Anal. Chem. 1996, 68, 3840-3844
Electrostatic Spraying: A Novel Technique for Preparation of Polymer Coatings on Electrodes Boy Hoyer,* Gunnar Sørensen,† Nina Jensen, Dorthe Berg Nielsen, and Bent Larsen
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Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark
A liquid flow emerging from a tip or a thin tube under the influence of a strong electric field will, due to charging of the dielectric liquid, break up into small droplets. Thus, if a polymer material is dissolved in the liquid, this electrodeposition technique can be utilized for producing polymer coatings on electrodes. The method was applied for in situ formation of ultrathin (∼3000 Å) cellulose acetate (CA) phase inversion membranes on glassy carbon electrodes. The purpose of the membrane was to protect the electrode surface from fouling by macromolecular species. The spraying liquid consisted of CA, acetone, and aqueous magnesium perchlorate as pore former, and the spraying voltage was 14 kV. Profilometric measurements showed that the thickness of the spray-cast membranes was much more uniform than that of similar membranes formed by solvent casting. By using cadmium and lead as test analytes and differential pulse anodic stripping voltammetry as detection method, it was found that the membranes prepared by spray casting offered better protection against interference from poly(ethylene glycol) (PEG) 6000 than those prepared by solvent casting. Also, the interference from PEG 2000 was significantly reduced. Experimental details of the electrostatic spraying technique are given. Electrodes coated with a thin layer of polymer have been intensively studied in the last decades1 and remain a highly active area of research.2 In electroanalytical chemistry, the main reasons for applying polymer films on electrodes are the attainment of electrocatalytic properties, preconcentration of the analyte in the film, and selective exclusion of interfering species.3 Probably, the most generally applicable and convenient method for coating electrodes with polymers is solvent casting, in which a small volume of a solution of the polymer is applied to the electrode surface, and the solvent is allowed to evaporate. However, this approach generally yields films of nonuniform thickness,4,5 although the situation is improved if the evaporation of the solvent is very slow.6 In contrast, polymer films of uniform thickness can be obtained by spin coating, in which a solution of the polymer is
applied to a rotating disk.7 The outward, radial flow of the solution is counteracted by the gradual increase in viscosity due to the evaporation of the solvent, leading to a uniform coverage of the disk.7 The method has been used for coating relatively large electrodes (2.5-cm diameter),8 but due to edge effects, it is not suitable for microelectrodes with a diameter of a few millimeters. Also, it is a prerequisite for the proper functioning of spin coating that the substrate is homogeneous, and electrodes fitted in insulating materials, therefore cannot be coated by this technique. In this work, the principle of electrostatic spraying has been applied for coating electrodes with polymers. A liquid stream emerging from a tip or a tube is under the influence of a strong electrical field, and owing to the electrostatic repulsion generated by the charging of the liquid, it breaks up into droplets.9 This effect can be utilized for generation of an aerosol with submicrometer-size droplets.10 By dissolving polymers in the spraying liquid, it is possible to deposit a polymer layer on electrically conducting substrates, including electrodes. Electrostatic spraying is widely used in industry and agriculture for dispersing paints and pesticides,9 but to the best of our knowledge, it has not previously been used for preparation of polymer-coated electrodes. Pneumatic spraying of size-exclusion membranes onto disposable test strips for clinical use has recently been described.11 It will be shown in the present work that electrostatic spraying yields polymer coatings with a much more uniform thickness than is obtained by solvent casting. Moreover, the apparatus for the electrostatic spray coating technique is inherently simple and lends itself for laboratory-scale preparation of modified electrodes. Our interest in developing a coating technique for electrodes superior to evaporative casting emanated from work toward the development of thin size-exclusion membranes for voltammetric electrodes.12,13 The purpose of the membrane is to prevent fouling of the electrode surface by discriminating against macromolecular species, which often cause severe adsorption interferences when complex samples are analyzed by electrochemical methods.14,15 Kuhn et al.16,17 and we12,13 have shown that cellulose acetate (CA) membranes prepared by the phase-inversion (PI) process hold
* To whom correspondence should be addressed. Fax: +45 86 19 61 69. E-mail:
[email protected]. † Institute of Physics and Astronomy, Aarhus University, Ny Munkegade, DK8000 Aarhus C, Denmark. (1) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, pp 191-368. (2) Ryan, M. D.; Bowden, E. F.; Chambers, J. Q. Anal. Chem. 1994, 66, 360R427R. (3) Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Chem. 1987, 59, 379A390A. (4) Emr, S. A.; Yacynych, A. M. Electroanalysis 1995, 7, 913-923. (5) Daum, P.; Murray, R. W. J. Phys. Chem. 1981, 85, 389-396. (6) Nakahama, S.; Murray, R. W. J. Electroanal. Chem. 1983, 158, 303-322.
