 Of course, usually you don't have to get it exactly. Let's try it. Without writing anything down, try to figure out what the square root of 33 is. Six, six minus a fourth, 5.75, minus 1, 200, 5.745, still two days, 5.7, this could be embarrassing, 5.7446. Somebody worked it out. I think that's still too big. The interesting thing, of course, is that after you get to 100, they just repeat. So you only need to know the square root's up to 100 and the world is your oyster. And also, you can then magically multiply huge numbers and amaze people because you can say, gee, what's 574 times 574, and you can just say, oh, it's about 33,000. You've already got them. Today, you just scale up, scale down so you can multiply things so forth and so on. I never used to be able to do that. I just practiced it. While I was out running, I'd try to pick a number and I'd try to get the square root. I'd get home and I'd be way off because I'd made a mistake keeping everything straight. And then after a while, I made fewer mistakes and then after a while, I didn't make very many at all and then it seemed pretty easy and that's the way things go. If you practice, you'll get better. If you don't practice, you'll stay the same. If you do something like watch TV, you'll get dumber. The reason why a lot of people can't follow verbal instructions, which are very important to be able to follow. Hey, small guy, do this, do that, so forth, buy. You can't then say to the boss, what did you say? Wait a minute, how do I do that? That's why they hired you. It's because you've been watching TV and if you think there's any content on TV, turn your back to the TV and just listen to the garbage and you'll realize you're training yourself not to listen. So if you've got time to watch TV, you aren't even close to studying hard enough. TV should be a zero for this whole quarter, even if it's something interesting, not as interesting as this course. Okay, let's get back. I do have one announcement and that is rather embarrassingly, I got the date wrong so I can calculate the square root of 33 but I can't get the super date right. And so to make up for that, I put the homework back on for colligative properties. Do this Friday, if you want to try it again, get a higher score than you got last time, great. You want to stand pat, take a week off, your choice, okay? But practice will make perfect and the next exam is going to have those freezing point and osmotic pressure, things on. And it's also going to have a graph, draw, need the distribution of speeds for xenon and helium. And I got all kinds of drawings there. That was supposed to be just a simple pen point. I got things that didn't start at zero, things that started up here and then went down. I got things that went off to infinity like the gas had infinite energy. If your line starts and there's a lot of atoms at zero velocity, that's called a solid. Think what the graph means. There's no way there are any atoms moving at zero in a gas. They can't, they just get knocked around. You've got a lot of atoms at zero, that's a solid, pretty much. Okay, let's, good, this is working. Osmotic pressure, colloids. And some, a little practical chemistry today. What do we talk about? Laundry detergent and shampoo. And boy, can you save a lot of money if you know a little chemistry. Oh, osmotic pressure. This is from last time. If we have a semi-permeable membrane, and of course the trick is, how do you fabricate a semi-permeable membrane that lets in some things and doesn't let in other things? That's the trick. But once you've got it, and you can sometimes use a cell membrane as a model, then you will find that because the Gibbs function of the side with a solution is lower, the pressure will build up. There will be a pressure difference. Imagine I've got two containers of water and they're connected by a hose and the water can run and they're level. And then I lower one of them. Well, it starts filling with water, right, from this side because that's what naturally happens. And that you can understand by gravitational potential energy. But now if I put something here that blocks things, and I move it, then the water presses on that thing and presses against it with a pressure. Okay? And likewise, if I put in something in solution, it lowers something sort of like this. It's a little harder to imagine, but it lowers the tendency of those solvent molecules who want to get out of there. And so the pure solvent will tend to flow into the side where you have the solution. And we can measure the pressure difference because pressure is dead easy to measure and very sensitive. And so osmotic pressure measurements are extremely good for measuring ion concentrations and also for measuring macromolecular concentrations. Because if I've got a big guy, then the big guy can't get through the membrane. The membrane could be real cheap to make because the big guy can't get through. So it's set up to do osmotic pressure. And if I have a set up like this, fresh water flows into the salty water side until the G functions equalize. And likewise, that will happen in U. You can see some kind of candy that's got sugar substitutes, sugar alcohols. And it says on the bag or on the whatever it is, it says overconsumption may lead to a laxative effect, right? That laxative effect is osmotic pressure. So you eat this stuff and your stomach can't metabolize it very quickly because the whole idea is it's not sugar. And so it goes through your system and into your large intestine and it changes the osmolarity in there. So it lowers that and then water rushes in from your plasma and the pressure is huge. And that's why you better be able to sprint because you aren't going to make it otherwise. And it's a very unnatural feeling, very unpleasant. And so they warn you about it. But it's harmless. It's a harmless effect. Could be very embarrassing but nothing worse than that. Okay. Here's a mock-up from a book on biochemistry. We've