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

Molecular Design Strategies to Enhance the Electroresponse of Polyelectrolyte Brushes: Effects of Charge Fraction and Chain Length Dispersity

Polyelectrolyte brushes are functional surface coatings that react to external stimuli. The response of these brushes in electric fields is nearly immediate as the field acts directly on the charges in the polyion, while the response to bulk stimuli such as temperature, acidity, and ionic composition is intrinsically capped by transport limitations. However, the response of fully charged brushes is limited because large field strengths are required to achieve a response. This limits the application of these brushes to architectures such as small pores or nanojunctions because small biases can generate large field strengths over small distances. Here, we propose a design strategy that enhances the response and lowers the field strength required in these applications. Our coarse-grained simulations highlight two approaches to increase the electroresponse of polyelectrolyte brushes: dispersity in the chain length enhances the electroresponse and a reduction in the number of charged monomers does the same. With these approaches, we increase the relative brush height variation from only 28% to as much as 227% since in partially charged brushes, more chains need to respond to screen the imposed field and the longer chains in disperse brushes can reorganize over large distances. Additionally, we find that disperse brushes show a stratified response where short chains collapse first and long chains stretch first because this stratification minimizes the change in conformational energy. We envision that our insights will enable the application of electroresponsive polyelectrolyte brushes in larger architectures or in small architectures using smaller biases, which could enable a stimulus-responsive pore size modulation that could be used for filtration and molecular separations.

Polyelectrolyte Complexation When Considering the Counterion-Mediated Hydrogen Bonding

In this work, we have investigated a pH-modulated complexation between two oppositely charged strong polyelectrolytes to demonstrate the effect of counterion-mediated hydrogen bonding (CMHB) on polyelectrolyte complexation. We have found that such a pH-modulated complexation cannot be understood without considering the CMHB. Thermodynamically, the effect of CMHB on the polyelectrolyte complexation is manifested by the alteration of both enthalpic and entropic contributions to the free energy change. The pH-dependent intrinsic ion-pairing and complex coacervation processes of the polyelectrolyte complexation can be understood when considering the CMHB. Our study demonstrates that both the extent of polyelectrolyte complex formation in bulk solutions and the formation of polyelectrolyte multilayers on surfaces are controlled by the pH-dependent intrinsic ion-pairing process. Furthermore, on the basis of the pH-dependent intrinsic ion pairing, the properties of the multilayers can be tuned by pH. This work provides a new strategy to control the polyelectrolyte complexation with counterions and will inspire new ideas for building advanced polyelectrolyte materials.

Ion-Specific Effects on Vesicle-to-Micelle Transitions of an Amino Acid Surfactant Probed by Chemical Trapping

Ion-specific effects widely exist in biological and chemical systems and cannot be explained by classical theories. The complexity of ion-specific effects in protein systems at the molecular level necessitates the use of mimetic models involving smaller molecules, such as amino acids, oligopeptides, and other organic molecules bearing amide bonds. Therefore, it is of theoretical value to determine the effect of additional salts on the aggregation transitions of acyl amino acid surfactants. Herein, the effects of specific tetraalkylammonium ions (TAA⁺) on sodium lauroyl glycinate (SLG) aggregation were studied by dynamic light scattering (DLS) and transmission electron microscopy. Although previous studies have shown that the kosmotropic TAA⁺ ions tend to induce micellar growth or micelle-to-vesicle transitions of some anionic surfactants, TAA⁺ addition in the present study induced partial vesicle-to-micelle transitions in SLG solutions. The chemical trapping (CT) method was employed to estimate changes in the interfacial molarities of water, amide bonds, and carboxylate groups during such transitions. The vesicle-to-micelle transitions were accompanied by a marked rise in interfacial water molarity and a decline in interfacial amide bonds molarity, suggesting that the hydrated TAA⁺ entered the interfacial region and disrupted hydrogen bonding, thus preventing the SLG monomers from packing tightly. Molecular dynamic simulation was also performed to demonstrate the salt-induced cleavage of amide-amide bonds between SLG headgroups. Furthermore, both CT and DLS results show that the ability of tetraalkylammonium cations to induce such transitions increased with increasing size and hydrophobicity of the cation, which follows the Hofmeister series. The current study offers critical molecular-level evidence for understanding the specific effects of tetraalkylammonium ions on the aggregation transitions of an acyl amino acid surfactant.

Phase Separation of Aqueous Poly(diisopropylaminoethyl methacrylate) upon Heating

Poly(diisopropylaminoethyl methacrylate) (PDPA) is a pH- and thermally responsive water-soluble polymer. This study deepens the understanding of its phase separation behavior upon heating. Phase separation upon heating was investigated in salt solutions of varying pH and ionic strength. The effect of the counterion on the phase transition upon heating is clearly demonstrated for chloride-, phosphate-, and citrate-anions. Phase separation did not occur in pure water. The buffer solutions exhibited similar cloud points, but phase separation occurred in different pH ranges and with different mechanisms. The solution behavior of a block copolymer comprising poly(dimethylaminoethyl methacrylate) (PDMAEMA) and PDPA was investigated. Since the PDMAEMA and PDPA blocks phase separate within different pH- and temperature ranges, the block copolymer forms micelle-like structures at high temperature or pH.