Adsorption of CTAB on Sapphire-cat High pH: Surface and ZetaPotential Measurements Combined with Sum-Frequency andSecond-Harmonic Generation

A Abdelmonem and YC Zhang and B Braunschweig and D Glikman and A Rumpel and W Peukert and T Begovic and XD Liu and J Lutzenkirchen, LANGMUIR, 38, 3380-3391 (2022).

DOI: 10.1021/acs.langmuir.1c03069

The adsorption of cetyltrimethylammonium bromide (CTA+Br-)on sapphire- csurfaces was studied at pH 10 below the surfactants'critical micelleconcentration. The evolution of interfacial potentials as a function of CTABconcentration was characterized by surface and zeta potential measurements andcomplemented by molecular dynamic (MD) simulations as well as by second-harmonic (SHG) and vibrational sum- frequency generation (SFG) spectroscopy.The changes in interfacial potentials suggest that the negative interfacial chargedue to deprotonated surface aluminols groups is neutralized and can be evenovercompensated by the presence of CTA+cations at the interface. However, SFGintensities from strongly hydrogen-bonded interfacial water molecules as well asSHG intensities decrease with both increasing CTAB concentration and the magnitude of the surface potential. They do not suggesta charge reversal at the interface, while the change in zeta potential is actually consistent with an apparent charge inversion. This canbe qualitatively explained by results from MD simulation, which reveal adsorbed CTA+cations outside afirst strongly boundhydration layer of water molecules, where they can locally distort the structural order and replace some of the interfacial watermolecules adjacent to thefirst layer. This is proposed to be the origin for the significant loss in SFG and SHG intensities withincreasing CTAB concentration. Moreover, we propose that CTA+can act as a counterion and enhance the occurrence ofdeprotonated surface aluminols that is consistent with the decrease in surface potential.

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