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Elastohydrodynamic effects with adsorbed layers in surface force measurements
LETTERS TO THE EDITOR Elastohydrodynamic Effects with Adsorbed Layers in Surface Force Measurements When two elastic surfaces carrying adsorbed layers are forced together under high loads, as in measurement with the surface forces apparatus, a bell-shaped deformation develops in the flattened contact zone. This behavior is due to the elastohydrodynamic "lubrication" of the adsorbed layer. Compression of molecules thus trapped between the surfaces may under some circumstances, e.g., in the cases of surfaces carrying weakly adsorbed proteins, lead to irreversible changes in the adsorbed layer and consequently in the surface interaction. © 1990AcademicPress,Inc. The surface force apparatus ( 1, 2) is becoming an increasingly more powerful and versatile tool for studying the properties of matter in thin films. It is being used not only for direct force measurements (3) but also to study adhesion (4), viscosity ( 5 ), friction (6), phase transitions (7), and adsorption isotherms (8). A proper understanding of the effects of the measuring process on the system under study is of great importance when it comes to correctly interpreting the observations. The mica substrate surfaces used in the surface force apparatus are glued with an epoxy resin to supporting silica discs mounted in a crossed-cylinder configuration. The force F between the surfaces is related to the free energy of interaction per unit area of flat plates G by (9) F ( D ) / R = 27rG(O),
R >> D,
[1]
where D is the surface separation and R is the geometric mean radius & t h e surfaces. When the surfaces are pressed together under high loads they deform and flatten, due mainly to a compression of the epoxy glue, which has the lowest Young's modulus ( ~ 2 × 109 N / m 2, the corresponding value for mica and silica is 5 × 10~°-101~ N / m 2 and 7 X 10 I° N / m 2) (10). At such high loads only part of the external force is used to overcome repulsive surface forces--the remainder is stored as elastic energy. The force at which surface deformation becomes noticeable depends not only on the thickness of the glue and on the local radius but also on details of the force law, including its gradient. A typical value is F / R ~ 10-20 m N / m . Once the flattened area is large enough that contributions to the force from regions outside it can be neglected, a mean pressure can be calculated from the area and the applied load. Note, however, that the pressure is not constant across the area of interaction but has a maximum at the center and decays to zero at the perimeter (for nonadhesive contact) (10). In this letter we report on some observations of general significance for work with the surface force apparatus, particularly in the many applications where adsorption from solution is involved (polymers ( 11 ), surfactants (8,
12), proteins ( 13-15 ), liquid mixtures ( 3, 16 ), etc.). As surfaces that are already flattened are pushed together under even larger loads a bell-shaped depression may develop in the center of the flattened contact zone. This often leads to irreversible (at least on a practical time scale) changes in the surface forces. Let us consider a few typical observations made during investigations of protein adsorption to mica surfaces. These results were obtained both with the old surface force apparatus ( 1 ) as well as with a new and much improved vers i o n - t h e "Mark IV" (2), using standard procedures described in the literature. The shape of the surfaces is deduced from the appearance of the "fringes of equal chromatic order" that emerge when incident white light passes through the system of back-silvered mica surfaces (17). The adsorption of the protein cytochrome c to mica surfaces from aqueous solution has been elaborated on in recent papers ( 14, 15). We are here concerned with what happens when the adsorbed layers are pressed together under high loads. With only a moderate applied load the surfaces flatten, as seen from the interference fringes shown in Fig. 1 (top). At higher loads the surfaces flatten further and then appear to move apart in the center with the fringes adopting a bell shape, as seen in Fig. 1 (bottom). As an example of changes in the surface forces brought about by the appearance of such a bell-shaped depression consider the force curves shown in Fig. 2. The adsorption of human serum albumin (HSA) on mica and the interactions between HSA-coated mica surfaces have recently been investigated and will be reported in detail elsewhere (18). The force measured when two mica surfaces are approaching each other in a solution containing 0.01 mg H S A / m l and 10 -3 MNaC1 is shown by the filled circles. At large separations a double-layer force is present whereas at separations below 50-60 A forces due to compression and dehydration of the albumin layer dominate. When the surfaces are pressed together under a strong force they first flatten and then, at higher applied loads, the characteristic bell shape appears. If the surfaces are separated and then brought together
291 0021-9797/90 $3.00 Journal of Colloid and Interface Science, Vol. 138, No. 1, August 1990
Copyright © 1990 by Academic Press, Inc. All rights of reproduction in any form reserved.
