Dissociation of Surface Functional Groups and Preferential Adsorption

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Dissociation of Surface Functional Groups and Preferential Adsorption of Ions on Self-Assembled Monolayers Assessed by Streaming Potential and Streaming Current Measurements Ruediger Schweiss,*,†,‡ Petra B. Welzel,† Carsten Werner,†,§ and Wolfgang Knoll‡ Institute of Polymer Research Dresden, Department Biocompatible Materials, Hohe Str. 6, D-01069 Dresden, Germany, and Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany Received December 12, 2000. In Final Form: April 6, 2001 The interfacial charge formation of self-assembled monolayers (SAMs) of thiol derivatives on gold was investigated by streaming potential and streaming current measurements in aqueous electrolyte solutions. All experiments were performed at a slit channel formed by two parallel sample plates. For the first time, ζ potentials of planar SAM-solution interfaces were determined experimentally. Surface pK values were calculated from the pH dependence of the ζ potential. Monolayers carrying ω-carboxylic moieties show a shift of the pK toward the alkaline direction as compared to the carboxy-terminated alkanethiol molecules in solution. Contact angle titrations using a captive bubble method confirmed the dissociation behavior of monolayers with acidic groups. Monolayers of methyl-terminated thiols exhibit ζ potential-pH plots that account for unsymmetrical ion adsorption. Models of the electric double layer are discussed to describe the interface between self-assembled monolayers and electrolyte solutions. Two contrary situations were found depending on the process which generates surface charges. For methyl-functionalized SAMs, the main part of the surface charge is compensated within the diffuse double layer. In contrast, at monolayer surfaces bearing carboxylic acid groups, the main part of the countercharge was found to be located in the stagnant part of the double layer.

Introduction Interfacial charge density at solid-liquid interfaces was found to be relevant for a number of phenomena such as adhesion, wetting, colloid stability, or biocompatibility.1-10 The dissociation of surface-confined functional groups and the specific adsorption of ions from solution have turned out to be the essential processes to generate surface charges. Self-assembled monolayers (SAMs) of alkanethiolates have been the subject of numerous interfacial studies.11-19 Various spectroscopic, microscopic, and electrochemical techniques were used to establish structureproperty relationships of these systems, and many ap* To whom correspondence should be addressed. E-mail: [email protected]. † Institute of Polymer Research Dresden. ‡ Max-Planck-Institute for Polymer Research. § University of Toronto, Department of Mechanical and Industrial Engineering, 5 King’s College Road, Toronto, Ontario, Canada, M5S 3G8. (1) Lyklema J. Fundamentals of Interface and Colloid Science; Academic Press: London, 1995; Vols. 1 and 2. (2) Surface and Interfacial Aspects of Biomedical Polymers; Andrade, J. D., Ed.; Plenum Press: New York, 1985. (3) Hunter, R. J. Foundations of Colloid Sciences; Clarendon Press: London, 1986. (4) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; Wiley: New York, 1982. (5) Ratner, B. D. In Biomaterials: Interfacial Phenomena and Applications; Cooper S. L., Peppas, N. A., Eds.; Advances in Chemistry Series 199; American Chemical Society: Washington, DC, 1982. (6) Grundke, K.; Jacobasch, H. J.; Simon, F.; Schneider, St. J. Adhes. Sci. Technol. 1995, 9, 9327. (7) Acid-base interactions: Relevance to Adhesion Science and Technology; Mittal, K. L., Anderson, H. R., Eds.; VSP: Zeist, The Netherlands, 1991. (8) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927. (9) Whitesides, G. M.; Biebuyck, H. A.; Folkers, J. P.; Prime, K. L. J. Adhes. Sci. Technol. 1991, 5, 57. (10) Dukhin, S. S.; Derjaguin, B. V. In Surface and Colloid Science; Interscience: New York, 1974; Vol. 7, pp 1-336.

plications in the fields of analytical science, lithography, and nanotechnology were demonstrated by further chemical and physical modifications of the monolayers. On the other hand, alkanethiol SAMs provide versatile model systems to establish correlations of microscopic structure and macroscopic properties because they are reproducible in fabrication and structure. Additionally, thiol SAMs form a chemically homogeneous surface with an approximately ideal two-dimensional structure with each group experiencing the same micro-environment. The density of these surface groups is predetermined by the surface coverage of the thiol molecules. These properties are especially advantageous for studying the acid-base equilibrium at solid-liquid interfaces. A number of techniques, for example, contact angle titration,21-23 quartz microbalance measurements,24 am(11) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (12) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (13) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (14) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (15) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (16) Ulman, A. Chem. Rev. 1996, 96, 1533. (17) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (18) Doblhofer, K.; Figura, J.; Fuhrhop, J. H. Langmuir 1992, 8, 1811. (19) Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1993, 9, 1775. (20) Holmes-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G. M. Langmuir 1985, 1, 725. (21) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (22) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741. (23) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675. (24) (a) Wang, J.; Frostman, L. M.; Ward, M. D. J. Phys. Chem. 1992, 96, 5224. (b) Shimazu, K.; Teranishi, T.; Sugihara, K.; Uosaki, K. Chem. Lett. 1998, 669.

