Influence of Cononsolvency on the Aggregation of Tertiary Butyl Alcohol in Methanol–Water Mixtures

Mechanism of poly(N-isopropylacrylamide) cononsolvency in aqueous methanol solutions explored via oxygen K-edge X-ray absorption spectroscopy

The cononsolvency mechanism of poly(N-isopropylacrylamide) (PNIPAM), dissolving in pure methanol (MeOH) and water (H2O) but being insoluble in MeOH-H2O mixtures, was investigated by O K-edge X-ray absorption spectroscopy (XAS). The cononsolvency emerges from the aggregation of PNIPAM with MeOH clusters, leading to the collapse of the hydrophobic hydration of PNIPAM.

Poly(N,N-diethylacrylamide)-endowed spontaneous emulsification during the breath figure process and the formation of membranes with hierarchical pores

The spontaneous emulsification for the formation of water-in-oil (W/O) or oil-in-water (O/W) emulsions needs the help of at least one kind of the third component (surfactant or cosolvent) to stabilize the oil-water interface. Herein, with the water/CS2-soluble polymer poly(N,N-diethylacrylamide) (PDEAM) as a surfactant, the spontaneous formation of water-in-PDEAM/CS2 emulsions is reported for the first time. The strong affinity between PDEAM and water or the increase of PDEAM concentration will accelerate the emulsification process with high dispersed phase content. It is demonstrated that the spontaneous emulsification of condensed water droplets into the PDEAM/CS2 solution occurs during the breath figure process, resulting in porous films with two levels of pore sizes (i.e., micron and submicron). The emulsification degree and the amounts of submicron-sized pores increase with PDEAM concentration and solidifying time of the solution. This work brings about incremental interest in spontaneous emulsification that may happen during the breath figure process. The combination of these two simultaneous processes provides us with an option to build hierarchically porous structures with condensed and emulsified water droplets as templates. Such porous membranes may have great potential in fields such as separation, cell culture, and biosensing.

Ethanol-Induced Aggregation of Nonpolar Nanoparticles in Water/Ethanol Mixed Solvents

The dispersity of nonpolar nanoparticles (NPs) in water/ethanol mixed solvents was studied using molecular dynamics simulations. Based on the rule of "like dissolves like," nonpolar NPs should be dispersed better in a solvent with a lower polarity. As the mole fraction of ethanol in a mixed solvent (R) increases from 0% (pure water) to 100% (pure ethanol), the polarity of the mixed solvent is indicated to decrease monotonically. However, the dispersity of nonpolar NP does not increase monotonically: it first decreases after the addition of a small fraction of ethanol (R < 8.0%) and then markedly increases as R further grows. When there is a small amount of ethanol, the ethanol molecules around aggregated NPs tend to simultaneously make contact with multiple NPs, which can increase the tendency of NP aggregation. Furthermore, with a considerable ethanol ratio, the interaction of the solvent with NPs becomes notably strong, which facilitates the dissolution of NPs. Our findings may help to better understand the mechanism of dispersion of NPs in mixed solvents and may provide a useful precipitation technology for NP production.

Poly(sulfobetaine)-Based Diblock Copolymer Thin Films in Water/Acetone Atmosphere: Modulation of Water Hydration and Co-nonsolvency-Triggered Film Contraction

The water swelling and subsequent solvent exchange including co-nonsolvency behavior of thin films of a doubly thermo-responsive diblock copolymer (DBC) are studied via spectral reflectance, time-of-flight neutron reflectometry, and Fourier transform infrared spectroscopy. The DBC consists of a thermo-responsive zwitterionic (poly(4-((3-methacrylamidopropyl) dimethylammonio) butane-1-sulfonate)) (PSBP) block, featuring an upper critical solution temperature transition in aqueous media but being insoluble in acetone, and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block, featuring a lower critical solution temperature transition in water, while being soluble in acetone. Homogeneous DBC films of 50-100 nm thickness are first swollen in saturated water vapor (H₂O or D₂O), before they are subjected to a contraction process by exposure to mixed saturated water/acetone vapor (H₂O or D₂O/acetone-d6 = 9:1 v/v). The affinity of the DBC film toward H₂O is stronger than for D₂O, as inferred from the higher film thickness in the swollen state and the higher absorbed water content, thus revealing a pronounced isotope sensitivity. During the co-solvent-induced switching by mixed water/acetone vapor, a two-step film contraction is observed, which is attributed to the delayed expulsion of water molecules and uptake of acetone molecules. The swelling kinetics are compared for both mixed vapors (H₂O/acetone-d6 and D₂O/acetone-d6) and with those of the related homopolymer films. Moreover, the concomitant variations of the local environment around the hydrophilic groups located in the PSBP and PNIPMAM blocks are followed. The first contraction step turns out to be dominated by the behavior of the PSBP block, whereas the second one is dominated by the PNIPMAM block. The unusual swelling and contraction behavior of the latter block is attributed to its co-nonsolvency behavior. Furthermore, we observe cooperative hydration effects in the DBC films, that is, both polymer blocks influence each other's solvation behavior.

