Anal. Chem. 1995, 67,3441 -3447
TemperaturelJump Investigation of Adsorption/ Desorption Kinetics at Methylated Silica/Solution Interfaces F. Y. Ren, 8. W. Waite, and J. M. Harris* Department of Chemistv, Univecsity of Utah, Salt Lake City, Utah 84 7 72
A temperature-jump relaxation technique is used to monitor reversible adsorptioddesorptionkinetics at the reversed-phaseCl-silica/solutioninterface. A Joule discharge is used to heat a packed bed of trimethylchlorosilane-derivatizedsilica gel on a microsecond time scale. Single-exponential relaxation kinetics are observed for adsorption of an ionic fluorescent probe, l-anilino-8naphlldendonate, to a C1-silicasurface h m methanol/ water solution. The relaxation rate increases with concentration of solute in solution, which shows that adsorption kinetics are detectable in the relaxation. The adsorptionrate of the ionic probe is slower than diffusioncontrolled, exhibiting significant influence Over the adsorption equilibrium constant. The adsorption rate of N-phenyl-1-naphthylamine is indistinguishable from the diffusion limit, indicating a negligible barrier to adsorption for this neutral species. Separations in reversed-phase liquid chromatography result from complex thermodynamic and kinetic processes involving the transfer of solutes between the mobile and stationary phases. Knowledge of kinetics in this process is important for the design of effective column materials and for fundamental understanding of the chemistry of bonded phases and the retention behavior of analytes. The kinetics of chromatographic retention on bonded hydrocarbon stationary phases are not well-understood. Two approaches to gaining information about adsorption/desorption kinetics in chromatographic systems have been developed. Chromatographic techniques have been used to investigate these kinetics by measuring plate heights versus flow velocity and correcting for the estimated contributions from dispersion and diffusion.' Despite questions raised about the assumptions underlying this method2 and errors associated with the fitting of peak shapes to determine kinetic parameters? two general conclusions can be made from band-broadening studies of adsorp tion/desorption kinetics: that slow rates of desorption dominate kinetic contributions to band broadening and that barriers to adsorption are much smaller and di€Ecult to detect in the shape of eluted peaks. A more direct approach to measuring adsorption/desorption rates is the use of relaxation kinetic methods. In relaxation kinetics, the equilibrium of a reversible process is shifted by a rapid change of conditions, such as temperature, pressure, or (1) Horvath, Cs.; Lin, H. J.J. Chromufogr. 1978,149,43-70. (2) h o l d , F. W.; Blanch, H. W.; Wilke, C. W.1. Chromufogr.1985,330,159166.
(3) Lenhoff, A. M. 1.Chromutgr. 1987,384,285-299.
0003-2700/95/0367-3441$9.00/0 0 1995 American Chemical Society
electric fieldO4Pressurejump techniques are particularly useful for studying adsorption or binding kinetics of charged species due to the large molar volume change associated with ionic solvation; these methods have been successfully adapted to study proton transfer rates at oxide ~ u r f a c e P -and ~ more recently to investigate ion-pair adsorption/desorption kinetics at alkylated silica surfaces!.g A temperaturejump perturbation can be used for studying adsorption kinetics of nonionic species, since most adsorption equilibria exhibit a nonzero enthalpy and are thus temperature dependent. A Joule discharge was recently adapted to rapid heating of packed-bed samples of porous silica getlo it was demonstrated that the itzterior surfaces of chromatographic silica gel could be heated in a few microseconds. Heating rates were measured to study the effects of surface modification and pore size on the connectivity of the pore network.'l More recently, the rates of solute sorption/desorption kinetics at alkylated-silica/ solution interfaces have been measured by both Joule discharge heating and laser temperaturejump methods.12J3 In the Joule heating study, relaxation kinetics for an ionic fluorescent probe, 1-anilino-haphthalenesulfonate (ANS) , and an uncharged probe of otherwise similar structure, 1-phenyl-1-naphthylamine(1-NPN), were measured at a C18modified silica surface.12For ANS, a twocomponent relaxation was observed, where one relaxation exceeded the rate of heating Q ? 5 x 1Oj s-'> and where the rate of the slower relaxation increased with increases in retention of the solute due to changes in solvent composition. A two-step adsorption/partition model was developed to explain these results, where the faster relaxation was assigned to partitioning of adsorbed probe into the C18 layer while the slower rate was assigned to initial adsorption of solute from solution. The rate of the slow relaxation varied linearly with the concentration of the (4) Bemasconi, C. F. Relamtion Kinetics; Academic Press: New York, 1976.
