Studies of Transport Phenomena and Electrostatic Interactions in Polyacrylate Gels

Self-Protection of Electrochemical Storage Devices via a Thermal Reversible Sol-Gel Transition

Thermal self-protected intelligent electrochemical storage devices are fabricated using a reversible sol-gel transition of the electrolyte, which can decrease the specific capacitance and increase and enable temperature-dependent charging and discharging rates in the device. This work represents proof of a simple and useful concept, which shows tremendous promise for the safe and controlled power delivery in electrochemical devices.

Sedimentation of a charged porous particle in a charged cavity

The sedimentation of a charged porous sphere at the center of a charged spherical cavity filled with an electrolyte solution is analyzed. The thickness of the electric double layers around the particle and cavity wall is arbitrary, and their relaxation effect is considered. Through the use of a set of linearized electrokinetic equations and a perturbation method, the ionic electrochemical potential energy, electric potential, and velocity fields in the fluid are solved with the fixed space charge density of the particle and surface charge density of the cavity as the small perturbation parameters, and an explicit formula for the sedimentation velocity is obtained. Due to the electroosmotic enhancement on the fluid recirculation in the cavity caused by the sedimentation-induced electric field, the presence of the surface charges on the cavity wall increases the sedimentation velocity of the porous particle. For the sedimentation of a porous particle in a cavity with their fixed charges of the same sign, the effect of electric interaction between the particle and cavity wall in general increases the sedimentation velocity. For the case of their fixed charges with opposite signs, the sedimentation velocity is increased/reduced if the magnitude of the fixed charge density of the cavity wall is relatively large/small. The effect of the surface charges at the cavity wall on the sedimentation of the porous particle increases with an increase in the permeability for fluid flow within the particle and with a decrease in the particle-to-cavity radius ratio (i.e., an increase in the surface area of the cavity wall relative to a given size of the particle, which enhances the fluid recirculation effect).

Temperature response in electrosensors and thermal voltages in electrolytes

Temperature sensation is increasingly well understood in several model organisms. One of the most sensitive organs to temperature changes is the functional electrosensor of sharks and their relatives; its extreme thermal responsiveness, in excised preparations, has not been mechanistically described. In recent years, conflicting reports have appeared concerning the properties of a hydrogel that fills the ampullae of Lorenzini. The appearance of a thermoelectric effect in the gel (or, using different methods, a reported lack thereof) suggested a link between the exquisite electrosense and the thermal response of the electroreceptors (or, alternately, denied that link). I review available electrophysiology evidence of the organ's temperature response, calculate a theoretical gel signal prediction using physical chemistry, analyze the strengths and weaknesses of the existing gel measurements, and discuss broader implications for the ampullae and temperature sensation.

Structural Changes of Polyacids Initiated by Their Neutralization with Various Alkali Metal Hydroxides. Diffusion Studies in Poly(acrylic acid)s

The changes in the three-dimensional structure of the poly(acrylic acid), PAA, induced by incorporation of various alkali-metal counterions have been evaluated by studying diffusion of an uncharged probe (1,1'-ferrocenedimethanol) in the polymeric media. The studies are supported by the measurements of conductivity and viscosity of the polymeric media. Solutions of linear PAA of four different sizes (molecular weights: 450,000, 750,000, 1,250,000, 4,000,000) were neutralized with hydroxides of alkali metals of group 1 of the periodic table (Li, Na, K, Rb, Cs) to the desired neutralization degree. The transport properties of the obtained polyacrylates were monitored by measuring the changes in the probe diffusion coefficient during the titration of the polyacids. The probe diffusivity was determined from the steady-state current of the probe voltammetric oxidation at disk microelectrodes. Diffusivity of the probe increases with the increase in the degree of neutralization and with the increase in viscosity. It reaches the maximum value at about 60-80% of the polyacid neutralization. The way the probe diffusion coefficients change is similar in all polyacid solutions and gels. The increase in the size of a metal cation causes, in general, an enhancement in the transport of probe molecules. The biggest differences in the probe diffusivities are between lithium and cesium polyacrylates. The differences between the results obtained for cesium and rubidium are not statistically significant due to lack of good precision of the voltammetric measurements. The measurements of the electric conductivity of polyacrylates and the theoretical predictions supplemented the picture of electrostatic interactions between the polyanionic chains and the metal cations of increasing size. In all instances of the PAAs, the viscosity of the solutions rapidly increases in the 0-60% range of neutralization and then becomes constant in the 60-100% region. With the exception of the shortest chain polyacid, the formation of a rigid medium (gel) has been observed in the experiments with all cations. After the end point of the titration was passed, a sudden drop in the viscosity and the disappearance of the gelatinous structure were seen. The largest value of viscosity has been recorded for the longest chain polyacid. The change in the cation of the strong base used did not affect the viscosity of the polymeric system.

