Anal. Chem. 1995, 67,1390-1399
Laser Temperature Jump Relaxation Measurements of Adsorption/Desorption Kinetics at Liquid/Solid interfaces S. W. Waite,t J. F. Holzwarth,* and J. M. Hads*st Fritz-Haber-lnstitut, Mau-Planck-Gesellschaft, Faradayweg 4- 6, D 14195 Berlin-Dahlem, Gennany, and Department of Chemistty, University of Utah, Salt Lake City, Utah 841 12
The iodine laser temperature jump method is used to study adsorptioddesorption kinetics at a methylated silica/solution intehce. A suspension of C1-derivatized fumed silica is used for the kinetic measurements. The colloidal silica does not significantly change the attenuation of near-IRradiation from the iodine laser and allows the surface site concentration to be varied so that adsorption and desorption rates can be determined. The temperature jump relamtion method was used to investigate the effect of electrolyte on adsorption of a charged solute (ANS) on a C1 silica surface. Adsorption equilibrium conditions were optimized to observe a maximum relaxation signal. Without electrolyte, the relaxation signal is biexponential, which is also reflected in a broad chromatographicpeak shape and a two-site sorption isotherm. When electrolyte is added, the relaxation signal is primarily single exponential, which agrees with the linear adsorption isotherm. The adsorption rate and equilibrium constant were found to increase si8nificantly with added electrolyte, which showed that adsorption kinetics can influence both band broadening and retention. The interface between liquids and dielectric solids and plays an important role in many analytical chemistry processes including adsorption,desorption,monolayer self-assembly,and reactions of immobilized reagents. Most of our Understanding of adsorption of molecules at dielectric liquid/solid interfaces has been acquired from chromatographic measurements which allow small differences in retention equilibria to be observed. The subtlety of molecular interactions at the interface and mechanisms of adsorption or partition cannot generally be resolved through equilibrium measurements alone. Spectroscopic methods, for example, have been used to probe the structure of stationary-phase ligands and sorbed solutes in reversed-phase chromatographic Direct measurements of adsorption and desorption rates are needed, however, for understanding the kinetic barriers for interaction at the interface to improve our knowledge of chromatographic retention mechanisms and kinetic sources of band broadening. Relaxation kinetic methods have been developed for reversible reactions in homogeneous liquids where the equilibrium of a reversible reaction is shifted by a rapid change of conditions such ~
~~~~~~~~~~
University of Utah. 4 Max-Planck-Gesellschaft. (1) Rutan, S. C.; Harris, J. M. /. Chromatrogr A 1993,656,197-215. (2) Sentell, K B.J. Chromafogr.A 1993,656,231-263. (3) Wirth, M. J. LC-GC 1994,12, 656-664. +
1390 Ana/ytica/ Chemistry, Vol. 67, No. 8, April 15, 1995
as temperature, pressure, or electric field strength.415 Several successful examples of using relaxation methods to study interfacial chemical kinetics have been published; these studies have employed a pressure jump approach. The pressure jump perturbation is suitable for studying sorption/desorption of charged species due to the large change in molar volume that accompanies the process. Proton-transfer rates on titanium and iron oxides617 and proton-transfer and adsorption rates for acetate on silicaalumina surfaces* have been determined by pressure jump techniques using conductivity detection. More recently, ion pair sorption/desorption rates at alkylated silica interfaces were measuredgJOby using fluorescence detection of the adsorbed probe. These latter studies showed that a distribution of desorption rates was correlated with chromatographic band broadening. In more recent work, a temperature jump relaxation approach has been developed for investigating sorption/desorption kinetics at alkylated silica/solution interface^.*^-'^ A temperature jump is a more versatile relaxation method since most chemical equilibria derive from a nonzero enthalpy and are thus temperature dependent. Joule discharge heating was adapted to packedbed samples of porous silica gel; the method was found to be capable of uniformly raising the temperature of the porous silica on a time scale of microseconds if particle size of the silica was 10pm or smaller and heating rates were kept below 2 x 105 s-1.11J2 Using this method, sorption/desorption relaxation kinetics were observed for a fluorescent probe, 1-anilindnaphthalenesulfonate (ANS), at a Clgmoditled silica surface.13 On a 100 ps time scale, a slow relaxation was detected having a rate that increased as retention of the solute was increased by changes in mobilephase composition. This behavior suggested that sorption kinetics were controlling the relaxation rate; the linear dependence of the rate on the concentration of the probe in the mobile phase verified (4) Eigen, M. Discus. Faraday SOC.1954,17, 194-205. (5) Bernasconi, C. F. Relawtion Kinetics; Academic Press: New York, 1976. (6)Ashida, M.: %saki, M.; Kan, H.; Yasunaga, T.; Hachiya, K; Inoue, T.J. Colbid Inte$uce Sei. 1978,67,219-225. (7) As"ian, R D.; %saki, M.; Yasunaga, T.; Schelly, Z. A J. Phys. Chem. 1981, 85, 3832-3835. (8) Ikeda, T.;Sasaki, M.; Hachlya, IC;Astumian, R D.; Yasunaga, T.: Schelly, Z . A J. Phys. Chem. 1982,86, 3861-3866. (9) Marshall, D. B.; Bums, J. W.; Conolly, D. E. J. Chromatogr. 1986,360, 13-24. (10) Marshall, D.B.; Bums, J. W.; Conolly, D. E. J. Am. Chem. SOC.1986,108, 1087- 1088. (11) Waite, S. W.; Harris, J. M., Ellison, E. H., Marshall, D. B. Anal. Chem. 1991, 63,2365-2370. (12) Ellison, E. H.; Waite, S. W.: Marshall, D. B.; Hanis, J. M. Anal. Chem. 1993, 65,3622-3630. (13) Waite, S. W.; Marshall, D. B.; Hanis, J. M. Anal. Chem. 1994,66,20522061.