got a solution and then we've got some membrane here that we've stretched over the bottom. And we throw in some solute into this test tube that are initially equal. And if we do nothing, if we do nothing, what we find is that the solution creeps up to a certain height. And the height depends on how much stuff we dissolve in there. It depends on how many moles of particles are in there. And that's important because if we measure it in grams and we know how many moles we know grams per mole, we might guess what it is. If instead of letting it creep up like that, we press on it so that it stays the same, we'll find that we have to press with a certain pressure to make it stay right where it's supposed to be and we can measure that pressure. And that pressure is called the osmotic pressure. We can measure it easily and it's a very good way to tell. And we use a Greek symbol, pi, which is sort of like pressure but fancier to indicate the osmotic pressure. And so the trick is we've got to find this semipermeable membrane that lets through the solvent but not the solute. And then if we can find that, then we can quantify the reduction of the G function by measuring this pressure. The solvent will always want to reside with the solute in solution and so it'll tend to build up on that side. As I said, if we allow the pressure in the compartment to increase, then eventually we'll counteract the tendency. The solvent's trying to push in but we're pushing back with the same amount of pressure so it can't come in. That's the idea and it works especially well with big solutes because big solutes are easy to fabricate a membrane that they can't get through. That's pretty straightforward. It can be much harder with something else and in that case it's not a good measure. Now, cellular membranes are semipermeable. They let in certain kinds of ions and keep out certain other kinds of ions and it's all carefully controlled. Usually the interior of a cell has much higher potassium and much lower sodium than the outside. But they have similar osmolarity. They have similar numbers of particles so that the cell membrane doesn't have a ton of pressure put on it one way or another. But of course, if you change the osmolarity, you can change the osmotic pressure experienced by the cell. And this is not a very good thing to do in general. Here is sequence here. I've got a red blood cell. If I have the correct amount of ions inside and outside because I'm not gulping down salt and I'm not gulping down water, I'm not walking around with a bottle of water drinking it all the time when I'm not thirsty, then my red blood cells will look like this shape and that's the good shape that they should have so they can squeeze through capillaries and small spaces and keep your blood flowing. If I go wild on salt, then my plasma has too much salt so it has a lower Gibbs function and the water rushes out of the cell and the cell crinkles up like a prune, the cremated form. And so it changes shape and then it gets these spikes on it and things with spikes on them can jam and if the cells jam in a capillary then the other cells on the other side remember these are delivering oxygen. Can't get any oxygen and they may die. And so you could have micro strokes going on all the time. Okay, okay, another neuron gone. You may not even notice. How are you going to notice one neuron disappearing? Probably not, but if you keep doing it you'll notice. On the other hand, if you go wild with water, you can't actually kill yourself with water. It's usually only people who are mentally ill who manage to do that, but they have an idea in their head and ideas are extremely powerful. That's why you have to watch them like a hawk. The idea is I'm purifying myself with all this pure rain water, blah, blah, blah and I'm drinking all the time even though I'm not very thirsty and I'm making my kidneys work like crazy against all this stuff and I'm also having my red blood cells puff up like these big balloons. And finally, if you keep doing it, they start popping like balloons. You don't want that, I assure you. Okay, when that starts happening, you're a goner. You can get to that point. These membranes are pretty tough, pretty tough stuff, but not infinitely tough. The some marathoners have died because they drink. All the time. So this is pretty interesting. Exercise associated hyponatremia, not enough sodium in your blood. Drinking oneself to death. This was April 9th, 2012, leave a comment. A 22-year-old male fitness instructor finished the 2007 London Marathon, collapsed, and despite immediate emergency medical care, died. His serum sodium was markedly low and the cause of death was found to be hyponatremia. Subsequent, this doesn't happen often, but just to let you know you can't overdo anything. Subsequent investigation established that he had been concerned about becoming dehydrated and that therefore drunk a large volume of fluid. This guy's 22, he's fit enough to run a marathon. That makes him in pretty good health. He's a fitness instructor. Okay, but he made a bad mistake. He had an idea in his head. He wasn't thirsty when he was drinking that, but he thought that he might perform better. And then they looked at the Boston Marathon finishers and found that 13 and 12% respectively of these two races had hyponatremia by the time they finished because they were drinking too much during the race. And it's interesting because this never used to be a problem. So when Mike Gratton won the London Marathon in 1983, he drank nothing. Usually the fastest guys don't drink anything because when you're running that hard, it's even hard to swallow in between the huge gulps of air and you might choke. And anyway, you don't want your stomach going slush, slush, slush, while you're trying to win 10,000 bucks or whatever it is today. And so they tend to, plus they finish more quickly. But they aren't out there six hours pounding their knees into jelly either. They're