Interaction mediated growth of chitosan and fatty acid binary system at air-water and air-solid interfaces

Chitosan (CHS), being a polysaccharide of rich physicochemical properties, has attracted a great deal of research worldwide. It is widely used in medicine, food preservation, cosmetics, antibacterial agent and biotechnology owing to its biocompatibility, biodegradability and polycationic nature. In most of these applications CHS has to interact with bio-membrane surfaces. Being biocompatible means that CHS can interact with biomolecules without degrading them. This unique characteristic brings CHS into spotlight as a green polymer. CHS is being considered in various countries as a dietary supplement of human consumption. However, concern has arisen about the fact that certain promotional campaigns advertise CHS as a fat-binding active agent towards all kinds of lipids with no supporting scientific proof. Therefore, understanding the interactions involved in the process of binding CHS with stearic acid (SA) is of paramount interest. But still the mechanism of these interactions is not well understood. In this work we have demonstrated that CHS, which is widely used in biomedical science owing to its biocompatibility and biodegradability, does not form self-supporting film individually at the air-water interface, whereas CHS mixed with SA does. However, there is still a lack of understanding about the surface orientation of CHS in the presence of lipids and fatty acids, the residence of CHS at the air-water interface, the interaction involved. Though a numerous literature exploring viscoelastic properties (e.g. elasticity or compressibility) of CHS-lipids and CHS-fatty acids hybrid systems is present, their thermodynamic (excess Gibb’s free energy of mixing) and thermal (temperature dependent) properties remain unexplored. CHS insertion causes an expansion of CHS -fatty acid hybrid monolayers and reduces the elasticity and makes the film heterogeneous. A similar expansion is observed with lowering of temperature. CHS interacts with the fatty acid monolayer by means of electrostatic, dipolar and hydrophobic interactions. Furthermore it was found from energy stability analysis that excess Gibb’s free energy is positive, which indicates the presence of repulsive interaction between CHS and stearic acid. The same nature was also confirmed via miscibility study. The results were rationalized in terms of a model (see figure) in which at low surface pressure CHS is situated at the interface, interacting with SA molecules via electrostatic and hydrophobic interactions whereas at high pressure CHS is mainly located at the subsurface beneath stearic acid molecules.

In the latter case the interaction is predominantly electrostatic yielding very small contribution to the surface pressure. We also extended these properties towards low temperatures (down to 8 0C) along with thermodynamic stability (Fig. 1c and Fig. 1d). A reduction of the temperature of subphase water allows more CHS molecules to reach the surface and reduced the elasticity and made the film heterogeneous. CHS endorses a local distortion of the fatty acid tails involving electrostatic, dipolar and hydrophobic interactions. The results could be rationalized in terms of a model in which at low surface pressure CHS is situated at the interface, interacting with stearic acid molecules via electrostatic and hydrophobic interactions whereas at high pressure CHS is mainly located at the subsurface beneath stearic acid molecules. Additionally, we have demonstrated that the CHS could be transferred easily onto solid supports employing the LB technique by mixing with fatty acid in subphase. Their detailed structure extracted from two complementary surface sensitive (precision ~ few angstrom) techniques, such as XRR (out-of-plane) and AFM (in-plane), is found strongly dependent on the CHS mole fraction and deposition pressure. By knowing the structure of solid supported films at various conditions, one can develop the organization or conformation of the pre-deposited films at the air-water interface. This might lead to different model structures, which may have implications in the biological applications of CHS in drug delivery and in bio sensing devices.