Interfacial charge of organic thin films characterized by streaming potential and streaming current measurements

Colloids and Surfaces A: Physicochemical and Engineering Aspects 195 (2001) 97 – 102 www.elsevier.com/locate/colsurfa

Interfacial charge of organic thin films characterized by streaming potential and streaming current measurements Ruediger Schweiss a,b, Petra B. Welzel a, Carsten Werner a,c,*, Wolfgang Knoll b a

Department Biocompatible Materials, Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany b Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany c Department of Mechanical and Industrial Engineering, Uni6ersity of Toronto, 5 King’s College Road, Toronto, Ont., Canada M5S 3G8

Abstract Self-assembled monolayers of alkanethiol compounds chemisorbed on flat gold surfaces were characterized by streaming potential and streaming current measurements in aqueous electrolyte solutions using a novel microslit electrokinetic setup. The alkanethiols analyzed were terminated with different functional groups. Depending on the type of alkanethiol used different mechanisms were relevant for the generation of interfacial charge: dissociation for thiols bearing ionizable surface groups and preferential adsorption of ions for methyl-terminated thiols. In all investigated cases, the zeta potential calculated from the streaming potential was significantly lower than the zeta potential obtained from the streaming current. This was due to a contribution of the conductivity of the underlying gold substrate to the surface conductivity. Based on the data obtained for the zeta potential and the surface conductivity, the surface charge of acidically functionalized monolayers was concluded to be compensated almost completely in the stagnant layer whereas for methyl-terminated monolayers a considerable part of the countercharge is localized in the diffuse, hydrodynamically mobile part of the electric double layer. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Streaming current; Self-assembled monolayers; Interfacial charge density; Zeta potential; Double layer

1. Introduction Self-organizing monomolecular assemblies on metal and semiconductor surfaces have attracted growing interest in the last decade [1 – 4]. Amidst them, numerous studies deal with self-assembled monolayers (SAMs) of organic sulfur compounds * Corresponding author. Tel.: + 49-351-4658-285; fax: + 49-351-4658-214. E-mail address: [email protected] (C. Werner).

on gold surfaces, since these systems are most widely characterized and correspond to a nearly ideal two-dimensional structure with almost exclusively terminal functional groups being exposed to the adjacent phase. Long-chain alkanethiolate monolayers exhibit crystalline packing of the hydrocarbon chains and are suggested to be impermeable to ions [2,7]. Additionally, functionalization of these layers is rather easy and they are not prone to hydrolysis in aqueous solutions.

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2. Experimental

Fig. 1. Monolayer systems that were investigated in this study: (A) mercaptoundecane phosphate, (B) mercaptoundecanoic acid, (C) octadecanethiol, and (D) thioctic acid.

For many processes that take place at such monolayer interfaces in aqueous electrolyte solutions, for instance grafting of polymers from the surface, adsorption of biomolecules and polyelectrolytes, electrochemical charge transfer and specific binding to the surface, profound knowledge of the interfacial charge and its mechanisms of formation is desirable [4– 8]. Therefore, a comprehensive characterization of electrosurface phenomena at flat solid – liquid interfaces formed of layered alkanethiols in aqueous electrolyte solutions is of high interest. For that aim, streaming potential and streaming current measurements were applied in this study. As a further benefit of investigating well-defined thin organic films with electrokinetic methods, we expect new insights into electrokinetic phenomena of solids with high coverage of surface functional groups and into the method of streaming potential and streaming current measurement itself.

SAMs of alkanethiols as shown in Fig. 1 were prepared by overnight deposition from 0.5 mM solutions in absolute ethanol on cleaned glass slides (10× 20 mm2) covered with evaporated gold (150 nm, primed with a 2 nm chromium layer to promote adhesion). The carriers were rinsed thoroughly with ethanol and dried under vacuum. The samples were examined by wetting measurements and ellipsometry to control monolayer formation. Advancing contact angles were measured using a Kru¨ ss DSA system (Hamburg, Germany) and ellipsometry was performed with a DRE ellipsometer (Dr Riss Ellipsometerbau, Ratzeburg, Germany). Results of ellipsometric and wetting measurements are shown in Table 1. For the electrokinetic experiments, two equally prepared substrates were glued to glass blocks and mounted into the microslit cell to form the streaming channel. In order to obtain sample parallelity the glass blocks were aligned by means of a light microscope. Further details of the microslit elektrokinetic setup can be found in [9,10]. All electrolyte solutions (potassium chloride, potassium hydroxide and hydrochloric acid) were prepared using deionized water, which was degassed under vacuum prior to use. Nitrogen 5.0 served as process gas for all experiments. The width of the slit channel was thoroughly adjusted by liquid flow measurements and was set to 509 0.1 mm. The maximum pressure applied across the slit channel was 200 mbar for all measurements. The whole setup was assembled under a laminar

Table 1 Advancing contact angles (sessile drops), ellipsometric thicknesses (nSAM =1.49) and electrokinetic properties of monolayers investigated Monolayera

A B C D

qadv (°)

319 2 129 3 1129 1 289 3

ottom-border\−29 a The letters A–D refer to Fig. 1.

d (A, )

24 22 26 10

IEP

4.10 90.10 4.26 90.10 3.96 90.05 4.36 90.10

n plateau (mV) n(Is )

n(Us )

−120 −113 – −199

−33 −30 – −29

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flow box. All electrokinetic experiments were performed at ambient temperature.