just like little birds sticking along. Very light, you hardly hear them. But then in 1975, the American College of Sports Medicine published a position statement advocating regular fluid intake during endurance events, suggesting this would reduce the risk of heat stroke. And then people started following this and whether they were thirsty or not, they started drinking like crazy. And that's when this problem appeared. So if you aren't thirsty, don't drink anything. Wait till you're thirsty. Okay, you'll get a signal that you need to drink. There's no danger, believe me. Okay, the danger is thinking, well, I don't need a signal for anything. And so I'm going to just decide what to do. Okay, okay, now I'm going to derive the osmotic pressure. Don't freak out because I won't require you to be able to derive it. But I don't like the way the book at this level pulls equations out of a half. Because if you pull an equation out of a half, it doesn't make much sense and it can be quite hard to remember it. You will have to remember it or write it down so that you've got it for the next exam. But it's easier if you see where it comes from. At least you can see the rationale. So we're going to call the solvent A and the solute B. And we can calculate when the solvent will have the same Gibbs function, so when things will be balanced on both sides of the membrane. We lower G by putting in some particles and then the pressure raises G back to the same level as the solvent. So I'll show you the calculation, but you do not need to be able to do it or do this whole derivation. It's so beautiful to write partial derivatives. You know why? When you write the funny D, it just makes you feel so much smarter instantaneously. And that's already the reason to keep taking math so that you can actually have a vantage point to look down on reality and understand things clearly. Mathematics is the clearest way to understand what's going on. That's why we invented it. But it is a foreign language. And like any foreign language, it can be hard to learn. The reason why it's harder than most natural languages is because it's so compressed, you can put so much information into a little equation that it's hard to unpack it and really say in words what it means. But in fact, you'll learn in physical chemistry, if you take it, that for any change, you can think of this as a change in the height of something, in G, it's given by a small change in pressure times the molar volume minus the molar entropy times the small change in temperature. Those are the two things that come in. And that means if I divide both sides by Dp and then I say Dt is 0, which I do by putting this little funny thing T out here, if Dt is 0 and I divided both sides by P, then I have Dg over Dp is equal to the molar volume. And I have to be mathematically rigorous. I have to use the funny D when I start saying the derivative is in this direction. In a graph, there is no direction. There's just left and right and it's clear what the derivative is. But in two dimensions, the derivative one way can be different. So if I'm walking on a trail, then I can be walking this way and it can be pretty flat. But if I walk that way, it can be very steep. So when I say I want to take the slope of the hill, I have to say if I'm taking the slope north and south or east and west or in some other funny direction. And here I'm taking the slope in whatever direction T is constant. That's just what the notation means. And we have an equation. The equation is that G for the pure liquid on one side at pressure P is equal to G of the other side in a solution with mole fraction XA because I've got some solute. And with an additional pressure, P plus pi, big pi for the osmotic pressure. And then I have to figure out what on earth this means. But this is the principle is that the two sides should be balanced, the G function. Well, I can take this side and I can expand it out. And again, you'll learn, you don't know this now, that the G function of a solution of mole fraction XA is equal to the G function under the same conditions of the pure liquid plus RT log XA. And since XA is between 0 and 1, the log is always negative. And that means that the G function for a solution is always lower compared to the G function of a pure liquid. That's just exactly how much it's lower. We can use the dependence of G on pressure. Now we take our liquid pure, we have the RT log XA. Now we've got P plus pi, we want to get it to P so we can compare with this. And this is equal to the G function at the initial pressure plus the integral from P to P plus pi of the molar volume. But in the very first few lectures, we decided that the molar volume of a liquid doesn't change much with pressure and pi is small. It's not lifting a car or anything like that, usually. So we're going to assume that the volume is of the solvent, the molar volume is not a function of pressure. And then you learn from calculus, if something is a constant, you can pull it outside the integral. So this is about equal to this plus pull this thing out and do the integral of Dp. Well, the integral of Dp is just P. So I take P plus pi minus P and I get that this thing is equal to the molar Gibbs function of a pure liquid plus pi V. Now this pi V starts looking suspiciously like an ideal gas kind of equation and it is actually related. So we put this together and what we said was that the G function of the solution at mole fraction XA of solvent and higher pressure is equal to the G function of the pure liquid on the other side. But we know what this thing is. This thing is in fact equal to this plus this plus that. Therefore, this plus that must be equal to zero and that gives us a relationship between the osmotic pressure, the molar volume of the solvent and the mole fraction of the solvent and the temperature and the equation and look, we get even R. It's kind of nice. The gas constant comes in minus RT log XA is equal to pi times the molar