Prediction of Molecular Affinity on Solid Surfaces via Three-Dimensional Solubility Parameters Using Interfacial Free Energy as Interaction Threshold
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Various adhesive and wetting phenomena in nature and practical applications originate from the interaction between the surfaces of materials and other substances. In this study, we developed a method for calculating the Hansen solubility parameters (HSP) of the surface of the solid materials to elucidate the surface interaction by quantitatively and visually representing adsorptivity with a three-dimensional vector. The HSP were derived from the interfacial free energy, which can easily be calculated from the contact angles of three organic solvents on the solid materials. The HSP for a glassy carbon (GC) surface calculated using our method were correlated with the adsorptivity on the GC surface of several organic molecules. The adsorptivity was evaluated using electrochemical impedance spectroscopy and molecular mechanics simulations; the latter of which also revealed that the HSP calculated for the Pt surface were highly correlated with its interactivity. Moreover, the HSP of the polytetrafluoroethylene (PTFE) surface obtained herein appropriately reflect the molecular structure of PTFE. The results underpin that our method enables the elucidation of various surface phenomena involving noncovalent interaction and allows the affinity between solid surfaces and tens of thousands of substances recorded in the HSP database to be predicted.Keywords:
Solid surface
Molecular mechanics
Interaction energy
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Understanding the molecular mechanism of protein adsorption on solids is critical to their applications in materials synthesis and tissue engineering. Although the water phase at the surface/water interface has been recognized as three types: bulk water, intermediate water phase and surface-bound water layers, the roles of the water and surface in determining the protein adsorption are not clearly identified, particularly at the quantitative level. Herein, we provide a methodology involving the combination of microsecond strengthen sampling simulation and force integration to quantitatively characterize the water-induced contribution and the peptide-surface interactions into the adsorption free energy. Using hydroxyapatite and graphene surfaces as examples, we demonstrate how the distinct interfacial features dominate the delicate force balance between these two thermodynamics parameters, leading to surface preference/resistance to peptide adsorption. Specifically, the water layer provides sustained repelling force against peptide adsorption, as indicated by a monotonic increase in the water-induced free energy profile, whereas the contribution from the surface-peptide interactions is thermodynamically favorable to peptide adsorptions. More importantly, the revealed adsorption mechanism is critically dictated by the distribution of water phase, which plays a crucial role in establishing the force balance between the interactions of the peptide with the water layer and the surface. For the HAP surface, the charged peptide exhibits strong binding affinity to the surface, due to the controlling contribution of peptide–surface interaction in the intermediate water phase. The surface-bound water layers are observed as the origin of bioresistance of solid surfaces toward the adsorption of charge-neutral peptides. The preferred peptide adsorption on the graphene, however, is dominated by the surface-induced component at the water layers adjacent to the surface. Our results further elucidate that the intermediate water phase significantly shortens the effective range of the surface dispersion force, in contrast to the observation on the hydrophilic surface.
Umbrella sampling
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The surface behavior of biologically or atmospherically relevant chemical compounds in aqueous solution has been studied using surface-sensitive X-ray photoelectron spectroscopy (XPS). The aim is to provide information on the molecular-scale composition and distribution of solutes in the surface region of aqueous solutions. In the first part, the distribution of solutes in the surface region is discussed, where in particular single molecular species are studied. Concentration-dependent studies on succinic acid and various alkyl-alcohols, where also parameters such as pH and branching are varied, are analyzed using different approaches that allow the quantification of surface concentrations. Furthermore, due to the sensitivity of XPS to the chemical state, reorientation of linear and branched alkyl-alcohols at the aqueous surface as a function of concentration is observed. The results are further discussed in terms of hydrophilic and hydrophobic interactions in the interfacial region, where the three-dimensional hydrogen bonded water structure terminates. In the second part, mixed solutions of compounds, both ionic and molecular, are inspected. Again concentration, but also co-dissolution of other chemical compounds, are varied and differences in the spatial distribution and composition of the surface region are discussed. It is found that the guanidinium ion has an increased propensity to reside at the surface, which is explained by strong hydration in only two dimensions and only weak interactions between the aromatic π-system and water. Ammonium ions, on the other hand, which require hydration in three dimensions, are depleted from the surface region. The presence of strongly hydrated electrolytes out-competes neutral molecules for hydrating water molecules leading to an enhanced abundance of molecules, such as succinic acid, in the interfacial region. The partitioning is quantified and discussed in the context of atmospheric science, where the impact of the presented results on organic loading of aerosol particles is emphasized.