10.1021/la001741g CCC: $20.00 © 2001 American Chemical Society Published on Web 06/06/2001

Dissociation and Preferential Adsorption on SAMs

perometry,25 voltammetry,26-29 laser-induced temperature jump studies,30 chemical force microscopy,31-35 electrochemical titration,36 or double-layer capacitance measurements,37 have been applied to determine the pK and the double-layer structure of surface-confined acids and bases. Values of the dissociation constants for monolayers carrying carboxylic acid groups were found in a wide range from 4.5 to 7.7. Several reasons were suggested for the shift of the pK of surface-confined ionizable groups as compared to the molecules dispersed in aqueous solution. First, electrostatic repulsion of the charged moieties may affect the pK values because of overlap of the local electric fields. This effect can be expected to be enhanced by a reduced dielectric susceptibility at the interface compared to the solution because of orientation of solvent molecules. A reduced permittivity of the interfacial layers may also lead to weaker solvation of the released protons, which is energetically unfavorable. Further, stabilizing effects by hydrogen bonding of the acid-base functions may influence the ionization process as well. In this paper, we present a new approach to meet the question of charge formation of SAMs with a technique that originates from colloid science. We determined the ζ potential (the potential at the hydrodynamic shear plane) of SAM-solution interfaces by measurement of the streaming potential and the streaming current at a rectangular slit channel which is formed by two parallel sample plates. The experimental setup, the so-called microslit electrokinetic setup, was recently developed in our group42,43 and has already been successfully applied for the characterization of polymer interfaces.44 Briefly, the hydrodynamically mobile part of the electric double layer is tangentially shifted in this experiment by the force exerted by the flowing liquid phase. The resulting charge separation can be quantified either by streaming (25) Cheng, Q.; Braither-Toth, A. Anal. Chem. 1992, 64, 1998. (26) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385. (27) White, H. S.; Peterson, J. D.; Cui, Q.; Stevenson, K. J. J. Phys. Chem. B 1998, 102, 2930. (28) Molinero, V.; Calvo, E. J. J. Electroanal. Chem. 1998, 445, 17. (29) Godinez, L. A.; Castro, R.; Kaifer, A. E. Langmuir 1996, 12, 5087. (30) Smalley, J. F.; Chalfant, K.; Feldberg, S. W.; Nahir, T. M.; Bowden, E. F. J. Phys. Chem. B 1999, 103, 1676. (31) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114. (32) (a) Kane, V.; Mulvaney, P. Langmuir 1998, 14, 3303. (b) Kokkoli, E.; Zukoski, C. F. Langmuir 2000, 16, 6029. (33) van der Vegte, E. W.; Hadziioannou, G. J. Phys. Chem. B 1997, 101, 9563. (34) (a) Zhang, H.; He, H. X.; Mu, T.; Liu, Z. F. Thin Solid Films 1998, 327-329, 778. (b) Zhang, H.; Zhang, H. L.; He, H. X.; Zhu, T.; Liu, Z. F. Mater. Sci. Eng., C 1999, 8-9, 191. (35) (a) He, H. X.; Huang, W.; Zhang H.; Li, Q. G.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 517. (b) Vezenov, D. V.; Noy, A.; Rosznyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006. (36) Zhao, J.; Luo, L.; Yang, X.; Wang, E.; Dong, S. Electroanalysis 1999, 11, 1108. (37) Kakiuchi, T.; Iida, M.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 5397. (38) (a) Smart, J. L.; McCammon, J. A. J. Am. Chem. Soc. 1996, 118, 2283. (b) Borkovec, M. Langmuir 1997, 13, 2608. (39) Aoki, K.; Kakiuchi, T. J. Electroanal. Chem. 1999, 478, 101. (40) Hunter, R. J. Zeta Potential in Colloid Science: Fundamentals and Applications; Academic Press: London, 1981. (41) Jacobasch, H. J. Oberfla¨ chenchemie faserbildender Polymere; Akademie-Verlag: Berlin, 1984. (42) (a) Werner, C.; Ko¨rber, H.; Zimmermann, R.; Dukhin, S. S.; Jacobasch, H. J. J. Colloid Interface Sci. 1998, 208, 329. (b) Ko¨rber, H.; Werner, C.; Jacobasch, H. J. Patent DE 197 49 429.3, December 11, 1997. (43) Zimmermann, R.; Jenschke, W.; Ko¨rber, H.; Werner, C. Tech. Mess. 2000, 9, 353. (44) (a) Werner, C.; Ko¨nig, U.; Augsburg, A.; Arnhold, C.; Ko¨rber, H.; Zimmermann, R.; Jacobasch, H. J. Colloids Surf., A 1999, 159, 519. (b) Zimmermann, R.; Werner, C. J. Phys. Chem. B, submitted. (c) Ko¨nig, U.; Ph.D. Thesis, Dresden, 2000.

