Molecular interaction of chitosan against hydrophobic and hydrophilic surfaces

Abstract

Chitosan, the second most abundant natural material after cellulose, possesses unique properties, including solubility in acidic conditions and adhesion to various substrates through physical bonding. While low molecular weight chitosan (LMW chitosan, <10 kDa) holds promise for biomedical applications such as antibacterial and gene/drug delivery due to its high water solubility, high molecular weight chitosan (HMW chitosan, >100 kDa) exhibits significant potential in tissue engineering and food packaging, owing to its high mechanical strength. Systematic studies on the adhesion between chitosan and substrates are rarely reported despite its diverse applications. The physical properties of chitosan, influenced by external conditions such as pH, salt concentration, temperature, and molecular weight, highlight the need for a comprehensive understanding to fully exploit its potential within biomedical applications. This study employed the surface forces apparatus (SFA) to probe the molecular interactions of chitosan with various functional group surfaces. Four different types of functionalized self-assembled monolayer surfaces (COOH, NH2, CH3, Phenyl) were investigated to explore hydrophilic/hydrophobic properties and identify interaction mechanisms. The findings revealed that as pH increases from 3.0 to 6.5, chitosan becomes half-protonated, leading to the formation of hydrogen bonding with hydrophilic surfaces. Additionally, strong hydrophobic interactions with the methyl SAM surface were observed due to low charge density. Understanding these molecular interactions provides valuable insights for optimizing formation of chitosan-based composites with various materials including biomaterials or synthetic materials, emphasizing the significance of probing chitosan's interaction with various functional group surfaces and its implications for binding efficiency against other molecules. In this thesis, to clarify the interaction mechanism of chitosan in detail and specifically, we investigate the interaction mechanism and interaction force between chitosan and four different types of self- assembled monolayer (SAM) surfaces (Ph(phenyl)-SAM, CH3-SAM, NH2-SAM, and COOH-SAM) using surface forces apparatus (SFA) with pH elevation, contact time, and molecular weight. Chapter 1 introduces the need for biopolymers and the excellence of chitin/chitosan, along with understanding of molecular interactions within biomaterials. This chapter explains that the efforts to determine molecular interactions between chitosan and other materials using analytical instruments, highlighting the suitability of the SFA for this purpose. In Chapter 2, the molecular interaction and mechanism studies of LMW chitosan against different functional groups are investigated using SFA with pH elevation and contact time. At pH 3.0, LMW chitosan can act as cation source with Ph-SAM. At pH 6.5, LMW chitosan exhibits strong adhesion force except Ph-SAM. Reduced charge density of LMW chitosan enhance adhesion force with methyl SAM. Strong electrostatic attraction between carboxylic SAM and LMW chitosan leads to having high adhesion energy. At pH 8.5, rigid rod structure makes LMW chitosan hard to interact with functionalized surfaces. Molecular interactions of LMW chitosan are mainly affected by charge density. In Chapter 3, the molecular interaction and mechanism studies of HMW chitosan against various functional groups with pH elevation and contact time are clarified. Soft detachment of HMW chitosan indicates that HMW chitosan has semi flexible polymer. Strong hydrophobic interaction cause chain entanglement of HMW chitosan. At pH 3.0, HMW chitosan shows high adhesion force with Ph-SAM and NH2-SAM. It indicates that HMW chitosan can act as cation source and strong hydrogen bonds can overcome electrostatic repulsion. At pH 6.5, HMW chitosan exhibits strong electrostatic attraction and hydrophobic interactions. Molecular interactions of HMW chitosan are mainly affected by chain entanglement. Chitosan has different main factor to molecular interaction mechanism with molecular weight. Interestingly, we discovered that both chitosan with the CH3-SAM had the strongest interaction energy, demonstrating the importance of hydrophobic attraction. It indicates that the polysaccharide back bone can act as hydrophobic sources even absent of N-acetyl group. Strong adhesion force observed between the chitosan and phenyl-SAM, probably due to cation-π interaction, elucidates the reason for the formation of robust composite in numerous chitin/chitosan with polyphenols such as tannins. The strong hydrogen bonding can overcome electrostatic repulsion. This phenomenon is often observed in ionic liquids, suggesting the possibility of utilizing chitosan as an ionic liquid. This thesis is focused on providing a comprehensive understanding for molecular interaction of chitosan against various functional groups. These findings provide insights for the fundamental understanding of biomaterial composites. They also show the possibility to adjust the strength of the interaction energy by manipulating the pH of the solution and specific functional groups. This opens up possibilities for optimizing formation of chitosan-based composites with various materials including biomaterials or synthetic materials.