A chromatographic estimate of the degree of surface heterogeneity of reversed-phase liquid chromatography packing materials. II-Endcapped monomeric C18-bonded stationary phase

In a previous report, the heterogeneity of a non-endcapped C30-bonded stationary phase was investigated, based on the results of the measurements of the adsorption isotherms of two neutral compounds (phenol and caffeine) and two ionizable compounds (sodium naphthalene sulfonate and propranololium chloride) by frontal analysis (FA). The same method is applied here for the characterization of the surface heterogeneity of two new brands of endcapped C18-bonded stationary phases (Gemini and Sunfire). The adsorption isotherms of the same four chemicals were measured by FA and the results confirmed by the independent calculation of the adsorption energy distribution (AED), using the expectation-maximization (EM) method. The effect of the length of the bonded alkyl chain was investigated. Shorter alkyl-bonded-chains (C18 versus C30) and the end-capping of the silica surface contribute to decrease the surface heterogeneity under the same experimental conditions (30% methanol, 25 mM NaCl). The AEDs of phenol and caffeine are bimodal with the C18-bonded columns while they are trimodal and quadrimodal, respectively, with a non-endcapped C30-bonded column. The "supersites" (adsorption energy > 20 kJ/mol) found on the C30-Prontosil column and attributed to a cation exchange mechanism completely disappear on the C18-Gemini and C18-Sunfire, probably because the end-capping of the silica surface eliminates most if not all the ionic interactions.     

Comparison of the adsorption mechanisms of pyridine in hydrophilic interaction chromatography and in reversed-phase aqueous liquid chromatography

The adsorption isotherms of pyridine were measured by frontal analysis (FA) on a column packed with shell particles of neat porous silica (Halo), using water–acetonitrile mixtures as the mobile phase at 295 K. The isotherm data were measured for pyridine concentrations covering a dynamic range of four millions. The degree of heterogeneity of the surface was characterized by the adsorption energy distribution (AED) function calculated from the raw adsorption data, using the expectation-maximization (EM) procedure. The results showed that two different retention mechanisms dominate in Per aqueous liquid chromatography (PALC) at low acetonitrile concentrations and in hydrophilic interaction chromatography (HILIC) at high acetonitrile concentrations. In the PALC mode, the adsorption mechanism of pyridine on the silica surface is controlled by hydrophobic interactions that take place on very few and ultra-active adsorption sites, which might be pores on the irregular and rugose surface of the porous silica particles. The surface is seriously heterogeneous, with up to five distinct adsorption sites and five different energy peaks on the AED of the packing material. In contrast, in the HILIC mode, the adsorption behavior is quasi-homogeneous and pyridine retention is governed by its adsorption onto free silanol groups. For intermediate mobile phase compositions, the siloxane and the silanol groups are both significantly saturated with acetonitrile and water, respectively, causing a minimum of the retention factor of pyridine on the Halo column.     

DOPA醌在Cu（100）表面吸附的密度泛函理论研究

采用广义梯度密度泛函理论(GGA)的BLYP方法结合周期性平板模型,以原子簇Cu41为模拟表面,对DOPA醌分子在Cu(100)表面不同位置的吸附模型进行了构型优化、能量计算以及Mulliken布居分析,结果表明通过相邻的羰基垂直吸附在表面的桥位是其最佳吸附方式,吸附能为247.2310kJ/mol;其次为顶位、顶位R45和穴位,吸附能分别为227.7162kJ/mol、220.7305kJ/mol和217.8456kJ/mol。Mulliken布居分析结果表明整个吸附体系发生了由Cu原子向DOPA醌分子的电荷转移。     

Heterogeneity of the surface energy on unused C18-chromolith adsorbents in reversed-phase liquid chromatography

Single-component adsorption isotherm data were acquired by frontal analysis (FA) for phenol and caffeine on a new C18-Chromolith column (Merck, Darmstadt, Germany), using a water-rich mobile phase (methanol/water, 15/85, v/v). These data were modeled for best agreement between the experimental data points and the adsorption isotherm model. The adsorption-energy distributions, based on the expectation-maximization (EM) procedure, were also derived and used for the selection of the best isotherm model. The adsorption energy distributions (AEDs) for phenol and caffeine converged toward a trimodal and a quadrimodal distribution, respectively. Energy distributions with more than two modes had not been reported before for the adsorption of these compounds on packed columns. The third high energy mode observed for both phenol and caffeine seems to be specific of the surface of the monolithic column while the first and second low energy modes have the same physical origin as the two modes detected on packed columns. These results suggest significant differences between the structures of the porous silica in these different materials.     

