Webinar: Experimental Methods for Measuring Vapor Pressures of Chemical Substances

Hello everyone and thank you for joining us for today's Surface Measurement Systems Online webinar. Today we will be exploring the topic Experimental Methods for Measuring Vapor Pressures of Chemical Substances. We are lucky enough today to have our very own Dr. Vladimir Martis delivering today's presentation. Our resident expert in vapor pressures, Dr. Vladimir Martis is the DBS product manager for Porous Materials and Vacuum product specialist at Surface Measurement Systems. He received his Master's degree in Materials Engineering from Trenchin University of Alexander Dubcek in Slovakia in 2004. He received his engineering doctorate degree in Molecular modeling and material Simulation from the Chemistry Department at the University College of London in 2012. Since joining SMS, Dr. Marthis has continued working on the development of advanced in situ experimental surface science techniques using molecules as probes instead of X rays for studying catalysts, zeolites, moths, polymers, freeze and spray dried materials, composites and glasses under relevant industrial conditions. Dr. Marthis has authored several papers in peer reviewed journals and presented at several international conferences. Now, before we pass over to Dr. Martis to begin his presentation, I just want to remind everyone of a few housekeeping rules before we proceed. If you could all keep your mics muted for the duration of the presentation, please, just to avoid any feedback from anyone else's end. And as you may know, at the end of today's session we'll have a live Q and a with Dr. Martis so you can pose any questions you have on this topic and any other questions that arise from the presentation. You can submit these questions in one of two ways. You can submit them in writing at the Questions tab, where I will pick them up and ask Dr. Martis in person at the end of the session. Or alternatively, you can raise your hand and. And at the end during the live Q and A, I will let you know when you can unmute your microphone and you can ask Dr. Martis your question yourself. So just to reiterate, you can submit your question for us to ask in the Questions tab in writing, or you can ask the question yourself over your mic just by raising your hand and waiting for us to call you up. So without further ado, I will hand the control over to Vladimir to begin his presentation. Thank you everyone.

Thank you John. So good afternoon everyone and thank you very much for dialing in this afternoon. Now you should be able to see my screen.

Yes we can Vlad.

