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Definition

Adsorption used involving the binding of molecules to a solid surface. This is a complex phenomenon. Most solid surfaces are not uniform, and have different binding sites. These sites usually have different adsorption energies and surface patterns. Adsorption at different binding sites may be independent of each other, or they may intact with neighboring sites. The binding forces between molecules and solid surfaces can be physical or chemical. Physi-adsorption is the physical attraction between binding sites and molecules. It is usually weak and reversible. In chemi-adsprotion, molecule is strongly attached to the solid binding site. This binding is usually much stronger than physical binding, thus needs a much greater energy to break the binding compared to the energy needed to break a physical binding. The heat of adsorption is different in physil and chemi adsorptions. Chemi-adsorption produces more adsorption heat than physiadsorption. Adsorption can also be distinguished by monolayer or multilayer adsorption. But these two patterns are not exclusive because multilayer often occurs before the completion of monolayer.

The objective of this experiment is to research the adsorption of different proteins on metal oxide adsorbents. The adsorption is affected by several chemical conditions, such as pH values, particle charges and temperatures. In this experiment, four proteins are used. They are bovine serum albumin (BSA), urease, lysozyme and chymotrypsinogen A. BSA and urease are negatively charged in solution. Lysozyme and chymotrypsinogen A are positively charged in solution. By using them, the effect of protein molecule charge on adsorption is revealed. Two metal oxides are used as adsorbents in this experiment. One is ZnO, the other one is TiO2. By using these two adsorbents, the effect of adsorbent charge in solution is revealed. The adsorption experiment is done in different pH environments, for example, pH=5, pH=6.5 and pH=8. By doing this, the effect of pH environments on adsorption is revealed. Also, adsorption is carried out at different temperatures. By comparing the data collected at different adsorption temperatures, the effect of adsorption temperatures is revealed.

Buffer preparation

Buffer solutions are resistant to pH changes, and they stabilize the protein molecule charge. The concentration of buffer solutions was 20mM at all pH values. Three buffer solutions were made at three pH values: pH=5, pH=6.5 and pH=8. The trizma base from Sigma-Aldrich was used to prepare the buffer solutions at pH=8 and pH=6.5. First, 2.4228g of trizma base were weighted using the Mettler Toledo analytical balance and placed into a 1000ml beaker. 1000ml DI water was added to the beaker to make the concentration around 20mM. Next, a magnetic stir bar was placed in the beaker and the solution was stirred for about 4 hours. A piece of parafilm was used to stop any evaporation. The original pH value of the buffer was around 10 to 11. After stirring, pure acetic acid was used to adjust the pH of the solution. A pH meter probe was placed into the solution to monitor the changes in pH. Pure acetic acid was added into the stirred solution drop wise. The acid addition stopped at the pH 7 and 8 otherwise. The solution was stirred for another half an hour to make sure that the solution was uniform. To make a buffered solution of pH=6.5, the addition of acid was ended when the pH was 6.5, other procedures remained the same. To make the buffer of pH=5, piperazine from Sigma- Aldrich was used instead of trizma base. 1.7228g of piperazine were added into the beaker to make the concentration around 20mM. Next, the solution was stirred for 4 hours using the same method mentioned above. After that, pure acetic acid was added into the solution drop wise. The addition was stopped when the pH was 5.

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