Water-Induced Interactions between Carbon Nanoparticles

The Effects of External Interfaces on Hydrophobic Interactions I: Smooth Surface

External interfaces, such as the air-water and solid-liquid interfaces, are ubiquitous in nature. Hydrophobic interactions are considered the fundamental driving force in many physical and chemical processes occurring in aqueous solutions. It is important to understand the effects of external interfaces on hydrophobic interactions. According to the structural studies on liquid water and the air-water interface, the external interface primarily affects the structure of the topmost water layer (interfacial water). Therefore, an external interface may affect hydrophobic interactions. The effects of interfaces on hydrophobicity are related not only to surface molecular polarity but also to the geometric characteristics of the external interface, such as shape and surface roughness. This study is devoted to understanding the effects of a smooth interface on hydrophobicity. Due to hydrophobic interactions, the solutes tend to accumulate at external interfaces to maximize the hydrogen bonding of water. Additionally, these can be demonstrated by the calculated potential mean forces (PMFs) using molecular dynamic (MD) simulations.

The Dependence of Hydrophobic Interactions on the Shape of Solute Surface

According to our recent studies on hydrophobicity, this work is aimed at understanding the dependence of hydrophobic interactions on the shape of a solute's surface. It has been observed that dissolved solutes primarily affect the structure of interfacial water, which refers to the top layer of water at the interface between the solute and water. As solutes aggregate in a solution, hydrophobic interactions become closely related to the transition of water molecules from the interfacial region to the bulk water. It is inferred that hydrophobic interactions may depend on the shape of the solute surface. To enhance the strength of hydrophobic interactions, the solutes tend to aggregate, thereby minimizing their surface area-to-volume ratio. This also suggests that hydrophobic interactions may exhibit directional characteristics. Moreover, this phenomenon can be supported by calculated potential mean forces (PMFs) using molecular dynamics (MD) simulations, where different surfaces, such as convex, flat, or concave, are associated with a sphere. Furthermore, this concept can be extended to comprehend the molecular packing parameter, commonly utilized in studying the self-assembly behavior of amphiphilic molecules in aqueous solutions.

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.

Water-mediated interactions destabilize proteins

Proteins are folded to avoid exposure of the nonpolar groups to water because water-mediated interactions between nonpolar groups are a promising factor in the thermodynamic stabilities of proteins-which is a well-accepted view as one of the unique effects of hydrophobic interactions. This article poses a critical question for this classical view by conducting an accurate solvation free-energy calculation for a thermodynamic cycle of a protein folding using a liquid-state density functional theory. Here, the solvation-free energy for a leucine zipper formation was examined in the coiled-coil protein GCN4-p1, a typical model for hydrophobic interactions, which demonstrated that water-mediated interactions were unfavorable for the association of nonpolar groups in the native state, while the dispersion forces between them were, instead, responsible for the association. Furthermore, the present analysis well predicted the isolated helical state stabilized by pressure, which was previously observed in an experiment. We reviewed the problems in the classical concept and semiempirical presumption that the energetic cost of the hydration of nonpolar groups is a driving force of folding.

Mitigation of graphene oxide toxicity in C. elegans after chemical degradation with sodium hypochlorite

Graphene oxide (GO) is a promising and strategic carbon-based nanomaterial for innovative and disruptive technologies. It is therefore essential to address its environmental health and safety aspects. In this work, we evaluated the chemical degradation of graphene oxide by sodium hypochlorite (NaClO, bleach water) and its consequences over toxicity, on the nematode Caenorhabditis elegans. The morphological, chemical, and structural properties of GO and its degraded product, termed NaClO-GO, were characterized, exploring an integrated approach. After the chemical degradation of GO at room temperature, its flake size was reduced from 156 to 29 nm, while NaClO-GO showed changes in UV-vis absorption, and an increase in the amount of oxygenated surface groups, which dramatically improved its colloidal stability in moderately hard reconstituted water (EPA medium). Acute and chronic exposure endpoints (survival, growth, fertility, and reproduction) were monitored to evaluate material toxicities. NaClO-GO presented lower toxicity at all endpoints. For example, an increase of over 100% in nematode survival was verified for the degraded material when compared to GO at 10 mg L^-1. Additionally, enhanced dark-field hyperspectral microscopy confirmed the oral uptake of both materials by C. elegans. Finally, this work represents a new contribution toward a better understanding of the links between the transformation of graphene-based materials and nanotoxicity effects (mitigation), which is mandatory for the safety improvements that are required to maximize nanotechnological benefits to society.

