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On the Possibility of Glue Contaminations in the Surface Force Apparatus
Journal of Colloid and Interface Science 210, 215–217 (1999) Article ID jcis.1998.5935, available online at http://www.idealibrary.com on
NOTE On the Possibility of Glue Contaminations in the Surface Force Apparatus A noninterferometric device has been employed to measure the surface forces between glass spheres. The aim was to investigate how the presence of a large area of the Epikote resin used with the surface force apparatus affects the interactions between clean glass spheres in pure water and in 10 mM NaCl. © 1999 Academic Press Key Words: surface force apparatus; Epikote resin; surface force measurements.
It is occasionally suggested that the thermosetting resin (Epikote 1004, Shell) used to glue the mica sheets to the polished silica discs in the surface forces apparatus (SFA) dissolves in aqueous solutions and hence constitutes a source for contaminants. It has further been argued that the dissolved glue would affect the interactions observed and therefore could nullify interpretations of the origin of these surface forces (1). However, in 1995 Pincet et al. (2) showed, using surface tension measurements, that the amount of glue dissolved depends on the surface area of the glue exposed to the solution and that the effect is negligible for the amounts usually used in the SFA. Only for large amounts, of the order of 15 mg glue powder, were they able to detect any change in surface tension beyond the experimental error in their Wilhelmy balance (0.5 nM z m21). Convincing as they are, the results still do not eliminate the possibility that the amount dissolved, however small it might be, may be sufficient to affect the surface forces. It is reasonable to assume that minute traces of glue, or of glue components, will dissolve in aqueous solutions despite its claimed insolubility in water. With the development of a noninterferometric device for measuring the surface forces acting between macroscopic surfaces which does not use any glue in the surface preparation, a tool is now available to shed some light on this issue. With this technique the interactions between any two surfaces may be monitored provided the surfaces are smooth enough for interpretation to be feasible (3). A bimorph, two piezoelectric slabs glued together with opposite polarization directions and contained in a teflon sheath, constitutes the force sensor; a charge proportional to its deflection is produced under the action of a surface force. One of the surfaces is mounted on the bimorph, the other is made to approach using a piezoelectric crystal. An experiment is conducted and analyzed in much the same way as with an AFM when used for force measurements (4), i.e., the surface separation is calculated from the known expansion of the piezo tube and the position of the “hard wall” observed when the surfaces reach contact. Thus, the reported surface separation is relative to the position of the hard wall reached in each force run. In the experiments presented here the resolution of the instrument was 0.035 mN z m21 with respect to the force and 0.3 nm for the surface separation. A well-defined geometry is used with known surface radii, allowing a direct comparison of the normalized forces using the Derjaguin approximation (5) (F(D)/R 5 2 p G f , where G f is the free energy of interaction per unit area
between flat surfaces and R 5 R 1 R 2 /(R 1 1 R 2 )) with those obtained employing the SFA and with theoretical predictions. In the work presented in this note glass surfaces were used, prepared by melting the end of a borosilicate glass rod (diameter 2 mm) in a burner using a butane– oxygen mixture. The surface energy of the glass causes the molten glass to form a spherical drop on the molten end with diameter '4 mm. It is for this study worth pointing out that no glue other than that present on the inserted disk is in contact with the solution in the chamber. The protocol for preparing surfaces for surface force measurements using the interferometric technique (6, 7) involves cleaning the supporting silica disks with acetone and/or ethanol followed by blow-drying in cleaned N2 gas, before placing them on a well-cleaned heating plate. Pieces of glue, about 1 mg, are then placed on the disks and the plate heated to 150°C before the glue is distributed over the convex disk surface using a pair of ethanol-rinsed and blow-dried tweezers. After removing air-bubbles in the glue layer with the tweezers, and allowing the glue to settle for approx. 20 min. a thin piece of mica (;1–3 mm thick and with an area of about 1 cm2) is placed on the glue layer. The disk is then quickly removed from the heat and allowed to cool off. The mica sheet has in most, if not all, instances a larger area than the area of the curved, glue-covered, surface of the disk
