Instrumentation and Operation

The AFM investigations were performed using a Park Scientific Autoprobe CP instrument with the Multitask Head (Institute of Physical and Theoretical Chemistry, University of Regensburg, Regensburg, Germany) and a Digital Instrument Nanoscope MultiMode instrument (Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, Australia). The devices are represented in Figure 3.10.

A) B)

Figure 3.10 – AFMs used in this work: A) Autoprobe CP instrument. Illustrated together with the optical microscope and video camera used for control. Reproduced from www.veeco.com. B) DI Nanoscope III instrument. Reproduced from www.eng.yale.edu/environmental/ facilities.html

Figure 3.11 – Cantilever chip with 4 cantilevers (left) and standard V-shaped cantilever (right)

The cantilevers from ThermoMicroscopes, Sunnyvale, CA were used on the Autoprobe CP instrument. The length of them was 180 µm and the leg width 25 or 38 µm (C-Ultralevers type A and B, respectively). Nanoscope III instrument used standard silicon nitride V-shaped cantilevers of length 200 µm with a leg width 40 µm (long, fat) (Digital Instruments, Santa Barbara, CA). The tip diameter, according to the

manufacturer’s data, did not exceed 10 nm, and the spring constant was in all cases below 0.6 N•m^-1. Cantilever chips and a scheme of a cantilever are presented in Figure 3.11.

New cantilevers have been used for every measurement performed on the Digital Instruments Nanoscope III. Before mounting, the cantilevers underwent plasma

treatment identical to the substrate (described above). When working on the Autoprobe CP instrument, each cantilever was used for more than one measurement. Between the measurements, cantilevers mounted on the cartridge, were soaked in a water-isopropyl alcohol mixture, rinsed with deionized water and dried in air. The cantilever integrity and condition was checked before every measurement using a 1µm calibration grating and adjusting feedback software parameters.

A fluid cell constructed and made at the Mechanical Shop of the Faculty of Chemistry and Pharmacy of the University of Regensburg was used during studies on the Park Scientific Autoprobe CP Microscope. Between the measurements, the cell was cleaned using a water-isopropyl alcohol mixture. The standard Contact Mode Fluid Cell was used while working with the Digital Instruments Nanoscope. Cleaning of this cell was performed using Millipore water and redistilled ethyl alcohol followed by drying with a stream of nitrogen gas. The filling of the standard Contact Mode Fluid Cell (Digital Instruments device) was performed by sample injection after mounting the cell

and before cantilever approach. The filling of the custom made fluid cell (Autoprobe CP) was made by adding 2.2 ml of sample into the opened cell before lowering the AFM head.

3.3.3.1 Software

The post-measurement processing of images and curves was performed using ProScan Version 1.6 Image Processing software of Autoprobe CP instrument,

NanoScope 4.42 GUI software, and Nanotec WSxM 1.2 software as well as Microcal Origin 4.1 and Microcal Origin 7G software.

3.3.3.2 Imaging

The soft-imaging method of Manne et al. and Senden et al. [114, 21, 28] was used.

The key to this method is fine control of the imaging force in the repulsive regime of the tip-sample interaction, enabling the adsorbed layer to be imaged without damage.

Adsorbed aggregates are generally only visible over a narrow range of applied force (<1 nN), with the substrate imaged at higher forces. Generally, the applied force on both instruments can be controlled directly using the operation software of the AFM. In all cases it was below 0.9 nN.

3.3.3.3 “Scratching”

For investigation of the properties of the adsorbed layer of the polymer-surfactant mixtures a special treating of the layer with the cantilever was applied. First, a scan image (typically, 10*10 µm² or 5*5 µm²) of the adsorbed mixture was acquired. Then, a smaller area (typically 1*1 µm²) in the middle of the field of vision was scanned by pushing the cantilever very hard, at the highest scan rate (60 Hz). After making this, the scanning was repeated at previous settings and on a larger area. This method is

downwards called “scratching”, since the cantilever may move the layer or its parts aside and expose the substrate surface. The method allows investigation of adhesion pattern of the adsorbed layer or clusters.

3.3.3.4 Acquirement and evaluation of force-distance curves

The force-distance curves were acquired on every sample, with an emphasis to visualize the difference between areas with different structures’ pattern. When

“scratching” had been applied, some curves were acquired inside and outside the scratched area. This allows making conclusions about the presence or absence of the polymer and the surfactant on the investigated surface, about the rigidity of the layer, its elasticity and hydration. The force-distance curves are presented without any further processing except “shifting” the curves by means of the Origin 7.0 software package in order to reflect precisely the zero lines and zero positions (elimination of feedback distortions).

4CHAPTER 4. RESULTS AND DISCUSSION

The results of our investigations are presented in this Chapter. First, the properties of the SDS / JR400 mixtures of different compositions in bulk solution are described: the ternary phase diagram established during the solubility studies is discussed and a description of the selection procedure of mixtures (samples) for further research is given. Then the results of DLS measurements performed on selected samples are represented. The second part of the chapter shows the results of imaging and

characterization of adsorbed polymer-surfactant layers at solid-liquid interfaces with the help of AFM. This part is divided into two sections: the first compares the structure of adsorbed layers at different surfaces and the properties of the corresponding samples in bulk solution. The second section deals with the results of the AFM measurements: the results of two series of “washing-off” experiments are presented consisting of

subsequent altering of the polymer-surfactant mixture composition with simultaneous monitoring the properties of the adsorbed layer.