Gas phase charged aggregates of bis(2-ethylhexyl)sulfosuccinate (AOT) and divalent metal ions: first evidence of AOT solvated aggregates

Abstract

Assembling and chelating properties of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) towards divalent metal ions have been investigated in the gas phase by electrospray ionization mass spectrometry. A variety of positively charged monometallated and mixed metal aggregates are formed. Interestingly, several ions contain solvent (MeOH, H(2)O) molecules and constitute the most abundant AOT cationic aggregates not containing sodium. These species are the first example of solvated AOT-metal ion aggregates in the gas phase. By increasing the surfactant aggregation number, the abundance of solvated species becomes lower than that of unsolvated ones. Decompositions of ionic species have been studied by tandem mass spectrometry, and their stability has been determined through energy resolved mass spectrometry. In contrast with positively charged AOT-alkaline metal ion aggregates, whose decompositions are dominated by the loss of individual surfactant molecules, AOTNa-divalent ion aggregates mainly dissociate through the cleavage of the AOT H(2)C-O bond followed by further intramolecular fragmentations. This finding, that is consistent with an enhanced chelation of divalent ions with AOT(-) head groups, has been taken as an indication that such aggregates are characterized by a reverse micelle-like organization with a ionic core formed by the metal cations interacting with the negatively charged surfactant polar heads, whereas the surfactant alkyl chains point outside.

Surfactant self‐assembling in the gas phase: bis(2‐ethylhexyl)‐sulfosuccinate divalent metal ion anionic aggregates

“…It is worth noting that, while we observed for the first time inclusion of solvent molecules in positively charged AOT‐M II aggregates, such as [AOTM II (CH 3 OH) 2 ] + , solvated species were not detectable when the overall charge of the aggregate was negative.…”

“…The monomeric positive aggregates with divalent metal ions must contain only one AOT – anion and one M II metal cation, whose most common coordination number is 6. However, [AOTM II ] + aggregates have never been observed in the gas phase, suggesting that one AOT – anion has four coordination sites available and two oxygen atoms from solvent molecules are required to complete the coordination sphere …”

“…In the case of positively charged AOTNa‐divalent ion aggregates, after the cleavage of the AOT H 2 C–O bond, further fragmentation occurs leading to the elimination of CO 2 . Such behavior was attributed to a strong interaction between divalent ions and surfactant head groups consistent with a reverse micelle‐like structure of the aggregates …”

“…Recently, it has also been shown that the charged reverse micelle‐like aggregates formed by AOTNa and AOT – salts in the gas phase are able to entrap some solutes. Small salt clusters included in [AOT n Li n+1 ] + (n = 1 – 4) aggregates and in [AOT 4 Na 3 ] – have been observed, as well as the entrapment of water or methanol in positively charged species formed by AOT 2 M II (M II = Ca 2+ , Mg 2+ , Co 2+ , Mn 2+ ) . Furthermore, Fang et al reported a thorough study on the inclusion of amino acids into multiply charged reverse micelles formed by AOTNa in the gas phase .…”

Mass Spectrometry Study of Multiply Negatively Charged, Gas-Phase NaAOT Micelles: How Does Charge State Affect Micellar Structure and Encapsulation?

“…Solvent molecules are not necessary for building a large gas-phase aggregate architecture [8], neither are they required for entrapping guest molecules (vide infra). In fact, while solvation of small gas-phase AOTdivalent metal ion clusters has been recently detected [43] , solvated NaAOT aggregates has not been reported yet.…”