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Cleaning of polymer and metal surfaces studied by ellipsometry
Cleaning of Polymer and Metal Surfaces Studied by Ellipsometry KJ E L L B)/,CKSTROM, * SVEN ENGSTR()M,* BJ()RN LINDMAN,* THOMAS ARNEBRANT,t TOMMY NYLANDER,# AND KARE LARSSONt • Physical Chemistry 1, p.o. Box 740, and tFood Technology, P.O. Box 740, S-220 07 Lund, Sweden Received August 31, 1983; accepted November 9, 1983 The removal of fat, i.e., triglycerides, from polyvinylchloride (PVC) and chromium-coated glass slides by aqueous surfactants in solution was studied by means of ellipsometry. It was found that it is possible to use PVC slides in the ellipsometer, in spite of their low reflectance compared to metal surfaces. When fat adsorbed on a plate is treated with surfactants in solution, the fat removal can be monitored continuously with the ellipsometer. By analysis of the ellipsometer readings it was possible to follow the cleaning mechanism, involving an initial adsorption of surfactant, solubilization of the fat, and diffusion of the solubilized fat from the surface. INTRODUCTION
The cleaning of hard surfaces, e.g., metals, glasses, polymers, makes up a widespread activity in society, both professionally and in household duties. An understanding of the cleaning process with respect to the performance of the different chemicals used in detergent mixtures may therefore be important to many, at least indirectly. The generally accepted mechanism of dirt removal from hard surfaces, as described in any freshman textbook on surface chemistry (1), includes the following steps: (i) adsorption of the detergent on the dirty surface, (ii) solubilization of the dirt by the detergent, and (iii) diffusion of the solubilized dirt from the surface. Several methods have been used to study cleaning efficiency on hard surfaces, e.g., gravimetrical (2), optical (3) and electrochemical (4) methods as well as radioactive decay (5), and contact angle (6) measurements. In Table I some of the characteristics of these methods are listed. Table I reveals that the simple gravimetric method is perhaps the most general, but it demands rather high amounts of adsorbed dirt. A disadvantage with the method is, however,
that it is difficult to follow the cleaning process kinetically. The kinetics can, on the other hand, be monitored by the electrochemical and the contact angle method. However, for the electrochemical method one is restricted to conducting materials. Ellipsometry is an optical method which utilizes the change of state of polarized light caused by an adsorbed film on a reflecting surface. The measured parameters can be used to evaluate the refractive index of the film, its thickness and the amount of adsorbed material (7-9). These quantities are of particular interest in the study of cleaning efficiency. With ellipsometry the kinetics of the cleaning process can be readily followed, and moreover, since the resolution with respect to the adsorbed layer is high (a few Angstrrms), one may be able to interpret the data from a molecular point of view. The most severe limitation of the method is the need for very planar and highly reflecting surfaces. Using laser light excitation, we have been able to measure on polyvinylchloride (PVC) slides which are available commercially, without any pretreatment. Some results on the study of fat removal from PVC and chromium-coated glass slides by exposure to surfactant solutions, obtained
549
Journal of Colloid and InterfaceScience, Vol. 99, No. 2, June 1984
0021-9797/84 $3.00 Copyright© 1984by AcademicPress,Inc. All rightsof reproductionin any form reserved.
550
BACKSTROM ET AL. TABLE I Characteristics of Methods Used to Study the Cleaning of Hard Surfaces Method
Surface
Dirt
Thickness (,~)
Continuous
Gravimetric Optical (Reflectance) Electrochemical Radioactive
Any Reflecting
Any Absorbing
> 105 > 104
Yes No
Conducting Any
-Any
Yes No
Contact angle Ellipsometer
Any Reflecting
Insulating Any containing radioactive atoms Any Any
-> 10
Yes Yes
Note. "Continuous" indicates whether the dirt removal can be followed kinetically or not.