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potential or by streaming current measurements across the slit channel. Experimental Section (i) Monolayer Preparation. 11-Mercaptoundecanoic acid (MUA) and 16-mercaptohexadecanoic acid (MHA) were purchased from Aldrich (Deisenhofen, Germany), and hexadecanethiol (HDT) and octadecanethiol (ODT) were obtained from Merck (Darmstadt, Germany). Polished glass slides (length (L) 20 mm, width (b) 10 mm, Berliner Glas GmbH, Germany) were cleaned and primed with a 2 nm chromium layer to promote adhesion. Then, a 150 nm gold layer was evaporated onto the substrates. Prior to thiol deposition, the gold substrates were cleaned with sulfurchromic acid and rinsed with deionized water and finally with a copious amount of absolute ethanol. After that, they were immersed for at least 16 h in a 0.5 mM solution of the corresponding thiol dissolved in absolute ethanol. After assembly, the substrates were rinsed thoroughly with ethanol and dried under vacuum. To control monolayer quality, the samples were examined by wetting measurements, ellipsometry, and reflection-adsorption FTIR spectroscopy. The thickness of the films was determined by an ellipsometer (DRE Dr. Riss Ellipsometerbau, Ratzeburg, Germany). Reflection-adsorption FTIR spectra were recorded by means of a Perkin-Elmer 1760X FTIR spectrometer (Norwalk, USA) equipped with a He-Ne laser (wavelength 632.8 nm, p-polarized light, angle of incidence 80°) and a MCT detector which was cooled with liquid nitrogen. These spectra yielded CH2-stretching vibrations at 2918 and 2850 cm-1 for HDT and 2919 and 2851 cm-1 for MHA which indicate crystalline packing and a high degree of orientation of the hydrocarbon chains.13-15 (ii) Electrokinetic Measurements. The key features of the microslit electrokinetic setup are the simultaneous determination of the streaming potential and the streaming current and the possibility of varying the height of the slit channel formed by the sample surfaces. The liquid flow proceeds between two containers passing the microslit and sensors for the measurement of pH, conductivity, and temperature. Streaming potential and streaming current were measured with an electrometer having a high ohmic resistance using Ag/AgCl electrodes installed at both ends of the streaming channel. All electrolyte solutions were prepared using deionized water (Milli-Q, resistivity 18 MΩ/cm) and by means of titrators equipped with standard electrolyte solutions (KCl, HCl, KOH). The solution pH was adjusted by addition of hydrochloric acid or potassium hydroxide. Nitrogen 5.0 served as process gas to generate liquid flow in all experiments. It excludes carbon dioxide from the atmosphere and thus maintains constant ionic strength and pH over long periods of time. The streaming potential and the streaming current were measured by applying pressure ramps up to 200 mbar in 20 mbar steps and in both directions of the slit channel. The gas pressure was controlled by means of a pressure sensor. Measurement procedure, data collection, and processing were performed with a software package written in Testpoint (Keithley Instruments, USA).43 The whole setup was assembled under a laminar flow box. Further details of the microslit electrokinetic setup may be found in refs 42 and 43. The substrates were glued to glass blocks, cleaned with a stream of argon, and mounted into the microslit cell. To obtain a rectangular slit channel, the samples were aligned by means of a light microscope. The height of the slit channel was thoroughly adjusted by flow measurements at four different pressure differences over the channel and was set to 50 ( 0.1 µm. (iii) Contact Angle Titrations. Contact angles of sessile drops were measured using a Kru¨ss DSA system (Hamburg, Germany). Contact angle titrations using the captive bubble method were performed with a Kru¨ss G640 goniometer (Hamburg, Germany) equipped with a camera in a 100 mL quartz glass cuvette containing the degassed electrolyte solution (3 × 10-4 M KCl) and a pH microelectrode. Captive bubbles were produced on the samples by a syringe with a micrometer screw. To change the solution pH, 0.1 M hydrochloric acid or potassium hydroxide solution was added. No buffer substances were used in the measurements. An average value of three bubbles was

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taken at each pH. Referring to the applied procedure, we simply state in the following the advancing angle of the captive bubble as receding contact angle of the liquid. Receding contact angles showed better reproducibility than advancing contact angles and were thus used for interpretations. Nevertheless, advancing contact angles showed the same trend as receding contact angles. All electrokinetic and contact angle experiments were performed at 23 ( 1 °C.

Theory Surfaces with acidic or basic functional groups undergo acid-base reactions in aqueous media which lead to the formation of interfacial charges:

-COOH T -COO- + H+surf

(1)

where H+surf is the concentrations of protons at the surface. One can define an equilibrium constant Ka as

ΓCOO- [H+]surf Ka ) ΓCOOH

(2)

The value of pH where one-half of the ionizable groups is charged is defined as the surface pK. For low surface potentials, the surface concentration of protons is commonly approximated by a Boltzmann distribution,32,57,58

(

[H+]surf ) [H+]bulk exp -

)

FΨsurf RT

The actual surface potential Ψsurf of the SAM-solution interface is not assessable through experiments. However, several methods such as double-layer capacitance or forcedistance measurements31-33 allow at least an estimation of the surface potential. (i) ζ Potential Determination. A number of polymers, inorganic surfaces, modified metal substrates, or biointerfaces have been successfully characterized using electrokinetic techniques.49-55 Electrokinetic measuring principles are nondestructive and can be applied for dispersions, fibers, membranes, and planar substrates as well. The ζ potential is highly sensitive to changes in the double-layer charge and can therefore be applied to monitor surface charge formation. We chose the electrokinetic approach because this method does not interfere with the ionization process as compared to force-distance measurements. Furthermore, the ζ potential is the potential that is relevant for many of the interfacial phenomena, and it allows us to quantify the charge density of the diffuse part of the double layer. It was recently assumed that the potential at the shear plane (ζ potential) can become very close to the interaction potential determined by force-distance measurements for certain systems.56 The ζ potential can be calculated from the pressure dependence of the streaming potential and the streaming current measurements across a capillary system (Smoluchowski equations):

