2887
Anal. Chem. 1993, 65, 2887-2893
Mechanistic Studies of Electroosmotic Control at the Capillary-Solution 1nterface Tung-Liang Huang, Pei Tsai, Chin-Tiao Wu, and Cheng S. Lee' Department of Chemical and Biochemical Engineering, University of Maryland Baltimore County Campus, Baltimore, Maryland 21228
The electrokinetic phenomena at the silica-solution interface under the influence of applied radial electric potential gradient were analyzed by a theory based on the Gouy-Chapman-SternGrahame (GCSG) model and the induced effect across the capillary wall. The effect of adsorbed ions at the silica-solution interface on the direct control of electroosmosis was studied with the application of lithium ions, tin(1V) ions, and dodecyltrimethylammonium bromide (DTAB). In addition,various organic coatings,including butyl phase, amino phase, and (glycidoxypropy1)trimethoxysilane-ethylene glycol diglycidyl ether (GOX-EGDE),were employed for investigatingthe effect of surface deactivationon the direct control of electroosmosis. The fundamental relationship between the microenvironment at the silicasolution interface and the direct control of electroosmosis obtained from our experimental and theoretical results is presented and discussed.
INTRODUCT10N In our recent studies, we demonstrated the direct control of electroosmosis in capillary electrophoresis by using a radial electric potential gradient across the capillary wall.'" The applied radial electric potential gradient affected the polarity and magnitude of the { potential at the capillary-solution interface and therefore controlled the direction and flow rate of electroosmosis. The extent of electroosmotic control with the application of a radial electric potential gradient, however, was found to be dependent on the magnitude of the { potential.2 As explained by the capacitor theory? the capacitance of the electrostatic diffuse layer increased with the increase in the { potential. Thus, the effectiveness of the applied radial electric potential gradient for controlling electroosmosis decreased with the increase in the { potential. Since the { potential is mainly determined by the solution pH? the effect of an applied radial electric potential gradient on electroosmotic control in capillary electrophoresisbecomes diminished at high solution pHs.
* To whom all correspondenceshould be addressed. Present address:
Analytical Instrumentation Center, Ames Laboratory-USDOE, and Department of Chemistry, Iowa State University, Ames, Iowa 50011. (1) Lee, C. S.; Blanchard, W.C.; Wu,C. T. Anal. Chem. 1990, 62, 1550-1552. (2) Lee, C. S.; McManigill, D.; Wu,C. T.;Patel, B. A d . Chem. 1991, 63,1519-1523. Lopes, T.; Patel, B. J.Chromatogr. 1991,559, (3) Lee, C. S.; Wu,C. T.; 133-140. (4) Wu,C. T.; Lopea, T.; Patel, B.; Lee, C. S. Anal. Chem. 1992,64, 886-891. (5) Wu,C. T.Miller, T. J.; Lee, C. S. Anal. Chem. 1992,64,2310-2311.
0003-2700/93/0385-2887$04.00/0
Our interest in extending the direct control of electroosmosis at high solution pHs requires a strategy to reduce the magnitude of the { potential at those pHs. This study therefore examines the effect of adsorbed ions and organic coatings at the silica-solution interface on the direct control of electroosmosis. A theory based on the Gouy-Chapman model697 was developed by Hayes and Ewine for describing the effect of an applied radial electric potential gradient on the { potential. For studying the fundamental relationship between the microenvironment at the capillary-solution interface and the direct control of electroosmosis, a theory based on the Gouy-Chapman-Stern4rahame (GCSG)model697 including the site-binding effect in the Stern layer and the Boltzmanndistribution of ions at the interface is presented and employed in this study.
