Facile Monitoring of Water Hardness Levels Using Responsive Complex Emulsions

Abstract

The cationic content of water represents a major quality control parameter that needs to be followed by a rapid, onsite, and low-cost method. Herein, we report a novel method for a facile monitoring of the mineral content of drinking water by making use of responsive complex emulsions. The morphology of biphasic oil-in-water droplets solely depends on the balance of interfacial tensions, and we demonstrate that changes in the surfactant effectiveness, caused by variations in the mineral content inside the continuous phase, can be visualized by monitoring internal droplet shapes. An addition of metal cations can significantly influence the surfactant critical micelle concentrations and the surface excess values and therefore induce changes in the effectiveness of ionic surfactants, such as sodium dodecyl sulfate. The morphological response of Janus emulsions droplets was tracked via a simple microscopic setup. We observed that the extent of the droplet response was dependent on the salt concentration and valency, with divalent cations (responsive for water hardness), resulting in a more pronounced response. In this way, Ca 2+ and Mg 2+ levels could be quantitatively measured, which we showcased by determination of the mineral content of commercial water samples. The herein demonstrated device concept may provide a new alternative rapid monitoring of water hardness levels in a simple and cost-effective setup.

Dynamic In Situ Monitoring of the Salt Counter-ion Effect on Surfactant Effectiveness Using Reconfigurable Janus Emulsions

“…Selectively influencing their effectiveness then allows to dynamically and controllably alter the interfacial tension equilibrium and thus droplet morphology. To this end, a series of active surfactants have been developed that can be selectively attached to specific droplet interfaces, e.g., in order to target specific stimuli, such as light, electric or magnetic fields, − environmental contaminants, , biomacromolecules, or bacteria . Based on the unique chemical-morphological-optical coupling inside reconfigurable complex droplets and depending on a fine-adjustment of the droplets’ starting morphology, marginal variations of the interfacial tension balance can then cause significant variations of the macroscopic optical properties of the emulsion.…”

Detection of PFAS and Fluorinated Surfactants Using Differential Behaviors at Interfaces of Complex Droplets

“…Previous work by Swager, Zarzar, and Zeininger has demonstrated the various methods of leveraging complex droplets for selective detection of analytes. − A key strategy used in this work relies on the coupling between their dynamic morphology and emission intensity as the result of the changing interfacial tensions. That is, depending on the morphology of the droplets, the emission intensity of the embedded fluorescent dye varies significantly.…”

“…Briefly, the change in the balance of the two surfactants (and the two interfacial tensions: γ HW and γ FW ) significantly alters the droplet morphologies. This transformation can be easily detected by simple optical micrographs from both the regular top-view images or a custom-built side-view microscope. , Zeininger and co-workers have demonstrated the use of this sensing method to measure the concentrations of Ca 2+ and Mg 2+ ions to measure water hardness and analyze the critical micelle concentrations of various surfactants. , Our method uses the effect of the change in morphology, namely, the internal interfaces between the H-oil and F-oil, on the direction of the fluorescent emission of the embedded perylene dye. , This transduction mechanism is demonstrated by the schematic in Figure a. The complex droplets are initialized at the Janus state with two hemispherical domains.…”

“…Of particular interest to sensing applications, this dynamic physical transformation can be visualized via an optical microscope or by a measurement of transmitted or emissive light. For example, droplet-based sensors have leveraged the change in transparency, , scattering efficiency, − and emission intensity , to detect enzyme activities, , foodborne pathogens, ,,, viruses, , and trace chemicals. ,, In these examples, the minute changes in interfacial behavior produce easily detectable signals and provide a novel and rapid alternative to a conventional tensiometer.…”

Multiplexed and continuous microfluidic sensors using dynamic complex droplets

“…[5][6][7][8][9][10][11] For example, they have previously been demonstrated as effective detection platforms for biological analytes-such as E. coli, 12,13 Listeria monocytogenes, 14,15 SARS-CoV-2, 16,17 and bacteriophages 18 -as well as environmental contaminants. 19,20 While these examples served as successful proofs-of-concept, crucial limitations exist that prevent the development of an on-site sensing platform for real-world samples and situations. Specifically, many of the current platforms lack the capacity to generate multiplexed or continuous sensing signals.…”