Supporting Information Specific ion effects on the interaction of hydrophobic and hydrophilic self assembled monolayers T. Rios-Carvajal*, N. R. Pedersen, N. Bovet, S.L.S. Stipp, T. Hassenkam. Nano-Science Center, Department of Chemistry, University of Copenhagen, Denmark * corresponding author: tatiana@nano.ku.dk The supporting information contains 8 pages and 1 figure. 1
Figure S1. Typical data from CFM experiments. a) A single force-distance curve, showing tip approach (red) and withdrawal (blue). The adhesion force (arrow) is extracted from each force curve to produce a pixel on the force map (b). The color code in the map begins at 0 nn with pale blue, through medium adhesion with darker blue, grey, black, red and finally ends with pink, representing high adhesion at 10 nn. c) The average adhesion force calculated over the entire force map is represented as a bar in the adhesion force graph and the range in force over all of the maps is represented by the standard error bars. 2
Theoretical approximation for adhesion forces The adhesion force, F AD, between the tip and substrate, can be described as a combination of three contributing forces: van der Waals, F vdw ; electric double layer, F EDL 1 and hydrophobic, F HI (Equation 3). 1 For the hydrophilic system, -CH 3 functionalised surfaces are assumed to be neutral and thus, F EDL ~0. Following this, adhesion force for the hydrophobic system is the result of a contribution of the F vdw and F HI. Stock et al. 2 estimated the theoretical F AD in the AFM interaction between two hydrophobic surfaces (in this case, a sphere on a plane) by combining the van der Waals interaction with the Hydra model proposed by Donaldson, 3 F =R () 4πγ H e, (S1) where, R eff represents the effective radius of the tip. The first expression in round brackets represents the van der Waals interaction, estimated from the Hamaker constant, A, as a function of the distance of the interaction, D, by, 4,5 A(D)= 2 +. (S2) ( ) ( ) A H-H represents the Hamaker constant for an interface between two hydrocarbon surfaces (4.5x10-21 J) and A Au-Au denotes the interaction of two gold surfaces across an aqueous medium (400x10-21 J). 6 T ML represents the thickness of the adsorbed organic monolayer. For our approximation, we used the value measured for 1-undecanethiol by ellipsometry, T ML = 11 Å, 7 which is expected to be close to the thickness of a monolayer of the alcanethiol used in our experiments because they have the same carbon chain length. The second expression in round brackets in Equation S1 describes the hydrophobic parameter and γ i represents the interfacial tension. We set it to the expected value for a -CH 3 SAM-water 3
surface, i.e. 45 mj/m 2. 2 The Hydra parameter, H y, refers to the hydrophobicity of the surfaces in contact. It quantifies the fraction of relative hydrophobic surface area, where H y =1 represents a fully hydrophobic, hydrocarbon surface. The exponential decay length, λ Hy, is an experimentally derived variable and for the conditions of hydrophobic interaction, its value is ~0.4 nm. 2 Following this approximation, we assumed the minimum distance of interaction in the system to be D=0.2 nm, as a result of steric repulsion, and we used the standard radius of curvature of the tip as an effective radius of interaction, R eff =30 nm. The theoretical adhesion force estimated under these conditions, using Equation S1, gives an adhesion force of ~10 nn, almost 20 times higher than the forces we measured. If an ion is between the surfaces, as is expected in an ion bridge, the distance (D) would be at least the diameter of the hydrated ion (~0.8 nm). For D=1.4 nm, Equation S1 gave F AD =650 pn, a value closer to the forces measured by CFM. The fact that the interaction distance is larger than the hydrated diameter of the interacting cations suggests that water and some partly hydrated cations and anions could affect the interaction between the hydrophobic tip and the surface. In the hydrophilic system, because of the presence of two charged surfaces, EDL force is expected to contribute to the adhesion force. A simple approximation could be made assuming Coulombic repulsion, using Equation 4. To keep the model as simple as possible, we assumed the dielectric constant of the medium to be the same as the bulk. Using the same distance of interaction that we estimated for the hydrophobic system, i.e. D=1.4 nm, we estimated the repulsive force resulting from the negative SAM layers using Equation 4 and assuming interaction between two carboxylates, to be 1.5 pn. For the estimated number of carboxylate groups interacting in the hydrophilic system (12-27 groups), the repulsive force would range 4