Heterogeneity of multiwalled carbon nanotubes based on adsorption of simple aromatic compounds from aqueous solutions

The surface heterogeneity of multiwalled carbon nanotubes (MWCNTs) is studied on the basis of adsorption isotherms from dilute aqueous phenol and dopamine solutions at various pH values. The generalized Langmuir–Freundlich isotherm equation was applied to investigate the cooperative effect of the surface heterogeneity and the lateral interactions between the adsorbates. The theoretical isosteric heats of adsorption were obtained assuming that the heat of adsorption profile reveals both the energetic heterogeneity of the adsorption system and the strength of the interactions between the neighboring molecules. The adsorption energy distribution functions were calculated by using algorithm based on a regularization method. The great advantage of this method is that the regularization makes no assumption about the shape of the obtained energy distribution functions. Analysis of the isosteric heats of adsorption for MWCNTs showed that the influence of the surface heterogeneity is much stronger than the role of the lateral interactions. The most typical adsorption heat is 20–22 kJ/mol for both phenol and dopamine. After purification of nanotubes, heat value for phenol dropped to 16–17 kJ/mol. The range of the energy distribution is only slightly influenced by the surface chemistry of the nanotubes in the aqueous conditions.     

The adsorption mechanism of nortryptiline on C18-bonded Discovery

The adsorption isotherms of an ionizable compound, nortriptyline, were accurately measured by frontal analysis (FA) on a C(18)-Discovery column, first without buffer (in an aqueous solution of acetonitrile at 15%, v/v of ACN), then with a buffer (in 28%, v/v ACN solution). The buffers were aqueous solutions containing 20 mM of formic acid or a phosphate buffer at pH 2.70. The linear range of the isotherm could not be reached with the non-buffered mobile phase using a dynamic range larger than 40,000 (from 1.2 x 10(-3) g/L to 50 g/L). With a 20 mM buffer in the liquid phase, the isotherm is linear for concentrations of nortriptyline inferior to 10(-3) g/L (or 3 micromol/L). The adsorption energy distribution (AED) was calculated to determine the heterogeneity of the adsorption process. AED and FA were consistent and lead to a trimodal distribution. A tri-Moreau and a tri-Langmuir isotherm models accounted the best for the adsorption of nortriptyline without and with buffer, respectively. The nature of the buffer affects significantly the middle-energy sites while the properties of the lowest and highest of the three types of energy sites are almost unchanged. The desorption profiles of nortriptyline show some anomalies in relation with the formation of a complex multilayer adsorbed phase of acetonitrile whose excess isotherm was measured by the minor disturbance method. The C(18)-Discovery column has about the same total saturation capacity, around 200 g of nortriptyline per liter of adsorbent (or 116 mg/g), with or without buffer. About 98-99% of the available surface consists in low energy sites. The coexistence of these different types of sites on the surface solves the McCalley's enigma, that the column efficiency begins to drop rapidly when the analyte concentration reaches values that are almost one hundred times lower than those that could be predicted from the isotherm data acquired under the same experimental conditions. Due to the presence of some relatively rare high energy sites, the largest part of the saturation capacity is not practically useful.     