Molecular Dynamics Studies of Poly(Lactic Acid) Nanoparticles and Their Interactions with Vitamin E and TLR Agonists Pam1CSK4 and Pam3CSK4

Poly(lactic acid) (PLA) nanoparticles (NPs) are widely investigated due to their bioresorbable, biocompatible and low immunogen properties. Interestingly, many recent studies show that they can be efficiently used as drug delivery systems or as adjuvants to enhance vaccine efficacy. Our work focuses on the molecular mechanisms involved during the nanoprecipitation of PLA NPs from concentrated solutions of lactic acid polymeric chains, and their specific interactions with biologically relevant molecules. In this study, we evaluated the ability of a PLA-based nanoparticle drug carrier to vectorize either vitamin E or the Toll-like receptor (TLR) agonists Pam₁CSK₄ and Pam₃CSK₄, which are potent activators of the proinflammatory transcription factor NF-κB. We used dissipative particle dynamics (DPD) to simulate large systems mimicking the nanoprecipitation process for a complete NP. Our results evidenced that after the NP formation, Pam₁CSK₄ and Pam₃CSK₄ molecules end up located on the surface of the particle, interacting with the PLA chains via their fatty acid chains, whereas vitamin E molecules are buried deeper in the core of the particle. Our results allow for a better understanding of the molecular mechanisms responsible for the formation of the PLA NPs and their interactions with biological molecules located either on their surfaces or encapsulated within them. This work should allow for a rapid development of better biodegradable and safe vectorization systems with new drugs in the near future.

Solvent-Solvent Correlations across Graphene: The Effect of Image Charges

Wetting experiments show pure graphene to be weakly hydrophilic, but its contact angle (CA) also reflects the character of the supporting material. Measurements and molecular dynamics simulations on suspended and supported graphene often reveal a CA reduction due to the presence of the supporting substrate. A similar reduction is consistently observed when graphene is wetted from both sides. The effect has been attributed to transparency to molecular interactions across the graphene sheet; however, the possibility of substrate-induced graphene polarization has also been considered. Computer simulations of CA on graphene have so far been determined by ignoring the material's conducting properties. We improve the graphene model by incorporating its conductivity according to the constant applied potential molecular dynamics. Using this method, we compare the wettabilities of suspended graphene and graphene supported by water by measuring the CA of cylindrical water drops on the sheets. The inclusion of graphene conductivity and concomitant polarization effects leads to a lower CA on suspended graphene, but the CA reduction is significantly bigger when the sheets are also wetted from the opposite side. The stronger adhesion is accompanied by a profound change in the correlations among water molecules across the sheet. While partial charges on water molecules interacting across an insulator sheet attract charges of the opposite sign, apparent attraction among like charges is manifested across the conducting graphene. The change is associated with graphene polarization, as the image charges inside the conductor attract equally signed partial charges of water molecules on both sides of the sheet. Additionally, using a nonpolar liquid (diiodomethane), we affirm a detectable wetting translucency when liquid-liquid forces are dominated by dispersive interactions. Our findings are important for predictive modeling toward a variety of applications including sensors, fuel cell membranes, water filtration, and graphene-based electrode materials in high-performance supercapacitors.

