How hydrophobic hydration responds to solute size and attractions: Theory and simulations

How do water-mediated interactions and osmotic second virial coefficients vary with particle size?

We examine quantitatively the solute-size dependences of the effective interactions between nonpolar solutes in water and in a simple liquid. The potential w(r) of mean force and the osmotic second virial coefficients B are calculated with high accuracy from molecular dynamics simulations. As the solute diameter increases from methane's to C60's with the solute-solute and solute-solvent attractive interaction parameters fixed to those for the methane-methane and methane-water interactions, the first minimum of w(r) lowers from -1.1 to -4.7 in units of the thermal energy kT. Correspondingly, the magnitude of B (<0) increases proportional to σα with some power close to 6 or 7, which reinforces the solute-size dependence of B found earlier for a smaller range of σ [H. Naito, R. Okamoto, T. Sumi and K. Koga, J. Chem. Phys., 2022, 156, 221104]. We also demonstrate that the strength of the attractive interactions between solute and solvent molecules can qualitatively change the characteristics of the effective pair interaction between solute particles, both in water and in a simple liquid. If the solute-solvent attractive force is set to be weaker (stronger) than a threshold, the effective interaction becomes increasingly attractive (repulsive) with increasing solute size.

Evidence for Entropically Controlled Interfacial Hydration in Mesoporous Organosilicas

At aqueous interfaces, the distribution and dynamics of adsorbates are modulated by the behavior of interfacial water. Hydration of a hydrophobic surface can store entropy via the ordering of interfacial water, which contributes to the Gibbs energy of solute binding. However, there is little experimental evidence for the existence of such entropic reservoirs, and virtually no precedent for their rational design in systems involving extended interfaces. In this study, two series of mesoporous silicas were modified in distinct ways: (1) progressively deeper thermal dehydroxylation, via condensation of surface silanols, and (2) increasing incorporation of nonpolar organic linkers into the silica framework. Both approaches result in decreasing average surface polarity, manifested in a blue-shift in the fluorescence of an adsorbed dye. For the inorganic silicas, hydrogen-bonding of water becomes less extensive as the number of surface silanols decreases. Overhauser dynamic nuclear polarization (ODNP) relaxometry indicates enhanced surface water diffusivity, reflecting a loss of enthalpic hydration. In contrast, organosilicas show a monotonic decrease in surface water diffusivity with decreasing polarity, reflecting enhanced hydrophobic hydration. Molecular dynamics simulations predict increased tetrahedrality of interfacial water for the organosilicas, implying increased ordering near the nm-size organic domains (relative to inorganic silicas, which necessarily lack such domains). These findings validate the prediction that hydrophobic hydration at interfaces is controlled by the microscopic length scale of the hydrophobic regions. They further suggest that the hydration thermodynamics of structurally heterogeneous silica surfaces can be tuned to promote adsorption, which in turn tunes the selectivity in catalytic reactions.

Structure and dynamics of nanoconfined water and aqueous solutions

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

Hydration of a small protein under carbon nanotube confinement: Adsorbed substates induce selective separation of the dynamical response

The co-involvement of biological molecules and nanomaterials has increasingly come to the fore in modern-day applications. While the "bio-nano" (BN) interface presents physico-chemical characteristics that are manifestly different from those observed in isotropic bulk conditions, the underlying molecular reasons remain little understood; this is especially true of anomalies in interfacial hydration. In this paper, we leverage atomistic simulations to study differential adsorption characteristics of a small protein on the inner (concave) surface of a single-walled carbon nanotube whose diameter exceeds dimensions conducive to single-file water movement. Our findings indicate that the extent of adsorption is decided by the degree of foldedness of the protein conformational substate. Importantly, we find that partially folded substates, but not the natively folded one, induce reorganization of the protein hydration layer into an inner layer water closer to the nanotube axis and an outer layer water in the interstitial space near the nanotube walls. Further analyses reveal sharp dynamical differences between water molecules in the two layers as observed in the onset of increased heterogeneity in rotational relaxation and the enhanced deviation from Fickian behavior. The vibrational density of states reveals that the dynamical distinctions are correlated with differences in crucial bands in the power spectra. The current results set the stage for further systematic studies of various BN interfaces vis-à-vis control of hydration properties.

