Multivalent Salt Solutions Research Articles

Synthesis, characterization, and swelling behaviors of a pH-responsive CMC-g-poly(AA-co-AMPS) superabsorbent hydrogel

New superabsorbent hydrogels were synthesized through free radical graft copolymerization of partially neutralized acrylic acid (AA) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) onto carboxymethyl cellulose (CMC) backbones. The structure and morphology of the synthesized superabsorbent hydrogels were characterized by Fourier transform infrared spectroscopy and scanning electron microscope. The effect of the molar ratio of AMPS to AA, the APS concentration, and the CMC content on swelling ratio was optimized. The swelling properties in various pH solutions and saline solutions as well as the swelling kinetics were also evaluated. Results showed that the introduction of AMPS enhanced the swelling capacity, swelling rate, and salt-resistance of the superabsorbent hydrogels. Moreover, the hydrogels exhibited smart swelling behavior in multivalent salt solutions and better reversible pH sensitivity in the pH 2.0 and 7.0 solutions, which makes the hydrogels available as a candidate for drug delivery systems.

Enhanced swelling properties of a novel sodium alginate‐based superabsorbent composites: NaAlg‐g‐poly(NaA‐co‐St)/APT

AbstractA novel alginate‐based superabsorbent composite with enhanced swelling properties was synthesized by simultaneously grafted copolymerization partially neutralized acrylic acid (NaA), styrene (St) onto the sodium alginate (NaAlg) backbones in the presence of attapulgite (APT). The structure and morphology of the synthesized superabsorbent composites were characterized by Fourier transform infrared spectroscopy (FTIR), ultraviolet‐visible (UV‐vis) spectra and Field emission scanning electron microscope (FESEM). Major factors affecting the swelling ratio according to Flory's theory were discussed. The superabsorbent composite synthesized under optimum reaction conditions exhibits the maximum swelling ratio of 587 g/g in distilled water and 73 g/g in 0.9 wt % NaCl solution. Compared with NaAlg‐g‐PNaA hydrogel, the simultaneous introduction of tiny amount of St and APT not only enhanced the swelling ratio but also increased the initial swelling rate. In addition, the effects of salt and pH medium on the swelling behaviors were investigated, and the results indicated that the composite showed smart swelling behavior in multivalent salt solution and better pH‐responsitivity in the pH 2.2 and 7.4 solutions, which makes it promising for drug delivery application. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Polyelectrolytes in multivalent salt solutions

We study conformational and electrophoretic properties of polyelectrolytes (PEs) in tetravalent salt solutions under the action of electric fields by means of molecular dynamics simulations. Chain conformations are found to have a sensitive dependence on the salt concentration C(s). As C(s) is increased, the chains first shrink to a globular structure and subsequently re-expand above a critical concentration C(s)*. An external electric field can further alter the chain conformation. If the field strength E is larger than a critical value E*, the chains are elongated. E* is shown to be a function of C(s) by using two estimators E(I)* and E(II)* through the study of the polarization energy and the onset point of chain unfolding, respectively. The electrophoretic mobility of the chains depends strongly on C(s), and the magnitude increases significantly, accompanying the chain unfolding, when E>E(II)*. We study the condensed ion distributions modified by electric fields and discuss the connection of the modification with the change of chain morphology and mobility. Finally, E* is studied by varying the chain length N. The inflection point is used as a third estimator E(III)*. E(III)* scales as N(-0.63(4)) and N(-0.76(2)) at C(s) =0.0 and C(s)*, respectively. E(II)* follows a similar scaling law to E(III)* but a crossover appears at C(s) =C(s)* when N is small. The E(I)* estimator fails to predict the critical field, which is due to oversimplifying the critical polarization energy to the thermal energy. Our results provide valuable information to understand the electrokinetics of PE solutions at the molecular level and could be helpful in micro/nanofluidic applications.

PH and Salt Concentration Sensitive Polyampholyte Microspheres: Synthesis and Properties

Polyampholyte microspheres with pH and salt concentration sensitive behaviors were prepared by acrylamide(AM),acryloylxyethyl trimethyl ammonium chloride(DAC) and methacrylic acid(MAA) in ethanol.The shape and conformation of microspheres were determined by transmission electron microscope and infrared spectroscopy,respectively.Meanwhile,the change behavior of the microsphere diameter in various pH solutions and in NaCl and CaCl2 salt solutions was investigated.The results indicated that the diameter of microspheres was 1 177 nm,and polydispersity index(PDI) value was 0.181 at the second step,which were greater than those(d=764 nm;PDI=0.069) at the first step.The diameter of microspheres increased with decreasing pH when the pH was lower than pH=4.3.Whereas when the pH was higher than 4.3,the diameter increased with increasing pH,and a minimum diameter was obtained when the pH was 4.3.At the isoelectric point,the dispersion exhibited a typical antipolyelectrolyte effect in univalent NaCl salt solution.However,in multivalent CaCl2 salt solution,the diameter of microspheres first increased and then decreased.

