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Investigation of Adhesion Properties of Polymer Materials by Atomic Force Microscopy and Zeta Potential Measurements
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
180, 232–236 (1996)
0294
Investigation of Adhesion Properties of Polymer Materials by Atomic Force Microscopy and Zeta Potential Measurements PETRA WEIDENHAMMER
AND
HANS-JO¨RG JACOBASCH 1
Institut fu¨r Polymerforschung, Postfach 120411, 01005 Dresden, Germany Received August 24, 1995; accepted December 14, 1995
MATERIALS AND METHODS AFM and zeta potential measurements were applied in order to investigate the influence of adsorption phenomena on the adhesion behavior of polymer materials. It is shown by means of zeta potential measurements that hydroxyl anions adsorb much stronger than chloride anions onto poly(etheretherketone) surfaces. The adhesion forces between the PEEK surface and the silicon nitride tip of the AFM is lower in potassium hydroxide solution than in potassium chloride solution corresponding to the different adsorption free energies of the anions. For electrolyte concentrations above 0.5 mmol/liter no adhesive contact between tip and sample can be established. We propose an explanation based on the action of competitive forces in the system PEEK–electrolyte solution– Si 3N4 . q 1996 Academic Press, Inc. Key Words: adhesion; adsorption; electrolytes; zeta potential; AFM.
INTRODUCTION
Adhesion of solids in the presence or absence of a liquid phase is a technologically and biologically important phenomenon. Adhesion due to dispersion forces can be quantitatively described by the van der Waals–Lifshitz theory (1); electrostatic interactions by Deryaguin’s theory as well as by semiempirical approaches included in the acid–base theory (2). The stability of colloidal systems is explained by the DLVO theory and the approach of steric stabilization (3). None of these theories contains practically applicable approaches allowing the determination of the influence of adsorption phenomena on adhesion forces. For example, the soil–fiber adhesion in the textile washing process is affected by water and detergent adsorption, the sticking of solids by fatty substances, and the strength of reinforced plastics by adsorbed water (4). It is necessary to find structure–property relationships between the adsorbability of additional substances and adhesion forces. In the current paper, electrokinetic and direct force measurements have been performed in order to investigate these correlations in a nonsymmetric system. 1
To whom correspondence should be addressed.
232
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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A technical, injection molded poly(etheretherketone) (PEEK) was investigated, which had been chosen due to its mechanical and chemical stability. The surface was not damaged by the atomic force microscope (AFM) probe and did not swell in contact with liquids. XPS measurements showed that the polymer consists of pure poly(etheretherketone) without any dissociable groups on the surface. A very small excess of hydrocarbon and C–O–R groups was found, probably due to hydrocarbon contaminants, and a small silicon peak due to silicone and silica groups (H.-J. Jacobasch, Y. Candidus, T. Zeiler, and F. Simon, manuscript in preparation). There was no indication for contamination by steel abrasion. The electrolyte solutions were prepared using Millipore water (pH 5.6); salts were provided by Liil GmbH Dresden (Germany). Force measurements were performed using a Nanoscope III AFM from Digital Instruments ( Santa Barbara, CA ) . The cantilever deflection caused by the forces between probe and polymer surface is detected via a laser beam on a split photo diode. Silicon nitride triangular cantilevers with integrated tips were used; the spring constants, according to the manufacturer, were 0.58 and 0.12 N /m, respectively. The tips were treated with UV light for 15 min before use. We will not present absolute adhesion force measurements here and, therefore, the surface roughness is of particularly small concern. For the present PEEK samples, it involves two components: one with a peak-to-valley roughness of 3 nm and a lateral periodicity of 80 nm and the other with a peak-to-valley roughness of 30 nm and a lateral periodicity of 5 mm. The AFM probe radius is significantly smaller than 100 nm; it ‘‘sees’’ the first component in the same way all over the surface and is not affected by the second. The zeta potential measurements were performed by an electrokinetic analyzer (EKA) (5–7) with a streaming potential cell for flat surfaces (5, 8, 9) constructed by Anton Paar KG (Graz, Austria). In order to characterize the Si 3N4 tips, force – distance
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measurements were performed with different Si 3N4 tips and freshly cleaved mica both immersed in an electrolyte solution. The pH value of the electrolyte solution was varied between 2.8 and 11.0. A long-range electrostatic repulsion always was observed for pH values higher than 3. Since mica is negatively charged for pH ú 2, we can conclude an isoelectric point for the Si 3N4 tip material of pHIEP õ 3. Silanol ( Si-OH ) groups are probably present on the tip surface and determine the pHIEP ( 10 ) . The surface potential can be estimated from ( 11 ) , where the zeta potential for Si 3N4 powders was determined to be about 050 mV for pH @ pHIEP ( in KCl solution, 1 mmol / liter ) . Therefore, the Si 3N4 tips should have a high negative surface potential in all experiments reported here ( pH ú 5.6 ) .