Temperature Effects on Water-Mediated Interactions at the Nanoscale

We perform molecular dynamics simulations to study the effects of temperature on the water-mediated interactions between nanoscale apolar solutes. Specifically, we calculate the potential of mean force (PMF) between two graphene plates immersed in water at 240 ≤ T ≤ 400 K and P = 0.1 MPa. These are thermodynamic conditions relevant to cooling- and heating-induced protein denaturation. It is found that both cooling and heating tend to suppress the attraction, and ultimate collapse, of the graphene plates. However, the underlying role played by water upon heating and cooling is different. Isobaric heating reduces the strength and range of the interactions between the plates. Instead, isobaric cooling stabilizes the plates separations that can accommodate an integer number of water layers between the graphene plates. In particular, the energy barriers separating these plate separations increase linearly with 1/ T. We also explore the sensitivity of the plates PMF to the water model employed. In the case of the TIP4P/2005 model, water confined between the plates crystallizes into a defective bilayer ice at low temperatures, whereas in the case of the SPC/E model, water remains in the liquid state at same thermodynamic conditions. The effects of varying water-graphene interactions on the plates PMF are also studied.

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.

Design Rules for Graphene and Carbon Nanotube Solvents and Dispersants

The constantly widening industrial applications of carbon-based nanomaterials puts into sharp perspective the lack of true solvents in which the materials spontaneously exfoliate to individual molecules. This work shows that the different geometry of graphene compared to that of carbon nanotubes can change the potency of a molecule to act as a solvent or dispersant. Through analysis of the structure/function relationships, we derive a number of design rules that will aid the identification of the best solvent or dispersant candidates.

Controlling the Graphene-Bio Interface: Dispersions in Animal Sera for Enhanced Stability and Reduced Toxicity

Liquid phase exfoliation of graphite in six different animal sera and evaluation of its toxicity are reported here. Previously, we reported the exfoliation of graphene using proteins, and here we extend this approach to complex animal fluids. A kitchen blender with a high-turbulence flow gave high quality and maximum exfoliation efficiency in all sera tested, when compared to the values found with shear and ultrasonication methods. Raman spectra and electron microscopy confirmed the formation of three- or four-layer, submicrometer size graphene, independent of the serum used. Graphene prepared in serum was directly transferred to cell culture media without post-treatments. Contrary to many reports, a nanotoxicity study of this graphene fully dispersed to human embryonic kidney cells, human lung cancer cells, and nematodes (Caenorhabditis elegans) showed no acute toxicity for up to 7 days at various doses (50-500 μg/mL), but prolonged exposure at higher doses (300-500 μg/mL, 10-15 days) showed cytotoxicity to cells (∼95% death) and reproductive toxicity to C. elegans (5-10% reduction in brood size). The origin of toxicity was found to be due to the highly fragmented smaller graphene sheets (<200 nm), while the larger sheets were nontoxic (50-300 μg/mL dose). In contrast, graphene produced with sodium cholate as the mediator has been found to be cytotoxic to these cells at these dosages. We demonstrated the toxicity of liquid phase exfoliated graphene is attributed to highly fragmented fractions or nonbiocompatible exfoliating agents. Thus, low-toxicity graphene/serum suspensions are produced by a facile method in biological media, and this approach may accelerate the much-anticipated development of graphene for biological applications.

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.

Structure and dynamics of water confined in a graphene nanochannel under gigapascal high pressure: dependence of friction on pressure and confinement

The friction coefficient at the water/graphene interface is dependent on the lateral pressure and nanochannel height under gigapascal high-pressure.

Path integral-GC-AdResS simulation of a large hydrophobic solute in water: a tool to investigate the interplay between local microscopic structures and quantum delocalization of atoms in space

We perform large scale quantum (path integral) molecular dynamics simulations of a C₆₀ -like molecule in water.

Universal Repulsive Contribution to the Solvent-Induced Interaction Between Sizable, Curved Hydrophobes

In addition to the direct attraction, sizable hydrophobes in water experience an attractive force mediated by interfacial water. Using simple geometric arguments, we identify the conditions at which the water-induced interaction between curved hydrocarbon surfaces becomes repulsive. The repulsive contribution arises from the thermodynamic penalty due to the emergence of the liquid/vapor boundary created as water gets expelled between curved hydrophobes. By augmenting the mean field approach with atomistic simulations of pristine and alkyl-coated graphitic nanoparticles in three distinct geometries, spherical, cylindrical and planar, immersed in water, we show the macroscopic thermodynamics remarkably works down to the molecular scale. The new insights improve the prediction and control of wetting and dispersion properties for a broad class of nonpolar nanoparticles.

