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Exploring Forces Between Individual Colloidal Particles with the Atomic Force Microscope

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Exploring Forces Between Individual Colloidal Particles with the Atomic Force Microscope 214 CHIMIA 2012, 66, No. 4 Laureates: awards and Honors, sCs FaLL Meeting 2011 doi:10.2533/chimia.2012.214 Chimia 66 (2012) 214–217 © Schweizerische Chemische Gesellschaft Exploring Forces between Individual Colloidal Particles with the Atomic Force Microscope Prashant Sinha§, Ionel Popa, Marco Finessi, Francisco J. M. Ruiz-Cabello, István Szilágyi, Plinio Maroni, and Michal Borkovec* §SCS-Metrohm Foundation Award for best oral presentation Abstract: Forces between individual colloidal particles can be measured with the atomic force microscope (AFM), and this technique permits the study of interactions between surfaces across aqueous solutions in great detail. The most relevant forces are described by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, and they include electrostatic double-layer and van der Waals forces. In symmetric systems, the electrostatic forces are repulsive and depend strongly on the type and concentration of the salts present, while van der Waals forces are always attractive. In asymmetric systems, the electrostatic force can become attractive as well, even when involving neutral surfaces, while in rare situations van der Waals forces can become repulsive too. The enormous sensitivity of the double layer forces on additives present is illustrated with oppositely charged polyelectrolytes, which may induce attractions or repulsions depending on their concentrations. Keywords: Atomic force microscope · Colloidal probe technique · DLVO theory · Double-layer forces · van der Waals forces 1. Introduction particles even at high solid concentrations, forces between colloidal particles (e.g. which are important in paints, foods, or osmometry, scattering). The idea to mea- Many phenomena involving colloi- cosmetics.[3,4] sure the respective forces directly evolved dal particles, which typically are in a size Themostrelevantforcesactingbetween from Derjaguin’s force balances used to range between nanometers and microm- colloidal particles across aqueous solu- measure forces between macroscopic ob- eters, are governed by mutual interaction tions can be often rationalized within the jects[12] and the subsequently developed forces. Water treatment plants rely on at- classical theory from Derjaguin, Landau, surface forces apparatus.[8] The latter ap- tractive forces between suspended particles Verwey, and Overbeek (DLVO).[5–8] paratus did provide first reliable measure- to induce formation of large aggregates This theory stipulates that interaction ments of DLVO forces between curved that can be separated by sedimentation.[1] forces can be approximated by the sum of mica sheets. The necessary miniaturization Papermaking equally requires attractive electrostatic double-layer repulsion and of the force balance was made possible forces between cellulose fibers and filler van der Waals dispersion interactions. The through the invention of the atomic force particles in order to rapidly form large ag- double-layer repulsion originates from the microscope (AFM) by Binnig and cowork- gregates in the paper machine.[2] Repulsive buildup of an osmotic pressure between ers.[13] First direct force measurements in- forces are essential to maintain stable and two overlapping diffuse layers. Such forces volving colloidal particles with the AFM easily flowing suspensions of colloidal only occur when at least one of the surfaces were independently realized by Ducker is charged, and they sensitively depend on and Butt.[14,15] While other techniques the presence of charged chemical spe- permitting force measurements involving cies. The van der Waals interaction, which individual colloidal particles have been is also referred to as the Casimir force, developed in the meantime, such as total originates from the dispersion attraction internal reflection microscopy (TIRM)[16] between fluctuating permanent or induced or optical tweezers techniques,[17] it is dipoles, and is mostly attractive. In some probably fair to state that the colloidal situations, non-DLVO forces may become probe technique is currently the most important, and they may include interac- promising one, and is being used in nu- tions due to inhomogeneous charge dis- merous laboratories worldwide.[9–11,18–26] tributions,[9] adsorbed polymer layers,[10] or depletion forces induced by suspended *Correspondence: Prof. Dr. M. Borkovec particles or polymers.