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Chemical functionalization of metal sulfides by adsorption of specially designed ligands is of primary importance in efficiently manipulating the physicochemical properties of the target surface. Despite the widely studied ligand–metal molecular systems and the generally used binding energy concept, reports on the bond character of ligand–metal sulfide and its implication on the design of ligands with enhanced functionalities are rather scarce. Herein, using density functional theory calculations, we studied the intrinsic binding mechanism of the typical S/O-terminated ligands and (100) surface of lead sulfide (PbS), with a particular emphasis on the ligand–PbS(100) bond character. Strikingly, the ligand–PbS(100) bond is found to be predominantly ionic with minor covalency, completely different from the conventional perception of covalent interaction. Further insights into the specific hydrophobization of PbS led us to demonstrate that the ligand electronegativity, rather than the commonly used binding energy, allows more accurate prediction of the relative hydrophobic functionality of the ligand toward PbS. Therefore, this study not only advances our fundamental understanding on the ligand–PbS(100) interaction but also provides new perspectives on the first-principles design of ligands.

Chemical functionalization of metal sulfides plays a critical role in many fields such as materials science and froth flotation. The commonly used thiol-bearing functionalized ligands are generally considered to bind with metal sulfides covalently, and the computational binding energy is widely used to evaluate the functionality of the ligands toward metal sulfides. Herein, we studied the surface chemistry of the model ZnS and its binding with typical S- and O-terminated ligands using density functional theory calculations with an emphasis on the resulting bond character. Surprisingly, it was found that the ligand–ZnS(110) bond is essentially ionic with limited covalency. This very fundamental finding was further extended to the hydrophobization of ZnS in the context of froth flotation and rationalized the previously unresolved phenomenon that the higher the ligand–ZnS(110) binding strength, the lower the hydrophobic functionality of the ligand toward ZnS. Meanwhile, instead of the binding energy, the electronegativity of the ligand was identified as an effective computational descriptor that can accurately predict the relative hydrophobic functionality of the ligand toward ZnS. This work, therefore, further advanced our understanding of the intrinsic ligand–metal sulfide binding mechanism and highlighted the importance of computational parameters, beyond the binding energy, in guiding the first principles design of ligands with enhanced functionalities or optimizing relevant industrial processes.

Owing to the increasing popularity of using waste disposal as a source material, inorganic salts such as CaCl2and MgCl2are frequently present in geopolymerization typically taking place in alkali media. Such contaminants influence the dissolution of the aluminosilicate source materials and consequently the properties of the geopolymers made. This work is particularly aimed at elucidating the dissolution mechanism of a well-known clay, namely, kaolinite, in alkali media with the presence of two aforementioned aqueous medium contaminants using molecular dynamics (MD) simulation. A series of MD simulations was carried out on model kaolinite, with its tetrahedral and deprotonated octahedral surfaces exposed to the alkali solutions containing neat Na+or neat K+cations at two concentrations, 3 and 5 M. Different concentrations of CaCl2and MgCl2contaminants (i.e., 0.1, 0.3, and 0.5 M) were added to such alkali solutions. Atomic density profiles show that all cations, including those from the contaminants adsorbed on the two basal surfaces, intensify the dissociation of the aluminate groups from the deprotonated octahedral surface. The dissociation mechanism is somewhat similar to that of the alkali media without contaminants, in which cations weaken the interaction between the aluminum and bridging oxygen atoms. The number of the aluminate groups dissociated decreased with increasing contaminant concentration. In fact, at the highest contaminant concentration used (i.e., 0.5 M), the number of dissociated aluminate groups was even lower than that of the system without contaminants. This observation was attributed to the fact that most of chloride anions remained in the bulk solution at 0.1 M but an increasing amount of chloride ions started to cluster around the cations at 0.5 M, thereby screening the interaction between cations and the deprotonated octahedral surface. As a result, the screening effect reduced the number of aluminate groups dissociated from the surface. Structural analyses of the deprotonated octahedral surface indicated that the crystallinity of the surface decreased with increasing simulation time and alkali solution concentration. No dissolution of the tetrahedral surface was observed for all systems studied.