0003-2700/95/0367-1390$9.00/0 Q 1995 American Chemical Society
this postulate. The results showed further that the adsorption rate of this ionic probe is slower than diffusion-limited and exhibits significant innuence over the equilibrium constant. The sorption rates of two neutral probes, however, were indistinguishablefrom a diffusion limit, indicating a negligible barrier to sorption. The results showed that similar retention equilibria can arise from different underlying kinetic mechanisms. Despite the success of the Joule heating temperature jump experiment, this approach has several shortcomingsfor investigating interfacial reaction kinetics. These problems include the inability to vary the concentration of surface sites in the measurement and the requirement of added electrolyte to allow Joule discharge heating; both of these shortcomings are addressed in the present work. In relaxation kinetic measurements, it is desirable to keep one of the reacting species in large excess. This allows a linearization of the rate equation so that the kinetic signal can be modeled as pseudo first order.5 Under these conditions, the kst-order relaxation rate for an adsorption equilibrium is given by
where k& is the adsorption rate constant, k d is the desorption rate constant, and [SI is the concentration of available surface sites for adsorbing the molecule at concentration [MI in solution. In order to determine the rate of adsorption and desorption, the concentration of M or S must be varied and a plot ofthe observed relaxation rate versus the concentration should yield a straight line, with a slope equal to kads and an intercept that can be related to the desorption rate constant if the concentration of surface sites is accurately known. It is advantageous to vary the concentration of the excess component, as its much larger concentration will have a significant effect on the observed rate. In the Joule heating experiment13 the concentration of the probe molecule [M] was varied in the micromolar range, and the concentration of surface sites was held constant. The density of surface sites in a packed bed of Cl&modifed silica was measured by frontal elution chromatographyto be 1.9 M, many orders larger than the probe solute. While it would have been preferable to vary the surface site concentration,this is not possible when silica gel is used due to the porous nature of the material. The diffusion distance for a small probe molecule on the time scale of the experiment (-100 ps) is only 600 nm, while the average particle diameter of the silica was 5 pm. Since 9% of the surface area of porous silica gel is within the internal pore ~tructure,'~ the probe only samples the interior of one silica particle during the relaxation experiment. Simply changing the density of silica particles does not change the effective concentration of surface sites available to the probe during the kinetic experiment. If the number of available surface sites can be reduced and their concentration controlled, then the rates of the observed relaxation signal can be slowed, allowing faster adsorption rates to be measured and the desorption rate to be determined. Recently, hydrophobic fumed (nonporous) silicas have become commerciallyavailable. These silicas are nonporous, with particle diameters in the 10 nm range (on the order of the pore size of porous silica) and surface areas of 100-200 m2/g. The silica surface has been chemically modified with a high density of (14)Roumeliotis, P.;Unger, K K ]. Chromatogr. 1978, 149, 211-224.
methyl groups during synthesis. Dispersions of fumed silica of this trpe for studies of heterogeneous reaction kinetics should allow controlled variation of the surface site concentration. From the diffusion rate of a small solute molecule in solution during a 100 ps lifetime of a typical temperature jump relaxation, the adsorbate would sample over 200 particles when colliding with 7 nm particles dispersed at a concentration of 5 mg/mL; this is a suf5ciently large number of sampled particles to allow a continuum model for the kinetics. Thus, the concentration of the surface sites can be easily varied by manipulating the particle concentration in the dispersion. The use of fumed silica particles provides a flexible approach to measure kinetics at liquid/solid interfaces and is one focus of this investigation. The use of Joule heating to achieve the fast temperature perturbation requires a high concentration of electrolyte to be added to the liquid phase to carry the current of the Joule discharge. The addition of electrolyte produces solution-phase conditions that are not normally encountered in reversed-phase chromatography, for example, and can perturb the adsorption equilibrium for some solutes. Producing temperature perturbations without added electrolyte would allow the investigation of interfacial kinetics under more normal chromatographic conditions and also should allow the influence of electrolyte on adsorption kinetics to be studied. An iodine laser temperature jump apparatus has been developed15J6 that produces temperaturejumps of several degrees in aqueous samples. The use of laser heating requires no electrolyte be added to the sample; furthermore, the optical properties of nanometer silica colloids are more compatible with laser heating than larger, highly scattering porous silica particles. This work, therefore, describes the measurement of interfacial kinetics at colloidal fumed silica/liquid interfaces by employing the iodine laser temperature jump technique. EXPERIMENTAL SECTION Materials. Two different fumed silicas were evaluated for use in the current experiment. Hydrophobic fumed silica, Aerosil R976, was obtained from the Degussa Corp. (Dublin, OH). This silica is nonporous and has a mean particle diameter of 7 nm and a surface area of 250 m2/g. It has been chemically modified with methyl groups during the manufacturing process to give a hydrophobic surface. The density of methyl groups on the surface was estimated from the mass fraction of carbon of the material, determined by microchemical analysis (M-H-W Labs); the material used was 1.44%carbon, corresponding to a high surface coverage of methyl groups of 4.8 pmol/m2. Cabosil NO,a hydrophilic fumed silica was used as received from the Cabot Corp. (Tuscola, IL). This silica has a surface area of 100 m2/g. The surface of the L90 silica was chemically modified with octadecyldimethylchlorosilane following a published procedure17to yield a hydrophobic C18 surface. The solvents for the temperature jump and chromatographic measurements consisted of methanol/water mixtures, with NaCl (15)Holzwarth, J. F.;Eck, J.; Gem,A In Spectroscopy and Dynamics ofMolecular Biological Systems Baily, P. M., Dale, R E., Eds.; Academic Press Inc.: London, 1985;pp 351-377. (16)Holzwarth,J. F.;Schmidt, A; WOW, H.;Volk, R]. Phys. Chem. 1977,81, 2300-2301. (17) Wong, A L.; Hunnicutt, M. L;Harris, J. M.Anal. Chem. 1991,63,10761081.