Thank you John. Yes, so thank you once again. Thank you very much for dialing in today and join this afternoon webinar about exploring the experimental methods for vapor pressure measurements. So first I can just give short introduction to the vapor pressures. It's just one slide is quite busy. So basically, let me define first the vapor pressure. So the vapor pressure is defined as the pressure exerted by vapor in thermodynamic equilibrium with its condensed phase, the given temperature in the core system. This can be the as a sample, as a substance can be either solid or liquid. The vanilla vapor pressure are then used to calculate the heat evaporation or evaporation sublimation. Now, the vapor pressure of a substance at a certain experimental temperature is usually characteristic of material, which can be defined by its properties like molecular mass, melting point and boiling point. The vapor pressure doesn't really depend on the amount of liquid or solid samples, as long as we can use only small amount of liquid or gas specimen, because that's going to be only the present in equilibrium with this vapor phase. Typical applications for vapor pressures are either the environmental or human health protection, vapor pressure deposition processes, lubricant, semiconductors, vacuum industry, heat transfer fluids, fragrance formulation, photovoltaic production processes, aerospace and automotive industry. Now let's move on to my next slide where I'm going to explain the vapor pressure of liquidities. So let me give you an example in which you have already observed the phenomenomena of vapor pressure at home. So we all heated up the pan, filled it with the water and with the clothes lid on the top on a gas cooker. And many times you forget about it. So you are in the middle of something, you're doing something else. And what you observe, you hear some racking noise from you have some running lid and the hot water is spilling out of the cooker. So what we observe here is a form of the thermodynamic point of view is the observing that the liquid is eta. And this molecule obtain enough kinetic energy to overcome forces holding them in the liquid. And they try the escaping into the gas phase. So during the, during this, during this process, what's happening as they escaping, they generate enough pressure to move the lid up, which allows the vapor to escape. So and as the vapor show, so once these gas molecules form, which creates the population of molecules in the gas phase, in the vapor phase above the liquid, they produce a pressure and the pressure is the, and this is called the vapor pressure of the liquid. Now the other important parameter is the relationship between volatility and the boiling point. So volatility describing generally how readily a substance. So we have the table here which shows different compounds varying from boron to hydrogen. But what we see, what we can see from these tables is as the boiling point is going down, the volatility goes up. So hydrogen is going to be very highly volatile compared to let's say mercury and which is the least volatile. And one of the use of the mercury was in the manometers in the YouTube shape manometers because the vapor pressure of the mercury is about like 0.0017 torque at 25 degrees Celsius. As you know the vapor pressure is also strongly correlated with the temperature. So the vapor pressure exponentially rise with increasing the temperature. And if you use a natural logarithmus to express this as non relationship as a linear we can use this Clausian Klepping equation where LNP is equal to vaporization over the universal gas constant R and temperature T and the C is the slope intercept is the constant for the equation. Now there are several methods to measure the vapor pressure. So here we're looking at the table which was produced by OECD guideline showing different methods for the vapor pressure. So you have the dynamic method, starting method, the effusion method or gravimetric method in this talks and each of these methods got its own certain recommended range and also the reproducibility range, whether they're suitable for liquids and solids. Now in this talk I'm going to focus mainly on that's an effusion method which is can be applied which is applicable in the range 10 or minus 15 Pascal up to 100 Pascal and the static method which is direct measurements of the vapor pressure which can be applied in the vapor pressure range from 100 to 100,000 Pascal. The reason we have to use these two methods in order to cover whole vapor pressure range because shortly you won't be able to if the effusion rate or the evaporation rate is very high then you won't be able to use the natson effusion method because your samples can evaporate fast completely before you'll be actually able to measure. So let's start with Natson effusion method. The Natson effusion method, it's a gravimetric method. So what it means. So we're using the thermal balance to measure the effusion rate or molecule into the vacuum. In this measurement sample is held in a small nasen cell which is called nasen which is a small orifice. And the molecule escaping through the direction of the vacuum. Now distills is a place inside to the metal pond and then the metal pond is from the ultra balanced onto the hang down wire. So what we measure during these measurements is the constant mass loss. And as you know, the effusion rate or evaporation rate is proportional to the actual vapor pressure material. So we continuously measure the mass loss of the sample during this process. So as you can see as a function of the time at constant temperature. What you see we observe in the muscle, which is usually the linear. So what these nut cell cell look like. So the picture here, on the top we have the lead and with a small orifice size. And at the bottom with the cups, then it's going to be placed your powder solid or a liquid. Right hand side you have the inside of the view. So once you know the sample, we screw the lid on top of the car and we place the nuts and seal into the metal pan. And then we hang it directly from the hangdown wire, which is suspended from the ultra balance. And this is also all placed inside the vacuum chamber. And then we just pull the vacuum. So during the measurements, we continuously weigh the sample and we measure the max loss rate under the high vacuum at different temperatures. So how we can calculate the vapor pressure of material from the industrial fusion vector. So you expose a sample to certain temperature, basically temperature, the temperature in a closed environment, temperature defines an evaporation rate. On this evaporation rate, which is expressed as DM DT value, we can calculate the vapor pressure using lossy Blackbeard equation. So for calculation of vapor pressure we're going to use this equation on the right. And for this we need to have knowledge of the molecular weight of the sample. So this is very important. The surface area or the orifice and the DMDT value or the evaporation rate which is obtained from this thermogravime metric measurements