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Precise differential measurements of monolayer surface potentials
JOUI~NAL OF COLLOID AND I N T E R F A C E SCI ENCE 2 3
(1967)
Letters to the Editors and manipulated only on one side, while the other remains clean; this, of course, is the con~ figuration used in measuring surface pressures in the conventional Langmuir film balance (1). In principle, two air electrodes could be mounted over the two parts of the trough, and their p o t e n tials compared directly. In practice, however, this arrangement has proved unsatisfactory, because it is difficult if not impossible to find two electrodes the surface characteristics and history of which match closely enough so that they drift together. The presence of two ionizing sources also enhances errors due to scattered ionization
Precise Differential Measurements of Monolayer Surface Potentials
Two methods are commonly used for measur~ ing the "surface p o t e n t i a l " due to an insoluble monolayer spread on a liquid surface (1). The reference electrode in the air above the surface m a y be moved vertically so t h a t the capacitance of the air gap is changed, inducing, in the external circuit, a current which depends on the Volta potential difference between the liquid surface and the reference electrode. Alternatively, the conductivity of the air gap may be increased by (vide infra). ionization with a small amount of an alphaThese difficulties are eliminated if a single emitting radioisotope, and the Volta potential ionizing air electrode is mounted so that it may difference measured by a d.c. method. be positioned over either portion of the trough With either method, there is serious difficulty • surface at will. In our apparatus, this mounting in maintaining an adequate reference electrode in has taken the form of an arm, about 4 inches the gas phase. Adsorption and desorption of gas long, pivoted on a post attached to the top of the phase components on the metal electrode can enclosure. The air electrode is fastened to one © alter the Volta potential appreciably during the end of the arm, a magnet to the other. With this course of an experiment (2, 3). This problem is arrangement the position of the air electrode is especially serious when the electrode is located readily controlled by another magnet outside the over a water surface in an atmosphere nearly enclosure. The arm is adjusted to be accurately saturated with water vapor. The common practice parallel to the water surface so that the resistance has been to use a noble metal for the reference of the air gap does not change appreciably as the electrode, and to age it under the experimental electrode is moved. conditions in an effort to stabilize its surface and A comparison of measured potentials obtained minimize changes in its Volta potential. E v e n when both portions of the surface are free of film with a well-aged electrode, however, differences is shown in Fig. 1. Movement of the electrode of several millivolts may be found in "cleanfrom one side of the barrier to the other in this surface" values o b t a i n e d before and after the experiment always gave potentials agreeing spreading and removal of a monolayer (4). Obviwithin 1.5 my., although the measured potential ously, these effects become even larger if the ranged over almost 40 my. during the period of composition of the gas phase is deliberately the experiment. An experiment in which a monoaltered during an experiment, as may be desirable, layer of stearyl alcohol was present on one side for example, in studies of chemical reactions in of the barrier, and potentials were recorded conmonomolecular films. Polymer coatings, which tinuously, is illustrated in Fig. 2. have been suggested to improve stability in the The measurement of cell potential may be a c vibrating electrode method (2), are not applicable complished in either of two basically different to the d.c. measurement technique because of the ways. The actual voltage is either opposed by a high resistance of the polymer. potentiometer until no current flows in the exterThe most satisfactory method for compensating nal circuit, or, if a sufficiently high impedance for reference electrode drifts when an ionizing voltmeter is available, the voltage may be meassource is used would appear to be differential ured directly with the meter. Bewig (5) has determination, wherein Volta potentials of clean pointed out that serious errors can arise in the and film-covered water surfaces are compared at use of the ionized air gap method if scattered the same time. The trough surface can be divided ionization can lead to current flow to surfaces by a barrier, with an insoluble monolayer spread 292
LETTERS
TO THE
EDITORS 5E01
-II,
I
5o0[
A
.~
293 I
J
49°I
\i
210
310 MINUTES
4/
510
60
Fro. 1. Measured potentials of americium air electrode (Am241 i m p r e g n a t e d foil, 2 cm. square, a c t i v i t y ~ 0 . 4 me.) w i t h respect to Ag/AgCI electrode in 0.01 M NaCI solution in trough, surface free of film. Open and solid circles represent m e a s u r e m e n t s on two sides of a barrier dividing the surface, read directly w i t h an elect r o m e t e r v o l t m e t e r . Triangles represent corresponding m e a s u r e m e n t s w i t h a nulling pot e n t i o m e t e r . After a conditioning period of 1 hr. in wet nitrogen, m e a s u r e m e n t s were made for 30 rain. in t h a t atmosphere. A t the p o i n t indicated, the gas flow was changed to wet oxygen, and m e a s u r e m e n t s were continued.