S
S
[1]
Solid–solid adhesion is broken and replaced by separated solid–detergent compounds if this is energetically favorable. The free energy of adhesion between the solids A and B in presence of a detergent C can be written as (13)
01
01
. [3]
F/ and F0 are the nonelectrostatic components of the adsorption free energies for cations and anions, respectively. C0 is the surface potential, z the zeta potential, d 0 the distance between shear plane and surface, c the electrolyte concentration, R the gas constant, F Faraday’s constant and T the absolute temperature. The maximum of the absolute value of z (c) occurs at
cmax Å exp
Asolid Bsolid / 2Cdetergent } AsolidCdetergent / BsolidCdetergent .
S
DD DD
1 F/ / Fz (c) 0 1 / r exp c RT
THEORY
A simple description has been used by one of the authors for the action of adsorbing substances on the adhesion of solids (4). An approach by Fava and Eyring (12) can be generalized for the interaction of two solids in the presence of a third substance, e.g., a detergent:
S
1 1 F0 0 Fz (c) ( C0 0 z (c)) Å 1 / r exp 4pd 0 c RT
S
F/ / F0 2RT
D
[4]
and zmax Å
F0 0 F/ . 2F
[5]
If cmax and zmax are measured, F/ and F0 can be calculated from [4] and [5]. For many initially uncharged polymers without dissociable groups, z -c functions similar to Fig. 1a have been measured (17–19 and references therein). The surface charge, which is acquired by the adsorption of electrolyte ions, increases with increasing Hamaker constant, decreasing hydrophilicity of the polymer and decreasing radius of the hydrated anion (4). RESULTS
DG 0 Å DG 0AB 0 DG 0AC 0 DG 0BC .
[2] Force Measurements
DG 0AB denotes the free energy necessary for establishing the adhesive junction between the solids A and B in absence of the third substance. DG 0AC and DG 0BC are the adsorption free energies of the species C on the solids A and B, respectively. Technical adhesion phenomena have been described quantitatively by this approach (4), though measures for the adhesion have been arbitrary ones. Therefore, it is necessary to establish a measurement of adhesion forces as a force per unit area. Besides Israelachvili’s surface forces apparatus (SFA) (14) the AFM (15) is capable of measuring force– distance functions as well as adhesion forces between almost every kind of solid substrate and a sharp probe. The free energy of adsorption of a dissolved species on a solid can be calculated from its adsorption isotherm. In case of ions, it can be obtained by concentration dependent zeta potential measurements. Considering a solid in presence of a dilute 1–1 electrolyte solution, Stern’s equation (16) describes the charging of the solid surface
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Figure 2 shows typical force–separation graphs for a Si 3N4 tip and the PEEK surface in KOH and KCl solutions of different concentrations. Since both interacting surfaces have negative surface potentials, their electrical double layers repel each other and the force decays exponentially with distance. The decay length is the Debye length 1 Å k
S
(i r`i e 2z 2i 110kT
D
01 / 2
[6]
which depends strongly on the electrolyte concentration r (20). For electrolyte concentrations below 0.3 mmol/liter the overall force curve becomes attractive for distances less than 10 nm (Fig. 2a). There is a discontinuity in the force curve when the gradient of the attractive force exceeds the cantilever force constant and the tip jumps into contact with the sample (21). The distance at which the jump occurs is a measure for the range of the attractive interaction. In the