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added to some samples. Triply distilled water was used in the solvent mixture for temperature jump relaxation measurements, and distilled, deionized, 18 MQ water was used for chromatographic measurements. Uvasol grade methanol (Merk, Damstadt Germany) was used for temperature jump experiments and Omnisolve grade methanol (Merk) in chromatographic measure ments. The sodium chloride was reagent grade Wllinckrodt) for the chromatographic measurements and proanalysis grade (Merk) for the temperature jump experiments. Two hydrophobic probes whose fluorescence intensity is sensitive to polarity were evaluated 1-Anilinegnaphthalenesulfonateammonium salt WS) (Aldrich) and N-phenyl-1-naphthylamine(1-NPN) (Aldrich). A fluorescence temperature probe was used &(N-(7-nitrobenz-2oxa-1,3-diazol-4-yl)aminohexanoic acid (NBD) (Molecular Probes). Fumed silica particles were suspended into a stable colloidal dispersion by the following procedure. A known amount of the silica was weighed and placed in an erlenmeyer flask. Tne amount of methanol needed for the h a l methanol/water (v/v) concentration was pipeted into the Erlenmeyer flask and a magnetic stir bar added. This mixture was stirred for -30 min to wet the entire hydrophobic silica surface. Water was then pipeted at a slow rate into the stimng mixture until the desired mixture ratio was achieved. Dissolved air was liberated with the addition of water. These air bubbles were removed by alternating sonication with stirring until no bubbles were visibly present. This mixture was covered and left to mix overnight. The surface probe was not added until a few hours before the experiments were to be performed. Upon the cessation of stirring, the particles slowly settled to the bottom of the flask it took several hours, however, before any observable silica particle concentration gradient could be detected. In order to assure a uniform suspension, the colloidal mixture was constantly stirred. It was noticed that occasionally a colloidal suspension of silica particles was not stable and there was a rapid settling; when this occurred, the pH of the colloidal silica mixtures was measured, and it was found that the unstable mixtures were at a pH less than 6, while those that were stable had a pH of 7-8. Upon adjustment of the low-pH samples to a pH of 7, these samples also became stable. This behavior can be attributed to electrostatic repulsion between the negatively charged particles responsible for the stable dispersion;18 at higher pH, deprotonation of residual surface silanols generates a higher surface charge density and increased colloid stability. AnalyticalMeasurements. Chromatographic columns were packed from an 2-propanol/silica slurry in the upward direction followed by a 50:50 methanol/water conditioning solvent using a Shandon column packer equipped with a Keystone Scientific sluny reservoir. The base silica was Licrosorb Si-60 (5 pm particle size, 60 A pore size), which was chemically modilied with trimethylchlorosilane to yield trimethylsilyl ligands on the surface at a density of 2.34 pmol/m2 as estimated from the 4.64% C from microanalysis. Care was taken in weighing the silica added to the slurry reservoir, and in weighing the remaining silica so that the packing density of the columns could be calculated. The column size was 150 mm x 4.6 mm for the C1 silica. Chromatographic retention measurements were performed on a HPLC system consisting of a 1x0 Model 2350 isocratic pump, 1 x 0 type 11 W/visible absorbance detector operated at 253 nm, and Hasteloy tubing to avoid corrosion from the chloride
containing solutions. When temperaturedependent retention was measured, an Eppendorf CH-30 column heater coupled with an Eppendorf K - 5 0 temperature controller was added to the system. Sorption isotherms were measured by frontal elutionlg using a previously describedI3 dual-pumping system and a Beckman Model 153 UV/visible detector operated at 365 nm. Ultraviolet and visible absorbance measurements of the fluorescent probe molecules were performed on a Hewlett-Packard 8452A photodiode array spectrophotometer. Fluorescence spectra of the probes were acquired at various temperatures with a Farrand Optical Model 801 scanning fluorometer equipped with a jacketed cell holder. Temperature control was provided by a Haake D3 constant-temperaturebath, which circulated a silicon oil through the jacketed cell. Near-IR absorbance measurements of the colloidal silica were carried out on a Cary 17 spectrophotometer to assess absorption and light scattering in the wavelength region of the iodine laser output. Laser Temperature Jump Experiments. Figure 1shows a block diagram of the iodine laser temperature jump system. The temperature jump is performed inside a black box which holds a quartz sample cuvette. The cuvette is thermostated to 25 "C by an external jacket through which water is circulated by a Haake F3 constant-temperaturebath. The box is constructed with four ports for light inlet and outlet and one port for sample introduction and removal. This design keeps stray light from the room from entering the experiment. Rapid heating of the sample was provided by an iodine laser constructed locally at the Fritz-Haber Institute of the Max-Planck Society and has an output of 1.4 J at 1.315pm. The laser pulse is an initial sharp spike followed by an exponential decay of the radiation with a lifetime of 2.4 ps; the laser can be fired at a rate
(18) Ross, s.:Morrison, I. D. Colloidal Systems and Interfaces; Wiley New York, 1988;Part m.
(19) Katti, A. M., Guiochon, G. A. In Advances in Chromatography, Giddings, J. C.,Ed.: Marcel Dekker: New York, 1991: Vol. 31,Chapter 1.
1392 Analytical Chemisfty, Vol. 67, No. 8, April 15, 1995
Biamotion Digital Tr o n sie nt Recorder
HP 9816 I
I
I
ce
Lamp
I I I
/
--" focusing lens
/
optical filters
i .-C
U 0
-
Figure 1. Schematic diagram ofthe iodine laser temperature jump experiment. For detailed description, see text.
of 0.2 minl. The near-IR output from the laser is weakly focused to a spot size of -6 mm in a quartz cuvette with a heating path length of 5 mm. This provides a temperature jump of -1.5 K A Schott RG 780 filter for the iodine laser radiation is placed at the entrance port to the sample enclosure to keep stray visible light from entering the sample. An Oriel 200 W Hg/Xe arc lamp was used to excite the fluorescence of the probe molecules. Schott U G l l and WG320 filters were placed between the lamp and the sample to isolate the strong mercury lines in the 365 nm region. The light from the arc lamp is collimated by a condenser in the lamp housing and focused by two focusing lenses to a spot size just larger than the spot size of the laser. Additionally, a water filter is placed in front of the arc lamp to absorb the infrared radiation from the iodine laser. This is necessary to ensure that the arc lamp is not struck by the high photon density laser pulses. Fluorescence was collected along a 10 mm path length and passed through Schott GG405 short-wavelength cut filters and Andover 550 short-wavelength pass filters to remove scattered light from the excitation source and to reduce the amount of interfering light at longer wavelengths. It was necessary to restrict the detection wavelength to the fluorescence range of the probe because the large photon density of the iodine laser causes a prompt emission from the sample due to nonlinear optical effects (ionization or continuum generation) which appears to be highest at the cell walls. This interference is detected as a light flash during the first few microseconds and can corrupt the measured kinetic signal. By filtering out all radiation except that in the wavelength region of the probe fluorescence, the amplitude of the background flash is greatly reduced. Addition of fumed silica did not detectably increase the amplitude of the flash, indicating that ionization at particle surfaces was not occurring. This was probably prevented by the particle size (7 nm) being much smaller than the wavelength of the excitation radiation (1315 nm). As a further test, the temperature change observed in the sample with particles was slightly smaller than without particles (see Results and Discussion), indicating that no additional heating mechanisms were brought with the addition of fumed silica. The filtered fluorescence emission from the sample was focused onto two photomultiplier tubes (PMT) (EM1 type 9558QA, Middlesex, UK) at 90" from the excitation and heating sources and 180" from each other. The voltages on the PMTs were adjusted so that each signal had a total amplitude of 500 mV; 5-10 transients were acquired for signal averaging. The slow repetition rate of the laser made it necessary to mix the sample by inversion of the cuvette between laser shots. This ensured that settling of the silica particles and changes in the local surface site concentration did not occur. A fresh sample of colloidal silica was transferred to the cell after each three temperature jumps to ensure that any degradation products did not build up and interfere with the kinetic signal. The output from the photomultipliers was conditioned by a Dialog (Dusseldorf, Germany) amplifier and sent to a Biomation 8100 digital transient recorder. Two different time bases were used for each measurement in order to observe the signal over a long time frame while still preserving a high sampling frequency for the shorter time domain. Relaxation traces were viewed on a Tektronix oscilloscope consisting of 7904 mainframe and 7822 plug-in. The rise time of these electronics was measured to be 0.4 ,us. Relaxation traces were transferred from the Biomation
A
Flgure 2. Chromatographicelution of 100pM ANS from a C1 silica column with 5050 methanoVwater mobile phase: (A) no added electrolyte; (B) 0.4 M NaCl in the mobile phase.