THEORY In this study, we combine the GCSG model proposed by Yates et ala7with an induced capacitive effectz.8to describe the electrokinetic phenomena at the silica-solution interface under the influence of a radial electric potential gradient. As shown in Figure 1,the GCSG modeW divides the electrostatic double layer into a diffuse layer and an ion-binding region. The ion-binding region is further divided into inner and outer capacitive layers having the permittivities of €1 and €2 and capacitances of C1 and CZ,respectively. The plane where the diffuse layer begins is called the outer Helmholtz plane (OHP), and the edge for the compact region of bound ionic species is called the inner Helmholtz plane (IHP). This model has an electroneutrality condition in the form of u,
+ up + u,j = 0
(1) where u,,, up, and Ud are the surface density of charge at the silica surface, IHP, and OHP, respectively. The acidic side of the titration curve for silica substrate is not accessibleby any electrokineticmeasurements. The point of zero charge (pzc) of silica substrate can only be established through the extrapolation method. Thus, the silica-solution interface can be treated with a single-site model; Le., dissociation of the silanol groups is considered as the sole ionization reaction of surface silanolgroups. The dissociation reaction of silanol groups at the silica surface can be expressed by the dissociation constant, KA,as
-
SiOH SiO- + Hs+; KA = [SiO-I [Hs+lIISiOHl (2) where the SiOH and SiO- are given as the surface functional groups per unit area and Hs+ are the protons in the surface plane, which are distinguished from the protons in the bulk phase of aqueous solution, Hb+. The direct binding of (6) Hunter, R. J. Zeta Potential in Colloid Science: Principles and Applications; Academic Press: New York, 1981; Chapter 2. (7) Y a w , D. E.;Levine,S.;Healy,T.W.J. Chem.Soc.,Faraday Tram. 1974, 70, 1807-1818. ( 8 ) Hayes, M. A.; Ewing, A. G.Anal. Chem. 1992,64,512-516. 0 1993 American Chemlcai Society
2888
'I'd
ANALYTICAL CHEMISTRY, VOL. 05, NO. 20, OCTOBER 15, 1993
is the applied radial potential gradient, and RI and Ro are the inner and outer radius of the capillary, respectively. Since, in this work, the applied radial electric potential gradient was only effective for 70% of the capillary length (28 cm of the 40-cm-longcapillary),the induced surface charge density, uv, is then multiplied by a factor of 0.7 for obtaining the actual induced charge density, d,, at the silica surface. Assuming that the interaction between asi and d, is additive! the total surface charge density, a, at the silica surface would then be the sum of I J S and ~ d,. Thus, eq 1can be rewritten as
-
asi
'I'P
+ d.,+ a@+ a d = 0
or
flsi
+ ( 0 . 7 ) ~+~o@ + c d = 0
(10) The charge densities and electrostatic potentials in the electrostatic double layer can be correlated as6
Water molecule
3, - \k, = adC,
(11)
3, - 3, = -u,/c,
I
Go
(12) On the basis of electroneutrality consideration: the charge density and the electrostatic potential at OHP can be correlated as
bd
op
Figure 1. Schematicdlagram of the GCSG model describinga possible potential dlstributlon and the locations of charges for the electrostatic double layer at the solld-aqueous interface (adapted from Davis et ai.18).
a d = (-2xekT/e) sinh(e\kd/2kT) (13) where K is the inverse Debye length, e is the permittivity of the aqueous solution, and 3, is the electrostatic potential at OHP. The latter is regarded as the {potential in this study. counterions to the surface charged sites can be described as Through eq 13 is developed for the symmetrical electrolytes, an exchange reaction with a surface binding constant, KM,as we may still use this equation for the unsymmetrical electrolytes,like phosphate buffer, without incurringtoomuch SiOH + Ms+ c+ SiOM + Hs+; error. The reason for this is that almost all of the surface KM = [ S i O M ] [ H ~ ~ l / [ S ~ O H ] [ M (3)~ ~ ] charge