292
LETTERS TO THE EDITOR
again the surface interaction is seen to have changed irreversibly. In particular, the thickness of the adsorbed layer has increased locally and the force due to compression and dehydration of the HSA layer is predominant already at separations of about 150 A (Fig. 2, empty circles). Qualitatively similar behavior has been observed in other systems where species adsorb to mica surfaces from solution, such as nonionic surfactants (12) and crude oils (16). We suggest that these observations may be rationalized by referring to the theory of elastohydrodynamic lubrication (19). The bell-shaped depression is a familiar consequence of bringing together two elastic surfaces separated by a thin layer of very viscous lubricant. The hydrodynamic pressure has a maximum in the center and this gives rise to the largest elastic deformation in this region, especially ifa large viscosity prevents the rapid outward flow of liquid. The appearance and the evolution with time of such bellshaped deformations has been studied by Roberts and Ta-
FIG. l. Photographs of the interference fringes showing the characteristic bell shape formed when surfaces carrying adsorbed cytochrome c are pressed together. The top photograph shows the appearance of a flattened contact area when the applied load is moderate ( ~ 3 0 m N / m ) . The bottom photograph shows the bulge that appears when higher loads ( ~ 300 raN/m) are applied. Journal of Colloid and Interface Science, Vol. 138, No. 1, August 1990
0
•
~
4
.O
0 o 0
&
0 0
:
o O
%o O~mc
20
x
40
60
D [nm] FIG. 2. The force (normalized by the mean radius of curvature of the surfaces) measured between mica surfaces coated with a layer of human serum albumin (HSA) immersed in a solution containing 0.01 mg HSA/ml and 10-3 M NaC1. Filled and unfilled circles represent the forces measured prior to and after formation of a bell-shaped depression, respectively.
bor, using soft rubber spheres approaching glass surfaces in various liquids (20). A similar, purely dynamic effect has been observed by us when hydrophobic surfaces are brought together in water. The very strong attractive forces cause the surfaces to jump together at high speed and water is initially trapped in a bell-shaped region between the surfaces. In this case, however, the liquid drains away rapidly, in less than a second. The use of a video camera is essential for observing such rapid changes in the surface shape. In the examples given here, because of the very large effective viscosity of the adsorbed protein molecules, the effect is apparently irreversible, at least on a practical time scale. The effect should depend on the rate at which the applied load is increased and would presumably vanish if the surfaces were pushed together infinitely slowly. When the adsorbed molecules are very strongly bound to the mica surface, like electrostatically anchored surfactants, the adsorbed layer will not yield (i.e., flow) even under a very high load and the contact region remains flat even at very high pressures. It is only when the adsorbed molecules are relatively loosely and not too densely bound to the mica surface, like albumin or cytochrome c at low concentrations, that the elastohydrodynamic deformation appears to occur. In such cases the bound molecules are able to flow into the center and collect as the local rise in pressure leads to the appearance of the characteristic bell shape illustrated in Fig. 1. The changes in the surface layer are often irreversible and affect the surface interaction. Hence, some caution is needed in order to distinguish changes in surface interaction caused by such dynamic effects from those related to changes in adsorption equilibrium.
