Adsorption and mixing behavior of ethers and alkanes at the solid/liquid interface

The behavior of binary mixtures of linear symmetrical ethers and alkanes adsorbed to a graphite surface from the bulk liquid mixtures is described on the basis of differential scanning calorimetry (DSC) data. Both the ethers and the alkanes are found to form solid monolayers when adsorbed from the liquid. In addition, the monolayer mixing behavior is addressed. The results indicate that there is good, essentially ideal, mixing in the monolayers for ethers and alkanes of the same overall chain length, where the chain length is equal to the total number of carbon and oxygen atoms in the molecule. However, a difference in chain length of more than one atom results in a variation of mixing behavior from nonideal mixing (for long pairs) to phase separation (for short pairs) on the graphite surface. Hence, we conclude that it is the relative chain lengths that control mixing behavior. The results are quantified using a regular solution model with a correction for preferential adsorption. The phase behavior of the mixed monolayers is also compared to the behavior of the bulk. Interestingly, we observe mixtures where the bulk and monolayer behavior are quite different, for example, phase separation in the bulk but essentially ideal mixing in the monolayer for mixtures of ethers and alkanes with the same chain lengths. At present, we attribute this mixing in the monolayer to dilution of the unfavorable ether oxygen-ether oxygen lone pair interactions by the coadsorbed alkanes. In addition, we find evidence for the preferential adsorption of the alkane over the ether. For example, heptane is preferentially adsorbed over dibutyl ether even though it contains two fewer atoms in the molecular chain. This contrasts with the preferential adsorption of alcohols over alkanes reported previously (Messe, L.; Perdigon, A.; Clarke, S. M.; Inaba, A.; Arnold, T. Langmuir 2005, 21, 5085-5093).     

Thermodynamic investigation of the adsorption of amides on graphite from their liquids and binary mixtures

We report a thermodynamic investigation of the adsorption of saturated and unsaturated (cis- and trans-) alkyl amides onto the surface of graphite from their pure liquids and from binary mixtures. We identify the formation of solid monolayers of the amides at temperatures when the bulk materials are liquid. The extent of this presolidification is much more extensive than other related materials, indicating that these amide layers are significantly more stable. The monolayer stability is found to be greatest for saturated amides. In addition, the stability of unsaturated amides is extremely sensitive to the location of the double bonds in the alkyl chain of the molecules, and trans isomers are found to be more stable than cis. We also address the preferential adsorption and mixing behavior of amide mixtures and amides mixed with other species coadsorbed onto graphite from binary solution. The results indicate that the amide molecules appear to be adsorbed with their principal axis parallel to the graphite surface and that amides are found to be strongly preferentially adsorbed with respect to alkanes. Interestingly the amides appear to mix rather better than might have been expected. There is also evidence of a number of other transitions in the adsorbates.     

Mixing in adsorbed monolayers: perfluorinated alkanes

The mixing behavior of binary combinations of perfluoroalkanes in the bulk and in solid monolayers adsorbed at the graphite/liquid interface, determined by calorimetry and powder diffraction, is reported. The perfluoroalkanes are found to generally have a smaller excess enthalpy of mixing on the surface than in the bulk, and their relative size ratio is a good parameter to predict the mixing behavior. The excess enthalpy of mixing for perfluoroalkanes is found to be significantly smaller than that of the closely related hydrocarbons. The preferential adsorption of longer homologues over shorter ones is observed. Interestingly, the extent of preferential adsorption with relative size ratio is very similar to that of the hydrocarbons. These results can be understood in terms of the increased compressibility and lower polarizability of the perfluoroalkanes compared to hydrocarbons.     

烷烃混合物在Cu-BTC中的吸附与分离

用巨正则系综Monte Carlo (GCMC)和构型导向Monte Carlo (CBMC)相结合的方法模拟了298 K下甲烷-乙烷-丙烷体系以及正丁烷-异丁烷体系在1,3,5-苯三甲酸铜(II) (Cu-BTC)中的吸附行为. 结果表明, Cu-BTC对丙烷以及异丁烷的吸附分离都有较好的选择性. 通过我们发展的“材料剖面成像”方法研究了烷烃混合物在Cu-BTC中不同压力下的吸附位点, 从而进一步分析了烷烃混合物在Cu-BTC中的分离性能. 结果发现, 在吸附过程中主要存在着两种效应, 即能量效应和尺寸效应的竞争. 在甲烷-乙烷-丙烷体系中, 较高压力下, 由于尺寸效应的影响, 丙烷主要吸附在主孔道中, 而对甲烷和乙烷组分, 能量效应占主导地位, 从而导致乙烷主要吸附在四面体孔内, 甲烷则主要吸附在三角形孔窗外. 在正丁烷-异丁烷体系中, 能量效应起主导作用, 从而使异丁烷主要吸附在四面体孔内, 而正丁烷主要吸附在主孔道中.     