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The attractive tip–sample interaction and the adhesion depend on the kind of ions present in the solution. Figure 3 shows the distributions of jump-in distances and adhesion forces in KCl and KOH solution for six force cycles each. Attractive interaction and adhesion are much stronger if K / and Cl 0 ions are present compared to K / and OH 0 ions. If the electrolyte concentration is increased to a value of 0.5 mmol/liter, the attractive interaction between tip and polymer disappears. There is neither a jump-in for small tipsample distances nor adhesion during the retraction cycle (Fig. 2b). This phenomenon is observed in both KOH and KCl electrolyte solutions at above the same ‘‘critical’’ concentration value, though the surface potentials of the PEEK surface are different. The effect is reversible; i.e., attractive interaction can be reestablished by reducing the bulk electrolyte concentration. Zeta Potential Measurements Figure 1a shows the dependence of the zeta potential of PEEK on the electrolyte concentration in KOH and KCl solution. At low electrolyte concentrations it corresponds to the adsorption isotherm for ions on the surface. The polymer is being negatively charged because anions are adsorbed in excess. When all adsorption sites are occupied, the isotherm reaches its saturation value. The zeta potential has a maximum at this concentration because the potential difference between surface and bulk phase decreases for higher bulk concentrations (review (4)). From the zeta potential curves in Fig. 1a it is obvious that the PEEK surface acquires a higher negative surface charge in the OH 0 than in the Cl 0 electrolyte. The bulk concentration cmax , at which the surface is completely covered with ions, is slightly different for both electrolytes. The adsorption free energies for OH 0 and Cl 0 ions onto the PEEK surface calculated from [4] and [5] are given in Fig. 1b. DISCUSSION
FIG. 1. ( a ) Dependence of the zeta potential of PEEK from the electrolyte concentration, determined from streaming potential measurements. The maximum occurs at ( 0.25 { 0.05 ) mmol / liter in the KOH and at ( 0.45 { 0.15 ) mmol / liter in the KCl solution. The extreme value of the zeta potential is ( 039.6 { 2.6 ) mV for KOH and ( 013.5 { 0.6 ) mV for KCl. ( b ) Nonelectrostatic components of the adsorption free energies for electrolyte ions on the PEEK surface according to [ 4 ] and [ 5 ] , determined from the z - c plot in ( a ) . F0 denotes the anion adsorption free energy; F/ denotes the cation adsorption free energy. The difference F0 0 F/ gives a measure for the amount of excess anion adsorption. The total F0 / F/ gives a measure for the adsorption of both anions and cations at the interface.
retraction cycle, there is adhesion between tip and sample. The tip does not pull off the sample until the energy stored in the deflected cantilever equals the adhesion energy.
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The force measurements show that the forces between the PEEK surface and the Si 3N4 tip both immersed in an electrolyte solution are strongly affected by the number and the kind of ions present in the solution. The PEEK surface acquires a certain surface charge and potential, so that electrostatic interactions could account for the effects observed. On the other hand, a layer of adsorbed ions can have an influence on the van der Waals attraction. The electrostatic interaction between two macrobodies carrying surface charges of the same sign may become attractive at small separations if two requirements are met: First, the surface potentials are different and, second, the surface potentials remain constant during the interaction (22). The first condition is fulfilled if the zeta potential is taken as a measure for the surface potential and the values are decreasing in the order: Si3N4 tip, PEEK in KOH, and PEEK in KCl. Provided also the second
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235
FIG. 2. (a) A force–distance plot for PEEK in 0.3 mmol/liter KCl solution. The zero distance is defined from the contact position between tip and sample determined from the constant compliance region in the original force curve. There is a jump-into-contact as well as an adhesion force. Similar force curves with different Debye lengths are observed in pure water and KOH and KCl solutions below 0.5 mmol/liter. (b) A force–distance plot for PEEK in 0.5 mmol/liter KOH solution. There is no attractive interaction nor an adhesive contact. The approach and the retraction curve are coinciding exactly. This force curve is typical for KOH and KCl solutions higher concentrated than 0.5 mmol/liter.
