Surface forces measurement for materials science

Introduction

The elucidation of structures and interactions is the most fundamental approach in materials science. There are many tools for characterizing structures; i.e. X-ray and neutron diffractions, nuclear magnetic resonance (NMR), various vibration spectroscopies, electron microscopy, etc. On the other hand, our understanding of interactions is still limited due to the lack of appropriate characterization techniques and also the complexity of interactions of multiple atom- and molecular systems. Surface forces measurement, which provides a force-distance profile (interaction potential), is a powerful tool for elucidating interactions. Indeed, the measurement was introduced in the 1930s to prove the dispersion force predicted by quantum mechanics [1], though real demonstration of a dispersion force with modern accuracy was realized only around 1970 by Tabor and Winterton [2], [3], Tabor and Israelachvili [4], and Derjaguin and his colleagues [5] following the pioneering efforts for the measurement by Overbeek et al. [6] and Derjaguin et al. [7] in the 1950s. The key to this achievement by Tabor is utilization of FECO (fringes of equal chromatic order) [2] for determining the surface separation distance and the deflection of a spring. The apparatus, which adapted the original design by Tabor, is called the “surface forces apparatus (SFA)” and is still used [8]. The interactions in water was then measured to demonstrate the DLVO force, which is the sum of the electric double layer repulsion and the van der Waals attraction, by Israelachvili and Adams [9].

The measurement was extended in the 1980s to study the interactions mostly involved in colloidal stability, the DLVO force [9] and the steric interactions of polymers [10], [11]; as well as those in the self-assembled organization of amphiphiles of micelles and bilayers [12], [13]. The measurement revealed an unexpected oscillation force attributed to the layer structures of liquid molecules confined between mica surfaces [14]. It shed new light on our understanding of surface interactions which had been mostly understood based on empirical models. Molecular assembling chemistry took off at a similar time, thus it was natural that many early studies were devoted to study molecular assemblies. The interactions between the hydrophilic head groups of amphiphiles were studied [12], [13], providing a physical and chemical foundation of amphiphilic assemblies. My early researches were concerned with the biomimetic functionalization employing molecular self-assemblies such as micelles [15], [16], liposomes [17], vesicles [18], self-assembled monolayers [19] and Langmuir-Blodgett films [20], so I became interested in understanding molecular interactions and surface forces. Our earlier studies concern the interactions between surfaces uniquely modified with the Langmuir-Blodgett films by taking advantage of our expertise in molecular architecture chemistry. Studied are: Long-ranged attraction between hydrophobic surfaces in aqueous media [21]; charge reversal upon adsorption of polyanions on cationic monolayers [22]; attraction between nucleic acid bases [23], [24]; and polyelectrolyte brushes [25], [26], [27], [28]. These studies have shown that the measurement not only directly demonstrates interactions, but can reveal unknown phenomena occurring at the interfaces as described in previous reviews [29], [30]. Encouraged by these studies, we have been making efforts to utilize the surface forces measurement as a common tool in the materials science field. This review describes a general overview and our efforts towards this goal with focus on instrumentation including the development of the first practical SFA for opaque samples and of the resonance shear measurement and its applications.

Surface forces measurement: advantages

The surface forces measurement employing SFA is important and has several advantages compared to other direct force measurements such as atomic force microscopy (AFM) and total reflection microscopy (TRM) as follows [8], [29], [30]:

1. It is essential to elucidate the molecular and surface interactions as fundamentals in the materials science field. Especially, the self-organization of molecules and particles has become an important part of recent material design because rather complicated structures and functions can be created using simple building blocks [30], [31], [32]. Self-assembly is also a key for the ingenuity of biological systems.

2. The surface force measurement is a powerful tool for characterizing interfaces. The surface forces sensitively change in response to interfacial properties. Therefore, the force profiles contain information about interfacial properties and their changes from the surface toward the bulk liquid. The simplest cases are (1) the electric double layer formed at the interface between a charged surface and an electrolyte solution, and (2) steric repulsion attributed to polymer adsorbed layers [10], [11]. The surface forces measurement directly monitors these characteristics, which are generally complicated to monitor. Alternative ways for characterizing them are the X-ray and/or neutron reflection methods, but both require large facilities and model data analysis. The SFA measurement is simple, direct and compact. Using the combination of the surface forces measurement and the vibration spectroscopy, we have found the hydrogen-bonded organization of molecules adsorbed on solid surfaces [34], [35].