(3) ζ(Us) )

[

0

The surface charge density, σ , is simply given by -FΓCOO-. In the case of self-assembled monolayers (thiols, for example), the concentration of the acidic moieties, ΓCOO-, is given by the surface concentration of the thiol molecules. Therefore, the surface charge density is dependent only on Ka, the concentration of thiol molecules, and on the pH:32

σ0 ) -FΓCOO- )

-FΓthiol × 10-pK ) -FΨsurf 10-pK + 10-pH exp RT -FΓthiolR (4)

[

]

where R is the degree of ionization of the acidic groups. The concentration of thiol moieties on the surface is known from the literature from helium diffraction,45 transmission electron diffraction,46 and reductive electrochemical desorption studies47,48 to be 7.7 × 10-10 mol/cm2, at least for a densely packed monolayer. This corresponds to one thiol molecule per 21.5 Å2. Equation 4 is valid if the surface sites are identical and their equilibrium constant is independent of the degree of dissociation. Because all thiol molecules are of the same chain length, a very smooth surface is obtained, with the carboxy groups being approximately coplanar. The ionization process generates a local potential at the terminating functions of the thiols which counteracts further dissociation of neighboring carboxy groups. As a consequence, the titration curves are shifted in the alkaline direction and are broadened compared to the same species in solution.38,39 (45) Chidsey, C. E. D.; Liu, G. Y.; Rowntree, P.; Scoles, G. J. J. Chem. Phys. 1989, 91, 4421. (46) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 456. (47) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (48) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687.

ζ(Is) )

]

2Kσ h dUs 0r dp

η κe +

η L dIs 0r A dp

(5)

(6)

where ζ(Us) is the ζ potential calculated from the streaming potential, ζ(Is) is the ζ potential obtained by the streaming current, η is the viscosity, L is the channel length, A is the cross-sectional area of the streaming channel, κe is the electrolyte conductivity, Kσ is the surface conductivity, and h is the channel height. If the influence of interfacial conductivity is not considered, only an apparent ζ potential is determined from the streaming potential which is lower than the actual one. This aspect must especially be considered for conducting samples. The ζ potential determined from the streaming current, however, is independent of the surface conductivity and is a function only of the channel geometry for a given system. At channel heights above about 40 microns and intermediate electrolyte concentrations, the surface conductivity is usually negligible for nonconducting samples. In the latter case, the surface conductivity can be determined by measuring the channel conductance at several channel heights and extrapolating to zero sample distance.42 From ζ potential, one can calculate the charge density of the (49) Weidenhammer, P.; Jacobasch, H. J. J. Colloid Interface Sci. 1996, 180, 232. (50) Jacobasch, H. J.; Simon, F.; Weidenhammer, P. Colloid Polym. Sci. 1998, 276, 434. (51) Minor, M.; van der Linde, A.; Lyklema, J. J. Colloid Interface Sci. 1998, 203, 177. (52) Bismarck, A.; Kumru, M. E.; Springer, J. J. Colloid Interface Sci. 1999, 217, 377. (53) Calvo, J. I.; Hernandez, A.; Pradanos, P.; Tejerina, F. J. Colloid Interface Sci. 1996, 181, 399. (54) Werner, C.; Jacobasch, H. J. Int. J. Artif. Organs 1999, 22, 160. (55) Lopes, M. A.; Monteiro, F. J.; Santos, J. D.; Serro, A. P.; Saramago, B. J. Biomed. Mater. Res. 1999, 45, 370. (56) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Phys. Chem. 1995, 99, 2114.


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diffuse layer. For an 1:1 electrolyte of the concentration c, it is given by

[

σ d ) x80rRTc sinh -

Fζ 2RT

]

(7)

Electroneutrality requires that the integrated charge density of the diffuse double layer is numerically equal to the charge density of the immobile layer σ i. If the double layer is purely diffuse, σ d equals the surface charge density σ 0:

σ0 ) -σ d

(8)

Equation 8 is valid only for low surface charge densities and low electrolyte concentrations. In the case of high surface concentrations of acid-base functions which are given for SAMs, this equation is obviously not applicable. In the latter case, the surface charge is compensated in part by ions in a stagnant layer at the interface OHP (outer Helmholtz plane). This situation refers to the Stern model of solid-liquid interfaces.61

σ IHP + σ OHP + σ d ) 0

(9)

(ii) Contact Angle Titration. Whitesides et al. introduced the use of contact angle titration for the characterization of polymer and SAM surfaces.9,17,20-22 The change of advancing contact angles of buffered droplets was monitored as a function of pH. Systems with acidic groups on the surface show a titration curve due to the increased wettability of charged interfaces, whereas hydrophobic samples reveal constant contact angles over a wide range of pH. One has to distinguish between reactive and nonreactive spreading. Reactive spreading is driven by the energy of protonation/deprotonation of the ionizable surfaces, and the curves for carboxylic acids show no plateau in the basic range. Therefore, it is solely possible to determine the onset of ionization. Nonreactive spreading occurs if the sample is preequilibrated with a buffered solution that is matched to the pH of the solution that is used for the spreading experiment. This was investigated in detail by Creager et al.23 The contact angle of the nonreactive spreading protocol is thus dependent only on the fraction of charged carboxylic acids on the surface:

cos θ ) cos θCOOH + R(cos θCOO- - cos θCOOH)