volume of the solvent. But we don't really want to focus on the mole fraction of the solvent. We want to focus on the mole fraction of the solute because that's what we shoved in there. And so we have to add one other thing. The mole fraction of solute and solvent has to be one. We substitute for the log of XA, the log of 1 minus XB, and as long as XB is small, any book on math will tell you that that's about equal to minus XB. So we can substitute for the log of XA minus XB, but we've got two minus signs so that's going to simplify things quite a bit if we do that. And so we can write this simplified equation. RT times the mole fraction XB is equal to pi V. Now we have a relationship between the mole fraction of solute and the osmotic pressure. And you notice it depends on the kind of solvent here because it depends on the molar volume of the solvent. If we've got a dilute solution, then the mole fraction XB, which is strictly speaking the number of moles of B over A plus B, we'll just assume that B is small so A plus B is pretty much A. And therefore the mole fraction of XB is pretty much the moles of solute divided by the moles of solvent. And the number of moles of solvent over the molar volume is just V, the volume of the solvent. And this gives us a kind of ideal gas equation called the van Poff equation. Pi V is equal to NB RT. We usually don't leave it like that. We usually isolate the osmotic pressure and we can make one more simplification to do that. The number of moles NB of solute divided by the molar volume of A, the solvent, is about equal to the number of moles of B divided by the volume of the solution because the solution doesn't change much in volume when we add a little bit of B. And this, the number of moles per liter of solution, if you like, is just the concentration of B. And usually we write that. There are two ways to write it. They're the same. The osmotic pressure is equal to the molarity of the solute times RT, or we just write C times RT, where C is the concentration of the solute. And the key is it's the number of particles. If you dissolve something that falls apart and gives a lot of particles, the osmotic pressure will be higher because the effective concentration will be higher. Now, if we have a semipermeable membrane and we have ocean water on one side and fresh water on the other side, there'll be incredible pressure. The fresh water will just try to push in there like crazy and we'll need to just put a lot of force on there to keep the fresh water from rushing in. But if we've got enough muscle, we can put even more force than that and push fresh water out. That's how we desalinate water by reverse osmosis. We just outmuscle the osmotic pressure and we force the thing to go backwards instead. If we're going to do that, we might want to know how much work we're going to have to do because if we've got to do work to do something like this, it's going to cost money because we're going to need a compressor and it's going to have to be pressurizing. And then how many liters of fresh water do we need to make? It could be a lot, so we might need a lot of muscle. In fact, I think up near Davis, they're proposing to put in two nuclear reactors to supply the muscle to desalinate water because by irrigating all the fields in the San Joaquin Valley over time, you start building up more and more and more salt, just the residual salt in the water. You keep irrigating like crazy. And so to sweep all that stuff out, they really need some good water and they need a lot of it. So they're proposing to put in two nuclear power plants. Well, they won't make any power. What they'll do is they'll desalinate water. Okay, they'll squeeze all the sodium out and then they'll regenerate the fields. If you don't do this from time to time, what happens is your crop yield keeps going down and down and down. Plants don't like salt. They don't have any salt in them, or at least not much. Plants have a lot of potassium, but they don't have sodium. You never see a fertilizer that says, lots of sodium, right? I think that's what the Romans did, right? They sowed salt, so that in the field, that means not only do we conquer you now, but we make sure you starve in the future. Because you can't grow anything, so try that on for size, and then we take off. Not a very popular move, but that was war. So the way this works then is we produce a lower entropy product. We're actually unmixing salt from water and we're gonna have to do work, but if we're willing to do the work, then we can produce what we want. Ocean water, recall, is about 3.5 weight percent salt, so about 35 grams per kilogram, or 600 millimolar, and human blood, recall, is about 150 millimolar salt. This process is gonna be energy intensive. You'll see why we need the two nuclear power plants, or reverse osmosis plants. They won't be generating any electricity, because of the work we have to do against the concentration gradient. However, we may need to do this, and here's why. The scientists in Crow Hall tell me that they're predicting far less snow in the Sierras. That means in the summer we'll run out of water. If you run out of water, it's suddenly serious. It's like running out of air. As long as you have enough air, there's no worry. The second you run out, it's a big worry. Instantaneous, and so you wanna guard against that. So could we get all of our water needs from the ocean? There's lots of water in there, you just can't drink it, right? So could we have just a massive series of reverse osmosis plants save the day for us? Allowing those 10 minute showers. Oh, the first thing is you stop doing the 10 minute showers. You're gonna be showering like they do in the Navy. Whoop, little water on, whoop, soap, soap, soap, no water, rinse off, that's it. Because they have to purify all the water they use on the subs and the ships, and they have rules about how you use it. You don't just leave the tap on. The capital costs for these