Micromechanical Characterization of Hydrogels Undergoing Swelling and Dissolution at Alkaline pH.

Wei Hu,Romain Jeantet,Ruben Mercadé-Prieto,Francois Martin,Xiao Chen

doi:10.3390/gels3040044

Abstract

The swelling of polyelectrolyte hydrogels usually depends on the pH, and if the pH is high enough degradation can occur. A microindentation device was developed to dynamically test these processes in whey protein isolate hydrogels at alkaline pH 7–14. At low alkaline pH the shear modulus decreases during swelling, consistent with rubber elasticity theory, yet when chemical degradation occurs at pH ≥ 11.5 the modulus decreases quickly and extensively. The apparent modulus was constant with the indentation depth when swelling predominates, but gradients were observed when fast chemical degradation occurs at 0.05–0.1 M NaOH. In addition, these profiles were constant with time when dissolution rates are also constant, the first evidence that a swollen layer with steady state mechanical properties is achieved despite extensive dissolution. At >0.5 M NaOH, we provide mechanical evidence showing that most interactions inside the gels are destroyed, gels were very weak and hardly swell, yet they still dissolve very slowly. Microindentation can provide complementary valuable information to study the degradation of hydrogels.

Highlights

Protein-rich fouling deposits are commonly removed in the food industry using cleaning-in-place (CIP) systems using alkaline-based solutions at high temperatures, and usually at Reynolds number values over 70,000 [1]
We report here an initial study showing the experimental capabilities of microindentation for the study of reactive swelling and dissolution, a very complex problem present in many other soft degradable hydrogels [9,10]
In neutral polymers at swelling swelling equilibrium, affine models predict that n = 3ν/(3ν − 1), where ν is the excluded volume equilibrium, affine models predict that n = 3ν/(3ν − 1), where ν is the excluded volume exponent [28]

Summary

Introduction

Protein-rich fouling deposits are commonly removed in the food industry using cleaning-in-place (CIP) systems using alkaline-based solutions at high temperatures, and usually at Reynolds number values over 70,000 [1]. Cleaning involves a chemical aspect, that is, alkali destroys chemical interactions between proteins, making the deposits weaker. Extensive swelling can occur at extreme pH, low or high, due to the increase in the protein charge, common to many polyelectrolyte hydrogels [2], and makes the deposits extremely weak. The removal of protein deposits on surfaces has been studied using many different techniques. Fluid dynamic gauging (FDG) can provide direct measurements of the deposit thickness [4], during swelling and dissolution, as well as the rupture shear of the soft material [5,6]; other studies quantify the concentration of proteins in solution to determine cleaning rates [3]

Methods

Results

Conclusion