Characterization
of Functionalized Chromatographic
Nanoporous Silica Materials by Coupling Water Adsorption and Intrusion
with Nuclear Magnetic Resonance Relaxometry
posted on 2024-01-08, 16:36authored byCarola Schlumberger, Carlos Cuadrado Collados, Jakob Söllner, Christoph Huber, Dorothea Wisser, Hsiao-Feng Liu, Chun-Kai Chang, Stephanie A. Schuster, Mark R. Schure, Martin Hartmann, J. Ilja Siepmann, Matthias Thommes
Nanoporous silica is widely used
as a support material
for chemically
bound stationary phases in chromatographic separations as well as
for catalytic applications. Tuning of textural properties and surface
chemistry of stationary phase materials (SPMs) is crucial to enhance
their selectivity to certain compounds and the resulting process efficiency.
Nanoporous silica supports are beneficial, as surface modifications
are possible with a large variety of hydrophilic and hydrophobic functional
groups, but their influence on the surface properties has not been
evaluated in detail. In this sense, the contact angle is a key parameter
for the assessment of surface chemistry, but its quantification in
pores is challenging and requires a combination of various experimental
techniques. This work demonstrates that combining water adsorption
and intrusion measurements allows for the determination of the effective
contact angle of adsorbed water on the pore walls for wetting, partial
wetting, and nonwetting situations using functionalized hydrophilic
and hydrophobic silica SPMs as model materials. Furthermore, NMR relaxometry
experiments reveal that the ratio of the spin–lattice (T1) to spin–spin (T2) relaxation time of the adsorbed water film (T1,ads.film/T2,ads.film ratio)
can be correlated with the effective adsorption strength of water
on the surface. Indeed, a linear correlation between the negative
inverse relaxation time ratio (−T2,ads.film/T1,ads.film) and the contact angle is
observed. Our work demonstrates that water vapor adsorption and water
intrusion experiments coupled with NMR relaxometry can be used as
complementary tools to quantify the wettability and surface chemistry
of nanoporous materials. This approach allows for addressing important
aspects of nanoscale wettability in the context of material synthesis
as well as for their application in chromatographic separation processes
and catalysis.