3. A unique characteristic of the measurement employing SFA is the large size of the monitored surface, i.e. a 30 μm contact diameter, which enables us to measure the interactions of many molecules at the same time. Interactions between multiple molecules, which are common in real materials, are not necessarily described by the sum of single molecular interactions when the single molecular measurement is an idealistic goal of the atomic force microscopy using a tip force probe [36]. The SFA measurement should bear a close relevance to real systems that consist of many molecules and/or in the condensed states. Our study on polyelectrolyte brush layers revealed their collective properties and the density dependent transition [25], [27].

In addition, a practical advantage of the SFA measurement is that it is possible to study long-range weaker forces due to large surface area. A better resolution in the separation distance is another advantage. The SFA commonly utilizes the crossed cylinder geometry for interacting surfaces. The measured force (F) normalized by the mean radius (R) of the cylinders or a sphere, F/R, is known to be proportional to the interaction energy, G_f, between flat plates (Derjaguin approximation), F/R=2πG_f [8]. This enables one to quantitatively evaluate the measured forces, e.g. by comparing them to a theoretical model.

Key approaches for materials science

In order to explore the potential of SFA in materials science, however, there are some restrictions and barriers, which we are making efforts to overcome as described below.

1. SFA for opaque samples: Broad applications of the forces measurement by the conventional SFA is restricted by the requirement of transparent samples for the distance determination employing the FECO method (Fig. 1). This has practically limited the availability of sample substrates to only mica and modified mica. In spite of many efforts of developing an apparatus for opaque samples [37], [38], [39], [40], there was no apparatus in practical use for opaque samples. We have recently completed the apparatus for non-transparent samples, which is called the twin path SFA [41]. This apparatus can be used not only for opaque sample surfaces such as metals and polymers, and but also for extending the measurement to electrochemistry [42], [43], [44], [45] and photochemistry [46], [47].

2. Preparation of flat and smooth surfaces: The SFA measurement requires flat and smooth surfaces. Therefore, the conventional SFA uses cleaved mica sheets as sample substrates because they need to also be transparent for the FECO and chemically stable. However, cleaving mica into thin sheets of 2~3 μm in thickness necessary for the FECO, cutting them into ca. 10×10 mm² pieces, and gluing back-silvered sheets on cylindrical silica disks by melted epoxy resin are tedious and demanding jobs. Moreover, the epoxy resin could dissolve in organic solvents. These are problems for broader applications of the measurement. We recently prepared silica substrates for the FECO by directly depositing gold and silica layers on cylindrical silica lenses [48]. For the twin-path SFA, we need to prepare flat and smooth surfaces of, preferably, all materials. Therefore, continuous efforts are required to prepare smooth and flat surfaces of various materials by adjusting the conditions of sputtering, atomic layer deposition, vacuum-deposition etc. We prepared surfaces, such as gold [42], [43], [44], platinum [45] iron [49] with their RMS roughness values of typically around 0.5 nm or less. Polymer films can be prepared by spin-coating.

3. Elucidation of interactions: It is not always possible to understand the measured interactions based on the simple fundamental knowledge of forces found in a textbook, because the interactions of different origins sometime overlap [25], [26], [27], or the surface properties change in complex manners [32], [33]. It has not been established how we can treat liquids when the surfaces interact in liquid medium of smaller thickness such as several nm to a few tens of nm: A continuous medium or as molecules? We need complementary characterization, model analysis and simulations to elucidate the observed forces [29], [30], [35].

Fig. 1:

A schematic drawing of a surface forces apparatus using FECO for the distance determination. Photographs of sample substrates, back-silvered mica sheet glued on silica disc (left) and silica sputtered on gold coated silica disc (right) are shown in the figure.

All of these are challenging, but rewarding to do since we can often find new knowledge. The shear measurement is another area where the SFA based forces measurement is unique and powerful. Various measurement methods [50], [51], [52], [53], [54], including the resonance method by us [52], [54], have been developed, and each of them has different advantages. The advantages of the resonance shear measurement (RSM) are: (1) High sensitivity and stability because we use the resonance mode; (2) Ability to continuously monitor nano-rheological to nano-tribological properties. It is possible to study confined liquids with their thickness from μm to zero (surface contact).

