Cation Effects on the Phase Transition ofN‐isopropylacrylamide Hydrogels

Abstract

Polymers formed from N -isopropylacrylamide (NIPAM) are highly water soluble and undergo a temperature-induced phase transition to an insoluble state. The phase behavior is determined by competing hydrophilic and hydrophobic forces. In this report, additional insight regarding the effect soluble metals have on the phase transition process is provided by showing that cation solvation aids with stabilization of hydrophobic forces. This reduces barriers to rehydration and decreases thermodynamic entropy and enthalpy, obtained with variable-temperature 1 H nuclear magnetic resonance spectroscopy of NIPAM hydrogels in D 2 O, NaCl, MgCl 2 , and CaCl 2 . For the series of cations studied, it is observed that the order of increasing effect to facilitate the phase transition is Ca 2+ < Mg 2+ < Na + . NaCl and MgCl 2 exhibited similar effects on the thermodynamics of the collapsing process. However, signifi cant differences in the phase transition thermodynamics are observed between MgCl 2 and CaCl 2 salt solutions. The infl uence on Stage 1 enthalpy and entropy values for CaCl 2 solutions is approximately half that of the MgCl 2 solutions. This difference is likely related to their charge density of Ca 2+ , which is approximately half that of Mg 2+ . make them attractive for use in various separations processes. These applications often occur in the presence of alkali and alkali earth metal cations, and a more complete understanding of hydrogel chemistry is provided by examining the role of metals in the phase transition LCST. [13][14][15][16][17][18][19][20][21] Tanaka et al. concluded that the degree of ionization of the polymer is an important aspect of the phase transition of ionic gels. [ 22 ] An additional stimulus to affect hydrogel phase transitions is the ionic strength of the solution in which it is immersed. Ohmine et al. investigated the effects of varying salt concentrations of NaCl, CaCl 2 , BaCl 2 , and MnCl 2 on the phase transition of partially hydrolyzed polyacrylamide gels. [ 23 ] With increasing concentrations of NaCl in water, the gels undergo a continuous transition to a dehydrated state. For NaCl, the phase transition began at 10 −3 mol L -1 concentrations, whereas the phase transition was initiated by much lower concentrations, 10 −5 -10 −6 mol L -1 , of the divalent salts investigated. The difference between the effectiveness of NaCl and MgCl 2, at inducing the phase transition, was attributed to the fact that only half as many divalent ions are needed to

Ion-specific binding of cations to the carboxylate and of anions to the amide of alanylalanine

“…7b (for other concentrations and waiting times, see Supplementary Figs. [18][19][20][21][22][23][24][25][26][27][28][29], the correlation between excitation and probing frequencies is reduced due to spectral diffusion, as evident from the increased vertical elongation of the detected signals 70 .…”

“…Model systems, like amide-rich polymers, have tremendously helped understanding the underlying principles of specific ion effects on proteins [24][25][26] , as the effect of salts on the phase transition of the polymers resembles the effect on proteins. Ionspecific macroscopically observable phase transitions have been traced back to microscopic interactions of ions with the amide backbone: spectroscopic studies and molecular dynamics simulations have confirmed direct ion-amide interactions 13,[27][28][29][30][31] .…”

Effect of Urea on Phase Transition of Poly(N-isopropylacrylamide) and Poly(N,N-diethylacrylamide) Hydrogels: A Clue for Urea-Induced Denaturation

“…Compared to other analytical techniques, such as dynamic light scattering (DLS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and differential scanning calorimetry (DSC), NMR is a powerful tool to detect both the chemical structure and dynamics at atomic level in qualitative or quantitative way. The HRMAS technology can sufficiently reduce the line broadening caused by the residual dipolar coupling (RDC) or chemical shift anisotropy (CSA) and produce high resolution NMR spectra in the heterogeneous system. , Thus, it is possible to conduct the sophisticated 1D and 2D NMR experiments for samples like tissues , and polymer hydrogels. − In addition, the nuclear Overhauser effect (NOE) is developed to study the weak intermolecular interaction based on the cross-relaxation rate, which depends on the proximity and correlation time of the interacting molecules. − In this paper, we have analyzed the solvent composition in hydration shell and investigated the interaction between the solvent and the polymers. By comparison, our study reveals that the NH group in the side chain of the polymer plays a critical role in forming and stabilizing the hydrogen bond between PNIPAM and urea.…”

Thermo‐Induced Double Phase Transition Behavior of Physically Cross‐Linked Hydrogels Based on Oligo(ethylene glycol) methacrylates

“…In general, two kinds of thermosensitive hydrogels can be defined: One of which shows insoluble to soluble phase transition in water at the upper critical solution temperature (UCST), and the other undergoes reverse phase transition at the lower critical solution temperature (LCST) . Among the polymers that exhibit a LCST, poly( N‐ isopropylacrylamide) (PNIPAM)‐based hydrogel exhibits a thermally reversible coil‐to‐globule phase transition near its LCST (32 °C) . For the LCST transition, polymer chains hydrate and adopt an extended conformation below the LCST, and then polymer chains dehydrate and undergo a sharp collapse above the LCST …”