condition, electrostatic attraction could account for the net attractive interaction at small separations in Fig. 2. It could also clarify the different range of attractive interaction in presence of OH0 and Cl 0 electrolytes, respectively. The higher the surface potential of the polymer, the higher the electrostatic barrier and the smaller the range of attractive electrostatic interaction. This is qualitatively the behavior we found in Fig. 3. For electrolyte concentrations above the ‘‘critical’’ value, however, we do not find any attractive interaction between the tip and the PEEK surface. If the attraction was electrostatic, there is no explanation
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why it should disappear when slightly increasing the surface potential. Besides, it should still display the different surface potentials of the PEEK in the different electrolyte solutions. Therefore, the former assumption of constant potential interaction can no longer be valid and electrostatic effects cannot account for our results. There must be another mechanism which correlates the ion adsorption phenomena with the attractive and adhesion forces. There are three experimental observations: First, the range of interaction and the amount of adhesion decrease with increasing electrolyte concentration. Second, they
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SUMMARY
We can conclude that there is an influence of adsorbing electrolyte ions on the adhesion of solids, which can be quantitatively characterized by zeta potential measurements. The larger the free energy of anion adsorption, the smaller is the attractive force between polymer surface and AFM probe. Complete surface coverage yields complete absence of attractive interaction. More experiments with different substrates and adsorbing substances will be necessary in order to explain the competitive interaction mechanism. ACKNOWLEDGMENTS The authors thank Yvonne Candidus for providing the PEEK material and Frank Simon and Franz-Josef Schmitt for valuable discussions.
REFERENCES
FIG. 3. Frequency distribution of jump-in-distance and adhesion force between AFM tip and PEEK surface in 0.3 mmol/liter KOH and KCl, respectively. In KOH, the attractive tip–sample interaction is reduced and the adhesion force is smaller.
are larger in presence of the electrolyte anion with the lower adsorption free energy. Third, they disappear at a bulk electrolyte concentration which corresponds to a complete coverage of the PEEK surface with both anions and cations. It seems to be obvious that adsorbed ions affect the attractive van der Waals interaction between the bare surfaces. The adsorption of ions onto the surface is supposed to happen due to a nonspecific van der Waals attraction, too. There is a competitive interaction between the ions and the Si 3N4 tip opposite the polymer surface similar to the equilibrium described in [1]. The ion adsorption free energy values given in Fig. 1b are a measure for the strength of interaction with the PEEK surface. The stronger the ion–PEEK interaction, the weaker is the tip–PEEK attraction. Above a ‘‘critical’’ concentration there is no more net attractive interaction between Si 3N4 tip and the PEEK surface. Though hard wall contact can be established (Fig. 2b), the ion layer prevents both surfaces from coming into adhesive contact.
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1. Dzjalosinskij, I. E., Lifshitz, E. M., and Pitaevskij, L. P., Adv. Phys. 10, 165 (1961). 2. Fowkes, F. M., and Patrick, R. L., ‘‘Treatise on Adhesion and Adhesives,’’ Vol. 1. Dekker, New York, 1967. 3. Verwey, E. J. W., and Overbeek, J. T. G., ‘‘Theory of the Stability of Lyophobic Colloids.’’ Elsevier, Amsterdam, 1948. 4. Jacobasch, H.-J., ‘‘Oberfla¨chenchemie faserbildender Polymerer.’’ Akademie-Verlag, Berlin, 1984. 5. Ribitsch, V., Jorde, C., Schurz, J., and Jacobasch, H.-J., Prog. Colloid Sci. 77, 49 (1986). 6. Jacobasch, H.