(7) Meyerhofer, D. J. Appl. Phys. 1978, 49, 3993-3997. (8) Schroeder, A. H.; Kaufman, F. B.; Patel, V.; Engler, E. M. J. Electroanal. Chem. 1980, 113, 193-208. (9) Bailey, A. G. Electrostatic Spraying of Liquids; John Wiley: New York, 1988. (10) Meesters, G. M. H.; Vercoulen, P. H. W.; Marijnissen, J. C. M.; Scarlett, B. J. Aerosol Sci. 1992, 23, 37-49. (11) Black, M.; Lin, L.; Guthrie, J. Electrochemical Sensors. PCT Int. Appl. WO 94/27140, 1994. (12) Hoyer, B.; Jensen, N. Talanta 1995, 42, 767-773. (13) Hoyer, B.; Jensen, N. Talanta 1996, 43, 1393-1400. (14) Brezonik, P. L.; Brauner, P. A.; Stumm, W. Water Res. 1976, 10, 605-612. (15) Treloar, P. H.; Christie, I. M.; Vadgama, P. M. Biosens. Bioelectron. 1995, 10, 195-201. (16) Kuhn, L. S.; Weber, S. G.; Ismail, K. Z. Anal. Chem. 1989, 61, 303-309. (17) Kuhn, L. S.; Weber, S. G. Electroanalysis 1991, 3, 941-948.
3840 Analytical Chemistry, Vol. 68, No. 21, November 1, 1996
S0003-2700(96)00550-1 CCC: $12.00
© 1996 American Chemical Society
Figure 1. (A) Diagram of spray-coating apparatus and (B) cross section of electrode mounted in the brass housing. The drawing is not to scale.
great promise for this purpose. The membranes are cast in situ on the electrode surface. In the PI process,18 the permeability of the membrane is controlled by adding a pore former to the casting solution. When the casting solvent has evaporated, the membrane is immersed into a nonsolvent gelation bath, where the polymer precipitates as the pore former is exchanged by nonsolvent. Generally, anisotropic membranes with a dense skin layer and a porous substructure are obtained.18 In our previous work, we have used the four-component system of Loeb and Sourirajan,19 in which acetone serves as solvent for CA and aqueous magnesium perchlorate is the pore former. The membranes were cast on glassy carbon, and a thin layer of mercury was subsequently plated on the electrode. It was found that the membrane coating eliminated interference from albumin (66 000 MW) and lysozyme (15 000 MW) in differential pulse anodic stripping voltammetry, while the interference from poly(ethylene glycol) (PEG) 6000 was reduced. These compounds all produce a large signal reduction at the conventional thin mercury film electrode (TMFE). The inevitable loss of sensitivity due to the diffusional resistance of the membrane compared favorably with studies in which preformed bulk membranes were mounted on the electrode.20 The present work is a continuation of our effort to lower the molecular weight cutoff of the membrane while keeping the loss of sensitivity at a minimum. The production of membranes with uniform thickness by electrostatic spraying is important in this context because mass transport will be reduced at the thicker parts of an inhomogeneous membrane without any accompanying improvement of the overall permselectivity. (18) Kesting, R. A. Synthethic Polymeric Membranes, 2nd ed.; John Wiley: New York, 1985. (19) Loeb, S. ACS Symp. Ser. 1981, 153 (Vol. I), 1-9. (20) Aldstadt, J. H.; Dewald, D. H. Anal. Chem. 1993, 65, 922-926.