215
FIG. 1. The interaction between two glass spheres in water (filled inverse triangles). Remaining symbols represent the interaction between glass surfaces at different times after the introduction of a glue-covered silica SFA disk to the measuring chamber (cf. text): 8 h (circles), 5 days (diamonds), 7 days (triangels), and 11 days (squares). Filled and unfilled symbols represents measurements on approach and separation, respectively. The inset shows the interaction on approach on a semilog scale, together with a theoretical prediction using DLVO theory. 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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which is on the order of 0.8 cm2 (r 5 0.5 cm). If for the sake of simplicity we consider the surface to be flat and circular, and use the ordinary amount of glue applied (1 mg, r 5 1.19 g/cm2), the glue below the mica would form a 1 z 1023 cm thick layer with an area of 7 z 1023 cm2 from which to dissolve. In the experiments presented here the same procedure was followed, except that no mica was placed on the glue-covered SFA disk. A disk prepared in this way delivers an area of glue approximately equal to 0.8 cm2 when placed in contact with a solution, i.e., about a factor one hundred larger than in a typical surface force experiment. Two types of experiments were conducted in duplicate. In each type of experiment the first step was to measure the interactions between the two freshly prepared (as described above) spherical glass surfaces in air. This was repeated after filling the chamber with the medium used in each experiment. If at this stage the behavior deviated from that which has been found to be normal, new surfaces were prepared and the procedure repeated. A silica disk, cleaned with chromosulfuric acid, was then placed in the measuring chamber and the interaction was measured again as a control of any intrinsic contaminants associated with the SFA silica disk. The disk was then covered with glue as described above and again placed in the measuring chamber, so as to present a large surface area of glue to the solution. The two sets of experiments differed only with respect to the medium used, water (Figs. 1 and 2) and 10 mM NaCl (Fig. 3). Figure 1 shows the observed interactions when the medium was Milli-Q water (conductivity , 18.2 MVcm). The different symbols corresponds to different times from the start of the experiment, ranging from before, to 11 days after, the introduction of the glue-covered disk. As is shown in Fig. 1 there is no effect from the presence of the disk during at least a week from the start of the experiment. In one of the two identical experiments performed in water, a lowering of the electrostatic repulsion, with a simultaneous emergence of a small adhesion, was observed after 11 days, cf. Fig. 2. Apart from this, no long-term effect of the presence of a glue-covered SFA disk could be detected. The interaction between the two glass surfaces is characterized by an electrostatic repulsion originating from the overlap of the diffuse layers formed outside the charged surfaces as shown in the inset to Fig. 1. The solid line in
FIG. 2. Results showing a unique feature obtained after a long time (11 days) in one experiment. The interaction between two glass spheres in water (circles) and after introduction of a cleaned silica SFA disk (squares). Inverse triangles show the interaction between two glass spheres in the presence of a glue-covered silica SFA disk after 5 days while triangles represent the interaction after an additional 6 days (11 days in total). Filled and unfilled symbols represents measurements on approach and separation, respectively.
FIG. 3. The observed interaction between two glass spheres in 10 mM NaCl is given by the thin solid line. The thick solid lines are the result of a theoretical estimate of the interaction according to DLVO theory, using an apparent surface potential of 45 mV and a decay length of 3.6 nm (to compare with 3.0 nm expected for 1 mM 1:1 electrolyte). The remaining lines represent the interaction between glass spheres at different times after placing a silica SFA disk covered with glue in the measuring chamber (cf. text): 1 day (———), 4 days (–––) and 7 days (- - -). There is no hysteresis between the interactions on approach and separation; only approach is presented for clarity.
