Effect of pH and Salt on the Adhesion between a Particle and the Air/Water Interface

Abstract

Information about the physical properties of the air/water interface has been obtained by measuring the forces between an air/water interface and a particle (probe) in the water with the Monolayer Particle Interaction Apparatus. A silica particle was modified to be positive charge by using N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride. The effect of the pH of the water on the forces was measured by adding HCl or NaOH to the water. The presence of a salt on the force curves was determined by adding NaCl to the water. In the absence of NaCl, the water surface was determined from the force curves to be positively charged at pH 2.0 and negatively charged for pH ≥ 3.0. The charges of interfaces for the pH 2.0 and pH ≥ 3.0 solutions were explained by the adsorption of H⁺ and OH^– ions, respectively, to the air/water interface. The surface of a 10 mM NaCl solution was seen from the force curves to be negatively charged at pH 2.0–7.0 and positively charged at pH 9.0. The addition of NaCl to the water therefore caused the charge pH 2.0 and pH 9.0 water surfaces to reverse. This charge reversal is explained by the specific adsorption of Na⁺ and Cl^– ions to the OH^– and H⁺ ions, respectively, that are at the air/water interfaces.