Citric Acid—A Dispersant for Aqueous Alumina Suspensions

Abstract

The interaction between citric acid and alumina in aqueous solution is characterized. Adsorption isotherms of the dispersant on the alumina surface, electrophoretic mobility of the alumina particles as a function of the citric acid concentration, and attenuated total reflection Fourier transform infrared (ATR‐FTIR) spectroscopy of the citratealumina surface complex have been used. The adsorption behavior of citric acid is dependent on the pH of the suspension and the concentration of the citric acid. The maximum amount of citric acid adsorbed on the alumina surface, 2.17 μ.mol/m2 at pH 3, decreases to 1.17 μmol/m2 at pH 8. The adsorption of citrate causes a highly negatively charged powder surface and a shift of the isoelectric point (IEP) to lower pH values. The IEP of alumina can be fixed at any pH value between 9 and 3 by proper adjustment of the citric acid concentration. In situ ATR‐FTIR spectroscopy of the citrate‐alumina surface complex gives evidence for a direct interaction between the carboxylate groups of the citrate and the surface aluminum(III) atoms. The rheological properties of alumina suspensions are studied as a function of the citric acid concentration. The data obtained from the viscosity and dynamic electrophoretic measurements correlate well and allow the construction of a stability map of alumina suspensions stabilized with citric acid. The influence of citric acid on the viscosity is discussed using the Derjaguin‐Landau‐Verwey‐Overbeek (DLVO) theory. The interaction potential between the particles is determined by the citrate adsorbed on the surface, leading to a negative particle charge, and the citrate anions remaining in the solution, resulting in an increase of the ionic strength. The adsorption of citric acid also creates a steric barrier that inhibits the complete mutual approach of the individual alumina particles.

Humic Acid as Dispersant of an Alumina Suspension and its Rheological Behaviour

“…However, at this pH the HA molecules are in an extended conformation 16 in which steric repulsion can operate. The efficiency of the electrosteric mechanism in an alumina suspension with polyelectrolytes was postulated by Tari et al 12 .…”

“…Polyacrylate salts are a wellestablished and optimised electrosteric dispersants for alumina suspensions, with one carboxylic group per repetition unit in the structure 23 . Surfacial charges are the responsible for the great repulsion capability of the dispersant 16 . On the other hand HA has a complex and not totally understood structure.…”

“…There is no ideal dispersant for all wet ceramic process 16 . Currently, dispersants derived from carboxylic acids dominate the market and citric acid has also been widely utilised.…”

Colagem de barbotina de aluminas submicrométricas comerciais

“…O ácido cítrico vem sendo usado como dispersante em suspensões de alumina, principalmente porque forma um ânion carboxilato na superfície, provocando mudanças na carga superficial e, consequentemente, a repulsão entre as partículas [11,12].…”

Estudo da adsorção de dispersante à base de poliacrilato em um sistema varistor comercial: caracterização físico-química

“…A adsorção específi ca de dispersantes contendo grupos carboxílicos à interface óxido/líquido é ocasionada pela substituição (reação de coordenação) de íons hidroxila (-OH -) ou grupos -OH 2 + presentes na superfície das partículas do óxido pelos íons carboxilato (-COO -) da molécula do dispersante [16]. É assumido que a substituição de grupos superfi ciais -OH 2 + se dê de forma preferencial e que a afi nidade dos polieletrólitos aniônicos por partículas apresentando grupos superfi ciais carregados positivamente é bem maior, porém não podemos dizer que esta afi nidade seja nula para partículas apresentando grupos superfi ciais carregados negativamente.…”