is balanced by the accumulation of counterions and relatively little by the repulsion of co-ions.6 Thus, the where Ms+ are the metal ions and assumed to lie in the counterions enjoy a greater significancethan the co-ions, and compact layer. we can treat an asymmetric electrolyte as symmetric with the Assuming a Boltzmann distribution of ions at the silicavalence of the counter ions, which are sodium ions in this solution interface, the concentration of surface ions may be study. related to the concentration of the same species in the bulk In order to obtain the theoretical value of the { potential phase by under the influence of a radial electric potential gradient [Hs+l = [€Ib+]exp(-e*dkT) (4) across the capillary wall, the self-consistent calculations of eqs 2-13 as a set of 12 nonlinearly dependent functions have where 3, is the electrostatic surface potential, e is the to be made. Due to the nonlinear characteristic of eqs 2-13, electronic charge, k is the Boltzmann constant, and Tis the an explicit expression for the { potential as a function of absolute temperature. For the metal ions, this becomes solution pH, electrolyte concentration, and applied radial electric potential gradient is unattainable. For the GCSG [Ms+l = [&+I exp(-e*,/kT) (5) model, a numerical methodology was therefore developed as where 3 0 is the electrostatic potential at the IHP and [Mb+] a computer program MINEQL by Westall et al.9 to obtain is the concentration of metal ions in the bulk phase. the { potential at the oxide-water interface. In this study, The surface charge density, usi, due to the dissociation of the computer program MINEQL was modified with the silanol groups and the silanol groups complexing with metal addition of eq 9 for the induced surface charge due to the ions is then given by capacitiveeffect. This modified program based on an iterative scheme allows the calculation of the { potential at various aSi = -e([SiO-l + [SIOMI) (6) solution conditions and applied radial electrical potential gradients. The electroosmotic mobility, pbo, can then be The total number of surfacesite density and the charge density obtained by using the Smoluchowski equationlo at the IHP are expressed as
N s = [SiOHl + [Si07 + [SiOMl
(7)
and
up = e[SiOMl
(8) respectively. With the application of a radialelectric potential gradient across the capillary wall, the induced surface charge and surface charge density, a,, due to the capacitive effect can be described as8 0 ,
= (esiVJRI)(l/h(RdRI))
(9)
where esi is the permittivity of the fused-silica capillary, V,
we0 = {(e/$ (14) where 7 is the viscosity of aqueous solution. The parameters needed for the modeling are the dissociation constant of fused silica KA,the complexation constant of ionized silanol groups with metal ions, KM,the surface site density of the silanol group, N,, and the capacitance of the inner and outer capacitive layers, C1 and Cz. The values of (9) Westall, J. C.; Zachary, J. L.; Morel, F. M. M. Tech. Note. No. 18, Water Quality Lab., Dept. of Civil Engineering,Massachusetts Institute of Technology, 1976. (IO)Whitehead, R.; Rice, C. C. J. Phys. Chem. 1965,69, 233-240.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993 2888
140 and 40 mF/cm2 were recently used by Scales et al.11 as C1 and CZin the prediction of the electrokinetic behavior of a Suprasil-grade fused silica. The values of C1 and Cp are kept constant in the calculation of the computer program MINEQL. The MINEQL program does not account for the potential changes in the values of C1and Czunder the influence of applied radial potential gradient. The magnitude of Ns was generally found to be 5 X 1014sites/cm2.12The values of K M and K A used in the calculation are discussed later in Results and Discussion. EXPERIMENTAL SECTION The experimental setup in an external;ground configuration for applying a radial electricpotential gradient acrossthe capillary wall was described previously