 All right, so let's talk about a topic that will bring in lots of the different ideas that we've talked about in physical chemistry, and that's this concept of adsorption, which sounds perhaps as a familiar word, but perhaps as just something you're likely to get confused with a very similar word. There's a difference between absorption and adsorption. In everyday English, absorption, something being absorbent, is a more common term. That means typically a liquid will be absorbed into the interior of something else, so it has an ab in front of it. Something drawn within another thing, adsorption, is when something sticks to the outside of something else. For example, if I use a sponge or a paper towel to soak up water, that's absorption. Adsorption would be, for example, if humidity condenses on the surface of this piece of glass that's right in front of me, that would be adsorption. It's not being drawn within the surface of the glass, it's sticking to the outside of the glass. So this concept of adsorption is what we're talking about. I can draw a cartoon of what I mean. So I've got some sort of surface, perhaps this glass lightboard. I've got some molecules in some phase near that surface, and I've got some molecules that have adsorbed or stuck onto the surface of the substrate. So just so that I can name the species or the phases, the species up that have not been adsorbed onto the surface are called the adsorbate species. What they're adsorbing onto is often called the substrate, or perhaps the absorbent. So I'll typically use the word substrate to refer to the surface onto which the adsorbate is adsorbing. So we're typically going to think of this as an equilibrium process. I've got an equilibrium, I've got molecules on the surface. At the moment that might desorb or come off the surface at some later time, I've got molecules up here in this other phase that are in the other phase right now, but may adsorb onto the surface. So when we're in equilibrium, I've got a steady amount of adsorbate molecules in this upper phase and adsorbed onto the surface. So the first question perhaps is who cares? Why do we need to know about adsorption? What are the applications of it? It's useful as it turns out in a lot of different fields of chemistry and engineering. One of those is catalysis, in particular heterogeneous catalysis. When I have a catalyst particle, heterogeneous catalysis, I might have a reaction taking place in the solution phase, but I have solid particles, solid catalyst particles at the surface of which the catalysis takes place. So the reactants have to adsorb onto the surface of the catalyst particle. The reaction takes place and then they desorb off of the catalyst particle. So the rate at which adsorption takes place or the amount of adsorbed species can be important in determining how well catalysis is going to work. It's also typically very important for all sorts of protein ligand binding, which might be, for example, another form of catalysis. If enzymatic catalysis is taking place, if an enzyme is catalyzing a reaction, then some ligand molecule is going to bind to the enzyme. So in this case, the substrate is some large protein molecule, some enzyme. The ligand binds to the active site of the protein, catalyzed chemical reaction takes place and then it desorbs or unbinds from the protein later on. It's also an essential part of understanding how chromatography works in analytical chemistry. So if you have a chromatographic separation taking place, molecules flow through a mobile phase and they occasionally bind or adsorb onto the stationary phase, they'll stay there for a while, then they'll desorb and move further down in the mobile phase. So that chromatographic separation occurs because some molecules have greater or lesser affinity for the stationary phase, so that's an adsorption process. There's also, there's a long list of these topics. We can talk about deposition, either gas phase molecules being deposited onto a surface or solution phase molecules precipitating or depositing onto a surface to coat a surface. These processes involving membranes, like for example, osmosis or filtration, deposition onto those membranes, in this case is a bad thing. It's not something that we want to happen, it's a bad thing. You can end up fouling or blocking that membrane if adsorption takes place and blocks the pores in that membrane. So there's quite a few important practical applications of this topic of adsorption. A few other vocabulary words that I'll mention, some terminology related to adsorption. It's often important to distinguish between two types of adsorption. We can talk about physisorption versus chemisorption and as you might be able to guess from the name, the difference between those is whether the interactions that give rise to the adsorption are physical interactions or chemical interactions. For example, with the picture I've drawn here, where some molecule sticks to the surface perhaps with Van der Waals interactions or just a polar attraction between the surface and the molecule and can desorbe without any chemical change to the molecule, that would be an example of physisorption. If I were to show you an example of chemisorption, that might be some other molecule, my triangle shaped molecule will bind to the surface and when it binds to the