Heat transfer across a fractal surface

The effects of surface irregularities and imperfections on the thermal resistance at a solid-liquid interface have been investigated using molecular dynamics. The molecular model comprises liquid argon confined between silver walls. The surface roughness was designed using fractal theory, introducing stochastic patterns of multiple scales that resemble realistic surface geometries. In agreement with most previous studies, we find that increasing the strength of the solid-liquid interactions monotonically reduces the thermal resistance across smooth interfaces. Yet, the behavior of the thermal resistance across rough surfaces is more complex. Following the initially anticipated decrease, the thermal resistance starts to increase once the strength of solid-liquid interaction increases past a threshold. We attribute the above behavior to two competing phenomena, namely, the area of the solid-liquid interface and the introduction of vibrational anharmonicities and localization of phonons resulting from the surface roughness. Finally, we demonstrate that, for the same fractal dimension and depth of surface roughness, different surfaces practically have the same thermal resistance, solid-liquid radial distribution function, and liquid density profiles. We conclude that the above fractal parameters are useful in deriving reduced models for properties related to the surface geometry.

Direct Measurement of Length Scale Dependence of the Hydrophobic Free Energy of a Single Collapsed Polymer Nanosphere

The physics underlying hydrophobicity at macroscopic and microscopic levels is fundamentally distinct. However, experimentally quantifying the length scale dependence of hydrophobicity is challenging. Here we show that the size-dependent hydrophobic free energy of a collapsed polymer nanosphere can be continuously monitored from its single-molecule force-extension curve using a novel theoretical framework. The hydrophobic free energy shows a change from cubic to square dependence of the radius of the polymer nanosphere at a radius of ∼1  nm-this is consistent with Lum-Chandler-Weeks theory and simulations. We can also observe a large variation of the hydrophobic free energy of each polymer nanosphere implying the heterogeneity of the self-assembled structures and/or the fluctuation of the water-polymer interface. We expect that our approach can be used to address many fundamental questions about hydrophobic hydration, which are otherwise inaccessible by ensemble measurements.

Water-mediated curvature change in graphene by single-walled carbon nanotubes

Novel nanostructured materials possessing new architectural segments can be synthesized using various combinations of graphene and carbon nanotubes (CNT) that can result in the generation of enhanced physico-chemical properties within the hybrids. Comprehending the various physical processes involved in the creation of these new segments is crucial for designing an optimized nanomaterial for a specific purpose. In this paper we report induced folding in a graphene sheet resulting from the physical interactions between water-mediated graphene and a CNT. Owing to robust binding interactions between the CNT and a compatible graphene sheet, the latter forms a second domed layer around the former culminating in a structure equivalent to a double-walled CNT. The induced curvature change in graphene by CNT was found to have a strong dependence upon their relative physical dimensions. For example, CNT possessing extremely small diameters are unable to induce any significant curvature changes in longer graphene sheets. The potential-of-mean force (PMF) between our reference graphene and CNT in water suggests a favorable binding interaction of -14.5 kcal mol-1. The breakdown of the PMF into direct graphene-nanotube interactions and water-mediated interactions reveals a huge reduction in the strongly attractive binding interactions between graphene and CNT by the water molecules.

