Thiourea as a “Polar Hydrophobic” Hydrogen-Bonding Motif: Application to Highly Durable All-Underwater Adhesion

• Published in: Journal of the American Chemical Society 2024-07, Vol.146 (30), p.21168-21175
• Main Authors: Kikkawa, Kohei, Sumiya, Yosuke, Okazawa, Kazuki, Yoshizawa, Kazunari, Itoh, Yoshimitsu, Aida, Takuzo
• Format: Article
• Language: English
• Subjects: adhesion; glass; hydrogen bonding; hydrophobicity; liquids; seawater; thiourea; urea; water solubility
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.4c07515

Here, we report that, in contrast to urea, thiourea functions as a “polar hydrophobic” hydrogen-bonding motif. Although thiourea is more acidic than urea, thiourea exchanges its N–H protons with water at a rate that is 160 times slower than that for urea at 70 °C. This suggests that thiourea is much less hydrated than urea in an aqueous environment. What led us to this interesting principle was the serendipitous finding that self-healable poly­(ether thiourea) adhered strongly to wet glass surfaces. This discovery enabled us to develop an exceptionally durable all-underwater adhesive that can maintain large adhesive strength for over a year even in seawater, simply by mechanically mixing three water-insoluble liquid components on target surfaces. Because thiourea is hydrophobic, its hydrogen-bonding networks within the adhesive structure and at the adhesive–target interface are presumed to be dehydrated. For comparison, a reference adhesive using urea as a representative "polar hydrophilic" hydrogen-bonding motif was durable for less than 4 days in water. Highly durable all-underwater adhesives are needed in various fields of marine engineering and biomedical sciences, but their development has been a major challenge because a hydration layer that spontaneously forms in water always inhibits adhesion.