For determine whether amino acids are adsorbed onto silica surface in the solid state, you can provide the following experiment.

However, for this experiment you need to have special equipment.

You need to press silica into a thin tablet, place it in a special cuvette, create a vacuum, and release the amino acid vapors. The amino acid is adsorbed on the silica. It is analyzed by the FTIR. Then you can create a vacuum again, and ideally also heat it up, to study the binding force of the amino acid to the surface, for example for modified silica.

Not exact, but similar DOI: 10.1039/c9ra07721a

Yongze Chen

The adsorption process is an equilibrium process. If you are conducting an experiment on the adsorption of amino acids on the surface of silicon dioxide, you need to make sure that equilibrium has been reached. The concentration of amino acids in water should reach a constant value after some time. Then you need to wash the precipitate with water, not alcohol, controlling the washing away of the remaining excess amino acids to the equilibrium value. Otherwise, the equilibrium value of the amount of amino acids during adsorption should be approximately equal to the amount of amino acids after washing.

The solid-solid reactions are very slow.

Amino acids have appreciable vapor pressures at T>150. Hypothetically, you cam remove AA from the mixture. While, an adsorption is reversible. Therefore, the removal of AA will results in desorption. Thus, you will never know the amount of adsorbed AA after grinding.

Any washing procedure exposes your sample to a solvent. Thus, you will get the equilibrium of AA on the surface and in solution.

In addition, you never know the rates of sorption/desorption.

Your concerns: "wouldn't you actually wash off these AA that are already adsorbed onto the surface?" Probably, yes

"Can this sufficiently remove free AAs that are not adsorbed onto the silica while keeping those adsorbed intact?" This can remove ALL AA. You can't keep those adsorbed intact.

Dear Dr. Yongze Chen

I'm afraid I don't agree with you in that you don't think the interaction of adsorbed AA onto silica surface is covalent via hydrogen bonding . In [1] , item 3.4. FTIR Analysis, figure 3 shows the function of relative changes in peaks area within the band in the region 2850-2950 cm-1 in terms of calcination temperatures. This region is often assigned to symmetric vibration of the C-H group, therefore such function indicates the intensity of the organic substitution in the obtained modified silica. It is clear that the temperature should not exceed 300oC to maintain reasonable modification and if you want to hold all of modification intensity and at the same time getting rid of washing solution, dry at less than 100oC.

Best regards .....

[1] Article Investigating the effects of calcination temperatures on the...