plants, working with seawater, are really high, and the operating costs have energy parts and maintenance issues. The part of the maintenance issue is that the membranes get fouled. You're trying to squeeze through water and you've got tiny holes that don't let salt through. Well, what if a load of seaweed and dirt and stuff comes through, then your membrane gets clogged and you have to swap it out. And then you have to keep making new membranes and then that costs you money. And where do you get the materials and where do you get the energy to do that when you've burned up all the fossil fuels a long time ago? And I got this estimate from the Army Corps of Engineers. The capital cost is $2,000 per meter cubed per day capacity. And the operating cost is $1.25 per meter cubed per day capacity. That's an estimate. A cubic meter is a lot of water. It's 258 U.S. gallons. But a day is a long time. People use water for all kinds of things. The public water use for 240 million U.S. residents in the year 2000 was 43 billion gallons of fresh water. That's a lot. 31% indoor use is toilets. 19% is showering. 25% is washing laundry. That's a lot of waste. You can cut way back on most of that if you're cognizant of it. So for Southern California, if we estimate 24 million people, we'd need to desalinate 430 million gallons of water. Well, the way I figure it, that would cost us $2 million a year, ongoing cost, and it would cost us $3.3 billion to build all the plants to get the thing going. So you can see that climate change if it comes about has real economic cost. Somebody has to come up with the 3.3 billion that somebody is you and me. The second you get a job and earn any money, there'll be people flying around you like crazy to get their share of it. Desertification, though, may make us have to do this. So we should consider it. People are planning these things even as we speak. If you go out to the aqueduct and you just go out to the last mountain range where they had those pipes, they have 6,000, 12,000 horsepower pumps running full tilt to pump the water up over the last mountain range and down so we can use it. Those are using about 100 megawatts of power continuous to do that because Northern California is not a lot higher than Southern California in terms of altitude. So you've got to pump the water all the way down here to do that. But it won't help you if there's no water to pump. It won't help you at all. They'll just be sitting there and you're going to have to get the water some other way. Okay, let's figure out how much muscle we need. Practice problem 28. What is the minimum pressure that we need to apply to a suitable semipermeable membrane in order to squeeze fresh water out of salt water? In other words, to do reverse osmosis. Now, let's have a look. We have to overcome the osmotic pressure. I'm going to assume a temperature, but it gets worse as the temperature gets higher. So if it's a hot day, it's going to be bad. The osmotic pressure depends on the number of particles. Remember, for each mole of sodium chloride we can estimate that there are two moles of particles. And if we estimate two moles of particles, that means that they move independently, which is not true for sodium chloride because sodium's plus, chloride's minus, they tend to stick together occasionally. So it doesn't really look like two. It looks like a little less and it gets a little bit fiddly, but you just measure it. You measure the osmotic pressure, you know what it is. If it's 0.6 molar, 0.6 moles per liter times two moles of particles for mole of sodium chloride, here's R, here's 300 kelvin, just a guess. Looks like about 29.6 atmospheres, 30 bar or 430 psi. 430 psi is quite a lot. One atmosphere is 14.7 psi. At this pressure, the flow is zero because all you've done is balance it. Now if you actually want to get things moving, the other way, you've got to push harder. You can't push too hard or you break the membrane. So there's a rate at which you can push. Luckily, i is a little bit less than 2 because the ions tend to congregate a little bit and the osmotic pressure turns out to be about 24 bar rather than 30 bar. So that's good. But still, you need about 40 to 70 bar to do reverse osmosis. So you need to take all those millions of gallons of water and you need to push on them like crazy and that takes a lot of energy to do that. Or rather, it takes a lot of power. Okay, now let's determine the mass of a macromolecule. We've got one here called calmodulin. It's a great protein. It grabs calcium. It does all kinds of things. And we've got 35 milligrams of it. Let's assume we have it without the calcium so we don't have to worry about the calcium coming off and making extra particles. And I've purified 35 milligrams of that protein, calmodulin, and I've dissolved it in 1 milliliter of water. And then I've measured the osmotic pressure. It's tiny, 41 Torr. But I can measure 41 Torr easily. If I measure that, then what's the molar mass of calmodulin? Okay, well I just say I know the osmotic pressure. Now I want to solve for the concentration. So again, I just assumed 300 K wasn't given in the problem. On a test, of course, I'd give you the temperature. I wouldn't leave it up to guessing. But let's just guess. What about room temperature on a warm day? The osmotic pressure was 41 Torr. And I said I don't like Torr, so I'm going to convert it to atmosphere by dividing by 760. Torrs go away. R, I know. T is 300 Kelvin. And so it turns out the concentration is 2.19 millimolar. 2.19 times 10 to the minus 3 molar. That means in a liter, if I have 35 milligrams in a milliliter, I have 35 grams in a liter. So 35 grams is 2.19 times 10 to the minus 3 moles. Because remember, this is moles per liter. So I have to figure out how many grams I have per liter. 