Competition between Osmotic Squeezing versus Friction-Driven Swelling of Gels.

Miyu Seii,Masao Doi,Yoshimi Tanaka,Tomoki Harano

doi:10.3390/gels7030094

Miyu Seii, Masao Doi + Show 2 more

Open Access

https://doi.org/10.3390/gels7030094

Copy DOI

Abstract

Some types of hydro-gels have almost the same equilibrium swelling volume in water and in ethylene glycol (EG), a highly viscous liquid completely miscible with water. Experiments showed that when a gel fully swollen with EG is immersed into a large amount of water, it temporarily swells up and then relaxes to the equilibrium volume in water. The temporary swelling is explained by the friction force exerted on the gel network from the outward EG flux In this paper, we experimentally show that the temporary swelling is suppressed by adding linear PEG (polyethylene glycol) in the outer water. Although the suppression seems to be explained by the osmotic pressure (i.e., by the same mechanism as the conventional osmotic squeezing), our theoretical analysis reveals that the effect of PEG is much stronger than that expected from the equilibrium osmotic pressure, implying that the PEG chains are condensed on the gel surface.

Highlights

In the 1970s, the concept of cooperative diffusion of gels was established [1]
Recent experimental and theoretical investigations, have revealed that for gels swollen in binary solvents, the molecular diffusion of the solvents couples strongly with the cooperative diffusion of the gel network even when the equilibrium swelling volume hardly depends on the mixing ratio of the binary solvent [3,4]
When an acrylamide gel saturated with ethylene glycol (EG), say “EG-swollen gel”, is immersed into a large amount of water, its volume once increases and decreases toward the equilibrium value even though the gel has almost the same equilibrium swelling volume in EG and in water; and the time scale of the initial temporary swelling is much faster than that expected from the cooperative diffusion constant and the gel size

Summary

Introduction

In the 1970s, the concept of cooperative diffusion of gels was established [1]. According to the concept, the time change of the gel volume (swelling/deswelling) is governed by a diffusion equation, but the diffusion constant is much (roughly two digits) smaller than that of the self-diffusion of the solvent (water for hydro-gels), the volume change is caused by solvent transport. Recent experimental and theoretical investigations, have revealed that for gels swollen in binary solvents, the molecular diffusion of the solvents couples strongly with the cooperative diffusion (i.e., the volume change) of the gel network even when the equilibrium swelling volume hardly depends on the mixing ratio of the binary solvent [3,4]. When an acrylamide gel saturated with ethylene glycol (EG), say “EG-swollen gel”, is immersed into a large amount of water, its volume once increases and decreases toward the equilibrium value even though the gel has almost the same equilibrium swelling volume in EG and in water; and the time scale of the initial temporary swelling is much faster than that expected from the cooperative diffusion constant and the gel size

Methods

Results

Conclusion