EXPERIMENTAL SECTION Apparatus. The spray coating apparatus is shown in Figure 1. The spraying solution is fed to the needle from a small, graduated glass buret with a screw-type Teflon stopcock. The tip of the stopcock was given a conical shape which allowed easy pressure mounting of standard injection needles. Before spraying, the glassy carbon electrode was inserted into a cylindrical brass housing (cf. Figure 1B). The glassy carbon was exposed to the spray through a hole (3.8-mm diameter with beveled edge) in the lid, which was 0.3 mm thick. The diameter of the hole had to be larger than that of the glassy carbon electrode due to small variations in the diameter of individual glassy carbon disks. Also, the glassy carbon was placed slightly eccentrically in the Teflon body in some electrodes. The ring of Teflon, which was thereby exposed, was masked off by a 0.05mm-thick disk of metal foil with a hole which closely matched the circumference of the glassy carbon. The disk was held in place by pressing the electrode against the lid of the brass housing. The electrode was connected to ground by a screw that fitted into the thread which is normally used for mounting the electrode on the rotor shaft. During spraying, the brass housing with the electrode was placed on a grounded metal plate. In the initial experiments, a modified 30-gauge injection needle (0.30-mm o.d., 0.14-mm i.d.) was used as the spraying tube. First, the needle was cut perpendicular to its axis by immersing the tip in an electropolishing bath, consisting of 92% (v/v) glacial acetic acid and 8% (v/v) perchloric acid (70% (w/w)), and applying +5 V to the needle versus a stainless steel cathode. Hereafter, the needle was lowered further into the bath and electropolished for a short period. Microscopic examination of the tip showed that this procedure yielded a regular and smooth edge. The second type of spraying tube was prepared from a 0.2-mm-o.d. and 0.1Analytical Chemistry, Vol. 68, No. 21, November 1, 1996
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mm-i.d. stainless steel tube (obtained from Goodfellow, Cambridge, U.K.), which was fastened in the tip of a 27-gauge injection needle with epoxy glue. The tube was then cut, and its edge was smoothed by electropolishing as described above. This type of spraying tube was used for preparation of all spray-coated electrodes. The high-voltage power supply for the electrostatic spraying technique can be built from rather inexpensive components because the current generated in the spray process is very small (typically in the low nanoampere range10), and the required output power is, therefore, minute. For the same reason, the electrical hazard can be lowered by inserting protective resistance into the high-voltage lead. Our power supply was based on a Model 4300 transformer from EMCO (Sutter Creek, CA) and equipped with 280 MΩ protective resistance. The spraying tube was on a positive potential relative to ground. Voltammetric measurements were performed with a programmable electrochemical analyser.21 The electrochemical cell comprised a Metrohm 628-50 rotating disk electrode unit with a 3-mmdiameter glassy carbon electrode embedded in a 10-mm-diameter Teflon cylinder. The reference electrode was a Radiometer K401 saturated calomel electrode (SCE), while the counter electrode was a glass-fitted platinum wire (1 cm long, 0.5 mm thick). The constant humidity environment needed for the pregelation drying of the PI membranes was established in 500-mL soft plastic bottles, in which the air was in equilibrium with an aqueous solution of lithium chloride. Each bottle contained the appropriate amount of the salt dissolved in 50 g of water. In this manner, a relative humidity from 13% to 100% (25 °C) can be obtained.22 The electrode tip was inserted into the bottle through a hole cut in the side. The thickness of the polymer coatings was measured with a Sloan DEKTAK 3030 profilometer using a stylus force of 0.1 mN. The roughness was determined by an algorithm which calculates the average vertical distance between the individual data points in the profile and a regression line fitted to all points in the selected part of the profile. The thickness was found by scraping off small pieces of the coating and measuring the step height across the borderline between the coated and bare parts of the electrode. Reagents. Buffers and supporting electrolyte media were prepared from Merck Suprapur reagents and triply distilled water, while other reagents were of analytical grade. The acetone was a special dry grade (Merck, maximum 0.01% (w/w) water). Cellulose acetate (39.8% acetyl content, 30 000 MW) was obtained from Aldrich. The composition of the casting and spraying solution for the PI membranes was 20 g of acetone, 0.250 mL of pore former solution, and 0.05 g of CA. Unless otherwise stated, the pore former was 0.9 M aqueous Mg(ClO4)2. Procedure for Preparation of Coated Electrodes. Prior to coating, the glassy carbon electrode was polished with 0.25-µm diamond paste, rinsed with ethanol, and dried with lens paper. Coating with CA by solvent casting was done by inverting the rotating disk electrode and applying 2.5 µL of the casting solution to the glassy carbon surface, while the electrode was spinning at 1000 rpm. When the acetone had evaporated, the electrode tip was placed in a bottle with 21% relative humidity for 10 min. The (21) Thomsen, K. N.; Skov, H. J.; Kryger, L. Anal. Chim. Acta 1989, 219, 105121. (22) Landolt-Bo¨rnstein Physikalish-Chemische Tabellen, 3rd supplementary volume; Springer Verlag: Berlin, 1936; p 2504.
