Особливості планування і реалізації проектів ресторанного бізнесу

Ресторанний бізнес є однією із найбільш значущих складових індустрії гостинності. Водночас, ресторанний бізнес, з одного боку, є одним із засобів високоліквідного використання капіталу, а з іншого − середовищем із високим ступенем конкурентності. У всьому світі він є одним із найбільш розповсюджених видів малого бізнесу, тому заклади та підприємства ведуть між собою постійну боротьбу за сегментацію ринку, за пошук нових та за утримання постійних споживачів їхньої продукції та послуг. Всі заклади та підприємства повинні мати високий рівень конкурентоспроможності та мати свою унікальність

Effects of Ambient Conditions on Solvent−Nanotube Dispersions: Exposure to Water and Temperature Variation

Dispersions of single walled nanotubes in N-methyl-2-pyrrolidone (NMP) have been exposed to water and variations in storage temperature. The subsequent degradation of dispersion quality has been monitored using sedimentation, UV−vis−NIR, and atomic force microscopy (AFM) measurements. Four parameters derived from AFM; the root-mean-square bundle diameter, the total number of dispersed objects (individuals and bundles) per unit volume of dispersion, the number fraction of individual nanotubes, and the number of individual nanotubes per unit volume of dispersion were used to quantitatively characterize the dispersion quality as a function of water content and storage temperature. In addition the positions of the nanotube absorption peaks were used to track dispersion quality, with redshifts taken as an indication of aggregation. It was found that water can rapidly shift the dispersion to a new but more aggregated equilibrium state. In particular the population of individual nanotubes falls to zero for relatively low amounts of added water. The dispersion quality decreases with increasing water content, reaching a plateau for all metrics by 20 vol % added water. In addition, it was also identified that low temperature treatment, i.e. −16, −18, −20, and −22 °C (all above the freezing point of NMP) does not influence the dispersion quality and stability regardless of the treatment time. However, freezing (−80 °C) or heating (80 °C) the dispersion leads to a substantial degradation of the dispersion quality and stability

New Solvents for Nanotubes: Approaching the Dispersibility of Surfactants

We demonstrate dispersion and exfoliation of nanotubes in two new solvents for nanotubes, cyclohexyl-pyrrolidone (CHP) and 1-benzyl-2-pyrrolidinone (NBenP). Both solvents are structural analogues of the well-known nanotube solvent N-methyl-pyrrolidone. Each solvent can disperse nanotubes at high concentrations, up to 3.5 mg/mL for CHP. The nanotubes in these dispersions are highly exfoliated, even at high concentration. In cyclohexyl-pyrrolidone dispersions, the root-mean-square bundle diameter was ∼3 nm for a concentration of 2 mg/mL. The bundle diameter fell as the concentration was reduced, reaching 1.5 nm at concentrations below 10−3 mg/mL. These dispersions have very large populations of individual nanotubes and small bundles. For CHP the total population of one-dimensional dispersed objects exceeded 100 μm−3 for concentrations >2 mg/mL. Of these ∼10% were individual SWNTs. However, as the concentration was reduced, the fraction of individual SWNTs increased to ∼80% for a nanotube concentration of 10−4 mg/mL. Like other successful nanotube dispersing solvents, both of these new solvents are characterized by surface tensions close to 40 mJ/m2. We believe their ability to disperse and exfoliate nanotubes is due to the low energetic cost of exfoliation in such solvents. Finally, their relative lack of toxicity makes these solvents much more user-friendly than traditional nanotube solvents such as N-methyl-pyrrolidone or dimethyl-formamide

Measurement of Multicomponent Solubility Parameters for Graphene Facilitates Solvent Discovery

We have measured the dispersibility of graphene in 40 solvents, with 28 of them previously unreported. We have shown that good solvents for graphene are characterized by a Hildebrand solubility parameter of δT ∼ 23 MPa1/2 and Hansen solubility parameters of δD ∼ 18 MPa1/2, δP ∼ 9.3 MPa1/2, and δH ∼ 7.7 MPa1/2. The dispersibility is smaller for solvents with Hansen parameters further from these values. We have used transmission electron microscopy (TEM) analysis to show that the graphene is well exfoliated in all cases. Even in relatively poor solvents, >63% of observed flakes have <5 layers