Electrochemical Time-of-Flight in crosslinked collagen matrix solution: Implications of structural changes for drug delivery systems

Electrochemical Time-of-Flight (ETOF) method was used for the first time to measure the diffusion coefficient of 4-hydroxy-TEMPO, a molecular probe in collagen I matrix solution as a function of its concentration and the extent of crosslinking with glutaraldehyde (GA). The values of the diffusion coefficient were correlated with Circular Dichroism (CD) and viscosity data to assess the changes of the structure of a collagen matrix. The low value of probe diffusion coefficient indicates that the molecular collagen contributes to large diffusion hindrance of the medium. The combined Brinkman or effective medium model and the Carman-Kozeny model were used to estimate the average diameter of a matrix pore as a function of collagen solution composition. We show that 0.5% and 1% (w/w) collagen matrix crosslinked with the addition of GA above 0.1% (v/w) forms matrix with pores larger than in native collagen. This result suggests the existence of a micro-phase separation in the crosslinked collagen matrix. The implications of these findings for the design of small molecule drug delivery systems are discussed.

Diffusion of Uncharged Probe Reveals Structural Changes in Polyacids Initiated by Their Neutralization: Poly(Acrylic Acids)

The diffusion studies of the uncharged probe (1,1'-ferrocenedimethanol) have been successfully applied for the evaluation of the changes in the three-dimensional structure of poly(acrylic acids) of various molecular weights (ranging from 2000 to 4,000,000 g/mol) during their neutralization with a strong base. The qualitative picture of the macromolecule arrangement during the titration of the polyacids has been obtained from the conductometric measurements. The characteristic changes in the poly(acrylic acid) conductivity are practically the same for all polyacids examined and are in a very good agreement with the predictions of our theoretical model of the polyelectrolyte conductance. The transformation of the polyelectrolyte solution into the gel-like or gel phase has been investigated more quantitatively by tracing the changes in the diffusion coefficient of the uncharged probe redox system. The probe diffusivities, D, were determined using steady-state voltammetry at microelectrodes for a wide range of neutralization degree, alpha, of the polyacids tested. The dependencies of D versus alpha are of similar shape for all poly(acrylic acids). The first parts of the dependencies reflect a rapid increase in D (up to neutralization degree of either 45% for the lowest molecular-weight poly(acrylic acid) or 75-80% for other polyacids). They are followed by the parts of a slight drop in the diffusion coefficient. The changes in the probe diffusivity become stronger as the molecular weight of poly(acrylic acid) increases. The maximum probe diffusion coefficients are greater than the initial values in the pure polyacid solutions by 14, 24, 19, 30, and 28% for poly(acrylic acid) of molecular weights of 2000, 450,000, 1,250,000, 3,000,000, and 4,000,000 g/mol, respectively. The variation in the probe diffusion coefficient qualitatively follows the line of the changes in the macroscopic viscosity of the polyelectrolyte system. This is in contrast to the predictions of the Stokes-Einstein relation and, therefore, suggests that the changes in the probe diffusion rate are mainly due to the structural changes in the polyacrylate medium and the macromolecular rearrangements induced by the chemical, acid-base reaction. By adapting the obstruction model for diffusion in homogeneous gels, the transport characteristics of the probe were converted into the structural characteristics of the polyelectrolytic systems. It has been found that the most ordered structure of the polyelectrolyte, or in other words the most permeable structure, is obtained when poly(acrylic acid) is neutralized at 75-80%.

