Mass Spectrometry of Surfactant Aggregates

Abstract

In contrast with the enormous amount of literature produced during many decades in the field of surfactant aggregation in liquid, liquid crystalline and solid phases, only a few investigations concerning surfactant self- assembling in the gas phase as charged aggregates have been carried out until now. This lack of interest is disappointing in view of the remarkable theoretical and practical importance of the inherent knowledge. The absence of surfactant-solvent interactions makes it easier to study the role of surfactant-surfactant forces in determining their peculiar self-assembling features as well as the ability of these assemblies to incorporate selected solubilizate molecules. Thus, the study of gas-phase surfactant and surfactant-solubilizate aggregates is a research subject which has exciting potential, including mass and energy transport in the atmosphere, origin of life and simulation of supramolecular aggregation in interstellar space. On the other hand, the structural and dynamic properties of surfactant aggregates in the gas phase could be exploited in a number of interesting applications such as atmospheric cleaning agents, transport and protection of pulmonary drugs or biomolecules and as nanoreactors for specialized chemical reactions in confined space. Spectrometric techniques, together with molecular dynamics simulations, have been the principal investigative tools in this field and appearto be particularly suited to gaining fundamental information on the structure and stability of surfactant-based supramolecular aggregates, charge state effects, entrapment of solubilizate molecules, preferential solubilization sites and chemical reactions localized in a single organized aggregate. The main aim of this review is to present the actual state of the art in this novel and exciting research field underlining the knowledge acquired up to now as well as the aspects needing a more deep understanding. Moreover, intriguing departures of the behavior of surfactant solutions under electrospray ionization conditions from that of ionic, polar and apolar analytes will be discussed.

Native Mass Spectrometry Characterization of Intact Nanodisc Lipoprotein Complexes

“…12 These results demonstrate that Nanodisc complexes in the gas phase closely mirror the properties of Nanodiscs in solution. This is likely due to the constraint from the MSP belt, which maintains the defined structure of the Nanodisc complex.…”

“…17–19 Given the important protective nature of these micelles, there has been significant interest in understanding the behavior of detergent and lipid complexes in the gas phase so as to improve the ionization. 8,9,12,20 …”

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

“…This provides further evidence that surfactant-surfactant interactions are an overwhelming driving force for self-assembling of NaAOT in the gas phase [15]. 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 contrast with enormous work devoted to NaAOT aggregates in solution, only a handful of investigations have been reported concerning NaAOT aggregates as charged species in the gas phase [8][9][10][11][12][13][14][15][16], including electrospray ionization (ESI) and matrix-assisted laser desorption/ionization mass spectra [10,14], time-offlight secondary ion mass spectra [16] and infrared multiple photon dissociation spectra [13] of singly and doubly positively charged NaAOT aggregates. These measurements revealed reverse micelle-like structures for these aggregates.…”

“…Elimination of solvents removes surfactant-solvent interactions, but the electrostatics between surfactant polar heads and their counterions remains. From this perspective, gas-phase aggregates provide a simplified system for better understanding the roles of surfactant-surfactant and surfactantcounterion interactions in their self-assembling and entrapping of guest molecules [15].…”