Electron Transfer through Monolayers of Mixed Long-chain Aliphatic Acids and Alcohols

Properties of monolayers of cetyl, stearyl, and eicosyl alcohols and stearic and behenic acids and their mixtures are studied, along with the kinetics of a redox reaction of hemin adsorbed on these monolayers. Three-dimensional computer models of corresponding monolayers are constructed and compared with their electrochemical behavior. Properties of monolayers of cetyl alcohol in acid and alkaline solutions and stearyl alcohol in acid solutions correspond to package of molecules in a stretched conformation oriented normally to the electrode surface; for the other one-component systems, the monolayer structure is less regular. The hemin redox reaction rate has no correlation whatsoever with the monolayer thickness and is defined by the length of the electron transfer path (over a chain of covalent bonds and through van der Waals contacts between molecules). The stronger the deviation of the conformation of molecules from the stretched one, the shorter the effective path of the electron transfer. In mixed films of stearic acid and cetyl alcohol (1 : 6), it is sometimes possible to keep molecules of stearic acid in a stretched conformation, thus inhibiting the electron transfer along the hydrocarbon chain of this molecule to a maximum extent. The other mixed systems, while making the conformation more stretched, fail to provide for a completely regular structure.     

Understanding the Anomalous Alkane Selectivity of ZIF-7 in the Separation of Light Alkane/Alkene Mixtures

C2 and C3 alkanes are selectively adsorbed from mixtures over the corresponding alkenes on the zeolite imidazolate framework ZIF-7 through a gate-opening mechanism. As a result, the direct production of the pure alkene upon adsorption and the pure alkane upon desorption in packed columns is possible. Herein, a detailed investigation of the step-wise adsorption and separation of alkanes and alkenes is presented, together with a rigorous performance assessment. A molecular picture of the gate-opening mechanism underlying the unprecedented selectivity towards alkane adsorption is proposed based on DFT calculations and a thermodynamic analysis of the adsorption-desorption isotherms.     

Monte Carlo simulation for the adsorption and separation of linear and branched alkanes in IRMOF-1

The adsorption and separation of linear and branched alkanes in the isoreticular metal-organic framework IRMOF-1 have been investigated using Monte Carlo simulation. For pure linear alkanes (C1-nC5), the limiting adsorption properties exhibit linear behavior with the alkane carbon number; the long alkane is preferentially adsorbed over the short alkane at low fugacities, whereas the reverse is found at high fugacities. For pure branched alkanes (C5 isomers), the linear isomer adsorbs more than its branched analogue. The adsorbed amounts of pure alkanes in IRMOF-1 are substantially greater than in a carbon nanotube bundle and in silicalite. For a five-component mixture of C1 to nC5 linear alkanes, the long alkane adsorption first increases and then decreases with increasing fugacity, whereas short alkane adsorption continually increases and progressively replaces the long alkane at high fugacity due to the size entropy effect. For a three-component mixture of C5 isomers, the adsorption of each isomer increases with increasing fugacity until saturation, though there is less adsorption of the branched isomer due to the configurational entropy effect. The adsorption selectivity among the alkanes in IRMOF-1 is smaller than in a carbon nanotube bundle and in silicalite.     

Behavior of binary alcohol mixtures adsorbed on graphite using calorimetry and scanning tunneling microscopy

The mixing behavior of binary combinations of linear alcohols adsorbed from their liquids is studied by calorimetry and scanning tunneling microscopy (STM). In particular, we consider combinations of primary alcohols that differ by a single methylene group. Where the shorter alcohol has an odd number of carbon atoms, the combination is found to mix, essentially, ideally on the surface. However, for combinations where the shorter alcohol has an even number of carbon atoms, we find that there is molecular complex formation for shorter members but ideal mixing for longer (n>12) homologues. This extends previous work in this area by the determination of the limits of surface molecular complex formation. We also exploit STM to address this unexpected complex formation.     