by the ellipsometric technique, are presented below. EXPERIMENTALS
The ellipsometer. The measuring instrument was a modified automated Rudolph thin film ellipsometer, Type 43603-200E. The instrument was equipped with computer-steered stepping motors on the analyzer and the polarizer, making it possible to measure one position every 4 sec and thus to follow the cleaning process continuously. A comprehensive description of the apparatus, the measuring and the theoretical evaluation of refractive index, thickness is given in Ref. (10). The calculation of the adsorbed mass from the refractive index and thickness is described in Ref. (7). Test-slidepreparation. The slides used were made of red PVC and glass, the latter with vacuum-deposited chromium on it with a film thickness of a few thousands Angstrrms. The precleaning of the slides was very important in order to obtain reproducible results. Both kinds of slides were subject to the following procedure: (i) cleaned in alkaline detergent solution, and (ii) rinsed in tap water and doubly destilled water. Furthermore, the PVC slides were finally cleaned in ethanol, while the chromium slides were treated with hot acetone and finally plasma-cleaned with a Harrick plasma cleaner, type PDC-3XG, connected to a vacuum-pump. The plasmaJournal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
cleaning removed all organic contamination and gave a hydrophilic character to the chromium surface, as obvious from water wettability (11). To obtain a layer of fat on the slides, they were dipped manually in solutions of either tripalmitin or triolein in n-octane. Measurements. After cleaning the slide was placed in a cuvette containing 4.5 ml doubly destilled water, and the optical properties of the slide were measured. The same procedure was repeated with the slide after fat deposition, and with the new optical parameters the amount of fat could be determined. Surfactant solution (0.5 ml) was then pipetted into the cuvette and the change in optical properties was recorded continuously every 4 sec. All measurements were carried out at 25 °C, both with and without agitation. RESULTS AND DISCUSSION
The removal oftripalmitin from chromium by an anionic surfactant is shown in Fig. 1. The appearance of the curves is typical for all experiments performed. It can be seen that the cleaning process follows the usually assumed pattern: At first there is an increase in the amount, m, on the surface, indicating that adsorption of the surfactant occurs, followed by a decrease, showing the removal of the fat. The rate of adsorption and desorption can easily be regulated by agitation of the detergent solution. Figure I also shows the cleaning protess at two different speeds of the magnetic
ELLIPSOMETRY
STUDY
OF POLYMER
AND
METAL CLEANING
551
140
1.0 100
0.6 •~ 60 0.2 2o r 0
__1 1
[__ 2 time, m i n u t e s
i 3
__
i 4
FIG. 1. The removal of tripalmitin from chromium at different agitation rates by 0.1% sodium oleate, pH 9.6, temp. 25°C. Surfactant added at I = 0; rn = the adsorbed amount on the surface. stirrer. The qualitative behavior of the systems is the same in both cases, but increased agitation accelerates adsorption as well as desorption. The reason for using a relative scale on the ordinate instead of an absolute one in Fig. 1 is the difficulty in obtaining a reproducible thickness of the fat layer by the manual dipping method. Since our interest in the present project is focused on floor-cleaning we are particularly interested in studying polymer surfaces. However, the applicability of the ellipsometry method was less evident for polymer material. At first the method requires that the substrate be highly reflecting, because the method is built upon the measurement of the reflected light from the surface. Therefore, the surface should be smooth and not too transparent. Second, it is required that the refractive index of the soil and the surfactant differ from the refractive index of the substrate by a certain amount, otherwise the ellipsometer cannot distinguish between the layers and the determination of the adsorbed a m o u n t becomes impossible. Figure 2 shows a removal curve for PVC. The shape of the curve is the same as for chromium, but our data suggests that it is m o r e difficult to eliminate the last part of the fat from PVC than from chromium. The reason probably is that the fat, being hydrophobic, is stronger adsorbed on the hydrophobic polymer than on the hydrophilic metal surface.
t i m e , minutes
FIG. 2. T h e r e m o v a l o f t r i p a l m i t i n f r o m P V C by 0.05%
sodium oleate, pH 9.6, temp. 25°C. Agitation rate 100 rpm. Surfactantadded at t = 0; m = the adsorbedamount on the surface. Until now we have investigated the anionic surfactants sodium oleate and sodium dodecyl sulfate (SDS) and the nonionic surfactant Berol 09 (nonylphenolpolyglycol ether with 9.5 mole EO). In Fig. 3 the behavior of SDS and Bero109, at the same concentration, is shown. The figure reveals that there is a clear difference between the surfactants regarding their ability to remove triolein from chromium. The nonionic surfactant is adsorbed on the fat surface to an appreciable a m o u n t and removes the fat very well. The adsorption of SDS, on the other hand, is low and no fat is removed by the surfactant. In a future work we intend to investigate several other surfactants with respect to their soil removal capacity under different conditions, e.g., p H and ionic strength.