293
LETTERS TO THE EDITOR REFERENCES 1. Israelachvili, J. N., and Adams, G. E., J. Chem. Soc. Faraday Trans. 1 74, 975 ( 1978 ). 2. Parker, J. L., Christenson, H. K., and Ninham, B. W., Rev. Sci. Instrum. 60, 3135 (1989). 3. Christenson, H. K., J. Dispersion Sci. Technol. 9, 171 (1988). 4. Christenson, H. K., J. Colloid Interface Sci. 121, 170 (1988). 5. Chan, D. Y. C., and Horn, R. G., J. Chem. Phys. 83, 5311 (1985). 6. Israelachvili, J. N., McGuiggan, P. M., and Homola, A. M., Science 240, 189 (1988). 7. Christenson, H. K., Fang, J., and Israelachvili, J. N., Phys. Rev. B 39, 11,750 (1988). 8. K6kicheff, P., Christenson, H. K., and Ninham, B. W., Colloids Surf 40, 31 (1989). 9. Derjaguin, B. V., Kolloid-Z. 69, 155 (1934). 10. Horn, R. G., Israelachvili, J. N., and Pribac, F., J. Colloid Interface Sci. 115, 480 (1987). 11. Patel, S. S., and Tirrel, M., Annu. Rev. Phys. Chem. 40, 597 (1989). 12. Rutland, M. W., and Christenson, H. K., Langmuir, in press. 13. Claesson, P. M., Arnebrant, T., Bergenstfihl, B., and Nylander, T., J. Colloid Interface Sci. 130, 457 (1989). 14. Afshar-Rad, T., Bailey, A. I., Luckham, P. F., Macnaughtan, W., and Chapman, D., Colloids Surf 31, 125 (1988). 15. K6kicheff, P., Pileni, M. P., and Ninham, B. W., in preparation.
16. Fang, J., and Christenson, H. K., J. Dispersion Sci. Technol. 11, 97 (1990). 17. Israelachvili, J. N., J. Colloid Interface Sci. 44, 259 (1973). 18. Blomberg, E., and Claesson, P. M., in preparation. 19. Moore, D. F., "The Friction and Lubrication of Elastomers," Pergamon, Oxford, 1972. 20. Roberts, A. D., and Tabor, D., Proc. R. Soc. London A 325, 323 (1971). EVA BLOMBERG PER M. CLAESSON The Surface Force Group Department of Physical Chemistry The Royal Institute of Technology S-IO0 44 Stockholm, Sweden The Institute for Surface Chemistry Box 5607 S-114 86 Stockholm, Sweden HUGO K. CHRISTENSON1 Department of Applied Mathematics Research School of Physical Sciences Australian National University G.P.O. Box 4 Canberra, ACT 2601 Australia Received February 6, 1990; accepted May 3, 1990
1TO whom correspondence should be addressed.
JournalofColloidand InterfaceScience.Vol. 138,No. 1, August1990
R >> D,
[1]
where D is the surface separation and R is the geometric mean radius & t h e surfaces. When the surfaces are pressed together under high loads they deform and flatten, due mainly to a compression of the epoxy glue, which has the lowest Young's modulus ( ~ 2 × 109 N / m 2, the corresponding value for mica and silica is 5 × 10~°-101~ N / m 2 and 7 X 10 I° N / m 2) (10). At such high loads only part of the external force is used to overcome repulsive surface forces--the remainder is stored as elastic energy. The force at which surface deformation becomes noticeable depends not only on the thickness of the glue and on the local radius but also on details of the force law, including its gradient. A typical value is F / R ~ 10-20 m N / m . Once the flattened area is large enough that contributions to the force from regions outside it can be neglected, a mean pressure can be calculated from the area and the applied load. Note, however, that the pressure is not constant across the area of interaction but has a maximum at the center and decays to zero at the perimeter (for nonadhesive contact) (10). In this letter we report on some observations of general significance for work with the surface force apparatus, particularly in the many applications where adsorption from solution is involved (polymers ( 11 ), surfactants (8,
