Specific Anion Effects on Charged-Neutral Random Copolymers: Interplay between Different Anion-Polymer Interactions.

The study of ion specificities of charged-neutral random copolymers is of great importance for understanding specific ion effects on natural macromolecules. In the present work, we have investigated the specific anion effects on the thermoresponsive behavior of poly([2-(methacryloyloxy)ethyl trimethylammonium chloride]-co-N-isopropylacrylamide) [P(METAC-co-NIPAM)] random copolymers. Our study demonstrates that the anion specificities of the P(METAC-co-NIPAM) copolymers are dependent on their chemical compositions. The specific anion effects on the copolymers with high mole fractions of poly(N-isopropylacrylamide) (PNIPAM) are similar to those on the PNIPAM homopolymer. As the mole fraction of PNIPAM decreases to a certain value, a V-shaped anion series can be observed in terms of the anion-specific cloud point temperature of the copolymer, as induced by the interplay between different anion-polymer interactions. Our study also suggests that both the direct and the indirect anion-polymer interactions contribute to the anion specificities of the copolymers. This work would improve our understanding of the relationship between the ion specificities and the ion-macromolecule interactions for naturally occurring macromolecules.

Anion Specificity of Polyzwitterionic Brushes with Different Carbon Spacer Lengths and Its Application for Controlling Protein Adsorption.

Both ion-specific interaction and carbon spacer length have strong effects on the properties of polyzwitterions. In this work, we have investigated the anion specificity of poly(sulfobetaine methacrylamide) (PSBMAm) brushes with different carbon spacer lengths. The effectiveness of anions to enhance the hydration of the PSBMAm brushes increases from kosmotropic to chaotropic anions. The interactions between the anions and the PSBMAm brushes are strongly influenced by carbon spacer length because the strength of inter/intrachain association of the PSBMAm brushes decreases with increasing carbon spacer length. The inter/intrachain association of the PSBMAm brushes with a longer carbon spacer is easier to break by the external anions in the high salt concentration regime. On the other hand, a longer carbon spacer is more favorable for the zwitterionic groups to form cyclic intramolecular structures. As a result, the addition of anions can more effectively enhance the hydration of the PSBMAm brushes with a medium-length carbon spacer compared with that of the PSBMAm brushes with a either shorter or longer carbon spacer in the low salt concentration regime, determined by the balance between the inter/intrachain association and the formation of cyclic intramolecular structures. Our study also demonstrates that both anion identity and carbon spacer length can be used to control protein adsorption on the surface of the PSBMAm brushes.

Cleaning with Bulk Nanobubbles.

The electrolysis of aqueous solutions produces solutions that are supersaturated in oxygen and hydrogen gas. This results in the formation of gas bubbles, including nanobubbles ∼100 nm in size that are stable for ∼24 h. These aqueous solutions containing bubbles have been evaluated for cleaning efficacy in the removal of model contaminants bovine serum albumin and lysozyme from surfaces and in the prevention of the fouling of surfaces by these same proteins. Hydrophilic and hydrophobic surfaces were investigated. It is shown that nanobubbles can prevent the fouling of surfaces and that they can also clean already fouled surfaces. It is also argued that in practical applications where cleaning is carried out rapidly using a high degree of mechanical agitation the role of cleaning agents is not primarily in assisting the removal of soil but in suspending the soil that is removed by mechanical action and preventing it from redepositing onto surfaces. This may also be the primary mode of action of nanobubbles during cleaning.

Ion specificities of artificial macromolecules.

Artificial macromolecules are well-defined synthetic polymers, with a relatively simple structure as compared to naturally occurring macromolecules. This review focuses on the ion specificities of artifical macromolecules. Ion specificities are influenced by solvent-mediated indirect ion-macromolecule interactions and also by direct ion-macromolecule interactions. In aqueous solutions, the role of water-mediated indirect ion-macromolecule interactions will be discussed. The addition of organic solvents to aqueous solutions significantly changes the ion specificities due to the formation of water-organic solvent complexes. For direct ion-macromolecule interactions, we will discuss specific ion-pairing interactions for charged macromolecules and specific ion-neutral site interactions for uncharged macromolecules. When the medium conditions change from dilute solutions to crowded environments, the ion specificities can be modified by either the volume exclusion effect, the variation of dielectric constant, or the interactions between ions, macromolecules, and crowding agents.

