Surface modification of colloidal silica particles using cationic surfactant and the resulting adsorption of dyes

Surfactant interactions, self-association, and self-organizing properties can play an important role in customizing colloidal particles surface. Colloidal silica (CS) is an essential biocompatible material, which makes it a suitable candidate for various studies related to surface modifications. In this work, colloidal silica (SiO2) was synthesized using a modified Stober's process. Point of zero charge (PZC) was measured by standard pH titrations. Field emission scanning electron microscopy (FE-SEM) was employed to examine the particle's morphology while phase integrity was confirmed using powder X-rays diffraction (XRD) technique. CS interactions with a cationic dye “methylene blue” (MB) and anionic dye “congo red” (CR) were investigated spectrophotometrically. Later on, CS was modified using cetyltrimethylammonium bromide (CTAB) and observed the changes in PZC and surface morphology, zeta potential and hydrodynamic diameter. The interactions of CTAB modified particles with MB and CR were reinvestigated. For both modified colloidal SiO2 (MCS) and unmodified colloidal SiO2 (CS), different models of adsorption and adsorption kinetics were applied to the UV–Vis spectroscopic data to quantify binding interactions, adsorption efficiency, changes in enthalpy (ΔH), entropy (ΔS), and free energy (ΔG) of the adsorption processes. CS has shown efficient adsorption of MB while MCS was found as a good adsorbent for CR. CTAB enabled the modification of an “anionic surface” of CS to an efficient “cationic surface” and this was demonstrated by significant absorption of an anionic dye CR on the CS surface as well as by the changes in zeta potential and hydrodynamic radius of particles before and after modifications by CTAB.