0.2
Berol 0 9
g
o.1
i
0
•
5
L
L
J
10
15
20
time, minutes
FIG. 3. T h e r e m o v a l o f triolein f r o m c h r o m i u m by 0.05% surfactant, p H 6.0, t e m p . 2 5 ° C . N o agitation. Surfactant a d d e d at t = 0; m = a d s o r b e d a m o u n t on the
surface. Journal of Colloid and Interface Science. Vol. 99, No. 2, June 1984
552
BACKSTROM ET AL. CONCLUSIONS
REFERENCES
We have shown that it is possible to follow continuously the removal of fat by detergent solutions from both PVC and chromium surfaces by the ellipsometry method. Our results also clearly reveal that the usually assumed cleaning mechanism, adsorption of surfacgnt, solubilization of fat, and desorption of solubilized fat, is the one actually occurring. For future studies the reproducibility of the experiments, especially regarding the amount of deposited dirt, deserves further attention. It is probably necessary to use some kind of mechanical dipping method of the slides, e.g., the Langmuir-Blodgett technique.
1. Shaw, D. J., "Introduction to Colloid and Surface Chemistry," p. 142. Butterworths, London, 1980. 2. Mankowich, A., J. Amer. Oil Chem. Soc. 38, 589 (1961). 3. Kiewert, E., Seifen, Oele, Fette, Wachse 107, 35 (1981). 4. Trautmann, M., Tenside Deterg. 18, 73 (1981). 5. Bourne, M. C., and Jennings, W. G., J. Amer. Oil Chem. Soc. 40, 517 (1963). 6. Ogino, K., and Agui, W., Bull. Chem. Soc. Jpn. 49, 1703 (1976). 7. Cuypers, P. A., Corsel, J. W., Janssen, M. P., Kop, K. M. M., Hermens, W. T., and Hemker, H. C., J. Biol. Chem. 258, 2456 (1983). 8. Neal, W. E. J., in "'Surface Contamination: Genesis, Detection, and Control" (K. L. Mittal, Ed.), Vol. 2, p. 749. Plenum, New York, 1979. 9. Cuypers, P. A., Hermens, W. T., and Hemker, H. C., Anal. Biochem. 84, 56 (1978). 10. Cuypers, P. A., "Dynamic Ellipsometry." Rijksuniversitetet, Limburg, 1976. 11. O'Kane, D. F., and Mittal, K. L., J. Vac. Sci. Technol. 11, 567 (1974).
ACKNOWLEDGMENTS The Swedish Work Environment Fund is gratefully acknowledged for sponsoring this project as well as Dr. Peter A. Cuypers, Professor Ingemar Lundstr6m, and Stefan Welin for fruitful and interesting discussions.
Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
The cleaning of hard surfaces, e.g., metals, glasses, polymers, makes up a widespread activity in society, both professionally and in household duties. An understanding of the cleaning process with respect to the performance of the different chemicals used in detergent mixtures may therefore be important to many, at least indirectly. The generally accepted mechanism of dirt removal from hard surfaces, as described in any freshman textbook on surface chemistry (1), includes the following steps: (i) adsorption of the detergent on the dirty surface, (ii) solubilization of the dirt by the detergent, and (iii) diffusion of the solubilized dirt from the surface. Several methods have been used to study cleaning efficiency on hard surfaces, e.g., gravimetrical (2), optical (3) and electrochemical (4) methods as well as radioactive decay (5), and contact angle (6) measurements. In Table I some of the characteristics of these methods are listed. Table I reveals that the simple gravimetric method is perhaps the most general, but it demands rather high amounts of adsorbed dirt. A disadvantage with the method is, however,
that it is difficult to follow the cleaning process kinetically. The kinetics can, on the other hand, be monitored by the electrochemical and the contact angle method. However, for the electrochemical method one is restricted to conducting materials. Ellipsometry is an optical method which utilizes the change of state of polarized light caused by an adsorbed film on a reflecting surface. The measured parameters can be used to evaluate the refractive index of the film, its thickness and the amount of adsorbed material (7-9). These quantities are of particular interest in the study of cleaning efficiency. With ellipsometry the kinetics of the cleaning process can be readily followed, and moreover, since the resolution with respect to the adsorbed layer is high (a few Angstrrms), one may be able to interpret the data from a molecular point of view. The most severe limitation of the method is the need for very planar and highly reflecting surfaces. Using laser light excitation, we have been able to measure on polyvinylchloride (PVC) slides which are available commercially, without any pretreatment. Some results on the study of fat removal from PVC and chromium-coated glass slides by exposure to surfactant solutions, obtained
549
Journal of Colloid and InterfaceScience, Vol. 99, No. 2, June 1984
0021-9797/84 $3.00 Copyright© 1984by AcademicPress,Inc. All rightsof reproductionin any form reserved.