Cononsolvency of thermoresponsive polymers: where we are now and where we are going

Cononsolvency is an intriguing phenomenon where a polymer collapses in a mixture of good solvents. This cosolvent-induced modulation of the polymer solubility has been observed in solutions of several polymers and biomacromolecules, and finds application in areas such as hydrogel actuators, drug delivery, compound detection and catalysis. In the past decade, there has been a renewed interest in understanding the molecular mechanisms which drive cononsolvency with a predominant emphasis on its connection to the preferential adsorption of the cosolvent. Significant efforts have also been made to understand cononsolvency in complex systems such as micelles, block copolymers and thin films. In this review, we will discuss some of the recent developments from the experimental, simulation and theoretical fronts, and provide an outlook on the problems and challenges which are yet to be addressed.

A tiny charge-scaling in the OPLS-AA + L-OPLS force field delivers the realistic dynamics and structure of liquid primary alcohols

We carry out molecular dynamics simulations for pure liquid primary alcohols ranging from methanol to 1-decanol under ambient conditions. Based on the OPLS-AA force field with the L-OPLS correction, we demonstrate that a few % increases in the partial charges deliver the realistic dynamics (self-diffusion coefficient and shear viscosity) and structure (density and X-ray scattering intensity) as well as enthalpy of vaporization and isothermal compressibility. The validity against thermal expansion coefficient, isobaric heat capacity, and static dielectric constant are also discussed.

Less-Ordered Hydration Shell around Poly(N,N-diethylacrylamide) Is Insensitive to the Clouding Transition

Raman multivariate curve resolution (Raman-MCR) is applied to examine how the hydration shell around poly(N,N-diethylacrylamide) (PDEAM) changes upon heating, in comparison with poly(N-isopropylacrylamide) (PNIPAM), both of which undergo a clouding transition near room temperature. We report that PDEAM possesses a less-ordered and smaller hydration shell than PNIPAM. Furthermore, the PDEAM hydration-shell structure is insensitive to the occurrence of clouding, indicating the coil-globule transition and aggregation of multiple chains can be achieved without the hydration-shell structural transformation.

Molecular-scale origins of solution nanostructure and excess thermodynamic properties in a water/amphiphile mixture

The molecular and nanoscale origins of nonideality in excess thermodynamic properties are essential to understanding cosolvent mixtures, yet they remain challenging to determine. Here, we consider a binary mixture of water and an amphiphile, N,N,N',N'-tetramethylmalonamide (TMMA), which is characterized by strong hydrogen bonding between the two components and no hydrogen bonding between amphiphiles. Using molecular dynamics simulation, validated with excess volume measurements and X-ray scattering, we identify three distinct solution regimes across the composition range of the binary mixture and find that the transition between two of these regimes, marked by the water percolation threshold, is closely correlated with minima in the excess volume and excess enthalpy. Structural analysis of the simulations reveals an interplay between local interactions and solution nanostructure, determined by the relative strength of the water-water and water-amphiphile hydrogen bonding interactions. By comparison with other amphiphiles, such as linear alcohols, the relative strength of like and unlike interactions between water and amphiphile affects the relationship between thermodynamics and structural regimes. This provides insight into how molecular forces of mutual solvation interact across length scales and how they manifest in excess thermodynamic properties.

