Optical tweezer platform for the characterization of pH-triggered colloidal transformations in the oleic acid/water system

Hypothesis: Soft colloidal particles that respond to their environment have innovative potential for many fields ranging from food and health to biotechnology and oil recovery. The in situ characterisation of colloidal transformations that triggers the functional response remain a challenge.

Experiments: This study demonstrates the combination of an optical micromanipulation platform, polarized optical video microscopy and microfluidics in a comprehensive approach for the analysis of pH-driven structural transformations in emulsions. The new platform, together with synchrotron small angle X-ray scattering, was then applied to research the food-relevant, pH-responsive, oleic acid in water system.

Findings: The experiments demonstrate structural transformations in individual oleic acid particles from micron-sized onion-type multilamellar oleic acid vesicles at pH 8.6, to nanostructured emulsions at pH < 8.0, and eventually oil droplets at pH < 6.5. The smooth particle-water interface of the onion-type vesicles at pH 8.6 was transformed into a rough particle surface at pH below 7.5. The pH-triggered changes of the interfacial tension at the droplet-water interface together with mass transport owing to structural transformations induced a self-propelled motion of the particle. The results of this study contribute to the fundamental understanding of the structure–property relationship in pH-responsive emulsions for nutrient and drug delivery applications.