Ion specificity of macromolecules in crowded environments.

Macromolecular crowding plays a significant role in the solubility and stability of biomacromolecules. In this work, the thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) has been employed as a model system to study the specific ion effects on the solubility of macromolecules in crowded environments of dextran and polyethylene glycol (PEG). Our study demonstrates that crowding agents can interact with either anions or PNIPAM chains. The chaotropic anion SCN(-) interacts with dextran but does not interact with PEG. Both Cl(-) and CH3COO(-) do not interact with dextran and PEG. On the other hand, dextran can interact with PNIPAM as a hydrogen-bond donor, whereas PEG interacts with PNIPAM as a hydrogen-bond acceptor. The salting-in effect exerted by SCN(-) on PNIPAM is weakened in the crowded environment of dextran but is strengthened in the crowded environment of PEG due to the distinct anion-crowder interactions. In parallel, the salting-out effect generated by Cl(-) and CH3COO(-) on PNIPAM is weakened by the crowding of dextran but is strengthened by the crowding of PEG because of the different macromolecule-crowder interactions. Our study reveals that the ion specificity of macromolecules is altered significantly changing from dilute solutions to crowded environments.

Cation-specific conformational behavior of polyelectrolyte brushes: from aqueous to nonaqueous solvent.

We have investigated changes in the cation-specific conformational behavior of poly(sodium styrenesulfonate) (PSS) brushes as the solvent changes from water to methanol using a quartz crystal microbalance with dissipation (QCM-D). A solvation to desolvation transition of the grafted chains accompanied by swelling to the collapse transition of the brushes is observed for Na(+). In the case of Cs(+), the brushes undergo solvation to desolvation to resolvation accompanied by swelling to collapse to reswelling transitions. The resolvation and reswelling transitions for Cs(+) are induced by the charge inversion of the brushes via van der Waals interactions between Cs(+) and the brushes. All of the transitions for monovalent cations become less obvious as the methanol content increases. For divalent Ca(2+) and trivalent La(3+), a solvation to desolvation to resolvation transition of the grafted chains accompanied by a swelling to collapse to reswelling transition of the brushes can be observed. The resolvation and reswelling of the brushes for the multivalent cations are induced by the charge inversion of the brushes via charge-image charge interactions. The extent of the transitions for the PSS brushes in the presence of multivalent cations is only slightly influenced by the methanol content.

Insight into the amplification by methylated urea of the anion specificity of macromolecules.

Methylated urea and sugar are chaotropic and kosmotropic osmolytes, respectively. In the present work, we have investigated the specific anion effect on the lower critical solution temperature (LCST) behavior of poly(N-isopropylacrylamide) (PNIPAM) in the presence of methylated urea or sugars. Differential scanning calorimetry studies revealed that tetramethylurea can adsorb onto the PNIPAM surface, but glucose is excluded from the PNIPAM surface. The specific anion effect on the LCST behavior of PNIPAM is amplified by methylated urea but not by sugars. The amplification of the anion specificity by methylated urea is attributed to an increased difference in the anion-specific polarization of hydrogen bonds, induced by the formation of PNIPAM/methylated urea complexes via hydrophobic interactions. As the number of methyl groups on the methylated urea increases, the extent of amplification of the anion specificity increases due to increasing hydrophobic interactions between the PNIPAM and methylated urea. Additionally, no amplification of the anion specificity is observed in the presence of urea because a PNIPAM/urea complex cannot be formed via hydrophobic interactions.

Ion specificity at a low salt concentration in water-methanol mixtures exemplified by a growth of polyelectrolyte multilayer.