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-80
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I 30
I
I 60 MINUTES
I
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Fio. 2. Recording of p o t e n t i a l w i t h electrode over the clean surface of 0.01 M NaC1 solution in t r o u g h (a) a n d w i t h monolayer of stearyl alcohol spread to its equilibrium spreading pressure (23°C.) (b). B o t h traces recorded w i t h 100 my. full-scale s e n s i t i v i t y ; monolayer p o t e n t i a l partially c o m p e n s a t e d w i t h p o t e n t i o m e t e r set at 450 my. M e a s u r e m e n t s b e g u n 25 min. after spreading of monolayer, in wet n i t r o g e n atmosphere. Gag supply changed to wet oxygen at p o i n t indicated.
(e.g., of the enclosure) which have V o l t a potentials different from t h a t of t h e surface u n d e r study. His analysis of the circuit indicates t h a t these errors are a f u n c t i o n of b o t h the extraneous Volta p o t e n t i a l a n d the resistance of the s t r a y c u r r e n t p a t h , a n d hence c a n n o t be e v a l u a t e d directly b y simple measurements. I t should be noted, however, t h a t the absolute m a g n i t u d e of the error depends on w h e t h e r t h e r e is c u r r e n t flow in t h e external circuit. Bewig's e q u a t i o n s provide a criterion for verifying the absence of such s t r a y conduction errors. If a high impedance v o l t m e t e r (such as the electrometer v o l t m e t e r used here, i n p u t impedance >1014 fl) is available, readings m a y be made b y b o t h the direct voltm e t e r a n d the nulling p o t e n t i o m e t e r method. If there is appreciable s t r a y conduction to surfaces the V o l t a p o t e n t i a l s of which differ from t h a t of the liquid surface, the readings will n o t agree. Only if t h e y do agree can it be assumed t h a t these errors are a b s e n t or e x p e r i m e n t a l l y insignificant. I
Whereas the occurrence of such s t r a y conduction errors is obviously largely d e p e n d e n t on the geometry of the system, we found t h a t considerable care was needed to eliminate t h e m in our system. Careful insulation of the air electrode m o u n t i n g was required. W h e n a small amerieimn foil electrode (activity ~ 0.1 me.) was used, differenees of 10-30 my. were n o t e d between p o t e n t i ometrie and direct voltage readings. W i t h t h e larger electrode used in the m e a s u r e m e n t s described here, readings always agreed w i t h i n the accuracy of the m e t e r reading. We are i n d e b t e d to A. F. R a z z a n o for skillful assistance in the performance of these experiments. REFERENCES
1 Koenig (6) has suggested t h a t t h e p o t e n t i o metrically b a l a n c e d p o t e n t i a l is not identical, in any event, to the directly m e a s u r e d V o l t a potential._The possible i m p o r t a n c e of this distinction should not be overlooked. If the difference be-
tween these q u a n t i t i e s is e x p e r i m e n t a l l y significant, the present analysis is not valid. The results suggest t h a t in the eases we have examined, at least, the difference is not greater t h a n experim e n t a l error.
1. e.g., GAINES, G. L., JR., " I n s o l u b l e Monolayers at Liquid-Gas Interfaces, Chapt. 3. Wiley-Interseience, New York, 1966.