over an IEEE bus for storage and analysis on a Hewlett-Packard 9816 computer. The data were fit to an exponential model by a least-squares routine previously described.20 This fitting routine was developed to fit noisy data with small relaxation amplitudes; it uses a Newton-Gauss method for the initial estimate of the fitting parameters, followed by a Levenberg-Marquardt nonlinear regression routine modified by the method of Jacobson.21 RESULTS AND DISCUSSION
Adsorption Characteristics of ANS at a C1 SurEace. In chromatographic measurements of ANS, it was found that electrolyte added to the solution phase significantly influences the retention equilibrium of this probe. On a C18 reversed-phase column the chromatographic capacity factor for the negatively charged ANS molecule decreased from a value of k' = 1.87 in 7030 methanol/water (v/v) to k' = 0.90 when sodium chloride was added at a concentration of 0.4 M. This resulting reduction in retention was not accompanied by any noticeable change in chromatographicpeak shape and is likely due to a simple saltingin effect in the solution phase (adding electrolyte at moderate concentrations lowers the activity of ionic solutes). In measurements of 1-NPN,the nonionic analog of ANS lacking the sulfonate group, the addition of sodium chloride did not measurably change retention on the C18 column. When adsorption of ANS was measured on a C1 reversedphase column, the retention of the probe increased with added electrolyte, where k' increased from 2.18 to 2.47 in 5050 methanol/water (v/v) when sodium chloride was added to a concentration of 0.4 M. A similar increase in retention was found when electrolyte was added to the solution phase at other methanol/water mixtures. It was also noted that the chromatographic peak shape of ANS drastically changed in the presence of electrolyte. Figure 2 shows the chromatograms of ANS with and without NaCl. It can be seen that when electrolyte is not present the ANS peak is broad and exhibits both fronting and tailing. When salt is present, the ANS elution profile narrows and becomes symmetric, showing that the broad shape in the absence of electrolyte originates in the retention process. The nonionic probe 1-NPN also exhibited a slight increase in retention in the presence of salt on a C1 column, but their peaks were narrow and symmetric under both conditions. The modifcation of the surface of silica gel with alkyl ligands leads to reaction with at most -50% of the surface silanols, due to steric hinderance between bound ligands.14 Residual silanol (20) Lerch, E.Masters Thesis, Free University, Berlin, 1983. (21) Botsaris, C. A; Jacobson. D. H. J Math. Anal. Appl. 1976, 54,217-229.
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sites are partially deprotonated at neutral pH, which leads to a negative surface charge that would repel a negatively charged ANS adsorbate. The C18 surface has an alkyl chain length of 18 carbons and a surface depth of -17 while a C1 surface has an alkyl chain length of one carbon and a surface depth of 5 5 A.l2 The surface potential due to deprotonated surface silanols is expected to play a much greater role on the C1 surface as compared with the C18 surface due to the double-layer thickness compared to the thickness of the alkyl ligands. The negative surface charge from the residual silanols causes a repulsion of the ANS from the C1 surface, reducing the retention equilibrium for adsorption. When sodium chloride is added to the solution phase, the thickness of the double layer is greatly reduced, which lowers the interfacial repulsion and increases retention of the negatively charged solute. The additional width and asymmetry of the chromatographicdata suggest that there is a dispersion in surface site interactions for adsorption of ANS to the C1 surface in the absence of electrolyte; this could be due to some sites experiencing a greater charge repulsion due to a variation in the density of the surface silanols. With the addition of ions to the mobile phase, a more homogeneous surface is realized by reducing the effect of surface charge density on retention. To check for the presence of a dispersion in adsorption interactions between ANS and a C1 surface in the absence of electrolyte, a adsorption isotherm was measured by frontal elution chr~matography.’~ In this method, a series of mobile phases with increasing solute concentrationare pumped through the column. M e r the solute comes to equilibrium with the surface the excess solute elutes off the column, and a “breakthrough” curve is observed. The amount of solute sorbed to the surface is given by19
where Qi+l is the moles of solute on the surface after breakthrough, C is the concentration of solute in the mobile phase, K+l is the breakthrough volume, V, is the system void volume, and a,is the surface area of the Clderivatized silica in the column. The adsorption isotherm for the uncharged probe 1-NPN is linear (correlation coefficient, 12 = 0.999 85) over a solution concentration range of 10 pM to 0.5 mM in 50:50 methanol/water (v/v), which greatly exceeded concentrations used in relaxation measurements. The linear isotherm behavior over this range is consistent with a single adsorption mechanism and the symmetrical elution peak shape of the uncharged probe. The adsorption isotherm of ANS in the absence of electrolyte is nonlinear, however, over this concentration range. In Figure 3, the adsorption isotherm for ANS is plotted over two concentration ranges in 30:70 methanol/water (v/v) without electrolyte present in the solution phase. In Figure 3 4 it can be seen that the isotherm starts to curve over, but at higher ANS concentrations (B) the isotherm becomes linear again. Additionally, it should be noted that near the point where the isotherm regains linearity the breakthrough curve shows two breakthrough waves; points higher or lower in [ANSI exhibit only one breakthrough point. This isotherm, along with the dual breakthrough behavior, is strong evidence of a two-site adsorption mechanism (also (22) Sander, L. C.; Glinka, C. J.; Wise, S. AAnaZ. Chem. 1990,62, 1099-1101.
1394 Analytical Chemistry, Vol. 67, No. 8, April 15, 1995
[ANSI (mM) I
.
