Forces between surfaces are a determining factor for the performance of natural as well as synthetic colloidal systems, and play a crucial role in industrial production processes. Measuring these forces is a scientific and experimental challenge and over the years several techniques have been developed to measure the interaction between surfaces directly as a function of their separation distance. Colloidal probe atomic force microscopy (colloidal probe AFM) offers the possibility to study such forces between virtually all kinds of surfaces. Furthermore, the time scale of the measurements can be short enough to monitor relaxation effects and to study the interaction at Brownian-like collision rates. Combining this with the original application of the AFM, namely the imaging of surfaces at nanometer resolution, makes the AFM a versatile instrument in surface science.
In this thesis the forces that play a role in colloidal systems, especially with respect to the role of surface groups and polymer layers are studied using colloidal probe AFM.
A Colloidal Probe (a silica particle) glued to an AFM cantilever.
An introduction to forces acting between (colloidal) surfaces is given in chapter one . In addition, this chapter presents a short overview of the development and various applications of the atomic force microscope, especially with respect to its application as a surface force apparatus. In colloidal probe AFM a micrometer-sized particle (the colloidal probe) is glued to the end of an AFM cantilever and is moved towards and from a flat surface with the use of a piezo element. The deflection of the cantilever is measured as a function of piezo position and reflects the forces acting between the surfaces. The chapter concludes with an overview of the various techniques to directly measure surface forces. A comparison of three of these techniques, i.e ., the surface force apparatus (SFA), colloidal probe AFM, and a relatively new technique called MASIF is made.
In chapter two the experimental ins and outs of the colloidal probe technique are described in detail. The chapter deals with topics such as colloidal probe preparation, cantilever calibration and conversion of the raw data into force-distance curves.
Chapter three presents colloidal probe force measurements on a silica-silica system in aqueous solutions of varying pH and electrolyte concentration. The results are compared to similar measurements by other authors and were found to be in good agreement with these earlier experiments, which confirmed the proper working of our surface force technique. The experimental data were fitted to the DLVO (Derjaguin, Landau, Verwey and Overbeek) theory. No indication whatsoever was found for Van der Waals interaction, which is in itself surprising but is in line with what is generally reported in literature. Most probably the Van der Waals interaction is obscured by non-DLVO short-range interactions, in particular hydration forces, and by surface roughness effects.
In the same chapter the interaction between gold-coated surfaces as a function of pH is described. For comparison, streaming potential measurements were performed as well. The zeta-potentials thus obtained for the gold-coated surfaces are in good agreement with the surface potentials derived from the gold-gold force measurements through Poisson-Boltzmann fits. As for the silica-silica systems, we found no evidence for a contribution of Van der Waals forces to the interaction. Of course, also in the gold-gold system the Van der Waals interactions may be partly hidden due to surface roughness or the presence of hydration layers. However, because of the high literature value for the Hamaker constant of gold, a significant contribution of the Van der Waals interaction was expected at distances up to 10 - 20 nm (!). The only possible conclusion is that the high Hamaker constant for bulk gold is not applicable for the systems studied, but the reason is not clear at all.
Finally, we studied the interaction between silica and gold surfaces. Overall, the results are in agreement with expectation. All experimental force curves are well in between the calculated Poisson-Boltzmann limits for two surfaces maintaining either constant charge or constant potential. In the case of dissimilar surfaces it is not possible to determine the potential of one of the surfaces from the interaction curves without knowledge of the potential of the other surface and of the charge regulation mechanisms. Depending on the latter, the interaction on approach between surfaces of opposite charge sign may change from attraction into repulsion, or repulsion between surfaces of the same charge sign may change into attraction. Indications of such phenomena was found for the gold-silica system around the i.e.p. of the gold surface, where the ratio between the surface charge densities is the most extreme.
In chapter four interaction forces are described between polymer-covered surfaces for different polymer chain lengths. The polymer used was poly(ethylene oxide) (PEO). The interaction on approach is dominated by electrostatic interaction. On separating the surfaces, however, a strong adhesion is observed, which is attributed to bridging. The adhesion shows a strong dependence on the chain length of the polymer. A linear relationship between the adhesion force and the surface coverage ( i.e ., the adsorbed amount in mass per unit area) is found. However, adhesion occurs only for chain lengths above a certain threshold value. In order for this bridging to occur the surfaces have to be pressed together to some extent. At some pH values electrostatic repulsion inhibits this bridging and no adhesion is found. In these cases bridging can be induced by increasing the electrolyte concentration or increasing the load-force.
The topic of chapter five is interactions between acid- and base-functionalised surfaces. Silica and gold-coated silica surfaces were modified with self-assembled monolayers with amine terminal groups and carboxylic acid terminal groups, respectively. Especially for the NH 2 modified silica surfaces, we found that variations in the pretreatment of the surface results into differences in the density of functional surface groups. The interaction upon approach between the different combinations of surface layers can be explained from electrostatics, assuming that for the NH 2 -NH 2 and COOH-COOH combinations the surface layers on the colloidal probe and the flat surface are not identical (due to differences in pretreatment of probe and flat surface). On approach the NH 2 -NH 2 system and the COOH-COOH system show the same trends: repulsive when the surface layers carry a large charge, but as the pH changes in the direction where more surface groups become uncharged the repulsion changes into an attraction. On retraction all combinations of modified surfaces show a pH dependent adhesion, the strongest between NH 2 and COOH surfaces. This is attributed to acid-base interaction (between -COO -and -NH 3+) and hydrogen bonding (between -NH 2 and -NH 3+and between -COOH and -COOH). As compared to literature data, the adhesion forces are low. Probably, the roughness of the surfaces, which reduces the real physical contact area, is the most important cause for this weak adhesion. Surface roughness may also lead to the large influence of the ionic strength on the adhesion force since a part of the adhesion force originates from electrostatic interaction especially just outside the actual contact area.
|Qualification||Doctor of Philosophy|
|Award date||22 Jan 2001|
|Place of Publication||S.l.|
|Publication status||Published - 2001|