Microdroplet Microreactors: From Passive Ion Carriers to Active Chemical Vessels
Sanjay Choudhary
Department of Chemistry, Government Degree College, Kathua, India.
Chandan Sharma
Department of Chemistry, Government Degree College, Kathua, India.
Sapna Sharma *
Department of Chemistry, Government Degree College, Paloura, India.
*Author to whom correspondence should be addressed.
Abstract
Microdroplets have moved from a supporting role in mass spectrometry to a contested but increasingly consequential class of chemical microreactors. Their historical identity as carriers that transfer ions from solution to the gas phase has been revised by evidence that reactions can proceed differently in small droplets than in corresponding bulk solutions, sometimes with substantial increases in product formation over millisecond-to-second timescales. This critical narrative review examines how that conceptual transition occurred, which mechanisms are supported by the strongest evidence, and how convincingly present platforms function as controllable reactors rather than analytically convenient reaction environments. Literature was selected through live searches of accessible multidisciplinary, chemical and biomedical scholarly indexes, supplemented by citation chasing and DOI-level verification. The synthesis distinguishes concentration effects produced by evaporation from genuine interfacial kinetic effects; evaluates surface enrichment, partial solvation, electric fields, charge, acidity, redox chemistry and gas uptake as reaction-specific contributors; and uses the hydrogen-peroxide controversy as a stress test for mechanistic claims. Evidence from charged electrospray droplets, levitated droplets, droplet-fusion arrangements and electrically unactivated droplets shows that no single mechanism explains all reported acceleration. The most defensible interpretation is that a microdroplet is a transient, non-equilibrium reactor trajectory whose volume, composition, surface-to-volume ratio, charge state, interfacial solvation and gas exchange evolve together. This perspective clarifies why apparent acceleration factors depend strongly on how reaction time, volume change, sampling and ionisation are defined. Preparative nebulisation, solvent-recycling systems, high-throughput reaction screening and interfacial electrochemistry demonstrate meaningful engineering progress, but analytical throughput and striking local rates do not by themselves establish scalable, energy-efficient synthesis. Future work should prioritise orthogonal product quantification, independent measurement of droplet histories, deliberately neutral or charge-controlled comparisons, standardised reporting of reactor productivity, and life-cycle-relevant process metrics. Microdroplets are therefore best regarded neither as passive ion carriers nor as universally catalytic interfaces, but as actively evolving vessels whose chemical function must be demonstrated for each reaction and reactor architecture.
Keywords: Air-water interface, charged droplets, electrospray ionisation, interfacial chemistry, reaction acceleration, reaction engineering, high-throughput synthesis, non-equilibrium chemistry.