between 18 and 40.5 pn, which is very low compared with the observed force measured with CFM, i.e. ~1 nn in the hydrophilic system. al. 8 : F EDL for the hydrophilic system was estimated using the approximation presented by Butt et F = (σ σ )e +2σ σ e, (S3) where R eff = 30 nm; σ T and σ S represent the surface charge densities of the tip and substrate and λ D represents the Debye length. For the solutions used in this work, the estimated Debye length from the approximation developed by Nylander and coworkers 9, λ = 1+ ( ) ( ), (S4) which takes into account the presence of an asymmetric polyelectrolyte on the interfacial interaction of charged surfaces. For Equation S4, F represents the Faraday constant and I, the ionic strength in molarity, M. R represents the gas constant and T, the temperature. N A represents Avogadro s number, c i, the molar (M) concentration of the electrolyte ion, i and z i, the charge of i. Using Equation S4, the estimated Debye length for the divalent cation solutions is λ DMe 2+ =0.32 nm. In our system, the tip and substrate were identical in composition so their surface charge was assumed to be the same. Thus Equation S3 is simplified to: F = () e +e. (S5) The surface charge is closely related to the surface potential, φ o, which can be expressed by the Grahame equation: 1 σ =8ε εkt sinh(eφ 2kT)NaCl+Me 2+e, (S6) 5
where T 0 represents room temperature, 25 o C, concentrations are expressed in M, φ o, in mv. We used the surface potential reported by Hu and Bard, φ o = -30 mv, 10 for a -COO(H) SAM in 10-3 M KCl solution at ph 8.2, giving an estimated surface charge of -0.048 Cm -2. Using previous assumptions from the well defined hydrophobic system, i.e. D=1.4 nm and R eff =30 nm, F EDL estimated for the hydrophobic system is ~4.9 pn, which is low compared with the adhesion forces measured using CFM but consistent with the high ionic strength of the solutions involved. We expected an important influence from the repulsive hydration interactions between hydrophilic surfaces because of the presence of one or more hydration layers that result in repulsive osmotic forces when two such surfaces approach and interact. 3 Using the Donaldson hydration model, 11 we estimated a theoretical hydration force for the hydrophilic system from the hydrophilic parameter (second expression in Equation S1), using a negative effective interfacial tension, i.e. Hy < 0. This means that because of the repulsive character of the hydrophilic force, work must be done for the -COO(H) surfaces to release their water and be brought into direct contact. 3 Donaldson and coworkers validated their model with data from previous CFM reports on mica surfaces, which demonstrated the reliability of their approximation for surfaces with high negative charge over a range of conditions, where the Hydra parameter, H y, was between - 0.01 and -0.1, depending on environment. Under these assumptions, the repulsive hydration force estimated for the hydrophilic system was ~62 pn, which gave an overall repulsive force, i.e. F C + F EDL+ F HI of ~100 pn. The van der Waals force, F vdw, that is active in the hydrophilic system is defined by: F =. (S7) Here, we used A COO(H) = 5x10-21 J, from the Hamaker constant reported by Preston et al. 12 for gold 12-mercaptododecanoic acid colloids, which have the same end group, -COO(H). They 6
differ from the functionalised surfaces used in our work only in carbon chain length. With the same D and R eff used for all the approximations, the estimated attractive vdw force was 12.7 pn. If we calculate the F AD for the hydrophilic system, as the sum of all the forces involved, the overall force is repulsive, on the order of 100 pn. REFERENCES (1) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.; Elsevier B.V.: Santa Barbara, California, 2011. (2) Stock, P.; Utzig, T.; Valtiner, M. Direct and Quantitative AFM Measurements of the Concentration and Temperature Dependence of the Hydrophobic Force Law at Nanoscopic Contacts. J. Colloid Interface Sci. 2015, 446, 244 251. (3) Donaldson, S. H.; Røyne, A.; Kristiansen, K.; Rapp, M. V.; Das, S.; Gebbie, M. A.; Lee, D. W.; Stock, P.; Valtiner, M.; Israelachvili, J. Developing a General Interaction Potential for Hydrophobic and Hydrophilic Interactions. Langmuir 2015, 31, 2051 2064. (4) Israelachvili, J. N. The Calculation of Van Der Waals Dispersion Forces between Macroscopic Bodies. Proc. R. Soc. London. A. Math. Phys. Sci. 1972, 331, 39 LP-55. (5) Parsegian, V. A. Van Der Waals Forces, First edit.; Cambridge University Press, Ed.; Cambridge.org: New York, 2006. (6) Kokkoli, E.; Zukoski, C. F. Interactions between Hydrophobic Self-Assembled Monolayers. Effect of Salt and the Chemical Potential of Water on Adhesion. Langmuir 1998, 7463, 1189 1195. 7
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