294
L E T T E R S TO T H E E D I T O R S
2. ]~EWIG, K. W., AND ZISMAN, W. A., V.S. Naval Research Laboratory Report 5383 (1959); Advan. Chem. Ser. 33, 100 (1961). 3. SCttULMAN, J. H., I~ANIS, L., AND BARUCI-I,M., U.S. D e p a r t m e n t of Commerce, O.T.S., PB report 150,864 (Chem. Abstr. 58, 8641d (1963)). 4. e.g., COCKBAIN,E. G., DAY, K. J., AND McMvLLEN, A. I., Proc. Intern. Congr. Surface Activity, 2nd London 1957, Vol. 1, 56. 5. BEWIG, K. W., Rev. Sci. Instr. 35, 1160 (1964). 6. KOENIG, F. 0., Proc. Meeting Intern. Comm. Electrochem. Thermodyn. Kinet. 3rd Milan, 1952, p. 299. J. A. BEI=tGERON G. L. GAINES, JR. General Electronic Research and Development Center Schenectady, New York Received May 9, 1966
Adsorption of Dyes to Etch Pits on Crystal Surface of Potassium Chloride Adsorption of several dyes to etch pits onto the crystal surface of potassium chloride was examined to clarify the relation between dislocation points and active centers of adsorption on solid surface. Dyes employed in this experiment were Sudan I I I , Methyl Violet, Methylene Blue, and Rhodamine 6G; they were selected because they are soluble in isopropyl alcohol. The etching of dislocations of potassium chloride was carried out by the Sakamoto method (1). The etchant solution was similar to that of the
rinse solution. The single crystal of potassium chloride was cleaved carefully by a razor into small specimens, which were about 2 X 10 X 10 mm. in size along their {100} plane. These specimens were immersed in doubly distilled acetone saturated with zinc chloride. The immersion times were about 30 seconds. The specimens were then rinsed with acetone. Thereafter, they were dried on blotting paper (special grade), and were stocked in the desiccator. This operation etches the dislocation points to proper sizes as shown in Fig. 1. The adsorption study was carried out as follows: The specimens of potassium chloride having the surface etched by the method described were immersed in isopropyl alcohol solution of the selected dyes mentioned in the preceding paragraph, under the conditions of different dye concentration and immersion times. The effect of isopropyl alcohol was previously ascertained not to enlarge the etch pits within the range of 15 hours immersion. After the immersed specimens were taken up from the dye soultions, the specimens were rinsed sufficiently with acetone again. Then, they were dried on blotting paper. Figures 2a, 2b, and 2c show the adsorption of Sudan I I I dye onto the etch pits on potassium chloride crystal. The state of Sudan I I I adsorbed along on the etching lines of each etch pit may be clearly observed in Fig. 2c. In order to obtain the clear adsorption pattern, the proper concentration and immersion time must be selected. Figure 2c was obtained after 10 hours immersion at 1 millimolar dye concentration.
FIG. 1. The state of etch pits on crystal surface of potassium chloride. X 450
(1967)
Letters to the Editors and manipulated only on one side, while the other remains clean; this, of course, is the con~ figuration used in measuring surface pressures in the conventional Langmuir film balance (1). In principle, two air electrodes could be mounted over the two parts of the trough, and their p o t e n tials compared directly. In practice, however, this arrangement has proved unsatisfactory, because it is difficult if not impossible to find two electrodes the surface characteristics and history of which match closely enough so that they drift together. The presence of two ionizing sources also enhances errors due to scattered ionization
Precise Differential Measurements of Monolayer Surface Potentials
Two methods are commonly used for measur~ ing the "surface p o t e n t i a l " due to an insoluble monolayer spread on a liquid surface (1). The reference electrode in the air above the surface m a y be moved vertically so t h a t the capacitance of the air gap is changed, inducing, in the external circuit, a current which depends on the Volta potential difference between the liquid surface and the reference electrode. Alternatively, the conductivity of the air gap may be increased by (vide infra). ionization with a small amount of an alphaThese difficulties are eliminated if a single emitting radioisotope, and the Volta potential ionizing air electrode is mounted so that it may difference measured by a d.c. method. be positioned over either portion of the trough With either method, there is serious difficulty • surface at will. In our