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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage
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, and Tue Hassenkam Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01720 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018
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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 Keywords: Adhesion force, ion bridging, chemical force mapping, hydrated ions.
prepared for Langmuir * corresponding author:
[email protected] ABSTRACT
Interactions between mineral surfaces and organic molecules are fundamental to life processes. The presence of cations in natural environments can change the behaviour of the organic compounds and thus alter the mineral-organic interfaces. We investigated the influence of Na+, Mg2+, Ca2+, Sr2+ and Ba2+ on the interaction between two model, self assembled monolayers (SAM), that were tailored to have hydrophobic -CH3 or hydrophilic -COO(H) terminations. Atomic force microscopy (AFM) in chemical force mapping (CFM) mode, where the tips were
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functionalised with the same terminations, was used to measure adhesion forces between the tip and substrate surfaces, to gather fundamental information about the role of these cations in the behaviour of organic compounds and the surfaces where they adsorb. Adhesion force between hydrophobic surfaces in 0.5 M NaCl solutions, that contained 0.012 M divalent cations, did not change, regardless of the ionic potential, i.e. the charge per unit radius, of the cation. For systems where one or the other surface was functionalised with carboxylate, -COO(H), mostly in its deprotonated form, -COO-, a reproducible change in the adhesion force was observed for each of the
ions.
The
trend
of
increasing
adhesion
force
followed
the
pattern:
Na+~Mg2+Mg2+>Ca2+, is known as the Hofmeister series.12–14 This series is an example of one of the best known specific ion effects.15 Behaviour specific to ions reflects that surface interactions depend on more than concentration, ionic strength and charge. The type of ion, i.e. its electron configuration, plays the central role. The availability of techniques that allow us to measure forces between surfaces at nanometre scale, now make it possible to relate submicrometre scale properties with macroscopic effects. One specific question is how ions influence organic compounds, on surfaces or in solution. Answers will contribute to deeper understanding of the controls on surface tension,13 colloid stability,14 and ionic interactions in aqueous systems.16 Applications range across many fields, from development of functional materials to remediation of contaminated soil and ground water and oil production.17 We used a rather new approach to study the effect of the ions in systems containing organic surfaces and aqueous solutions, namely atomic force microscopy (AFM) in chemical force mapping (CFM) mode. In our experiments, AFM tips were functionalised with a self assembled monolayer (SAM)18. In the experiments, we moved the tip toward and away from a model surface and the rupture forces between the tip and sample were measured during exposure to a range of solutions. CFM19 is a promising approach because one can investigate nanoscale events with resolution high enough that even the rupture force of molecular bonds can be measured.20 In our research group, we have used this technique to investigate interactions between various organic compounds10,11 and minerals, such as quartz,21,22 illite,8 hematite,23 mica24 and calcite24,25 and composite materials such as bone26 and chalk.7
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We investigated the change in adhesion between hydrophilic and hydrophobic surfaces, in solutions containing Na+ and one of Mg2+, Ca2+, Sr2+ or Ba2+. We functionalised gold coated AFM tips and flat, gold covered silicon wafers with -CH3 and -COO(H) SAMs to produce hydrophobic or hydrophilic surfaces. These functional groups represent hydrocarbons and other organic compounds. These groups are very common in natural environments and in the fulvic and humic acids in natural waters.27 EXPERIMENTAL DETAILS Sample preparation – Self assembled monolayers To produce the SAMs, we used molecules with our chosen functional group on one end and thiol on the other. The sulfur of thiol adsorbs tightly to gold, exposing the functional group to the solution. To produce the gold substrates, we used two types of silicon wafers: i) 1 cm2 square silicon wafers, on which the SAM was built and ii) gold coated disk wafers (from Platypus Technologies) that were 10 cm in diameter with a 50 nm gold layer deposited over a titanium adhesion layer,28 from which the gold film was taken. We first cleaned the small, square silicon wafers in an ultraviolet (UV) ozone chamber for 20 min. Then, we glued them, with the clean silicon surface facing down, onto the gold coated silicon disk, using EPOTek 353ND. To cure the glue, the sandwich (Si square-glue-Au-Ti-Si) was heated at 120 °C, in air for 30 min. After cooling, the squares could be pulled away from the gold coated disk, producing a freshly stripped, flat, clean, gold surface, that was used immediately for SAM formation. The self assembled monolayer was prepared by immersing a freshly stripped gold substrate and the UV ozone treated AFM gold coated Biolevers (Olympus BL-RC-150VB) to a 4-5 mM absolute ethanol (≥99.8% semiconductor grade PURANAL™) solution containing the desired precursor: 1-uncanethiol (98%, Sigma Aldrich), for a -CH3 functionalised SAM substrate, and
