ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
483
Surface Analyses by a Triboelectric Charging Technique Harry W. Gibson," John M. Pochan, and F. C. Bailey Webster Research Center, Xerox Corporation, Webster, New York
A device for measuring triboelectric charge exchange is described. Triboelectric charging is a surface phenomenon. It is sensitive to the chemical (molecular) structure of the surface. The utility of the technique for detection of surface contamination by mass transfer and of chemical transformations of surfaces is demonstrated by description of several examples. Not only can such changes be detected, but by a knowledge of the relationship between triboelectric charging and molecular structure, deductions as to the nature of the new surface species can be made.
Triboelectrification is a well known phenomenon ( 1 , 2). I t is essential in the processes of xerography (3, 4 ) , electrostatic painting ( 5 ) ,and electrostatic separations (6),among others (6-8). While most of the work on triboelectric charging is directed toward these ends, the sensitivity of triboelectrification t o the presence of foreign species on the surface also offers the opportunity for its utilization in analytical techniques for surfaces. T h e present communication describes the utility of triboelectrification t o detect surface contamination in one system in some detail. Then several examples of the sensitivity of the technique to chemical alterations of surfaces are discussed. EXPERIMENTAL Triboelectric Charging Measurements. The device of Figure 1 was employed. The angle of the incline was 45'. The drop height from the hopper to the film was 1 cm. The path length was 7 inches. The hopper of 1-inch width was centered on the width (4 inches) of the film. The electrometer was a Keithley Model 610C (solid state) operated in the fast feedback mode on the lo-' coulomb scale. A Mettler P1200N balance was employed for mass measurements. The entire setup except for the electrometer was housed in a dry box maintained a t zero humidity under a positive pressure of air passed through several drying columns. Films and beads were dried as indicated in the vacuum antechamber of the dry box prior to examination. Film Preparation. Films were cast from solutions as indicated by use of a motorized Gardner Laboratory draw bar coater, typically with 10 wt % solutions and an 8-mil draw bar. Metal Beads. The 100-pm nickel berry beads were obtained from Sherritt-Gordon Co. The 100-pm steel beads were acquired from Nuclear Metals Co. All beads were washed, dried and stored as per Table I. Organic Materials. J. Yanus of Xerox Corporation supplied a sample of bis(4-diethylamino-2-methylphenyl)phenylmethane (1). General Electric Co. supplied Lexan 145 (2) in pellet form, which was precipitated once from chloroform into methanol prior to use. R. L. Schank of Xerox Corporation provided a sample of acetone oxime blocked toluene-2,4-diisocyanate ( 5 ) . At temperatures above 110 "C, this blocked diisocyanate efficientllreverts to acetone oxime and toluene-2,4-diisocyanate (9). Dow 666U polystyrene that had been precipitated once from tetrahydrofuran into methanol was employed in the oxidation experiment. For the sulfonation experiment, a free-standing film of Dow Trycite, type 1000 was used. Reaction of Blocked Diisocyanate 5 and Copolymer 7. A film was prepared as described above from the following solution: 0.4751 g of 7 (0,288 mequiv of hydroxyl) ( I O ) , 0.498 g of 5 (0.312 mequiv of isocyanate groups) and 10 mL of THF. The film was dried a t room temperature in vacuo for several hours. Then it 0003-2700/79/035 1-0483$01 . O O / O
14580
Table I. Effect of Exposurea to 1 o n the Chargingb of Metal Beads' vs Fresh Lexan Filmsd bead charge (nC/g)e clean bead bead exposed clean bead bead (1st) to 1 ( 2 n d ) (3rd) 100-pm t 1 . 2 4 i 0.09 -1.32 i 0.05 + 0 . 5 9 5 * 0.055 nickel berry 100-Mm -1.64 * 0.13 -2.86 z 0.15 -2.10 * 0.16 steel Exposed by cascading clean beads once over a pure film of 1 o n cascade device (Figure 1). Film of 1 was cast from 10 w t % CH, C1, solution o n t o brush grained aluminum using an 0.008-inch draw bar; dried a t 25 " C , 1 mm Hg for 48 h. Thickness 20 pm. Charging measurements o n cascade device of Figure 1 at zero humidity. Washed successively with n-hexane, acetone, water, methanol, methylene chloride, carbon tetrachloride, chloroform, and diethyl ether; dried at 45 "C, 4 mm Hg for 64 h ; 25 C, 1 mm Hg for 6 h ; stored in grounded capped aluminum bottles in dry box at 0% relative humidity. Films cast o n t o brush-grained aluminum substrates using 10% by weight methylene chloride solutions and 0.008-inch draw bar; dried a t 25 OC, 1 mm for 16-63 h ; thickness 20 pm. Fresh in this context means that the film has not been contacted with anything prior t o the experiment. e Average of 3 to 9 determinations (2-8 g each) t standard deviation.
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was heated in an oven at 185 ' C incrementaly. After each heating period, the triboelectric charging capacity of the film was determined. When this reaction was carried out on an NaCl disk, the infrared spectrum underwent a sharpening and increase in intensity of the 1715 cm-' (C=O) peak, loss of the 3360 cm-I (OH) peak, and slight enhancement of the weak 3420 cm-' (NH) peak.
DISCUS SI 0 N T r i b o e l e c t r i f i c a t i o n M e a s u r e m e n t s . T h e device used t o measure the triboelectrification is shown in Figure 1. I t consists of a grounded inclined plane. A film of the substance to be examined is mounted on this plane. Typically these films are cast from solution onto a n aluminum sheet, b u t freestanding films can be used. At the top of the incline is a grounded metal hopper which contains metal beads. T h e metal beads are allowed t o cascade down the film and are caught in a metal receptacle isolated from the surroundings by a metal Faraday cage. T h e receptacle is not grounded but rather is attached to an electrometer, which is used to measure the charge on the beads. By measuring the mass of the beads, the charge t o mass ratio (Q/M) characteristic of the film and metal bead is obtained. Q / M is independent of mass over the range of masses employed. T h e device is an adaptation (11) of a n apparatus previously reported (6). Good reproducibility is generally obtained with the device. For example, using six Lexan polycarbonate films on aluminum substrates and four to seven individual charging determinations on each film, the average charging value was +1.11 f 0.14 nanocoulombs (nC) per gram of 100-pm diameter nickel berry. Our experience of eight years of such measurements indicates t h a t the precision of the technique as estimated by standard deviation is 10% or less of the average value from 0 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 4 , APRIL 1979 METALLIC ( S E E L , NICKEL. ZINC, E T C . 100, Z50p DIAMETER) B E D S
METAL A
+
t---
HOP
LEVEL FERMI
METAL B
ORGANIC SOLID LUMO
AE:
I
A&
LEVEL FERMI
l
AE:
--
-*-
--
--
HOMO-+--
-c
~
~~
c
& RECEPTACLE METAL
METAL A
A E ~ A E ~:.ORGANIC
METAL B
AE: < A E ~ :.ORGANIC
CHARGES POSITIVELY
ln(Q/M) ~ A E ;
ELECTROMETER
Figure 1. Device for measurement of triboelectric charging
CHARGES NEGATIVELY
I n ( P / M ) aAE4
Figure 2. Relationship of molecular orbital energy levels, metal Fermi
levels, and triboelectric charging film to film. For a given film the standard deviation is usually less, on the order of 5 % , except for very high ( > l o nC/g) or very low (