3. Results and discussion Zeta potentials were calculated from the Smoluchowski equations:



n(Us )= n(Is )=

n

2K | h dUs , m0mr dp

p se +

(1)

p L dIs , m0mr A dp

(2)

where n(Us ) is the zeta potential derived from the streaming potential and n(Is ) is the zeta potential that is obtained from the streaming current, L, the channel length and A, the cross-section area of the streaming channel, h, the channel width and K |, the surface conductivity. The charge density of the diffuse layer can be calculated using the Gouy Chapman Stern Grahame (GSCG) model [11 – 16]:



| d = 8m0mr RTsinh −

n

Fn

celectrolyte. 2RT

(3)

Electroneutrality requires that the charge density of the diffuse part of the double layer | d is numerically equal to the charge density of the immobile layer | i. The inner layer charge density can be considered as the sum of the surface charge density | IHP and the charge density in the stagnant part of the double layer | OHP | i = | IHP + | OHP = −| d.

(4)

For the analyzed samples it is justified to apply Eqs. (3) and (4), respectively, since the diffuse layer can be assumed to be completely hydrodynamically mobile (i.e. no consideration of surface roughness). For all SAM samples, significant differences of the zeta potential calculated from the streaming potential (Eq. (1)) and the zeta potential derived from streaming current (Eq. (2)) were observed. This effect was most pronounced for monolayers carrying ionizable groups. The zeta potential obtained from streaming potential was significantly

Fig. 2. Zeta potential from streaming current n(Is ) of (A) mercaptoundecanephosphate, (B) mercaptoundecanoic acid, (C) octadecanethiol, and (D) thioctic acid monolayers on gold vs pH. Electrolyte 0.3 mM potassium chloride.

lower in all cases. A major contribution of additional conductivity of the streaming channel exceeding the conductivity of the electrolyte solution is evident from such a behavior. The additional conductivity can be calculated equating Eqs. (1) and (2). The notation of Eq. (1) assumes that any additional conductivity can be attributed to the solid–liquid interface. For a mercaptoundecanoic acid SAM an additional conductivity K | of about 160 nS was obtained. Despite of a high density of charge carriers in the double layer, this high value cannot be explained as a surface conductivity. Therefore, we have to conclude that there exists a contribution of the conducting gold surface to the total additional conductivity of the sample. The zeta potential obtained from streaming current is not affected by these phenomena and is therefore used in the following for the calculation of charge densities. Fig. 2 represents zeta potential data (streaming current, calculated from Eq. (2)) of SAMs with different functional groups. The isoelectric points (IEPs), plateau values of zeta potential and monolayer data are summarized in Table 1.

3.1. Ionogenic surfaces Surprisingly, for the carboxylic acid and phosphate terminated SAMs, IEPs were found above

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pH 4.0 which is rather high for surfaces bearing acidic groups. Grafted polymers with high densities of similar functionalities commonly exhibit IEPs markedly below pH 4.0 [14,16]. Nevertheless, all these monolayers show a pronounced plateau of the zeta potential in the alkaline range characteristic of acidic surfaces (Fig. 2(A), (B) and (D)). One may conclude that there is a deviation of the acid– base character of spatially confined acidic functions compared to the corresponding species dissolved in aqueous solutions. Surface confined acidic groups appear less acidic. This may be attributed to electrostatic interactions of the surface groups of the monolayer and a reduced permittivity at the interface [17,18]. Similar conclusions were drawn in earlier studies of the corresponding SAMs by different methods such as direct force measurements or contact angle titrations [4,18]. Since the surface concentration of ionogenic groups is equal to the surface coverage of the thiol molecules which is 7.7× 10 − 10 mol cm − 2 [19], this enables to calculate the surface charge density assuming full ionization at high pH. For a monolayer of a monobasic acid one gets IHP

| high pH = − FGthiol

(5)

This surface charge would be equal to about 74 mC cm − 2. From the zeta potential, we derive the charge density via Eq. (3). At high pH, the charge density of the diffuse layer determined from the zeta potential is about 0.95 mC cm − 2. Assuming full dissociation of the monolayer, this very low diffuse layer charge density indicates that the main potential drop at the interface has to occur in the stagnant part of the double layer ( cIHP − cOHP  COHP , Fig. 4(A)). However, the assumption of a fully dissociating monolayer should be critically reconsidered. In the case of thioctic acid the surface charge density due to deprotonation of the carboxy group is assumed to be significantly lower, since two thiol groups anchor at the surface and because of this a dense packing as known from n-alkanethiols is not given in this case. This also implies that such monolayers might allow for ion penetration. Nevertheless, the thioctic acid SAM was found to give a similar IEP as compared to

the mercaptoundecanoic acid SAM. The absolute plateau value of the zeta potential was found to increase for these monolayers. Since | IHP should be even lower in this case, this may indicate that a larger part of the countercharge is located in the diffuse layer compared to a mercaptoundecanoic acid SAM. This agrees well with predictions of the Gouy–Chapman theory that a high surface charge density is accompanied by a high concentration of counterions in the stagnant layer. On the other hand, a low surface charge density will mainly be compensated in the diffuse layer. Therefore, a thioctic SAM surface may be described by a charge distribution which is in between the distinct states schematically shown in Fig. 4. However, the similar IEP does not support this interpretation at first glance. It might be concluded that for monolayer (D) unsymmetrical ion adsorption occurs as a second charge formation process. It might be that parts of the hydrocarbon backbone are exposed to the outermost surface, which is reflected in the higher contact angle of the thioctic SAMs compared to the mercaptoundecanoic acid monolayers.

3.2. Unpolar surfaces Octadecanethiol SAMs (C) form a hydrophobic surface without dissociable sites. This surface is charged in aqueous solutions by preferential adsorption of ions from the solution. Fig. 3b shows the inner layer charge densities of an octadecanethiol SAM vs different electrolyte compositions calculated from the zeta potential using Eqs. (2)–(4). It is demonstrated that potassium and chloride ions do not significantly charge the surface whereas the samples in potassium hydroxide solutions exhibit zeta potential vs electrolyte concentration plots that are characteristic of preferential hydroxide adsorption. In turn, hydrochloric acid creates positive values of the zeta potential above an electrolyte concentration of 10 − 4 M. Altogether, the data indicate the preferential adsorption of hydroxide ions as compared to the hydronium ion. This hypothesis is also confirmed by the fact that the zeta potential of this surface in deionized water has a negative value due to the preferential adsorption of hy-

R. Schweiss et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 195 (2001) 97–102

Fig. 3. Inner layer charge densities calculated from n(Is ) of (a) and (c) octadecanethiol-SAM on gold vs pH, and (b) different electrolyte compositions.

droxide ions which are created by the self-dissociation of water (2H2O lH3O+ +OH−, Kw = 10 − 14 mol2 l − 2). Further corroboration is obtained from zeta potential vs pH plots at different concentrations of the background electrolyte potassium chloride. These plots and the corresponding charge densities do not show any extremum value (Fig. 3(a)) and the IEP is independent on the potassium chloride concentration, which means that there is no contribution of background electrolyte adsorption. This behavior has already been observed for hydrophobic polymer surfaces [20– 23]. The surface concentration of preferential adsorbed ions on nonpolar surfaces is assumed to be much lower than the surface concentration of

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acidic groups in an acidically functionalized SAM. This was recently shown in detail by advanced electrokinetic measurements at nonpolar fluoropolymer films based on the estimation of | i by zeta potential and surface conductivity data [22]. However, considering lower inner layer charge densities, the zeta potential values of this methyl-terminated SAM are even higher in magnitude than those for a COOH-terminator monolayer. Therefore, we conclude that in this case a major part of the countercharge is to be located in the diffuse, hydrodynamically mobile part of the double layer ( cIHP − cOHP B cOHP , Fig. 4).

4. Conclusions Streaming potential and streaming current measurements of flat solid surfaces have proven to be suitable for the characterization of self-organizing organic adsorbates. Monomolecular films with acidic functions show a characteristic plateau of the zeta potential in the alkaline pH range. However, the high density of surface ionizable groups seems to influence their acid–base character. This is reflected in a rather high IEP compared to acid-functionalized polymer surfaces bearing similar functions. Hydrophobic monolayers behave just as nonpolar polymer films and show zeta potential characteristics that indicate preferential adsorption of hydroxide ions from the solution. Therefore, these surfaces are negatively charged above pH 4.0.

Fig. 4. Models for the structure of the electric double layer at monolayer surfaces. (a) COOH-terminated n-alkanethiol monolayer, and (b) CH3-terminated monolayer. IHP and OHP denote the inner and the outer Helmholtz planes.

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Major differences between the zeta potential obtained from streaming potential and zeta potential derived from streaming current measurements were observed. Beyond the generally known effect of surface conductivity, the conducting gold underlayer affects the zeta potential obtained from streaming potential measurements as well.

Acknowledgements The authors are grateful to Ralf Zimmermann, Thomas Kratzmu¨ ller and Gretl Dworschak who were involved in some steps of this work.

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