Free energetics of carbon nanotube association in aqueous inorganic NaI salt solutions: Temperature effects using all-atom molecular dynamics simulations

In this study, we examine the temperature dependence of free energetics of nanotube association using graphical processing unit-enabled all-atom molecular dynamics simulations (FEN ZI) with two (10,10) single-walled carbon nanotubes in 3 m NaI aqueous salt solution. Results suggest that the free energy, enthalpy and entropy changes for the association process are all reduced at the high temperature, in agreement with previous investigations using other hydrophobes. Via the decomposition of free energy into individual components, we found that solvent contribution (including water, anion, and cation contributions) is correlated with the spatial distribution of the corresponding species and is influenced distinctly by the temperature. We studied the spatial distribution and the structure of the solvent in different regions: intertube, intratube and the bulk solvent. By calculating the fluctuation of coarse-grained tube-solvent surfaces, we found that tube-water interfacial fluctuation exhibits the strongest temperature dependence. By taking ions to be a solvent-like medium in the absence of water, tube-anion interfacial fluctuation shows similar but weaker dependence on temperature, while tube-cation interfacial fluctuation shows no dependence in general. These characteristics are discussed via the malleability of their corresponding solvation shells relative to the nanotube surface. Hydrogen bonding profiles and tetrahedrality of water arrangement are also computed to compare the structure of solvent in the solvent bulk and intertube region. The hydrophobic confinement induces a relatively lower concentration environment in the intertube region, therefore causing different intertube solvent structures which depend on the tube separation. This study is relevant in the continuing discourse on hydrophobic interactions (as they impact generally a broad class of phenomena in biology, biochemistry, and materials science and soft condensed matter research), and interpretations of hydrophobicity in terms of alternative but parallel signatures such as interfacial fluctuations, dewetting transitions, and enhanced fluctuation probabilities at interfaces.

Evidence of entropically driven C60 fullerene aggregation in aqueous solution

In the present work, we report the first experimental evidence of entropically driven C60 fullerene aggregation in aqueous solution, occurring with nearly zero enthalpy change.

On the origin of C₆₀ fullerene solubility in aqueous solution

In this work, we report that the surface hydroxylation of C60 molecules is the most likely mechanism for pristine C60 fullerenes/C60 fullerene aggregate stabilization in water, being independent of the method of C60 fullerene aqueous solution preparation.

Free energetics of rigid body association of ubiquitin binding domains: a biochemical model for binding mediated by hydrophobic interaction

Weak intermolecular interactions, such as hydrophobic associations, underlie numerous biomolecular recognition processes. Ubiquitin is a small protein that represents a biochemical model for exploring thermodynamic signatures of hydrophobic association as it is widely held that a major component of ubiquitin's binding to numerous partners is mediated by hydrophobic regions on both partners. Here, we use atomistic molecular dynamics simulations in conjunction with the Adaptive Biasing Force sampling method to compute potentials of mean force (the reversible work, or free energy, associated with the binding process) to investigate the thermodynamic signature of complexation in this well-studied biochemical model of hydrophobic association. We observe that much like in the case of a purely hydrophobic solute (i.e., graphene, carbon nanotubes), association is favored by entropic contributions from release of water from the interprotein regions. Moreover, association is disfavored by loss of enthalpic interactions, but unlike in the case of purely hydrophobic solutes, in this case protein-water interactions are lost and not compensated for by additional water-water interactions generated upon release of interprotein and moreso, hydration, water. We further find that relative orientations of the proteins that mutually present hydrophobic regions of each protein to its partner are favored over those that do not. In fact, the free energy minimum as predicted by a force field based method recapitulates the experimental NMR solution structure of the complex.