Ll
4.0
II
.’Ok 0
O! 1
”*
0!3 O ’V 4 015 o:6 [ANSI
017
(mM)
Figure 3. Adsorption isotherm for ANS on a C1 silica surface from 30:70methanoVwater solution. A. [ANSI ranging from 10 to 100 pM. B. [ANSI ranging from 10 pM to 0.7 mM. Data are fit to eq 3.
suggested by the chromatographic peak shape), where a small population of surface sites saturates, following which the adsorp tion equilibrium is dominated by a second population of sites of much higher concentration on the surface. The data in the low concentration range fit a Langmuir isotherm through 0.1 mM; when the higher concentration range data in Figure 3B are included, however, a term must be added to account for the linear dependence of adsorbed molecules in the higher ANS concentration range:
acm
Q=-
1
+ bC, + mCm
(3)
The least-squares fit of the combined data in Figure 3B to eq 3 is shown as the solid line in the figure. The calculated fitting parameters are a = 3.3 pL/m2, b = 6 x lo3 M-I, and m = 6.2 ,uL/m2. If we invert eq 3 (1/Qversus l/Cm) and examine the intercept where C, equals infinity, we find that the concentration of surface sites that saturate is small, a/b = 0.5 nmol/m2. This number should only be taken as a rough estimate since its uncertainty is comparable to its value. From the adsorption equilibrium constants for the two sites, a and m,respectively, it is clear that the low concentration site that saturates exhibits weaker adsorption of ANS to the surface. The dominate surface site produces a -2-fold stronger adsorption, although the concentration of these sites cannot be determined due to the linearity of the isotherm over this concentration range (limited by the dynamic range of the chromatographic detector).
The nonlinear adsorption isotherm behavior of ANS on a C1 silica surface in the absence of electrolyte is contrasted with its linear adsorption over this same concentration range from 30:70 methanol/water containing 0.4 M NaCl. The isotherm is linear (correlation coefficient, 12 = 0.9998) and the equilibrium constant for adsorption was nearly 4 times greater,23consistent with the increased chromatographic capacity factor when electrolyte was added to a 5050 mobile phase as discussed above. The effect of electrolyte on the adsorption equilibrium of negatively charged ANS is clearly significant. It provides an interesting case for investigation with the laser temperature jump method, since electrolyte may be added but is not required as in previous Joule heating temperature jump In this work, therefore, the iodine laser is used to produce fast temperature perturbations by optically heating the sample, which allows the effects of electrolyte on the kinetics of retention to be studied. Nonporous fumed silica particles are employed in this study for two reasons: first, the highly scattering nature of porous silica gel makes it impossible to evenly heat a sample by optical means, so a dispersion of weakly scattering colloidal particles is used instead; second, the number density of particles in the colloidal suspension allows the concentration of surface sites to be varied. Both of these attributes of fumed silica are tested in the next section, followed by characterization and optimization of the surface probe. Suspensionand Laser Heating of Fumed Silica Particles. The chromatographic data above suggest that the addition of electrolyte effects retention of ANS on C18 and C1 surfaces very differently. To examine this affect in the adsorption/desorption kinetics, it would be interesting to compare these two silicas in relaxation kinetics measurements. An attempt was made, therefore, to suspend both C1 (Degussa R976 hydrophobic fumed silica) and ClMerivatized fumed silica particles in methanol/water solutions. The suspension of the C1 silica was easy and straightforward as described in the Experimental Section. The suspension of the C18 fumed silica particles proved much more difficult. The unmoditied Cabosil L90 was a light, fluffy powder easily dispersed in water, but derivatization of the surface of this silica with C18 ligands causes the material to aggregate, most likely due to the loss of the repulsion of the negative surface potential at the shear plane18due to the presence of the large C18 ligands on the surface. While, the aggregates were partially broken up by sonication, it was not possible to completely disperse this silica, since aggregates of the silica remained that were visible by eye and were therefore larger than the wavelength of visible light. The aggregation of the C18 silica and difticulty forming a stable suspension is evidence that the charged double layer due to surface silanols does not provide a strong enough repulsive field to keep the hydrophobic particles from associating. T i e present study is therefore limited to examining adsorption kinetics at the C1 silica/solution interface, until a suitable means of dispersing Clgderivatized fumed silicas can be found. The optical properties of the fumed silica colloidal suspensions are critical to the potential success of uniform heating by pulsed laser excitation. A major concern is the effect of light scattering by the silica particles on the transmission of the 1.315 pm wavelength radiation from the iodine laser. If the silica suspension (23) Ren, F.; Waite, S. W.; Harris, J. M. Resented at the Engineering Foundation Conf. on Surface Characterization of Adsorption and Interfacial Reactions, Kona, January 10, 1994 manuscript in preparation.
W U
e s n 4
1.5-
1.0-
-0
e p VJ
0.5
I
0
-0
f
0.20
W c I]
In
.-
0.15
0
e
v
e
LL
0.10
1000
1100 1200 Wavelength (nm)
1300
00
Figure 4. Effect of fumed silica on near-IR attenuation: (A) (Lower curve) absorbance of 30:70 methanol/water solution, 1 .O cm path length: (top curve) absorbance/scattering of 10 mg/mL R976 fumed silica in 30:70methanoVwater. (6)Scattering from silica colloid fit to Rayleigh theory (heavy line).
is strongly scattering at this wavelength, then uneven heating of the sample will occur creating temperature gradients and shock waves that make kinetic measurements impossible. To determine the effect of the colloidal particles on the transmission of near-IR radiation, the absorbance of the methanol/water solvent was compared with the combined absorbance and scattering of a colloidal silica dispersion. Figure 4A compares the spectrum of the absorbance of a 30: 70 methanol/water (v/v) solution in a 1 cm path length cell from 1.0 to 1.4 pm with the absorbance and scattering of the colloidal dispersion in the same solvent. From these results, it is clear that the attenuation of radiation across a 1.0 cm path of the colloidal silica sample is very similar to that of solvent alone. The attenuation for the colloid sample deviates from the solvent at shorter wavelengths, which is typical of scattering by particles that are much smaller than the wavelength of the radiation. As a test of this concept, the difference in the measured absorbance was converted to sample transmittance, T, and the fraction scattered, (1 - T ) , was plotted to see how the scattering varied with wavelength. The results are plotted in Figure 4B, along with least-squares (one-parameter) fit to Rayleigh theoryz4accounting for the l / A 4 decrease in the fraction scattered at higher wavelengths. The ability of Rayleigh theory to account for the wavelength dependence of the scattering is strong evidence that the particles are indeed well dispersed, since signiticant aggregation would have produced the onset of Mie scattering and deviation from a simple l/A4 dependence on wavelength. The fraction of light scattered by the colloid at the iodine laser line, 1.315pm, is estimated to be 8.3%,which adds little attenuation to the absorbance by the solvent at this wavelength. (24) Chu, B.Laser Light Scutten'ng, 2nd ed.; Academic Press: New York, 1991; Chapter 2.
