio-inspired anti-freezing materials in a greener and eco-friendly way.