and different temperatures. So the MDT value is obtained at least for five different temperatures. And during these measurements we obtain linear mass loops. And then we feed that value here and we automatically calculate the vapor pressure of the solid at a certain at the experimental temperature. For this method we're using the apparatus which is called vapor pressure analyzer. So what we have here, so we have the temperature enclosure which allows to control the temperature. The temperature is between 10 to 80 degrees. Inside we have the vacuum stand and under the dome here we have ultra balance. From the balance beam we are suspending hanging down the wire. And on the left hand side we have the sample. So in this case the sample is placed inside the nuts and fusion cell. On either side is a reference to counterbalance the sample site. And this is connected to the vacuum line and at the end of the vacuum line we have the sets of the turbo molecular pump and the rotary pump to maintain a very high vacuum. And the pressure is measured using the vacuum pressure gauge. So this is the typical setup for this measurement. Now some of the samples, especially the oils, they're not going to evaporate at low temperatures below 80. So if we have this kind of samples, what we do, we increase the temperature of the sample locally up to let's say 400 degrees. And we measure the evaporation rates at different temperature, let's say from 80 up to 300 degrees Celsius means that validity and pressure at each temperature at least for two or three hours. So this is the typical setup for the measurements of vasile fusion method. So here are just a brief picture of the system, what we have here, this is set of the pumps, here's the temperature enclosure and inside is the vacuum system. It's a fully automated system controlled by PC where we run just automated method to run the different. Let's move to the example of the NASA data. So system is usually calibrated using a reference material like benzoic acid because its vapor pressure is well known. So in typical experiments, once you load the sample into an absence hull, we pour high vacuum and then we expose the sample to different temperatures and we wait at each temperature for several hours. In this case, we waited about three hours at each stage. So I call each these temperature steps stage. So solid line is from incubator, solid yellow is from the preheater. So in this case the preheater is just measuring the temperature of the sample under the Latin soil. And what you observe as you expect as we increasing the temperature, the effusion rate or the evaporation of rates of the molecule is faster now and from this data now, so then you have the evaporation rates at different temperatures from this data. Now we can calculate using the equation just shown earlier, vapor pressure of the benzoic acid at different temperatures. Now as we do this, we automatically calculate the heat or vaporization using classic Keppen equation. And then here we get logarithmic available pressure versus less temperature. So we expect the heat of vaporization for benzene gas is about 88, 88 kilogram, about between 88, 87 kilopascal kilojoules. Now so what's the absolute value of the vapor pressure for this material? So the experimental range was between 30 to 60 degrees. We have the evaporation rates in the column marked as the mvt. We have the vapor pressure obtained by using our laxative effusion method in this column here and then vapor pressure from the literature. As you can see, there is a really good agreement between measured vapor pressures and vapor pressures obtained from the literature of the publications. And also noted the vapor pressure size varieties which was in this case it was 167 micro. So we have the cells these different sizes of the orifice. It is important depending on type of material you use for evaporation rates. We expect so if you have highly water materials, you want to use a small orifice size. When you have let's say material which is less volatile like the oils, you use a higher orifice size to get a good data. Next example is so basically the data automatically calculated using our by using our analysis software. So in this case we generate the report and absolute value which provides you all the vapor pressure value in the pascal as well heat of vaporization in the Jupiter. So these are automatically. Now I move to some examples. Let's say vapor pressure naphthalene. So you might be familiar with naphthalene you're always used to happy to keep the coals closed range. So basically the vapor pressure is at 25 degrees is a nice linear evaporation rate is a positive 1.8 Pascal. And the rate of production agency reports 11.6. Now another example of this method is for vapor pressure also is like pesticides and fungicide. So these are environmental to tackle the environmental issues. So we look at the four different pesticides are ametrin, bifetrinechlorac and azo 6 strobin and again nuts and method and the literature values are in very good agreement. And you can go down to 8.71 10 -10 Pascal. So can switch as example of one of these mass data here. So it's very useful method. Okay, another interesting application the vapor pressure decreases or lubricating oils. This is mainly for the vacuum palm oil. So and it may because the function is for lubrication, cooling, corrosion protection, noise reduction. And one of the requirements is the low vapor pressures. Because if the high vapor pressures especially with draining it's going to cause the issue with the process. So here look at the bi vapor pressure over the oil A so what we see here we have the first we have the effusion rate on the top graph. When we increasing the temperature we go from 80 all the way up to 230 degrees showing solid yellow line. And the mass loose rate of evaporation rate are showing solid red line. What you observe at low temperatures it's we can't measure this vapor pressure, because it's so low, so the system, the microbiome is not able to detect it. And then once the temperature is increased above 100 degrees. So we are able to calculate the vapor pressure of this material and it's the total vapor pressure for this oil. Same story applies to the same oil, but is the vapor oil C. Again we have in this case, the vapor pressure measuring range for this oil had to be increased up to 200 degrees and the pressure method from 200 degrees to 260 degrees. So in order to get a good evaporation rate. So as you can see, it's very important to find the right temperature. So the vapor pressure, when you extrapolate the vapor pressure to 20 degrees, the value go down to 10 -15 Pascal. Now the other implication is vapor pressure ocean. So here look at the silicon vapor pressure and what you observe here, this other opposite. So in this case you want to go to lower temperatures because the evaporation rates are quite High at 25 degrees, as you can see here from the evaporation rates on the left hand side. So 25 degrees, you have very high evaporation rate. Then you get the 30 degrees. The change in