Effect of self-assembly of fullerene nano-particles on lipid membrane

Carbon nanoparticles can penetrate the cell membrane and cause cytotoxicity. The diffusion feature and translocation free energy of fullerene through lipid membranes is well reported. However, the knowledge on self-assembly of fullerenes and resulting effects on lipid membrane is poorly addressed. In this work, the self-assembly of fullerene nanoparticles and the resulting influence on the dioleoylphosphtidylcholine (DOPC) model membrane were studied by using all-atom molecular dynamics simulations with explicit solvents. Our simulation results confirm that gathered small fullerene cluster can invade lipid membrane. Simulations show two pathways: 1) assembly process is completely finished before penetration; 2) assembly process coincides with penetration. Simulation results also demonstrate that in the membrane interior, fullerene clusters tend to stay at the position which is 1.0 nm away from the membrane center. In addition, the diverse microscopic stacking mode (i.e., equilateral triangle, tetrahedral pentahedral, trigonal bipyramid and octahedron) of these small fullerene clusters are well characterized. Thus our simulations provide a detailed high-resolution characterization of the microscopic structures of the small fullerene clusters. Further, we found the gathered small fullerene clusters have significant adverse disturbances to the local structure of the membrane, but no great influence on the global integrity of the lipid membrane, which suggests the prerequisite of high-content fullerene for cytotoxicity.

C60 fullerene aggregation in aqueous solution

In the present work we develop a novel approach for quantification of the energetics of C60 fullerene aggregation in aqueous media in terms of equilibrium aggregation constant KF. In particular, it is shown that the experimental determination of the magnitude of KF is possible only within the framework of the 'up-scaled aggregation model', considering the C60 fullerene water solution as a solution of fullerene clusters. Using dynamic light scattering (DLS) data we report the value, K(F) = 56,000 M(-1), which is in good agreement with existing theoretical estimates and the results of energetic analyses. It is suggested that the proposed 'up-scaled model' may be used in any instances of non-specific aggregation resulting in formation of large spherical particles. The measurement of the translational diffusion coefficient and the dimensions of the light scattering particles using a DLS approach with respect to C60 fullerene aggregates is found to contain significant systematic errors originating from the interaction effect that is well-known for micellar solutions. As a result, corrections to the equations associated with DLS data are proposed.

Free energetics of carbon nanotube association in pure and aqueous ionic solutions

Carbon nanotubes are a promising platform across a broad spectrum of applications ranging from separations technology, drug delivery, to bio(electronic) sensors. Proper dispersion of carbon nanotube materials is important to retaining the electronic properties of nanotubes. Experimentally it has been shown that salts can regulate the dispersing properties of CNTs in aqueous system with surfactants (Niyogi, S.; Densmore, C. G.; Doorn, S. K. J. Am. Chem. Soc.2009, 131, 1144-1153); details of the physicochemical mechanisms underlying such effects continue to be explored. We address the effects of inorganic monovalent salts (NaCl and NaI) on dispersion stability of carbon nanotubes.We perform all-atom molecular dynamics simulations using nonpolarizable interaction models to compute the potential of mean force between two (10,10) single-walled carbon nanotubes (SWNTs) in the presence of NaCl/NaI and compare to the potential of mean force between SWNTs in pure water. Addition of salts enhances stability of the contact state between two SWNT's on the order of 4 kcal/mol. The ion-specific spatial distribution of different halide anions gives rise to starkly different contributions to the free energy stability of nanotubes in the contact state. Iodide anion directly stabilizes the contact state to a much greater extent than chloride anion. The enhanced stability arises from the locally repulsive forces imposed on nanotubes by the surface-segregated iodide anion. Within the time scale of our simulations, both NaI and NaCl solutions stabilize the contact state by equivalent amounts. The marginally higher stability for contact state in salt solutions recapitulates results for small hydrophobic solutes in NaCl solutions (Athawale, M. V.; Sarupria, S.; Garde, S. J. Phys. Chem. B2008, 112, 5661-5670) as well as single-walled carbon nanotubes in NaCl and CaCl2 aqueous solutions.

Insertion of short amino-functionalized single-walled carbon nanotubes into phospholipid bilayer occurs by passive diffusion

Carbon nanotubes have been proposed to be efficient nanovectors able to deliver genetic or therapeutic cargo into living cells. However, a direct evidence of the molecular mechanism of their translocation across cell membranes is still needed. Here, we report on an extensive computational study of short (5 nm length) pristine and functionalized single-walled carbon nanotubes uptake by phospholipid bilayer models using all-atom molecular dynamics simulations. Our data support the hypothesis of a direct translocation of the nanotubes through the phospholipid membrane. We find that insertion of neat nanotubes within the bilayer is a “nanoneedle” like process, which can often be divided in three consecutive steps: landing and floating, penetration of the lipid headgroup area and finally sliding into the membrane core. The presence of functional groups at moderate concentrations does not modify the overall scheme of diffusion mechanism, provided that their deprotonated state favors translocation through the lipid bilayer.