Differentiating Defect and Basal Plane Contributions to the Surface Energy of Graphite Using Inverse Gas Chromatography

Historically, reported values for the surface energy of graphite have covered a very wide range. Here, we use finite-dilution inverse gas chromatography (FD-IGC) to show that the dispersive component of the surface energy of graphite has contributions from edge and basal plane defects as well as from the hexagonal carbon lattice. The surface energy associated with the defect-free hexagonal lattice is measured at high probe-coverage to be 63 ± 7 mJ/m², independent of graphite type. However, the surface energy measured at low probe coverage varied from 125 to 175 mJ/m² depending on the graphite type. Simulation of the FD-IGC output for different binding site distributions allows us to associate this low-coverage surface energy with the binding of probe molecules to high energy defect sites. Importantly, we find the rate of decay of surface energy with probe coverage to carry information about the defect density. By analyzing the dependence of these properties on flake size, it is possible to separate out the contributions of edge and basal plane defects, estimating the basal plane defect content to be ∼10¹⁵ m^–2 for all graphite samples. Comparison with simulation gives some insights into the basal plane and defect binding energy distributions

Quantitative Evaluation of Surfactant-stabilized Single-walled Carbon Nanotubes: Dispersion Quality and Its Correlation with Zeta Potential

Stable dispersions of single-walled carbon nanotubes in deionized water were prepared using six common surfactants: sodium dodecylbenzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), lithium dodecyl sulfate (LDS), tetradecyl trimethyl ammonium bromide (TTAB), sodium cholate (SC), and Fairy liquid (FL). For all nanotube dispersions (CNT = 1 mg/mL), the optimum concentration of surfactant was found to be close to CSurf = 10 mg/mL by measuring the fraction of nanotubes remaining after centrifugation for a range of surfactant concentrations. The aggregation state of each nanotube−surfactant dispersion was characterized as a function of nanotube concentration by AFM analysis of large numbers of nanotubes/bundles deposited onto substrates. The dispersion quality could then be quantified by four parameters: the saturation value (at low concentration) of the root-mean-square bundle diameter, the maximum value of the total number of dispersed objects (individuals and bundles) per unit volume of dispersion, the saturation value (at low concentration) of the number fraction of individual nanotubes, and the maximum value of the number of individual nanotubes per unit volume of dispersion. According to these metrics, the dispersion quality of the six surfactant−nanotube dispersions varied as SDS > LDS > SDBS > TTAB > SC > Fairy liquid. It was found that each of these dispersion-quality metrics scaled very well with the measured ζ-potential of the surfactant−nanotube dispersion. This confirms that dispersion quality is controlled by the magnitude of electrostatic repulsive forces between coated nanotubes

High Quality Dispersions of Functionalized Single Walled Nanotubes at High Concentration

Single walled nanotubes are difficult to disperse in solvents, with dispersion quality limited by nanotube bundling at high concentration. We quantitatively study dispersions of singlewall nanotubes, functionalized with the bulky molecules PABS, PEG, and ODA, in common solvents. TGA measurements coupled with AFM analysis of deposited nanotubes shows almost complete coverage of the functionalities along the nanotube body. The best solvents are characterized by Hildebrand solubility parameters that are close to those of the functional groups. At low concentration, the dispersions contain predominately individual functionalized SWNTs as evidenced by root-mean-square bundle diameters of ∼3−4 nm. This can be compared with the measured diameter of individual functionalized nanotubes of ∼3 nm. These nanotubes display very weak concentration dependent aggregation when dispersed in common solvents. Root-mean-square bundle diameters of only ∼5−6 nm were observed at concentrations as high as 1 mg/mL. This translates into >100 bundles per cubic micron of solvent, much higher than observed in other systems. These results have practical implications for the production of well dispersed polymer-nanotube composites that would be expected to display high interfacial stress transfer

Multicomponent Solubility Parameters for Single-Walled Carbon Nanotube−Solvent Mixtures