the inset shows a theoretical prediction from DLVO theory. The experimental data fall within the limits obtained if an apparent surface potential of 85 6 10 mV and a Debye length of 90 nm (corresponding to 1.1 z 1025 M 1:1 electrolyte) was used. The effect of NaCl at a concentration of 10 mM on the interactions can be seen in Fig. 3. Similarly, there does not seem to be any effect of dissolved glue on the measured interactions in this case either, and again the interactions are those expected from DLVO theory. We note, however, that the interaction seems to be fractionally more long-ranged than theoretically predicted. This is a feature which we have encountered in other experiments using glass surfaces in salt solutions (J. C. Fro¨berg, manuscript in preparation) and we attribute this to the nature of the surface and emphasize that this long-ranged tail is present both prior to, and following, the introduction of the glue-covered disk. Attempts have been made to describe the nature, and the long term stability, of the glass surface in water and in saline solutions (8, 9), but as yet no fully satisfactory account is available. We consider this issue to be of considerable importance but not relevant to the present issue. To conclude, we have found that over a period of at least one week there are no observable effects of dissolved glue on the measured forces between glass surfaces either in water or in 10 mM NaCl. We are therefore confident that the glue used to attach the mica sheets to the silica disks in the SFA will not affect the measured forces under these conditions for the normal duration of an SFA experiment.
REFERENCES 1. Parsegian, V. A., and Gershfeld, N. L., Biophys. J. 64, 222A (1993). 2. Pincet, F., Perez, E., and Wolfe, J., Langmuir 11, 373 (1995).
217
NOTE 3. Parker, J. L., Prog. Surf. Sci. 47, 205 (1994). 4. Ducker, W. A., Senden, T. J., and Pashley, R. M., Nature (London) 353, 239 (1991). 5. Derjaguin, B., Kolloid. Zh. 69, 155 (1934). 6. Israelachvili, J. N., and Adams, G. E., J. Chem. Soc., Faraday Trans. 1 74, 975 (1978). 7. Parker, J., Christenson, H., and Ninham, B., Rev. Sci. Instrum. 60, 3135 (1989). 8. Vigil, G., Xu, Z., Steinberg, S., and Israelachvili, J., J. Colloid Interface Sci. 164, 367 (1994). 9. Yaminsky, V. V., Ninham, B. W., and Pashley, R. M., Langmuir 14, 3223 (1998).
Johan C. Fro¨berg1 Thomas Ederth Laboratory for Chemical Surface Science Department of Chemistry, Physical Chemistry, Royal Institute of Technology SE-100 44 Stockholm, Sweden Institute for Surface Chemistry Box 5607 SE-114 86 Stockholm, Sweden Received May 18, 1998; accepted October 19, 1998 1 To whom correspondence should be addressed at Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden.
NOTE On the Possibility of Glue Contaminations in the Surface Force Apparatus A noninterferometric device has been employed to measure the surface forces between glass spheres. The aim was to investigate how the presence of a large area of the Epikote resin used with the surface force apparatus affects the interactions between clean glass spheres in pure water and in 10 mM NaCl. © 1999 Academic Press Key Words: surface force apparatus; Epikote resin; surface force measurements.
It is occasionally suggested that the thermosetting resin (Epikote 1004, Shell) used to glue the mica sheets to the polished silica discs in the surface forces apparatus (SFA) dissolves in aqueous solutions and hence constitutes a source for contaminants. It has further been argued that the dissolved glue would affect the interactions observed and therefore could nullify interpretations of the origin of these surface forces (1). However, in 1995 Pincet et al. (2) showed, using surface tension measurements, that the amount of glue dissolved depends on the surface area of the glue exposed to the solution and that the effect is negligible for the amounts usually used in the SFA. Only for large amounts, of the order of 15 mg glue powder, were they able to detect any change in surface tension beyond the experimental error in their Wilhelmy balance (0.5 nM z m21). Convincing as they are, the results still do not eliminate the possibility that the amount dissolved, however small it might be, may be sufficient to affect the surface forces. It is reasonable to assume that minute traces of glue, or of glue components, will dissolve in aqueous solutions despite its claimed insolubility in water. With the development of a noninterferometric device for measuring the surface forces acting between macroscopic surfaces which does not use any glue in the surface preparation, a tool is now available to shed some light on this issue. With this technique the interactions between any two surfaces may be monitored provided the surfaces are smooth enough for interpretation to be feasible (3). A bimorph, two piezoelectric slabs glued together with opposite polarization directions and contained in a teflon sheath, constitutes the force sensor; a charge proportional to its deflection is produced under the action of a surface force. One of the surfaces is mounted on the bimorph, the other is made to approach using a piezoelectric crystal. An experiment is conducted and analyzed in much the same way as with an AFM when used for force measurements (4), i.e., the surface separation is calculated from the known expansion of the piezo tube and the position of the “hard wall” observed when the surfaces reach contact. Thus, the reported surface separation is relative to the position of the hard wall reached in each force run. In the experiments presented here the resolution of the instrument was 0.035 mN z m21 with respect to the force and 0.3 nm for the surface separation. A well-defined geometry is used with known surface radii, allowing a direct comparison of the normalized forces using the Derjaguin approximation (5) (F(D)/R 5 2 p G f , where G f is the free energy of interaction per unit area