Interactions of a fluoroaryl surfactant with hydrogenated, partially fluorinated, and perfluorinated surfactants at the air/water interface

A novel surfactant containing pentafluorophenyl moiety attached at the terminal position of undecanol (11,11-difluoro-11-(pentafluorophenyl)undecan-1-ol, abbr. PBD) was synthesized and employed for the Langmuir monolayer characterization and miscibility studies with a semifluorinated alkane (perfluorodecyleicosane, abbr. F10H20) and four alcohols differing in the degree of fluorination in their hydrophobic chains: octadecanol (C18OH), perfluorooctyldecanol (F8H10OH), perfluoroisononyldecanol (iF9H10OH) and 1H,1H-perfluorooctadecanol (F18OH). Pure monolayers of all of the investigated surfactants as well as their mixtures were investigated with surface pressure-area isotherms complemented by Brewster angle microscopy (BAM) images. PBD was found to form stable Langmuir monolayers of liquid-expanded character. Characteristic dendritic structures were formed at the very early stage of compression and remained up to the vicinity of collapse, where 3D crystallites appeared. 2D miscibility studies revealed that PBD forms mixed monolayers with the investigated semifluorinated alkane (F10H20) as well as with perfluorinated alcohol (F18OH) within the whole composition range, do not mix with octadecanol to the fully hydrogenated alcohol, whereas it is partially miscible (up to a certain surface pressure value) with the studied semifluorinated alcohols. The analysis of the miscibility derived from the surface pressure-area isotherms (collapse pressure vs composition dependencies) agrees well with BAM images. Molecular interactions in the investigated systems have been quantified with interaction parameter, alpha.     

Molecular simulation of adsorption and separation of mixtures of short linear alkanes in pillared layered materials at ambient temperature

Grand canonical Monte Carlo and configurational-bias Monte Carlo techniques are carried out to simulate the adsorption of ternary and quaternary mixtures of short linear alkanes, involving methane, ethane, propane, and n-butane, in pillared layered materials at ambient temperature, T=300 K. In the simulation, a pillared layered pore is modeled by a uniform distribution of pillars between two layered walls built by making two separate talc lamellas parallel each other with a given size of interlayer distance. The interaction between fluid molecules and two layered walls is measured by storing potentials calculated in advance at a series of grid points. The interaction between fluid molecules and pillars is also calculated by a site-to-site method. The potential model proposed in this work is proved to be effective because of the simulation result being good agreement with the experimental data for the adsorption of nitrogen at 77 K. Then, the adsorption isotherms of mixtures of short linear alkanes in pillared layered pores with three different porosities psi=0.98, 0.93 and 0.85, and three pore widths H=1.02, 1.70 and 2.38 nm at 300 K are obtained by taking advantage of the model. The simulation results tell us that the longer chain component is preferentially adsorbed at low pressures, and its adsorption increases and then decreases as the pressure increases while the shorter chain component is still adsorbed at high pressures. Moreover, the sorption selectivity of pillared layered materials for the longest chain component in alkane mixtures increases as the mole fraction of methane in the gas phase increases. The selectivity of pillared layered materials for the longest chain component in alkane mixtures also increases as the pore width decreases and the porosity increases.     

New thermodynamic/electrostatic models of adsorption and tension equilibria of aqueous ionic surfactant mixtures: application to sodium dodecyl sulfate/sodium dodecyl sulfonate systems

The nonideal adsorbed solution (NAS) theory has been formally extended to adsorption at the air/water interface from aqueous mixtures of ionic surfactants, explicitly accounting for the surface potential of the adsorbed monolayer with the Gouy-Chapman theory. This new ionic NAS (iNAS) theory is thermodynamically consistent and, when coupled to a micellization model, is valid for concentrations below and above the mixed cmc. Counterion binding is incorporated into the model using two fractional binding parameters, beta(sigma) for the adsorbed monolayer and beta(m) for the micelles. The regular solution theory is used to model the nonideal interactions within the adsorbed monolayer and within the mixed micelles. New tension data for an equimolar mixture of sodium dodecyl sulfate (SDS) and sodium dodecyl sulfonate (SDSn) at two salinities fit this model well when mixing is ideal. The total surface densities, the surface compositions, and the surface potentials for the mixed monolayers are calculated. When there is no added salt, at total surfactant concentrations below the mixed cmc, the adsorbed monolayer is enriched in SDSn, but at total concentrations at and above the mixed cmc, the adsorbed monolayer is nearly an equimolar mixture. In the presence of 100 mM NaCl, the adsorbed monolayer is nearly an equimolar mixture, independent of the total surfactant concentration.     