Analytical Chemistry, Vol. 67, No. 8, April 15, 1995
1395
As a practical test of whether the iodine laser pulse could indeed heat a silica particle dispersion, a dispersion of fumed silica particles (8 mg/mL) was prepared in a 1090 methanol/water solution containing the fluorescence thermometry probe 6-(N-(7nitrobenz-2-oxa-1,3-diazol-4yl) aminohexanoic acid, previously employed in Joule heating studies of porous silica.*zWhen compared to laser heating of the same solution without silica particles, it was found that the addition of 8 mg/mL silica produced a temperature rise that was -15% less than the solvent and thermometry probe alone; this difference could be due in part to the effect of scattering by the colloid, which could remove a small fraction of the radiation from the excitation path without heating the sample. More importantly, the result shows that the addition of fumed silica brings no additional heating mechanisms into the sample such as dielectric breakdown or other nonlinear optical effects at the colloid particle/solution interface. Characterhation and Optimization of Suface Probe. The observation of a relaxation signal from the equilibrium between adsorbate on the chemicallymodified silica surface and in solution requires that several conditions be met. First there must be a way to distinguish a probe molecule on the surface from one in the liquid phase. For the ANS probe molecule used in this work, a much higher fluorescence intensity is exhibited in a nonpolar environment as compared with a polar environment, so that adsorption to a hydrophobic surface is accompanied by an increase in fluorescence. Second, it is necessary that the intensity and spectral characteristics of fluorescence for the probe molecule be temperature independent. If the fluorescence changes with temperature, then the relaxation signal is convoluted with the thermal response, making interpretation of the data very difficult. The fluorescence spectra of ANS was measured at 25 and 36 "C in both polar and nonpolar solvents. It was found that the fluorescence of ANS did not exhibit a temperature sensitivity and could be used in relaxation kinetic experiments. Third, in order to observe a relaxation signal, there must be a finite enthalpy of adsorption of the probe to the surface, since the temperature dependence of the equilibrium constant depends on the enthalpy according to the van't Hoff equation, 8 In K/aT = AH"/RP. One can determine the enthalpy of an adsorption equilibrium by measuring chromatographicretention as a function of temperature:
+ + In 4
Ink'=- -AH @ RT R
(4)
where k' is the measured capacity factor and 4 is the phase ratio. A plot of the log of the capacity factor versus l/Tyields a straight line with a slope equal to -AH/R. For ANS in 50:50 methanol/ water (v/v) with 0.4 M NaCl adsorbing onto a C1 surface, the van't Hoff plot taken over a 30 K range is linear (+ = 0.996) and AH of adsorption is -16.3 (5 0.8) kJ/mol; for the case of no added electrolyte, the enthalpy change accompanying ANS adsorption is larger, AH = -31 (f3)kJ/mol. Since addition of electrolyte produces greater ANS retention (see Table l), electrolyte in solution leads to a smaller entropy loss upon ad~orption.~~ Under both conditions, ANS exhibits a large negative enthalpy of adsorption to the surface; as the temperature is increased in a temperature jump, therefore, the equilibrium will shift toward (25) Dorsey, J. G.; Cooper, W. T.Anal. Chem. 1994,66. 857A-867A.
1396 Analytical Chemistry, Vol. 67, No. 8, April 15, 1995
Table 1. Chromatographic Retention of ANS on C l Slfica*
k' k'/(l
+ k')
6040
methanol/water (v/v) 50:50
4060
0.57 0.36
2.18 0.69
3.98 0.80
methanol/water (v/v)
k' k'/(l a
+ k')
+ 0.4 M NaCl
60:40
50:50
40:60
0.95 0.51
2.47 0.71
6.82 0.87
Uncertainty in k' is less than 1%.
more probe in the liquid phase. On the basis of the direction of the equilibrium shift and the environment sensitivity of the probe, one would expect to observe a decrease in fluorescence after the perturbation as the population of molecules in the more polar solution phase is increased. A fourth requirement that must be met in order to observe relaxation kinetic signals is that the equilibrium not be too onesided, so that a change in the equilibrium constant makes a significant change in the population of molecules on the surface. As shown in previous work,5J3the maximum population relaxation occurs when there are an equal number of probe molecules on the surface and in the liquid phase; this condition is met when the ratio k'/(l k') = 0.5. The capacity factor k' can be varied by adjusting the mobile-phase composition; chromatographic measurements were carried out in order to determine where the equilibrium should be poised. Chromatographic retention data for ANS on a C1 column are summarized in Table 1. When a fumed silica dispersion is used, the effective value of k' will be reduced due to the smaller amount of silica surface per unit volume of solvent compared to a packed column. The reduction in silica surface in the colloid dispersion makes it necessary to increase the water fraction of the solution phase to increase the fraction of adsorbed molecules toward a value of 0.5, where a strong relaxation amplitude can be observed. To predict the capacity factor k' under solvent conditions where retention is so strong that chromatographic measurement is impractical, one can plot In k' versus the concentration of water in the mobile phasez6and extrapolate to a higher percent water. The log of the capacity factor data in Table 1varies linearly with increasing water concentration. Extrapolatingthese results to a mobile phase composed of 1090 methanol/water, one predicts for ANS k' = 82 with no added electrolyte and k' = 130 with 0.4 M NaCl in the solution. The reduction in the effective k' when changing from the packed column of porous C 1 silica to the dispersed fumed C1 silica is estimated by the following procedure. Accounting for the change in surface area of the porous silica due to C1 deri~atizationl~ and having measured the packing density and mobile-phase volume of the column, the ratio of surface area of the C1-derivatized porous silica to the mobile-phase volume was determined to be 408 m2/mL. The reduction in effective k' for a fumed silica dispersion is proportional to the reduction in the
+
(26) Karger, B. L.; Gant, J. R; Hartkopf, A; Weiner. P. H. I. Chromafogy, 1976, 128, 65-78.
