Retention of ionizable compounds in reversed-phase liquid chromatography. Effect of the ionic strength of the mobile phase and the nature of the salts used on the overloading behavior

The retention mechanism of the protonated cation in propranolol chloride on C18-Xterra was investigated using mobile phases of various compositions. Accurate adsorption data were measured by frontal analysis, with a mixture of methanol and water (25% methanol), with no salt, as the mobile phase. The experimental isotherm has at least two inflection points, at concentrations of about 0.2 and 6.0 g/L, respectively. This precludes the modeling of these data with a simple convex-upward isotherm (e.g., Langmuir). The adsorption energy distribution or relationship between the number of sites on the adsorbent surface and the energy of adsorption on these sites was calculated by assuming Moreau isotherm behavior (S-shaped isotherm). This model has never been applied to describe the surface heterogeneity of any RPLC adsorbent. The calculation converged toward a bimodal energy distribution. Accordingly, the bi-Moreau model is the simplest theoretical model accounting for the adsorption data of propranolol from a mobile phase without salt. The complex-overloaded band profiles of propranolol measured in the presence of increasing concentrations of a supporting salt (KCl) in the mobile phase demonstrate that the same isotherm model applies also under these conditions, as was merely assumed in a previous work. The elution band profiles of propranolol calculated with the bi-Moreau isotherm model for solutions of salts of different natures (CaCl2, CsCl, Na2SO4) in the same mobile phase agree very well with the experimental band profiles.     

Adsorption of zinc(II) from an aqueous solution onto activated carbon.

Adsorption isotherms were measured experimentally for Zn(II) adsorption from aqueous solution onto commercial activated carbons C, F-400, F-300 and Centaur HSL in a batch adsorber. The effects of carbon type and solution pH on adsorption isotherms were evaluated in this work. Nearly three times as much Zn(II) adsorbed onto C carbon as on the other three carbon types. The adsorption isotherm for Zn(II) was dependent on solution pH since Zn(II) did not adsorb to carbon below pH 2, and the adsorption isotherm increased as pH increased from 3 to 7. The adsorption isotherm of Zn(II) on C carbon was temperature independent while on F-400 the isotherm showed unusual behavior as temperature increased.     

Effect of the surface heterogeneity of the stationary phase on the range of concentrations for linear chromatography

The range of sample sizes within which linear chromatographic behavior is achieved in a column depends on the surface heterogeneity of the RPLC adsorbents. Two widely different commercial adsorbents were tested, the end-capped XTerra-C18 and the non-end-capped Resolve-C18. Adsorption isotherm data of caffeine were acquired by frontal analysis. These data were modeled and used to calculate the adsorption energy distribution (AED). This double analysis informs on the degree of surface heterogeneity. The best adsorption isotherm models are the bi-Langmuir and the tetra-Langmuir isotherms for XTerra and Resolve, respectively. Their respective AEDs are bimodal and quadrimodal distributions. This interpretation of the results and the actual presence of a low density of high-energy adsorption sites on Resolve-C18 were validated by measuring the dependence of the peak retention times on the size of caffeine samples (20-microL volume, concentrations 10, 1, 0.1, 1 x 10(-2), 1 x 10(-3), 1 x 10(-4), and 1 x 10(-5) g/L). The experimental chromatograms agree closely with the band profiles calculated from the best isotherms. On Resolve-C18, the retention time decreases by 40% when the sample concentration is increased from 1 x 10(-5) to 10 g/L. The decrease is only 10% for Xterra-C18 under the same conditions. The upper limit for linear behavior is 1 x 10(-4) g/L for the former adsorbent and 0.01 g/L for the latter. The presence of a few high-energy adsorption sites on Resolve-C18, with an adsorption energy 20 kJ/mol larger than that of the low-energy sites while the same difference on Xterra is only 5 kJ/mol, explains this difference. The existence of adsorption sites with a very high energy for certain compounds affects the reproducibility of their retention times and a rapid loss of efficiency in a sample size range within which linear behavior is incorrectly anticipated.     