(10)

where R represents the degree of dissociation. Most of those contact angle titrations have been performed under inert liquids (cyclooctane, for example ) with sessile drops. In difference to that approach, we are using a captive bubble technique in diluted electrolyte solutions similar to those applied in the electrokinetic experiments. The surfaces are assumed to be in equilibrium with the electrolyte solution. Pre-equilibration of the samples in pH-matched solutions is therefore not required. This situation has to be considered as a nonreactive spreading experiment. (57) Healy, T. W.; White, L. R. Adv. Colloid Interface Sci. 1978, 9, 303. (58) Smith, C. P.; White, H. S. Langmuir 1993, 9, 1. (59) Andreu, R.; Fawcett, W. R. J. Phys. Chem. 1994, 98, 12753. (60) Fawcett, W. R.; Fedurco, M.; Kovacova, Z. Langmuir 1994, 10, 2403. (61) Stern, O. Z. Elektrochem. 1924, 30, 508.

Figure 1. ζ Potential measurements of mercaptoundecanoic acid (MUA, left) and hexadecanethiol (HDT, right) SAMs on gold as a function of pH. ζ Potential was derived from streaming potential (open circles) and streaming current (solid circles). Electrolyte: 0.3 mM potassium chloride.

Results and Discussion (i) ζ Potential Measurements. Self-assembled monolayers of MUA, MHA, HDT, and ODT were investigated by streaming potential and streaming current measurements. Potassium chloride served as background electrolyte to adjust the ionic strength. ζ Potentials were calculated by means of eqs 5 and 6. Self-assembled monolayers of long-chain alkanethiolates were stable at the applied experimental conditions and gave reproducible results during a series of pH-dependent measurements. However, some preliminary experiments with hydrophilic short-chain thiols (mercaptopropanesulfonic acid, for example) indicated some desorption of thiol molecules during the experiments which became obvious by the shift of the isoelectric point. Especially for the carboxy-terminated SAMs, there are very large differences between the ζ potential determined from the streaming current and the ζ potential obtained from the streaming potential (Figure 1). As we operated at a slit channel height of 50 microns, this discrepancy cannot be solely interpreted by any influence of the surface conductivity. A contribution of the bulk conductivity of the underlying gold substrate may be the most plausible explanation for such a behavior. In contrast to polymer films which display almost a complete identity of ζ potentials calculated from the streaming current and the streaming potential under these conditions, layered systems consisting of self-assembled monolayers of thiols on gold reveal a significant conductivity. For hydrophobic alkanethiol monolayers, the difference of ζ(Us) and ζ(Is) is much smaller. That is, these monolayers show more effective dielectric “shielding” of the gold support. The charge transfer may proceed through defect sites or pinholes in the monolayer. One may treat this effect on the ζ potential in an analogeous manner to a surface conductivity. In this case, Ktot describes the sum of the contributions of the surface conductivity Kσ and the conductivity of the underlying gold layer Kgold. Using eqs 5 and 6, one obtains42a

Ktot ) Kσ + Kgold )

[

]

1 dIs′ L - κeh 2 dUs′ b

where

dIs′ )

dIs dp

dUs′ )

dUs dp

(11)

4308


Schweiss et al. Table 1. Electrokinetic Data of Different SAMs: Isoelectric Points, Surface pK Values from ζ Potential and Contact Angle Measurement, and Plateau Values of ζ Potential Obtained from Streaming Potential (Us) and Streaming Current (Is)

Figure 2. ζ Potentials from streaming current ζ(Is) of an ODT SAM. ζ Potential vs pH at different potassium chloride concentrations (top) and vs concentrations of different electrolytes (bottom).

Because the ζ potential from the streaming current is independent of the surface or substrate conductivity, we are using the latter in the further interpretation of ζ potential data. Figure 1 shows the pH dependence of the ζ potential of carboxy- (MUA) and methyl-terminated alkanethiol (HDT) SAMs. The ζ potential of the hydrophobic HDT SAM versus solution pH shows a rather linear plot, which indicates that the interfacial charge is established by unsymmetrical ion adsorption. Isoelectric points (zero value of the ζ potential) were almost exactly at about pH 4.0 which was also found for nonpolar polymers without ionizable surface groups.6,49,50,52 It was very astonishing that hexadecanethiol and octadecanthiol SAMs showed a lower isoelectric point than acidic monolayers. Both systems showed positive ζ potentials at low pH, which has to be attributed to proton adsorption in the double layer. Figure 2 shows ζ potential plots of an octadecanethiol SAM under varying electrolyte compositions. The isoelectric point was independent of the KCl electrolyte concentration (Figure 2, top). That means that there is no contribution of the potassium chloride to the charging process. This is also confirmed by measurements of ζ potential as a function of the electrolyte concentration of potassium chloride, hydrochloric acid, and potassium hydroxide (Figure 2, bottom). The negative ζ potential does not increase in magnitude with increased potassium chloride concentration. The ζ potential as a function of the potassium hydroxide concentration shows a plot which is characteristic of a negative charging of the surface. Hydrochloric acid creates positive ζ potentials above a concentration of about 10-4 mol/L. Therefore, we suggest a preferential adsorption of ions in a row OH- > H3O+ . Cl- ≈ K+.44b The well-known decrease of ζ potential at high ionic strength is to be attributed to a compression of the double layer. As expected for carboxylic-functionalized thiols, the ζ potential reaches a plateau in the alkaline range. This accounts for the dissociation of the surface-confined carboxy groups. The plateau value of the ζ potential of