surface it chemically binds. If I have a valent bond to the surface, when it binds to the surface, then there's a chemical change taking place and that would be chemisorption. Even more drastic case of chemisorption would be if I have a molecule like water that when it binds to the surface chemically dissociates and binds as a hydrogen binding at this site and a OH binding at this site, that's again a chemical change that takes place on the surface and that can drastically change the way that we think about the adsorption, so that would be a case of chemisorption as well. One more way of distinguishing different types of adsorption that I'll mention is what exactly this phase is of the adsorbate species when it's not adsorbed to the surface and in what phase we find the substrate. So again, because there's so many different applications of this, there's many different phases in which adsorption can happen. We can be talking about a gas adsorbing onto a solid, for example, in the example I gave you of water adsorbing onto this glass surface, deposition onto a solid surface, that's often gas adsorbing onto a solid. But this upper phase doesn't have to be the gas phase. This can be a solution phase. These isolated molecules that I've drawn here might be solute species in a solution that adsorb onto a solid surface, so it might be some solute species in a solution phase that adsorb onto some solid surface that might be, for example, what's going on in a chromatography application or in a heterogeneous catalysis application. In fact, we can even have dissolved species in solution, dissolving, I'm sorry, adsorbing onto other solute species within the same solution. That's probably more like what's going on in most cases of enzymatic catalysis, where the substrate to which the ligand binds is itself a solute in an aqueous solution. So we've got a large protein in solution, small ligand molecule binds to that enzyme, and then when it desorbs, it goes back into solution. So that would be a case where we're talking about solution phase solute binding onto a different solute in the same solution. So there's a big variety of different phases that both the adsorbate and the substrate can be in. The last thing I'll mention in this general tour of the features of adsorption is a little bit about the thermodynamics of adsorption. So in general, for this adsorption process, specifically if I'm talking about let's say an adsorbate species unbound becoming bound to the surface. So the adsorption process is the process I'm talking about here. So when a molecule adsorbs and becomes attached onto the surface, typically the energy change for that reaction, whether it's energy or enthalpy that we prefer to think about, that's going to be a negative number. The enthalpy is going to be lower in the adsorbed state than it is in the dissociated state. That energy provides the driving force for this reaction. There's an attractive interaction between the adsorbate and the substrate. Otherwise, the adsorption wouldn't occur. Entropy, you might stop and think yourself about what the sign of the entropy is going to be. In the dissociated, desorbed state, these molecules, whether they're in the gas phase or solution phase, they have a lot of configurational entropy. They can move throughout the gas, they can move throughout the solution. There's a lot of different positions they can be, so they have relatively high entropy. When they're bound to the surface, however, there's many fewer states that they can occupy. There's many fewer positions they can occupy. So I've confined them quite a bit when they're adsorbed onto the surface. So I've reduced their entropy when they're adsorbed. So when the molecule binds, adsorbs onto the surface, its entropy goes down. What that means, in turn, is if we want to ask about the free energy of that reaction. If the enthalpy is negative and the entropy is also negative, negative number minus another negative number, what's the sign of that going to be? It depends. Maybe this negative number dominates, the enthalpy dominates the T delta S term and the overall, this might be a negative number or if the entropy dominates, that means the molecules really prefer to be up in the desorbed state because the entropy term is more important than perhaps a weak binding enthalpy and that will mean the delta G of adsorption is a positive number. So the other thing it means is that I can switch the delta G of this adsorption process based on the temperature. If I increase the temperature, I can cause any desorption process to be unfavorable with a positive delta G. If I decrease the temperature far enough, I can cause any adsorption process to be favorable with a negative delta G of adsorption. So what that means is rather than thinking about this as purely a one-way chemical reaction, since there's a delta G that's going to change sign, we're going to end up with an equilibrium between desorbed and adsorbed species and that equilibrium can shift one way or the other depending on the temperature and other conditions of the system that we prepare. So this has introduced a lot of terminology in this general idea of absorption. The next step is going to be getting a little more quantitative to see what we can understand about the thermodynamics and the equilibrium of how adsorption proceeds.