Complex Behavior of Aqueous α-Cyclodextrin Solutions. Interfacial Morphologies Resulting from Bulk Aggregation

Abstract

The spontaneous aggregation of α-cyclodextrin (α-CD) molecules in the bulk aqueous solution and the interactions of the resulting aggregates at the liquid/air interface have been studied at 283 K using a battery of techniques: transmission electron microscopy, dynamic light scattering, dynamic surface tensiometry, Brewster angle microscopy, neutron reflectometry, and ellipsometry. We show that α-CD molecules spontaneously form aggregates in the bulk that grow in size with time. These aggregates adsorb to the liquid/air interface with their size in the bulk determining the adsorption rate. The material that reaches the interface coalesces laterally to form two-dimensional domains on the micrometer scale with a layer thickness on the nanometer scale. These processes are affected by the ages of both the bulk and the interface. The interfacial layer formed is not in fast dynamic equilibrium with the subphase as the resulting morphology is locked in a kinetically trapped state. These results reveal a surprising complexity of the parallel physical processes taking place in the bulk and at the interface of what might have seemed initially like a simple system.

Cyclodextrin Films with Fast Solvent Transport and Shape‐Selective Permeability

“…Here, we used a hydrocarbon‐based terephtaloyl chloride (TC) solution and cyclodextrins in the aqueous phase instead of the conventional diamines (e.g., m ‐phenylenediamine, p ‐phenylenediamine, piperazine). This approach presents several challenges: (i) cyclodextrins are much larger and bulkier than diamines, (ii) the hydroxyl (OH) groups of cyclodextrins are less reactive than amines, and have different reactivities among them, and (iii) cyclodextrins tend to agglomerate in aqueous solutions . Nevertheless, by optimizing preparation conditions, we produced continuous β‐CD films on top of a commercial polyacrylonitrile (PAN) porous support ( Figure ).…”

Propofol adsorption at the air/water interface: a combined vibrational sum frequency spectroscopy, nuclear magnetic resonance and neutron reflectometry study

“…Two different data acquisition approaches were used to determine: (1) the adsorption isotherm and (2) the interfacial structure of the adsorbed molecules at the air/water interface, following the methodology adopted in ref. 39 In the first case, measurements were made only in ACMW, and the data were reduced only over 4.5-12 Å to restrict the q z -range to 0.01-0.03 Å -1 . This approach reduces the sensitivity of the analysis to details of the structure at the interface.…”

Rings, Hexagons, Petals, and Dipolar Moment Sink-Sources: The Fanciful Behavior of Water around Cyclodextrin Complexes

“…As a consequence, the increase of mass in AA force fields could reduce the sampling even if the time step is increased. The mass increase of just polar Hs from 1 to 2 Da is justified based on the experimental fact that only the polar H atoms are spontaneously deuterated (thus doubling their mass) when they are solvated in deuterated water [1,23]. In the present work, we propose to also test the mass increase of just the polar H up to 4 Da.…”

“…The behavior of native cyclodextrins (CDs) in aqueous solution and at the air/water interface is much more complex than expected just considering their apparently simple molecular structure [1][2][3]. Consequently, the correct prediction of their properties and skills such as their solubility, their ability to adsorb at interfaces, or to encapsulate a variety of molecules is not straightforward; not to mention their propensity to aggregate forming different patterns in the bulk solution and at the water/air interface [4][5][6].…”