Succinic acid
Branching (polymer chemistry)
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The present article reports a molecular dynamics study of ammonium based ionic liquids confined between surfaces of amorphous carbon. The ionic liquids studied herein are composed of alkylammonium cations combined with alkylsulfonate anions which due to their environmentally acceptable character are suitable candidates for lubrication. A model was built from first principles describing the interaction between ionic liquids and an amorphous carbon surface. A set of interaction parameters was obtained by fitting density functional theory potential energies of the interaction between fragments of ionic liquids and a cluster of diamond, with a site–site potential function. Molecular dynamics simulations using the developed potentials were performed, and the structure at the solid–liquid interface was analyzed, as well as the orientational order of the alkyl side chains with respect to the surface. Finally, by applying shear and load to the system we predict the friction coefficient at different values of shear velocities.
Amorphous carbon
Carbon fibers
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Protein Adsorption
Polymer Adsorption
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Understanding protein adsorption onto solid surfaces is of critical importance in the field of bioengineering, especially for applications such as medical implants, diagnostic biosensors, drug delivery systems, and tissue engineering. This study proposed the use of molecular dynamics simulations with potential of mean force (PMF) calculations to identify and characterize the mechanisms of adsorption of a protein molecule on a designed surface. A set of model systems consisting of a cardiotoxin (CTX) protein and mixed self-assembled monolayer (SAM) surfaces were used as examples. The set of mixed SAM surfaces with varying topographies were created by mixing alkanethiol chains of different lengths. The results revealed that CTX proteins underwent similar conformal changes upon adsorption onto the various mixed SAMs but showed distinctive characteristics in free energy profiles. Enhancement of the adsorption affinity, i.e., the change in free energy of adsorption, for mixed SAMs was demonstrated by using atomic force microscopic measurements. A component analysis conducted to quantify the physical mechanisms that promoted CTX adsorption revealed contributions from both SAMs and the solvent. Further component analyses of thermodynamic properties, such as the free energy, enthalpy, and entropy, indicated that the contribution from SAMs was driven by enthalpy, and the contribution from the solvent was driven by entropy. The results indicated that CTX adsorption was an entropy-driven process, and the entropic component from the solvent, i.e., the hydrophobic interaction, was the major driving force for CTX adsorption onto SAMs. The study also concluded that the surfaces composed of mixtures of SAMs with different chain lengths promoted the adsorption of CTX protein.
Protein Adsorption
Cardiotoxin
Self-assembled monolayer
Hydrophobic effect
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Performance and safety of materials in contact with living matter are determined by sequential and competitive protein adsorption. However, cause and consequences of these processes remain hard to be generalized and predicted. In a new attempt to address that challenge, the authors compared and analyzed the protein adsorption and displacement on various thoroughly characterized polymer substrates using a combination of surface-sensitive techniques. A multiple linear regression approach was applied to model the dependence of protein adsorption, desorption, and exchange dynamics on protein and surface characteristics. While the analysis confirmed that protein properties primarily govern the observed adsorption and retention phenomena and hydrophobicity as well as surface charge are the most relevant polymer surface properties, the authors have identified several protein-surface combinations that deviate from these patterns and deserve further investigation.
Protein Adsorption
Surface protein
Dynamics
Surface charge
Protein Dynamics
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Abstract Recent development of the interfacial nanochemistry in the solvent extraction systems is reviewed, as well as the development of measurement methods for interfacial reactions. The studies on specific reactions at the interface of various metal extraction systems have elucidated essential modes of the interfacial reactions, including the interfacial adsorption, the interfacial complexation, and the interfacial aggregation. The catalytic role of the liquid-liquid interface in the solvent extraction kinetics and the molecular recognition ability of the self-assembled metal complexes at the interface were emphasized. Spectroscopic studies on the rotational and translational dynamics of fluorescent molecules, a single molecule in some cases, at the interface are also discussed. The utility of the molecular dynamics simulations is demonstrated in the discussions of the solvent structure in the interfacial region and of the stability of surface active molecules at the interface.
Nanochemistry
Surface forces apparatus
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Quantification of interfacial composition and interfacial energy is essential for understanding prevalent phenomena such as purification and adhesion. However, for high-energy planar solid surfaces, traditional approaches for determining both parameters are inadequate. We take advantage of interface-sensitive spectroscopy to calculate the interfacial composition for acetone-chloroform, tetrahydrofuran-benzene, and N,N-dimethylformamide (DMF)-benzene mixtures. We calculate the differences in interfacial energy for the two components of each mixture from the adsorption isotherms and compare with that obtained from acid-base and dispersive interactions. The interfacial energy calculated using interfacial segregation agrees with the interfacial energy calculated by acid-base and dispersive interactions. The comparison illustrates how molecular interactions control macroscopic interfacial segregation. In all three mixtures, acid-base interactions dominate interfacial segregation. Comparing the two approaches for DMF-benzene mixtures leads to evidence of DMF dimerization in benzene. Using the present approach, the interfacial composition and interfacial energy can now be understood for interfacial behaviors including wetting and self-assembly.
Dimethyl formamide
Base (topology)
Tetrahydrofuran
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Carbon fibers
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