12), proteins ( 13-15 ), liquid mixtures ( 3, 16 ), etc.). As surfaces that are already flattened are pushed together under even larger loads a bell-shaped depression may develop in the center of the flattened contact zone. This often leads to irreversible (at least on a practical time scale) changes in the surface forces. Let us consider a few typical observations made during investigations of protein adsorption to mica surfaces. These results were obtained both with the old surface force apparatus ( 1 ) as well as with a new and much improved vers i o n - t h e "Mark IV" (2), using standard procedures described in the literature. The shape of the surfaces is deduced from the appearance of the "fringes of equal chromatic order" that emerge when incident white light passes through the system of back-silvered mica surfaces (17). The adsorption of the protein cytochrome c to mica surfaces from aqueous solution has been elaborated on in recent papers ( 14, 15). We are here concerned with what happens when the adsorbed layers are pressed together under high loads. With only a moderate applied load the surfaces flatten, as seen from the interference fringes shown in Fig. 1 (top). At higher loads the surfaces flatten further and then appear to move apart in the center with the fringes adopting a bell shape, as seen in Fig. 1 (bottom). As an example of changes in the surface forces brought about by the appearance of such a bell-shaped depression consider the force curves shown in Fig. 2. The adsorption of human serum albumin (HSA) on mica and the interactions between HSA-coated mica surfaces have recently been investigated and will be reported in detail elsewhere (18). The force measured when two mica surfaces are approaching each other in a solution containing 0.01 mg H S A / m l and 10 -3 MNaC1 is shown by the filled circles. At large separations a double-layer force is present whereas at separations below 50-60 A forces due to compression and dehydration of the albumin layer dominate. When the surfaces are pressed together under a strong force they first flatten and then, at higher applied loads, the characteristic bell shape appears. If the surfaces are separated and then brought together
291 0021-9797/90 $3.00 Journal of Colloid and Interface Science, Vol. 138, No. 1, August 1990
Copyright © 1990 by Academic Press, Inc. All rights of reproduction in any form reserved.
292
LETTERS TO THE EDITOR
again the surface interaction is seen to have changed irreversibly. In particular, the thickness of the adsorbed layer has increased locally and the force due to compression and dehydration of the HSA layer is predominant already at separations of about 150 A (Fig. 2, empty circles). Qualitatively similar behavior has been observed in other systems where species adsorb to mica surfaces from solution, such as nonionic surfactants (12) and crude oils (16). We suggest that these observations may be rationalized by referring to the theory of elastohydrodynamic lubrication (19). The bell-shaped depression is a familiar consequence of bringing together two elastic surfaces separated by a thin layer of very viscous lubricant. The hydrodynamic pressure has a maximum in the center and this gives rise to the largest elastic deformation in this region, especially ifa large viscosity prevents the rapid outward flow of liquid. The appearance and the evolution with time of such bellshaped deformations has been studied by Roberts and Ta-
FIG. l. Photographs of the interference fringes showing the characteristic bell shape formed when surfaces carrying adsorbed cytochrome c are pressed together. The top photograph shows the appearance of a flattened contact area when the applied load is moderate ( ~ 3 0 m N / m ) . The bottom photograph shows the bulge that appears when higher loads ( ~ 300 raN/m) are applied. Journal of Colloid and Interface Science, Vol. 138, No. 1, August 1990
0
•
~
4
.O
0 o 0
&
0 0
:
o O
%o O~mc
20
x
40
60
D [nm] FIG. 2. The force (normalized by the mean radius of curvature of the surfaces) measured between mica surfaces coated with a layer of human serum albumin (HSA) immersed in a solution containing 0.01 mg HSA/ml and 10-3 M NaC1. Filled and unfilled circles represent the forces measured prior to and after formation of a bell-shaped depression, respectively.
