Dipole-Induced water adsorption on surfaces.

Abstract

Water is present on almost any surface exposed to air. Both vapor and liquid water modify and determine the properties of molecules and materials (friction, adhesion, folding, reactivity...). However, there is still an important lack of knowledge about how water interacts with surfaces at the sub-micrometer level. Such interactions will determine the final macroscopic properties of surfaces and compounds. In addition to these facts, water also plays a central role in determining the structural conformation and the properties of biomolecules, such as proteins. During the last decade, much attention has been driven into achieving a deeper understanding in how water interacts with proteins. Nowadays, water is considered, not as the solvent media where proteins are placed, but as a proper part of the protein itself. Many theoretical studies have been performed recently, but it is still necessary to extract more information with direct experiments. Scanning Probe Microscopy (SPM) has opened the door to powerful measures at the nanometer level that allow us to follow processes and detect properties in scales not achieved until recently. Atomic Force Microscopy (AFM) is a member of the SPM family, with multiple operational modes able to sense different surface properties, that turn it into a very versatile tool. During this thesis work, I have studied the interaction of water with several surfaces, using different AFM modes._x000D_ The study began by describing how water affects different crystal surfaces of several amino acids: L-alanine, D-alanine, L-valine, D-valine, DL-Valine and L-leucine were studied by means of AFM imaging using several modes. These amino acids were chosen for their structural simplicity and their importance in the human-body biomolecules. The study revealed the importance that the amino acid dipoles play in their interaction with water. The structural changes on amino acid surfaces due to vapor and liquid water action on them have been also studied. From this study we described a new 2D landscape on the L-alanine (011) surface as a consequence of its interaction with water. Also, the enantiomeric recognition of L- and D-valine has been described in a easy experiment using AFM. The electric field generated by some amino acid crystals has been studied as a possible factor of water freezing (as reported for some amino acids at the macroscopic level). I studied the effect of the natural electric field of several crystals on the water molecules present in the media as a function of relative humidity and temperature._x000D_ The importance of the dipole-dipole interactions in these processes drove me towards ferroelectric materials. In the last part of this thesis work, PZT2080 ferroelectric thin films have been used due to that their dipoles can be oriented by means of AFM in a controlled way. I have used these surfaces to study the influence of their dipoles in the ordering of water. From this study, the optimum experimental conditions to ensure a the polarization in a near a 100% effectiveness of a PZT2080 region (using its PFM phase signal as reference) with a minimum charge injection. KPFM imaging revealed differences of several tens of mV on polarized regions for slight temperature decreasing, in a controlled and reproducible manner. This demonstrates the effectiveness of the polarized regions to order the nearby water molecules when the loss of temperature decreases their thermal energy.

Date of Award	1 Jun 2012
Original language	English
Supervisor	Albert Verdaguer Prats (Director) & Jordi Fraxedas Calduch (Director)