evaporation is only small area 35. Also there's no significant change in evaporation. But when you increase the temperature all the way to 40 degrees, the sample completely evaporated. So there's nothing left in the pan. So but we were still. So in a small narrow temperature range between 25 and 35 degrees, we were able to calculate vapor pressure of this silicon, which was about 3.3 Pascal at 25 degrees. The other application is vapor pressure, like basic chemicals like stabilizers, neutralizers. So here we look at the vapor pressure curve of 3 isopropanolamine and again it was measured for narrow temperature range from 40 to 90 degrees. Those ones go higher to temperature. And then we were able to evaporate extrapolate value for 20 degrees, which was about 0.006 Pascal. Now, so that was all done for the nuts and fusion method driving by treatment. Now I'm going to move to the static method. So static method is a little bit different. So static method we're going to look at the evaporation and condensation in a closed system. So what you can undo here. So on the, on the left hand side we have the system, we have the flask filled in with the water, connected with the pressure gauge. And on the top we have a thermometer. Okay, so what we measure here, so this Is the static method. So there is no moving part. So static or so in a closed system means that air cannot coming in and water cannot go out at the same. So inside the flask the water cannot come out and air cannot coming in. So this means completely isolated. That's why we have this stop plug. On the top is thermometer and it's connected to closed cage. So initially, so when you pour the water close the valve. We measure the pressure of the dry air in the flask. So the pressure is almost zero. So the razor wave evaporation is almost non existent. Now if we set the temperature to the window at 20 degrees, the rates of evaporation will increase. So we have the water molecules escaping from the liquid and they start, I mean there is enough evaporation. Then some of the molecules then they will start condensation. So the evaporation, the rates of condensation in this case will be defined by the rates of evaporation. And in general, what we have here, the evaporation will be greater the rate of condensation for the liquid water. So once what we see here, afterwards we will be observed to measure the vapor pressure, the vapor pressure of the liquid. So as you can see now, the brigade moved and as a result the rate of the condensation is increasing. Now what we observing, once we reach the rate of evaporation in condensation, we reach the dynamic equilibrium. And that dynamic equilibrium, the both rates will be equal. Okay, so this is how we're going to measure it. And at some point we will saturate because it's the closed system. So we're going to measure the saturation pressure of water inside this oil system. So now how are we going to do these experimentally? So for this we have the. I showed a schematic of the static vapor pressure apparatus which is very similar what you saw in the previous picture. So basically what you have, you have the liquids filled in in the flask. So the vapor pressure is a zero. So the vapor pressure to the vapor, the liquid mips are evaporate at some point after a few minutes. And all this is inside the temperature control enclosure. This is very, very important because temperature enclosure will define your evaporation rate at a certain temperature, let's say 25 degrees. Then we have the chamber, small chamber, when you have isolated by such other valves. And then if you measure the increase in the pressure. So when we open the valve, we measure the increase in the pressure inside this vacuum chamber using two pressure sensors. 10 to 100,000 to and all these sensors, thermomolecular bulb. So what's happening here? Due to the experiment, we remove the air. So this you cannot measure three, just to make sure, just to make it clear, you cannot measure three samples at the same time. So what we're measuring here, first we basically outgass solvent number one. So we open the valve, we pull the vacuum, the pressure will decrease during the outgassing process. We remove all the air above the molecule till we reach the rates of the wave operation and condensation are nickel. So once we reach that state, we close the valve. We close the valve here, which will isolate the pump and the pressure inside the vacuum chamber will reach till saturation pressure. Once the experiment is completed, we can repeat the same experiment on the solvent using the solvent number two. So again we finish with the outgassing of the solvent and then we close the valve and we measure the vapor pressure. And then we can do the same experiments for the solvent number three. And this can be done at three or four different temperatures because the evaporation rates and the vapor pressure will change because of the temperature. So what it looks like. So these solvents look like this. They are connected directly through the celestial flask to the vacuum valve and then we just evaporate in the liquid. So at the bottom, what you see here is the sample one. So you have the liquid sitting at the bottom. So large surface area due to larger evaporation rate. But the number the vapor pressure and the liquid in a closed system won't change. So you're not going to get any losses. And then you have the second liquidity at the bar. Now I show first like with the example. Let me show you now from the example of the measure. Now, the static vapor pressure method is direct measurement auto pressure. So that's an effusion method, which is gravimeter, is indirect measurement. So in this case, once we outdone the public layout gassing, we closed the wall, we observe the vapor pressure. So here's the vapor pressure of cyclohexane at 25 degrees, which was 98.34 Celsius. Another example is vapor pressure of n octane at 40 degrees. And the vapor pressure is about 31.94. Now we can measure also vapor pressure of, let's say quite highly volatile compound like vapor pressure of cyclopentane at 37.9 degrees. And this vapor pressure is about 511 torr, which is about 668.2 kilopascal. Another example is vapor pressure of the for decalin at 25 degrees and its vapor pressure is about 27. As you can see, it's very straightforward method for the vapor pressure. Also, the main benefit of this method is that it can be applied to the mixtures with the different molecular weights distributions. So in this case we look at the origamous. So we see stem allows you to measure the multiple samples. So in this case, sample one, I have a vapor pressure 4.9 Torr. Then you outgast the vapor pressure. Sample, sample, sample. Now, so this comes to my slides. So I just. In conclusion, I hope I show you enough data to convince you that the both static and center fusion methods are powerful methods to measure the vapor pressure of various substances, varying from the liquids to solids. And lastly, I would like to thank you for your attention. And if you have any question, please don't hesitate to ask.