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Analytical Chemistry, Vol. 68, No. 21, November 1, 1996
electrode was then quickly withdrawn from the bottle and immersed in an ice water gelation bath (0 °C) for 30 min. This transfer was done in less than 1 s. Mercury was deposited on the glassy carbon/CA substrate by electrolysis for 10 min at -1 V vs SCE in a deaerated acetate buffer spiked with 7.5 × 10-5 M mercury(II). In the following, this electrode preparation will be referred to as PI-TMFE III in order to facilitate comparison with our previous work. Coating of the electrode with CA by electrostatic spraying was done with the following settings: spraying voltage, 14 kV; spraying volume, 200 µL; spraying time, 10 ( 1 min; distance from tip of tube to electrode, 3.5 cm. The linear and the volume flow rates of the liquid are respectively 4 cm/s and 0.3 µL/s. The electrode surface was covered with a metal plate during the initial adjustment of the spray. After spraying, the electrode was removed from the brass housing, inserted into a bottle with saturated acetone vapor for 5 s, and conditioned in 21% relative humidity for 10 min. The subsequent treatment was the same as that for the electrodes coated by solvent casting. This electrode preparation is called PI-TMFE IV in the following. A conventional TMFE was prepared in the same manner without the polymer coating and with only 2.5 × 10-5 M Hg(II) in the plating solution. Procedure for DPASV Measurements. All measurements were performed in 0.1 M acetate buffer spiked with 2.0 × 10-7 M Cd(II) and 2.0 × 10-7 M Pb(II). Solutions were deaerated with argon for 5 min prior to the DPASV measurement. The working electrode was rotated at 750 rpm during deposition, while stripping was carried out in a quiescent solution following a 15-s rest period. The stripping signals were recorded in differential pulse mode with the following instrumental settings: deposition potential, -1000 mV vs SCE; deposition time, 2 min; scan range, -1000 to -50 mV vs SCE; pulse height, 50 mV; pulse width, 20 ms; sampling time, 2 ms; pulse repetition time, 0.24 s; effective scan rate, 12.3 mV/s. A freshly prepared electrode was preconditioned by performing two deposition/stripping cycles in the acetate buffer. In the interference experiments, the surfactant concentration in the test solution was raised incrementally in the following sequence: 1, 2, 5, 10, and 20 ppm. Two stripping measurements were carried out after each addition. In this manner, the dependence of the DPASV peak current on the surfactant concentration could be studied, and the adsorption of surfactant onto the working electrode was closer to equilibrium. RESULTS AND DISCUSSION Characteristics of the Electrostatic Spray-Coating Technique. The description given in the following is primarily founded on observations made with the acetone-based CA solutions discussed previously as the spraying liquid. In general, the droplet size and the spatial distribution of the spray depend on the viscosity, surface tension, and conductivity of the liquid,9,10,23 and the values of the experimental parameters need to be optimized for each system. The fineness and stability of the spray increased with the voltage, and the spray appeared as a regular cone with a top angle of ∼90° at the optimal value. At even higher voltages, the aerosol formation shifted from the tip of the emerging liquid cone to a number of points on the edge of the tube. This phenomenon (23) Cloupeau, M.; Prunet-Foch, B. J. Electrost. 1989, 22, 135-159.
Table 1. Effect of Surface-Active Compounds on the DPASV Peak Current for Cadmium and Lead Cd surfactant added nonec PEG 6000 PEG 2000
Pb
electrode type
mean change of peak currenta/%
SD of change of peak current/%
mean change of peak currenta/%
SD of change of peak current/%
nb
TMFE PI-TMFE IIId PI-TMFE IV TMFE PI-TMFE IIId PI-TMFE IV TMFE PI-TMFE IV
-16 -9 +1 -85 -41 -27 -56 -17
7 3 2 0.4 2 7 12 4
-4 -13 -1 -38 -9 -5 -8 -2
6 2 4 0.6 0.1 2 6 4
11 2 4 3 2 7 9 9
a Change in DPASV peak current after incremental addition of 20 ppm surfactant (see Experimental Section). b Number of individual electrode preparations. c Change in signal after the same number of DPASV runs as in the experiments in which surfactant was added. These data reflect the signal stability of the electrode. d Data from ref 13.