SAM

IEP

pK (ζ)

ζplat (Is)/ mV

ζplat (Us)/ mV

pK (contact angle)

MUA MHA HDT ODT

4.26 ( 0.06 4.34 ( 0.07 4.00 ( 0.04 3.95 ( 0.03

5.15 5.20

-113 -109

-29 -40

5.32 5.25

Figure 3. ζ Potential from streaming current ζ(Is) of a MHA SAM at different potassium chloride concentrations.

carboxylic SAMs was reproducible to about (6 mV for both chain lengths. Isoelectric points (IEP) varied in the range of pH 4.32 ( 0.08 (Table 1). At a pH value above the IEP, ionization of carboxylic moieties starts. No substantial difference of the ζ potential versus pH dependence (i.e., the titration behavior) was found for mercaptoundecanoic and mercaptohexadecanoic acid except for the ζ potential derived from streaming potential measurements. The ζ potential calculated from streaming potential was lower for mercaptoundecanoic acid SAMs which corresponds to a higher conductivity contribution of the gold substrates. The surface pK values for carboxylic-terminated monolayers were determined to be 5.15 ( 0.2 (MUA) and 5.20 ( 0.2 (MHA) by fitting the ζ potential data to a titration curve. The results are in good agreement with data from force-distance measurements described in the literature.32-35 The dependence of the ζ potential of MHA on pH at different background electrolyte concentrations (Figure 3) also reflects the behavior found in atomic force microscopy (AFM) experiments.32 Titration plots of carboxylic acids dispersed in solutions usually extend over 2 units of pH and reveal pK values of 4.5-4.7. Titration plots obtained from ζ potential curves of carboxyterminated SAMs used in this work are broadened compared to them. Similar results have been obtained by other experimental methods.31-33 The broadening has been explained by retarded ionization due to the local electric field. A controversial discussion has arisen concerning the shift of the pK of surface-confined ionizable groups compared to the same moieties in solution. This phenomenon has also been attributed to effects of hydrogen bonding and a reduced dielectric permittivity near the surface. The influence of interactions between the acidic groups was recently modeled theoretically.38,39 Bain et al. investigated mixed SAMs with methyl and carboxy terminal groups of equal chain length by means of contact angle titrations. They found that ionization starts at higher pH if the thiols bearing carboxylic acids are diluted with


Langmuir, Vol. 17, No. 14, 2001 4309

Figure 4. Model of the electrochemical double layer of a SAM/solution interface. IHP denotes the inner Helmholtz plane, and OHP denotes the outer Helmholtz plane.

methyl-functionalized thiols.21 Creager et al. found that if the carboxy functions are mixed with a methyl component of higher chain length, this also shifts the pK toward the alkaline range.23 Both findings would explain the impact of the dielectric environment on the ionization, though, even for pure carboxy SAMs, shifts of the pK by 3 units of pH were proposed.24,29,31 If we take the isoelectric point as a criterion, we have to suggest a significant shift. The IEP is most of all sensitive to the acidity/basicity of the surface and to the density of acidic groups on the surface. A possible example for comparison to our carboxy-terminated SAMs is poly(acrylic acid) (PAAc). ζ Potential measurements of PAAc show a characteristic isoelectric point of pH 2.5.44c Therefore, one may suggest a major deviation of the ionization behavior of spatially confined acid-base functions from the situation given in solutions. However, it is important to note that PAAc is subject to swelling in aqueous solutions and adopts a rather three-dimensional structure. Therefore, comparisons with monolayers are to be made with caution. On the other hand, by fitting the ζ potential plots to a sigmoidal titration curve, we could not give evidence for an intrinsic shift of the surface pK of COOH groups by several units of pH. Several approaches have been proposed to calculate surface pK values from ζ potential data.1,40,41,64,65,67 Most of them are restricted to low ζ potentials and do not seem to be useful in our case. Additionally, a possible ion adsorption onto the protonated carboxy surface was not considered. Strictly speaking, only an apparent surface pK is obtainable from the pH where the ζ potential reaches half of the plateau value. The charge densities of the diffuse double layer were calculated by means of eq 7. In the plateau of the ζ potential, the carboxy groups are assumed to be fully dissociated. Applying eq 7 yields a diffuse layer charge density of 0.95 ( 0.05 µC/cm2. In the case of a pure diffuse (62) Lyklema, J.; Overbeek, J. Th. G. J. Colloid Sci. 1961, 16, 501. (63) Donath, E.; Voigt, A. J. Colloid Interface Sci. 1986, 109, 122. (64) Bo¨rner, M.; Jacobasch, H. J.; Simon, F.; Churaev, N. V.; Sergeeva, I. P.; Sobolev, V. D. Colloids Surf., A 1994, 85, 9. (65) Bo¨rner, M.; Jacobasch, H. J. Proc. Symp. Electrokin. Phenomena 1987, 231, Institute of Polymer Research Dresden. (66) Welzel, P.; Rauwolf, C. Manuscript in preparation. (67) Lyklema, J.; Rovillard, S.; de Coninck, J. Langmuir 1998, 14, 5659.