Effect of chain stiffness on ion distributions around a polyelectrolyte in multivalent salt solutions

Ion distributions in dilute polyelectrolyte solutions are studied by means of Langevin dynamics simulations. We show that the distributions depend on the conformation of a chain while the conformation is determined by the chain stiffness and the salt concentration. We observe that the monovalent counterions originally condensed on a chain can be replaced by the multivalent ones dissociated from the added salt due to strong electrostatic interaction. These newly condensed ions give an important impact on the chain structure. At low and at high salt concentrations, the conformation of a semiflexible chain is rodlike. The ion distributions show similarity to those for a rigid chain, but difference to those for a flexible chain whose conformation is a coil. In the midsalt region, the flexible chain and the semiflexible chain collapse but the collapsed chain structures are, respectively, disordered and ordered structures. The ion distributions, hence, show different profiles for this three chain stiffness with the curves for the semiflexible chain lying between those for the flexible and the rigid chains. The number of the condensed multivalent counterions, as well as the effective chain charge, also shows similar behavior, demonstrating a direct connection with the chain morphology. Moreover, we find that the condensed multivalent counterions form triplets with two adjacent monomers and are localized on the chain axis at intermediate salt concentration when the chain stiffness is semiflexible or rigid. The microscopic information obtained here provides a valuable insight to the phenomena of DNA condensation and is very useful for researchers to develop new models.

Effects of valences of salt ions at various concentrations on charged dendrimers

The static properties of charged dendrimers in salt solutions with explicit salt ions are investigated by coarse-grained molecular dynamics simulations. The emphasis is placed on the effects of valences of ions with different salt concentrations. We find that the conformations of dendrimers are of weak dependence on ionic strength in monovalent salt solution, while dendrimers in multivalent salt solution display structural transitions with increasing the salt concentration. At each salt concentration, the higher the valence of salt ions is, the more compact the dendrimers become. We also witness that dendrimers show the dense-core conformation accompanied by a back-folding terminal shell under all the conditions studied and that their shapes are also not sensitive to valences and concentrations of salt ions. Furthermore, charge layering shows no clear difference among different salt concentrations in a monovalent salt solution, whereas it undergoes multi-state transitions in multivalent salt solution with the addition of multivalent salt ions and with the change of valences at a given salt concentration, implying that multivalent salt ions have a significant influence on the interior conformations of charged dendrimers in comparison with univalent salt ions. Moreover, overcharging of dendrimers is observed in multivalent salt solution at high salt concentrations but not in a monovalent salt solution.

Synthesis and characterization of pH‐ and salt‐responsive hydrogels based on etherificated sodium alginate

AbstractHydrogels composed of etherificated sodium alginate (ESA), sodium acrylic acid (NaAA), and poly (vinyl alcohol) (PVA) were synthesized by aqueous solution polymerization. The effects of reaction variables such as terminal pH, ions, and ionic strength on hydrogel swelling ratio (SR) were determined and compared. SR was influenced strongly by pH and ionic strength. SR increased with increasing pH but tended to decrease with PVA content. At a given ionic strength, SR of ESA/NaAA/PVA hydrogel was dependent on the valence of anion; SR was higher in multivalent anion salt solution than in monovalent anion salt solution, i.e., SRK2SO4 > SRKCl and SRNa2SO4 > SRNaCl. The swelling kinetic of the hydrogels showed Fickian kinetic diffusion in acidic media and non‐Fickian behavior in alkaline media. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Preparation of porous polyacrylate/poly(ethylene glycol) interpenetrating network hydrogel and simplification of Flory theory

In this article, we successfully designed and fabricated polyacrylate/poly(ethylene glycol) interpenetrating network (PAC/PEG IPN) hydrogel by a two-step technique. The influences of synthesis parameters, such as crosslinker dosage and neutralization degree of PAC, on the equilibrium swelling ratio (SR) of the PAC/PEG IPN hydrogel were investigated. Based on Flory theory, a simplified experiential formula $$ Q_{{{\text{H}}_{2} {\text{O}}}}^{5/3} = 14.72I^{ - 1} + 207.43 $$ was proposed. The results revealed that the theoretical SR of the PAC/PEG IPN hydrogel was consistent with experimental value in the presence of univalent salt solution. In this case, the SR of the hydrogel can be calculated without measurement. While for multivalent salt solutions, the theoretical and experimental results are inconsistent owing to chelations between carboxylic and carboxylate groups of the PAC/PEG hydrogels and the cations in the solution.