Molecular heterogeneity in aqueous cosolvent systems

Aqueous cosolvent systems (ACoSs) are mixtures of small polar molecules such as amides, alcohols, dimethyl sulfoxide, or ions in water. These liquids have been the focus of fundamental studies due to their complex intermolecular interactions as well as their broad applications in chemistry, medicine, and materials science. ACoSs are fully miscible at the macroscopic level but exhibit nanometer-scale spatial heterogeneity. ACoSs have recently received renewed attention within the chemical physics community as model systems to explore the relationship between intermolecular interactions and microscopic liquid-liquid phase separation. In this perspective, we provide an overview of ACoS spatial segregation, dynamic heterogeneity, and multiscale relaxation dynamics. We describe emerging approaches to characterize liquid microstructure, H-bond networks, and dynamics using modern experimental tools combined with molecular dynamics simulations and network-based analysis techniques.

Spectroscopically Quantifying the Influence of Salts on Nonionic Surfactant Chemical Potentials and Micelle Formation

The influence of two salts (NaSCN and Na₂SO₄) on the micellization of a nonionic surfactant (1,2-hexanediol) is quantified using Raman multivariate curve resolution spectroscopy, combined with a generalized theoretical analysis of the corresponding chemical potential changes. Although the SCN^- and SO₄^2- anions are on opposite ends of the Hofmeister series, they are both found to lower the critical micelle concentration. Our combined spectroscopic and theoretical analysis traces these observations to the fact that in both salt solutions the ions have a greater affinity for (or are less strongly expelled from) the hydration shell of the micelle than the free surfactant monomer, as quantified using the corresponding chemical potentials and Wyman-Tanford coefficients. This probe-free experimental and theoretical analysis strategy may readily be extended to micelle formation processes involving other surfactants, salts, and cosolvents, as well as to other sorts of aggregation and binding processes.

Solvent Effects on the Self-Assembly of an Amphiphilic Polypeptide Incorporating α-Helical Hydrophobic Blocks

The self-assembly of biological molecules is an important pathway to understanding the molecular basis of complex metabolic events. The presence of a cosolvent in an aqueous solution during the self-assembly process can promote the formation of kinetically trapped metastable intermediates. In nature, a category of cosolvents termed osmolytes can work to strengthen the hydrogen-bond network of water such that the native states of certain proteins are favored, thus modulating their function and stability. However, identifying cosolvents that act as osmolytes in biomimetic applications, such as the self-assembly of soft materials, remains challenging. The present work examined the effects of ethanol (EtOH) and acetonitrile (ACN) as cosolvents on the self-assembly of the amphiphilic polypeptide PSar₃₀-(l-Leu-Aib)₆ (S30L12), which incorporates α-helical hydrophobic blocks, in aqueous solution. The results provided a direct observation of morphological behavior of S30L12 as a function of solvent composition. Morphological transitions were investigated using transmission electron microscopy, while the packing of peptide molecules was assessed using circular dichroism analyses and evaluations of membrane fluidity. In the EtOH/H₂O mixtures, the EtOH strengthened the hydrogen-bond network of the water, thus limiting the hydrophobic hydration of S30L12 assemblies and enhancing hydrophobic interactions between assemblies. In contrast, ACN formed self-associated nanoclusters in water and at the hydrophobic cores of peptide assemblies to stabilize the edges exposed to bulk water and enhance the assembly kinetics. Fourier transform infrared (FT-IR) analysis indicated that both EtOH and ACN can modify the self-assembly of biomaterials in the same manner as osmolyte protectants or denaturants.

A cosolvent surfactant mechanism affects polymer collapse in miscible good solvents

The coil-globule transition of aqueous polymers is of profound significance in understanding the structure and function of responsive soft matter. In particular, the remarkable effect of amphiphilic cosolvents (e.g., alcohols) that leads to both swelling and collapse of stimuli-responsive polymers has been hotly debated in the literature, often with contradictory mechanisms proposed. Using molecular dynamics simulations, we herein demonstrate that alcohols reduce the free energy cost of creating a repulsive polymer-solvent interface via a surfactant-like mechanism which surprisingly drives polymer collapse at low alcohol concentrations. This hitherto neglected role of interfacial solvation thermodynamics is common to all coil-globule transitions, and rationalizes the experimentally observed effects of higher alcohols and polymer molecular weight on the coil-to-globule transition of thermoresponsive polymers. Polymer-(co)solvent attractive interactions reinforce or compensate this mechanism and it is this interplay which drives polymer swelling or collapse.

