Uncharged Gels Research Articles

Electrophoretic mobility of a charged spherical colloidal particle in an uncharged or charged polymer gel medium

A theory is developed of the electrophoretic mobility of a charged spherical colloidal particle in a gel medium due to the long-range hydrodynamic particle-gel interaction by using the Brinkman-Debye-Bueche model and the general mobility expression is derived. On the basis of the obtained general mobility expression, a simple analytic mobility expression for particles with low zeta potential is derived, which is found to be in excellent agreement with the exact mobility equations expressed in terms of exponential integrals derived by Allison et al. for an uncharged gel (J Colloid Interface Sci 313: 328, 2007) and by Li et al. for a charged gel (J Colloid Interface Sci 423: 129, 2014). The relative error is less than 1.6%. An approximate mobility expression for a particle with arbitrary zeta potential in an uncharged gel medium applicable for large κa > 30, where a is the particle radius and κ is the Debye-Huckel parameter, is also derived.

A dynamic light scattering study of hydrogels with the addition of surfactant: a discussion of mesh size and correlation length

We report a study of the interaction of sodium dodecyl sulfate (SDS) with hydrogels made of hydroxyethylcellulose (HEC) and hydrophobically modified hydroxyethylcellulose (HMHEC) by means of dynamic light scattering. The results reveal that the interaction between the hydrogels and SDS only affects the cooperative diffusion, while the slow relaxation of the hydrogels was not influenced by the interaction. Gel swelling due to the presence of SDS is also examined by measuring the weight change of the gel. The main observation is that when the gel swells due to increasing SDS concentration, the cooperative diffusion becomes faster. As such, we raise the question of whether the correlation length obtained from DLS is still proportional to the mesh size inferred from gel swelling. In the discussion, we attribute the cause to the quantity of bound surfactants. The mutual diffusion of SDS micelles could be detected at high SDS concentrations, and the diffusion coefficient of SDS in the gel matrix was determined. HEC (or HMHEC) gel swells due to increasing SDS concentration, while the cooperative diffusion becomes faster. This indicates that the HEC (or HMHEC) gel can swell until the osmotic pressure equals the modulus when the charge effect dominates over the effect of configurational entropy for uncharged gels interacting with charged surfactants.

Nanoparticle gel electrophoresis: Bare charged spheres in polyelectrolyte hydrogels

Nanoparticle gel electrophoresis has recently emerged as an attractive means of separating and characterizing nanoparticles. Consequently, a theory that accounts for electroosmotic flow in the gel, and coupling of the nanoparticle and hydrogel electrostatics and hydrodynamics, is required, particularly for gels in which the mesh size is comparable to or smaller than the particle radii. Here, we present an electrokinetic model for charged, spherical colloidal particles undergoing electrophoresis in charged (polyelectrolyte) hydrogels: the gel-electrophoresis analogue of Henry’s theory for electrophoresis in Newtonian electrolytes. We compare numerically exact solutions of the model with several independent asymptotic approximations, identifying regions in the parameter space where these approximations are accurate or break down. As previously assumed in the literature, Henry’s formula, modified by the addition of a constant electroosmotic flow mobility, is accurate only for nanoparticles that are small compared to the hydrogel mesh size. We derived an exact analytical solution of the full model by judiciously modifying the theory of Allison et al. [15] for uncharged gels, drawing on the superposition methodology of Doane et al. [14] to account for hydrogel charge. This furnishes accurate and economical mobility predictions for the entire parameter space. The present model suggests that nanoparticle size separations (with diameters ≲40nm) are optimal at low ionic strength, with a gel mesh size that is selected according to the particle charging mechanism. For weakly charged particles, optimal size separation is achieved when the Brinkman screening length is matched to the mean particle size.

Hydrophobicity and Counterion Effects on the Binding of Ionic Surfactants to Uncharged Polymeric Hydrogels

Gel swelling experiments have been used to study the binding of ionic surfactants to a series of nonionic alkylacrylamide hydrogels of increasing hydrophobicity. The binding of hexadecyl trimethylammonium (C16TA+) to uncharged gels is sensitive to both the hydrophobicity of the gel and the counterion to the surfactant. There is a minimum hydrophobicity threshold below which binding of the surfactant does not occur, and this is influenced by the counterion to the surfactant. The surfactant concentration at the onset of binding, the critical association concentration (cac), decreases with increasing gel hydrophobicity. The maximum swelling of the gel (at intermediate network hydrophobicity) increases in the order of the Hofmeister series of anions, bromide (Br-) < chloride (Cl-) < acetate (Ac-). At higher gel hydrophobicity, differences in swelling are no longer observed on changing the counterion. A minimum hydrophobicity threshold was also found for the binding of the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl-di(ethylene oxide)-sulfate (SD-(EO)2-S). Differences in the swelling behavior with network hydrophobicity are explained in terms of the degree of saturation of the gel with surfactant at the cmc.

