# Surface-water hydrolysis reaction & reaction/adsorption kinetics model

I didn't major in Chem or Chem Eng when I was an undergraduate, so I have a rather limited background in this field, please forgive me if my questions are too basic or very wrong.

I wish to model a hydrolysis reaction between an amorphous silica glass surface and water. Pure silica glass and pure water. No acid, no other atoms or molecules. To do this, I ran some MD scale simulations. From MD models, I can get the number of decreasing H2O and increasing OH, and surface number density of OHs.

Then, I wish to apply some reaction kinetics models to get reaction constants. Then, I wish to predict the number of surface OHs at the different temperatures & pressure conditions, after a specific amount of time for each temperature.

I can control the initial concentration of water from the MD model. My target temperature is high (800~1500K) so all water will be superheated steam. And I will assume very high relative humidity near the surface, so the steam vapor pressure will be assumed as 1 atm (atmospheric pressure).

Hydroxyl formation reaction on the silica glass surface with water is very well known:

1) X-Si-O-Si-X + H2O (steam vapor for my case) --> X-SiOH + SiOH-X
and reversely,
2) X-SiOH + SiOH-X --> X-Si-O-Si-X + H2O
Where X is O-Si-O-Si-O...


Now, I wish to apply some kinetics models. But which one should I need to apply for?

I was considering the Langmuir Isotherm, especially the modified Langmuir model that considers the lateral interactions because I suspect there might be OH-OH and OH-H2O H-bond networks to attract more hydrolysis. Equations for Langmuir or modified Langmuir are well known.

However, I see some journal papers researching Alkoxysilanes, and they use the typical rate of reaction equation (rate = k[A]^x[B]^y) to the surface hydrolysis reaction. In terms of reaction kinetics, hydrolysis of Alkoxysilanes is really the same with silica surface + water hydrolysis.

So... I'm really confused. Which model should I need to apply for surface hydrolysis reaction? I thought the rate of reaction equation should be applied to the liquid-liquid or liquid-gas or gas-gas reactions, and use Langmuir isotherm for interfacial models of solid-gas or solid-liquid. But am I wrong?

Another curious point is the order of adsorption (or order of reaction). Since OH formation is the dissociative reaction and requires one bridge oxygen site on the surface and one water molecule, I think I need to assume the reaction as the second order. Interestingly, I really look through a lot of paper, I couldn't find the paper that describes the order of reaction for the hydrolysis reaction between pure silica surface and pure water. And interestingly, some papers assumed the OH formation reaction as the first order. So... may I ask what would be the best approximation for the silica surface-water hydrolysis?

Lastly, how can I define surface coverage from the amorphous surface? In crystal surface, it is very easy to estimate the possible maximum reaction site. But in amorphous, it is not easy like that. I could use reaction density (number per area) but still, I'm not sure about this one.