resulted in the formation of small jets, and the spray became anisotropic. These observations are in general agreement with results from previous work.9,10,23 The droplet size of the spray increased with the flow rate. However, for a given voltage, there was a lower limit to the flow rate, below which the spray became unstable due to formation of small jets at the edge of the tube. This minimum flow rate increased with the voltage. The modified 30-gauge injection needle was acceptable as a spraying nozzle, but the thinner 0.2-mm-o.d. tube was superior because it yielded a finer spray, and it was used for preparation of all electrode coatings in this work. A deposit of CA gel tended to build up in the tube in spite of frequent flushing with acetone, but cleaning the tube each day by drawing a thin wire through it prevented any problems with clogging. The final parameter settings chosen for preparation of the CA coatings (see above) yielded a fine, stable, reproducible, and symmetrical spray. The spatial distribution of the CA deposition was evaluated by placing a thin, mirror-like stainless steel plate on top of the brass housing and observing how the coating evolved. It was clear that maximum CA deposition occurred immediately below the tube, and the deposition decreased symmetrically with distance away from this point. The correlation between the appearance of the spray and the deposition on the plate was further supported by the observation that an irregular pattern was formed on the plate when the apparatus was operated under conditions which yielded a spray with jets. The trajectories of the droplets in the spray are quite sensitive to perturbations of the electrical field. Therefore, care must be taken that the electrode and its housing are properly grounded. Also, the insulating material, in which electrodes are normally fitted, must be fully shielded off by metal. For example, we found that the area of Teflon, which was exposed in the gap between the glassy carbon electrode and the brass housing (see Experimental Section), was sufficient to cause edge effects on the coating. This problem disappeared when a ring of metal foil, which closely fitted the glassy carbon, was placed on top of the Teflon. In one experiment, a small piece of rubber was deliberately placed on the electrode surface prior to spraying, and the resulting irregularity in the coating was more pronounced than a mere “shadow” effect. Profilometric Examination of CA Coatings Prepared by Solvent Casting and Electrostatic Spraying. The ratio R between the maximum and minimum thickness of each membrane
ranged from 2.6 to 4.6 for solvent-cast CA membranes prepared without rotation during application and evaporation of the solution. Generally, the thickness increased from the center of the electrode (∼1000-1500 Å) toward the edge (∼4000-6000 Å), but thin areas close to the edge were also found on some coatings. Rotation of the electrode during application and evaporation of the solution did not change this pattern noticeably. It should be noted that the rotation does not make the coating technique equivalent to spin coating because the casting solution is confined to the glassy carbon due to the hydrophobic nature of the surrounding Teflon material. Obviously, the formation of the polymer layer by solvent casting is a chaotic and poorly reproducible process. The roughness of the coatings was typically 30 Å. The thickness of the spray-coated CA membranes (∼3000 Å) was much more uniform, and the above-mentioned ratio R was reduced to a value ranging from 1.2 to 1.4. There was no systematic variation in the thickness with the distance from the center of the electrode. Most likely, the residual variation in thickness can be attributed to imperfect alignment of the spraying tube with the center of the electrode and to small variations in the spatial distribution of the spray. In both cases, the homogeneity of the film could be improved further if desired by moving or turning the electrode slightly during spraying. The treatment of the coating with saturated acetone vapor after spraying typically reduced the roughness from 250 to 30 Å. Thus, the short exposure to the solvent is sufficient to dissolve and smooth the top layer of the coating, which also became much more glossy. Antifouling Properties and DPASV Response Characteristics of the CA-Coated Electrodes. Compared to the solventcast PI-TMFE III, the spray-coated PI-TMFE IV allowed a further reduction of the severe interference from PEG 6000 on the cadmium signal at the naked TMFE (Table 1). Also, the interference from PEG 2000 on the cadmium signal was significantly reduced at the PI-TMFE IV. Although PEG 2000 should penetrate the membrane barrier more easily than PEG 6000 owing to its lower molecular weight, it was found that the latter caused the largest signal suppression at the PI-TMFE IV for cadmium as well as lead. The explanation is probably that PEG 6000 is the worst interferent, as can be seen from the data obtained with the TMFE, and the quantity of PEG 6000 which passes the membrane may actually be lower than in the case of PEG 2000. Table 1 also shows that the PI-TMFE IV has a better signal stability than the TMFE as well as the PI-TMFE-III. Analytical Chemistry, Vol. 68, No. 21, November 1, 1996
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Table 2. DPASV Peak Current of Cadmium and Lead Obtained with the PI-TMFE and TMFE Cd
Pb
mean peak SD of peak mean peak SD of peak electrode type current/µA current/µA current/µA current/µA
na
TMFE PI-TMFE IIIb PI-TMFE IV
20 8 18
a
5.6 1.6 1.0
2.0 0.3 0.3
11.7 2.4 2.1
2.3 0.6 0.6
Number of individual electrode preparations. b Data from ref 13.