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11-mercaptoundecanoic acid (99%, Sigma Aldrich), for a -COO(H) SAM substrate. Tips and surfaces were submerged for at least 17 h to allow a monolayer to form. Prior to use in the CFM experiments, tips and surfaces were rinsed in ethanol (absolute ethanol ≥ 99.8% semiconductor grade PURANAL™) for 30 minutes to remove the excess, unbonded thiol molecules from the surface. Solution preparation A stock solution of 0.5 M NaCl was used as the reference solution for the baseline experiments and for preparing the other four solutions, that each contained 12 mM of a divalent cation chloride: MgCl2, CaCl2, SrCl2 or BaCl2. For all solutions, pH was adjusted from 5.5 to 8.2-8.3 with 0.2 M NaOH. pH was measured using a MetrOhm 827 pH meter, while stirring. The pH electrode was calibrated at pH 4, 7 and 9 with standard buffers at the same temperature as the solutions. All solutions were equilibrated with air prior to use. All chemicals were Reagent Grade or better, supplied by Sigma-Aldrich Chemicals, except the MgCl2, which was supplied by Merck. We used ultrapure deionized water from a Millipore purification system (MilliQ, resistivity >18.2 MΩ∙cm) for all solutions. Chemical force mapping (CFM) A MFP-3D Asylum Research atomic force microscope was used to collect force maps, following the procedure described previously by Hassenkam et al.7 We mapped areas of 5x5 µm2 with 30x30 data point coverage, with an indentation force of 500 pN. The dwell time was set to 0.1 s, the scan rate to 4 Hz and scan velocity to 6.38 µm/s. The spring constant of the functionalised gold cantilevers was between 20-30 pN/nm and the nominal radius of curvature was ~30 nm.
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In CFM, the position of the tip with respect to the surface at each point is described by a force curve. We use one datum from each curve (adhesion force, Figure S1a, Supporting Information) to create each pixel on the adhesion force map (Figure S1b, Supporting Information). We can assume that our scanned surfaces were essentially homogenous because SAMs ought to have the same composition over the whole surface. Thus, we used the average adhesion from the 900 curves that we collected for each map to represent that measurement in the adhesion force plot (Figure S1c, Supporting Information). To ensure stable instrument function, similar experimental conditions and to minimise uncertainties in the mapping procedure, we used the same tip and substrate to record at least four force map series in each of the five solutions, following the procedure reported by Rimmen et al.10. Before each experiment, the deflection sensitivity of the cantilever was determined by ramping the cantilever against a very stiff surface and the spring constant was estimated by fitting a Lorentzian function to the thermal spectrum.29 We investigated the behaviour of three different systems: a hydrophobic system, where both interacting surfaces (tip and substrate) were functionalised with -CH3; a hydrophilic system where both surfaces were functionalised with -COO(H) and an asymmetric system, where the tip was functionalised with -COO(H) and the surface was functionalised with -CH3 to examine the hydrophilic-hydrophobic interaction. At the pH of our experiments (~8.3), it is expected than most of the carboxylic acids were in a deprotonated (-COO-) form.30 Based on previous work, we estimated that absolute adhesion forces would not be affected by the location of the hydrophobic surface (i.e. tip or surface).19,31,32 The use of a reference solution (NaCl) allowed us to compare between the systems by removing the specific effects present for each system, that could affect the measurements.
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In each experiment, we explored how the solutions affected the adhesion forces. We sequentially investigated the influence of the reference 0.5 M NaCl solution, then the test solutions which contained 0.5 M NaCl and 0.012 M of one of each of the four divalent cations, Mg2+, Ca2+, Sr2+, Ba2+. Each solution was tested at least 4 times, after the reference solution was reinjected, to record detachment of the cation. Then the pure NaCl solution was replaced with a solution containing a new cation. The experiment ended when adhesion maps from each of the solutions had been collected. In a separate series, we changed the order of solution addition, to test if the observed cation effect was an artefact from the previous solution. RESULTS AND DISCUSSION Figure 1 presents a summary of all of the average adhesion force measurements obtained for the three systems: hydrophobic, hydrophilic and asymmetric or mixed. Each bar represents the average adhesion from one force map and the range in force over all of the maps is represented by the standard error bars (which are very small).
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a 2000
System 1 - Hydrophobic - CH3-CH3
FAD (pN)
1500 1000
Na+ Ba2+ Sr2+ Ca2+ Mg2+
500 0
b FAD (pN)
2000
System 2 – Hydrophilic - COO(H)-COO(H)
1500 1000 500 0
c
2000
System 3 – Asymmetric - CH3-COO(H)
1500
FAD (pN)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1000 500 0
Figure 1. Average adhesion force, FAD, in pN for the systems (a) hydrophobic, (b) hydrophilic and (c) asymmetric or mixed. Table 1 summarizes the adhesion force measurements for the three systems, ionic radius, hydrated radius33–35 and the absolute adhesion, which is defined as: ∆F = F − F . (1) The ratio of response that quantifies the extent of adhesion that the divalent cation contributes to each system is: % Response =
!"#. %&'()*&+ (-./') !"#. %&'()*&+ (-./')
x 100%. (2)
Table 1. Summary of all adhesion force data (used to produce Figure 1), ionic and hydrated radii.29, 31 FAD and ∆FAD are in pN, Resp represents the % response.