Numerical solutions of the adsorption integral equation utilizing the spline functions

Detailed studies dealing with numerical solutions of the fundamental integral equation representing the overall adsorption isotherms are carried out utilizing different types of spline function. The successive use of the smoothing cubic spline functions is recommended to calculate the energy distribution function from the adsorption isotherm. A new computer program EDCAIS (energy distribution computation from adsorption isotherm utilizing the smoothing spline functions) is developed for quasi-Langmuir local isotherms involving multilayer correction. EDCAIS is tested using the theoretical adsorption isotherms relating to five types of energy distribution: constant distributions and distributions composed of one, three and five gaussian peaks.     

Ce改性金属有机骨架材料对氟的吸附

利用水热合成法，将金属Ce与合成金属有机骨架材料(MOFs)所需的反应前体混合，通过“一锅法”和“两步法”分别合成性能不同的Ce/MOF-5材料。采用SEM、XRD、BET等对合成材料进行表征。结果表明:不同的方法合成的Ce/MOF-5形貌有较大差异，对氟吸附性能也不同。并测定了初始浓度、pH值、吸附时间对F?吸附效果的影响。实验表明，通过“一锅法”合成的Ce/MOF-5材料对F?的吸附在pH=7、吸附时间为60 min左右即可达到吸附平衡，吸附量为109.6 mg·g?1，符合准二级动力学模型和Freundlich等温吸附模型。      

Adsorption mechanism in RPLC. Effect of the nature of the organic modifier

The adsorption isotherms of phenol and caffeine were acquired by frontal analysis on two different adsorbents, Kromasil-C18 and Discovery-C18, with two different mobile phases, aqueous solutions of methanol (MeOH/H2O = 40/60 and 30/70, v/v) and aqueous solutions of acetonitrile (MeCN/H2O = 30/70 and 20/80, v/v). The adsorption isotherms are always strictly convex upward in methanol/water solutions. The calculations of the adsorption energy distribution confirm that the adsorption data for phenol are best modeled with the bi-Langmuir and the tri-Langmuir isotherm models for Kromasil-C18 and Discovery-C18, respectively. Because its molecule is larger and excluded from the deepest sites buried in the bonded layer, the adsorption data of caffeine follow bi-Langmuir isotherm model behavior on both adsorbents. In contrast, with acetonitrile/water solutions, the adsorption data of both phenol and caffeine deviate far less from linear behavior. They were best modeled by the sum of a Langmuir and a BET isotherm models. The Langmuir term represents the adsorption of the analyte on the high-energy sites located within the C18 layers and the BET term its adsorption on the low-energy sites and its accumulation in an adsorbed multilayer system of acetonitrile on the bonded alkyl chains. The formation of a complex adsorbed phase containing up to four layers of acetonitrile (with a thickness of 3.4 A each) was confirmed by the excess adsorption isotherm data measured for acetonitrile on Discovery-C18. A simple interpretation of this change in the isotherm curvature at high concentrations when methanol is replaced with acetonitrile as the organic modifier is proposed, based on the structure of the interface between the C18 chains and the bulk mobile phase. This new model accounts for all the experimental observations.     

金属-有机骨架材料MOF-199对甲醛气体吸附行为的研究

初步探讨了金属-有机骨架材料MOF-199对甲醛气体的吸附性能.采用分光光度法测定MOF-199对甲醛气体的吸附量,研究了吸附量与吸附温度及吸附时间的关系,并探讨了MOF-199对甲醛的吸附机理,提出了一种测定MOF-199对甲醛吸附量的方法.结果表明,在50℃、6h的吸附条件下,MOF-199对甲醛气体的吸附量最大,达到83.84mg/g;MOF-199对甲醛气体具有较好的吸附效果.     

Adsorption of Pb(II) and Cd(II) metal ions from aqueous solutions by mustard husk

The adsorption of Pb(II) and Cd(II) metal ions on mustard husk has been found to be concentration, pH, contact time, adsorbent dose and temperature dependent. The adsorption parameters were determined using Langmuir and Freundlich isotherm models. The adsorption isotherm studies clearly indicated that the adsorptive behavior of Pb(II) and Cd(II) metal ions on mustard husk satisfies not only the Langmuir assumptions but also the Freundlich assumptions, i.e. multilayer formation on the surface of the adsorbent with an exponential distribution of site energy. Ion exchange and surface complexation are the major adsorption mechanisms involved.

The applicability of Lagergren kinetic model has also been investigated. Thermodynamic constant (k_ad), free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) were calculated for predicting the nature of adsorption. The results indicate the potential application of this method for effluent treatment in industries and also provide strong evidence to support the adsorption mechanism proposed.     