Heats of immersion of titanium dioxide pigments in alcohol—water mixtures

The heats of immersion of partially dried anatase and rutile pigments in mixtures of water with methanol, ethanol, and n-propanol were measured by a differential calorimetric method. The anatase heats of immersion could best be explained by assuming preferential adsorption of the alcohols, the effect being greatest for n-propanol The rutile pigment, however, appeared to adsorb water preferentially in methanol—water and ethanol—water mixtures over the whole concentration range. In propanol—water mixtures the rutile pigment preferentially adsorbed water below an alcohol mol fraction of 0.25, and preferentially adsorbed propanol at mol fractions of alcohol greater than 0.25. The differences in behaviour between the two pigments may be explained qualitatively from the point of view of their surface morphology.     

Use of alkane monolayer templates to modify the structure of alkyl ether monolayers on highly ordered pyrolytic graphite

Scanning tunneling microscopy (STM) has been used to investigate the structure of pure and mixed monolayers formed by adsorption of long-chain alkanes and/or ethers on highly ordered pyrolytic graphite. Application of a pure phenyloctane solution of simple alkanes, such as tritriacontane, CH3(CH2)31CH3, produced a monolayer within which the individual molecular axes were oriented perpendicular to the lamellar axes. In contrast, a pure solution of symmetrical long-chain ethers, such as di-n-hexadecyl ether, CH3(CH2)15O(CH2)15CH3, produced a monolayer within which the molecular axes were oriented at an angle of approximately 65 degrees relative to the lamellar axes. The compositions of the overlying solutions were then gradually changed either from pure alkanes to nearly pure ethers or from pure ethers to nearly pure alkanes. When ethers replaced alkanes in the monolayer, the ethers conformed to the orientation within the existing alkane layer, rather than adopting the characteristic orientation of pure ether monolayers. However, when alkanes were incorporated into monolayers that had been formed from pure ether solutions, the orientation of the molecules within the monolayer converted to that characteristic of pure alkanes. Alkane monolayers thus acted as templates for subsequent ether layers, but ether monolayers did not act as templates for alkane layers.     

Semifluorinated chains at the air/water interface: studies of the interaction of a semifluorinated alkane with fluorinated alcohols in mixed langmuir monolayers

Mixtures of a semifluorinated alkane 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-henicosafluorotriacontane (abbr. F10H20) and different alcohols were investigated at the air/water interface using surface pressure-area isotherms complemented with BAM images. In our studies, octadecanol and its fluorinated derivatives differing in the degree of fluorination were researched. To verify the influence of an iso branching of the fluorinated segment in an alcohol molecule, the properties of perfluorooctyldecanol and perfluoro-iso-nonyldecanol in mixtures with F10H20 were compared. From the isotherms datapoints, the excess of free energy of mixing (DeltaG(exc)) together with the interaction parameter (alpha) were calculated. On the basis of the additivity rule and BAM images, phase diagrams for all of the investigated systems were constructed. It occurs that F10H20 mixes with the fully hydrogenated alcohol, octadecanol, within the whole range of alcohol mole fractions, whereas it is completely immiscible with its perfluorinated analogue. Regarding the mixtures of F10H20 with semifluorinated alcohols, it turned out that these systems exhibit limited miscibility, i.e., are miscible at a low semifluorinated alcohol proportion, whereas upon increasing alcohol content, the systems start to demix. It may be concluded that the molecular packing in mixed monolayers is the key factor determining the miscibility of F10H20 of the investigated alcohols.     

Phase behavior studies of quaternary systems containing N-lauroyl-N-methylglucamide/alcohol/alkane/water

The three-phase behavior of quaternary systems comprising N-lauroyl-N-methylglucamide (MEGA-12)/alcohol/alkane/water has been studied using epsilon-beta fishlike phase diagrams. From the epsilon-beta fishlike phase diagrams a series of phase inversions Winsor I (2) --> III (3) --> II (2) were observed, and the hydrophilic-lipophilic balanced (HLB) plane equation for the quaternary system was deduced. Some physicochemical parameters, such as the mass fraction of alcohol in the HLB interfacial layer, A S, the coordinates of the start (beta B, epsilon B) and end points (beta E, epsilon E) of the middle-phase microemulsion, the mass fractions of MEGA-12 and alcohol in the total system (C S and C A), and the solubilities of MEGA-12 and alcohol in oil phase (S O and A O), were calculated. The effects of different alcohols, alkanes, and NaCl concentrations in the aqueous phase on the phase behavior and solubilization capacity were investigated, which indicates that alcohol with longer and alkane with shorter hydrocarbon chains have a larger solubilization capacity. NaCl concentration has little influence on the phase behavior.     