Infrastructure in the electric sense: admittance data from shark hydrogels

Elasmobranchs (sharks, skates, and rays) possess an electrosensory system with an infrastructure of canals connecting the electrosensors to the environment. The electrosensors and canals are filled with a uniform hydrogel, but the gel's function has not yet been determined. We present electrical admittance spectra collected from the hydrogel from 0.05 to 100 kHz, covering the effective range of the electrosensors. We have taken samples of this gel, postmortem, from Triaenodon obesus and Carcharodon carcharias; for purposes of comparison, we have synthesized a series of collagen-based hydrogel samples. The shark hydrogels demonstrate suppressed admittance when compared to both seawater and collagen gels. In particular, collagen hydrogels with equivalent ion concentrations are roughly 2.5 times more polarizable than the shark samples. We conclude that the shark hydrogels strongly localize ionic species, and we discuss the implications for the related roles of the gel and the canals in the electric sense. The gel-filled canals appear better suited to fostering voltage differences along their length than to providing direct electrical contact to the seawater environment.

Diffusiophoretic mobility of charged porous spheres in electrolyte gradients

The diffusiophoretic motion of a polyelectrolyte molecule or charged floc in an unbounded solution of a symmetrically charged electrolyte with a uniform prescribed concentration gradient is analytically studied. The model used for the particle is a porous sphere in which the density of the hydrodynamic frictional segments, and therefore also that of the fixed charges, is constant. The electrokinetic equations which govern the electrostatic potential profile, the ionic concentration distributions (or electrochemical potential energies), and the fluid velocity field inside and outside the porous particle are linearized by assuming that the system is only slightly distorted from equilibrium. Using a regular perturbation method, these linearized equations are solved for a charged porous sphere with the density of the fixed charges as the small perturbation parameter. An analytical expression for the diffusiophoretic mobility of the charged porous sphere in closed form is obtained from a balance between its electrostatic and hydrodynamic forces. This expression, which is correct to the second order of the fixed charge density of the particle, is valid for arbitrary values of kappaa and lambdaa, where kappa is the reciprocal of the Debye screening length, lambda is the reciprocal of the length characterizing the extent of flow penetration inside the particle, and a is the particle radius. Our result to the first order of the fixed charge density agrees with the corresponding solution for the electrophoretic mobility obtained in the literature. In general, the diffusiophoretic mobility of a porous particle becomes greater as the hindrance to the diffusive transport of the solute species inside the particle is more significant.

Preparation and characterization of thermoresponsive poly(N-isopropylacrylamide-co-acrylic acid) hydrogels: studies with electroactive probes

A new method of preparing thermoresponsive hydrogels consisting of N-isopropylacrylamide (NIPA) and acrylic acid (AA), with a well-defined concentration of an electroactive probe, 1,1'-ferrocenedimethanol (Fc(MeOH)2), is described, and a comparison of the physical and electrochemical properties of NIPA-AA gels with those of aqueous solutions is presented. The NIPA-AA gels undergo a discontinuous volume phase transition at 45 degrees C; this transition results in a release of approximately 40% of the solution mass from the gel phase. Characterization of hydrogels with electroactive probes is performed using electroanalytical techniques and FTIR and UV/vis spectroscopies. Steady-state voltammetry and chronoamperometry at platinum disk microelectrodes are used to measure the diffusion coefficient of Fc(MeOH)2 in gels under a wide range of experimental conditions. Similar diffusion coefficients for Fc(MeOH)2 in NIPA-AA gels are obtained by either electroanalytical technique at temperatures lower than 20 degrees C. The uncertainty in the Fc(MeOH)2 concentration in the gels, resulting from the discontinuous volume change transition, necessitated the use of concentration-independent chronoamperometric data (i.e. the chronoamperometric response divided by the steady-state current obtained at sufficiently long times) to obtain reliable diffusion coefficient values for Fc(MeOH)2. For temperatures above the volume phase transition, changes of concentration of Fc(MeOH)2 are detected in a copolymeric collapsed phase.