Twin-path SFA for opaque samples

We constructed a new surface forces apparatus for measuring the interactions between two opaque substrates and/or in nontransparent liquids [41]. The small displacement of a surface, the bottom one for this apparatus, was measured by the modified two-beam (called “twin path”) interferometry technique using the phase difference between the laser light reflected by a fixed mirror and that by a mirror on the back of the bottom surface unit (Fig. 2). In a previous attempt using the two-beam interferometry for SFA, the working distance range was not great, typically ca. 20 nm, for the high resolution of 0.2 nm in distance [39]. Such a short working range of 20 nm is not long enough for studying the surface forces, so the apparatus was claimed to be one for tribology. On the other hand, in the case of the twin-path SFA, the interference light is split into four parts of which the phases are shifted by π/2 and their intensities are monitored by a four-sectored photodiode (Fig. 2). This setup enabled us to determine the distance with a resolution of 0.2 nm while maintaining the long working range of longer than 5 μm [29], [41]. The twin-path apparatus is computer controlled, thus relatively easy to use compared to the conventional SFA.

Fig. 2:

A schematic drawing of the twin-path surface forces apparatus for opaque samples. Top: Laser light (λ=670 nm) goes through the window at the bottom of the chamber and is reflected at the back of the disk holder. The reflected light is monitored by the twin-path unit. The surface distance is controlled by a surface drive system. Bottom: In the twin-path unit, the +1^st and −1^st order beams separated by the grating are reflected at the bottom of the disk holder and at the fixed mirrors, respectively. They are then recombined on the diffraction grating in the front of the four-sectored photodiode which monitors the recombined and phase shifted lights. The signals monitored by the photodiode are analyzed by a PC shown at the bottom of the top figure.

This apparatus can be used for opaque surfaces such as metals and polymers [49], [55]. One major area for applications is, so far, for electrochemistry using metal electrodes as substrates [42], [43], [44], [45]. The electric double layer formed on charged surfaces is important in the performance of an electrode. The modification of gold surfaces by self-assembling monolayers (SAM) using thiol derivatives is possible [43].

Spectroscopic SFA has been developed by placing a fluorescence microscope above the SFA, since the space above the SFA, which has been used for the distance determination in the case of the conventional SFA, is now free [46], [47]. The twin-path SFA can significantly extend the scope of the measurement.

Electrochemical SFA

Recently, broad interest has been raised regarding electrochemical devices such as batteries and sensors [56]. The surface potential and surface charge density of the electrodes determine their performances, and they are sensitively changed depending on the interactions between the electrode surface and the ions in the electrolyte solutions. However, quantitative evaluation of the surface potential and charges has been difficult. The surface force measurement can effectively evaluate them based on the electric double layer forces. Naturally, the development of an electrochemical surface forces apparatus (EC-SFA) has been reported [55], [56]. The previous studies used mica as one surface and mercury [55] or gold electrodes [56] as the other, because FECO used for the distance determination in these studies required at least one surface to be transparent. Therefore, it is only possible to regulate the applied potential of one surface (electrode), and the quantitative analysis of the interaction is more difficult compared to the measurement of two identical surfaces.

We have constructed a technique which can measure the interactions between two identical opaque electrodes employing the twin-path surface forces apparatus [38]. This electrochemical SFA can not only provide a simpler setting, but also enhance the possibility for studying electrodes of various materials, such as gold and platinum, and with chemical modifications. For example, many thiol-terminated compounds can be employed to modify the gold surfaces by self-assembled monolayers (SAM) of the thiol compounds [43]. The large surface area of the electrodes enables us to run the cyclic voltammetry in-situ in SFA and to learn more about the relation between the interactions and the electrochemical reactions [43], [45]. We applied this system to also determine the distribution of counterions between the Stern layer and the Gouy-Chapman layer.