(5) Ashida, M.; Sasaki, M.; Kan, H.; Yasunaga, T.; Hachiya, K.; Inoue, T.].Colloid Interface Sci. 1978,67,219. (6) Astumian, R D.; Sasaki, M.; Yasunaga.T.; Schelly, Z. A]. Phys. Chem. 1981, 85,3832-3835. (7) Ikeda, T.;%saki. M.; Hachlya, K.; Astumian, R D.; Yasunaga, T.; Schelly, Z. A. J. Phys. Chem. 1982,86,3861. (8) Marshall, D. B.; Bums, J. W.; Conolly, D. E. ]. Chromufogr, 1986,360, 13-24. (9) Marshall, D.B.; Bums, J. W.; Conolly, D. E.]. Am. Chem. Soc. 1986,108, 1087- 1088. (10) Waite, S.W.; Harris, J. M.; Ellison, E. H.; Marshall, D. B. Anal. Chem. 1991, 63,2365-2370. (11) Ellison, E.H.; Waite, S.W.; Marshall, D. B.; Harris, J. M.Anul. Chem. 1993, 65,3622-3630. (12) Waite, S.W.; Marshall, D. B.; Harris, J. M. Anal. Chem. 1994,66,20522061. (13) Waite, S. W.; Holzwarth, J. F.; Hanis, J. M. Anal. Chem. 1995,67,13901399.
Analytical Chemistry, Vol. 67, No. 19, October 1, 1995 3441
solute in solution as expected for an adsorption step, and the rate constant was less than diffusion-limited. The adsorption rate of a neutral probe, however, was indistinguishable from the diffusion limit, indicating a negligible barrier to adsorption for the nonionic solute. To establish whether this two-step adsorption/partition mechanism is a reasonable kinetic model for sorption of ionic species at reversed-phase chromatographic interfaces, the present work considers adsorption/desorption kinetics at a methylated silica/solution interface. Using a Clderivatized silica surface, the kinetics of adsorption can be investigated free of any intercalation or partition into the surface ligands. In addition, attaching shorter ligands to the surface would eliminate any slow heating effects on the kinetic response that arise from pores blocked by longer chain ligands." An initial study of ANS adsorption kinetics in a suspension of methylated fumed silica was carried out using an iodine laser temperaturejump te~hnique.'~ The temperature-jump relaxation study was used to determine the effect of electrolyte on adsorption of a charged solute onto a C1-silica surface since Joule heating requires the addition of electrolyte while laser heating does not. Without electrolyte, the relaxation signal was biexponential, which was also reflected in a broad chromatographic peak shape and a two-site adsorption isotherm; when electrolyte was added, the relaxation signal became a nearly pure single exponential and the adsorption rate increased beyond the capabilities of the measurement. The effect of added electrolyte on the relaxation could be explained by compression of the electrical double layer at the silica surface, which reduces the charge repulsion between negatively charged ANS and deprotonated surface silanols, leading to a more homogeneous surface environment. Despite the utility of the laser temperature-jump technique for investigating the effect of electrolyte on adsorption/desorption kinetics, the method could not generate excursions in temperature of more than a few degrees, and the stability and scattering of colloidal dispersions limited the concentration of surface sites present in the sample. These restrictions combined to lower the sensitivity for detecting relaxation transients so that adsorption kinetics at C1-silica surfaces could only be collected for a singlesolvent condition corresponding to very high solute retention. Since the adsorption kinetics were homogeneous with electrolyte in the solvent, a more thorough investigation of these kinetics is possible by Joule discharge heating, which requires electrolyte to carry the current. In this work, therefore, a Joule heating temperature-jump perturbation is used to investigate adsorption/ desorption kinetics of ionic and nonionic solute probes at a C1silica surface in contact with a variety of methanol/water solutions. The adsorption rate of the ionic ANS probe is indistinguishable from the rate of the initial adsorption step observed at ClSsilical solution interfaces;I2 lack of a kinetic barrier to adsorption of neutral solutes to both C1 and C18 surfaces indicates that the nature of the adsorption barrier for the charged solute, ANS, is likely due to ionic solvation, which must be displaced to accommodate adsorption to the hydrophobic surface. The results show that similar adsorption equilibria can arise from quite different underlying kinetic mechanisms. EXPERIMENTAL SECTION