Table 2. Estimated Capacity Factors and Surface Concentrations for ANS on R976 Fumed Silica versus the Mass Fraction of Dlspersed Silica in la90 MethanoWateP
with 0.4 M NaCl
with no NaCl [R9761,
Asudvol,
m2/mL
[surface],b mM
k'
10 8 6 4 2
2.5 2.0 1.5 1.0 0.5
5.5 4.4 3.3 2.2 1.1
0.50 0.40 0.30 0.20 0.10
mg/mL
k'/(l
+ k')
0.33 0.28 0.23 0.17 0.09
[ANslads,
mM
k'
k'/(l+ k')
0.033 0.028 0.023 0.017 0.009
0.80 0.64 0.48 0.32 0.16
0.44 0.39 0.32 0.24 0.14
[ANslads,
mM
0.044 0.039 0.032 0.024 0.014
Initial ANS solution concentration is 100pM,corresponding to the highest [ANS]a&for the kinetic experiments. Adsorption site concentration in the silica colloidal dispersion.
surface area per unit volume in meters squared per milliliter, which is determined by the specific surface area of the fumed silica (250 m2/g) and the mass of colloid per unit volume in the dispersion. The effective k' values predicted for ANS in equilib rium with various dispersions of the C1 fumed silica in 10:90 methanol/water are listed in Table 2. Due to the large (>lo@ fold) reduction in surface area compared to a packed bed of porous silica, the capacity factors are small, and the fraction of molecules on the surface in all cases is less than 0.5. One could increase this fraction by increasing the amount of silica in the suspension, or by further shifting the adsorption equilibrium by increasing the fraction of water. It was not possible to increase the amount of silica significantly above 10 mg/mL due to difficulties in dispersing particles, which would tend to aggregate at the air/ water interface. It was also not possible to increase the water concentration above 90% by volume again due to difficulties in wetting the hydrophobic particle surface. Nevertheless, the fraction of adsorbed molecules was large enough in these silica suspensions in 10:90 methanol/water to observe measurable relaxation amplitudes as discussed below. A final requirement to obtain readily interpretable relaxation kinetic signals is that the concentration of one of the reacting species be kept in large excess and therefore pseudo~onstant.~ In an adsorption kinetic experiment, the reacting species are the adsorbate in solution and available sites for adsorption on the surface. An attempt to measure the site density for ANS adsorption on a C1 surface using a chromatographicisotherm was not successful since the dynamic range of the detector was not sufficient to allow high enough concentrations to observe the rollover in the retention. An estimate of the surface site density for ANS on the fumed silica surface was made by modeling the adsorption of the probe onto the surface. On a C1-derivatized silica surface, ANS has been found to lay flat with its aromatic rings parallel to the which is reasonable since it cannot partition into a methylated layer. The molecular dimensions of ANS are 7.0 A x 7.3 A,13yielding a surface area of 5.1 x m2/molecule or 3.1 x lo5 m2/mol. The packing of adsorbed molecules in a disordered array is generally not sufficientlyclose as to cover the entire area of the surface due to inefficiencies in packing and charge repulsion between adsorbates. The packing of ANS was estimated to yield a surface area per adsorption site of 4.5 x lo5 m2/mol, based on a previous study of the w2/3 fractionalcoverage by ANS on a C18 surface measured by a frontal elution isotherm.13 Since the surface area of the R976 fumed silica (27) Hlady, V.; h. Y.-S.; Andrade, J. D. Presented at the 198th National ACS Meeting, Miami, FL, September 11, 1989.
is 250 m2/g, it should provide a surface site concentration in colloidal dispersion of 5.5 x lo-' mol/g. Table 2 lists the surface site concentrations of R976 in the ranges that are used in the kinetic experiments. It also lists the concentration of ANS on the surface in 10:90 methanol/water (v/v) solution when the solution concentration of ANS is 100 pM, producing the highest [ANsIad, in the kinetic experiments; the surface concentration of ANS is estimated from the chromatographic k' data, corrected for the smaller surface area in the colloidal dispersion. It is seen from Table 2 that the concentration of adsorbed ANS is much less than the concentration of surface sites in the dispersion; these conditions ensure that an excess of surface sites controls the rate constant for the relaxation and that the kinetics can be accurately modeled as pseudofirst order. TemperatureJump Relaxation Results. Temperature jump relaxation data were acquired for ANS and C1-derivatizedfumed silica dispersions in solvent compositions of 30:70,20:80, and 10: 90 methanol/water (v/v). The relaxation amplitudes for the 30: 70 and 20:80 solvent compositions were very small, and no observable dependence of the relaxation rate on the concentration of the surface sites could be detected. This is likely due to the unfavorable positioning of the equilibrium,making the relaxation amplitudes too small to overcome the noise of the signal. In the 1090 methanol/water case, the relaxation signal was clearly detected, and a biexponential response was observed. Figures 5 and 6 show relaxation traces for ANS adsorption/desorption kinetics with R976 silica at a dispersed concentration of 8 mg/ mL (4.5 mM surface sites) in 10:90 methanol/water without electrolyte, and with 0.4 M NaCl, respectively. In these relaxation traces, two time bases were used to detect relaxation kinetics over a wider range of response time; the sampling time changes at the midpoint of the relaxation trace. In the first few microseconds, a background flash is observed which interferes with the initial kinetic signal. The large photon density of the iodine laser causes dielectric breakdown at the surfaces of the cell walls, as discussed above; fluorescence is gathered from the center region of the sample to avoid any artifacts associated with the cell walls. M e r the flash has decayed, the tail of the relaxation response in each time window is fit to an exponential model by a least-squaresfitting routine described in the Experimental Section. In the presence of electrolyte (Figure 6), the amplitude of the second, slower relaxation is greatly reduced, and the relaxation rate of the fast component increases and becomes too fast to be measured (due to the initial background). The increase in relaxation rate of the fast component combined with the near disappearance of the slow relaxation component Analytical Chemisrry, Vol. 67, No. 8, April 15, 1995
1397
A
TaMe 3. Relaxation Rates for I00 pM AN$ Adsorption at R976 Fumed Silica In la90 Methanowater (vhr) Solvent.
mg/mL
[surface], mM
robs, ps
8 6 4 2
4.5 3.3 2.2 1.14
3.1 4.0 5.5 10.5
[R976],
0
fast
slow Schem,
ps
relaxation Schem, p~
1.9 & 1.3 3.2 f 1.5 4.9 f 1.9 10.2 f 3.1
198 230 261
The uncertainties in the observed relaxation times are f20%.
Table 4. Rates for the Slow Relaxation of 100 pM ANS Adsorption at R976 Fumed Silica in 1O:W Methanol/ Water (vhr) and 0.4 M NaCl
LR9761, mg/mL
[surface],mM 4.5 3.3 2.2 1.14
Tchemf
ms
0.36 0.40 1.00 1.45
The errors in the relaxation times are f20%.