between flat surfaces and R 5 R 1 R 2 /(R 1 1 R 2 )) with those obtained employing the SFA and with theoretical predictions. In the work presented in this note glass surfaces were used, prepared by melting the end of a borosilicate glass rod (diameter 2 mm) in a burner using a butane– oxygen mixture. The surface energy of the glass causes the molten glass to form a spherical drop on the molten end with diameter '4 mm. It is for this study worth pointing out that no glue other than that present on the inserted disk is in contact with the solution in the chamber. The protocol for preparing surfaces for surface force measurements using the interferometric technique (6, 7) involves cleaning the supporting silica disks with acetone and/or ethanol followed by blow-drying in cleaned N2 gas, before placing them on a well-cleaned heating plate. Pieces of glue, about 1 mg, are then placed on the disks and the plate heated to 150°C before the glue is distributed over the convex disk surface using a pair of ethanol-rinsed and blow-dried tweezers. After removing air-bubbles in the glue layer with the tweezers, and allowing the glue to settle for approx. 20 min. a thin piece of mica (;1–3 mm thick and with an area of about 1 cm2) is placed on the glue layer. The disk is then quickly removed from the heat and allowed to cool off. The mica sheet has in most, if not all, instances a larger area than the area of the curved, glue-covered, surface of the disk
215
FIG. 1. The interaction between two glass spheres in water (filled inverse triangles). Remaining symbols represent the interaction between glass surfaces at different times after the introduction of a glue-covered silica SFA disk to the measuring chamber (cf. text): 8 h (circles), 5 days (diamonds), 7 days (triangels), and 11 days (squares). Filled and unfilled symbols represents measurements on approach and separation, respectively. The inset shows the interaction on approach on a semilog scale, together with a theoretical prediction using DLVO theory. 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
216
NOTE
which is on the order of 0.8 cm2 (r 5 0.5 cm). If for the sake of simplicity we consider the surface to be flat and circular, and use the ordinary amount of glue applied (1 mg, r 5 1.19 g/cm2), the glue below the mica would form a 1 z 1023 cm thick layer with an area of 7 z 1023 cm2 from which to dissolve. In the experiments presented here the same procedure was followed, except that no mica was placed on the glue-covered SFA disk. A disk prepared in this way delivers an area of glue approximately equal to 0.8 cm2 when placed in contact with a solution, i.e., about a factor one hundred larger than in a typical surface force experiment. Two types of experiments were conducted in duplicate. In each type of experiment the first step was to measure the interactions between the two freshly prepared (as described above) spherical glass surfaces in air. This was repeated after filling the chamber with the medium used in each experiment. If at this stage the behavior deviated from that which has been found to be normal, new surfaces were prepared and the procedure repeated. A silica disk, cleaned with chromosulfuric acid, was then placed in the measuring chamber and the interaction was measured again as a control of any intrinsic contaminants associated with the SFA silica disk. The disk was then covered with glue as described above and again placed in the measuring chamber, so as to present a large surface area of glue to the solution. The two sets of experiments differed only with respect to the medium used, water (Figs. 1 and 2) and 10 mM NaCl (Fig. 3). Figure 1 shows the observed interactions when the medium was Milli-Q water (conductivity , 18.2 MVcm). The different symbols corresponds to different times from the start of the experiment, ranging from before, to 11 days after, the introduction of the glue-covered disk. As is shown in Fig. 1 there is no effect from the presence of the disk during at least a week from the start of the experiment. In one of the two identical experiments performed in water, a lowering of the electrostatic repulsion, with a simultaneous emergence of a small adhesion, was observed after 11 days, cf. Fig. 2. Apart from this, no long-term effect of the presence of a glue-covered SFA disk could be detected. The interaction between the two glass surfaces is characterized by an electrostatic repulsion originating from the overlap of the diffuse layers formed outside the charged surfaces as shown in the inset to Fig. 1. The solid line in
FIG. 2. Results showing a unique feature obtained after a long time (11 days) in one experiment. The interaction between two glass spheres in water (circles) and after introduction of a cleaned silica SFA disk (squares). Inverse triangles show the interaction between two glass spheres in the presence of a glue-covered silica SFA disk after 5 days while triangles represent the interaction after an additional 6 days (11 days in total). Filled and unfilled symbols represents measurements on approach and separation, respectively.