double layer,58 the integrated diffuse layer charge density numerically equals the surface charge density. In the case of a fully charged monolayer, the surface charge density is given by -FΓthiol, which corresponds to 74.3 µC/cm2. Calculating the surface charge σ0 by eqs 7 and 8 and inserting into eq 4 yields the surface potential Ψsurf which would be expected for a purely diffuse layer. The obtained value (using pK ) 5.3 and pH ) 8) is -270 mV which is considerably higher as compared to the measured ζ potential. This undoubtedly indicates that the SAMsolution interface cannot be adequately described by a simple diffuse layer model. Mulvaney et al. recently presented models that describe AFM force-distance curves of mercaptoundecanoic acid assembled onto gold substrates in electrolyte solutions.32 The interaction potentials obtained from these AFM measurements were comparable to our ζ potential data. One can postulate an ion-free zero-order Stern layer (ZOS), in which the surface potential drops linearly because of the hydration of ions at the interface. In our case, the potential drop within the Stern layer which is required to fit the experimental data would be about 100-200 mV (depending on the pH) for the fully dissociated state. The second approach is based on a competitive binding of protons and cations in a purely diffuse layer in order to account for the low experimental interaction potential. The ZOS model and the competitive site binding model should be considered as boundary situations. Both explanations, an immobile Stern layer consisting of solvent molecules and charge compensation by adsorption of cations within the latter, may simultaneously contribute to the low surface charge density and surface potential determined by ζ potential measurements. Fawcett et al. recently presented an electrochemical picture of the SAM-electrolyte interface to predict double layer capacitance on SAM-modified gold electrodes.59,60 The carboxy groups are assumed to be located in the inner Helmholtz plane (IHP). The OHP intersects the center of the adsorbed counterions. The potential drops linearly from the IHP to the OHP (Figure 4). In the generally accepted double-layer models,1,3,40,61,62 the shear plane is located at the edge of the layer of adsorbed counterions. The boundary between the Stern layer and the diffuse layer is assumed to coincide with the hydrodynamic solidliquid boundary which is characterized by the ζ potential.

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Schweiss et al.

Table 2. Charge Densities of the Double Layer of MHA SAM/Solution Interfaces in the Fully Dissociated State (Plateau Value of ζ Potential) at Different Potassium Chloride Concentrations cKCl [mol/L]

σd [µC/cm2]

σOHP [µC/cm2]

2 ΓeOHP K+ [mol/cm ]

3 × 10-4 3 × 10-3

0.95 0.94

73.5 73.4

7.61 × 10-10 7.60 × 10-10

This also implies that the measured ζ potential almost equals the potential at the outermost edge of the outer Helmholtz plane ΨOHP as demonstrated in Figure 4. It is obvious that the assumption of a discrete shear plane on a molecular level is an oversimplification, but it has proven to be very useful for data evaluation and for the description of macroscopic properties. The correlation between the ζ potential and the OHP potential requires the surface to be ideal. Effects of roughness, porosity, or swelling can lead to significant deviations between the ζ potential and the potential of the OHP.63 However, because self-assembled monolayers fulfill most requirements of a model surface, ζ potentials will give a good estimation of the potential of the OHP. On the basis of the double-layer model presented in Figure 4, we can calculate the excess charge density caused by adsorbed counterions in the case of fully dissociated carboxy groups. As already stated above, the charge density of the IHP (or surface charge density σ0) for the fully dissociated state is simply -FΓthiol and the charge density of the diffuse layer is available by substituting ζplateau to eq 7. Therefore, we are able to calculate σOHP from eq 9. The charge density of the OHP allows for the determination of excess charge within the latter caused by the adsorbed potassium ions. This is given by

ΓeOHP K+ )

σOHP F

(12)

The results are shown in Table 2. The data reveal that only about 1-2% of the surface charge is compensated in the diffuse part of the double layer. At least for full ionization of the carboxylic acids, the concentration of potassium ions obviously has no influence on the charge density of the OHP. This strictly argues against the competitive site binding model.32 The knowledge of the charge densities of the double layer further allows for a calculation of the expected surface conductivity:

Kσ ) Kσ,OHP + Kσ,d

(13)

The surface conductivity may be divided up into contributions from the stagnant layer Kσ,OHP and the diffuse layer Kσ,d. Because the charge compensation is almost completely due to potassium ions in the stagnant layer, the surface conductivity is dominated by a contribution of the OHP and can be calculated by eq 14. OHP uK+ Kσ ) σK+

(14)

The maximum ion mobility of the potassium ions in the double layer uK+ is approximated by the ion mobility in the bulk, which is 7.6 × 10-8 m2 V-1 s-1.68 From eq 11, we obtain the total interfacial conductivity Ktot. The results are shown for a mercaptohexadecanoic acid SAM in Table 3. Apparently, a considerable part of Ktot is caused by the bulk gold conductivity despite a relatively high surface (68) Handbook of Chemistry and Physics, 78th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1997.