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0.35 Hydrated radius (nm)
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0.43
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Radius
CH 3-CH3
COO(H)-COO(H)
ΔF AD Resp (pN) (%)
F AD (pN)
ΔF AD Resp (pN) (%)
CH3-COO(H)
ionic (nm)
hyd (nm)
F AD (pN)
F AD (pN)
Na+ Mg2+ Ca2+ Sr2+
0.095
0.358
545 ± 4
0
0
228 ± 24
0
0
0.065
0.43
515 ± 5
-30
5
478 ± 22
250
110
298 ± 21 -149
33
0.099
0.412
561 ± 2
16
3
1010 ± 42
782
343
734 ± 28
287
64
0.118
0.412
570 ± 5
25
5
904 ± 69
676
297
573 ± 11
127
28
Ba2+
0.135
0.403
517 ± 6
-27
5
1384 ± 100 1156 507
976 ± 19
530 119
446 ± 6
ΔF AD Resp (pN) (%) 0
0
Hydrophobic system There is no significant difference in the adhesion force measured in the hydrophobic system (Figure 1a) regardless of which cations are in solution. The average adhesion force is ~550 pN, with variation of 5% or less. This was expected.21,36 The nonpolar character of the methyl surfaces limits interaction with the ions in the solution. Hydrophobic, -CH3, functionalised surfaces are assumed to be neutral. The cations are therefore not expected to bind specifically to -CH3. CFM measurements confirm the expectations: all the cations induce the same adhesion force, ~550 pN, implying that no specific interaction between cations and surfaces take place in the system. The adhesion force, FAD, is a complex interaction involving dispersion forces, hydration forces, structural forces, etc. To simplify our model and separate the contribution of each force from the differences observed between the systems, we have assumed that the bulk of the FAD, is mainly a combination of three forces: van der Waals, FvdW; electric double layer, FEDL33 and hydrophobic interaction, FHI:33 FAD= FvdW + FEDL + FHI. (3) The first two forces are defined by the well known DLVO theory33,37 as independent contributions and therefore can be added for a specific interacting distance for each system.
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Hydrophobic forces were approximated by a model proposed by Donaldson and coworkers,38 based on a Hydra parameter (Hy), which describes the degree of hydrophobicity (Supporting Information, Equation S1). We assumed that the surface charge density for the -CH3 surfaces was negligible, FEDL ~0, reducing its contribution to the force to a combination of FvdW and FHI. Assuming a minimum distance of interaction in the hydrophobic system to be D=0.2 nm (steric repulsion) and using the standard radius of curvature of the tip as an effective radius of interaction (Reff=30 nm), we estimated the adhesion force (using Equation S1) to be ~10 nN for the hydrophobic system, almost 20 times higher than the forces we measured. The discrepancy could be explained by one or both of the assumptions made for the tip radius and the distance of interaction. According to several papers,7,11,19,39 tip radius typically ranges between 8-50 nm and could be increased by functionalisation. However even if the true radius of curvature were to be half the reported value, the adhesion force estimated for the hydrophobic system using Equation S1 would still be 10 times higher than the measured one. For the interaction distance, our assumption of 0.2 nm did not consider other ions between the tip and surface. If an ion interacts with both surfaces, the distance between them would be larger, at least the diameter of the hydrated ion (~0.8 nm). Using D=1.4 nm and Reff=30 nm, our estimated force, FAD~650 pN, is reasonably close to the measured average adhesion force, FAD~550 pN. An interaction distance larger than the hydrated radii of the interacting cations suggests that water and partly hydrated cations and anions also affect the interaction between the hydrophobic tip and surface. Although -CH3 surfaces have no specific charge, interaction of water with the aliphatic chain of the SAM could lead to a slightly negative charge over the surface. It has been reported40–43 that air bubbles, oily groups and hydrophobic surfaces spontaneously acquire a diffuse negative
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charge in water as a result of hydroxide ion adsorption. This behaviour is a result of suppression of the dipole moment of water molecules, which induces an increase in hydroxide ion affinity to surfaces where dipole moment fluctuations are smaller than in bulk water.44 Water behaves similarly at all inert hydrophobic interfaces, with preferential hydroxide ion adsorption, giving a slight negative charge at neutral pH. A diffuse negative charge on both tip and surface would increase the interaction distance. Our measurements are consistent with this explanation. The fact that the force between the -CH3 surfaces is mainly caused by hydrophobic interaction45 (FvdW~100 pN; FHI~ 550 pN) would mean that such systems are more affected by surface interactions than ion-surface complexes.
Hydrophilic system There is strong interaction between hydrophilic surfaces (Figure 1b), when the Na+ solution also contains a small amount of divalent cation. The increase in adhesion force follows the pattern: Mg2+
