The concept of ‘Smart Emulsions’ has captured the attention of research due to the applicability in industrial practices where temporary emulsions are preferred. Switchable surfactants can function as a switch to adjust the emulsion stability when assisted by an external trigger. Many studies of switchable surfactants have provided insights on the reversibility of bulk emulsion properties. However, the interfacial phenomena within these emulsions at a molecular level is yet to be explored. This understanding is crucial as emulsion stability depends on both interfacial chemical composition and molecular arrangement.
This study utilizes the pH-dependent charge switchability of N-dodecylpropane-1,3-diamine (DPDA) to modify the emulsion stability. DPDA is identified as a surfactant that can exhibit diverse self-assembly in water. Herein, a study was conducted to understand the interfacial molecular organization at air-water and dodecane(oil)-water interfaces using sum frequency generation spectroscopy, elaborated with other techniques.
The packing of interfacial alkyl chains had most defects when water was at low pH. As shown in the figure, this observation was from the low intensity ratio between the CH₃ symmetric stretch (SS) at ~2880 cm^-1 and CH₂ SS at ~2850 cm^-1. At the same pH, the maximum hydrogen (H) bonding between the interfacial water molecules was determined by the highest intensity of OH SS. At pH=7, interfacial alkyl chains were well-ordered while water molecules also had better H-bonding. This influenced the most stable oil-water emulsion. When water was at high pH, water molecules had less H-bonding and relatively less ordered alkyl chains at the interface, giving desirable conditions for demulsification. This was compatible with the observation in bulk emulsion preparation, confirming both alkyl chain packing and water H-bonding at the interface contribute to the overall emulsion stability.

Emma Kubacki

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