-J., Prog. Org. Coat. 17, 115 (1989). 7. Schurz, J., Jorde, C., Ribitsch, V., Jacobasch, H.-J., Ko¨rber, H., and Hanke, R., in ‘‘Proc. Elektrokinetische Erscheinungen ’85,’’ p. 221. Akademie der Wiss, der DDR, Inst. f. Technologie d. Polymere, Dresden, 1985. 8. van Wagenen, R. A., and Andrade, J. D., J. Colloid Interface Sci. 76, 305 (1980). 9. Jacobasch, H.-J., and Schurz, J., Prog. Colloid Polym. Sci. 77, 40 (1988). 10. Bergstro¨m, L., and Bostedt, E., Colloids Surf. 49, 183 (1990). 11. Nitzsche, R., and Friedrich, H., in ‘‘Proc. Electrokinetic Phenomena ’89,’’ p. 123. Academy of Sciences of the GDR, Institute of Polymer Technology, Dresden, 1990. 12. Fava, A., and Eyring, H., J. Phys. Chem. 60, 890 (1956). 13. Jacobasch, H.-J., Angew. Makromol. Chem. 128, 47 (1984). 14. Israelachvili, J. N., and Tabor, D., Proc. R. Soc. London, Ser. A 331, 19 (1972). 15. Binnig, G., Quate, C. F., and Gerber, Ch., Phys. Rev. Lett. 56, 930 (1986). 16. Stern, O., Z. Elektrochem. 30, 508 (1924). 17. Bo¨rner, M., Jacobasch, H.-J., Simon, F., Churaev, N. V., Sergeeva, I. P., and Sobolev, V. D., Colloids Surf. A85, 9 (1994). 18. Jacobasch, H.-J., Faserforsch. Textiltech. 20, 4 (1969). 19. v. Stackelberg, M., Kling, W., Benzel, W., and Wilke, F., Kolloid-Z. 135, 67 (1954). 20. Israelachvili, J. N., ‘‘Intermolecular and Surface Forces.’’ Academic Press, London, 1992. 21. Landman, U., Luedtke, W. D., Burnham, N. A., and Colton, R. J., Science 248, 454 (1990). 22. Bell, G. M., and Peterson, G. C., J. Colloid Interface Sci. 41, 542 (1972).
coidal
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180, 232–236 (1996)
0294
Investigation of Adhesion Properties of Polymer Materials by Atomic Force Microscopy and Zeta Potential Measurements PETRA WEIDENHAMMER
AND
HANS-JO¨RG JACOBASCH 1
Institut fu¨r Polymerforschung, Postfach 120411, 01005 Dresden, Germany Received August 24, 1995; accepted December 14, 1995
MATERIALS AND METHODS AFM and zeta potential measurements were applied in order to investigate the influence of adsorption phenomena on the adhesion behavior of polymer materials. It is shown by means of zeta potential measurements that hydroxyl anions adsorb much stronger than chloride anions onto poly(etheretherketone) surfaces. The adhesion forces between the PEEK surface and the silicon nitride tip of the AFM is lower in potassium hydroxide solution than in potassium chloride solution corresponding to the different adsorption free energies of the anions. For electrolyte concentrations above 0.5 mmol/liter no adhesive contact between tip and sample can be established. We propose an explanation based on the action of competitive forces in the system PEEK–electrolyte solution– Si 3N4 . q 1996 Academic Press, Inc. Key Words: adhesion; adsorption; electrolytes; zeta potential; AFM.
INTRODUCTION
Adhesion of solids in the presence or absence of a liquid phase is a technologically and biologically important phenomenon. Adhesion due to dispersion forces can be quantitatively described by the van der Waals–Lifshitz theory (1); electrostatic interactions by Deryaguin’s theory as well as by semiempirical approaches included in the acid–base theory (2). The stability of colloidal systems is explained by the DLVO theory and the approach of steric stabilization (3). None of these theories contains practically applicable approaches allowing the determination of the influence of adsorption phenomena on adhesion forces. For example, the soil–fiber adhesion in the textile washing process is affected by water and detergent adsorption, the sticking of solids by fatty substances, and the strength of reinforced plastics by adsorbed water (4). It is necessary to find structure–property relationships between the adsorbability of additional substances and adhesion forces. In the current paper, electrokinetic and direct force measurements have been performed in order to investigate these correlations in a nonsymmetric system. 1
To whom correspondence should be addressed.