Investigations on physicochemical properties of halophosphate surfaces from liquid adsorption measurements

Investigations of the adsorption and thermodesorption of n-octane and water from phosphor samples are presented. The dispersion and non-dispersion components of the surface free energy were calculated. Correlations between adsorption capacity and the components of surface free energy and luminous characteristic of the examined phosphors were found. The luminous quality of phosphors increased with the increasing hydrophilic nature of their surfaces.     

Adsorptive removal of methyl orange and methylene blue from aqueous solution with a metal-organic framework material, iron terephthalate (MOF-235)

An iron terephthalate (MOF-235), one of the metal-organic frameworks (MOFs), has been used for the removal of harmful dyes (anionic dye methyl orange (MO) and cationic dye methylene blue (MB)) from contaminated water via adsorption. The adsorption capacities of MOF-235 are much higher than those of an activated carbon. The performance of MOF-235 having high adsorption capacity is remarkable because the MOF-235 does not adsorb nitrogen at liquid nitrogen temperature. Based on this study, MOFs, even if they do not adsorb gases, can be suggested as potential adsorbents to remove harmful materials in the liquid phase. Adsorption of MO and MB at various temperatures shows that the adsorption is a spontaneous and endothermic process and that the entropy increases (the driving force of the adsorption) with adsorption of MO and MB.     

Heterogeneity analysis of single-walled carbon nanotubes from the adsorption equilibria of nitrogen and benzene

Nitrogen adsorption/desorption isotherms and gravimetric methods were employed to examine the structural and adsorption properties of selected adsorbent. The equilibrium data of benzene were also obtained at three different temperatures (303.15, 313.15, and 323.15 K) with pressures up to 7 kPa. The results of nitrogen and benzene sorption isotherm revealed that SWCNTs exhibit type II with the features of type I. The Toth and UNILAN models were found to provide a reasonable correlation between the adsorption isotherm data. In addition, the adsorption second virial coefficient and the isosteric heat of adsorption were determined by using these isotherm models. The isosteric heat of adsorption and adsorption energy distribution indicated that SWCNTs have energetically and structurally heterogeneous surfaces.     

Characterization 0f solid-liquid adsorption systems by using Gaussian energy distribution

Theoretical foundations of the isotherm equations derived for adsorption from binary liquid mixtures composed of molecules of different sizes on heterogeneous surfaces characterized by Gaussian energy distribution are discussed. On the basis of numerical simulation studies the correlation between adsorption parameters of the equations in question and the parameters of the Gaussian energy distribution are established. These studies allow characterization of the Gaussian-type adsorbents on the basis of excess adsorption data.     

Modeling of the separation of the enantiomers of 1-phenyl-1-propanol on cellulose tribenzoate.

The competitive adsorption isotherms of rac-1-phenyl-1-propanol on cellulose tribenzoate were measured by competitive frontal analysis. The experimental data were fitted to four different isotherm models: Langmuir, Bilangmuir, Langmuir-Freundlich, and Tóth. The fittings of the experimental data to all four models were satisfactory. It was excellent in the case of the Langmuir-Freundlich and the Tóth models. Overloaded elution profiles calculated with the Tóth isotherm were in good agreement with the experimental profiles in all the different experimental conditions investigated. This work extends to the case of binary mixtures the equivalence between the general rate and the lumped pore diffusion models already demonstrated for pure compounds when the ratio between the Stanton and the Biot numbers exceeds 5. The adsorption energy distribution for the Tóth isotherm was also calculated.     

Localized adsorption on heterogeneous surfaces

The general theory of localized adsorption on patchwise heterogeneous surfaces, in both the submonolayer and the multilayer range, is presented in this work.Exact and approximate mathematical methods for finding the binding energy distribution are considered for the Langmuir adsorption isotherm.It is shown that all the methods described (related to the solution of Fredholm integral equations of the first kind) can also be applied to the Brunauer-Emmett-Teller multilayer isotherm through the definition of the “enhanced pressure”.The physical interpretations are applied to the Dubinin-Radushkevich isotherm which consequently must be modified, with respect to the original formulation, in the very low pressure limit and in the multilayer range.     