35. And it turns out to have a molecular weight of about 16,000 grams per mole. So it's a big guy. That's typical for these guys. Okay. Now suppose we had Calmodulin and we wanted to do freezing point depression. You'll see why osmotic pressure is far superior for these guys. For big guys, osmotic pressure is the way you select to do it. You never use boiling point elevation if you can avoid it because usually when you boil protein, you know what happens when you boil an egg? It changes, right? It reacts with itself and turns into a mess. And if you cool the egg down, it doesn't come back to where it was. You do an irreversible cross-linking of the material. And if you go out on Sundays and let UV light hit your face all day long, which you can easily do down here, then you will be frying your face like an egg and you'll have all kinds of irreversible cross-linking and you'll look old. And in the days before sunscreen, when people were sunbathing a lot, if you want to see the results of that, go out to Palm Springs, you know. And don't go out on Halloween because you won't be able to tell who's got a costume. Let's take the same solution and let's measure the freezing point depression instead. Let's just see what it would be. So 41 Torr, I assure you, is dead simple to measure. Any pressure meter will measure that. That's nothing. Now let's take our same 35 milligrams of Cal modulin, dissolve it in one milliliter of water, and let's do an experiment to try to determine its molar mass but it's not a good experiment. We will see why. We'll try to measure the freezing point depression. Well, we know the concentration from before is 2.19 millimolar and the molality and the molarity in dilute solutions are the same. So we can guess it's 2.19 times 10 to the minus 3 molal and then we look up the cryoscopic constant, K sub F, for water and find it's 1.86 degrees C per molal. And so the freezing point depression delta T is 2.19 times 10 to the minus 3 molal times 1.86 degrees per molal, 4.07 times 10 to the minus 3 degrees. That's going to be hard to measure. You're going to watch it freeze and you're going to say it froze four one-thousandth of a degree C. You know what? You've got to have a very, very accurate way to measure temperature. And at that level, the temperature on the edge of the container and the middle of the container and oh, it's a nightmare. Hard to control. This will never work. That's why they don't use freezing point depression to measure the molar mass of polymers and biopolymers. Osmotic pressure is much, much better way to do it. Okay, let's talk now, we talked about solutions. Let's talk about something that's not a solution. Let's talk about colloids. Colloids are interesting materials, but they are not solutions. What they are are small particles of one kind of material that are dispersed in another kind of material. And the size of the particles is much bigger than atomic or molecular dimensions usually. The size can be of micron size and things that are of micron size can scatter light and so often times colloidal solutions can look opaque. They aren't clear like a solution of salt and water is translucent. The light goes through. If I have milk in a glass, I look at it. It's got fat droplets dispersed in there and so I can't see through it. That's a colloid. There are lots of examples of this. Examples are fog, which are small liquid water droplets dispersed in the atmosphere in a gas. Fog makes it hard to see. Smoke, which are solid particles dispersed in a gas. And mayonnaise, which is like milk, which is liquid droplets dispersed in another liquid. There are lots of different examples of this. We can stabilize. Sometimes we can make it colloid and then if we let it sit around and sit around and sit around and sit around and separate if we take milk and let it sit around then the cream would rise to the top hence the saying the cream rises to the top and then they'd scoop off the cream and make butter and then drink the rest of the milk or they'd try to stir it back in if they wanted to have richer milk. Not so good for your heart but if we want to stabilize it what we can do is we can add a surface active agent a surfactant detergents and things like that and we can use them to stabilize the colloid and keep it and that is in fact exactly how detergents and soaps work. They take things like grease that are not going to tend to dissolve in water and they turn the grease into small particles that do dissolve in water and then you wash them away. There you go. By the way, if you're washing your hands how should you do it? I've washed a lot of people wash their hands and I've concluded we need a tutorial. Because it's not very good. First of all, do you wash them with the strongest disinfectant so far? 99.99% antibacterial. What's it? To make sure that they're really clean. No. Not unless you're a surgeon. When you do that, you kill all the beneficial bacteria on your skin as well. That's like landscaping by spraying your whole yard with Roundup. If you spray your whole yard with Roundup the first thing that comes back is weeds because you've killed everything. What you'd prefer to have is something like a very dense lawn that you maintain and that you don't wipe out all the time and then that dense lawn keeps the bad guys, the weeds, from coming in and likewise you don't want your skin to be too clean because you want the good guys there standing guard. Okay? So you just want to wash with soap and water and warm water makes colloids more effectively than cold water so it'll make it easier to clean your hands and that's just what mom told you to do, right? You don't use hot water, really hot because then your skin swells and then you can have other issues. You're more likely to get stuff in and have your skin get irritated and you don't wash your hands any more than absolutely necessary because you are stripping off the oil on your skin which is guarding you from a lot of bad guys and if you keep doing that you're likely to get some kind of dermatitis or irritation on your skin like people who wash their hands all the time get. These surfactants