Uptake and translocation mechanisms of cationic amino derivatives functionalized on pristine C60 by lipid membranes: a molecular dynamics simulation study

Bioactive molecules, cationic peptides among them, are nowadays well-recognized in modern pharmacology for their drug potential. However, they usually suffer from poor translocation across cell membranes, and specific drug carriers should be designed to circumvent this problem. In the present study, the uptake mechanism of fullerene bearing cationic ammonium groups by membranes modeled as lipid bilayers is investigated using extensive molecular dynamics simulations and free-energy calculations. Three main results issued from this work can be drawn. First, the fullerene core appears to be a good drug vector since it greatly enhances the uptake of the cationic groups by the membrane. Second, we show that the amino derivatives should be deprotonated at the lipid headgroup level in order to fully translocate the membrane by passive diffusion. Finally, the fullerenes bearing too many cationic groups display mostly a hydrophilic character; thus, the lipophilic fullerene core is not anymore effective as an insertion enhancer. Therefore, the lipid bilayer appears to be very selective with respect to the amount of amino groups conjugated with C(60).

Rationalization of the behavior of solid-liquid surface free energy of water in Cassie and Wenzel wetting states on rugged solid surfaces at the nanometer scale

The present work aims to contribute to the understanding at a molecular level of the origin of the hydrophobic nature of surfaces exhibiting roughness at the nanometer scale. Graphite-based smooth and model surfaces whose roughness dimension stretches from a few angstroms to a few nanometers were used in order to generate Cassie and Wenzel wetting states of water. The corresponding solid-liquid surface free energies were computed by means of molecular dynamics simulations. The solid-liquid surface free energy of water-smooth graphite was found to be -12.7 ± 3.3 mJ/m(2), which is in reasonable agreement with a value estimated from experiments and fully consistent with the features of the employed model. All the rugged surfaces yielded higher surface free energy. In both Cassie and Wenzel states, the maximum variation of the surface free energy with respect to the smooth surface was observed to represent up to 50% of the water model surface tension. The solid-liquid surface free energy of Cassie states could be well predicted from the Cassie-Baxter equation where the surface free energies replace contact angles. The origin of the hydrophobic nature of surfaces yielding Cassie states was therefore found to be the reduction of the number of interactions between water and the solid surface where atomic defects were implemented. Wenzel's theory was found to fail to predict even qualitatively the variation of the solid-liquid surface free energy with respect to the roughness pattern. While graphite was found to be slightly hydrophilic, Wenzel states were found to be dominated by an unfavorable effect that overcame the favorable enthalpic effect induced by the implementation of roughness. From the quantitative point of view, the solid-liquid surface free energy of Wenzel states was found to vary linearly with the roughness contour length.

Stabilization of aqueous carbon nanotube dispersions using surfactants: insights from molecular dynamics simulations

Techniques for separating bundles of carbon nanotubes into homogeneous dispersion are still under development, although a few methods have been successful at the laboratory scale. Understanding the effective interactions between carbon nanotubes in the presence of dispersing agents will provide the necessary information to develop better methods and also to refine the existing ones. We present here results from all-atom molecular dynamics simulations for aqueous flavin mononucleotide (FMN), which has been found experimentally to efficiently separate single-walled carbon nanotubes (SWNTs) based on diameter and chirality. We report results for the aggregate morphology of FMN on SWNTs of different diameters, as well as the potential of mean force between (6,6) SWNTs in the presence of aqueous FMN. The results are compared to the potential of mean force between SWNTs in aqueous sodium dodecyl sulfate (SDS). Our detailed analysis is used to explain the role of FMN, water, and sodium ions in providing a strong repulsive barrier between approaching SWNTs.