How can I prolong the existence of hydroxyl ions(OH-) in ionized water?

Question

And what will happen to the alkalinity of water and the smaller molecule cluster when OH- is gone?

Answer 1

Keep it away from free hydrogens.

Answer 2

water is always ionized to some extent... the pH of the water measures the hydrogen ion concentration and consequently the OH ion concentration. Water that is neutral has a pH of 7 which means that the H+ concentration is 10^-7 mol/l and the OH- concentration is 10^-7.

As the OH- concentration increases, the pH increases- the water becomes more basic. Unless you are doing something physically to the water (like electrolysis) and constantly removing the H+ ions, pure water (no contaminants) will always be near pH7.

Not sure what you mean by "smaller molecule clusters".