Oxypropylene chain length affects the physicochemical properties of ortho ester-based gemini surfactants

In this study, we synthesized ortho ester-based surfactants, with oxypropylene chain lengths from 1 to 3000, from a short-chain ortho ester, propylene glycol, and polyoxyethylene stearyl ether. We subsequently reacted these ortho ester surfactants with propylene glycol as a spacer to yield gemini surfactants. We characterized the surface activities of each of these gemini surfactants in terms of its critical micelle concentration (CMC), surface excess concentration (ΓCMC), surface area demand per molecule (ACMC), surface tension at the CMC (γCMC), and standard free energy of micellization per mole of monomer unit (ΔGm). Increasing the oxypropylene chain length had a significant influence on the surface activity. Overall, these ortho ester-based gemini surfactants possess excellent surface properties, providing low contact angles and emulsion stability.

The contact angle of this solution decreased upon increasing the concentration of the surfactant, reaching a constant value within a narrow concentration range. This phenomenon, which is similar to the behavior of the surface tension and fluorescence, was due to greater size of the oxypropylene chain in the spacer decreasing the concentration of the surfactant at the liquid surface.Figure optionsDownload as PowerPoint slideHighlights
► These ortho ester-based gemini surfactants possess excellent surface properties.
► An increase in the concentration of the surfactant led to a gradual decrease in the value of I1/I3.
► Increasing the oxypropylene chain length led to a decrease in the CMC.
► The contact angle on glass decreased from 35° to 15° upon increasing the oxypropylene chain length to 3000.
► The modified ortho ester-based gemini surfactants yielded more-stable emulsions than did unmodified 1820.