Interfacial properties of cetyltrimethylammonium-coated SiO2 nanoparticles in aqueous media as studied by using different indicator dyes

In this paper, we compare the properties of SiO2/water interface modified by cetyltrimethylammonium bromide (CTAB) with those of CTAB spherical micelles. The suspension of uniform silica nanoparticles coated with CTAB adlayer was investigated by using a set of acid–base indicators. The study of the colloidal system has been provided using electron microscopy and dynamic light scattering methods; the diameter of the initial SiO2 particles in dried state was ca. 40 nm. The increase in the ζ-potential value of nanoparticles from −34 to +(37–54) mV on going from pure silica suspension to the CTAB-containing system points on the silica surface recharging and formation of surfactant bilayer (or multilayer) on the silica/water interface. To obtain further information about the interfacial surfactant adlayer, the behavior of different indicator dyes has been studied in CTAB-modified SiO2 suspension. Comparison of the indices of apparent ionization constants, i.e., pKaa values of phenol red, bromothymol blue, and fluorescein with those determined in CTAB micellar solutions have confirmed the supposition about certain similarity between CTAB-covered silica nanoparticles and common spherical surfactant micelles. However, the experiments on kinetics of bromophenol blue fading, as well as the spectral properties of methyl orange and solvatochromic Reichardt's indicator and some other data revealed the specificity of surfactant-coated silica nanoparticles, presumably, originating from their surface morphology.

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