Development of the Top Layer Method for Measuring the Diffusion Kinetics of Small Molecules in Hydrogels
Date
2023-04-11
Authors
Wang, Guanqiao
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Abstract
Hydrogels have a wide range of applications, such as in everyday personal care products, for the recovery of oil in extraction processes, and as materials for the development of drug delivery vehicles. The design of these materials benefits from the knowledge on how small molecules move within the gel, and the understanding on how the structure of the gel affects the mobility of small molecules. I compared the properties of two types of supramolecular gels: a physical polymeric gel and a low molecular weight gelator (LMWG) supramolecular gel.
The polymeric gel under investigation is a triblock copolymer, Pluronic F-127. It is a thermo-reversible gelator that is composed of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene ocide) (PEO-PPO-PEO) subunits. The polymer forms micelles at low temperatures in solution and a transition into a cubic phase occurs when the temperature increases, resulting in a gel.
The LMWG gel under investigation is a bile salt, sodium deoxycholate (NaDC), whose size is significantly smaller than F-127. NaDC forms thixotropic and temperature-sensitive hydrogels within a narrow pH range. The NaDC molecules self-associate to form aggregates that form a gel when the pH is close to neutral, entrapping pools of aqueous solutions.
I compared the diffusion of small molecules in these two hydrogels. I have established a reproducible method to measure diffusion kinetics of small molecules in hydrogels over the centimetre length scale, using UV-vis spectroscopy. I prepared a set volume of gel in a 10 mm × 10 mm quartz cuvette and added a thin layer of dye solution on top. The absorbance of the sample at a fixed position in the gel is measured periodically to yield the kinetics curves for the diffusion of dyes through the gel.
The results showed that the polymer concentration greatly affected the diffusion of small molecules in hydrogels. Higher polymer concentration slows the diffusion of small molecules in the hydrogels. The concentration of the dye in the top layer solution did not affect the diffusion rate but it affected the amplitude of the absorbance in the kinetics curves. The charge on the dye molecules may interact with the gelators and slow down the diffusion. The negative dye fluorescein sodium salt was found to diffuse faster than methylene blue in NaDC hydrogels, due to the charge interaction between the dye and gelator molecules.
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Keywords
Polymer, Hydrogel, Diffusion, Sodium Deoxycholate, Kinetics, Pluronic, F-127