are cool they're engineered so that they have a part that's like grease and they have a part that likes to be in water and the thing that likes to be in water is a charge because then the water can line up near the charge and says great and that's how you design a detergent and here's one that you'll see in everything we'll see it in some ingredients here sodium laurel sulfate you'll see that on the ingredient list start looking at things you buy and understanding what's in them and then you may not buy everything just based on the marketing you can make it from palm oil or you can make it from petroleum products but it has this long greasy tail and then it has a charged head because the sodium just drifts off into the water and then I have this so this is an anion and this is called an anionic surfactant you'll see that listed when the manufacturer doesn't want to tell you what their secret formula is they just say anionic surfactant you figure it out we don't want you competing with us SDS promotes the formation of micelles which are approximately spherical drops with an oily interior in this case and a charged exterior this is what happens I've got some grease or oil and it will form two phases with water and I can't get it to dissolve in the water so I add some detergent when I add the detergent what happens is the tails of the detergent say hey there's grease they hate being in the water so there's grease around they plunge into the grease like that but then they have these charged heads don't like to be in the grease so then the whole thing lifts off like a helium balloon this big drop of the grease but now it can dissolve in the water as a colloid, not a solution but it will still wash away and so you get the thing clean and that's all you need to do is there's the oil and grease that you want to get rid of and there's your surfactant there are lots of variations you wouldn't want to use a sulfate group very often in fact I was very surprised to see that Colgate I think it's called total or complete or something anyway one of the ones that says it does everything has sodium laurel sulfate as one of the ingredients that's probably a little bit too strong a detergent and I think some people get mouth irritation or they get redness around their mouth because when you brush your teeth like a wild man foams like crazy and then you get it around here and it's a little bit too strong and you tend to have this skin irritation and you don't realize why it's what it's being caused by but if you switch toothpaste it may go away it may also get gum irritation so this is just regular soap it's got a wimpy head group it's not as strong as a detergent kind of wimpy this is what you want to use if it'll do the job to clean your hands this is one that doesn't have any charge this is one that Dow invented I think called Triton and then X depends on X and a number depends on how many ethylene oxide units they have this is used in a lot of products this is a factor and then this one you'll see in shampoos this one's not a great detergent really but what this one is is it's a foaming agent and the idea is that people don't think their hair is getting clean unless there's bubbles in foam so if you have a perfectly good detergent that doesn't foam people say ah it's not working it's not working it's working perfectly so they add this other up to cost sometimes they only add the foaming agent that would be called a mild or hypoallergenic you know it just doesn't have as strong a detergent as sodium lauryl sulfate which some things have they don't put this into laundry detergent however because you don't want it foaming coming out like a cartoon out of the dishwasher so they leave it out they specifically pick detergents that don't foam for that and that's why you don't see it foam but the detergent is very strong in laundry detergent real commercial products have a lot of ingredients and some impurities and the reason why you add these are as follows first like I told you to create foam you're getting cleaner this is a psychological impression it has nothing to do with reality to pH the detergent you don't want to give somebody a detergent pH 11 they have a bad reaction to it to stop microbial growth they put in things that stop bugs bugs can actually grow on soap mold can grow on soap it's amazing but it can and so they put in stuff and then you're putting that stuff all over yourself you are I'm not to work in hard water if there are ions the ions tend to go to the head group and stick now it's not charged now the soap doesn't work so they put in something to grab all the calcium ions and make sure the soap works water softener they control the viscosity again it's an impression if I pour out something and it's too wimpy and thin I don't like it so they just put something in there to make it thicker they put in a load of ingredients to impress uneducated people because I guess I'm to buy it hey look at all that great stuff all natural strict 9 is natural natural means nothing you've got to know if it's good or bad and to add a clean smell I got news for you clean doesn't smell like anything because there's nothing there if it's clean so the clean smell is dirt of a type you're adding something else but again it gives the impression that you might be clean and the impression is important because you have an idea in your head that you want to be clean just yesterday Procter and Gamble was on the defensive because the claim is that Tide this gentle and clear whatever it is contains carcinogens this is the headline it's kind of hard to read the small print so let's see what they did last year in its dirty secrets report that doesn't sound unbiased environmental group women's voices for the earth sent 20 different cleaning products out to an independent lab to find out what, if anything the products contained beyond the ingredients listed on their labels the results included a number of surprising discoveries including