550
BACKSTROM ET AL. TABLE I Characteristics of Methods Used to Study the Cleaning of Hard Surfaces Method
Surface
Dirt
Thickness (,~)
Continuous
Gravimetric Optical (Reflectance) Electrochemical Radioactive
Any Reflecting
Any Absorbing
> 105 > 104
Yes No
Conducting Any
-Any
Yes No
Contact angle Ellipsometer
Any Reflecting
Insulating Any containing radioactive atoms Any Any
-> 10
Yes Yes
Note. "Continuous" indicates whether the dirt removal can be followed kinetically or not.
by the ellipsometric technique, are presented below. EXPERIMENTALS
The ellipsometer. The measuring instrument was a modified automated Rudolph thin film ellipsometer, Type 43603-200E. The instrument was equipped with computer-steered stepping motors on the analyzer and the polarizer, making it possible to measure one position every 4 sec and thus to follow the cleaning process continuously. A comprehensive description of the apparatus, the measuring and the theoretical evaluation of refractive index, thickness is given in Ref. (10). The calculation of the adsorbed mass from the refractive index and thickness is described in Ref. (7). Test-slidepreparation. The slides used were made of red PVC and glass, the latter with vacuum-deposited chromium on it with a film thickness of a few thousands Angstrrms. The precleaning of the slides was very important in order to obtain reproducible results. Both kinds of slides were subject to the following procedure: (i) cleaned in alkaline detergent solution, and (ii) rinsed in tap water and doubly destilled water. Furthermore, the PVC slides were finally cleaned in ethanol, while the chromium slides were treated with hot acetone and finally plasma-cleaned with a Harrick plasma cleaner, type PDC-3XG, connected to a vacuum-pump. The plasmaJournal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
cleaning removed all organic contamination and gave a hydrophilic character to the chromium surface, as obvious from water wettability (11). To obtain a layer of fat on the slides, they were dipped manually in solutions of either tripalmitin or triolein in n-octane. Measurements. After cleaning the slide was placed in a cuvette containing 4.5 ml doubly destilled water, and the optical properties of the slide were measured. The same procedure was repeated with the slide after fat deposition, and with the new optical parameters the amount of fat could be determined. Surfactant solution (0.5 ml) was then pipetted into the cuvette and the change in optical properties was recorded continuously every 4 sec. All measurements were carried out at 25 °C, both with and without agitation. RESULTS AND DISCUSSION
The removal oftripalmitin from chromium by an anionic surfactant is shown in Fig. 1. The appearance of the curves is typical for all experiments performed. It can be seen that the cleaning process follows the usually assumed pattern: At first there is an increase in the amount, m, on the surface, indicating that adsorption of the surfactant occurs, followed by a decrease, showing the removal of the fat. The rate of adsorption and desorption can easily be regulated by agitation of the detergent solution. Figure I also shows the cleaning protess at two different speeds of the magnetic
ELLIPSOMETRY
STUDY
OF POLYMER
AND
METAL CLEANING
551
140
1.0 100
0.6 •~ 60 0.2 2o r 0
__1 1
[__ 2 time, m i n u t e s
i 3
__
i 4
FIG. 1. The removal of tripalmitin from chromium at different agitation rates by 0.1% sodium oleate, pH 9.6, temp. 25°C. Surfactant added at I = 0; rn = the adsorbed amount on the surface. stirrer. The qualitative behavior of the systems is the same in both cases, but increased agitation accelerates adsorption as well as desorption. The reason for using a relative scale on the ordinate instead of an absolute one in Fig. 1 is the difficulty in obtaining a reproducible thickness of the fat layer by the manual dipping method. Since our interest in the present project is focused on floor-cleaning we are particularly interested in studying polymer surfaces. However, the applicability of the ellipsometry method was less evident for polymer material. At first the method requires that the substrate be highly reflecting, because the method is built upon the measurement of the reflected light from the surface. Therefore, the surface should be smooth and not too transparent. Second, it is required that the refractive index of the soil and the surfactant differ from the refractive index of the substrate by a certain amount, otherwise the ellipsometer cannot distinguish between the layers and the determination of the adsorbed a m o u n t becomes impossible. Figure 2 shows a removal curve for PVC. The shape of the curve is the same as for chromium, but our data suggests that it is m o r e difficult to eliminate the last part of the fat from PVC than from chromium. The reason probably is that the fat, being hydrophobic, is stronger adsorbed on the hydrophobic polymer than on the hydrophilic metal surface.
