-[narrator] Studying adsorption, huh? Got a big test
coming up? Staying up all night studying. Let's see how we can make sure you get
some shut-eye. At first, adsorption looks like it's just
stuff sticking to other stuff. But if you take a closer look, you'll see that
there's more to adsorption than that. Adsorption is the adhesion of particles
on free surfaces. Adsorption happens when atoms, ions, or molecules attach to the
surface of a substance without entering it. They hang out on the surface. An example would be chemicals in a liquid sticking to a solid surface. If we zoom in closely, closer, almost, there. You've probably seen an image like this in your
textbook before, ions, which are chemicals with a charge that are attracted to soil particles. It's a bit more complicated than just stuff sticking onto other stuff. The outer sphere and inner sphere complexes are attaching in different
ways. It looks like some of them can easily be knocked off or exchanged with
other compounds in the solution, while others are holding on tightly. You can clearly see at this view how the hydration sphere surrounds the outer
sphere complexes and not the inner sphere ones. Can you see what the
difference is? Water. We call it hydration. Water is dipolar, which means a water molecule has a positive side and a negative side. An ion bound by outer sphere adsorption is surrounded by a hydration sphere, a
collection of water molecules that are physically attracted to the ion because
of its charge. Hydration spheres can form around both positive and negative ions
because water has a positive and negative end. We see water molecules
gathering around a cation and an anion clearly showing the different ways they
attach based on polarity. Opposites attract just like with magnets. Since water has a positive and a
negative side, it can align with either type of charged ions. This same physical
force attracts the water molecules with the ion in tow
towards the charged solid surface and soils. The water is acting as a buffer
between the ion and the solid mineral surface. The ion sticks to the surface
but only loosely, so it's easy to remove again. This is what happens during ion
exchange. It's like a chocolate-covered pretzel. The chocolate is stuck to the
pretzel, but you can easily chip it off. Although, that seems like a lot of work
to eat a plain pretzel. We say hydration sphere, but the ion doesn't need to be completely surrounded. Even just one water molecule between the ion and the
surface is considered outer sphere adsorption. Outer sphere adsorbed ions
are easy to remove. They can be knocked loose by physical force, reabsorbed back into solution, absorbed by plants, or even leached away and replaced by other ions with a similar charge. Since this adsorption process is entirely physical,
it is called physisorption. Without water though, something different happens. The
ion shares some electrons with the surface. This is called a covalent bond,
ligand exchange, or anion penetration. This means that a brand-new substance
forms at the boundary of the surface. The formation of a strong covalent bond
during adsorption creates a new chemical compound, compound, so this process is called
chemisorption. Monodentate inner sphere complexes attach at one point. Bidentate complexes attach at two points. Polydentate complexes attach
at three or more points. In adsorption, the more dentate links, the stronger the
bond and the harder it is to free the adsorbed ion. And in chemisorption,
you don't even need a charged surface. It's like baking a cake. You start out with flour, eggs, milk, and sugar. But once you bake it, you can't separate the
ingredients. So to review, physisorption is like the chocolate covered pretzel. Chemisorption, which is fusion, is like the chocolate cake. Anyway, this
new substance locks the ion away so it can't be knocked away from the surface
or taken up by plants or bacteria. The rate at which these happen is different
too which is why we have to run so many adsorption experiments. Outer sphere
adsorption is a physical process. It's fast and all it needs to occur are
dipolar molecules like water and a charged surface. Inner sphere adsorption
is a chemical bond so it only happens in more specific circumstances. You've got
this. With the naked eye, adsorption may look like just one process. But when we
get down to the molecular level, we can see it's actually very complex. These two
processes have a lot of implications: Whether fertilizer and nutrients can be
used by plants, what percentage of impurities can be
caught by a water filter, and much more. It's all part of the wonderful world of
adsorption. Oh sorry. Now you can get some well-deserved sleep. (upbeat music)