the presence of 1,4-dioxane a solvent the EPA calls a probable carcinogen so there you go the concentration of 1,4-dioxane is essentially zero but a good analytical chemist can find a needle in a hay sack if you're worried about this then you want to stay 50 miles away from anybody who's breathing out cigarette smoke because it's about the same kind of relative risk but nevertheless it gets a lot of press and if you say to me I want to buy detergent but I'm going to rinse off my clothes at the end of the day and I want it to have no dioxane at all first of all that's impossible to make something 100% pure but I want to have it so low, so low we can do it but do you want to pay 100 bucks for a bottle of detergent because it costs money to do that like it costs money to give you fresh water not cheap what about shampoo wow look at all these ingredients there's so many that I had to blow it up first ingredient is water there's the profit you call it aqua it sounds better sodium laurel sulfate oh my goodness see sodium laurel sulfate then all these other guys and then a bunch of just I don't know just whatever they could put into the garbage disposal here they put in I guarantee you this stuff was cheap somebody was getting rid of lemon peels and they said yeah well we'll make lemon shampoo man we'll take that stuff off your hands what are all these ingredients doing here's what they're doing this is cleaning your hair this is doing nothing this is making your hair foam up to give you tons of bubbles okay, laurel sulfate and the coconut and this here, this one here is in a lot of ingredients it's a mild detergent it adds viscosity and it's an anti-static type of thing they can humectants they can attract water to your hair when there's a little residual left on your hair and then your hair doesn't fly out and when you comb it it doesn't go flying out okay I don't have to worry about that pH citric acid another one to control the viscosity and so forth so you can see I should have marked that okay here's the chemist's approach you go to a supply house and you buy S.V.S. and you're done with it and since it's a buck a ton or something you can save an awful lot of money now you might say well all these products are doing a lot of good to my hair and to which I say until the 70s nobody washed their hair more than about once a week this is a recent thing and then Farrah Fawcett and Christie Brinkley and they said you know if we can just convince people their hair is dirty all the while we can sell a ton of shampoo you look on the thing it says lather, rinse, repeat do it again complete waste and all the water pollution as well well if you do that if you wash your hair a lot what happens is your sebaceous glands go crazy because they're trying to keep your hair oily enough so that it is protective the way it's supposed to be and you keep stripping all the oil off with detergent forming these micelles and so they respond by saying golly what a weird world we're living in let's produce a ton of oil and of course if you stop doing that they stop doing it too but if you never stop doing it then you never know that you just say oh my hair will be a mess by the end of the week but after a year it won't be less may be more here so the question is how much hair do you want to keep I can tell you I've lost a lot you may think well you've got your hair but see you don't know where I started so I can tell you one thing the 70s were a wild and crazy time there were all kinds of things you didn't have to worry about and people didn't worry about them and they went crazy I'll tell you what okay let's just close this out by looking at phospholipids here are three examples of phospholipids we tend not to use phosphate phosphate makes a great detergent but we tend not to use it because phosphate is a nutrient for growth of algae and other things and if you pollute rivers with phosphate what happens is the microorganisms go crazy and they use up all the oxygen in the water and then all the fish die and float to the top and that happened plenty in the 60s when people used phosphate detergent and just dumped dumped it in the river so they switched to sulfate and usually you look for stuff that's phosphate free this is kind of an interesting one that has them back to front in a big sphere it's called a liposome they use this in some products like skin cream to put various things on the inside and in some drug delivery things they put things on the inside and then it's a time release that kind of leaks out over time and this is our mycel again with phospholipids they can form those and this might be a cell membrane that has head groups pointing out into the plasma and the cell and then there's a greasy interior that segregates the outside of the cell from the inside and cells these things just form naturally so the first cells probably just formed because there was a certain concentration of these molecules around and then they started forming something and once you've got an inside and an outside then what goes on on the inside can be different chemistry than the outside and then of course you can get going ok so here's our summary for today colligative properties they depend on the number of particles but not their type at least for dilute solutions freezing point depression boiling point elevation and osmotic pressure are all colligative properties colloids have larger particles and they may be formed by using molecules with specific properties like surfactants and commercial formulations when you look at them closely are driven by many other considerations other than safety and efficacy in other words they're driven by whether they sell not by whether they're safe or effective they're probably safe enough but ok I will be punching a button to distribute the scan exams today have a look at them if anything's wrong we've missed at it I'll give you instructions how to fix 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