Synergistic Adsorption and Molecular Arrangement of Mixed Surfactants at the Air/Water Interface

The self-assembly of mixed surfactantsat the air/water interfaceplays a crucial role in mineral flotation separation. This study aimedto investigate the molecular arrangement and synergistic adsorptionof mixed surfactants at the air/water interface using molecular dynamics(MD) simulation and surface tension measurement. The stability ofmixed surfactant solutions was also measured using dynamic light scatteringand zeta potential. We investigated the mixture of octylhydroxamicacid (OHA) and dodecanol (DOD) surfactants at different weight ratios.The surface activity of the OHA/DOD mixture was characterized in detail,including the relative concentration distribution, structural properties,and radial distribution function. The MD simulations revealed thatOHA/DOD was inserted into the water as a headgroup at the air/waterinterface, and the carbon chain extended into the air. A strong synergisticeffect was observed between the two surfactants, and a dense monolayerfilm was formed at the air/water interface, demonstrating higher surfaceactivity. With an increase in the DOD ratio, the hydrogen bond betweenthe OHA headgroup and water molecules became stronger. The order ofsurface activity of the mixture was OHA:DOD = 1:1 > OHA:DOD = 2:1> OHA:DOD = 4:1. In the temperature range from 24 to 30 degrees C,thesurface tension decreased and the critical micelle concentration increasedwith increasing temperature. The addition of DOD changed the conformationof the mixed surfactants, which was beneficial to the aggregationof OHA/DOD complexes. When OHA:DOD = 1:1, the average particle sizewas the largest and the formed micelles were the most stable. Theexperimental results are consistent with the surface tension and simulationresults.