by Holloway and his co-workers.13 In contrast to the coaxial configuration,'4 only one capillary was required in this external grounding setup for the direct control of electroosmosis. The external surface of the silica capillary was coated with a conductive layer such as silver by vacuum deposition and then connected to the ground. Based on the external grounding, the electric field across the buffer solution in the capillary exerted the radial electric potential gradient upon the capillary wall. Since the applied radial electric potential gradient was varied along the capillary, an average radial electric potential gradient is calculated by averaging the radial electric potential gradients at each end of the capillary. Ion Adsorption. To investigate the effect of adsorbed ions and surfactants on the direct control of electroosmosis,a 10 mM sodium phosphate buffer was used as a blank solution for comparison. Lithium ions, tin(1V) ions, and dodecyltrimethylammonium bromide (DTAB) were selected as the adsorbing agentsfor modifying the {potentialat the silica-solution interface. To ensure equilibriumwas reached between the silicaand solution phases, the phosphate buffer in the absence or presence of adsorbing agents was loaded into the capillary at least 6 h before the measurements were carried out. Organic Coatings. Capillaries modified with butyl phase (a neutral and hydrophobic coating), amino phase (a charged coating),and (glycidoxypropyl)trimethoxysilane-ethylene glycol diglycidylether (GOX-EGDE, a neutral and hydrophiliccoating) were used in this study. Capillary tubings coated with amino phase or butyl phase were obtained from Dr. Sally A. Swedberg in Hewlett Packard Laboratories. The capillaries coated with GOX-EGDE were gifta from Professor Fred E. Regnier at Purdue University. The detail in the preparation of these organiccoatings were given elsewhere.1c16 Before the measurementsof electroosmoticmobility,the coated capillaries were washed with 0.1 N nitric acid for 15 min, then with distilled water for 15 min, and finally with the running buffer for 1h. The pH of the buffer solution was adjusted with 0.1 N hydrochloric acid or 0.1 N sodium hydroxide. Dimethyl sulfoxide obtained from Sigma (St.Louis, MO) was used as the electroosmoticflow marker. The flow rate of electroosmosis was assigned as positive when the direction of flow was toward the cathodicend. In this study, a constant electric field equal to 200 V/cm was applied to drive the electroosmotic flow at various radial electric potential gradients. Nitric acid, hydrochloric acid, sodium hydroxide, sodium phosphate, and DTAB were purchased from Sigma. Lithium chloride and tin(1V) chloride were obtained from Aldrich Chemical Co. (Milwaukee, WI). Silica capillaries with a 50-pm i.d. and 150-pm o.d., which were made from a Suprasil-grade fused silica, were obtained from Polymicro Technologies (Phoenix, AZ). (11) Scales, P. J.; Grieser, F.;Healy, T. W. Langmuir 1992,8,965-974. (12) Van der Voort, P.; Gillis-DHamere, I.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1990,86,3751-3760. (13) Holloway, R. R.; Keely, C. A.; Lux, J. A.; McManigill, D.; Young, J. E. The Fourth InternationalSymposiumon High PerformanceCapillary Electrophoresis, 1992; Poster Paper PT-27. (14) Swedberg, S. A. Anal. Biochem. 1990,185, 51-56. (15) Swedberg, S. A. U.S.patent 4,931,328, June 5, 1990. (16) Towns, J.; Bao, J. M.; Regnier, F.E. J. Chromatogr. 1992,599, 227-237.
-1 - -0 21 2
-8
-4
0
4
8 -pH3
12
Applied Radial Potential Gradient (kV)
Electroosmoticmobilityagainst appibd radlei electric potential gradient in 10 mM phosphate buffer at pH 3 or pH 7: (0)and dashed line, experimental and predicted data without the addition of 1 mM lithium chloride; (0)and solid line, experimental and predicted data with the addition of 1 mM lithium chloride. The expertmental error In measuring the electroosmotic mobility was about 0-3% for over five runs. Figure 2.
