Spontaneous and Ion-Specific Formation of Inverted Bilayers at Air/Aqueous Interface

Abstract

Developing better separation technologies for rare earth metals, an important aspect of a sustainable materials economy, is challenging due to their chemical similarities. Identifying molecular scale interactions that amplify the subtle differences between the rare earths can be useful in developing new separation technologies. Here, we describe ion-dependent monolayer to inverted bilayer transformation of extractant molecules at the air/aqueous interface. The inverted bilayers form with Lu 3+ ions but not with Nd 3+ . By introducing Lu 3+ ions to preformed monolayers, we extract kinetic parameters corresponding to the monolayer to inverted bilayer conversion. Temperature-dependent studies show Arrhenius behavior with an energy barrier of 40 kcal/mol. The kinetics of monolayer to inverted bilayer conversion is also affected by the character of the background anion, although anions are expected to be repelled from the interface. Our results show the outsized importance of ion-specific effects on interfacial structure and kinetics, pointing to their role in chemical separation methods.

Elucidating Trivalent Ion Adsorption at Floating Carboxylic Acid Monolayers: Charge Reversal or Water Reorganization?

“…For example, light and heavy lanthanides interact completely differently with DHDP, a phospholipid similar to DMPA but lacking the OH group. Although light lanthanides adsorb at DHDP monolayers without overcharging, heavier lanthanides cause the formation of inverted bilayers. , …”

“…The stability of colloids depends on the ions in the solution . Chemical separations require selective ion adsorption and transfer. − A molecular scale understanding of ion–surface water interactions is crucial for addressing the key challenges in these fields.…”

“…Although light lanthanides adsorb at DHDP monolayers without overcharging, heavier lanthanides cause the formation of inverted bilayers. 10,13 The behavior of ions at charged interfaces is a central question in separation science, energy storage, and atmospheric chemistry. Due to the complexity inherent to the interfaces, a thorough description requires consolidation of results from various techniques that are sensitive to different aspects of the interfaces.…”

Elucidating Trivalent Ion Adsorption at Floating Carboxylic Acid Monolayers: Charge Reversal or Water Reorganization?

“…3 Chemical separations require selective ion adsorption and transfer. [4][5][6][7][8][9][10][11][12][13] A molecular scale understanding of ion-surface-water interactions is crucial for addressing the key challenges in these fields.…”

“…Although light lanthanides adsorb at DHDP monolayers without overcharging, heavier lanthanides cause formation of inverted bilayers. 10,13 Behavior of ions at charged interfaces is a central question in separation science, energy storage, and atmospheric chemistry. Due to the complexity inherent to the interfaces, a thorough description requires consolidation of results from various techniques that are sensitive to different aspects of the interfaces.…”