On-Off of Co-non-solvency for Poly(N-vinylcaprolactam) in Alcohol-Water Mixtures: A Molecular Dynamics Study

Poly(N-vinylcaprolactam) (PVCL) exhibits co-non-solvency in aqueous solutions of 2-propanol but not in methanol. What distinguishes the impact of these two cosolvents on the polymer conformational stability? We report a molecular dynamics simulation study on PVCL 50-mer and monomers dissolved in methanol-water and 2-propanol-water mixtures. We show that the alcohol-concentration dependence of the effective attraction between a pair of PVCL monomers closely resembles the conformational changes in a single PVCL 50-mer as well as the experimentally observed behavior for PVCL chains. We also found that, at the co-non-solvency maximum, the monomer-monomer attraction works over a long-range beyond the solvent-separated distance. Then, we correlate the long-range attraction to the appearance of a dense alcohol concentration accumulated between the monomers. Furthermore, we distinctly demonstrate that the co-non-solvency of PVCL monomers can be switched on/off by artificially tuning the alcohol size while keeping the energetic parameters. Thus, we conclude that the magnitude of the excluded volume effect in alcohol accompanying the gain in translational entropy of the solvent is crucial to the occurrence of PVCL polymer co-non-solvency.

Understanding alcohol aggregates and the water hydrogen bond network towards miscibility in alcohol solutions: graph theoretical analysis

Under ambient conditions, methanol and ethanol are miscible in water at all concentrations, while n-butanol is partially miscible. This is the first study to quantitatively examine the miscibility of butanol and compare with miscible alcohols by employing molecular dynamics simulations and graph theoretical analysis of three water-alcohol mixtures at various concentrations. We show how distinct alcohol aggregates are formed, thereby affecting the water structure, which established the relationship between the morphological structure of the aggregates and the miscibility of the alcohol in aqueous solution. The aggregates of methanol and ethanol in highly concentrated solutions form an extended H-bond network that intertwines well with the H-bond network of water. n-Butanol tends to self-associate and form large aggregates, while such aggregates are segregated from water. Graph theoretical analysis revealed that the alcohol aggregates of methanol and ethanol solutions have a morphological structure different from that of n-butanol, although there is no significant difference in morphology between the three pure alcohols. These two distinct alcohol aggregates are classified as water-compatible and water-incompatible depending upon their interaction with the water H-bond network, and their effect on the water structure was investigated. Our study reveals that the water-compatible network of alcohol aggregates in methanol and ethanol solutions disrupts the water H-bond networks, while the water-incompatible network of n-butanol aggregates does not considerably alter the water structure, which is consistent with the experimental results. Furthermore, we propose that miscible alcohols form water-compatible networks in binary aqueous systems while partially miscible alcohols form water-incompatible networks. The bifurcating hypothesis on the alcohol aggregation behavior in liquid water is of critical use to understand the fundamental issues such as solubility and phase separation in solution systems.

Liquid-Liquid Phase Separation Produces Fast H-Bond Dynamics in DMSO-Water Mixtures

Liquid-liquid phase separation is common in complex mixtures, but the behavior of nanoconfined liquids is poorly understood from a physical perspective. Dimethyl sulfoxide (DMSO) is an amphiphilic molecule with unique concentration-dependent bulk properties in mixtures with water. Here, we use ultrafast two-dimensional infrared (2D IR) spectroscopy to measure the H-bond dynamics of two probe molecules with different polarities: formamide (FA) and dimethylformamide (DMF). Picosecond H-bond dynamics are fastest in the intermediate concentration regime (20-50 mol % DMSO), because such confined water exhibits bulk-like dynamics. Each vibrational probe experiences a unique microscopic environment as a result of nanoscale phase separation. Molecular dynamics simulations show that the dynamics span multiple time scales, from femtoseconds to nanoseconds. Our studies suggest a previously unknown liquid environment, which we label "local bulk", in which despite the local heterogeneity, the ultrafast H-bond dynamics are similar to bulk water.

