Simulation Evidence of Nanobubble Clusters of Gas in Water: A Nanoscale Solvation Mechanism

Previous experiments have shown that when water is mixed with dissolvable organic molecules (DOMs), it can lead to insoluble gas-containing nanoclusters. However, molecular insights into the gas-containing nanoclusters are still lacking. Herein, we show the first large-scale simulation evidence of stable nanobubble (NB) clusters (∼2 nm radius) of insoluble gas in water mixed with DOMs by using electrically neutral interface models. Notably, our molecular dynamics simulations demonstrate a dissolution-nanobubble-dissolution transition for the (insoluble) gas in the water/DOM solutions (as the mole fraction of DOMs is increased), a phenomenon observed across most DOM types excluding extreme polarity cases. The latter evidence suggests that the formation of gas-containing NB clusters is likely a generic phenomenon for water mixed with a modest amount of DOMs. The re-entrant dissolution behavior can be attributed to the dual roles of DOMs, i.e., for promoting and stabilizing NB clusters at low gas mole fraction and generating continuous nanostructures of the gas beyond the NB formation threshold. Our independent theoretical analysis confirms the stability of NB clusters in water/DOM solutions. Akin to micelle formation, NB formation results in increased solubility of insoluble gas in water/DOM solutions. From a fundamental solution chemistry perspective, the formation of NB clusters can be viewed as a nanoscale solvation mechanism that differs markedly from the conventional molecular solvation mechanism for the insoluble gas in water.