لخّصلي

This study used NMR, a non-destructive, fast, and accurate method requiring small sample volumes, to analyze water uptake by silica gel. Hydrogen proton resonance, induced by a radio frequency pulse in a magnetic field, measures the transverse relaxation time (T2); longer T2 indicates greater water mobility. A VTMR20-010V-I NMR instrument with specific parameters (e.g., 21 MHz RF frequency, CPMG sequence) was used. Silica gel samples were analyzed at 40°C and 85% relative humidity. Pre- and post-adsorption NMR signals were compared, integrating signal areas against T2 to determine water content changes. This data, combined with material quality changes, allowed deduction of the specific water adsorption reaction equation (detailed in a later section).

NMR is nondestructive and noninvasive and requires small sample volume
has high detection accuracy and fast speed. It detects water by
applying a radio frequency (RF) pulse to the sample in a constant
magnetic field to resonate hydrogen protons. Some low-energy
Hydrogen protons absorb energy and transit to a high-energy state.
When the RF pulse is turned off, the protons release the absorbed FR
wave in a nonradiative form, and the energy returns to the ground state
to attain the Boltzmann equilibrium. This process is a relaxation process.
and the time constant to complete the relaxation process is the relaxation
tion time [28]. The transverse relaxation time T2 is often used to detect
the state of water in a material. The larger the T2, the higher the degree
of freedom of water in a sample.
VTMR20-010V-I NMR instrument from Suzhou Niumag Analytical
Instrument Co., Ltd. was used in this study. The silica gel samples were
placed on the center of the RF coil at the center of the permanent magnet
for sampling in a constant temperature and humidity climate chamber
with a temperature of 40◦C and a relative humidity of 85%. The probe
coil was 10 mm, the magnet strength was 0.5 T, and the temperature of
The magnet was 35°C ±0.01°C. The parameters were set as follows:
main RF frequency SF = 21 MHz, offset O1 = 579483.7 Hz, hard pulse
90◦pulse width P1 =3.2 μs, hard pulse 180◦pulse width P2 = 6.2 μs,
sampling frequency SW = 333.333 KHz, repeat sampling interval time
TW = 500 ms, analog gain RG1 = 20 db, digital gain DRG1 =3, pre
amplification gain PRG =3, echo time TE = 0.08 ms, number of echoes
NECH = 500, and cumulative sampling times NS = 32. The transverse
relaxation time was measured by Carr-Purcell-Meiboom-Gill (CPMG)
sequence, and then the T2 inversion spectrum was obtained by T2
inversion with NMR inversion software. The form and content of water
binding in the sample could be obtained by analyzing the position and
area of the wave peak on the T2 inversion spectrum.
NMR is often used to detect the water status and content of food.
Here, it was employed to detect the water uptake by silica gel. The NMR
signal values were detected before and after water adsorption on silica
gel. The integral area of the signal values with the transverse relaxation
time as the abscissa was calculated. Through the above method, we
analyzed the internal reaction and change in water content during water
Adsorption by silica gel. The change in material quality before and after
the reaction will affect the expression of the chemical equation. Com
bined with the change in water content, we deduced the specific reaction
equation for water adsorption. This work is further described in section