t i m e , minutes
FIG. 2. T h e r e m o v a l o f t r i p a l m i t i n f r o m P V C by 0.05%
sodium oleate, pH 9.6, temp. 25°C. Agitation rate 100 rpm. Surfactantadded at t = 0; m = the adsorbedamount on the surface. Until now we have investigated the anionic surfactants sodium oleate and sodium dodecyl sulfate (SDS) and the nonionic surfactant Berol 09 (nonylphenolpolyglycol ether with 9.5 mole EO). In Fig. 3 the behavior of SDS and Bero109, at the same concentration, is shown. The figure reveals that there is a clear difference between the surfactants regarding their ability to remove triolein from chromium. The nonionic surfactant is adsorbed on the fat surface to an appreciable a m o u n t and removes the fat very well. The adsorption of SDS, on the other hand, is low and no fat is removed by the surfactant. In a future work we intend to investigate several other surfactants with respect to their soil removal capacity under different conditions, e.g., p H and ionic strength.
0.2
Berol 0 9
g
o.1
i
0
•
5
L
L
J
10
15
20
time, minutes
FIG. 3. T h e r e m o v a l o f triolein f r o m c h r o m i u m by 0.05% surfactant, p H 6.0, t e m p . 2 5 ° C . N o agitation. Surfactant a d d e d at t = 0; m = a d s o r b e d a m o u n t on the
surface. Journal of Colloid and Interface Science. Vol. 99, No. 2, June 1984
552
BACKSTROM ET AL. CONCLUSIONS
REFERENCES
We have shown that it is possible to follow continuously the removal of fat by detergent solutions from both PVC and chromium surfaces by the ellipsometry method. Our results also clearly reveal that the usually assumed cleaning mechanism, adsorption of surfacgnt, solubilization of fat, and desorption of solubilized fat, is the one actually occurring. For future studies the reproducibility of the experiments, especially regarding the amount of deposited dirt, deserves further attention. It is probably necessary to use some kind of mechanical dipping method of the slides, e.g., the Langmuir-Blodgett technique.
1. Shaw, D. J., "Introduction to Colloid and Surface Chemistry," p. 142. Butterworths, London, 1980. 2. Mankowich, A., J. Amer. Oil Chem. Soc. 38, 589 (1961). 3. Kiewert, E., Seifen, Oele, Fette, Wachse 107, 35 (1981). 4. Trautmann, M., Tenside Deterg. 18, 73 (1981). 5. Bourne, M. C., and Jennings, W. G., J. Amer. Oil Chem. Soc. 40, 517 (1963). 6. Ogino, K., and Agui, W., Bull. Chem. Soc. Jpn. 49, 1703 (1976). 7. Cuypers, P. A., Corsel, J. W., Janssen, M. P., Kop, K. M. M., Hermens, W. T., and Hemker, H. C., J. Biol. Chem. 258, 2456 (1983). 8. Neal, W. E. J., in "'Surface Contamination: Genesis, Detection, and Control" (K. L. Mittal, Ed.), Vol. 2, p. 749. Plenum, New York, 1979. 9. Cuypers, P. A., Hermens, W. T., and Hemker, H. C., Anal. Biochem. 84, 56 (1978). 10. Cuypers, P. A., "Dynamic Ellipsometry." Rijksuniversitetet, Limburg, 1976. 11. O'Kane, D. F., and Mittal, K. L., J. Vac. Sci. Technol. 11, 567 (1974).
ACKNOWLEDGMENTS The Swedish Work Environment Fund is gratefully acknowledged for sponsoring this project as well as Dr. Peter A. Cuypers, Professor Ingemar Lundstr6m, and Stefan Welin for fruitful and interesting discussions.
Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984