Reduction of water-mediated repulsion drives poly(N-vinylcaprolactam) collapse upon heating

Thermo-sensitive aqueous polymers undergo a coil-to-globule transition on heating, with drastic chemical and structural changes. We performed molecular dynamics simulations for PVCL in water to study the driving forces for the polymer's collapse.

Empirical S=O stretch vibrational frequency map

Dimethyl sulfoxide (DMSO) water mixtures have been widely studied due to their unique concentration-dependent bulk properties. Here, we present an empirical spectroscopic map for the sulfinyl (S=O) stretching mode. The model can be used to interpret infrared (IR) absorption and ultrafast two-dimensional infrared (2D IR) spectra and quantify hydrogen bond populations and lifetimes by directly connecting spectroscopic measurements with structures and dynamics from molecular dynamics simulations. The electrostatic map is directly parameterized against experimental absorption spectra in the S=O stretching region (980-1100 cm^-1) of dilute DMSO in water. A comparison of center peak frequencies shows that the map performs well across the entire DMSO concentration range, accurately reproducing the ∼10 cm^-1 red-shift per hydrogen bond observed in the experiments. We further benchmark the map by comparing experimental and simulated 2D IR spectra generated by direct numerical integration of the Schrödinger equation. We expect that this empirical frequency map will provide a quantitative platform for investigating intermolecular interactions, microscopic heterogeneity, and ultrafast dynamics in complex liquid mixtures containing DMSO.

Direct Nanoscopic Measurement of Laminar Slip Flow Penetration of Deformable Polymer Brush Surfaces: Synergistic Effect of Grafting Density and Solvent Quality

A detailed quantitative nanoscopic description of soft surfaces under dynamic flow is lacking, despite its importance. To better understand the role of surface texture in nanoscopic mass transport in complex media, we used Förster resonance energy transfer in combination with total internal reflectance fluorescence microscopy (FRET-TIRFM) to directly measure laminar slip flow penetration depth (slip length) on poly(N-isopropylacrylamide) (pNIPAM) thin films (50-110 nm) of different grafting densities (0.60, 0.38, and 0.27 chain/nm²) in solvents of different qualities created via cononsolvency in situ. Nontrivial synergistic interplay of grafting density and solvent quality on slip length was observed. Slip lengths are typically tens of nm (40-100 nm), increasing and then reaching a plateau with applied linear flow velocity (192-2,952 μm/s) regardless of experimental system. Slip length was systematically larger for lower density films, but the effect of grafting density was more significant in a good solvent than a poor solvent. Interestingly, however, the stagnant film thickness (polymer swollen thickness minus the slip length) collapsed to almost a singular value for a given grafting density regardless of solvent quality, likely suggesting a large gradient of segmental mobility at nonequilibrium. Moreover, we found that slip flow penetrates into soft pNIPAM surfaces more deeply in a good solvent than in a poor solvent and that this behavior was general and independent of grafting density. This behavior is counter to the notion that less interaction between a fluid (probe) and a solid surface promotes slip.

Hydration-Shell Vibrational Spectroscopy

Hydration-shell vibrational spectroscopy provides an experimental window into solute-induced water structure changes that mediate aqueous folding, binding, and self-assembly. Decomposition of measured Raman and infrared (IR) spectra of aqueous solutions using multivariate curve resolution (MCR) and related methods may be used to obtain solute-correlated spectra revealing solute-induced perturbations of water structure, such as changes in water hydrogen-bond strength, tetrahedral order, and the presence of dangling (non-hydrogen-bonded) OH groups. More generally, vibrational-MCR may be applied to both aqueous and nonaqueous solutions, including multicomponent mixtures, to quantify solvent-mediated interactions between oily, polar, and ionic solutes, in both dilute and crowded fluids. Combining vibrational-MCR with emerging theoretical modeling strategies promises synergetic advances in the predictive understanding of multiscale self-assembly processes of both biological and technological interest.