How do glycerol and dimethyl sulphoxide affect local tetrahedral structure of water around a nonpolar solute at low temperature? Importance of preferential interaction

Glycerol and dimethyl sulphoxide (DMSO) have vital roles in cryoprotection of living cells, tissues, etc. The above action has been directly linked with disruption of hydrogen (H-) bond structure and dynamics of water by these cosolvents at bulk region and around various complex units, such as peptide, amino acid, protein, and lipid membrane. However, the disruption of the local structure of the water solvent around a purely hydrophobic solute is still not studied extensively. The latter is also important in the context of stabilization of protein from cold denaturation. Through all-atom molecular dynamics simulation, we have investigated the comparative effect of glycerol and DMSO on the orientational order of water around a nonpolar solute at -5 °C. A steady reduction of the tetrahedral order of water is observed at bulk (>10 Å distance from the solute) and solute interface (<5.5 Å distance from the solute) with increasing the cosolvent concentration. Contrasting roles of glycerol and DMSO have been evidenced. While DMSO affects the H-bond structure of the interfacial water more than that of the bulk water, glycerol affects the water structure almost uniformly at all regions around the solute. Furthermore, while glycerol helps to retain water molecules at the interface, DMSO significantly reduces the water content in that region. We have put forward a plausible mechanism for these contrasting roles of these cosolvents. The solute-cosolvent hydrophobic-interaction-induced orientational alignment of an interfacial cosolvent molecule determines whether the involvement of the cosolvent molecules in H-bonding with solvent water in the interface is akin to the bulk region or not.

Molecular Shape and the Hydrophobic Effect

This review focuses on papers published since 2000 on the topic of the properties of solutes in water. More specifically, it evaluates the state of the art of our understanding of the complex relationship between the shape of a hydrophobe and the hydrophobic effect. To highlight this, we present a selection of references covering both empirical and molecular dynamics studies of small (molecular-scale) solutes. These include empirical studies of small molecules, synthetic hosts, crystalline monolayers, and proteins, as well as in silico investigations of entities such as idealized hard and soft spheres, small solutes, hydrophobic plates, artificial concavity, molecular hosts, carbon nanotubes and spheres, and proteins.

Solvation of Carbon Nanoparticles in Water/Alcohol Mixtures: Using Molecular Simulation To Probe Energetics, Structure, and Dynamics

Molecular dynamics simulations were used to examine the solvation behavior of buckminsterfullerene and single-walled carbon nanotubes (SWCNT) in a range of water/alcohol solvent compositions at 1 atm and 300 K. Results indicate that the alcohols assume the role of pseudosurfactants by shielding the nanotube from the more unfavorable interactions with polar water molecules. This is evident in both the free energies of transfer (ΔΔG_water→xOH = -68.1 kJ/mol and -86.5 kJ/mol for C₆₀ in methanol and ethanol; ΔΔG_water→xOH = -345.6 kJ/mol and -421.2 kJ/mol for the (6,5)-SWCNT in methanol and ethanol) and the composition of the solvation shell at intermediate alcohol concentrations. Additionally, we have observed the retardation of both the translational and rotational dynamics of molecules near the nanoparticle surface through use of time correlation functions. A 3-fold increase in the residence times of the alcohol molecules within the solvation shells at low concentrations further reveals their surfactant-like behavior. Such interactions are important when considering the complex molecular environment present in many schemes used for nanoparticle purification techniques.

Studies on electrostatic interactions within model nano-confined aqueous environments of different chemical nature

We study the potential of mean force for pairs of parallel flat surfaces with attractive electrostatic interactions by employing model systems functionalized with different charged, hydrophobic and hydrophilic groups. We study the way in which the local environment (hydrophobic or hydrophilic moieties) modulates the interaction between the attractive charged groups on the plates by removing or attracting nearby water and thus screening or not the electrostatic interaction. To explicitly account for the role of the solvent and the local hydrophobicity, we also perform studies in vacuo. Additionally, the results are compared to that for non-charged plates in order to single out and rationalize the non-additivity of the different non-covalent interactions. Our simulations demonstrate that the presence of neighboring hydrophobic groups promote water removal in the vicinity of the charged groups, thus enhancing charge attraction upon self-assembly. This role of the local hydrophobicity modulating electrostatic interactions is consistent with recent qualitative descriptions in the protein binding context.