RESULTS AND DISCUSSION
Ion Adsorption. Lithium Ions. The effect of an applied radial electric potential gradient on the direct control of electroosmosis in the solution condition of 10mM phosphate buffer and pH 3 was investigated with and without the addition of 1 mM LiC1. As shown in Figure 2, the value of electroosmotic mobility was varied from 1.8 X 10-4 cm2/V.s with the application of a -8-kV potential gradient to -1.4 X 10-4 cm2/V.s in the presence of a 8-kV potential gradient. With the addition of 1 mM LiC1, the absolute value of electroosmotic mobility at various radial electric potential gradients was slightly reduced by about (0-0.2) X lo-" cm2/ V-s. Milonjic" applied the double-extrapolation method of Davis18and reported the values of ion-binding constant,~ K M , for Li+ and Na+ as 6.5 and 6.9, respectively. Since these two binding constants are quite close to each other, we might expect that the binding behavior of Li+ is similar to that of Na+ and attribute the reduced electroosmotic mobility in the presence of LiCl solely to the increased ionic strength. Depending on the crystallinity, impurity, extent of hydration, and aqueous solution in which the surface ionization was determined, the values of pKA for silica have been reported in the wide range of 3.5-8.2.'' Instead of choosing a reported value of PKAfrom the literature, it was decided to regard PKA as a fitting parameter in the model. As shown in Figure 2, the prediction based on a PKA value of 6.3 was in good agreement with the experimental results. In fact, the PKA value of 6.3 used in this study was quite close to 5.8 as the PKA value reported by Scales et al." for a Suprasil-grade fused silica. The combination of GCSG model with the induced capacitive effect seemed to describe well the electrokinetic behavior at the silica-solution interface under the influence of an applied radial electric potential gradient. However,the inherent assumptions used in the GCSG model, such as the discrete charge effect and the direct binding of counterions exclusivelythrough Coulombicinteraction,'Jl and the limitations of GCSG model as discussed by Scales et al.," should be noticed. The effect of an applied radial electric potential gradient on the direct control of electroosmosis in the same 10 mM phosphate buffer but at pH 7 was studied. It is of interest (17) Milonjic, S. K. Colloid Surf. 1987, 23, 301-312. (18) Davis, J. A.; James, R. 0.;Leckie, J. 0. J.Colloid Interface Sci. 1978,63,480-499.
2890
0 9
ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993
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Applied Radial Potential Gradient (kV) Figure 3. Electroost??otIcmobility against applied radlal electric potential gradlent In 10 mM phosphate buffer: (0)and (0)experimental data at pH 3 In the absence and presence of 0.3 mM tin chloride; (A)and (0)experimental data at pH 7 In the absence and presence of 0.3 mM tin chlorde; dashed and solld Ilnes, predicted data In the absence and presence of 0.3 mM tln chlorlde. The experimental error in measurlng the electroosmotic mobllity was about 0-5% for over five
runs.
to note that the extent of electroosmoticcontrol at pH 7 was much less than the control at pH 3. This observation was attributed to a larger amount of dissociated surface silanol groups, usi, at pH 7 compared to that at pH 3. As shown in eq 10, the percentage in the contribution of surface induced charge (under the influence of an applied radial electric potential gradient), uv, to the charge density at OHP, Ud, became less in the presence of a larger value of usi. Because of this, the effectivenessof the applied radial electric potential gradient on Ud and the f potential (see eq 13) was reduced by raising the solution pH. Similar observations and conclusions were also reached by our laboratory2 and Ewing's group19 with the application of the capacitor theory and the GouyChapman model, respectively. Tin l o w . The effect of an applied radial electric potential gradient on electroosmotic control at 10mM phosphate buffer and pH 3 was examined in the absence and presence of a saturated concentration of tin chloride. The saturated concentration of tin chloride in 10 mM phosphate buffer was measured to be about 0.3 mM. As shown in Figure 3, the measured electroosmotic mobilities were barely changed with the addition of tin chloride in the phosphate buffer. Since fewer ionized silanol groups were available for binding with tin ions at pH 3, the effect of tin chloride on the f potential was therefore insignificant. As shown in Figure 3, the value of electroosmoticmobility with the application of a 0-kV potential gradient was reduced from 5.5 X 10-4 cm2/V.s in 10 mM phosphate buffer at pH 7 to 2.2 X 10-4 cm2/V.s in the same solution condition, but with the addition of 0.3 mM tin chloride. The concept of direct binding of counterions to the surface charged sites used in eq 3 is only applicable for indifferent ions such as lithium and sodium ions.8 For specifically adsorbed solutes such as tin ions in this study, the contribution of tin ions to the charge density at IHP, up, was used as the fitting parameter to compare with the experimental measurements. A value of 9.7 pC/cm2 for the contribution of tin ions to uj3 obtained from the fitting was much greater than 0.15 pC/cm2as the contribution of sodium ions to uj3 (see eqs 3 and 7) through the direct binding of indifferent ions at the silicaaolution interface. Even though the f potential was significantly reduced in the presence of tin chloride, the extent of electroosmotic control was only slightly increased. This (19)Hayes, M.A.; Kheterpd, I.; Ewing, A. G. Anal. Chem. 1993,65, 27-31.