232
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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JCIS 4180
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6g10$$$$41
A technical, injection molded poly(etheretherketone) (PEEK) was investigated, which had been chosen due to its mechanical and chemical stability. The surface was not damaged by the atomic force microscope (AFM) probe and did not swell in contact with liquids. XPS measurements showed that the polymer consists of pure poly(etheretherketone) without any dissociable groups on the surface. A very small excess of hydrocarbon and C–O–R groups was found, probably due to hydrocarbon contaminants, and a small silicon peak due to silicone and silica groups (H.-J. Jacobasch, Y. Candidus, T. Zeiler, and F. Simon, manuscript in preparation). There was no indication for contamination by steel abrasion. The electrolyte solutions were prepared using Millipore water (pH 5.6); salts were provided by Liil GmbH Dresden (Germany). Force measurements were performed using a Nanoscope III AFM from Digital Instruments ( Santa Barbara, CA ) . The cantilever deflection caused by the forces between probe and polymer surface is detected via a laser beam on a split photo diode. Silicon nitride triangular cantilevers with integrated tips were used; the spring constants, according to the manufacturer, were 0.58 and 0.12 N /m, respectively. The tips were treated with UV light for 15 min before use. We will not present absolute adhesion force measurements here and, therefore, the surface roughness is of particularly small concern. For the present PEEK samples, it involves two components: one with a peak-to-valley roughness of 3 nm and a lateral periodicity of 80 nm and the other with a peak-to-valley roughness of 30 nm and a lateral periodicity of 5 mm. The AFM probe radius is significantly smaller than 100 nm; it ‘‘sees’’ the first component in the same way all over the surface and is not affected by the second. The zeta potential measurements were performed by an electrokinetic analyzer (EKA) (5–7) with a streaming potential cell for flat surfaces (5, 8, 9) constructed by Anton Paar KG (Graz, Austria). In order to characterize the Si 3N4 tips, force – distance
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measurements were performed with different Si 3N4 tips and freshly cleaved mica both immersed in an electrolyte solution. The pH value of the electrolyte solution was varied between 2.8 and 11.0. A long-range electrostatic repulsion always was observed for pH values higher than 3. Since mica is negatively charged for pH ú 2, we can conclude an isoelectric point for the Si 3N4 tip material of pHIEP õ 3. Silanol ( Si-OH ) groups are probably present on the tip surface and determine the pHIEP ( 10 ) . The surface potential can be estimated from ( 11 ) , where the zeta potential for Si 3N4 powders was determined to be about 050 mV for pH @ pHIEP ( in KCl solution, 1 mmol / liter ) . Therefore, the Si 3N4 tips should have a high negative surface potential in all experiments reported here ( pH ú 5.6 ) .
S
S
[1]
Solid–solid adhesion is broken and replaced by separated solid–detergent compounds if this is energetically favorable. The free energy of adhesion between the solids A and B in presence of a detergent C can be written as (13)
01
01
. [3]
F/ and F0 are the nonelectrostatic components of the adsorption free energies for cations and anions, respectively. C0 is the surface potential, z the zeta potential, d 0 the distance between shear plane and surface, c the electrolyte concentration, R the gas constant, F Faraday’s constant and T the absolute temperature. The maximum of the absolute value of z (c) occurs at
cmax Å exp
Asolid Bsolid / 2Cdetergent } AsolidCdetergent / BsolidCdetergent .
S
DD DD
1 F/ / Fz (c) 0 1 / r exp c RT
THEORY
A simple description has been used by one of the authors for the action of adsorbing substances on the adhesion of solids (4). An approach by Fava and Eyring (12) can be generalized for the interaction of two solids in the presence of a third substance, e.g., a detergent:
S
1 1 F0 0 Fz (c) ( C0 0 z (c)) Å 1 / r exp 4pd 0 c RT
S
F/ / F0 2RT
D
[4]
and zmax Å
F0 0 F/ . 2F
[5]
If cmax and zmax are measured, F/ and F0 can be calculated from [4] and [5]. For many initially uncharged polymers without dissociable groups, z -c functions similar to Fig. 1a have been measured (17–19 and references therein). The surface charge, which is acquired by the adsorption of electrolyte ions, increases with increasing Hamaker constant, decreasing hydrophilicity of the polymer and decreasing radius of the hydrated anion (4). RESULTS
DG 0 Å DG 0AB 0 DG 0AC 0 DG 0BC .