The electric double layer of a charged surface is viewed as having a fixed charge of one sign on the surface (one layer) and the diffuse layer of oppositely charged ions (counterions) neutralizing the surface charge (Fig. 3). The diffuse layer is considered to consist of the relatively more strongly bound Stern layer in close proximity to the surface and a more diffuse layer at greater distances (Gouy-Chapman layer). The surface charge and potential of the electrode are determined by the distribution of the counterions between the Stern layer and the Gouy-Chapman layer for a surface of fixed charge given by various processes, such as ion dissociation and adsorption, and redox reactions. Therefore, it is important to know the distribution of counterions between the Stern layer and the Gouy-Chapman layer. However, the direct quantitative determination is difficult because of the difficulty of determining the number of fixed charges present on the surface. To overcome this problem, we chemically modified the electrodes with redox molecules and evaluated the charges produced on the surface by cyclic voltammetry.

Fig. 3:

A schematic drawing of the electric double layer consisting of the Stern layer of the bound counterions and the Gouy-Chapman layer of the diffused counterions (a) and the electric double layer repulsion due to the osmotic pressure of the counterions (b).

An electrochemical SFA was employed to measure the interactions between gold electrodes modified with self-assembled monolayers of ferrocene alkyl thiol (Fc-SAM) in a 1 mM aqueous electrolyte as shown in Fig. 4. The cyclic voltammogram provided the concentration of the oxidized ferrocene (ferocenium cation, Fc⁺-SAM) to be 3.1±0.2 molecules/nm². The double layer repulsion in both cases of the Fc-SAM (at the applied potential E=0 V vs. Ag/AgCl) and Fc⁺-SAM (E=0.8 V) electrodes was observed as shown in Fig. 5. The repulsion increases when the ferrocene molecules are oxidized and charged. The effective surface charge density (σ) evaluated from the double layer repulsions between the Fc⁺-SAM electrodes in 1 mM aqueous KClO₄ is 0.0027±0.0002 C/m² (0.017±0.001 charge/nm²). This means that 99.5% of the ferocenium cations are neutralized by the ClO₄⁻ ions in the Stern layer and only 0.5% of the ClO₄⁻ ions are in the diffused Gouy-Chapman layer. It is known that the counterion binding is different between the various counterions. The degree of dissociation (α_d) of CF₃SO₃ has been determined using the same method to be 3.2%, which is the highest among the ions studied in the order SO₄²⁻>NO₃⁻>ClO₄⁻, indicating that most of the positive charges of the ferrocenium cation were compensated by formation of an ion pair with counter anions in the Stern layer. This is, to the best of our knowledge, this is the first precise determination of the Stern layer, which is one of the most fundamental concept in electrochemistry.

Fig. 4:

Schematic drawings of an electrochemical surface forces apparatus, and of the oxidized ferrocene-modified gold electrodes.

Fig. 5:

(a) Cyclic voltammogram of the ferrocene-modified surfaces in 10 mM aqueous KClO₄ using conventional electrochemical cells; (b) Force profiles between the ferrocene-modified surfaces in a 1 mM aqueous electrolyte solution at various applied potentials. The different shapes of symbols in the graphs represent repeated experiments. Red symbols represent the data at the applied potential of 0.8 V vs Ag/AgCl in aqueous KNO₃, blue symbols at the applied potential of 0.8 V in KClO₄, and green symbols at the applied potential of 0 V in KClO₄. The solid line denotes the theoretical fits to the Poisson−Boltzmann equation of the electric double-layer force for the constant charge model.

Ion adsorption on the electrodes depends on the ion species as well as the kind of electrodes such as gold and platinum. This approach should provide a more quantitative and realistic view for the ion adsorption on electrodes, and assist in designing electrochemical devices. Taking advantage of characterizing the electrode during the electrochemical reaction, the proton adsorption and the subsequent hydrogen evolution on the platinum electrode were studied (Fig. 6) [45].

Fig. 6:

A drawing of the proton adsorption and the hydrogen evolution on a platinum electrode studied by the electrochemical SFA at the various electrochemical potentials. Proton adsorption began to occur at a electrochemical potential E of ca. 0 V, thus the surface potential (absolute value) remained low, and saturated at E=−0.1 V resulting in the minimum surface potential. Hydrogen generation began at −0. 2 V, thus the potential increased.

