Surface properties of tetra(ethylene oxide) dodecyl amide compared with poly(ethylene oxide) surfactants. 1. Effect of the headgroup on adsorption
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The surfactant tetra(ethylene oxide) n-dodecyl amide (TEDAd) was compared with the poly(ethylene oxide) n-alkyl ethers C12E5, C10E5, and C12E4. The adsorption to hydrophobic liquid-vapor and solid-liquid interfaces, as well as the wetting behavior on a hydrophobic surface, was investigated. The amide group is more hydrophilic than the ethylene oxide group, raising the cloud point and critical micelle concentration (cmc). The incorporation of an amide group decreases the adsorbed amount at a fixed surfactant concentration. For all studied surfactants the adsorbed amount was significantly lower on the solid-liquid interface than on the liquid-vapor interface at a fixed concentration below the cmc, but the maximum adsorbed amount was only slightly lower on the solid-liquid interface. The surface pressure vs area/molecule isotherms reveal attractive interactions between the amide groups in the adsorbed layer, which reduces the surface pressure at a fixed area/molecule. The advancing contact angles were only dependent on the surface tension at the liquid-vapor interface for C12E5 and TEDAd. The calculated adsorbed amount from the advancing contact angle agreed with the adsorbed amount on the liquid-vapor interface, while the adsorbed amount calculated from the receding contact angle was closer to the equilibrium value obtained with ellipsometry on the solid-liquid interface. The experimental results are supported by mean-field lattice model calculations, and due to the unfavorable interaction between the hydrocarbon region and the amide group in the adsorbed layer, the boundary between the hydrophobic and hydrophilic regions in the adsorbed layer becomes better defined in the presence of an amide group. This might be one of the explanations for the differences found in surface force measurements, and this will be described in part 2 of this investigation.