Physical origin of peak tailing on C18-bonded silica in reversed-phase liquid chromatography

Single component isotherm data of caffeine and phenol were acquired on two different stationary phases for RPLC, using a methanol/water solution (25%, v/v, methanol) as the mobile phase. The columns were the non-endcapped Waters Resolve-C18, and the Waters XTerra MS C18. Both columns exhibit similar C18 -chain densities (2.45 and 2.50 micromol/m2) and differ essentially by the nature of the underivatized solid support (a conventional, highly polar silica made from water glass, hence containing metal impurities, versus a silica-methylsilane hybrid surface with a lower density of less acidic free silanols). Thirty-two adsorption data points were acquired by FA, for caffeine, between 10(-3) and 24 g/l, a dynamic range of 24,000. Twenty-eigth adsorption data points were acquired for phenol, from 0.025 to 75 g/l, a dynamic range of 3000. The expectation-maximization procedure was used to derive the affinity energy distribution (AED) from the raw FA data points, assuming a local Langmuir isotherm. For caffeine, the AEDs converge to a bimodal and a quadrimodal distribution on XTerra MS-C18 and Resolve-C18, respectively. The values of the saturation capacity (q(s,1) approximately equal to 0.80 mol/l and q(s,2) approximately equal to 0.10 mol/l) and the adsorption constant (b1 approximately equal to 3.11/mol and b2 approximately equal to 29.1 l/mol) measured on the two columns for the lowest two energy modes 1 and 2, are comparable. These data are consistent with those previously measured on an endcapped Kromasil-C18 in a 30/70 (v/v), methanol/water solution (q(s,1) = 0.9 mol/l and q(s,2) = 0.10 mol/l, b1 = 2.4 l/mol and b2 = 16.1 l/mol). The presence of two higher energy modes on the Waters Resolve-C18 column (q(s,3) approximately equal to 0.013 mol/l and q(s,4) approximately equal to 2.6 10(-4) mol/l, b3 approximately equal to 252 l/mol and b4 = 13,200 l/mol) and the strong peak tailing of caffeine are explained by the existence of adsorption sites buried inside the C18-bonded layer. It is demonstrated that strong interactions between caffeine and the water protected bare silica surface cannot explain these high-energy sites because the retention of caffeine on an underivatized Resolve silica column is almost zero. Possible hydrogen-bond interactions between caffeine and the non-protected isolated silanol groups remaining after synthesis amidst the C18-chain network cannot explain these high energy interactions because, then, the smaller phenol molecule should exhibit similarly strong interactions with these isolated silanols on the same Resolve-C18 column and, yet, the consequences of such interactions are not observed. These sites are more consistent with the heterogeneity of the local structure of the C18-bonded layer. Regarding the adsorption of phenol, no matter whether the column is endcapped or not, its molecular interactions with the bare silica were negligible. For both columns, the best adsorption isotherm was the Bilangmuir model (with q(s,1) approximately equal to 2 and q(s,2) approximately equal to 0.67 mol/l, b1 0.61 and b2 approximately equal to 10.3 l/mol). These parameters are consistent with those measured previously on an endcapped Kromasil-C18 column under the same conditions (q(s,1) = 1.5 and q(s,2) = 0.71 mol/l, b1 = 1.4 l/mol and b2 = 11.3 l/mol). As for caffeine, the high-energy sites are definitely located within the C18-bonded layer, not on the bare surface of the adsorbent.     

Determination of single component isotherms and affinity energy distribution by chromatography

Adsorption isotherm data were acquired by frontal analysis (FA) and large sample-size band profiles were recorded for phenol and caffeine. For both compounds, the isotherm data fit well to the Langmuir, Toth, and Bi-Langmuir models of adsorption. The Langmuir model must be dismissed because it does not predict accurately the overloaded band profiles. However, profiles calculated using the unimodal Toth and the bimodal Bi-Langmuir models are indistinguishable. The expectation-maximization procedure was used to calculate directly the affinity energy distribution (AED) from the raw FA data points. For both compounds, the AED converges to a bimodal distribution at high numbers of iterations. This result, which shows the high sensitivity of the EM method, suggest that the Bi-Langmuir model makes better physical sense than the Toth model. This model also permits a detailed investigation of the properties of active sites, a feature often evoked in chromatography but so far rarely the topic of a quantitative investigation.     