Diffusivity of Ions in Agarose Gels and Intervertebral Disc: Effect of Porosity

The effect of tissue porosity on ion (sodium, potassium, and chloride) diffusivity in agarose gels and porcine intervertebral disc tissues was investigated using an electrical conductivity method. An empirical, constitutive model for diffusivity (D) of solutes in porous fibrous media was proposed: D/Do = exp[-alpha(r(s)/k(1/2))beta] where r(s) is the Stokes radius of a solute, kappa is the Darcy permeability of the porous medium, Do is the diffusivity in free solution, alpha and beta are two positive parameters whose values depend on material structure. It is found that alpha = 1.25 +/- 0.138, beta = 0.681 +/- 0.059 (95% confidence interval, R2 = 0.92, n = 72) for agarose gels and alpha = 1.29 +/- 0.171 and beta = 0.372 +/- 0.088 (95% confidence interval, R2 = 0.88, n = 86) for porcine annulus fibrosus. The functional relationship between solute diffusivity and tissue deformation was derived. Comparisons of our model prediction with experimental data on diffusion coefficients of macromolecules (proteins, dextrans, polymer beads) in agarose gels in the literature were made. Our results were also compared to the data on ion diffusivity in charged gels and in cartilaginous tissues reported in the literature. There was a good agreement between our model prediction and the data in the literature. The present study provides additional information on solute diffusivity in uncharged gels and charged tissues, and is important for understanding nutritional transport in avascular cartilaginous tissues under different mechanical loading conditions.

Monte Carlo Simulations of Defect-Free Cross-Linked Gels in the Presence of Salt

Properties of polyelectrolyte gels in equilibrium with a reservoir of salt solution described within the primitive model have been investigated using Monte Carlo simulations. The polyelectrolyte gel was modeled as a charged defect-free three-dimensional network of diamondlike topology with explicit simple ions. The deswelling upon addition of salt was investigated for gels with different charge density, cross-linking density, chain flexibility, and counterion valence. All polyelectrolyte gels underwent a deswelling upon addition of salt. In most systems, however, even at high salt concentration, the equilibrium volumes remained significantly larger than those of the corresponding uncharged gels, and the salt concentration in the gel remained smaller than that in the reservoir. The salt content in the gel was larger as compared to that predicted by an ideal Donnan equilibrium due to a more negative excess chemical potential of the salt in the gel as compared to that in the reservoir. The predictions of the...

Charge-Induced Microphase Separation in Polyelectrolyte Hydrogels with Associating Hydrophobic Side Chains: Small-Angle Neutron Scattering Study

The structure of amphiphilic hydrogels of copolymers of acrylic acid and n-alkylacrylate swollen by D2O was studied by small-angle neutron scattering (SANS). For most of the uncharged gels, a scattering peak is observed. It is attributed to the correlation between hydrophobic domains formed by self-assembled n-alkyl side chains. From the SANS data, the aggregation number of the hydrophobic domains was derived. It increases with increasing length of hydrophobic groups and also as a result of absorption of hydrophobic additives that are solubilized inside hydrophobic domains. The most important observation of this work consists of the fact that the introduction of charged groups into the gel leads to microphase separation with the formation of hydrophobic regions including several densely packed hydrophobic domains that alternate with hydrophilic regions swollen by water where most of the charged repeat units and counterions are located. The size of the hydrophobic regions decreases with increasing charge c...

Comparative swelling of gels and polymer brush layers

The swelling of chemically end-grafted polystyrene brushes in a marginal solvent is investigated as a function of molecular weight M w, grafting density σ, and solvent quality τ = ( T − θ)/ T by specular neutron reflection and these swelling results are compared to previous measurements on swollen polymer networks. This comparison is motivated by the view of a polymer brush as a variety of “gel” where the crosslinks (grafting points) are constrained to lie on a surface. In accordance with this analogy, the temperature dependence of the brush and the network swelling is found to be qualitatively similar and the relative degree of brush and network swelling is found to diminish with an increase of σ and the cross-link density v, respectively. The network swelling data are taken from a previous study by Zrinyi and Horkay [1] who found an apparent universal master curve describing the swelling of uncharged gels with temperature. An attempt to describe the swelling of polymer brushes having a range of molecular weights by similar reduced variables led to a fairly good description of the brush swelling data, but the scaling curves for the gels and the brushes do not exactly coincide.

EXTRACTION OF COPPER, CADMIUM AND RELATED METALS WITH POLY(SODIUM ACRYLATE - ACRYLIC ACID)HYDROGELS.

ABSTRACT The extraction of copper, cadmium and related metals (M2+) with poly(sodium acrylate - acrylic acid) PAA hydrogels has been studied. pH variations are consistent with a cation exchange process. Saturation of the gel is achieved for a metal/carboxylate ratio R ≈ 1/2 and a gel swelling of ≈ 40 which is that of the uncharged gel : (-COO)2M complexes are expected to be formed, but also complexes of higher stoichiometry (R = 1/3 for Cu and Cd, R = 1/4 for Pb) at low metal concentrations. The selectivity is that observed in liquid-liquid extraction of metal cations with fatty carboxylic acids (Pb > Cu > Cd > Zn > Ni ≈ Co). Metal stripping from the gel is readily achieved by washing with 0.1 M HNO3. PAA hydrogel extraction allows to remove cadmium from a diluted aqueous solution down to a final concentration of 5 ppb.

High-resolution two-dimensional electrophoresis with isoelectric focusing in immobilized pH gradients

Due to the high reproducibility of pH gradient slope and width, immobilized pH gradients (IPG) have been used as the first dimension of two-dimensional techniques in order to generate maps of constant spot position in the p M r . However, when coupling IPG to SDS (sodium dodecyl sulphate) gels two problems were encountered: vertical streaking, due to incomplete zone solubilization in SDS, and horizontal streaking, due to spot fusion along the pH axis caused by the electroendosmosis of the charged Immobiline gels. Two methodical modifications are herewith described to overcome these drawbacks: (a) the SDS equilibrium time of the first-dimension gel has been prolonged to at least 30 min; (b) the SDS electrophoresis gel has been cast together with a starting gel, containing 2.5 mM of each Immobiline species used in the first dimension, which serves as a transition from the charged to the uncharged gel.