Question regarding Silica Gel, Salt, and Vapor Pressure

Factual Questions
1

I’ve been in, for lack of a better term, an argument with some folks on the behavior of Silica Gel when used as a buffer to regulate humidity in a specific range, rather than just as a desiccant.

There are several commercial offerings used in Museums that moderate Relative Humidity to keep it in a specific range - usually somewhere between 40% and 60% depending on what is being housed. It is my understanding that the Amorphous Silica Gel Beads used are regulating a set point based on conditioning with salts to change the vapor pressure of the water that is adsorbed and re-released. I am being told that the pore size in the Silica Gel beads is solely what is regulating RH. I’m not clear on how pore size has an effect on regulating the release and re-adsorption of humidity from the beads.

If one wanted to have ‘conditioned beads’ that regulate at 65% would the pore size be changed, or would the salt content be modified to something like sodium nitrate which has a listed constant humidity at 70 degrees F of 66%

Anyone have the Straight Dope?

2

sorry couldn’t find much. This pdf has some references if you really want to track them down.
www.apsnyc.com/pdf/silica_gel_SW_2003.pdf
The pore size is directly related usualy to the surface area which is probably the most important factor. At first sight the pore size shouldn’t change the vapour pressure, but would influence via the surface area the amount of water that can be absorbed. Adding salts would definitely have an effect.

3

I’d seen that apsnyc link in my googling, and e-mailed them. It appears most of the manufacturers are loath to divulge how their products buffer RH. They did mention they don’t salt treat their product.

I’ve heard mention of Van der Waals forces playing a role at the molecular level with the Silica Gel and I’m trying to determine if salt is even a worthwhile component.