Table 3. Calculated Surface Conductivities of a Mercaptohexadecanoic Acid SAM at pH 9.2 Ktot [nS]

Kσ,OHP [nS]

Kσ,d [nS]

Kgold [nS]

129

56

0.59

72.4

Figure 5. Contact angle titrations (receding contact angle, captive bubble technique) of MUA (solid circles) and HDT (open circles) SAMs in 0.3 mM potassium chloride.

conductivity in the stagnant layer. The opposite situation is given for monolayers without ionizable groups. Most of the surface charge is assumed to be compensated in the diffuse layer in this case. This stems from the fact that the ζ potential values of methyl-terminated SAMs are even higher than those for carboxylic acid functionalized SAMs although the surface charge density should be lower. On the basis of our data, we can determine only the excess of hydroxide ions adsorbed in the stagnant layer. As surface conductivity data are not available because of interference of the contribution of the gold support, we are not able to determine the total ion density in the inner layer. For a HDT SAM, the total interface conductivity Ktot was determined to 26 nS. Because the ζ potentials of HDT and MHA are comparable, we may estimate that the conductivity contribution of the gold is 4-5 times higher for the acid-functionalized monolayer. It becomes obvious that the contribution of the gold to the total channel conductivity is in the same order of magnitude as the effect of surface conductivity. Additionally, it is not clear in which way the former is dependent on the pH. As a consequence, this fact strongly interferes with any straightforward interpretation of double-layer effects based on surface conductivity data in the analyzed cases. (ii) Contact Angle Titrations. Contact angle titrations were performed in order to verify the dissociation constants of the monolayer functionalities. Figure 5 depicts contact angle titrations (captive bubble technique) of mercaptoundecanoic acid and hexadecanethiol SAMs. Receding contact angles decreased beginning at about pH 4 which indicates ionization. The contact angles drop rather linearly with increasing pH to level out in the alkaline range. This behavior represents a titration curve described by the nonreactive spreading model. The surface pK for the COOH SAMs calculated from the measurements were in the range from 5.20 to 5.35. That is, good agreement between ζ potential plots and the contact angle titrations was observed. Self-assembled monolayers of n-alkanethiols exhibit a wetting behavior independent of pH. In contrast to the ζ potential method, unsymmetrical adsorption of ions is apparently not reflected by the contact angle. Ion adsorption and ionic strength do not remarkably


affect the contact angle. This is in good agreement with Whitesides et al. who found that contact angle titration plots are independent of the buffers that are used to match the pH.21 Similar results were obtained for nonpolar polymer surfaces without ionizable groups.66 As a potential explanation of the distinct difference of the ζ potential and the contact angle results for the case of unsymmetrical ion adsorption, recent studies by Lyklema et al.67 have demonstrated high degrees of lateral mobility of adsorbed ions which might be seen as an indication of the potential disturbance of the interfacial charge carrier distribution by manipulations such as contact angle experiments. It was also shown by the performed experiments that the captive bubble method is very suitable for nonreactive spreading of ionizable surfaces. It is no longer necessary to operate with buffered systems and under inert liquids. The freshly prepared samples can directly be immersed in the desired electrolyte solution to establish equilibrium. Conclusions It was demonstrated for the first time that streaming potential and streaming current measurements provide a powerful method for the description of interfacial charge formation at self-assembled monolayers of thiols in aqueous electrolyte solutions. Both preferential adsorption of ions (hydrophobic methyl-terminated SAMs) and acidbase reactions of surface functional groups (carboxyterminated SAMs) can be monitored by streaming potential and streaming current measurements. Streaming potential measurements are affected in part by bulk conductivity of the gold substrate. The contribution of the substrate to the total channel conductivity depends on the type of SAM and on the solution pH. Surface pK values of carboxylic acid terminated SAMs determined from ζ potential measurements were found to be comparable to the interaction potential of AFM forcedistance measurements of SAM surfaces described in the literature. The isoelectric points indicated a significant deviation of the acid/base character toward the acidic behavior as compared to the species in solution. In contrast, only a slight shift of the pK of about 0.7 units of pH compared to the species in solution was found for carboxylic acid monolayers analyzing the ζ potential versus pH plots as a titration curve. Several models of the electrochemical double layer were discussed to fit the experimental results. Two boundary situations were found for carboxy- and methyl-terminated monolayers. For acidic SAMs, the main part of the surface charge is compensated within the stagnant layer. In contrast, methyl-terminated SAMs show preferential adsorption of ions and the countercharge is predominantly located in the diffuse layer.

Langmuir, Vol. 17, No. 14, 2001 4311

Whereas for carboxy-terminated SAMs charge formation reflects in contact angle data, preferential ion adsorption in the case of methyl-functionalized SAMs has no detectable influence on the macroscopic contact angle. Further verification of the ionization constants could be done by potentiometric titration of SAM-modified gold colloids. Additionally, to elaborate a more adequate model of the interface, double-layer capacitance could be the issue of further investigations. Acknowledgment. The authors are very indebted to Ralf Zimmermann and Thomas Kratzmu¨ller (both at the Institute of Polymer Research Dresden) for fruitful discussions. Appendix: List of Symbols A R b c 0 r F Γ ΓOHP e h η Is L Kσ Ktot κe Ka p θ R σ0 σOHP σd T u Us Ψsurf ζ(Is) ζ(Us)

sample area degree of dissociation channel width electrolyte concentration permittivity of free space dielectric number Faraday constant surface concentration excess concentration in the outer Helmholtz plane channel height viscosity streaming current channel length surface conductivity total interfacial conductivity electrolyte conductivity surface dissociation constant pressure difference over the slit channel contact angle gas constant surface charge density () σIHP) charge density in the outer Helmholtz plane charge density in the diffuse layer temperature ion mobility streaming potential surface potential ζ potential from streaming current ζ potential from streaming potential

LA001741G

Dissociation of Surface Functional Groups and Preferential Adsorption

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