I
'
6
'
10
Applied Radial Potential Gradent (kV)
Flgure 4. ElectroosmoticmobHltyagalnst applied radlal electric potential gradlent In 10 mM phosphate buffer and pH 7: (0)and dashed ilne, experlmental and predlcted data without the addltion of 1 mM DTAB (0)and solld line, experimental and predicted data with the addltlon of 1 mM DTAB. The experimentalerror In measulng the electrwsmotlc
mobility was about 0-5% for over 5 runs.
observation was attributed to a larger value of up as mainly contributed by the specific adsorption of tin ions in the compact layer. As shown in eq 10, the percentage in the contribution of surface induced charge (under the influence of an applied radial electric potential gradient), a,, to the charge density at OHP, Ud, became less in the presence of a larger value of up The effect of surface induced charge on the f potential was diminished by the shielding effect of adsorbed tin ions at IHP. In this case, the extent of electroosmotic control predicted by the theory was less than the range of electroosmotic control measured in the experiment. This discrepancy between the theory and experimental results was attributed to the assumption of specific adsorption of tin ions in the compact layer. It was possible that the tin ions to some extent were attracted and trapped into the silica layer due to the strong Coulombic attraction with the negatively charged silanol groups. As discussed previously, the reduction in the surface charge density through the direct complexation of tin-silanol ion pairs would enhance the effectiveness of applied radial electric potential gradient on the direct control of electroosmosis. DTAB Surfactants. As shown in Figure 4, the electroosmotic mobility in the capillary was decreased significantly with the addition of 1mM DTAB in the solution condition of 10 mM phosphate buffer and pH 7. A value of 3.8 pC/cm2 as the charge density at IHP, Ug, due to the adsorption of DTAB was obtained from the fitting with the experimental measurements. Again, a larger value of up contributed to the reduction in the f potential and the lack of enhancement in the extent of electroosmotic control. Organic Coatings. Butyl Phase-Coated Capillaries. The structures of various organic coatings on the silica surface were shown in Figure 5. The electroosmotic mobility in a capillary coated with butyl phase was studied in 10 mM phosphate buffer with the application of various radial electric potential gradients. As shown in Figure 6, the extent of electroosmotic control under the influence of an applied radial electric potential gradient was almost constant at various solution pHs. The electroosmotic mobility in the butyl phasecoated capillary was changed from 2.2 X 1V cm2/V.s with the application of a -10.8-kV potential gradient to about -1 X 10-4 cm2/V-s in the presence of a 5-kV potential gradient. This range of electroosmotic control was similar to the extent of electroosmotic control in a silica capillary with the solution condition of 10mM phosphate buffer and pH 3.299 In addition, the electroosmoticmobility in the butyl phase-coated capillary
ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993
2891
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OH
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1
1
4 1-0 -SI - 0 4 1 -0-SI
-H 1-0 -SI - 0 4
I I-0-SI
-
Flgure 7. Electroosmotic mobility in the capillary coated with QOXEGDE or amino phase at 10 mM phosphate buffer against various solution pHs: (0)experimental data for GOX-EQDE coating: (A) experimental data for amino phase: solid line, the predictlon. The experimental error in measuringthe electroosmotk mobility was about 0-3% for over five runs at each solution pH.
was located about 10 A away from the silica substrate. By I
i
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(b) Flgure 5. Schematic representation of structures of (a) GOX-EQDE (taken from ref 20) and (b) butyl and amino phases.