Thermodynamics of hydration of fullerols [C60(OH)n] and hydrogen bond dynamics in their hydration shells

Molecular dynamics simulations of fullerene and fullerols [C₆₀(OH)_n, where n = 2-30] in aqueous solutions have been performed for the purpose of obtaining a detailed understanding of the structural and dynamic properties of these nanoparticles in water. The structures, dynamics and hydration free energies of the solute molecules in water have been analysed. Radial distribution functions, spatial density distribution functions and hydrogen bond analyses are employed to characterize the solvation shells of water around the central solute molecules. We have found that water molecules form two solvation shells around the central solute molecule. Hydrogen bonding in the bulk solvent is unaffected by increasing n. The large decrease in solvation enthalpies of these solute molecules for n > 14 enhances solubilisation. The diffusion constants of solute molecules decrease with increasing n. The solvation free energy of C₆₀ in water is positive (52.8 kJ/mol), whereas its value for C₆₀(OH)₃₀ is highly negative (-427.1 kJ/mol). The effects of surface hydroxylation become more dominant once the fullerols become soluble.

Note: Nonpolar solute partial molar volume response to attractive interactions with water

The impact of attractive interactions on the partial molar volumes of methane-like solutes in water is characterized using molecular simulations. Attractions account for a significant 20% volume drop between a repulsive Weeks-Chandler-Andersen and full Lennard-Jones description of methane interactions. The response of the volume to interaction perturbations is characterized by linear fits to our simulations and a rigorous statistical thermodynamic expression for the derivative of the volume to increasing attractions. While a weak non-linear response is observed, an average effective slope accurately captures the volume decrease. This response, however, is anticipated to become more non-linear with increasing solute size.

Effects of confinement on the structure and dynamics of an intrinsically disordered peptide: a molecular-dynamics study

In vivo, proteins and peptides are exposed to radically different environments than those in bulk. Because of the abundance of other cellular components, proteins perform their function in crowded and confined spaces. Confinement has been shown to alter the structure, dynamics, and folding of proteins that possess a native fold. Little is known, however, of the effects of confinement on biologically important intrinsically disordered proteins or peptides (IDP). Here, we use extensive molecular dynamics simulations to investigate the effects of confinement in an IDP, the Aβ21-30, a central folding nucleus of the full length amyloid β-protein. In this study, we report results derived from 107 μs of molecular dynamics simulations that subjected the Aβ21-30 to two types of confinement: hydrophilic and hydrophobic pores. Results show that turn structures are enhanced as a function of decreasing pore size (increasing confinement) over other structures, including coils, β-hairpins, and bridges. However, the percentage occurrence of the dominant hydrogen bond between amino acids Asp23 and Ser26 shown to stabilize the turn in bulk simulations does not increase as a function of confinement signifying a disconnect between structure and internal hydrogen bonding. Differences in structure and dynamics of the decapeptide due to hydrophilic and hydrophobic confinement are more apparent at the extreme confinement conditions, where a reduction of the available phase space in hydrophilic confinement is explained in terms of interactions between the decapeptide and a layer of water at the interface between the decapeptide and the surface of the pore, and a smaller size of the decapeptide in the hydrophobic pores is rationalized in terms of peptide-surface interactions.

Surface Polarity and Nanoscale Solvation

Linear solvation theories are well established to describe electrostatic hydration of small solutes when the hydration free energy is dominated by the electrostatic free energy of the solute multipole. In contrast, hydration of nanometer solutes is driven by surface hydration. We address the question of whether the linear-response thermodynamics established for small multipolar solutes applies to surface hydration. To this end, molecular dynamics simulations are carried out on a model C180 solute that carries no global multipole, but the surface of which is decorated with radially pointing dipoles. Linear response is dramatically violated in this case. Further, two crossovers in the solvation thermodynamics are discovered as the surface polarity is increased. Both transformations produce strongly nonlinear solvation response. The second, more collective, crossover leads to a dramatic slowing down of the interfacial dynamics, reaching the time-scales of nanoseconds. Our picture offers the possibility of flipping water domains at interfaces of nanoparticles and biomolecules.

