Thermo‐responsive gels in the presence of monovalent salt at physiological concentrations: A Monte Carlo simulation study

Abstract

In this work, slightly charged thermo-responsive gels in the presence of salt at concentrations close to physiological conditions have been simulated within a coarse-grained model widely used in the last decade. These simulations allow differentiate charge and salt effects, which are antagonist and coupled in many real systems because the degree of ionization might depend on the electrolyte concentration. An analysis in terms of the different contributions to osmotic pressure is also presented, which highlights the role played by excluded volume effects. In addition, our results also permit us to test some predictions based on the ideal Donnan equilibrium, a common assumption made to justify the swelling behavior of gels and microgels in the presence of salt. More specifically, simulations show that, for the slightly charged gels simulated here, such an assumption overestimates the concentration of salt inside collapsed gels and underestimates the excess of osmotic pressure associated to the additional electrolyte. KEYWORDS: computer modeling; drug delivery systems; gels; hydrogels; microgels; Monte Carlo simulations INTRODUCTION Thermo-responsive gels and microgels have gained considerable attention in different fields of biotechnology due to the ability of these materials to swell or shrink in response to temperature changes. Many researchers 1-3 have studied diverse applications of these gels, such as the controlled drug release. As potential therapeutic carriers, temperature-sensitive microgels should work at physiological conditions, that is, at ionic strengths of about 150 mM. Consequently, it should be desirable to explore how such ionic strengths can modify their thermal response.

The effect of electrosteric interactions on the effective charge of thermoresponsive ionic microgels: Theory and experiments

“…This result is not surprising given the low values of salt concentrations explored in this work, which scarcely contribute to the free energy of the system. However, for large amounts of salt, the LCST transition is expected to shift to lower temperatures . This is due again to two combined effects that favors the particle shrinking: the screening of the microgel bare charge and the increase of the osmotic pressure over the microgel surface exerted by the ions from outside.…”

“…However, for large amounts of salt, the LCST transition is expected to shift to lower temperatures. 64,65 This is due again to two combined effects that favors the particle shrinking: the screening of the microgel bare charge and the increase of the osmotic pressure over the microgel surface exerted by the ions from outside.…”

Computer Simulations of Static and Dynamical Properties of Weak Polyelectrolyte Nanogels in Salty Solutions

“…Therefore, intelligent employment of these materials can be used to achieve goals, like, e.g., drug delivery [7]. External stimuli can for example be temperature [8,9] or pH [1].…”

Recent developments in electrokinetics of salt-free concentrated suspensions

“…To overcome these limitations, some macroscopic (i.e., continuum-) descriptions have been developed that consider interactions in an average sense by mean field approaches [6][7][8][9][10][11][12][13][14][15][16][17]. Other macroscopic descriptions also account for ionic excluded volume effects, in very good agreement with Monte Carlo simulations considering explicitly the study of particle-ion or ion-ion correlations [18][19][20][21][22][23][24], mainly in equilibrium. In situations where such correlations are negligible, mean-field approaches like the cell model used in this work still continue to be promising ways to obtain theoretical predictions for most electrokinetic phenomena observed with concentrated suspensions.…”