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assigning a value of zero for both KA(the dissociation constant of surface silanol groups) and K M (the binding constant of counterions to the surface charged sites) in the theory, the prediction of electroosmotic mobility at various radial electric potential gradients was carried out and compared with the measurements. The assumption of K M equal to zero implied that there was no electrolyte species adsorbed on the butyl phase-modified silica surface. As shown in Figure 6, the prediction was in good agreement with the experimental results and further supported our hypothesis of the complete deactivation of surface silanol groups and the lack of adsorption of electrolyte species at the silica surface coated with butyl phase. GOX-EGDE-Coated Capillaries. As shown in Figure 7, the electroosmotic mobility in a silica capillary coated with GOX-EGDE was measured in 10 mM phosphate buffer at various solution pHs. The electroosmotic moblity was increased linearly from 0.3 X 10-4 cm2/V.s at pH 3 to about 1 X 10-4 cmZ/V.s at pH 5 and then reached a plateau region, where the electroosmoticmobility was kept constant around 1 X 10-4cm2/V.s. The electroosmoticmobility would be close to zero and insensitive to the solution pH if all the surface silanol groups were deactivated with GOX-EGDE coating. Some surface silanol groups might remain active even with GOX-EGDE coating, based on the titration curve shown in Figure 7 and the structure of cross-linking polyethers shown in Figure 5. The GOX-EGDE coating consisted of hydrophilic crosslinked polyethers, with a thickness of about 30-40 A.22 The distance between the silica subtrate and the slipping plane was therefore assumed to be 35 A in the calculation. The percentage of the active silanol groups on the coated surface was used as an adjusted parameter in the theory. As shown in Figure 7, the prediction based on 33% active silanol groups was in good agreement with the experimental measurements at various solution pHs. In this case, the magnitude of N,as the density of surface silanol groups was reduced from 5 x 10" sites/cm2 l2 for silica to 1.65 X 10" sites/cm2 for GOXEGDE-coated surface. The electroosmotic mobility in the GOX-EGDE-coated capillary was barely affected by using various radial electric (20) Bao, J. M.;Regnier, F. E. The Third Internntional Symposium on High PerformanceCapillary Electrophoresis, San Diego 1991; Poster PM-26. (21) PersonalcommunicationwithProfeaaorF. E. Regnier,Department of Chemistry, Purdue University.
2892
ANALYTICAL CHEMISTRY, VOL. 65, NO. 20,OCTOBER 15, 1993
potential gradients across the capillary wall. This experimental observation was explained by the fact that the contribution of induced surface charges (under the influence of an applied radial electric potential gradient) to the f potential decreased with the increase in distance between the silica surface and the slipping plane at the interface of the GOX-EGDE/aqueous solution. The effect of the applied radial electric potential gradient on the { potential and the electroosmotic mobility was diminisheddue to the thick layer (about 30-40 A) of GOX-EGDE coating. Amino-Coated Capillaries. The electroosmotic mobility in an amino phase-coated capillaryat 10 mM phosphate buffer was measured as a function of solution pH. As shown in Figure 7, the absolute value of electroosmotic mobility in the direction toward the anodic end was decreased from 4 X 10-4 cm2/V.s at pH 3 to 2 X 10-4 cm2/V.s at pH 5. The direction of electroosmosis was then reversed and toward the cathodic end when the solution pH was increased to pH 7. The isoelectric point (PI)of the amino-coated silica surface could not be determined due to the irreproducible measurements of electroosmotic mobility between pH 5 and 7. The p& value of primary amine groups was reported to be 11.7.22 For a fused-silica capillary fully covered with amino phase, the direction of elecroosmosis would then be toward the anodic end at most solution pHs. Based on the results shown in Figure 7, the theory was employed for investigating the percentageof active silanol groupsremaining on the silica surface coated with the amino phase. It was assumed that the slipping plane was located about 10 A away from the silica substrate. The percentage of surface-active silanol groups was used as a fitting parameter in the calculation. As shown in Figure 7,the prediction based on 70 7% active silanol groups was in good agreement with the experimental measurementsat various solution pHs. In this case,the magnitude of N , as the density of surface silanol groups was reduced from 5 X 10'4 sites/cm2 12 for silica to 3.50 X 1014 sites/cm2for the amino-coated surface. The capillaries coated with amino phase were examined as received. Two reasons could possibly account for the low surface coverage in the amino-coated capillary. First, the Coulombicrepulsion between the charged amino groups might prevent the amino groups from being bound closely on the silica surface. Second, the positively charged amino head groups might be adsorbed strongly to the negatively charged silanol groups and reduce the number of silanol groups available for the coating reaction. The extent of electroosmoticcontrol in the capillariescoated with amino phase was studied with the application of various radial electric potential gradients and is shown in Figure 8 for solution pHs of 3 and 5. By comparing the experimental results shown in Figure 6 with the measurements summarized in Figure 8, we found the extent of electroosmotic control in the capillary coated with butyl phase was always greater than that observed in the amino-coatedcapillary. This conclusion was attributed to the fact that the surface charge density and the f potential were greater in the amino-coated capillary than in the capillary coated with butyl phase. With the application of a 0-kV potential gradient, the amino-coated capillary exhibited larger values of electroosmoticmobility than those measured in the capillary coated with butyl phase. As discussed previously, the effect of induced surface charge (upon the application of a radial electric potential gradient) on the { potential and the electroosmotic mobility became diminished with the increase in the surface charge density as contributed by both silanol and amino groups. (22) Cantor, C. R.; Schimmel, P. R. Biophysical Chemistry, Part I: The ConformotionofBiological Macromolecuks;W. H. Freeman &. Co.: San Francisco, 1980; Chapter 2.