0 I
2 I
0 I
.
6
8 1 0 8 .
Time
a
508
308 I
1
1
1
(pi)
Figure 5. Relaxation response of 100,uM ANS and 8 mg/mL R976 hydrophobic silica to an iodine laser temperature jump. Solvent is 10:90 methanovwater. Note that, at 8 ps, the time scale changes from 2 to 100 ,&division to allow a larger time window to be observed: (A) least-squares fit of an exponential decay to the fast relaxation data; (B) least-squares fit to the slow relaxation.
L
* a
o ‘
2
4
6
8 1 0 8
I
I
I
I
308
508
b
correlates well with the observed chromatographic peak shapes of Figure 2. The addition of electrolyte causes the chromatographic peak shape to narrow and become more symmetrical. As discussed above, adding electrolyte to the solution can compress the double layer and decrease the charge repulsion between 1398 Analytical Chemistry, Vol. 67,No. 8, April 15, 1995
residual surface silanols and adsorbed ANS. This effect is observed kinetically by the much smaller amplitude of the slow relaxation, confirming the earlier postulate that electrolyte compresses the double layer and reduces the effects of charge repulsion. The sites with a slower relaxation rate appear to correspond to this small repulsive population that appears in the isotherm at low ANS concentrations without added electrolyte. From these results, it is seen that the rates of adsorption and desorption of molecules at the surface can drastically effect the peak shape in chromatographic processes. To calculate the adsorption and desorption rates at the surface, it is necessary to vary the concentration of surface sites and measure the relaxation rate over a series of site concentrations. Equation 1shows that a plot of relaxation rate versus surface site concentration should yield a straight line with a slope equal to the adsorption rate constant and an intercept that is proportional to the desorption rate constant. Table 3 lists the observed relaxation rates for both the slow and fast relaxation in the absence of NaCl, and Table 4 lists the slow relaxation rate with 0.4 M NaCl electrolyte. The fast relaxation event exhibited a lifetime that was close to the 2.4 p s decay of the heating impulse from the iodine laser. It was therefore necessary to correct the observed relaxation response to obtain the actual relaxation rate. This correction was done by the root-mean-square method, derived from the convolution of an exponential source function with an exponential decay,28given by &hem = (rob? - rhea?) 1’2, where &hem is the corrected relaxation time due to the chemical relaxation, robs is the observed relaxation time, and theat is the 2.4 $3 heating time of the iodine laser; this approach to deconvolution is recommended when fitting to noisy relaxation kinetic d a h B The uncertainty in theat = 30%, and this error has a greater effect on the results at higher surface site concentrations due to their faster relaxation. Figure 7 shows a plot of the corrected chemical relaxation rate for adsorption/desorption of ANS versus surface site concentration (28) Turner, D.H.In Investigation of Rates and Mechanisms of Reactions, 4th ed.; Bemasconi, C.F., Ed.; Wiley: New York, 1986;Part 11, p 159.
-
1 01
I
A
9)
4-
0
LT
01
I
1
2
3
4
5
(Surface Sites1 (mM)
Figure 7. Relaxation rate dependence on the concentration of
surface sites. Solution is 10:90 methanoVwater containing 100 pM ANS.
of suspended C1 colloid. The slope for the fast relaxation yields a rate constant k&,fast = 1.0 (f0.4) x 108 M-l s-l for ANS adsorption to a C1 silica surface in the absence of electrolyte. This rate is more than 1 order of magnitude slower than diffusioncontrolled which indicates that adsorption of ANS to the C1 surface has a significant energy barrier. A previous temperature jump study of adsorption of ANS to a C18 surface also showed a finite adsorption barrier that was attributed to changes in ionic solvation upon adsorption.13 Due to the uncertainties in the relaxation rates, the intercept could not be distinguished from zero, and a desorption rate constant could not be determined. When sodium chloride was added to the solution phase, the fast relaxation rate became indistinguishablefrom the heating time. The faster rate corresponds to an increase in the adsorption rate, since the relaxation is dominated by adsorption kinetics (see Figure 7); this correlates well with the higher capacity factor in the presence of electrolyte and indicates that changes in the adsorption rate can influence the retention equilibrium. The slow relaxations in both cases were much slower than the heating time so that deconvolution was not needed. The adsorption rate for the slow relaxation rate was found to be kads,s:ow = 5 (k2)x lo5 M-l s-l in the absence of electrolyte and 7 (f4) x lo5 M-l s-l in the presence of 0.4 M NaCl. Despite the smaller amplitude of the slow relaxation in the presence of NaC1, the slow adsorption rate that is observed is comparable to the rate in the absence of electrolyte. This indicates that there is still a small population of the kinetically slow adsorption sites in the presence
of electrolyte having indistinguishablekinetic behavior, but their concentration is so reduced that it does not detectably affect the adsorption isotherm or the chromatographic peak shape. Because of the large difference in time scales between the fast and slow relaxations, the small remaining population of slow sites could be detected in the adsorption kinetic experiment. The desorption rate for the slow relaxation is 2.6 (f0.3) x 103 s-l when no electrolyte is present. The desorption rate could not be distinguished from zero when electrolyte was added due to the greater uncertainty in the data arising from the smaller amplitude of the relaxation. The effect of electrolyte on adsorption of a charged solute (ANS) on a C1 silica surface is significant. Without electrolyte, a two-site sorption isotherm is observed, and the corresponding relaxation signal is biexponential. When electrolyte is added to the system the isotherm is linear, which is also observed for a nonionic probe (1-NPN) with or without electrolyte. The relaxation signal for ANS in the presence of electrolyte is less biexponential; the fast relaxation rate was found to increase, and the amplitude of the slower relaxation rate was greatly reduced. The effect of added electrolyte on the slow relaxation can 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. The faster adsorption rate in the presence of electrolyte agrees with increased retention of the probe on the surface and shows that adsorption kinetics can influence both band broadening and retention equilibria. ACKNOWLEDGMENT
This research was supported in part by grants from the US Department of Energy and from the Deutsche Forschungs Gemeinschaft. Fellowship support (to S.W.W.) from the ACS Division of Analytical Chemistry and Perkin-Elmer Corp. is acknowledged. Fumed silica samples were received as gifts from Degussa Corp. and Cabot Corp. Assistance with acquisition of adsorption isotherm and silica scattering data by Feiyan Ren and Stephanie Jura is gratefully acknowledged. Received for review November February 7, 1995.@
1,
1994.
Accepted
AC941061 G @Abstractpublished in Advance ACS Abstracts, March 15, 1995.
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