bor, using soft rubber spheres approaching glass surfaces in various liquids (20). A similar, purely dynamic effect has been observed by us when hydrophobic surfaces are brought together in water. The very strong attractive forces cause the surfaces to jump together at high speed and water is initially trapped in a bell-shaped region between the surfaces. In this case, however, the liquid drains away rapidly, in less than a second. The use of a video camera is essential for observing such rapid changes in the surface shape. In the examples given here, because of the very large effective viscosity of the adsorbed protein molecules, the effect is apparently irreversible, at least on a practical time scale. The effect should depend on the rate at which the applied load is increased and would presumably vanish if the surfaces were pushed together infinitely slowly. When the adsorbed molecules are very strongly bound to the mica surface, like electrostatically anchored surfactants, the adsorbed layer will not yield (i.e., flow) even under a very high load and the contact region remains flat even at very high pressures. It is only when the adsorbed molecules are relatively loosely and not too densely bound to the mica surface, like albumin or cytochrome c at low concentrations, that the elastohydrodynamic deformation appears to occur. In such cases the bound molecules are able to flow into the center and collect as the local rise in pressure leads to the appearance of the characteristic bell shape illustrated in Fig. 1. The changes in the surface layer are often irreversible and affect the surface interaction. Hence, some caution is needed in order to distinguish changes in surface interaction caused by such dynamic effects from those related to changes in adsorption equilibrium.
293
LETTERS TO THE EDITOR REFERENCES 1. Israelachvili, J. N., and Adams, G. E., J. Chem. Soc. Faraday Trans. 1 74, 975 ( 1978 ). 2. Parker, J. L., Christenson, H. K., and Ninham, B. W., Rev. Sci. Instrum. 60, 3135 (1989). 3. Christenson, H. K., J. Dispersion Sci. Technol. 9, 171 (1988). 4. Christenson, H. K., J. Colloid Interface Sci. 121, 170 (1988). 5. Chan, D. Y. C., and Horn, R. G., J. Chem. Phys. 83, 5311 (1985). 6. Israelachvili, J. N., McGuiggan, P. M., and Homola, A. M., Science 240, 189 (1988). 7. Christenson, H. K., Fang, J., and Israelachvili, J. N., Phys. Rev. B 39, 11,750 (1988). 8. K6kicheff, P., Christenson, H. K., and Ninham, B. W., Colloids Surf 40, 31 (1989). 9. Derjaguin, B. V., Kolloid-Z. 69, 155 (1934). 10. Horn, R. G., Israelachvili, J. N., and Pribac, F., J. Colloid Interface Sci. 115, 480 (1987). 11. Patel, S. S., and Tirrel, M., Annu. Rev. Phys. Chem. 40, 597 (1989). 12. Rutland, M. W., and Christenson, H. K., Langmuir, in press. 13. Claesson, P. M., Arnebrant, T., Bergenstfihl, B., and Nylander, T., J. Colloid Interface Sci. 130, 457 (1989). 14. Afshar-Rad, T., Bailey, A. I., Luckham, P. F., Macnaughtan, W., and Chapman, D., Colloids Surf 31, 125 (1988). 15. K6kicheff, P., Pileni, M. P., and Ninham, B. W., in preparation.
16. Fang, J., and Christenson, H. K., J. Dispersion Sci. Technol. 11, 97 (1990). 17. Israelachvili, J. N., J. Colloid Interface Sci. 44, 259 (1973). 18. Blomberg, E., and Claesson, P. M., in preparation. 19. Moore, D. F., "The Friction and Lubrication of Elastomers," Pergamon, Oxford, 1972. 20. Roberts, A. D., and Tabor, D., Proc. R. Soc. London A 325, 323 (1971). EVA BLOMBERG PER M. CLAESSON The Surface Force Group Department of Physical Chemistry The Royal Institute of Technology S-IO0 44 Stockholm, Sweden The Institute for Surface Chemistry Box 5607 S-114 86 Stockholm, Sweden HUGO K. CHRISTENSON1 Department of Applied Mathematics Research School of Physical Sciences Australian National University G.P.O. Box 4 Canberra, ACT 2601 Australia Received February 6, 1990; accepted May 3, 1990
1TO whom correspondence should be addressed.
JournalofColloidand InterfaceScience.Vol. 138,No. 1, August1990