[2] Force Measurements
DG 0AB denotes the free energy necessary for establishing the adhesive junction between the solids A and B in absence of the third substance. DG 0AC and DG 0BC are the adsorption free energies of the species C on the solids A and B, respectively. Technical adhesion phenomena have been described quantitatively by this approach (4), though measures for the adhesion have been arbitrary ones. Therefore, it is necessary to establish a measurement of adhesion forces as a force per unit area. Besides Israelachvili’s surface forces apparatus (SFA) (14) the AFM (15) is capable of measuring force– distance functions as well as adhesion forces between almost every kind of solid substrate and a sharp probe. The free energy of adsorption of a dissolved species on a solid can be calculated from its adsorption isotherm. In case of ions, it can be obtained by concentration dependent zeta potential measurements. Considering a solid in presence of a dilute 1–1 electrolyte solution, Stern’s equation (16) describes the charging of the solid surface
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Figure 2 shows typical force–separation graphs for a Si 3N4 tip and the PEEK surface in KOH and KCl solutions of different concentrations. Since both interacting surfaces have negative surface potentials, their electrical double layers repel each other and the force decays exponentially with distance. The decay length is the Debye length 1 Å k
S
(i r`i e 2z 2i 110kT
D
01 / 2
[6]
which depends strongly on the electrolyte concentration r (20). For electrolyte concentrations below 0.3 mmol/liter the overall force curve becomes attractive for distances less than 10 nm (Fig. 2a). There is a discontinuity in the force curve when the gradient of the attractive force exceeds the cantilever force constant and the tip jumps into contact with the sample (21). The distance at which the jump occurs is a measure for the range of the attractive interaction. In the
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The attractive tip–sample interaction and the adhesion depend on the kind of ions present in the solution. Figure 3 shows the distributions of jump-in distances and adhesion forces in KCl and KOH solution for six force cycles each. Attractive interaction and adhesion are much stronger if K / and Cl 0 ions are present compared to K / and OH 0 ions. If the electrolyte concentration is increased to a value of 0.5 mmol/liter, the attractive interaction between tip and polymer disappears. There is neither a jump-in for small tipsample distances nor adhesion during the retraction cycle (Fig. 2b). This phenomenon is observed in both KOH and KCl electrolyte solutions at above the same ‘‘critical’’ concentration value, though the surface potentials of the PEEK surface are different. The effect is reversible; i.e., attractive interaction can be reestablished by reducing the bulk electrolyte concentration. Zeta Potential Measurements Figure 1a shows the dependence of the zeta potential of PEEK on the electrolyte concentration in KOH and KCl solution. At low electrolyte concentrations it corresponds to the adsorption isotherm for ions on the surface. The polymer is being negatively charged because anions are adsorbed in excess. When all adsorption sites are occupied, the isotherm reaches its saturation value. The zeta potential has a maximum at this concentration because the potential difference between surface and bulk phase decreases for higher bulk concentrations (review (4)). From the zeta potential curves in Fig. 1a it is obvious that the PEEK surface acquires a higher negative surface charge in the OH 0 than in the Cl 0 electrolyte. The bulk concentration cmax , at which the surface is completely covered with ions, is slightly different for both electrolytes. The adsorption free energies for OH 0 and Cl 0 ions onto the PEEK surface calculated from [4] and [5] are given in Fig. 1b. DISCUSSION
FIG. 1. ( a ) Dependence of the zeta potential of PEEK from the electrolyte concentration, determined from streaming potential measurements. The maximum occurs at ( 0.25 { 0.05 ) mmol / liter in the KOH and at ( 0.45 { 0.15 ) mmol / liter in the KCl solution. The extreme value of the zeta potential is ( 039.6 { 2.6 ) mV for KOH and ( 013.5 { 0.6 ) mV for KCl. ( b ) Nonelectrostatic components of the adsorption free energies for electrolyte ions on the PEEK surface according to [ 4 ] and [ 5 ] , determined from the z - c plot in ( a ) . F0 denotes the anion adsorption free energy; F/ denotes the cation adsorption free energy. The difference F0 0 F/ gives a measure for the amount of excess anion adsorption. The total F0 / F/ gives a measure for the adsorption of both anions and cations at the interface.
retraction cycle, there is adhesion between tip and sample. The tip does not pull off the sample until the energy stored in the deflected cantilever equals the adhesion energy.