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9
Flguo8. ElectroosmoticmoMlttyaga~appWedradiei~potentiei gradlent In the capillary coeted with amino phase at 10 mM phosphate buffer: (A)experknentai data at pH 8; (0)experlmentai data at pH 5; solid line, the predlction. The expevhmntal error in measwlng the electroosmotic mobillty was about 0-5% for over five runs.
The prediction for electroosmotic control in the aminocoated capillary was calculated and compared with the experimental results. As shown in Figure 8, the prediction was in good agreementwith the trend of electroosmoticcontrol at various radial electric potential gradients. However, the absolute value of predicted electroosmotic mobilitywas lower than the experimental data by about 1X lo-" cm2/V-s. The theory developed in this study did not take into account the interaction between the surface charged groups. The low surface coverage of amino-coated capillary and the chargecharge interaction between the amino and silanol groups might well contribute to the discrepancy between the prediction and the experimental results.
CONCLUSION In this study, we examined the effect of adsorbed ions and organiccoatings on the direct control of electroosmosis at the capillary-solution interface. The adsorption of tin ions and DTAB at the silica-solution interface greatly reduced the { potential and the electroosmotic mobility. However, the reduction in the f potential did not enhance the extent of electroosmotic control under the influence of an applied radial electric potential gradient. This unexpected observationwas analyzed and explainedby a theory based on the combination of a GCSG model and the induced capacitive effect at the silicaaolution interface. The specific adsorption of tin ions and DTAB at the silicaaolution interface contributed to the significant increase in the surface charge density at IHP, up The shielding effect provided by the larger value of U,9 a t IHP not only reduced the {potential but also diminishedthe effect of a radial electric potential gradient on the direct control of electroosmosis. The theory developed in this study was able to describe the electrokinetic phenomena at the silica-solution interface under the influence of an applied radial electric potential gradient. In the presence of simpleelectrolytes (or indifferent electrolytes), the extent of electroosmoticcontrol was mainly dependent on the amount of dissociated surface silanol groups, ogi. Thus, the deactivation of surface silanol groups with the use of butyl coating enhanced the effectiveness of an applied radial electric potential gradient for the direct control of electroosmosis at various solution pHs. In addition to surface deactivation, the thickness in the GOX-EGDE coating and the percentage of active silanol groups remaining on the surface coated with amino phase were important considerations for the electronic adjustment of electroosmosis in capillary electrophoresis.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993
ACKNOWLEDGMENT We thank Dr. Sally A. Swedbergforproviding the capillaries coated with butyl and amino phases. We also thank Professor Fred E. Regnier and Jim M.Bao for providing the capillaries coated with GOX-EGDE. Support for this work by Hewlett Packard Laboratories, NSF Instrumentation Development Program (DIR-9105016),and NSF Research Initiation Award
2893
(CTS-9108875) is gratefully acknowledged. C.S.L. is a National Science Foundation Young Investigator (BCS9258652). for review December 31, 1992. Accepted
1993.a a Abstract published in Advance
1,
ACS Abstracts, September 1,1993.