Using a molecular dynamics simulation, the behavior of water and cyclohexane confined in a model clay (kaolinite) nanopore (∼4–6 nm) was studied at 298.15 K and 1 atm. On a dry clay basis, the water weight concentration ranged from 6 to 30% while that of cyclohexane was kept at 14%. The formation of water bridge(s) connecting the two basal surfaces was observed at all concentrations. Upon analyzing the surface potentials using the Gouy–Chapman theory, the observation was found to be attributed to the overlapping of the positive potential (octahedral) and the negative potential (tetrahedral) of the two surfaces in the interior region of such a confined environment. The wettability of the tetrahedral sheet was found to be dependent on the water content. Larger areas of this surface would become water-wet with the increase in water concentration to minimize the contact area between the oil and water phases. Addition of sodium chloride to the aqueous phase at the concentrations of 0.1, 0.5, and 1 M substantially improved the wetting of basal surfaces. At the highest salt concentration, breakage of the water bridge was observed, and a phase-separated, three-layer structure (water–cyclohexane–water) was formed within the nanopore because of the screening effect of the adsorbed counterions.

Molecular dynamics simulation was used to study the role of water in the intercalation of CO2with a model Mg–Al–Cl-hydrotalcite mineral at ambient pressure and temperature. The ClayFF force field was used along with a model Mg–Al–Cl-hydrotalcite containing different amounts of water (H2O) and carbon dioxide (CO2) molecules in its interlayer spacing. It was observed that high CO2content, say 3.85 mmol g–1, could be achieved at low water concentrations or even without the presence of water. However, high water concentrations (e.g., 2 H2O molecules/hydrotalcite unit cell, the maximum allowed water concentration observed experimentally) could also yield similar CO2content, but in this case, the presence of water led to a significant interlayer spacing expansion (from 23.0 Å (no water) to 28.5 Å). The expansion was likely due to the change in the orientation distribution of the CO2molecules. Analyzing the orientation of CO2molecules revealed that they preferred to orientate parallel to the mineral surface at low water concentrations. However, as water concentration increased, CO2molecules exhibited a wider range of orientations with a significant fraction of them orienting more or less perpendicular to the mineral surface, especially at high CO2contents. The observed change in the orientation of CO2was attributed to the dipole interaction between H2O and CO2molecules and the reduced interaction between CO2and the hydroxyl groups on hydrotalcite. Also, it was observed that water molecules formed extensive hydrogen bond networks. All of the above findings seem to explain the contradicting results reported in the literature that water is needed under certain conditions to increase the amount of CO2captured by hydrotalcites. Here, we showed that high amounts of CO2can be intercalated with the presence of water.

Single molecule force spectroscopy (SMFS) was utilized to study single-molecule interactions between synthesized polymers and MoS2surfaces that provide the guidance to determine candidate polymers for efficient aqueous exfoliation of bulk MoS2into single-layer nanosheets. The technique allowed the direct quantification of the adhesion force of single polymer molecules with both basal and edge surfaces of MoS2. Compared with two studied neutral polymers, highly water-soluble cationic poly(vinylbenzyl trimethyl ammonium chloride) (PVBTA) was shown to be more promising for MoS2exfoliation as a result of strong single-molecule interactions with both basal and edge surfaces of MoS2. In 1 mM NaCl solution of pH around 5.5, the measured single-molecule adhesion forces on basal and edge surfaces were around 59 and 51 pN, respectively. Such strong adhesion force led to high performance of PVBTA in exfoliating MoS2bulk material into single-layer sheets. Compared with reported approaches in the literatures, an order of magnitude higher concentration of single-layer MoS2sheets in water was prepared using an order of magnitude less treatment time in this study. This research demonstrated that SMFS is a powerful tool to select appropriate surface-active polymers for effective exfoliation of MoS2, which could be applied to a variety of practical applications, such as water adhesion  and dispersing.

Exploration aiming to understand how a single polymer chain interacts with interested solid–water interface and its dependence on the environmental stimulus is critical, as it is important for a variety of scientific research and practical applications, such as underwater adhesives or smart systems for controlled attachment/detachment. Here, by single molecule force spectroscopy (SMFS), results on underwater adhesion of a single stimuli-responsive polymer chain on a model hydrophobic surface are reported. Salt concentration was used as an effective stimulus for the transition of the stimuli-responsive polymer from hydrophilic (solvophilic) hydrated state to hydrophobic (solvophobic) state in SMFS experiment. The probed equilibrium single-molecule adhesion force that changed from 45 to 76 pN with increasing salt concentration were free of other interactions that are responsible for cohesions, indicating that solvent quality is critical in determining the strength of single-molecule interfacial adhesion. Moreover, the derived simple quantitative thermodynamic model by combining single-molecule detachment mechanics with hydration free energy illustrated that higher single-molecule adhesion force was resulted from higher solvation/hydration free energy. The experimental study and theoretical analysis presented in this work quantitatively revealed the role of hydrophobic and more general solvophobic interactions in single-molecule level and shed light on the molecular structure optimization for desired applications, such as underwater adhesives or smart interfaces.