Free Solution Electrophoresis of Homopolyelectrolytes

We investigate the behavior of single polyelectrolytes in multivalent salt solutions under the action of electric fields through computer simulations. The chain is unfolded in a strong electric field and aligned parallel to the field direction, and the chain size shows a sigmoidal transition. The unfolding electric field E* depends on the salt concentration and scales as V (-1/2) with V being the ellipsoidal volume occupied by the chain. The magnitude of the electrophoretic mobility of chain drastically increases during the unfolding. The fact that E* depends on the chain length provides a plausible mechanism to separate long charged homopolymers by size in free solution electrophoresis via the unfolding transition of globule polyelectrolytes condensed by multivalent salt.

Synthesis of polyacrylate/poly(ethylene glycol) hydrogel and its absorption properties for heavy metal ions and dye

AbstractA novel polyacrylate/poly(ethylene glycol) (PAA/PEG) hydrogel was synthesized by a simple two‐step aqueous prepolymerization method. The deswelling properties were investigated in the case of changing the neutralization degrees of PAA, the pHs and the concentration of multivalent salt solutions. Methyl orange (MO) dye can be adsorbed and released using the swelling–deswelling property of the PAA/PEG hydrogel. The PAA/PEG hydrogel can adsorb MO of 29 mg/g at equilibrium and release about 80% in 30 min. Based on the deswelling property, the dyes in wastewater can be reclaimed and reused. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

Superabsorbent polymeric materials. XI. Effect of nonionic monomers on the swelling behavior of crosslinked poly(sodium acrylate‐co‐nonionic monomers) in aqueous salt solutions

AbstractA series of xerogels based on sodium acrylate, nonionic monomers such as 2‐hydroxyethyl methacrylate (HEMA) and poly(ethylene glycol) methacrylate (PEGMA), and N,N′‐methylene bisacrylamide were prepared by inverse suspension polymerization. The results indicate that the water absorbencies for these two gel series were effectively improved by the addition of a small amount of nonionic monomer (HEMA or PEGMA). The initial absorption rates in deionized water were faster for the PEGMA gels than for the HEMA gels. Scanning electron microscopy showed that the spherical particle size was smaller for the PEGMA gels than for the HEMA gels. In addition, the water absorbency of the gels in various salt solutions decreased with increasing ionic strength, especially for the multivalent salt solutions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3666–3674, 2004

Effective Macroion Charge and Stability of Highly Asymmetric Electrolytes at Various Salt Conditions

We study electrostatic mechanisms of the destabilization of highly asymmetric electrolytes. For this purpose, we perform primitive model Monte Carlo simulations of charged macroions immersed in multivalent salt solution. At low salt concentration, the macroion effective charge is reduced due to multivalent counterion adsorption. At high salt concentrations, the macroions become overcharged so that their apparent charge has the opposite sign to the stoichiometric one. The inverted charge increases to a saturation value upon further increase of the salinity. The system remains stable at low as well as at very high salt concentrations. In the intermediate region, close to the macroion isoelectric point, we observe macroion aggregation. The obtained phase behavior closely resembles polyelectrolyte-induced instability of colloidal dispersions.

Polyelectrolytes in Multivalent Salt Solutions: Monomolecular versus Multimolecular Aggregation

We analyze the segregation of strongly charged chains of N monomers of size a in the presence of multivalent salts with valence z as a function of the concentration of monomers φ in the solutions. The multivalent ions of opposite charge condense along the monomers and induce monomer attractions that lead to the formation of dense finite size aggregates (micelles) of chains and multivalent ions when their valence z > 1. We compute the number density of chains in aggregates with p = 1, 2, 3, 4, ... chains by equating the chemical potential of all the p-aggregated chains and of the free and condensed ions in the aggregates. At low concentration of multivalent salts m, we observe monomolecular (p = 1) precipitated chains when φ < φ*, where φ* ≅ mz. When φ increases above φ*, the chains redissolve in the solution and adopt stretched conformations. As m increases above a critical value m**, the number density of aggregates with p > 1 chains increases such that there are more aggregates with p* > 1 chains when φ** < φ < φ*. If we only include the surface free energy in the analysis, we find p* = ∞. However, if we include the chain entropy of confining p chains in a region R(p) = (pN)1/3a in the free energy, p* can be finite and greater than or equal to one. This situation arises when the chains are constrained to stretched conformations, such as in metastable toroidal and in spherical coil aggregates observed in long double-stranded DNA in multivalent ions.

