Research Article 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 solu- tions 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. Keywords: Equilibrium-dispersive model / Frontal analysis / Solvent adsorption DOI 10.1002/jssc.201000341 1 Introduction In RP HPLC with binary, water–organic mobile phase, an excess amount of the organic modifier is adsorbed on the stationary phase. The amount of adsorbed organic modifier strongly depends on the nature and the concentration of modifier in the eluent. The measurement of equilibrium isotherms is one of the most popular methods to characterize the thermodynamic parameters of the mole- cular interactions between solvent molecules and the stationary phase. The isotherm should give useful informa- tion about the retention of organic compounds and the properties of the stationary phase in the chromatographic columns [1, 2]. There are various static and dynamic chromatographic methods for isotherm measurement of the organic modifiers [1–4]. The separation of compounds in RPLC results from their different distributions between the mobile phase (usually aqueous solution of organic solvent) and the hydrophobic bonded layer. Solute molecules may adsorb on the stationary bonded phase surface. An alternative mechanism of the distribution is a partition between mobile and bonded phase [5]. The total amount of adsorbed organic solvent depends on the chemical parameters of bonded phases: the length and the number of organic ligands [6], the coverage density [7],] and the presence of polar group in the ligand structure [8]. During the recent years, the minor disturbance method has been developed and extensively used for the character- ization of solvent adsorption in HPLC [9, 10]. The adsorp- tion of organic solvents was under extensive research in the past decades [2, 10–13]. This method consists in introducing and measuring a small perturbation in a biphasic system under equilibrium. The composition of the investigated organic modifier in the mobile phase is changed stepwise from zero to 100 percent. The small perturbation is intro- duced by injecting pure organic modifier at each mobile phase composition, and the retention time is measured [2, 14–16]. The empirical evaluation of adsorption isotherms represents the basis for the modeling and the optimization of separations in preparative or semi-preparative chroma- tography [1, 17]. Frontal analysis (FA) is probably the most straightforward and certainly the most accurate method for measuring isotherms [17]. The interpretation of isotherms versus excess isotherms on liquid–solid interfaces is critical and contains simplifications. It has been shown by Schay [18] that if the mole fraction of the analyte in the solution is significantly smaller than unity, an approximation can be made that the adsorption process can be modeled with a function, the maximum or limiting value of which corre- sponds to the adsorption capacity of the adsorbents. Those functions are the isotherm models. The isotherms of methanol, acetonitrile, and THF were measured by McCormick and Karger [12] using gas chromatography in the range of 0–30% of organic modifier. The experimental technique of adsorption isotherm Pe ´ ter Vajda 1 Szymon Bocian 2 Bogus$aw Buszewski 2 Attila Felinger 1 1 Department of Analytical and Environmental Chemistry, University of Pe ´ cs, Pe ´ cs, Hungary 2 Department of Environmental Chemistry and Bioanalytics, Faculty of Chemistry, Nicolaus Copernicus University, Torun, Poland Received May 11, 2010 Revised June 29, 2010 Accepted September 1, 2010 Abbreviations: AED, adsorption energy distribution; FA, frontal analysis Correspondence: Professor Attila Felinger, Department of Analytical and Environmental Chemistry, University of Pe ´ cs, Ifju ´ sa ´g u ´ tja 6, H-7624 Pe ´ cs, Hungary E-mail: felinger@ttk.pte.hu Fax: 136-72-501-518 & 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com J. Sep. Sci. 2010, 33, 3644–3654 3644