Improved Temperature Behavior of PNIPAM in Water with a Modified OPLS Model

We test the OPLS/AA force field for a single PNIPAM 40-mer in aqueous solution using replica exchange molecular dynamics simulations and find that the force field fails to reproduce the experimental temperature behavior. To resolve this issue, we apply a modification on the partial charges previously suggested to reproduce the liquid-liquid phase separation of NIPAM aqueous solutions. The modified force field features stronger amide-water electrostatic interactions than the original OPLS model, predicts a weaker water-mediated monomer-monomer attraction, and reproduces the experimental coil-globule collapse enthalpy of PNIPAM in water. We revisit the cononsolvency problem of PNIPAM in methanol/water mixtures with the modified model and show that the dependence of the coil-globule collapse enthalpy on methanol concentration follows the experimental trend of the lower critical solution temperature. The calculations with the modified force field confirm that polymer dehydration is the determining factor for chain collapse in the cononsolvency regime.

Collective Transformation of Water between Hyperactive Antifreeze Proteins: RiAFPs

We demonstrate, by molecular dynamics simulations, that water confined between a pair of insect hyperactive antifreeze proteins from the longhorn beetle Rhagium inquisitor is discontinuously expelled as the two proteins approach each other at a certain distance. The extensive striped hydrophobic–hydrophilic pattern on the surface, comprising arrays of threonine residues, enables water to form three independent ice channels through the assistance of hydroxyl groups, even at 300 K. The transformation is reminiscent of a freezing–melting transition rather than a drying transition and governs the stable protein–protein separation in the evaluation of the potential of mean force. The collectivity of water penetration or expulsion and the hysteresis in the time scale of ten nanoseconds predict a potential first-order phase transition at the limit of infinite size and provide a new framework for the water-mediated interaction between solutes.

Multiple binding modes of a moderate ice-binding protein from a polar microalga

Ice-binding proteins (IBPs) produced by cold-tolerant organisms interact with ice and strongly control crystal growth. The molecular basis for the different magnitudes of activity displayed by various IBPs (moderate and hyperactive) has not yet been clarified. Previous studies questioned whether the moderate activity of some IBPs relies on their weaker binding modus to the ice surface, compared to hyperactive IBPs, rather than relying on binding only to selected faces of the ice crystal. We present the structure of one moderate IBP from the sea-ice diatom Fragilariopsis cylindrus (fcIBP) as determined by X-ray crystallography and investigate the protein's binding modes to the growing ice-water interface using molecular dynamics simulations. The structure of fcIBP is the IBP-1 fold, defined by a discontinuous β-solenoid delimitated by three faces (A, B and C-faces) and braced by an α-helix. The fcIBP structure shows capping loops on both N- and C-terminal parts of the solenoid. We show that the protein adsorbs on both the prism and the basal faces of ice crystals, confirming experimental results. The fcIBP binds irreversibly to the prism face using the loop between the B and the C-faces, involving also the B-face in water immobilization despite its irregular structure. The α-helix attaches the protein to the basal face with a partly reversible modus. Our results suggest that fcIBP has a looser attachment to ice and that this weaker binding modus is the basis to explain the moderate activity of fcIBP.

Connections between the Anomalous Volumetric Properties of Alcohols in Aqueous Solution and the Volume of Hydrophobic Association

The partial molar volumes of alcohols in water exhibit a non-monotonic dependence on concentration at room temperature, initially decreasing with increasing concentration before passing through a minimum and rising to the pure liquid plateau. This anomalous behavior is associated with hydrophobic interactions. We report molecular simulations of short chain alcohols and alkanes in water to examine the volumetric properties of these mixtures at infinite dilution over a range of temperatures. Our simulations find this anomaly disappears at a crossover temperature, above which the solute volume only varies monotonically with concentration. A Voronoi volume analysis of solution configurations finds that solutes in clusters take up less space than individual solutes at low temperature and more space at elevated temperatures. These changes in cluster volumes are subsequently shown to correlate with the derivative of the solute partial molar volume with respect to solute concentration. The changes in solute volume upon nonpolar solute association impact the response of molecular-scale hydrophobic interactions for assembly with increasing pressure.

The Interplay of Structure and Dynamics in the Raman Spectrum of Liquid Water over the Full Frequency and Temperature Range

While many vibrational Raman spectroscopy studies of liquid water have investigated the temperature dependence of the high-frequency O-H stretching region, few have analyzed the changes in the Raman spectrum as a function of temperature over the entire spectral range. Here, we obtain the Raman spectra of water from its melting to boiling point, both experimentally and from simulations using an ab initio-trained machine learning potential. We use these to assign the Raman bands and show that the entire spectrum can be well described as a combination of two temperature-independent spectra. We then assess which spectral regions exhibit strong dependence on the local tetrahedral order in the liquid. Further, this work demonstrates that changes in this structural parameter can be used to elucidate the temperature dependence of the Raman spectrum of liquid water and provides a guide to the Raman features that signal water ordering in more complex aqueous systems.