Examination of the surface heterogeneity of reversed-phase packing materials with solvent adsorption

The adsorption isotherms of acetonitrile, ethanol, 2-propanol, and THF were measured using frontal analysis on six columns packed with octadecyl RP stationary phase. The effect of the bonding density of the end-capped octadecyl bonded phase on the adsorption properties was measured. Adsorption isotherm data were collected from aqueous solutions of the four organic modifiers. The isotherm model for solvent adsorption was selected using two independent parameter estimation methods, the regression analysis and adsorption energy distribution. The fitted isotherm parameters were tested by modeling of overloaded elution bands with the aid of the equilibrium-dispersive model of chromatography. The surface heterogeneity estimations and the effect of the silanol groups on the adsorption of solvents were based on those data.     

Monte Carlo simulations of surface energy of the open tetrahedral surface of 2:1-type phyllosilicate

Monte Carlo simulations were employed to investigate the surface energy of the open tetrahedral surface of 2:1-type phyllosilicate. Argon was selected as the probe molecule. The adsorption isotherm was simulated and the adsorption potential map was calculated. Both the density and energy distributions of adsorbed atoms were derived at different pressures to explore the adsorption mechanism. It is found that there exist two kinds of energetic sites: minima (-15.5 kJ/mol) corresponding to the centers of six-membered rings and platform points (-8.0 kJ/mol) corresponding to the edges and vertexes of hexagons. They are primary and secondary adsorption sites, respectively. The implications for experiments and future studies are discussed. Current results are applicable for understanding surface energy properties of other clay minerals, since they have very similar basal surfaces.     

Polymer adsorption on heterogeneous surfaces

The adsorption of polymers on an energetically heterogeneous surface can be modelled using a self-consistent field lattice theory. The surface consists of sites having different adsorption energies. It turns out that the distribution of the sites over the surface is one of the main parameters determining the adsorption behaviour.     

On the dynamics of H2 adsorption on the Pt(111) surface

The dynamics and kinetics of the dissociation of hydrogen over the hexagonal close packed platinum (Pt(111)) surface are investigated using Car–Parrinello molecular dynamics and static density functional theory calculations of the potential energy surfaces. The calculations model the reference energy‐resolved molecular beam experiments, considering the degrees of freedom of the catalytic surface. Two‐dimensional potential energy surfaces above the main sites on Pt(111) are determined. Combined with Car–Parrinello trajectories, they confirm the dissociative adsorption of H₂ as the only adsorption pathway on this surface at H₂ incindence energies above 5 kJ/mol. A direct determination of energy‐resolved sticking coefficients from molecular dynamics is also performed, showing an excellent agreement with the experimental data at incidence energies in the 5–30 kJ/mol range. Application of dispersion corrections does not lead to an improvement in the prediction of the H₂ sticking coefficient. The adsorption reaction rate obtained from the calculated sticking coefficients is consistent with experimentally derived literature values.     

Effect of the endcapping of reversed-phase high-performance liquid chromatography adsorbents on the adsorption isotherm

The retention mechanisms of n-propylbenzoate, 4-t ert-butylphenol, and caffeine on the endcapped Symmetry-C(18) and the non-endcapped Resolve-C(18) are compared. The adsorption isotherms were measured by frontal analysis (FA), using as the mobile phase mixtures of methanol or acetonitrile and water of various compositions. The isotherm data were modeled and the adsorption energy distributions calculated. The surface heterogeneity increases faster with decreasing methanol concentration on the non-endcapped than on the endcapped adsorbent. For instance, for methanol concentrations exceeding 30% (v/v), the adsorption of caffeine is accounted for by assuming three and two different types of adsorption sites on Resolve-C(18) and Symmetry-C(18), respectively. This is explained by the effect of the mobile phase composition on the structure of the C(18)-bonded layer. The bare surface of bonded silica appears more accessible to solute molecules at high water contents in the mobile phase. On the other hand, replacing methanol by a stronger organic modifier like acetonitrile dampens the differences between non-endcapped and endcapped stationary phase and decreases the degree of surface heterogeneity of the adsorbent. For instance, at acetonitrile concentrations exceeding 20%, the surface appears nearly homogeneous for the adsorption of caffeine.     