It appears from Table 2 that the DPASV peak currents obtained with the TMFE are reduced by a factors of respectively 3.6 and 4.2 for cadmium and lead at the PI-TMFE III, while the corresponding factors for the PI-TMFE IV are 5.7 and 4.8. This dampening of the signal compares favorably with previous work, in which the sensitivity of ASV was reduced by a factor as high as 18 when the electrode was protected by a bulk CA membrane.20 Also, it has previously been reported that the response of an amperometric detector was reduced between 10 and 20 times when a base-hydrolyzed CA film was applied to the working electrode.24 The influence of the concentration of magnesium perchlorate in the pore former on the discriminative power of the PI-TMFE IV toward PEG 2000 is shown in Figure 2. PEG 2000 mainly interferes with the cadmium signal (cf. Table 1), and the best rejection of this interference was obtained with 0.9 M Mg(ClO4)2 in the pore former. It is not understood why the effect of PEG 2000 is more pronounced when the pore former concentration is lowered to 0.7 M Mg(ClO4)2, because the membrane should, in principle, become less permeable.18 However, a similar effect has been observed for the albumin interference.12 The DPASV peak currents obtained with 0.7, 0.9, and 1.1 M Mg(ClO4)2 as pore former were approximately in the ratio 1:2:4 for cadmium as well as lead. These results emphasize the point that a gain in permselectivity of the membrane is not necessarily offset by a loss of sensitivity. (24) Wang, J.; Hutchins, L. D. Anal. Chem. 1985, 57, 1536-1541. (25) Francis, S. P.; Di Luzio, F. C.; Gillam, W. S.; Kotch, A. Fabrication and Evaluation of New Ultrathin Reverse Osmosis Membranes; Research and Development Progress Report No. 177, Office of Saline Water, United States Department of the Interior; U.S. Government Printing Office: Washington, DC, 1966. (26) Riley, R. L.; Lonsdale, H. K.; Lyons, C. R.; Merten, U. J. Appl. Polym. Sci. 1967, 11, 2143-2158. (27) Sørensen, G., to be published.
3844 Analytical Chemistry, Vol. 68, No. 21, November 1, 1996
Figure 2. Effect of the concentration of magnesium perchlorate in the pore former on the discriminative power of the PI-TMFE IV toward PEG 2000. 2, cadmium; 9, lead. Apart from the varied parameter, the standard formulation of the spraying solution was used (see Experimental Section). The ordinate axis shows the change in the DPASV peak current after incremental addition of 20 ppm PEG 2000. Each point represents the average of not less than five electrode preparations.
At present, we are exploring the possibility of lowering the molecular weight cutoff of the ultrathin CA membranes further by optimizing the steps in the preparation procedure which follow the spray coating. The feasibility of achieving this goal is supported by previous work which showed that CA membranes with a thickness as low as 1000 Å can be used for desalination by reverse osmosis.25,26 Unlike spray coating, the membrane preparation methods employed in these studies are not applicable for in situ formation of a protective coating on an electrode surface. In conclusion, our results have shown that electrostatic spraying yields CA membranes of high quality and with uniform thickness. However, several solvents (including ethanol, 2-propanol, and 1-butanol) can be atomized by electrostatic spraying,27 and the technique, therefore, has a much wider scope for preparation of polymer-coated electrodes than the application described in the present study. The optimum settings of the spray parameters for each liquid, including the voltage and the flow rate, can vary considerably, but they are easily established by visual observation of the spray. Received for review June 4, 1996. Accepted August 16, 1996.X AC9605509 X
Abstract published in Advance ACS Abstracts, October 1, 1996.
