Properties of Water Bound in Hydrogels

Abstract

In this review, the importance of water in hydrogel (HG) properties and structure is analyzed. A variety of methods such as ¹H NMR (nuclear magnetic resonance), DSC (differential scanning calorimetry), XRD (X-ray powder diffraction), dielectric relaxation spectroscopy, thermally stimulated depolarization current, quasi-elastic neutron scattering, rheometry, diffusion, adsorption, infrared spectroscopy are used to study water in HG. The state of HG water is rather non-uniform. According to thermodynamic features of water in HG, some of it is non-freezing and strongly bound, another fraction is freezing and weakly bound, and the third fraction is non-bound, free water freezing at 0 °C. According to structural features of water in HG, it can be divided into two fractions with strongly associated and weakly associated waters. The properties of the water in HG depend also on the amounts and types of solutes, pH, salinity, structural features of HG functionalities.

Keywords: cryogels; freezing-melting point depression; hydrogels; interfacial phenomena; strongly and weakly associated water; strongly and weakly bound water.

Figure 1

Confocal laser scanning microscopy (CLSM) images of HEMA-AGE hydrogel (sample A) in (a) hydrated and (b) dried states (scale bar 150 μm) with the pore (c) size and (d) wall thickness distributions (reproduced from Ref. [47] with permission from The Royal Society of Chemistry).

Figure 2

Wall thickness distributions with (a) Fiji and (b) ImageJ, and (c) pore size distributions for HEMA-AGE HG A, B, C and D (Table 1) (reproduced from Ref. [47] with permission from The Royal Society of Chemistry).

Figure 3

PSDs calculated from the DSC data for HEMA-AGE (A, B, C, and D samples) and G gels (hydration h = m_w/m_d where m_w is the weight of water evaporated in DSC measurements up to 160 °C and m_d is the residual weight of heated sample) at (a) high and (b,c) low hydration (reproduced from Ref. [47] with permission from The Royal Society of Chemistry).

Figure 4

¹H NMR spectra, of water adsorbed by gelatin gel recorded at different temperatures: (a) initial freeze-dried (0.3 wt % H₂O) in CDCl₃ (solid lines) and in a mixture CDCCl₃:CD₃CN 3:1 at (a, dashed-dotted lines) 0.8 wt % and (b) 10 wt % of water. Signal at 0 ppm corresponds to tetramethylsilane added as a standard; signal at 7.2 ppm corresponds to residual CHCl₃ (reproduced from Ref. [47] with permission from The Royal Society of Chemistry).

Figure 5

¹H NMR spectra, recorded at different temperatures, of water bound in gelatin gel at hydration h = 1 g per gram of dried gelatin in different media: (a) air (solid lines) and C₆D₆:CD₃CN = 6:1 (dashed-dotted lines), (b) C₆D₆ (solid lines) and CDCl₃:CD₃CN = 3:1 (dashed-dotted lines) (reproduced from Ref. [47] with permission from The Royal Society of Chemistry).

Figure 6

(a) Amount of unfrozen water (C_uw) as a function of temperature; (b) derivative dC_uw/d (ΔG), and (c) pore size distribution (NMR cryoporometry) for G gel in different media (reproduced from Ref. [47] with permission from The Royal Society of Chemistry).

Figure 7

Theoretical ¹H NMR spectra of water bound to models of partially hydrated gels with cross-linked HEMA-AGE (2373 atoms) with 1192 H₂O, collagen (two triple coils (1639 atoms) and 1032 H₂O) and fibronectin (8–9 Fn)—collagen (3200 atoms) with 827 H₂O (geometry optimized with PM6 method).

Figure 8

Chemical shifts of water molecules in clusters: pure (curves 1, 2, and 4), with dissolved NaCl (curve 3) and bound to PVA fragments cross-linked by glutaraldehyde (curve 5). Computational models are based on calculations using PM6 and PM7 methods and correlation functions based on DFT and PM6 or PM7 calculations of the same water clusters (adapted from [13]).

Figure 9

Amount of unfrozen water (C_uw) as a function of temperature; and changes in the Gibbs free energy of interfacial water versus C_uw at different concentrations of collagen in the hydrogel (adapted from [76] with permission, Copyright 2006, Elsevier).

Figure 10

The free surface energy as a function of the collagen concentration in the CG hydrogel (adapted from [76] with permission, Copyright 2006, Elsevier).

Figure 11

(a) Temperature dependence of the TSD current for the initial collagen hydrogel and “free” (bulk) water; (b) temperature dependences of the TSD current and the amounts of unfrozen water (C_uw) (NMR) for initial collagen HG (98.5 wt % of water) (c) distribution function of the activation energy of relaxation in these systems; and (d) incremental pore size distributions for the initial CG HG calculated on the basis of ¹H NMR and TSDC data (adapted from [76] with permission, Copyright 2006, Elsevier).

Figure 12

Pore size distribution calculated using ¹H NMR cryoporometry and CLSM methods (reproduced from Ref. [44] with permission from The Royal Society of Chemistry).

Figure 13

Diffusion kinetics through a collagen HG membrane (~1 mm in thickness) for (a) BPTI. Curve 1 is for an initial concentration of 1.23 mg/mL in the feeder cell (OD₂₈₀ = 0.09), curves 2–4 are for an initial concentration of 2.46 mg/mL (OD₂₈₀ = 0.18); curve 2—BPTI run after the first BPTI run; curve 3—BPTI run after Fg; curve 4—BPTI run after Fg and BSA; (c) BSA and BSA (with twice concentration) after the first BSA run, (e) Fg (initial concentration 1.7 mg/mL); curves (b,d,f) show the corresponding distribution functions of the diffusion coefficient f(D) for (b) BPTI, (d) BSA, and (f) Fg (reproduced from Ref. [44] with permission from The Royal Society of Chemistry).

Figure 14

Changes in frequency (1) and auto-gain controller voltage (2) on two injections (A and B) of 0.1 mL aliquot of 3T3 fibroblast suspension (2.0 × 10⁶ cell mL⁻¹) upon an unsupported section of CG HG laid the surface of a 10 MHz gold coated crystal. Flow injection rate 0.01 mL min⁻¹, 37 ± 0.1 °C, pH 7.2, PBS (reproduced from Ref. [44] with permission from The Royal Society of Chemistry).

Figure 15

¹H NMR spectra of water bound to (a–c) CM1 and (c) CM2 (dot-dashed lines) at hydration h = 2.3 wt % in different media: (a) air (solid lines) CDCl₃ (dot-dashed lines), (b) CD₃CN (solid lines) CD₃CN:CDCl₃ = 1:2.6 (dot-dashed lines), (c) CD₃CN:CDCl₃ = 1:5.

Figure 16

Temperature dependence of the amount of (a) SAW and (b) WAW and the relationships between changes in the Gibbs free energy and the amounts of SAW and WAW in HA/A-300 composites CM1 and CM2 (*).

Figure 17

¹H NMR spectra of CM1 with adsorbed aqueous solutions (150 mg/g) of (a) 18% HCl and (b) 16% H₂O₂ in CDCl₃ medium.

Figure 18

(a,c,e) Relationships between the amounts of unfrozen water and changes in the Gibbs free energy and (b,d,f) the corresponding PSD (NMR cryoporometry with GT equation at k_GT = 67 K nm) for (a,b) MCC, (c,d) MCC/A-300 (5.6:1), and MCC/TiO₂ (3:1) (adapted from [13]).

Figure 19

PSD (k_GT = 70 K nm) for A-300/PVP systems (adapted from [13]).