Measurement and theoretical analysis of the adsorption of supercritical methane on superactivated carbon

Adsorption/desorption isotherms of supercritical methane on superactivated carbon have been measured in the range of 0-10 MPa and 233-333 K (20 K interval). The reversibility of the physical adsorption process is acknowledged. The heat of adsorption of 16.5 kJ/mol is determined from the isotherms, and a new modeling strategy for isotherms with maximum is presented. The model yields fits to the experimental isotherms with precision of ?%, maintaining the constancy of the characteristic energy of adsorption. The exponent of the model equation expresses the pore size distribution feature of the adsorbent. The density of the supercritical adsor-bate is evaluated as a parameter of the model. It is shown that the conventional isotherm theory works too at supercritical condition if the limit state of supercritical adsorption is introduced into isotherm modeling.     

Hydrogen adsorption on nickel (100) single-crystal face. A Monte Carlo study of the equilibrium and kinetics

We propose a model of the dissociative adsorption of hydrogen on nickel single-crystal face. In this model, we treat the Ni(100) surface as a strongly correlated energetically heterogeneous surface, because the density functional theory (DFT) studies indicate that hydrogen atoms may adsorb either on hollow sites (energetically more favorable, binding energy 48 kJ/mol H) or bridge sites with the binding energy less by 11 kJ/mol H. The essential assumption of the proposed model is that the dissociation of the hydrogen molecule is possible only over the topmost Ni atom, and the resulting H atoms may adsorb either on two free hollow sites (but the adjacent bridge sites must be free) or two bridge sites (the adjacent hollow sites must be free). If the above condition is not fulfilled, then the dissociation and adsorption are impossible. The second assumption is that the rate (probability) of the associative desorption is limited by the rate of diffusion of H atoms on the surface. This is because the two H atoms desorb, giving an H2 molecule, only when they meet on two adjacent hollow-bridge sites. Our model recovers very well the behavior of the experimental equilibrium adsorption isotherms as well as kinetic isotherms. As a result, we stated that hydrogen atoms are not completely free on the surface, but they cannot also be considered localized at room and elevated temperatures. Additionally, while analyzing the kinetic adsorption isotherms, we stated that the rate-limiting step during the dissociative adsorption of H2 is the disintegration of the activated complex and the subsequent adsorption of hydrogen atoms.     

Numerical determination of the adsorption isotherms of tryptophan at different temperatures and mobile phase compositions

Single-component adsorption isotherm data were acquired by frontal analysis (FA) for tryptophan on a C(18)-Kromasil packed column, using acetonitrile-water solutions of various compositions (2.5, 5, and 7.5% ACN+1% acetic acid) and at five different temperatures between 25 and 65 degrees C. The adsorption isotherm model accounting best for these data is the bi-Moreau model, showing that two types of adsorption sites coexist on the surface and that strong adsorbate-adsorbate interactions take place. Large concentration band profiles of tryptophan were obtained for the three mobile phase compositions, at five different temperatures and the best values of the adsorption isotherm coefficients were determined by the inverse method (IM) of chromatography. The advantages and drawbacks of using the FA and the IM for determining the coefficients of the adsorption isotherm of tryptophan under the experimental conditions selected are discussed. The results of the FA and IM measurements are in good agreement. Both indicate that the retention time of tryptophan decreases rapidly with increasing acetonitrile concentration in the mobile phase as well as the saturation capacities of the two types of adsorption sites, with the highest values of the two saturation capacities being found for the lowest ACN content and the lowest temperature. The adsorption constant on the low-energy sites decreases with increasing acetonitrile content and temperature. In contrast, the adsorption constant on the high-energy sites increases with increasing ACN content of the mobile phase but decreases with increasing temperature. The solute-solute interaction parameters for the low and the high-energy adsorption sites increase rapidly with increasing ACN concentration in the mobile phase and with increasing temperature.     

