Characterization of hygroscopic behavior of single inorganic salt particles across the Hofmeister series

The hygroscopic behavior of inorganic salts is central to understanding ion-specific effects (SIEs) across biological, atmospheric, and industrial systems. In this study, we systematically investigate single particles of trisodium citrate (Na 3 C 6 H 5 O 7 ), sodium dihydrogen phosphate (NaH 2 PO 4 ), sodium chlorite (NaCl), sodium bromide (NaBr), sodium nitrate (NaNO 3 ) and sodium thiocyanate (NaSCN), representing kosmotropic, neutral, chaotropic anions along the Hofmeister series. Individual particles were levitated into a Paul Electrodinamic Trap (PET) and probed with Cavity Ring-Down Spectroscopy (CRDS) under controlled dehydration-hydration cycles. Particle radius was retrieved from elastic scattering at 473 nm, while extinction cross sections were measured at 532 nm, enabling complementary parameterizations of hygroscopic growth. The results reveal three distinct behaviors: kosmotropic salts exhibit limited growth and quasi-reversible hydration; neutral salts undergo abrupt deliquescence and efflorescence with strong hysteresis; and chaotropic salts display continuous growth with enhanced water uptake but minimal hysteresis. Growth factors, optical responses, and derived hygroscopicity parameters ( κ GF and κ emp ) collectively demonstrate systematic increases along the Hofmeister series, with NaCl standing out as a deviation from the Hofmeister trend, showing higher hygroscopicity than salts of similar position in the series. These findings establish an experimental benchmark for ion-specific hygroscopicity and highlight both the strengths and limitations of Hofmeister- based interpretations. Extending this framework to organic and biomolecular systems will further elucidate the interplay between composition and ion-organic-water interactions. • Single particle levitation with PET-CRDS enables precise hygroscopic characterization across the Hofmeister series. • Three hygroscopic regimes: kosmotropic show weak reversible growth; neutral abrupt transitions: chaotropic continuous cycle. • Hygroscopic parameters ( κ GF and κ emp ) increase across Hofmeister series, with NaCl and NaSCN as key anomalies.