Resonance shear measurement

It is important to understand the lateral force in addition to the normal force. The shear measurement is another area where the SFA based forces measurement is powerful. Various measurement methods [50], [51], [52], [53], [54], including the resonance method by us [52], [54], have been developed, and each of them has different advantages. The resonance shear measurement (RSM), developed by us, monitors the mechanical resonance signal of the upper unit, in which the upper surface is hung by a pair of vertical leaf springs as shown in Fig. 7 and laterally moved using a four-sectored piezo tube by applying a sinusoidal voltage of a given frequency. The shear resonance peak changes its frequency and amplitude depending on the properties of the opposed surface and liquids between them. We use the resonance peaks for the surface for separation in air (called as AS peak) and in contact (SC peak) as references for discussing changes in the raw data and for determining the mechanical parameter of the model analysis. Advantages of RSM are: (1) High sensitivity and stability because we use the resonance mode; (2) Ability to continuously monitor nano-rheological to nano-tribological properties of confined liquids and solid surface and solid-liquid interfaces in their separation distance from μm to zero (surface in contact). This method is used to determine the increase in the effective viscosity of water confined between mica surfaces at the separation distance below 1 nm, which was in dispute at the time of this study [56].

Fig. 7:

A schematic drawing of the resonance shear measurement (RSM): left, apparatus; right, resonance curves.

The most thoroughly studied system by RSM is ionic liquids, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][NTf2]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]), confined between silica surfaces [55], [56], [57]. The RSM revealed that [C4mim][NTf2], which has a bulk viscosity (58.4 mPa s) lower than that of [C4mim][BF4] (124.6 mPa s), exhibits a higher effective viscosity than [C4mim][BF4] when they are confined in a gap below ca. 10 nm (Fig. 8) [57]. Our molecular dynamics (MD) simulation of these ionic liquids on amorphous silica surfaces showed a checkerboard-like ordering of ions (both the anion and cation are in the same layer) for [C4mim][NTf2] and alternative anion and cation layers for [C4mim][BF4] [58]. These different structures could reasonably explain the properties of the nano-confined ILs under shear obtained by the RSM. The anion–cation alternate layers of [C4mim][BF4] could be sheared in a laminar manner, while the checkerboard structure of [C4mim][NTf2] showed higher fluctuations in the inner layers due to the strong electrostatic repulsion between the same ions induced by the shear. Consequently, the latter shows a higher apparent viscosity (in other words, friction) than the former. These simulated structures have been confirmed by an X-ray diffraction study of confined ionic liquids between silica at a thickness of 2 nm (Fig. 9) [59].

Fig. 8:

Plots of the effective viscosity vs. the separation distance of inonic liquids, [C4mim][NTf2] (a) and [C4mim][BF4] (b), confined between silica surfaces. The viscosity is estimated using the RSM data. Inserts show the resonance curves at various surface separations.

Fig. 9:

Structures of ionic liquids, [C4mim][NTf2] and [C4mim][BF4], confined between silica surfaces determined by the molecular dynamics (MD) simulation and X-ray diffraction.

The RSM has been also used to study various complex soft matters including: Model and commercial lubricants to study their behavior at the boundary lubrication region [60], [61]; a mechanism of the viscosity increase of the viscosifier, surfactant modified calcium carbonate nanoparticles in dioctyl phthalate, for sealants [62]; interfacial deformation contributed to friction of gels [63], [64]; confinement effect on electric field induced orientation of nematic liquid crystal [65]. These studies have demonstrated that the RSM is able to reveal unknown features of confined liquids, solid-liquid interfaces and surfaces as well as polymers and other complex systems when they are organized at the interface.

Closing remarks

This article describes unique features of surface forces measurements, their recent developments and applications. It is hoped that the readers will be introduced to the broad applications of the forces measurement. Recent development of apparatus has opened the door for newer applications. The surface forces are sensitively modified depending on changes in phenomena and conditions at the solid-liquid interfaces, thus sometime performing the experiments and understanding the forces are challenging. However, this sensitivity can be regarded as advantage. We can learn from changes when we properly organize the experiments and analysis with the help by additional complementary characterizations and simulation. This situation is very similar to calorimetry, in my opinion, thus we may call the measurement as forcemetry. We need to think more how one can effectively use forcemetry for materials science.