Hydrophobicity of proteins and interfaces: insights from density fluctuations

Macroscopic characterizations of hydrophobicity (e.g., contact angle measurements) do not extend to the surfaces of proteins and nanoparticles. Molecular measures of hydrophobicity of such surfaces need to account for the behavior of hydration water. Theory and state-of-the-art simulations suggest that water density fluctuations provide such a measure; fluctuations are enhanced near hydrophobic surfaces and quenched with increasing surface hydrophilicity. Fluctuations affect conformational equilibria and dynamics of molecules at interfaces. Enhanced fluctuations are reflected in enhanced cavity formation, more favorable binding of hydrophobic solutes, increased compressibility of hydration water, and enhanced water-water correlations at hydrophobic surfaces. These density fluctuation-based measures can be used to develop practical methods to map the hydrophobicity/philicity of heterogeneous surfaces including those of proteins. They highlight that the hydrophobicity of a group is context dependent and is significantly affected by its environment (e.g., chemistry and topography) and especially by confinement. The ability to include information about hydration water in mapping hydrophobicity is expected to significantly impact our understanding of protein-protein interactions as well as improve drug design and discovery methods and bioseparation processes.

The science of dispersing carbon nanotubes with surfactants

Self-assembled structures adsorbed on carbon nanotubes and other nanofibres offer a plethora of opportunities to endow them with new functions and to integrate them into devices and materials. At the same time they are key to solve the greatest problem in carbon nanotube utilisation--debundling and individualisation. Success will inevitably require an understanding of the underlying structure-function relationship of the adsorbed surfactant layer. Computer simulations are ideally suited to develop this understanding as they enable us to study the structure-function relationship in great detail. Combining the results from mesoscale and atomistic simulations we begin to develop this understanding and derive a number of recommendations for optimal dispersion design.

On the role of London dispersion forces in biomolecular structure determination

A DNA dodecamer and the methyladenine···methylthymine (mA···mT) complex in aqueous environment have been studied by means of molecular dynamics simulation, with a modified force field accounting for the hypothetical absence of London dispersion forces. Under these conditions, the mA···mT complex is preserved, while the double-helical DNA oligomer passes via an extended, ladder-like intermediate to a collapsed structure. The results are interpreted in terms of stability and specificity of the structure of studied complexes. While the hydrophobic effect of the solvent accounts for the sufficient stabilization of the complex, the appearance of the native biomolecular conformation is attributed to the London dispersion forces. Thus, the London dispersion seems to provide the native structure of a biomolecular complex with the largest additional stabilization, preferring it among several (or many) possible aggregated structures. The observations are affected by the construction of the modified force field, and this effect is discussed thoroughly. The fundamental issues are the coupling of the components of the Lennard-Jones potential and the way to separate them. Based on the observations, the description of nonbonded interactions with the current biomolecular force fields is discussed. It is proposed that a novel force field composed of physically correct components to describe nonbonded interactions could exhibit more favorable performance in certain up-to-date applications.

Size-dependent hydrophobic to hydrophilic transition for nanoparticles: a molecular dynamics study

The physical properties of nanoscale materials often vary with their size, unlike the corresponding bulk material properties, which can only be changed by modifying the material composition. In particular, it is believed that hydration phenomena are length scale dependent. The manifestation of hydrophobicity over multiple length scales plays a crucial role in self-assembly processes such as protein folding and colloidal stability. In the case of particles composed of a bulk hydrophobic material, it is well known that the free energy of hydration monotonically increases with particle size. However, the size-dependent free energy of hydration for particles composed of a bulk hydrophilic material has not been studied. Here we show that the free energy of hydration is not a monotonic function of particle size, but rather, changes sign from positive to negative as the particle size increases. In other words, the particle is hydrophobic at small size and hydrophilic at large size. This behavior arises from a purely geometrical effect caused by the curvature of the particle-water interface. We explore the consequences of this phenomenon on colloidal stability and find that it dictates the shape of colloidal aggregates.