apparatus, this mounting in maintaining an adequate reference electrode in has taken the form of an arm, about 4 inches the gas phase. Adsorption and desorption of gas long, pivoted on a post attached to the top of the phase components on the metal electrode can enclosure. The air electrode is fastened to one © alter the Volta potential appreciably during the end of the arm, a magnet to the other. With this course of an experiment (2, 3). This problem is arrangement the position of the air electrode is especially serious when the electrode is located readily controlled by another magnet outside the over a water surface in an atmosphere nearly enclosure. The arm is adjusted to be accurately saturated with water vapor. The common practice parallel to the water surface so that the resistance has been to use a noble metal for the reference of the air gap does not change appreciably as the electrode, and to age it under the experimental electrode is moved. conditions in an effort to stabilize its surface and A comparison of measured potentials obtained minimize changes in its Volta potential. E v e n when both portions of the surface are free of film with a well-aged electrode, however, differences is shown in Fig. 1. Movement of the electrode of several millivolts may be found in "cleanfrom one side of the barrier to the other in this surface" values o b t a i n e d before and after the experiment always gave potentials agreeing spreading and removal of a monolayer (4). Obviwithin 1.5 my., although the measured potential ously, these effects become even larger if the ranged over almost 40 my. during the period of composition of the gas phase is deliberately the experiment. An experiment in which a monoaltered during an experiment, as may be desirable, layer of stearyl alcohol was present on one side for example, in studies of chemical reactions in of the barrier, and potentials were recorded conmonomolecular films. Polymer coatings, which tinuously, is illustrated in Fig. 2. have been suggested to improve stability in the The measurement of cell potential may be a c vibrating electrode method (2), are not applicable complished in either of two basically different to the d.c. measurement technique because of the ways. The actual voltage is either opposed by a high resistance of the polymer. potentiometer until no current flows in the exterThe most satisfactory method for compensating nal circuit, or, if a sufficiently high impedance for reference electrode drifts when an ionizing voltmeter is available, the voltage may be meassource is used would appear to be differential ured directly with the meter. Bewig (5) has determination, wherein Volta potentials of clean pointed out that serious errors can arise in the and film-covered water surfaces are compared at use of the ionized air gap method if scattered the same time. The trough surface can be divided ionization can lead to current flow to surfaces by a barrier, with an insoluble monolayer spread 292
LETTERS
TO THE
EDITORS 5E01
-II,
I
5o0[
A
.~
293 I
J
49°I
\i
210
310 MINUTES
4/
510
60
Fro. 1. Measured potentials of americium air electrode (Am241 i m p r e g n a t e d foil, 2 cm. square, a c t i v i t y ~ 0 . 4 me.) w i t h respect to Ag/AgCI electrode in 0.01 M NaCI solution in trough, surface free of film. Open and solid circles represent m e a s u r e m e n t s on two sides of a barrier dividing the surface, read directly w i t h an elect r o m e t e r v o l t m e t e r . Triangles represent corresponding m e a s u r e m e n t s w i t h a nulling pot e n t i o m e t e r . After a conditioning period of 1 hr. in wet nitrogen, m e a s u r e m e n t s were made for 30 rain. in t h a t atmosphere. A t the p o i n t indicated, the gas flow was changed to wet oxygen, and m e a s u r e m e n t s were continued.
8o
,-, FI
I I I I
I I I I
~lll l[ll
Ill
~_ 9O i10
[
I\
.480~-
-80
I
\I
I
I
I I
I I
I I
I I tf I
..,j
70
6O
1
I 30
I
I 60 MINUTES
I
I 90
Fio. 2. Recording of p o t e n t i a l w i t h electrode over the clean surface of 0.01 M NaC1 solution in t r o u g h (a) a n d w i t h monolayer of stearyl alcohol spread to its equilibrium spreading pressure (23°C.) (b). B o t h traces recorded w i t h 100 my. full-scale s e n s i t i v i t y ; monolayer p o t e n t i a l partially c o m p e n s a t e d w i t h p o t e n t i o m e t e r set at 450 my. M e a s u r e m e n t s b e g u n 25 min. after spreading of monolayer, in wet n i t r o g e n atmosphere. Gag supply changed to wet oxygen at p o i n t indicated.
