ACID-BASE PROPERTIES OF THE SURFACE OF NANOSCALE FE3O4

Abstract

Nanosized magnetite is widely used to create magnetically sensitive nanocomposites for biological and medical applications. Its low toxicity and the possibility of surface functionalization make it a unique object for research. Nanosized magnetite (Fe3O4) was synthesized by the sol-gel method, using Fe(II) and Fe(III) chlorides and 25% ammonia solution as precursors. The morphological characteristics of magnetite nanoparticles (MNPs) were determined by electron microscopy (JEOL 1200 EX with a tungsten filament (accelerating voltage 120 kV). According to the obtained data, MNPs have a spherical shape with an average diameter of 6,76 ± 1,54 nm without significant aggregation. X-ray structural studies were carried out by powder X-ray diffraction (DRON-UM1 with Fe filter CuKα-radiation, Bragg- Brentano focusing, in the range 2θ 20–60° with a step of 0,1°, exposure 1 s.). The diffraction patterns show reflections (at 2θ = 30,1; 35,6; 44; 53,3; 57,4; 62,8 with interplanar distances 2,96; 2,52; 2,05; 1,71; 1,60; 1,47), which correspond to the crystalline phase of magnetite. The acid-base properties of the surface were investigated by pH-metry: the pH value of the surface isoelectric state (pHIEP), the total concentration and the Ki and pK values of surface hydroxyl groups active in the pH range 4–9, and the dependences of the proportion of surface hydroxyl groups on the pH of the medium in a 0,015 M NaCl solution were calculated. It was found that for nanoparticles of synthesized samples, neutral hydroxyl groups prevail, which makes it possible to form centers of both acidic and basic types. Thus, for Fe3O4 up to pH = 6, basic centers (С = 155,56 ·10−5 mol·g−1) are active, the strength of which decreases with decreasing pH (pK = 2,65 – 4,48). Acid centers in a very small amount (4,93·10− 6 mol·g− 1) are fixed in the pH range 6,5–9 and exhibit weakly acidic properties (pK = 10,95–11,24).