Surface Activity Study of Water-Soluble Silk Fibroin Prepared using Cocoons and Ca(NO3)2 · 4H2O

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Abstract
The objective of this study was to analyze the relationship between structural changes and surface-activity of water-soluble silk fibroin prepared by treatment with calcium nitrate tetrahydrate (Ca(NO3)2 · 4 H2O). Ca(NO3)2 · 4 H2O, is a hygroscopic compound at room temperature and a suitable solvent upon melting at 100 °C, which was traditionally used as a solvent for dissolving cocoons or silk. The cocoons or silk were optimally dissolved by Ca(NO3)2 · 4 H2O when using a 40 % (w/w) Ca(NO3)2 solution, a 1 : 10 ratio of cocoons or silk to solvent and a dissolving time of 69 min. The results showed that the hydrophobic region of the silk fibroin was destroyed, resulting in the exposure of the hydrophobic groups. The emulsifying ability and the emulsion stability as well as the foaming ability and the foam stability, and the γCMC and CMC of soluble silk fibroin were 92.8 %, 97.3 %, 213.3 %, 88.1 %, 65.83 mN/m and 0.42 mg/mL, respectively. The molecular conformation of silk fibroin chains was the β-sheet, as shown by the intense amide I–III bands at 3 163 cm−1, 1 627 cm−1, 1 518 cm−1, and 1 231 cm−1. The random coil/α-helix structure induced from Ca(NO3)2 convert to β-sheet conformation. Owing to the calcium nitrate's dissolution, silk fibroin can be dissolved in water by changing its structure, and shows excellent surface activity.