Influence of Metal Ions on the Melting Temperature, Modulus, and Gelation Time of Gelatin Gels: Specific Ion Effects on Hydrogel Properties

Abstract

The impact of ions on hydrogel strength is not well understood, in particular with regards to specific ion effects for cations. Herein, we find that divalent and monovalent cations in most cases reduce the modulus and melting temperature while increasing the gelation time of gelatin hydrogels. This behavior is in contrast to the well-known stiffening effect of trivalent metals. The melting temperature, the logarithm of the gelation time, and the logarithm of the amplitude of the complex modulus were found to follow a power law dependence on ionic strength: kI x . The power law exponent, x, was found to be universal within the groups of monovalent and divalent cations. The prefactor k depended linearly on the ionic radius, which was used as a proxy for ion polarizability. The slope of this linear dependence was different for monovalent and divalent cations.

Hofmeister Effects of Monovalent Sodium Salts in the Gelation Kinetics of Gelatin

“…In ref. 22 and in the case of monovalent metal salts, a straight line of the plots was obtained, with a slope equal to k.…”

“…The time after which the gel state is reached, , will be analyzed as a function of the ionic strength for each of the NaX electrolytes. The data will be plotted in a log-log scale as in reference 22 , namely:…”

“…The time after which the gel state is reached, τ, was analyzed as a function of the ionic strength for each of the NaX electrolytes. The data was plotted in a log–log scale as in ref , namely, where τ 0 is the extrapolated gelation time at zero ionic strength (i.e., in ultra pure water), I is the ionic strength (equal to the salt concentration C in the case of 1:1 electrolytes), and k is a characteristic constant characterizing the gelation of gelatin in the presence of the NaX electrolyte. In ref and in the case of monovalent metal salts, a straight line of the log τ versus log I plots was obtained with a slope equal to k .…”

“…Indeed, an alternative model to the solute induced structural changes suggests that the specific ionic hydration and the formation of cation–anion pairs in solution is at the origin of easily measurable colligative properties of aqueous solutions, namely reduction of the freezing point and of the vapor pressure of water, quantified by Raoult’s law. , Hence before reaching a definitive theoretical understanding of the nature of the electrolyte on the structure of water and the subsequent effect on other solutes, additional experimental quantitative measurements are required. In particular, there are surprisingly few articles dealing with the influence of the nature of the electrolyte on the gelation kinetics of hydrogels like gelatin. − The structure, the melting temperature, the chirality, and the mechanical properties of self-assembling molecules used to form supramolecular hydrogels are also strongly affected by the nature of the anion . The aim of the present investigation is to investigate the influence of different sodium salts with only monovalent anions on the gelation kinetics of bovine skin gelatin (at a constant mass fraction of 10% w/v) and to correlate the observed strong anion dependence, in agreement with the Hofmeister series, with molecular parameters of the salt, namely, its molecular polarizability and the viscosity B coefficients of the anions.…”

“…Hence before reaching a definitive theoretical understanding of the nature of the electrolyte on the structure of water and the subsequent effect on other solutes, additional experimental quantitative measurements are required. In particular, there are surprisingly few articles dealing on the influence of the nature of the electrolyte on the gelation kinetics of hydrogels like gelatin [19][20][21][22] . The structure, the melting temperature, the chirality and the mechanical properties of self-assembling molecules used to form supramolecular hydrogels are also strongly affected by the nature of the anion 23 .…”