Flexible linear polyelectrolytes in multivalent salt solutions: Solubility conditions

Polyelectrolytes such as single and double stranded DNA and many synthetic polymers undergo two structural transitions upon increasing the concentration of multivalent salt or molecules. First, the expanded-stretched chains in low monovalent salt solutions collapse into nearly neutral compact structures when the density of multivalent salt approaches that of the monomers. With further addition of multivalent salt the chains redissolve acquiring expanded-coiled conformations. We study the redissolution transition using a two state model [F. Solis and M. Olvera de la Cruz, {\it J. Chem. Phys.} {\bf 112} (2000) 2030]. The redissolution occurs when there is a high degree of screening of the electrostatic interactions between monomers, thus reducing the energy of the expanded state. The transition is determined by the chemical potential of the multivalent ions in the solution $\mu$ and the inverse screening length $\kappa$. The transition point also depends on the charge distribution along the chain but is almost independent of the molecular weight and degree of flexibility of the polyelectrolytes. We generate a diagram of $\mu$ versus $\kappa^2$ where we find two regions of expanded conformations, one with charged chains and other with overcharged (inverted charge) chains, separated by a collapsed nearly neutral conformation region. The collapse and redissolution transitions occur when the trajectory of the properties of the salt crosses the boundaries between these regions. We find that in most cases the redissolution occurs within the same expanded branch from which the chain precipitates.

Collapse of flexible polyelectrolytes in multivalent salt solutions

The collapse of flexible polyelectrolytes in a solution of multivalent counterions is studied by means of a two state model. The states correspond to rodlike and spherically collapsed conformations. We focus on the very dilute monomer concentration regime where the collapse transition is found to occur when the charge of the multivalent salt is comparable (but smaller) to that of the monomers. The main contribution to the free energy of the collapsed conformation is linear in the number of monomers N, since the internal state of the collapsed polymer approaches that of an amorphous ionic solid. The free energy of the rodlike state grows as N ln N, due to the electrostatic energy associated with that shape. We show that practically all multivalent counterions added to the system are condensed into the polymer chain, even before the collapse.

Superabsorbent polymeric material. V. Synthesis and swelling behavior of sodium acrylate and sodium 2-acrylamido-2-methylpropanesulfonate copolymeric gels

A series of xerogels based on sodium acrylate (SA), sodium 2-acrylamido-2-methyl propanesulfonate (NaAMPS), and N,N-methylene-bis-acrylamide (NMBA) are prepared by inverse suspension polymerization. The water absorbency and swelling behavior for these high absorbent polymers in deionized water and transition salt solutions are investigated. Experimental results indicated that the absorbency in deionized water increases with an increase of the NaAMPS content in the copolymeric gels, which is related to the degree of charge density of the network and the strength of hydrophilic group. The extent of crosslinking agent also influenced the swelling capacity because of elastic chain force of the polymer chain. The absorbency in chloride salt solutions decreases with an increase in the ionic strength of salt. But the decrease of absorbency is different in monovalent and multivalent salt solution. This behavior can be accounted for in terms of counterion condensation or screening effect for monovalent cations, as well as complexation for multivalent cations. The swelling rates in various salt solutions for these xerogels are also investigated. At last, SA-NaAMPS copolymeric gels were used for ion adsorption. But the result showed that the adsorptive amount of transition metal ions for SA-NaAMPS copolymeric gels was lower than that for pure poly(SA) gel. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 229–237, 1998

Reversal of bubble charge in multivalent inorganic salt solutions—effect of lanthanum

It was discovered recently that the charge of gas bubbles in aqueous solution can be reduced and even reversed by the addition of multivalent inorganic salts. In this work, the effect of LaCl 3 on the charge of bubbles was investigated and found to have an effect similar to that previously observed with AlCl 3 and MgCl 2. The electrokinetic behavior of gas bubbles was affected slightly by LaCl 3 additions at low concentrations but changed markedly at high concentration (1 × 10 −3 mol dm −3) of LaCl 3. A reversal of bubble charge was observed and this is attributed to the specific adsorption of La 3+ in the lower pH region and to the formation of La(OH) 3(s) precipitate at the gas—liquid interface in the alkaline pH region.

Reversal of bubble charge in multivalent inorganic salt solutions—Effect of aluminum

The charge of bubbles in aqueous solution is important in determining their interactions with particles, droplets, and other bubbles. While bubbles are known to be negatively charged in such media as water and sodium chloride solutions, they are found here to exhibit unusual positive surface charge characteristics in solutions of trivalent aluminum cations. Analysis of the results indicates that the reversal of bubble charge can be attributed to specific adsorption of Al +3 and its hydroxo complexes at the gas-liquid interface in the low pH range and to precipitation of aluminum hydroxide in the intermediate pH range.

Reversal of bubble charge in multivalent inorganic salt solutions—Effect of magnesium

An experimental technique has been developed to study the electrokinetic behavior of gas bubbles in aqueous electrolyte solutions. It was found that bubbles are negatively charged in NaCl solutions whereas, under certain conditions, they become positively charged in MgCl 2 solutions. The observed charge reversal in the pH range of 9 to 11 is attributed to the precipitation of Mg(OH) 2(s) at the gas-liquid interface while the reduction in the magnitude of the zeta potential of the bubble observed in the acidic pH range is considered to be due to the specific adsorption of Mg +2.