(e.g., of the enclosure) which have V o l t a potentials different from t h a t of t h e surface u n d e r study. His analysis of the circuit indicates t h a t these errors are a f u n c t i o n of b o t h the extraneous Volta p o t e n t i a l a n d the resistance of the s t r a y c u r r e n t p a t h , a n d hence c a n n o t be e v a l u a t e d directly b y simple measurements. I t should be noted, however, t h a t the absolute m a g n i t u d e of the error depends on w h e t h e r t h e r e is c u r r e n t flow in t h e external circuit. Bewig's e q u a t i o n s provide a criterion for verifying the absence of such s t r a y conduction errors. If a high impedance v o l t m e t e r (such as the electrometer v o l t m e t e r used here, i n p u t impedance >1014 fl) is available, readings m a y be made b y b o t h the direct voltm e t e r a n d the nulling p o t e n t i o m e t e r method. If there is appreciable s t r a y conduction to surfaces the V o l t a p o t e n t i a l s of which differ from t h a t of the liquid surface, the readings will n o t agree. Only if t h e y do agree can it be assumed t h a t these errors are a b s e n t or e x p e r i m e n t a l l y insignificant. I
Whereas the occurrence of such s t r a y conduction errors is obviously largely d e p e n d e n t on the geometry of the system, we found t h a t considerable care was needed to eliminate t h e m in our system. Careful insulation of the air electrode m o u n t i n g was required. W h e n a small amerieimn foil electrode (activity ~ 0.1 me.) was used, differenees of 10-30 my. were n o t e d between p o t e n t i ometrie and direct voltage readings. W i t h t h e larger electrode used in the m e a s u r e m e n t s described here, readings always agreed w i t h i n the accuracy of the m e t e r reading. We are i n d e b t e d to A. F. R a z z a n o for skillful assistance in the performance of these experiments. REFERENCES
1 Koenig (6) has suggested t h a t t h e p o t e n t i o metrically b a l a n c e d p o t e n t i a l is not identical, in any event, to the directly m e a s u r e d V o l t a potential._The possible i m p o r t a n c e of this distinction should not be overlooked. If the difference be-
tween these q u a n t i t i e s is e x p e r i m e n t a l l y significant, the present analysis is not valid. The results suggest t h a t in the eases we have examined, at least, the difference is not greater t h a n experim e n t a l error.
1. e.g., GAINES, G. L., JR., " I n s o l u b l e Monolayers at Liquid-Gas Interfaces, Chapt. 3. Wiley-Interseience, New York, 1966.
294
L E T T E R S TO T H E E D I T O R S
2. ]~EWIG, K. W., AND ZISMAN, W. A., V.S. Naval Research Laboratory Report 5383 (1959); Advan. Chem. Ser. 33, 100 (1961). 3. SCttULMAN, J. H., I~ANIS, L., AND BARUCI-I,M., U.S. D e p a r t m e n t of Commerce, O.T.S., PB report 150,864 (Chem. Abstr. 58, 8641d (1963)). 4. e.g., COCKBAIN,E. G., DAY, K. J., AND McMvLLEN, A. I., Proc. Intern. Congr. Surface Activity, 2nd London 1957, Vol. 1, 56. 5. BEWIG, K. W., Rev. Sci. Instr. 35, 1160 (1964). 6. KOENIG, F. 0., Proc. Meeting Intern. Comm. Electrochem. Thermodyn. Kinet. 3rd Milan, 1952, p. 299. J. A. BEI=tGERON G. L. GAINES, JR. General Electronic Research and Development Center Schenectady, New York Received May 9, 1966
Adsorption of Dyes to Etch Pits on Crystal Surface of Potassium Chloride Adsorption of several dyes to etch pits onto the crystal surface of potassium chloride was examined to clarify the relation between dislocation points and active centers of adsorption on solid surface. Dyes employed in this experiment were Sudan I I I , Methyl Violet, Methylene Blue, and Rhodamine 6G; they were selected because they are soluble in isopropyl alcohol. The etching of dislocations of potassium chloride was carried out by the Sakamoto method (1). The etchant solution was similar to that of the
rinse solution. The single crystal of potassium chloride was cleaved carefully by a razor into small specimens, which were about 2 X 10 X 10 mm. in size along their {100} plane. These specimens were immersed in doubly distilled acetone saturated with zinc chloride. The immersion times were about 30 seconds. The specimens were then rinsed with acetone. Thereafter, they were dried on blotting paper (special grade), and were stocked in the desiccator. This operation etches the dislocation points to proper sizes as shown in Fig. 1. The adsorption study was carried out as follows: The specimens of potassium chloride having the surface etched by the method described were immersed in isopropyl alcohol solution of the selected dyes mentioned in the preceding paragraph, under the conditions of different dye concentration and immersion times. The effect of isopropyl alcohol was previously ascertained not to enlarge the etch pits within the range of 15 hours immersion. After the immersed specimens were taken up from the dye soultions, the specimens were rinsed sufficiently with acetone again. Then, they were dried on blotting paper. Figures 2a, 2b, and 2c show the adsorption of Sudan I I I dye onto the etch pits on potassium chloride crystal. The state of Sudan I I I adsorbed along on the etching lines of each etch pit may be clearly observed in Fig. 2c. In order to obtain the clear adsorption pattern, the proper concentration and immersion time must be selected. Figure 2c was obtained after 10 hours immersion at 1 millimolar dye concentration.
FIG. 1. The state of etch pits on crystal surface of potassium chloride. X 450