FIG. 3. The observed interaction between two glass spheres in 10 mM NaCl is given by the thin solid line. The thick solid lines are the result of a theoretical estimate of the interaction according to DLVO theory, using an apparent surface potential of 45 mV and a decay length of 3.6 nm (to compare with 3.0 nm expected for 1 mM 1:1 electrolyte). The remaining lines represent the interaction between glass spheres at different times after placing a silica SFA disk covered with glue in the measuring chamber (cf. text): 1 day (———), 4 days (–––) and 7 days (- - -). There is no hysteresis between the interactions on approach and separation; only approach is presented for clarity.
the inset shows a theoretical prediction from DLVO theory. The experimental data fall within the limits obtained if an apparent surface potential of 85 6 10 mV and a Debye length of 90 nm (corresponding to 1.1 z 1025 M 1:1 electrolyte) was used. The effect of NaCl at a concentration of 10 mM on the interactions can be seen in Fig. 3. Similarly, there does not seem to be any effect of dissolved glue on the measured interactions in this case either, and again the interactions are those expected from DLVO theory. We note, however, that the interaction seems to be fractionally more long-ranged than theoretically predicted. This is a feature which we have encountered in other experiments using glass surfaces in salt solutions (J. C. Fro¨berg, manuscript in preparation) and we attribute this to the nature of the surface and emphasize that this long-ranged tail is present both prior to, and following, the introduction of the glue-covered disk. Attempts have been made to describe the nature, and the long term stability, of the glass surface in water and in saline solutions (8, 9), but as yet no fully satisfactory account is available. We consider this issue to be of considerable importance but not relevant to the present issue. To conclude, we have found that over a period of at least one week there are no observable effects of dissolved glue on the measured forces between glass surfaces either in water or in 10 mM NaCl. We are therefore confident that the glue used to attach the mica sheets to the silica disks in the SFA will not affect the measured forces under these conditions for the normal duration of an SFA experiment.
REFERENCES 1. Parsegian, V. A., and Gershfeld, N. L., Biophys. J. 64, 222A (1993). 2. Pincet, F., Perez, E., and Wolfe, J., Langmuir 11, 373 (1995).
217
NOTE 3. Parker, J. L., Prog. Surf. Sci. 47, 205 (1994). 4. Ducker, W. A., Senden, T. J., and Pashley, R. M., Nature (London) 353, 239 (1991). 5. Derjaguin, B., Kolloid. Zh. 69, 155 (1934). 6. Israelachvili, J. N., and Adams, G. E., J. Chem. Soc., Faraday Trans. 1 74, 975 (1978). 7. Parker, J., Christenson, H., and Ninham, B., Rev. Sci. Instrum. 60, 3135 (1989). 8. Vigil, G., Xu, Z., Steinberg, S., and Israelachvili, J., J. Colloid Interface Sci. 164, 367 (1994). 9. Yaminsky, V. V., Ninham, B. W., and Pashley, R. M., Langmuir 14, 3223 (1998).
Johan C. Fro¨berg1 Thomas Ederth Laboratory for Chemical Surface Science Department of Chemistry, Physical Chemistry, Royal Institute of Technology SE-100 44 Stockholm, Sweden Institute for Surface Chemistry Box 5607 SE-114 86 Stockholm, Sweden Received May 18, 1998; accepted October 19, 1998 1 To whom correspondence should be addressed at Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden.