Cationic and Nonionic Surfactant Adsorption on Thiol Surfaces with

6802

Langmuir 2002, 18, 6802-6806

Cationic and Nonionic Surfactant Adsorption on Thiol Surfaces with Controlled Wettability K. Boschkova*,†,‡ and J. J. R. Stålgren† Surface Chemistry, Department of Chemistry, Royal Institute of Technology, Drottning Kristinas va¨ g 51, SE-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden Received December 7, 2001. In Final Form: May 6, 2002 We have shown that thiolated surfaces work very well as model substrates in adsorption measurements using the quartz crystal microbalance-dissipation. Functionalized self-assembled monolayers were prepared from mixtures of hydrophobically, SH-C16, and hydrophilically, SH-C16OH, terminated thiols, which allowed the interfacial energy of the surfaces to be changed in a systematic way. The prepared thiol surfaces were used as substrates for adsorption of a cationic (DTAB, dodecyltrimethylammonium bromide) and a nonionic (C12EO8, octa(ethylene oxide) mono(n-dodecyl ether)) surfactant. When the fraction of methyl groups at the surfaces was increased, the adsorption for both DTAB and C12EO8 was increased. In particular, there is a transition from a micellar surfactant layer to a surfactant monolayer at 25-50% surface coverage of SH-C16 groups. In addition, the role of the counterion in the adsorbed surfactant layer for the charged surfactant is discussed in terms of contribution to the mass and viscoelastic response determined by the quartz crystal microbalance.

Introduction Adsorption of cationic surfactants at solid surfaces has been extensively studied through the years. Most studies are limited to a number of solid surfaces, such as silica, graphite, mica, and quartz,1,2 to mention only a few. In recent years, self-assembled monolayers (SAMs) at solid surfaces have become common for fundamental studies of interfacial and surface phenomena. From an application point of view, thiol monolayers are used as model surfaces for anticorrosion agents, alignment of liquid crystals, photoresists, biosensors, molecular recognition, in chemical force microscopy, and as model surfaces for surface phenomena as reviewed by others.3-4 It is well established that alkanethiols form well-ordered,5 stable, and reproducible monolayers at gold surfaces. Surface properties can easily be changed by functionalization of the thiols; examples of different functionalities at the terminal position are CH3, OH, and COOH, as well as thiols attached to cholesterol. In this study the use of a combination of hydrophobic and hydrophilic thiols enabled a systematic change of the surface hydrophobicity of the substrate. Only a limited number of systematic studies of surfactant adsorption at thiol surfaces have been reported. Surfactant assemblies and interaction forces due to adsorption of a nonionic surfactant (C12EO8, octa(ethylene oxide mono(n-dodecyl ether)) at thiol-modified surfaces exposing mixtures of hydrophobic and hydrophilic groups have been studied using the atomic force microscope.6 From interaction force measurements and atomic force microscopy (AFM) images, the structures of the layers * To whom correspondence should be addressed. E-mail: [email protected]. † Royal Institute of Technology. ‡ Institute for Surface Chemistry. (1) Chorro, M.; Chorro, C.; Dolladille, O.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 1999, 210, 134. (2) Wangnerud, P.; Berling, D.; Olofsson, G. J. Colloid Interface Sci. 1995, 169, 365. (3) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (4) Ulman, A. Chem. Rev. 1996, 96, 1533. (5) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (6) Grant, L. M.; Ederth, T.; Tiberg, F. Langmuir 2000, 16, 2285.

were deduced. It was found that micellar adsorption takes place on the hydrophilic surfaces and monolayer adsorption on the more hydrophobic ones. One of the purposes of this study was to determine whether adsorption measurements using the QCM-D (quartz crystal microbalance-dissipation) could reveal complementary information. Of particular interest is the viscoelastic behavior (as obtained from the dissipation parameter from the QCM-D) of the surfactant films. In addition, we also address the role of the counterion for ionic surfactant adsorption as quantified by the use of the QCM-D. This is done indirectly by interpreting dissipative changes upon changing the surface hydrophobicity as a result of associated counterions within the adsorbed layer structure. Depending on the hydrophobicity of the surface, the Br- ion will be more or less incorporated in the adsorbed layer structure, thereby changing the rigidity of the layer as interpreted from changes of the dissipation values. Materials Materials. C12EO8 was purchased from Nikko Chemicals, Japan, and used without further purification. Dodecyltrimethylammonium bromide (DTAB) (>99%) was obtained from SigmaAldrich Sweden AB and was used as received. All solutions were prepared from 10(cmc) (critical micellar concentration) batch solutions and equilibrated for at least 48 h after dilution to 1.2(cmc), where cmc(DTAB) ) 15 mM7 and cmc(C12EO8) ) 0.092 mM.8 The water used in the experiments was treated by a Milli-RO 10 Plus pretreatment unit, including depth filtration, carbon adsorption, and decalcination preceding reverse osmosis. This treatment was followed by a Milli-Q plus185 unit, which treats the water with UV light (185 nm + 254 nm) before being further purified by a Q-PAK unit consisting of an activated carbon unit followed by a mixed bed ion exchanger and finally an Organex cartridge. The outgoing water is filtered through a 0.2 µm filter. Preparation of SAMs. Thiols were adsorbed from 1 mM solutions in ethanol, where the adsorption time was at least 24 (7) Evans, D. F.; Allen, M.; Ninham, B. W.; Fouda, A. J. Solution Chem. 1984, 88, 5084. (8) Brinck, J.; Jo¨nsson, B. Langmuir 1998, 14, 1058.

10.1021/la011774b CCC: $22.00 © 2002 American Chemical Society Published on Web 08/02/2002

Surfactant Adsorption on Thiol Surfaces

Langmuir, Vol. 18, No. 18, 2002 6803

h. Thiohexadecane, HS(CH2)15CH3, “SH-C16OH” (Fluka, >95%), and thiohexadecanol, HS(CH2)16OH, “SH-C16OH”, were used as received (>99.5%, courtesy of Prof. Liedberg, Linko¨ping University). The mixed solutions were prepared in molar fractions. It has been shown that mixtures of molecules with equal chain lengths have almost the same SAM composition as the solution.9 Before the measurements the thiol surfaces were soaked in ethanol solution for at least 24 h to remove physisorbed thiols. Just before the measurements the surfaces were further cleaned in a 2% HellmanEx solution. Measurements of the contact angles of water were performed using a Fibro DAT 1100 system (Fibro Systems AB, Sweden, for example, described by Gerdes et al.10) at the different SH-C16 and SH-C16OH surfaces, and the results were in good agreement with previous observations from Bahr et al.11

Methods Quartz Crystal Microbalance. The quartz crystal microbalance used is a QCM-D from Q-Sense, Gothenburg, Sweden. The technique is described in detail elsewhere.12 The technique allows for simultaneous measurement of changes in resonance frequency and energy dissipation. The change in resonance frequency is a measure of the adsorbed amount, which can be calculated using the Sauerbrey relation (eq 1,

∆m ) -C(∆f/n)

(1)

where ∆m is the adsorbed mass, C is a constant characteristic of the crystal, in our case, 0.177 mg/(m2 Hz), ∆f is the change in frequency, and n is the shear wavenumber).13 This relation rests on the assumption that the deposited mass forms a thin rigid film and that the mass sensitivity is uniform over the entire surface. Equation 1 has been supported by experimental data up to mass loadings (massadsorbed/masscrystal) of approximately 2%.14 There are various models for converting the frequency shift to mass loadings, and these models give the same result up to approximately 5%.15 The change in dissipation is a measure of the viscoelastic properties of the adsorbed layer. The energy dissipation is measured on the basis of the principle that when the driving power to a piezoelectric oscillator is switched off, the voltage over the crystal decays exponentially and a damped oscillating signal is recorded. Hence, before disconnection of the driving oscillator, we obtain f, and D is obtained after the disconnection. The dissipation factor is defined by

D ) Edissipated/2πEstored

Figure 1. Mean values of frequency shifts from the first overtone, f15 MHz, as a function of surface hydrophobicity for 1.2(cmc) DTAB. The second axis displays the frequency shifts, converted to adsorbed amount (mg/m2). The error bars indicate the spread in the results.

(2)

where Edissipated is the energy dissipated during one oscillation and Estored is the energy stored in the oscillating system. The surfaces used for these experiments were hydrophilic gold surfaces with a 3D surface roughness of 2 ( 2 nm, which was determined using a profilometer, Zygo View 5000. The fluid cell was temperature controlled to 25 ( 0.02 °C. The QCM cell, crystal, and tubing were cleaned using a mild alkali cleaning solution of 2% HellmanEx II (Hellma GmbH & Co.) solution for 1 h. The system was then rinsed with water, and the crystal was also dismounted and cleaned with ethanol. After the cell was reassembled, the tubing and the crystal were then rinsed with excess water while the frequency and dissipation values were monitored. Note that the cell volume is approximately 1.0 mL, so allowing 2 mL of solution through the cell is sufficient to ensure a complete exchange of the solution. The injection time was kept constant at approximately 90 s, and at each injection 2 mL of surfactant solution was supplied to the cell. In total two injections, with 30 min of equilibrium time after each injection, were made (9) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (10) Gerdes, S.; Cazabat, A.-M.; Stro¨m, G. Langmuir 1997, 13, 7258. (11) Bahr, M.; Tiberg, F.; Zhmud, B. V. Langmuir 1999, 15, 7069. (12) Rodahl, M.; Ho¨o¨k, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. Rev. Sci. Instrum. 1995, 66, 3924. (13) Sauerbrey, G. Z. Phys. 1959, 155, 206. (14) Pulker, H. K. Z. Angew. Phys. 1966, 20, 537. (15) Mecea, V.; Bucur, R. V. Thin Solid Films 1979, 60, 73.

Figure 2. Frequency response after rinsing with water normalized against frequency response upon adsorption for 1.2(cmc) DTAB. to saturate the surface and to minimize any depletion effects. At least five independent measurements were performed for each surface composition and surfactant, and the results are shown as mean values from all measurements.

Results Adsorption of 1.2(cmc) DTAB. The frequency shift from the first overtone upon adsorption of DTAB at different thiol-modified surfaces, exposing methyl (-CH3) and hydroxyl (-OH) groups, is displayed in Figure 1. The most hydrophilic surfaces, 0% SH-C16 and 25% SH-C16, display frequency shifts of 20 ( 3 Hz, whereas the hydrophobic surfaces, 75% SH-C16 and 100% SH-C16, display frequency shifts of 35 ( 2 Hz. For 50% SH-C16 there is a transition point from a lower to a higher frequency shift, displaying a shift of 37 ( 4 Hz. After two injections with surfactant solutions a third injection was made with water to desorb the surfactant from the surface. In Figure 2 the frequency shift after rinsing with water is normalized to the frequency shift observed after adsorption and plotted versus surface hydrophobicity. It is observed that 20 ( 10% of the adsorbed surfactant is still present at the surface after rinsing. Information about the rigidity of the surfactant layer is obtained from the dissipation factor, where a high dissipation factor indicates a nonrigid adsorbate structure. Adsorption of a monolayer at the surface will in general

6804

Langmuir, Vol. 18, No. 18, 2002

Boschkova and Stålgren

Table 1. Area per Surfactant (Å2) for 1.2(cmc) DTAB and 1.2(cmc) C12EO8 on 0%, 25%, 50%, 75%, and 100% SH-C16 Surfaces, Assuming Monolayer Adsorptiona surface hydrophobicity, SH-C16 (%) 0 25 50 a

surface hydrophobicity, SH-C16 (%)

area per surfactant C12EO8 DTA+ 28 ( 1 36 ( 2 17 ( 2

56 ( 5 39 ( 1 34 ( 1

75 100

area per surfactant C12EO8 DTA+ 18 ( 1 20 ( 1

35 ( 2 36 ( 1

The counterion is not included in the area per surfactant calculation for DTAB.

Figure 3. Dissipation displayed as a function of surface hydrophobicity for 1.2(cmc) DTAB.

give rise to a small change in the dissipation factor, typically within (0-1) × 10-6,16 in contrast to vesicle adsorption, which typically gives a higher dissipation factor of approximately 3 × 10-6.16,17 In Figure 3 the dissipation change occurring upon DTAB adsorption is plotted versus surface hydrophobicity. The DTAB adsorption to the most hydrophilic surface, 0% SH-C16, gives rise to a dissipation change of (1.2 ( 0.7) × 10-6, whereas adsorption to the most hydrophobic one, 100% SH-C16, results in a significantly larger change in dissipation, (2.9 ( 0.2) × 10-6. From the frequency shift the adsorbed amount (mg/m2) is calculated using eq 1 and displayed in Figure 1 vs surface hydrophobicity. The more hydrophilic surfaces, 0% SHC16 and 25% SH-C16, display adsorbed amounts of 1.4 ( 0.1 and 1.0 ( 0.1 mg/m2. At equal surface compositions of hydroxyl and methyl groups there is a transition regime, giving rise to an adsorbed amount of 2.2 ( 0.2 mg/m2. The most hydrophobic surfaces, 75% and 100% SH-C16, display adsorbed amounts of 2.1 ( 0.1 and 1.9 ( 0.1 mg/ m2. The packing density of surfactants at the surface, expressed as area per surfactant, is shown in Table 1, where the area per surfactant, A (Å2), assuming a monolayer adsorption, is calculated using eq 3,

f15 MHz 1 )CNA A 3M

(3)

where f15 MHz is the frequency from the first overtone (Hz), M is the molar weight of the surfactant, DTA+ (228 g/mol) (without counterion), C is a constant, 0.177 mg/(m2 Hz), and NA is Avogadro’s constant, 6.022 × 1023 molecules/ mol. The apparent area per molecule for DTA+ on 0% SHC16 is 28 ( 1 Å2. Upon further increasing the hydropho(16) Keller, C. A.; Kasemo, B. Biophys. J. 1998, 75, 1397. (17) Johnsson, M.; Bergstrand, N.; Edwards, K.; Stålgren, J. J. R. Langmuir 2001, 17, 3902.

Figure 4. Mean values of frequency shifts from the first overtone, f15 MHz, as a function of surface hydrophobicity for 1.2(cmc) C12EO8. The second axis displays the frequency shifts, converted to adsorbed amount (mg/m2). The error bars indicate the spread in the results.

bicity to 25% SH-C16, the coverage decreases somewhat to 36 ( 2 Å2, possibly as a result of a more distorted micellar structure due to the underlying substrate. The DTA+ adsorption at the most hydrophobic surfaces displays an apparent area per surfactant of close to 20 Å2, which is close to the area per molecule of the underlying thiol surface (21.4 Å2). This may at first imply an almost perfectly packed monolayer, considering that the error in the area per molecule determination is approximately 2 Å2 (an estimation of ∆F/F for these experiments gives a maximum of ∆F/F ≈ 0.1, which implies an error of 2 Å2 for 20 Å2). However, as will be discussed below, a further analysis indicates that the area per molecule in fact is larger than indicated above and the adsorbed layer is less compact. Adsorption of 1.2(cmc) C12EO8. The frequency shift from the first overtone, f15 MHz, occurring as a result of C12EO8 adsorption is plotted in Figure 4 vs surface hydrophobicity. Just as for DTAB the surface hydrophobicity was varied by using thiol-modified surfaces exposing methyl and hydroxyl groups. It is observed that adsorption to the most hydrophilic samples, 0% SH-C16 and 25% SH-C16, results in lower frequency shifts of 22 ( 2 and 32 ( 1 Hz, respectively, whereas 50% SH-C16, 75% SHC16, and 100% SH-C16 all result in about 36 ( 2 Hz. It is concluded that adsorption to the more hydrophobic surfaces results in the largest frequency shifts. The frequency shift after rinsing with water normalized to the frequency before rinsing as a function of surface hydrophobicity is shown in Figure 5. After rinsing, 33 ( 12% of the adsorbed surfactant is still present at the surface. In Figure 6 the dissipation change upon adsorption is plotted versus surface hydrophobicity. There is no correlation between dissipation change and surface hydrophobicity for the C12EO8 adsorption at the different

Surfactant Adsorption on Thiol Surfaces

Figure 5. Frequency response after rinsing with water normalized against frequency response upon adsorption for 1.2(cmc) C12EO8.

Figure 6. Dissipation displayed as a function of surface hydrophobicity for 1.2(cmc) C12EO8.

surfaces. Since the dissipation is low, it is concluded that no large structures are formed at the surface. From the frequency shift the adsorbed amount (mg/m2) is calculated using eq 1 and displayed in Figure 4 vs surface hydrophobicity. The adsorbed amount on the hydrophilic surface, 0% SH-C16, is 1.3 ( 0.1 mg/m2, whereas it increases to 1.9 ( 0.1 mg/m2 on the 25% SH-C16 surface. At equal surface compositions of hydrophilic and hydrophobic groups, 50% SH-C16, and for the more hydrophobic surfaces the adsorbed amount equals 2.1 ( 0.2 mg/m2. There is a gradual change in the adsorbed amount, indicating no sharp transition from a micellar to a monolayer state of the adsorbed layer. The packing density expressed as area per molecule is calculated using eq 3 and given in Table 1. The area per molecule for C12EO8 adsorption is about 56 ( 5 Å2 at the 0% SH-C16 surface. At the 25% SH-C16 surface the packing of the nonionic surfactant increases to 39 ( 1 Å2. For 50% SH-C16, 75% SH-C16, and 100% SH-C16 the coverage further increases to around 35 ( 2 Å2. Discussion Frequency Shift. The frequency shifts for the two different surfactants, C12EO8 and DTAB, compared at the same surface hydrophobicity are rather similar, especially for the SH-C16OH surface and the two most hydrophobic surfaces (75% SH-C16 and 100% SH-C16). In the 25% SH-C16 and 50% SH-C16 region there is a discrepancy, where the nonionic surfactant displays a continuous frequency change whereas DTAB shows an abrupt dis-

Langmuir, Vol. 18, No. 18, 2002 6805

continuous frequency change. This might be explained by a simple geometrical argument considering the balance between the hydrophilic headgroup and hydrophobic tail. The cationic surfactant, DTAB, has a small headgroup giving rise to a sharp transition from the micellar to the monolayer state, whereas the more bulky and flexible headgroup, C12EO8, results in a wider transition regime. In this study we find no evidence for a formation of a complete surfactant bilayer for any of the surface compositions. This statement is based upon comparison of the frequency response for the different substrates with the most hydrophobic surfaces, where it is established that a monolayer does form. This is in contrast to previous observations for C12EO8 adsorption on a 50% SH-C16 surface where a bilayer is observed from AFM measurements.6 Effect of Rinsing. Comparing the bound strength of the surfactant to the underlying surface (i.e., the frequency after/frequency before rinsing with water), the nonionic surfactant is less prone to desorb under rinsing with water than the cationic surfactant. It is also observed that more surfactant is left after rinsing at the surface compared to corresponding ellipsometry measurements on the nonionic surfactant, C12EO8.18 This, we believe, is due to different rinsing conditions, where a typical rinse in the QCM measurements consists of two injections of 2 mL of water and the rinsing step in the ellipsometry measurements is a continuous flow of 10 mL/min with stirring at 300 rpm for 5-10 min. In the QCM there is no stirring, which means that the transport from the surface is slow. Dissipation. From the dissipation factor it is observed that DTAB adsorbs in a less rigid structure (higher dissipation) than C12EO8, and this result is not easily understood since C12EO8 packs less densly than DTAB. Possibly, this can be explained by the role of the counterion, in this case the Br- ion. In particular, the question is whether the Br- ion is strongly associated with the adsorbed surfactant layer. In a surfactant layer, some of the counterions will be located in the electrical double layer whereas others are located close to the surfactant headgroup. If these latter ions are strongly associated with the adsorbed surfactant, i.e., they follow the surfactant layer during the oscillation of the surface, they will contribute to the adsorbed mass (see the discussion below), whereas others that are less strongly bound to the surfactant layer will contribute to an increase in dissipation. In other words, the coupling between the charged surfactant layer and the oppositely charged diffuse double layer increases the energy dissipation rate. Furthermore, there are no reasons why a monolayer should give rise to a more viscous layer than a micellar structure (as seen from the higher dissipation values); this also indicates the incorporation of more loosely bound ions in the adsorbed layer. Adsorbed Amount. Previous adsorption measurements using ellipsometry with C12EO8 on SiO219 show an adsorbed amount of 1.8 µmol/m2. The latter value is to be compared to the one obtained at the hydrophilic surface, 0% SH-C16, in this study, about 3 µmol/m2. The QCM-D technique measures the mass oscillating with the crystal, and thus any bound or trapped water will contribute to the adsorbed amounts determined by the QCM-D. On the other hand, the ellipsometer is only sensitive to the surfactant. Hence, in general one expects a slightly larger (18) Tiberg, F.; Brinck, J. In ACS Symposium Series; Sharma, R., Ed.; American Chemical Society: Washington, DC, 1995; Vol. 615, p 231. (19) Brinck, J.; Tiberg, F. Langmuir 1996, 12, 5042.

6806

Langmuir, Vol. 18, No. 18, 2002

adsorbed amount from the QCM-D than from the ellipsometer. This will be further investigated in a future paper. Area per Molecule Determination. In determining the area per molecule for DTAB, the counterion, Br-, was assumed to not contribute to the mass of the adsorbed layer. However, if we assume that Br- is incorporated into the surfactant layer and contributing to the frequency response, there will be a difference in the area per molecule of about 26% (1-228.4/308.3). Assuming that the counterions contribute to the effective mass of the surfactant will thus give rise to a less densely packed monolayer, i.e., 25 Å2 for the 100% SH-C16 surface. This estimate considers a perfect degree of counterion association, β, to the monolayer structure. Self-diffusion measurements for Br- in decanol and CTAB20 (in a micellar solution) give βBr ) 0.73. From surface force measurements it has been observed that between 80% and 90% of the bromide and chloride counterions appear to be bound to mica surfaces.21 This makes us estimate the actual contribution of the Brto the area per molecule calculation to a maximum of 90%. Any trapped water will decrease the area per molecule further. Since the lifetime of a counterion at the micellar surface is small, likely smaller than the residence time for a surfactant molecule in a micelle (about 10-6 s),22 there are reasons to believe that the Br- ion will not be measured during the QCM measurements. Converting the first overtone, 15 MHz to period, gives 7 × 10-8 s, which is on the same order as the residence time for a counterion at the micellar surface. Fabre et al. suggest that the residence time of a counterion in hexadecyltrimethylammonium chloride and bromide micellar solutions is approximately 10-8 s. In addition, the electrostatic potential outside a flat surface is larger than outside micelles, which should support an even longer residence time. Furthermore, it is more difficult for ions to diffuse from a flat surface than from a spherical micelle, for geometrical reasons. All (20) Fabre, H.; Kamenka, N.; Khan, A.; Lindblom, G.; Lindman, B.; Tiddy, G. J. T. 1980, 84, 3428. (21) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Brady, J.; Evans, D. F. J. Phys. Chem. 1986, 90, 1637. (22) Wennerstro¨m, H.; Lindman, B. Phys. Rep. 1979, 52, 1.

Boschkova and Stålgren

together, this makes the assumption regarding the Brion contributing to the adsorbed mass sensed by the crystal reasonable. Conclusions We have shown that thiol surfaces work very well as model substrates for investigating surfactant adsorption using the QCM. After having established a cleaning procedure for removing the excess physisorbed thiols at the gold surfaces, the thiol surfaces are very stable in water. This is seen from the low drift of the frequency signal in pure water, which also makes up the baseline of the experiments. Functionalized thiol surfaces seem to be very promising and open up new possibilities for adsorption studies using the QCM-D. When the fraction of methyl groups at the surfaces was increased, the adsorption for both DTAB and C12EO8 was increased, which is in agreement with previous observations by Grant et al.6 on the C12EO8 system. In particular, there is a transition from a micellar surfactant layer to a surfactant monolayer at 25-50% surface coverage of SH-C16 groups. In the 25% SH-C16 and 50% SH-C16 the nonionic surfactant displays a continuous frequency change, whereas DTAB shows an abrupt discontinuous frequency change. Our hypothesis is that the counterion is associated within the adsorbed surfactant layer, contributing to the adsorbed mass as sensed by the QCM; however, the degree of association to the different surfaces remains unclear. To conclude, we believe that results from charged surfactant systems should be interpreted carefully. Acknowledgment. We thank Prof. P. Claesson for valuable discussions. B. Liedberg is acknowledged for providing us with the SH-C16OH thiol. K.B. gratefully acknowledges financial support from the Swedish Research Council for Engineering Sciences. J.J.R.S. acknowledges financial support from the SSF, Colloid and Interface Technology Program. LA011774B