Soy protein films with the hydrophobic surface created through non-covalent interactions

Different plasticizers (P) such as glycerol, 1,2 propanediol, trimethylol propane, thiodiglycol, and formamide (30% by weight w.r.t. soy protein) were used to prepare soy protein films (coded as SP) by compression molding at 140 °C and 20 MPa. The SP films were immersed in 0.5% (w/v) benzilic acid solution for 26 h to get arylated soy protein films (SP-B) with the evolution of CO2, leading to the formation of diphenylhydroxymethane (DPHM) on the surface. Lotus leaf like structure, on the surface of SP-B film, with high hydrophobicity has been created, supported by the existence of nano/micro spheres. The SP-B and SP films were characterized by Fourier transformed infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, tensile testing, optical transmittance, contact angle, and biodegradation experiments. The mechanical properties, transmittance, and contact angle of SP-B films increased significantly, compared with SP. The SP-B films exhibited high water resistance in the range of 67–82%. These results indicated attractive interactions (often termed as π-interactions) of DPHM aromatic groups took place in the SP-B films resulting in the formation of nanospheres or microspheres. This work provided a good way for increasing the surface hydrophobicity and the mechanical properties of the protein materials through non-covalent interactions.