Anal. Chem. 1983, 55, 1365-1370
1365
Electroosmotic Propulsion of Eluent through Silica Based Chromatographic Media Timothy S. Stevens” and Hernan J.
Cortes
Dow Chemical USA, Michigan Divislon, AnaJytlcal Laboratorles, 674 Building, Midland, Michigan 48640
Electrooqnosls !s the flow of llquld along a caplllary or through porous medla by the appllcatlon of an electrlc field. Used as an alteriiatlve to a pump In llquld chromatography (LC), electroocmosls generates flow wlthout the need to apply oxternal pressure to the eluent, and flow rate Is Independent of column length wlth a glven potentlal gradlent. The veloclty of eluent flow Is glven by the Helmholtz-von Smoluchowskl equatlon when the pore slze through the LC medle is relatlvely large, e.&, 20 pm, and can be 1 cm 8-’ at a field strength of 900 V cm-‘. However, when the hlghly efflclent small dlnmeter LC packlngs are used (dp = 10 pm), Rhe relatlvely small pore size of the bed (-2 pm) causes a dgnlflcantly reduced flow. Of the eluents studled (methanol, water, methanol-water, and acetonltrlle) methanol was least affected by a reductlon of packlng diameter.
Electroosmosis is represented as an excellent means of propelling eluent through a liquid chromatography (LC) column, especially through a column packed with relatively small particles where flow would otherwise be difficult to obtain (I). It is also represented as a means of reducing band spreading due to its favorable flat flow velocity profile vs. the parabolic flow velocity profile observed with pumped flow (2, 3). Electroosmosis-chromatography was brought to our attention by an evaluation ( 4 ) made of a technique named “electro molecular propulsion” (portrayed as an invention involving semiconducting solvents (5)). This evaluation concluded that the technique was actually a combination of electrop horesis and paper chromatography using electroosmosis as a means of propelling the eluent through the paper, much lihe the work of Mould and Synge in 1954 using collodion membranes (6). The subject of this contribution is the study of eluent propulsion through silica based chromatographic packing using electroosimosis and a clarification of the limitations involved. THEORY OF ELECTROOSMOSIS Electroosmosis is usually explained using the SternGouy-Chapman model of the electrical double layer a t a charged interface (7, 8). Considering a silica gel surface in contact with water, some of the silicic acid groups are ionized causing the surface to be negatively charged. In solution is an equivalent of hydronium ions. The hydronium ions are arranged into two layers. The layer next to the surface is tightly blound by electrostatic forces and is called the ‘‘compact region”. Thermal motion causes some of the ions in the compact, region to diffuse further out into solution to form the “diffuse region” of the double layer. If an electric field is imposed tangentially to the surface, the ions in the diffuse region will migrate toward the oppositely charged electrode and carry water along with them. This is called electroosmotic flow. Figure 1 is ca representation often made ( I , 7) when explaining electroosmosis and electrophoresis. The term q0 is the electrical potential at the interface between the solid and liquid. Out into solution decreases as represented by the curve. It decreases linearly in the compact region and then
+
exponentially in the diffuse region. The term +d is the potential at the interface between the compact and diffuse regions of the double layer. The distance out in solution from this interface to a point where = O.371Cid is conventionally termed the “thickness of the double layer”, 6 (7). If the bulk solution flows tangentially to the surface, a “plane of shear” is established a t oir just outside the interface between the compact and diffuse regions. The plane of shear@ not at the surface of the solid1 because the ions in the compact region have associated solvation spheres that are held stationary. The potential at the plane of shear is called the zeta potential, {. The addition of an added electrolyte to the bulk solution compresses the double layer as represented by the dashed curve in Figure 1 (with a corresponding reduction of {). Equation 1 (the Helmholtz-von Smoluchowski equation) is given (1,2,3, 7) for calculating the velocity of electroosmotic flow
+
where V is the linear velocity of eluent flow through the passageway, Z is the dielectric constant of the eluent, 9 is the viscosity of the eluent, E is the electrical field strength down the passageway, and { is the zeta potential. Silica-water systems are reported to have a zeta potential of about 150 mV. A linear velocity fo:r water of 9 mm s-l would be predicted by eq 1a t a field strerrgth of 900 V cm-l. The linear velocity of eluent pumped at 2.7 mL m i d through a 4.6 mm i.d. packed column is also about 9 mm s-l. Since field strengths of 900 V cm-l are obtainable across normal length LC columns, electroosmosis is offered as an alternative to pumped flow (1-3). The flow velocity profile across a passageway is reported (2, 7) to be different for electroosmotic flow than for laminar flow as shown in Figure 2. A characteristic parabolic velocity profile results from laminar flow. With electroosmosis, the velocity increases through the double layer and then is flat to the center of the passageway. This flat fronted profile results in less band spreading with electroosmosis LC than pumped flow as slhown by Pretorius et al. for 75-125 pm packing using an eluent of methanol-water and with a 500 pm i.d. capillary uoing an eluent of methanol (2). Tsuda et al. recently confirrned this effect with capillary columnR as small as 132 pm i.d. by using an eluent of water containing dissolved Na2HP04 (9).
EXPERIMENTAL SECTION The apparatus uried is shown in Figure 3. An electrical potential was applied t o the coiled platinum wire electrodes using an electrophoresis power supply (Brinkman Pherograph, T’ype Mini 68, Brinkman Lnstruments, Westburg, NY).The tubing used was Microline No. 18160, 0.040 in. i.d. X 0.070 in. 0.d. (Thermoplastics Scientifics Inc., Warren, NJ). The tube length was 2 cm for all experiments. The 10-mL Pyrex glass beakers had 0.065 in. holes drilled in them near their bottoms, and the tubing was pulled into the holes resulting in a press fit between the tube and beaker. The following ch~omatographicpackings were slurry packed into the tube (held in place by plugs of silanized glass wool at each end): (a) Bio-8il-A silica gel, 100-200 mesh, dp = 100 pm
0003-2700/83/0355-1~65$0l.50/00 1983 American Chemical Society
1366
* ANALYTICAL CHEMISTRY, VOL.
55, NO. 8,
JULY 1983
Sulk Solution
0 Compact Region
1
Diffuse Region
Distance
-
Table I. Electroosmotic Flow Observed for Various Silica Based Chromatographic Packings Using an Eluent of 85%Methanol, 15%Water calcd current linear passageway obsd,a velocity, radius, pm packing mms-’ PA Bio-Sil-A, 10 9 2.6 dp 100 pm