Adsorption energy distribution from the Aranovich-Donohue lattice density functional theory

We propose a new methodology projected for the estimation of the adsorption energy distribution from the monolayer part of a single nitrogen adsorption isotherm determined at 77 K based on the lattice density functional theory (DFT) via the Aranovich-Donohue formalism. At first sight, the presented approach is computationally more difficult than a classical one. However, it is more flexible and comprehensible. Next, we developed a numerical program and used it for the estimation of the adsorption energy distribution from the experimental data on carbon black samples. The main nitrogen molecule-carbon black surface interaction energy can be estimated as approximately 7-8 kJ/mol, but the heterogeneity of the investigated materials differs significantly. Furthermore, we compare the results obtained from the lattice DFT via the Aranovich-Donohue formalism with the solution of the integral equation with the kernel represented by the classical monolayer localized Fowler-Guggenheim isotherm equation. The similarity between these two independent approaches is observed. The proposed methodology can be used for the investigation of the energetic heterogeneity of not only the carbonaceous materials but also the other "flat-surfaced" solids.     

Pressure-induced effects in the heterogeneous adsorption of insulin on chromatographic surfaces

The effect of increasing the average column pressure (ACP) on the heterogeneous adsorption of insulin variants on a C18-bonded silica was studied in isocratic reversed-phase HPLC. Adsorption isotherm data of lispro and porcine insulin obtained for values of the ACP ranging from 57 to 237 bar were fitted to the Langmuir-Freundlich and the Tóth equation. The resulting isotherm parameters, including the equilibrium adsorption constant and the heterogeneity index, were next used for the calculation of distribution functions characterizing the energy of interactions between the adsorbed insulin molecules and the stationary phase. It was observed that increasing the pressure by 180 bar causes a broadening of the distribution functions and a shift of the position of their maximum toward lower interaction energies. These findings suggest that, under high pressures, the insulin molecules interact with the stationary phase in a more diversified way than under low pressures. Additionally, the most probable value of the energy of the insulin-surface interactions becomes lower when the ACP increases. The pressure-induced changes in the interaction of insulin variants with the hydrophobic surface are attributed to a possible conformational flexibility of the molecular structure of this protein.     

Theoretical and experimental studies on the adsorption of aromatic compounds onto cellulose

The adsorption of several aromatic compounds over microcrystalline cellulose was studied by molecular modeling and experimentally using gas chromatography. Experimental adsorption enthalpies were obtained from an equation based on Clausius-Clapeyron formalism and the temperature dependence of retention volume at infinite dilution. Four different cellulose surfaces (three crystalline (110, 100, and 010) and one amorphous) were modeled. Overall strong agreement was observed between the experimental and theoretical work with 84% of the adsorbate-cellulose systems having differences between measured and predicted values of less than 20%. Based on both calculated and experimental data, a morphology for the microcrystalline cellulose as a weighted combination of the four surfaces was proposed: 39% (110), 28% (100), 10% (010), and 23% amorphous. By adopting this distribution, differences between experimental and weighted average predicted adsorption energies were 10% or less for 14 out of 17 compounds; a maximum of 15% was observed for guaiacol. Experimental results for monosubstituted aromatic compounds revealed that adsorption enthalpies are related to the hydrophilic/hydrophobic character of the substituent groups: 3.5 kJ mol(-1) for a methyl group, 15.7 kJ mol(-1) for a double bond, 21.0 kJ mol(-1) for a methoxyl group, 22.8 kJ mol(-1) for a carbonyl group, and 27.6 kJ mol(-1) for a hydroxyl group. These tendencies were confirmed by modeling, except for the aldehyde carbonyl group, where an overestimation of 10.8 kJ mol(-1) was observed. Analysis of experimental and predicted adsorption enthalpies of multisubstituted aromatic compounds suggests that the efficiency of their interaction with cellulose depends on a compromise between the roughness of the cellulose surface and their conformational adaptability.