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The force measurements show that the forces between the PEEK surface and the Si 3N4 tip both immersed in an electrolyte solution are strongly affected by the number and the kind of ions present in the solution. The PEEK surface acquires a certain surface charge and potential, so that electrostatic interactions could account for the effects observed. On the other hand, a layer of adsorbed ions can have an influence on the van der Waals attraction. The electrostatic interaction between two macrobodies carrying surface charges of the same sign may become attractive at small separations if two requirements are met: First, the surface potentials are different and, second, the surface potentials remain constant during the interaction (22). The first condition is fulfilled if the zeta potential is taken as a measure for the surface potential and the values are decreasing in the order: Si3N4 tip, PEEK in KOH, and PEEK in KCl. Provided also the second
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FIG. 2. (a) A force–distance plot for PEEK in 0.3 mmol/liter KCl solution. The zero distance is defined from the contact position between tip and sample determined from the constant compliance region in the original force curve. There is a jump-into-contact as well as an adhesion force. Similar force curves with different Debye lengths are observed in pure water and KOH and KCl solutions below 0.5 mmol/liter. (b) A force–distance plot for PEEK in 0.5 mmol/liter KOH solution. There is no attractive interaction nor an adhesive contact. The approach and the retraction curve are coinciding exactly. This force curve is typical for KOH and KCl solutions higher concentrated than 0.5 mmol/liter.
condition, electrostatic attraction could account for the net attractive interaction at small separations in Fig. 2. It could also clarify the different range of attractive interaction in presence of OH0 and Cl 0 electrolytes, respectively. The higher the surface potential of the polymer, the higher the electrostatic barrier and the smaller the range of attractive electrostatic interaction. This is qualitatively the behavior we found in Fig. 3. For electrolyte concentrations above the ‘‘critical’’ value, however, we do not find any attractive interaction between the tip and the PEEK surface. If the attraction was electrostatic, there is no explanation
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why it should disappear when slightly increasing the surface potential. Besides, it should still display the different surface potentials of the PEEK in the different electrolyte solutions. Therefore, the former assumption of constant potential interaction can no longer be valid and electrostatic effects cannot account for our results. There must be another mechanism which correlates the ion adsorption phenomena with the attractive and adhesion forces. There are three experimental observations: First, the range of interaction and the amount of adhesion decrease with increasing electrolyte concentration. Second, they
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SUMMARY
We can conclude that there is an influence of adsorbing electrolyte ions on the adhesion of solids, which can be quantitatively characterized by zeta potential measurements. The larger the free energy of anion adsorption, the smaller is the attractive force between polymer surface and AFM probe. Complete surface coverage yields complete absence of attractive interaction. More experiments with different substrates and adsorbing substances will be necessary in order to explain the competitive interaction mechanism. ACKNOWLEDGMENTS The authors thank Yvonne Candidus for providing the PEEK material and Frank Simon and Franz-Josef Schmitt for valuable discussions.
REFERENCES
FIG. 3. Frequency distribution of jump-in-distance and adhesion force between AFM tip and PEEK surface in 0.3 mmol/liter KOH and KCl, respectively. In KOH, the attractive tip–sample interaction is reduced and the adhesion force is smaller.
are larger in presence of the electrolyte anion with the lower adsorption free energy. Third, they disappear at a bulk electrolyte concentration which corresponds to a complete coverage of the PEEK surface with both anions and cations. It seems to be obvious that adsorbed ions affect the attractive van der Waals interaction between the bare surfaces. The adsorption of ions onto the surface is supposed to happen due to a nonspecific van der Waals attraction, too. There is a competitive interaction between the ions and the Si 3N4 tip opposite the polymer surface similar to the equilibrium described in [1]. The ion adsorption free energy values given in Fig. 1b are a measure for the strength of interaction with the PEEK surface. The stronger the ion–PEEK interaction, the weaker is the tip–PEEK attraction. Above a ‘‘critical’’ concentration there is no more net attractive interaction between Si 3N4 tip and the PEEK surface. Though hard wall contact can be established (Fig. 2b), the ion layer prevents both surfaces from coming into adhesive contact.
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