Hydrophobizing of gypsum by silanes (Part 2)

7 Energetically most favorable conformations of the silane molecules propyltriethoxysilan (EEE) and propyldiethoxysilanol (EEO) on the (020) gypsum cystal face. Atoms in the gypsum crystal face as van der Waals model (blue = Ca, red = O, yellow = S), silane molecules as stick-and-ball model (light brown = Si, red = O, turquoise = CHn); a), b) top view; c), d) oblique view; e), f) schematic sketch
7 Energetically most favorable conformations of the silane molecules propyltriethoxysilan (EEE) and propyldiethoxysilanol (EEO) on the (020) gypsum cystal face. Atoms in the gypsum crystal face as van der Waals model (blue = Ca, red = O, yellow = S), silane molecules as stick-and-ball model (light brown = Si, red = O, turquoise = CHn); a), b) top view; c), d) oblique view; e), f) schematic sketch
8 Energetically most favorable conformations of dimer (left) and trimer (right) on the (020) gypsum surface. Gypsum surface as van der Waals model (blue = Ca, red = O, yellow = S), silane molecules as stick-and-ball model (light brown = Si, red = O, ­turquoise = CHn); top view
8 Energetically most favorable conformations of dimer (left) and trimer (right) on the (020) gypsum surface. Gypsum surface as van der Waals model (blue = Ca, red = O, yellow = S), silane molecules as stick-and-ball model (light brown = Si, red = O, ­turquoise = CHn); top view
9 Single (black) and average values (red) of the interaction energies per silicon atom (Eint) for the different silane species in their most favorable conformation at the (020) gypsum surface in the absence of solvent
9 Single (black) and average values (red) of the interaction energies per silicon atom (Eint) for the different silane species in their most favorable conformation at the (020) gypsum surface in the absence of solvent
10 Fifty snapshots of a proceeding simulation of adsorbed silane molecules in water. Gypsum crystal face as van der Waals model (blue = Ca, red = O, yellow = S), silane molecule single-colored with the color representing simulation time (red = start of the simulation, over white, till blue = end of the simulation) and the solvent water is not shown for clarity
10 Fifty snapshots of a proceeding simulation of adsorbed silane molecules in water. Gypsum crystal face as van der Waals model (blue = Ca, red = O, yellow = S), silane molecule single-colored with the color representing simulation time (red = start of the simulation, over white, till blue = end of the simulation) and the solvent water is not shown for clarity
11 Single (black) and average values (red) of the free energy of desorption for the different monomeric silane species from the (020) gypsum crystal face in the presence of the water solvent
11 Single (black) and average values (red) of the free energy of desorption for the different monomeric silane species from the (020) gypsum crystal face in the presence of the water solvent
Oblique view of the energetically most favor­able conformations of the silane molecules EEE and EEO on the (020) gypsum surface as van der Waals model (blue = Ca, red = O, yellow = S), silane molecules as stick-and-ball model (light brown = Si, red = O, turquoise = CHn)
Oblique view of the energetically most favor­able conformations of the silane molecules EEE and EEO on the (020) gypsum surface as van der Waals model (blue = Ca, red = O, yellow = S), silane molecules as stick-and-ball model (light brown = Si, red = O, turquoise = CHn)

The effectiveness of silanes as known water repellents for gypsum building materials had been the topic of the experimental studies on the example of propyltriethoxysilane in Part 1 of this article (ZKG 7-8/2013, pp. 72-78). Part 2 considers the involved processes on a molecular level, using molecular dynamic simulations. The results are interpreted together with the findings of Part 1 to obtain an understanding of the working mechanism of the hydrophobizing effect of silanes, especially alkyltriethoxysilanes, using the example of propyltriethoxysilane.

2.4 Molecular dynamic (MD) simulations
The MD simulations were carried out using the GROMACS-software package [22, 23] together with the GROMOS96 force field [24]. Force field parameters for calcium ions were taken from Åquist [25]. For silanes the parameters were derived from the GROMOS96 force field parameters. GROMOS96 is a united atom force field, thus different CHn-groups are treated as one atom, for example.

The simulations were based on the assumption that the silane molecules adsorb on the gypsum surface to achieve the hydrophobizing effect. The adsorption/desorption processes were...

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TEXT Dipl. Chem. Andrea Winkler, Dipl. Nat. Susanne Fritz, Dr. Thomas Aberle, Dr. Daniela Freyer, Prof. Dr. Wolfgang Voigt

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