[11] Nevertheless, the Department of Inorganic, Analytical and Applied simple DLVO picture is capable of ratio- 2. Colloidal Probe Technique Chemistry Sciences II nalizing interaction forces in a wide variety 30, Quai Ernest-Ansermet of systems, and illustrative examples will The key idea is to replace the sharp CH-1211 Geneva 4 be discussed in the present article. AFM-tip with a colloidal particle with a di- Tel.: +41 22 379 6405 For a long time, only indirect meth- ameter of a few µm (see Fig. 1a). This parti- Fax: +41 22 379 6069 E-mail: [email protected] ods were available to address interaction cle is normally glued to the cantilever in the Laureates: awards and Honors, sCs FaLL Meeting 2011 CHIMIA 2012, 66, No. 4 215 dry state with a micromanipulator.[14,15,21] must be obtained from force measure- intermediate situation, where the surface Alternative in situ techniques have been ments in asymmetric systems or another charge regulates in a less pronounced fash- developed permitting the attachment of technique (e.g. electrophoresis). The solid ion. This situation can be incorporated into particles in solution without a drying-re- lines shown in Fig. 2 are fits obtained from the DLVO theory by introducing a regula- wetting step.[9,20] With a standard AFM one the solution of the full Poisson-Boltzmann tion parameter, which describes the capac- approaches the colloidal probe vertically to equation. From these calculations, one can ity of the surface to regulate its charge. For the substrate and measures the cantilever extract a surface charge density of 3.5 mC/ CC this parameter is unity, for CP zero, and deflection by means of a reflected light m2. This charge density is very similar to for the realistic situation this parameter beam. As the force constant of the canti- the value obtained by electrophoresis, but normally assumes values in between. In lever can be measured independently, for its magnitude is substantially smaller than the case of extreme charge regulation, this example through its thermal fluctuations, the one expected from conductometric da- parameter can even become negative.[28] the deflection can be converted to the in- ta. The latter discrepancy indicates that the The effects of different boundary con- teraction force. Typical force resolution surface charge is partially neutralized by ditions are illustrated in Fig. 3. For a sym- achievable with colloidal probe AFM are strongly bound counter ions. metric system, the CC condition leads to few tens of pN, but the resolution can be When two charged surfaces approach, strongest repulsion, since the charge re- improved by subsequent averaging of indi- they will have the tendency to decrease the mains constant down to contact. The CP vidual force profiles. While the separation magnitude of their surface charge density condition leads to a weaker repulsion, and distance can also be measured with TIRM in order to reduce the overall free energy. now the surfaces become neutral at con- independently,[27] one normally obtains the This decrease may result from the disso- tact. The CR condition is most realistic distance from the contact point between the ciation of surface groups as well as the ion and describes the experimental data down probe and the substrate (i.e. onset of con- adsorption or desorption from the surface. to small separations with a regulation pa- stant compliance). For solid substrates, the This aspect can be considered quantitative- rameter of about 0.3. In any case, charge latter technique yields separation distances ly by considering the appropriate bound- regulation effects are not dramatic in a with an accuracy better than nanometers. ary conditions. For constant charge (CC) symmetric system. With this setup one can measure the conditions, the charge will remain constant In asymmetric systems, however, the force between a colloidal particle and upon approach, while for constant poten- effect of charge regulation can be really flat substrate (asymmetric system, Fig. tial (CP) conditions, the potential will re- important. Such an example is shown in 1a).[14,15,27] Attaching another particle to main constant but the surface charge will Fig. 3b where forces between a neutral la- the substrate, one can measure the inter- be reduced upon approach. The constant tex particle with a diameter of 3.3 µm and action between two colloidal particles regulation (CR) conditions describe the a negatively charge sulfate particle with a (symmetric system, Fig. 1b).[20,21] In this case, the particles have to be centered lat- Fig. 1. Colloidal probe erally prior to the measurement, which atomic force micros- can be achieved with an AFM mounted copy (AFM). Force on an inverted optical microscope. By measurement be- attaching one type of particles to the tip tween (a) particle and and another type to the substrate, the inter- planar substrate, (b) action between two different particles can two similar colloidal be investigated (asymmetric system, Fig. particles, and (c) two 1c).[20] dissimilar colloidal particles. 3. Double Layer Repulsion Fig. 2. Force profiles When particles are highly charged, between two ami- interaction forces are dominated
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