By use of a quartz crystal microbalance with dissipation (QCM-D), we have investigated the specific ion effect on the growth of poly(sodium 2-acrylamido-2-methylpropanesulfonate)/poly(diallyldimethylammonium chloride) multilayer at a salt concentration as low as 2.0 mM in water-methanol mixtures. QCM-D results demonstrate that specific ion effect can be observed in methanol and water-methanol mixtures though it is negligible in water. Moreover, the specific ion effect is amplified as the molar fraction of methanol (xM) increases from 0% to 75% but is weakened again with the further increase of xM from 75% to 100%. Nuclear magnetic resonance measurements reveal that the counterion-polyelectrolyte segment interactions may not account for the observed ion specificity. By extending the Collins' concept of matching water affinities to methanol and water-methanol mixtures, we suggest that the ion-solvent interactions and the resulted counterion-charged group interactions are responsible for the occurrence of the specific ion effect. The conductivity measurements indicate that water and methanol molecules may form complexes, and the change of relative proportion of complexes with the xM causes the amplification or weakening of the specific ion effect.

Modulation of the Binding Affinity of Polyzwitterion-Conjugated Protein by Ion-Specific Effects in Crowded Environments.

Macromolecular crowding could influence the binding affinity of the polyzwitterion-conjugated proteins. Herein, the hydrolysis of N-succinyl-Ala-Ala-Pro-Phe p-nitroanilide by the poly(carboxybetaine) conjugated α-chymotrypsin (PCT) was employed as a model system to investigate the modulation of the binding affinity of the polyzwitterion-conjugated proteins by the ion-specific effects in the crowded environments. In comparison with the bare α-chymotrypsin (BCT), the binding affinity of the PCT to the peptide is stronger in the dilute solutions but becomes weaker in the crowded environments. Our study demonstrates that the kosmotropic Ac- anion is incapable of achieving a stronger enzymatic binding affinity of the PCT than the BCT in the highly crowded environments. By contrast, the binding affinity of the PCT can be enhanced to be stronger than that of the BCT by the chaotropic SCN- anion in the crowded environments.

Mechanistic insights into amplification of specific ion effect in water-nonaqueous solvent mixtures.

Ethylene glycol (EG) and hydrogen peroxide (H2O2) can act as both hydrogen-bond donors and acceptors in the formation of solvent complexes with water molecules. In the present work, we have systematically investigated the ion-specific lower critical solution temperature (LCST) behavior of poly(N-isopropylacrylamide) (PNIPAM) in H2O­EG and H2O­H2O2 mixtures. The results obtained from turbidity measurements show that the specific anion effect is amplified with the increasing molar fraction of EG (x(EG)) but is independent of the molar fraction of H2O2 (x(H2O2)). The studies of Raman spectra and differential scanning calorimetry indicate that the discrepancy in amplification of specific anion effect between H2O­EG and H2O­H2O2 mixtures is due to the difference in the anion­solvent complex interactions rather than the anion­polymer or solvent­polymer interactions. On the other hand, the specific cation effect can also be amplified with the increasing x(EG) but changes only slightly with the x(H2O2). The discrepancy in amplification of specific cation effect between the two types of solvent mixtures is attributed to the difference in the solvent­polymer interactions.

Specific anion effect in water-nonaqueous solvent mixtures: interplay of the interactions between anion, solvent, and polymer.

Ethanol (EtOH) and dimethylsulfoxide (DMSO) are polar protic and aprotic organic solvents, respectively. In the present work, we have investigated the anion-specific lower critical solution temperature (LCST) and upper critical solution temperature (UCST) behaviors of poly(N-isopropylacrylamide) (PNIPAM) in the H2O-EtOH and H2O-DMSO mixtures. The turbidity and differential scanning calorimetry studies show that the LCST for the anions follows the Hofmeister series at the molar fraction of EtOH (xE) or DMSO (xD) of 6%. At xE of 26%, the UCST for the anions also follows the Hofmeister series because the dominating interactions for the UCST behavior are similar to that for the LCST behavior in the H2O-EtOH mixtures. In the H2O-DMSO mixture at xD of 70%, an inverted V-shaped anion series is observed for the UCST behavior of PNIPAM. Our studies demonstrate that the specific anion effect on the phase transition behaviors of PNIPAM is influenced not only by the anionic polarization of hydrogen bonding between solvent molecules and PNIPAM but also by the anion adsorption on the PNIPAM chain surface.