Materials. The substrate silica gel used in this study was Licrosorb Si-60, 5pm particle size, having a mean pore diameter of 60 A and a surface area of 550 m2/g (by N? BET). Trimethylchlorosilane (TMCS) was purchased from Petrarch. Methanol, 3442
Analytical Chemisrry, Vol. 67, No. 19, October 1, 1995
toluene, chloroform, tetrahydrofuran, and acetonitrile solvents were all spectral grade (OmniSolve). The hydrophobic ionic probe, 1-anilino-gnaphthalenesulfonate ammonium salt (ANS; Aldrich) and nonionic probe, N-phenyl-1-naphthylamine(1-NPN; Aldrich) were used as fluorescence probes for sorption/desorption studies. The fluorescence of both ANS and 1-NPN exhibits a negligible sensitivity to changing temperature; both produce greater fluorescence intensity in nonpolar envir~nments.'~J~ Since the excitation and emission wavelengths of the two probes fall in the region where silica is transparent, the fluorescence signals report the adsorption/desorption kinetics in the interior of the silica particles where the vast majority of the surface area is located. Solutions of the probes were prepared of methanol (OmniSolve), and glass distilled, B M Q water with sodium chloride (Mallinckrodt) was added electrolyte for both temperature-jump and chromatographic experiments. Synthesis of Bonded Phase. The silica gel was surfacemodfied by TMCS according to the following procedure. An amount (3 g) of silica gel was placed in a dry round-bottom flask; the flask was evacuated to submillitorr pressures, heated to 110 "C, and kept under vacuum for 24 h. Dry toluene (30 mL, stored over molecular sieves) and 3 mL of triethylamine (Fisher Scientific) were added to the reaction vessel. A 5fold molar equivalent excess of TMCS (based on a silanol density of 8 pmol/ m2 l4 ) was dissolved in 10 mL of toluene and transferred to the vessel. The reaction mixture was heated to reflux for 2 h. After cooling, the derivatized silica was washed with 6 x 150 mL of toluene, chloroform, tetrahydrofuran, acetonitrile, and methanol. The derivatized silica was then air-dried, placed in a clean roundbottom flask, evacuated to submillitorr pressure, and heated to 50 "C under vacuum for 24 h to remove all traces of the solvent. Elemental carbon and hydrogen analysis was performed by M-H-W Laboratories Phoenix, AZ). The silica was found to have a specific fraction of 4.6%carbon and 1.4%hydrogen, corresponding to 2.5 pmol/m2 based on the elemental analysis and the N2 BET surface area. ChromatographicMeasurements. All chromatographic data were obtained with a high-performance liquid chromatography system consisting of a Beckman Model 210 injector,an 1x0 Model 2350 isocratic pump, a 150 mm x 4.6 mm i.d. Licrosorb 5pm Si60 C1 column packed in-house (see below), and an 1 x 0 Model 229 W/visible absorbance detector operated at 253 nm. An Eppendorf CH-30 column heater coupled with an Eppendorf TC50 temperature controller was employed to measure temperaturedependent retention. Mobile-phase flow rate for all retention measurements was 1.0 mL/min. In addition, adsorption isotherm experiments were performed via the frontal elution method'j using a dual-pump systemI2 with detection by a Beckman Model 153 W/visible detector operated at 365 nm. Deuterium oxide was used as a dead-volume marker. The column used in the chromatographic measurements was packed from a 2-propanol/Cl-silica gel slurry using a Shandon column packer equipped with a Keystone Scientific slurry reservoir; the column was then conditioned with 50/50 methanol/water solvent. Spectral grade methanol (OmniSolve) was used without further purification. The mobile phase used for these studies was (14) Unger, K K. Porous Silica; Elsevier: Amsterdam, 1979. (15) Katti, A. M.; Guiochon, G. A. In Advances in Chromatography; Giddings, J . C., Ed.; Marcel Dekker: New York, 1991;Vol. 31, Chapter 1.
Table 1. Adsorptlon Enthalpies for Fluorescent Surface Probes on C1.Silica versus Solvent Composition
b
MeOH/H2OU (v/v)
low
u-
50/50
40/60 30/70
ANS 0.9976 0.9981 0.9969
AH W m o l ) -16.3 f 0.8 -17.9 f 0.5 -19.5 f 0.5
NPN
M e r
Mwr
$b
70/30
50/50
