You should use a setup similar to the McBain-type balance depicted in the attached figure. The column hosting the zeolite-containing balance is highly evacuated and kept at constant temperature by the colored, thermostated service fluid. The water reservoir is connected to the column through the upper chamber. You can measure pressure changes by means of pressure transducers and water vapor adsorbed amounts by measuring the elongation of the balance spring. Please keep in mind that water vapor adsorbed monolayers are very hard to determine because of lateral interactions between water molecules that imply non-BET behavior at low partial pressures.

Dear Mukherjee:

Theoretically, under the vapor pressure of the water at certain temperature, the water should be retained as gas-phase. My idea is to make a standard water vapor gas with desired pressure as follows:

(1) evacuate completely (using dry pump/ not rotary vain oil pump/ scroll pump is recommended) any flask which you already knew the volume of.

(2) calculate the amount of water (in liquid) that can be evaporated with desired pressure in the evacuated flask. You can use simply PV = nRT, where P is desired pressure, V is flask volume and T is room temperature. The P of the water vapor should be under water vapor pressure (google the table of water vapor pressure at certain temperature) at your room temperature. Over this pressure, few amount of water droplet should be remained.

(3) inject the water with the calculated amount into the evacuated empty flow with blocking the influx of any gases for outside of the flask. And then, close the flask. The water should be evaporated completely after few minutes.

(4) you can obtain a flask including water gas with known pressure.

For this method, you need to design your own flask.

Or~~~

There is better method.

(1) buy the dried gas cylinder which can adapt appropriately to your system. (if you cannot buy completely dried one, you can evacuate the cylinder overnight and then fill the cylinder with dry nitrogen of ambient pressure).

(2) inject the water (liquid-phase) with certain amount (see above how to calculate the amount of water to become vapor with desired pressure in the cylinder) into this cylinder.

(3) the water droplet should be existed in bottom of the cylinder. After closing the gas cylinder, cool down the bottom of the cylinder using liquid nitrogen to freeze the water completely. With maintaining liquid N2 cooling, directly evacuate the cylinder 30 min ~ 1 h. And then, close completely the cylinder. Then, warm up the gas cylinder to room temperature. The water inside the cylinder should be completely become vapor gas.

Is this enough answer for you?

If you cannot understand well, please give me e-mail again.

Best regards,

Kanghee Cho

Through Cahn balance one can adsorpe water in a controlled manner in zeolites. However, adsorption is a multilayer. Through BET equation it is well proved one.

An alternative option would be to use a commercial adsorption device such as those for the measurement of N2 sorption isotherms. I don't know if this option is available on all of them but, at least one (Belsorp-max, from Bel Japan) is equipped for the adsorption of vapors such as water. Recording first a complete H2O adsorption isotherm would be helpful for your study because you would be able to determine whether or not you can adsorb a monolayer (or something close to a monolayer) of water. Indeed, adsorption occurs often as multilayer (H2O could start to adsorb on adsorbed H2O before a monolayer of H2O is formed on the surface). The mode of adsorption will depend on the affinity of the water with the surface (compared to the affinity of water with itself, look at the BET model for more details-C parameter of the BET model). If the affinity of water for the surface is high (hydrophilic surface) you may be able to form something close to a monolayer (and you will observe a knee on the adsorption isotherm at low partial pressure). On the other side if the surface is hydrophobic the multilayer will form. Silica surface tends to become more hydrophobic when it is pretreated at high temperature, you could therefore vary the pretreatment temperature of the porous sample and mesure H2O adsorption isotherms to determine the optimum pretreatment temperature.

Dear colleagues,

I apologize to be unable to read all the above contribution because of the lack of time now. I just straightforward to the way we prepared water vapour at the time of these experiments. First we used deionised water by ions exchange collumn, a amount of this water, much larger than needed, is transfered to a borosilicate glass tube. Then we put it to boiling temperature at atmospheric pressure for some seconds leaving an exhaust valve open to the ambiant. Doing so we eliminate species "lighter" than water as air gas for instance. After closing the exhaust valve, and leaving the glass tube relax to ambiant temperature (~20°C), we connect the tube to a primary vacuum pump until reboiling for some seconds at a temperature lower than ambiant . Then we connect the glass tube to calibrate borolisicate glass vessel previously evacuated to a secondary vacuum (10^-5 -10^-6 Pa or 10^-10-10^-11 bar). This glass vessel is equiped with a high sensitivity high precision pressure gage. We let the vapour flow to the vessel until it reach a pressure well above the pressure needed in experiment chamber. This pressure is calculated taking int account the adsorption in the zeolite sample (and on the chamber wall and tubing). Its value is optimised to get best possible precision of the adsorbed amount. We then close the valve to the liquid water tube, and open a valve to the chamber containing the adsorbant under high vacuum. We let the pressure of the glass vessel decrease gently (to avoid "blowing" the adsorbant sample particules) to the right value corresponding to the desired transfered water amount . This adsorbant sample as been previously in-situ regenerated by gentle heating under vacuum up to about 350°C followed by a slow thermal relaxation under vacuum pumping to the desired experiment temperature. All this is made possible by : i) calibration of all the volume by a perfect gas (Helium or other light and low interacting molecule) ii) calibration of the adsorption capacities of all the tubing and sample chamber by a blank experiment with the same adsorbate (here water) at the same temperature and pressure. Iterations are necessary to get the right condition. iii) a very precise thermal regulation of the lab and all the parts of the experiment. iv) a reference vessel calibrated by liquid pure water weighing.

I wrote this by memory because I work now on another subject (photovoltaïcs) but you can find details in the old papers of my team. I'll join a list of papers. Feel free to ask other questions, as far as I find the time, I'll give you answers.

Very interesting devlopment technologically.