We have measured the dispersibility of single-walled carbon nanotubes in a range of solvents, observing values as high as 3.5 mg/mL. By plotting the nanotube dispersibility as a function of the Hansen solubility parameters of the solvents, we have confirmed that successful solvents occupy a well-defined range of Hansen parameter space. The level of dispersibility is more sensitive to the dispersive Hansen parameter than the polar or H-bonding Hansen parameter. We estimate the dispersion, polar, and hydrogen bonding Hansen parameter for the nanotubes to be D> = 17.8 MPa1/2, P> = 7.5 MPa1/2, and H> = 7.6 MPa1/2. We find that the nanotube dispersibility in good solvents decays smoothly with the distance in Hansen space from solvent to nanotube solubility parameters. Finally, we propose that neither Hildebrand nor Hansen solubility parameters are fundamental quantities when it comes to nanotube−solvent interactions. We show that the previously calculated dependence of nanotube Hildebrand parameter on nanotube diameter can be reproduced by deriving a simple expression based on the nanotube surface energy. We show that solubility parameters based on surface energy give equivalent results to Hansen solubility parameters. However, we note that, contrary to solubility theory, a number of nonsolvents for nanotubes have both Hansen and surface energy solubility parameters similar to those calculated for nanotubes. The nature of the distinction between solvents and nonsolvents remains to be fully understood

Large Populations of Individual Nanotubes in Surfactant-Based Dispersions without the Need for Ultracentrifugation

Stable dispersions of single-walled carbon nanotubes have been produced using the surfactant sodium dodecylbenzene sulfonate (SDBS). Atomic force microscopy analysis shows that, on dilution of these dispersions, the nanotubes exfoliate from bundles, resulting in a concentration-dependent bundle diameter distribution which saturates at Drms ≈ 2 nm for concentrations, CNT < 0.05 mg/mL. The total bundle number density increases with concentration, saturating at ∼6 bundles per μm3 for CNT > 0.05 mg/mL. As the concentration is reduced the number fraction of individual nanotubes grows, approaching 50% at low concentration. In addition, partial concentrations of individual SWNTs approaching 0.01 mg/mL can be realized. These values are far superior to those for solvent dispersions due to repulsion stabilization of the surfactant-coated nanotubes. These methods facilitate the preparation of high-quality nanotube dispersions without the need for ultracentrifugation, thus significantly increasing the yield of dispersed nanotubes

Solvent Exfoliation of Transition Metal Dichalcogenides: Dispersibility of Exfoliated Nanosheets Varies Only Weakly between Compounds

We have studied the dispersion and exfoliation of four inorganic layered compounds, WS₂, MoS₂, MoSe₂, and MoTe₂, in a range of organic solvents. The aim was to explore the relationship between the chemical structure of the exfoliated nanosheets and their dispersibility. Sonication of the layered compounds in solvents generally gave few-layer nanosheets with lateral dimensions of a few hundred nanometers. However, the dispersed concentration varied greatly from solvent to solvent. For all four materials, the concentration peaked for solvents with surface energy close to 70 mJ/m², implying that all four have surface energy close to this value. Inverse gas chromatography measurements showed MoS₂ and MoSe₂ to have surface energies of ∼75 mJ/m², in good agreement with dispersibility measurements. However, this method suggested MoTe₂ to have a considerably larger surface energy (∼120 mJ/m²). While surface-energy-based solubility parameters are perhaps more intuitive for two-dimensional materials, Hansen solubility parameters are probably more useful. Our analysis shows the dispersed concentration of all four layered materials to show well-defined peaks when plotted as a function of Hansen’s dispersive, polar, and H-bonding solubility parameters. This suggests that we can associate Hansen solubility parameters of δ_D ∼ 18 MPa^1/2, δ_P ∼ 8.5 MPa^1/2, and δ_H ∼ 7 MPa^1/2 with all four types of layered material. Knowledge of these properties allows the estimation of the Flory–Huggins parameter, χ, for each combination of nanosheet and solvent. We found that the dispersed concentration of each material falls exponentially with χ as predicted by solution thermodynamics. This work shows that solution thermodynamics and specifically solubility parameter analysis can be used as a framework to understand the dispersion of two-dimensional materials. Finally, we note that in good solvents, such as cyclohexylpyrrolidone, the dispersions are temporally stable with >90% of material remaining dispersed after 100 h