I never work with acetonitrile. Nevertheless at the same temperature, the pressure in solvent medium may be higher then in pure water. For example, in comparison of equal molar quantities of pure water and water solution of H2C2O4. The pressure in the 2-nd case is higher due to the reaction H2C2O4 = H2O+CO2+CO. CO is almost ideal gas.

Dear Yuxing Wang,

only thing that I found what may help you is a paper (unfortunately in German) about a solvothermal synthesis in acetonitrile. There at least you can find characteristical parameters for such reactions. They write that acetonitrile behaves quite similar to methanol.  I ' ve added the pdf to my note.

Best regards from Bayreuth,

Wolfgang

Dear Yuxing Wang,

I've no experience on acetonitrile solvothermal reactions, so I cannot be of help for solving the practical problems. What I can answer is your question about the pressure of the vapor when a solvent is not one but is a mixture of solvents in which reaction takes place. Please consider the PT phase diagrams of the mixture as well as the possible gas products or by products that could derive from your chemical reaction (not only from decomposition of the solvent).

Good luck for your experiments!

Cristina

Dear Yuxing Wang,

I've no experience on acetonitrile solvothermal reactions, so I cannot be of help for solving the practical problems. What I can answer is your question about the pressure of the vapor when a solvent is not one but is a mixture of solvents in which reaction takes place. Please consider the PT phase diagrams of the mixture as well as the possible gas products or by products that could derive from your chemical reaction (not only from decomposition of the solvent).

Good luck for your experiments!

Cristina

The pressure of the gas phase under subcritical conditions is determined by the equilibrium with the liquid, if pyrolysis of the liquid molecules does not take place. However, heating the liquid leads to an expansion of its volume and to evaporation. If the liquid fills most of the autoclave's volume (for water this filling is more than 32.6%), the expansion of the liquid volume can lead to the complete filling of the vessel and the disappearance of the gas phase. In this case, the pressure in the vessel will be determined by the expansion of the liquid with increasing temperature. In supercritical conditions, the pressure is determined by the filling factor of the free space of the autoclave.

There are hydrothermal bombs available in the market with pressure guage. But how much presuure is enough for the reaction to know is not easy. Should see the literature for any phase diagram of such systems.

This paper might be of interest. "Vapor Pressures of Acetonitrile Determined by Comparative Ebulliometry" J. Chem. Eng. Data 2004, 49, 486-491

http://pubs.acs.org/doi/pdf/10.1021/je034144f

Thanks you all for the kind reply. I think Dr. Yurii Ivakin answered my question. In the attached paper, there are a diagram of density of water as a function of temperature and pressure (figure 2), and another diagram of temperature dependence of the pressure in autoclave at different fill factor (figure 5). Basically, density of water decreases much faster with increasing temperature than increasing pressure, so much that the pressure effect may be ignored at the magnitude of vapor pressure for the purpose of estimating density of water. If fill factor is high, expansion of liquid phase will eliminate all the space for gas phase at a temperature lower than the critical point (374 C for water). For example, if the fill is 80%, the inflection point on the curve corresponds to a temperature that corresponds to 0.8 g/cc in figure 5. Below the temperature, vessel pressure is determined by vapor pressure; above the temperature, pressure is dictated by expansion of liquid.

Am i understanding this right, Dr. Ivakin?

Pune, India

Dear Wang,

First of all, the temperature pressure relation of a solvent under solvothermal condition is well elaborated in the literature as well as my other friends has explained to you.

In normal solvothermal condition, the pressure is generated autogeneously inside the reaction chamber kept at isothermal condition. Specially integrated pressure vulve autoclave is available in the market with the help of which you can monitor the autogeneous inside pressure at a particular temperature for a particular solvent. Nonetheless, there is also autoclaves without pressure gauge, but in this case you have to calculate inside pressure from literature (cursor search this time) for a particular solvent system under supercritical (solvothermal) condition. Also, if you use microwave for solvothermal reaction, specially built equipment (Milestone, Ultraclave unit) is there, where you can manually adjust your desired pressure and temperature with the help of additional N2 input pressure and microwave energy, respectively. 

Most importantly, for your rectilinear temperature (220 dC), the choice of autoclave is very important. As per my experience, most Teflon autoclave deform/degrade at 220 dC. It is better to use Titanium autoclave for reaction above 220 dC which can handle temperature upto 400 dC and even more higher pressure than ordinary autoclave.

Hope these information will help you for your research.

To answer my own question about acetonitrile. I first found the Density-temperature relationship of acetonitrile. 

Kmol/m^3 = 1.3064/(.22597^(1+(1-T/545.5)^ .28678))

At 220C, the density is 0.508g/cc, which is 64.5% of the density at room temperature. At a fill factor below 64.5%, the pressure is just vapor pressure of acetonitrile at 220C, about 25 bar. At a fill factor above 64.5%, liquid acetonitrile fills the vessel entirely and the pressure is much higher. I still don't know how to calculate the pressure though. With water, I guess I can read off the pressure from figure 5 in my previous attached paper by drawing a vertical line and finding the intercept. Not sure if there is such data for acetonitrile.

Like says V. G. Thomas, be aware of unexpected reactions. I would tell my negative experience. For recrystallization of two polycyclic aromatics I took DMSO and 180C, filling about 80% in two autoclaves. One autoclave was Ok. Another one contained brominated aromatic - this autoclave get exploded. At room temperature this compounds are nonreactive. At 180C looks like gases like SO2, HBr, C2H6 formed and rise pressure above 100 atm.

I agree that caution much be taken in designing solvothermal reaction, especially when it is easy to exceed the safety limit of the reactor by overfilling, let alone we don't fully understand what will happen a lot of time. 

Dear Yuxing Wang,

take care to an important point. If the reaction container (autoclave, Parr reactor, or any other closed vessel) is completely filled by the solvent/solution, due to the volume increase generated by the temperature increase, an enormous hydrostatic pressure is generated. I quote here a paragraph from Parr reactors instructions: “The loading limits prescribed for these bombs are purposely conservative to ensure that the energy released from the sample will not overstress the bomb. The user must also remember that when a water-based solution is heated to 250 °C it expands to fill a space approximately 25% larger than its volume at room temperature. If there is insufficient space to accommodate this expansion, the tremendous hydrostatic pressure which will be generated will destroy the bomb.” For example, taking into account that the usual Parr bomb volume is about 23 mL, 19 mL of water, heated at 250 °C, would expand to about 23.75 mL, thus occupying the whole volume, and eliminating the vapor. At this point the enormous hydrostatic pressure, would cause the explosion of the reactor. Thus, maintain the starting volume of your solution at a safety level. All this, independently from the other points previously stressed by colleagues, as possible reactions with formation of unexpected dangerous products as gases.