Layer-by-layer surface freezing of linear alcohols at the graphite/liquid interface

Differential scanning calorimetry (DSC), incoherent elastic neutron scattering, and neutron diffraction are used to demonstrate the presence of adsorbed solid multilayers of linear alcohols at the graphite-liquid alcohol interface. All alcohols studied (C(5)-C(18)) are found to form at least one monolayer. In addition all the even alcohols investigated (C(6)OH to C(18)OH) show multilayer formation. However, only the short odd alcohols (C(5)OH to C(11)OH) clearly exhibit additional features indicating multilayer formation.     

Adsorption of alkyl trimethylammonium bromides at the air/water interface

A number of features of the adsorption of alkyl trimethylammonium bromides with nc=10,12,14, and 16 at the air/water interface were studied. First, the adsorption isotherms were calculated from experimental surface tension vs concentration curves by means of the Gibbs equation. Second, a novel method was used to estimate the adsorption free energy change. From the analysis of these data it was concluded that the hydrophobic driving force for the adsorption first increases with increasing adsorbed amount and then levels off in a plateau, which holds true for all four homologues. This peculiar behavior was interpreted by the formation of a thin liquid-like alkane film at the air/water interface once a certain adsorbed amount is exceeded. The hydrophobic contribution to the standard free energy change of adsorption was compared with those values previously determined for alkyl sulfate homologues. This comparison suggests that the alkyl trimethylammonium type surfactants behave as if their alkyl chain was approximately one methylene group shorter than those of the corresponding alkyl sulfates.     

Poly(ethylene oxide)s hydrophobically modified. Adsorption and spreading at the air-water interface

A comparative study of spread and adsorbed monolayer of poly(ethylene oxide)s of different molecular weight hydrophobically modified with alkyl isocyanates of different length chain is reported. The modification of the polymer was carried out according to reported procedures. The polymers obtained were studied at the air-water interface by Langmuir isotherms for spread monolayers and by Gibbs isotherms for the adsorption process. Isotherms obtained are interpreted in terms of the hydrophobic and hydrophilic balance of the polymers. Limiting area per repeating unit (A(0)) and collapse pressure (pi(c)) from spread monolayers were obtained. Spread monolayers of the hydrophobically modified polymers show larger collapse pressure values than unmodified polymer monolayers. In the adsorption process the excess surface concentration Gamma(infinity), area per repeat unit sigma, and efficiency of the adsorption were determined. The values of the area occupied per repeat unit in adsorbed monolayer (sigma) were larger than those of the spread monolayer. The efficiency of the adsorption of poly(ethylene oxide)s increases with the hydrophobic modification and with the alkyl chain length.     

Bulk and adsorbed monolayer phase behavior of binary mixtures of undecanoic acid and undecylamine: catanionic monolayers

Differential scanning calorimetry (DSC) and X-ray powder diffraction (PXRD) have been used to determine the phase behavior of the binary mixtures of undecanoic acid (A) and undecylamine (B) in the bulk. In addition, we report DSC data that indicates very similar behavior for the solid monolayers of these materials adsorbed on the surface of graphite. The two species are found to form a series of stoichiometric complexes of the type AB, A(2)B, and A(3)B on the acid rich side of the phase diagram. Interestingly, no similar series of complexes is evident on the amine rich side. As a result of this complexation, the solid monolayers of the binary mixtures exhibit a very pronounced enhancement in stability relative to the pure adsorbates.     

On the interaction of dipalmitoyl phosphatidylcholine with normal long-chain alcohols in a mixed monolayer: a thermodynamic study

This study investigated the mixed monolayer behavior of dipalmitoyl phosphatidylcholine (DPPC) with normal long-chain alcohols at the air/water interface. Surface pressure–area isotherms of mixed DPPC/C₁₈OH and DPPC/C₂₀OH monolayers at 37°C were obtained and compared with previous results for the mixed DPPC/C₁₆OH system. The negative deviations from additivity of the areas and the variation of the collapse pressure with composition imply that DPPC and long-chain alcohols were miscible and formed non-ideal monolayers at the interface. At lower surface pressures, it seems that the attractive intermolecular force was dominant in molecular packing in the mixed monolayers. At higher surface pressures, the data suggest that the molecular packing in mixed DPPC/C₁₆OH monolayers may be favored by the packing efficiency or geometric accommodation. Furthermore, negative values of excess free energy of mixing were obtained and became significant as the hydrocarbon chain length of alcohols increased, which indicates there were attractive interactions between DPPC and long-chain alcohols. In each free energy of mixing–composition curve, there was only one minimum and thus a phase separation did not exist for mixed DPPC/long-chain alcohol monolayers.