Thank you, Vlad. That was very enlightening. We have our first question here, but before we get into that, I would just like to remind our audience that there are two ways to submit your questions. Please do so in writing through the questions tab.

Or.

Or you can raise your hand and ask the question over the microphone directly to Dr. Martis himself. So please do so now while we're addressing this other question. Just for now, Vlad, we have the question what is the best method to measure vapor pressure on volatile materials?

So the best. Really good question. So the best method to measure the vapor pressure of water material is the static metal. Because the evaporation rates, ambient temperatures are very high. And it is the easiest way to do the measurement. Because for the starting method we will. For natural fusion method, the sample will be evaporating fast before we will be able to measure. Able to obtain any measurable. Before obtaining any good vapor pressure data. So starting method is the method of the choice.

Great. Thank you. We have a hand raised by Takayuki Toyamasu. So, Takayuki, I will unmute your microphone now. If you would like to unmute yourself and ask a question. We can see you're unmuted, Takayuki, but we can't hear you. Okay, we'll just give him a moment to arrange his microphone. In the meantime, we have another question submitted. Oh no, that's just a thank you from the audience, Vlad, rather than a question. So Takayuki, are you able to ask a question? It looks like he might be having trouble with his microphone. If he is able to.

If you're.

If you're able to hear us, Takayuki, please do submit your question in the questions panel and we Will address it there if you submit there. A question has just come in, Vlad, from a Jason Perman who also says thank you for a wonderful presentation. And Jason asks, are there any solvents to where you can't measure the vapor pressure?

This is the, this is the good question. So we can measure only vapor pressure of solvents which have the vapor pressure within the range given by the measuring method. So if the vapor pressure of the solvents is highly volatile then. And it is outside of our measuring range. So let's say it's above 1 bar or 100,000 Pascal. We might not be, we won't be able to measure it because it's going to be too high for our operators. So in general, we can measure the vapor pressure of the solvents using either static or natson method, depending on the solvents characteristic, which will be defined by the boiling point and also by the molecular weight. So generally we can measure most of the solvents, but as I said, it all depends whether it's going to be fit beyond the vapor pressure range. Is he interested in.

Okay. And would that be the same for an ionic solution?

Jason's asked the ionic solution the way we can measure the vapor pressure of the. We should be able to measure the vapor pressure of the. I'm not sure what's the vapor pressure of the ionic solution. So I mean in theory, the best way to just try if it's, let's say if it's, if the pressure is going to be up to one bar, so we should be able to measure it or if the vapor pressure. I measured the ionic liquids in the past and ionic liquids got extremely low vapor pressure. So if you talk about ionic liquids. So the vapor pressure needs to be measured using Naxane fusion method because it is extremely low. So we can measure. Easily measure it if you're measuring the ionic solution. So this must be probably done in using the static method because vapor pressure might be probably higher than 100 pa. So I'm not sure what. So what vapor pressure do you expect? Let's say 25 degrees for this solution. Brilliant.

Thank you. We'll just wait a few more minutes for any more questions to be submitted. Okay. It doesn't look like we have any more questions coming in, Vlad, so I think we can, we can finish up there. Thank you again to our audience for being with us today engaging on this webinar. Is there anything you'd like to add before we finish, Vlad?

I just would like to thank you to all our attendees that have found the time to join this webinar and if they have any questions regarding the measurements or they would like to try to measure vapor pressure of the samples, either solids or liquids, they should get in touch and we can have discussion. And if possible I'm happy to do some preliminary work, proof of concept and once again, thank you very much for your attention.

And thank you Vlad for a great presentation. Yes, just further to what Vlad said, if anyone has any requests for any more information about what's been discussed today or wants to engage in taking some vapor pressure measurements, please do contact us@scienceurfacemeasurementsystems.com and we also invite you to attend all our upcoming educational events. We have two more webinars coming up this month, one focused on BET surface area measurements by Zorptra methods at ambient conditions and this will be taking place in one week from today. And we will also be having a webinar on the moisture sorbet and dry kinetic by DDS analysis which will be taking place in two weeks time, two to three weeks time. You can find out more about these on our website www.surfacemeasurementsystems.com and you can find more info on those there in the webinars page. Thank you again to all of our attendees for joining us today and we hope wherever you are you have a wonderful afternoon, evening or possibly even morning. Thank you all and take.