|Ph.D Student||Zohar Ohad|
|Subject||Some aspects of the Interaction between Charged Surfaces|
and Ions in Spatially Confined Geometrics
|Department||Department of Physics||Supervisor||Professor Uri Sivan|
|Full Thesis text|
Interactions between charged particles in solution play a fundamental role in many branches of science and technology, including colloidal science, polymer science, molecular biology, biotechnology etc. A long standing puzzle in this field is the attractive interaction between similar particles in solution in the presence of multivalent ions, also known as "like-charge attraction". Examples include DNA condensation by Z=3 and 4 cations and aggregation of charged fibers, proteins, polymers, and colloids.
In the first part of the research we set out to measure, for the first time, the attractive interaction as a function of separation between two similar particles in the presence of multivalent cations. Using an Atomic Force Microscope we have been able to accurately measure force vs. distance curves between two silica surfaces in the presence of various salts. We have found that for pure NaCl the interaction was always repulsive. Upon addition of cobalt hexamine ions, Co(NH3)6 , the repulsion was gradually suppressed and a pronounced attraction developed at short distances. Higher concentrations of cobalt hexamine turned the attraction back into repulsion. The analysis revealed an exponential dependence of the force upon separation with characteristic lengths considerably shorter than the corresponding calculated Debye-Hückel lengths. Several previously proposed models for like-charge attraction were compared with the experimental data, but none could account for the measured force curves. As such, the measured force curves facilitated indispensable testing of models for the widely observed attraction between similarly charged objects in the presence of multivalent ions.
In the second part of the research, we investigated electrostatic control of ion transport through nanopores. Electrostatic control of ion transport has many potential usages, including smart membranes, water purification systems, miniaturized chemical analysis systems (also known as "lab-on-a-chip"), bio-technological systems etc. Nanopores with controlled surface potential provide control over the relative ion densities near the surface and thus also over their ion-conductivities. Nanopores were constructed by drilling holes in a specially fabricated membrane using a focused ion beam. The membrane consisted of a conductive layer (gold) sandwiched between two insulating layers (silicon nitride). The pores formed this way were reduced in diameter by electrochemical gold deposition from solution, to a typical diameter of 20 nm. Impedance spectroscopy measurements conducted on single nanopores of diameters ranging between 4 -70 nm revealed a wealth of information, including the ionic resistance of the pore, the electrical resistance and capacitance of the membrane and of the silicon-solution interface.