离子交换纤维对绿原酸吸附特性的研究

研究了绿原酸在自制的强碱性阴离子交换纤维上的静态吸附行为，结果表明：该离子交换纤维对绿原酸的静态饱和吸附容量为252．2mg／g；等温吸附曲线服从Langmuir等温吸附式和Freundlich经验式；表观活化能Eα=15．498kJ／mol；表观速率常数随温度的升高而增大。用一定体积比的稀HCl-甲醇复合溶剂可将纤维上的绿原酸洗脱。     

Adsorption and thermo desorption characteristics of hydrocarbons on single walled carbon nanotubes

We investigated the heterogeneous adsorption and thermal desorption behaviors of acetone, n-hexane and trichloroethylene (TCE) on single walled carbon nanotubes (SWCNTs). Adsorption isotherms for selected molecules on SWCNTs were measured using a quartz spring balance at temperatures ranging from 303.15 to 323.15 K. Thermal gravimetric desorption experiments were also conducted at different heating rates (2-10 K/min) to obtain information about the interaction strength of hydrocarbons with SWCNTs surfaces. The adsorption isotherm data were analyzed successfully with the temperature dependent Toth equation. To obtain the adsorption and desorption energy distribution functions (AED/DED) for hydrocarbons and nitrogen, the integral equation with Fowler-Guggenheim isotherm (for AED) and first order desorption rate equation (for DED) were solved using the generalized nonlinear regularization method. It was found that Henry's constants, the isosteric heats of adsorption, and the pattern of energy distribution function were highly dependent on the polarizability.     

Dealumination of zeolites: Part V. Influence of the hydrothermal treatment of offretite on its pore structure and acid properties

The pore distribution of a series of offretite samples dealuminated through self-steaming at various temperatures was characterized by adsorption of nitrogen, n-hexane, and 3-methylpentane, and their acidity, by ammonia adsorption followed by calorimetry and by ammonia thermodesorption. From the analysis of the nitrogen adsorption by the α_s-method, it was concluded that during dealumination at least a fraction of the microporous system, characteristic of offretite network, is converted into supermicropores (0.7−2 nm). The aluminum extraframework species resulting from the framework dealumination have practically no effect on the adsorption of nitrogen, but limit that of the n-hexane and, above all, that of 3-methylpentane. The dealuminated samples are much less acidic than is the starting offretite and the number of acid sites shown by ammonia thermodesorption is much smaller than the number of acid sites estimated from the unit cell formula. This can be related to the partial destruction of the zeolite framework owing to the formation of supermicropores, to the dehydroxylation, and maybe to the neutralization of the acid sites by cationic aluminum extraframework species created during dealumination process.     

The adsorbate-adsorbate association model in localized adsorption of gases on random heterogeneous surfaces

In this paper we discuss the adsorbate-adsorbate association model, which assumes the formation of higher associates parallel to a heterogeneous surface with a random distribution of adsorption sites. The model calculations of the adsorption isotherms and isosteric heats were made using quasi-gaussian and exponential energy distributions. The critical properties of the adsorption isotherm equations, involving energetic heterogeneity and dispersion and association interactions in the surface phase, were also studied.     

Surface energy heterogeneity and heterogeneous adsorption of benzene on double-walled carbon nanotubes

We examined the adsorption properties of double walled carbon nanotubes (DWCNTs) synthesized by using a catalytic chemical decomposition vapor method. To analyze the structural and energetical surface characteristics of the DWCNTs, nitrogen and benzene adsorption isotherms were measured. The Toth isotherm equation was used to correlate the adsorption data and evaluate the thermodynamic properties such as Henry's constant and the adsorption second virial coefficient. The existence of heterogeneous surface in DWCNTs was confirmed by the analysis of pore size distribution, the isosteric heat of adsorption, and the adsorption energy distribution determined for nitrogen and benzene.     

Low-pressure adsorption of single gases on heterogeneous solid surfaces

A general isotherm equation is developed, to describe the low-pressure adsorption of single gases on the heterogeneous surfaces of real solids. This is a new approach, which is close to the Condensation Approximation, but yields correctly Henry's Law at very low adsorbate pressure. Special attention is given to the adsorption systems exhibiting the skew-gaussian distribution of the adsorption energy, which is responsible for the Dubinin-Raduskevich behaviour of the low pressure adsorption isotherms. The theory is illustrated by suitable model calculations, which explain all the deviations observed experimentally, from the traditional DR plot.