Hydration-Shell Transformation of Thermosensitive Aqueous Polymers

Although water plays a key role in the coil-globule transition of polymers and biomolecules, it is not clear whether a change in water structure drives or follows polymer collapse. Here, we address this question by using Raman multivariate curve resolution (Raman-MCR) spectroscopy to investigate the hydration shell structure around poly(N-isopropylacrylamide) (PNIPAM) and poly(propylene oxide) (PPO), both below and above the cloud point temperature at which the polymers collapse and form mesoscopic polymer-rich aggregates. We find that, upon clouding, the water surrounding long PNIPAM chains transforms to a less ordered and more weakly hydrogen bonded structure, while the water surrounding short PNIPAM and PPO chains remains similar above and below the cloud point. Furthermore, microfluidic temperature jump studies demonstrate that the onset of clouding precedes the hydration-shell structural transformation, and thus the observed water structural transformation is associated with ripening of aggregates composed of long-chain polymers, on a time scale that is long compared to the onset of clouding.

Comment on "Relating side chain organization of PNIPAm with its conformation in aqueous methanol" by D. Mukherji, M. Wagner, M. D. Watson, S. Winzen, T. E. de Oliveira, C. M. Marques and K. Kremer, Soft Matter, 2016, 12, 7995

In a recent paper, Mukherji et al. describe the collapse of poly(N-isopropyl acrylamide) in methanol-water mixtures based on experiments and molecular dynamics simulations. The conclusion of their work is that chain collapse is dominated by enthalpic bridging interactions while entropic effects play no major role. Here we show that this claim arises from an improper interpretation of preferential binding and the corresponding thermodynamic data presented. When interpreted correctly, the data instead provide evidence for repulsive enthalpic interactions of methanol with the polymer, supporting the emerging view of entropic chain collapse.

Reply to the 'Comment on "Relating side chain organization of PNIPAm with its conformation in aqueous methanol"' by N. van der Vegt and F. Rodriguez-Ropero, Soft Matter, 2017, 13, DOI: 10.1039/C6SM02139E

In a comment van der Vegt and Rodriguez-Ropero (vdVRR) challenged our explanation of the co-non-solvency effect of PNIPAm in aqueous methanol solutions. They argue, based on a careful selection of published studies including some of their own, that direct repulsions between the different constituents are sufficient to understand this phenomenon. According to vdVRR, the emerging view of entropic collapse, put forward by Flory (1910-1985) to explain common polymers in poor solvents, would be enough to explain co-non-solvency. In this reply we attempt to bring this discussion into firmer grounds. We provide a more comprehensive view of available experimental, numerical and theoretical results and review basic concepts of physical chemistry and of statistical mechanics of polymer collapse that show how methanol mediated attractions between chain monomers are required to understand this fascinating behavior.

Influence of co-non-solvency on hydrophobic molecules driven by excluded volume effect

We demonstrate by molecular dynamics simulation that co-non-solvency of hydrophobic molecules arises from solvent-excluded volume effect.

Preferential adsorption of the additive is not a prerequisite for cononsolvency in water-rich mixtures

NMR studies reveal the distinct molecular interactions accounting for cononsolvency.

Relating side chain organization of PNIPAm with its conformation in aqueous methanol

Combining nuclear magnetic resonance (NMR), dynamic light scattering (DLS), and μs long all-atom simulations with two million particles, we establish a delicate correlation between increased side chain organization of PNIPAm and its collapse in aqueous methanol mixtures. We find that the preferential binding of methanol with PNIPAm side chains, bridging distal monomers along the polymer backbone, results in increased organization. Furthermore, methanol-PNIPAm preferential binding is dominated by hydrogen bonding. Our findings reveal that the collapse of PNIPAm is dominated by enthalpic interactions and that the standard poor solvent (entropic) effects play no major role.