Molecular dynamics (MD) simulation and density functional theory (DFT) were applied to study the wetting behavior of nanoscale thin films of n-heptane, toluene, pyridine, and water at different thicknesses on the two model basal surfaces of kaolinite at room temperature. Despite the hydrophilicity of the basal surfaces, water exhibited the lowest affinity for them, but was statistically comparable to that of n-heptane, as quantified by the corresponding calculated monomolecular layer works of adhesion. The results are consistent with the simulations in which water molecules in monomolecular layer originally sandwiched between a monomolecular layer of aromatic compounds and the two basal surfaces departed from the surfaces, while the aromatic molecules migrated to the surfaces. However, such behavior was not observed in the cases in which water thin films contained multiple molecular layers of water. Interestingly, the corresponding calculated multimolecular layer works of adhesion of water were the highest among the compounds of interest. Analysis of the simulation data on both basal surfaces suggests that such observation is attributed to the water/water long-range charge–charge (69%, averaged over the two basal surfaces) and short-range hydrogen-bond (31%) interactions. Here, interfacial hydrogen bonds play a relatively minor role in the wetting behavior of water.

Molecular dynamics simulation was applied to study the relative growth rates of seven crystal faces of α-Al 2O 3in the presence of 1,4-butanediol. In particular, the adsorption energy of 1,4-butanediol on various crystal faces was computed using 200 solvent molecules. The following sequence was obtained: ( Eads(012) > Eads(001) > Eads(102) > Eads(113) > Eads(110) > Eads(111) > Eads(010)). The above sequence is in the reverse order of the experimentally observed growth rates of the respective faces. The two faces that exhibit the lowest adsorption energy (i.e., (111) and (010) faces) are the ones that do not emerge in the final morphology of α-Al 2O 3particles in practice. On examination of the data, it was found that 1,4-butanediol molecules preferentially adsorb onto the crystal faces that have larger numbers of octahedral faces per unit area (e.g., the (001) face). In other words, the binding of the growth units of α-Al 2O 3onto such crystal faces is less favorable than the other crystal faces. In addition to the adsorption energy, our results show that the growth habits are also influenced by the mobility of 1,4-butanediol near the crystal growth faces. In fact, the computed diffusion coefficients of 1,4-butanediol near the (012) and (102) faces are the lowest compared to those of the (001), (110), and (113) faces. This may explain why (012) and (102) faces do not emerge unless moderate stirring is applied. Nevertheless, the above conclusions were draw based upon cleaved α-alumina surfaces without relaxation (i.e., surface energy effects of the solid substrate were not included).

Adsorption behavior of two normal alkanes (C11and C200) with explicit hydrogens on a relaxed -Al2O3(0001) surface at 150 °C was studied using classical molecular dynamics (MD) simulation along with the use of the COMPASS force field. Prior to the MD simulations, first principle density functional theory (DFT) calculations were carried out to relax the alumina (0001) surface that was created by cleaving the corresponding crystal structure. It was found that orientation of the adsorbed segments and the number of carbons adsorbed seem to be insensitive to the chain length. The computed adsorption energy per mole of adsorbed CH2agrees well with those obtained from inverse gas chromatography measurements. Also, both simulation and experimental results showed that the adsorption energy decreased with increasing chain length. It was observed that molecular planes containing the skeletal carbons of the alkane segments that formed the first adsorption layer tend to orient parallel to the alumina surface, regardless of chain length. However, there existed a small amount of C11segments adopting the perpendicular orientation. For both C11and C200, the average distance between the neighboring adsorbed segments was determined to be 4.6 Å which is slightly lower than the distance between two adjacent normal alkane molecules in their crystalline form but is equal to the inter-atomic distance between the aluminum atoms in the surface layer of the alumina surface. The present work suggests that both the orientation and adsorption energy of normal alkanes on the alumina (0001) surface depends on whether the surface is relaxed or not.