Highly viscoelastic films at the water/air interface: α-Cyclodextrin with anionic surfactants

High Interfacial Viscoelasticity of Aqueous Mixed Dodecyltrimethylammonium Bromide-Sodium Dodecyl Sulfate Surfactants Forming Inclusion Complexes with α-Cyclodextrin

Mixtures of anionic-cationic surfactants have shown high synergistic effects in the bulk solution and at the liquid/air interface. These studies have been limited to a reduced concentration range, where there is no formation of aggregates or precipitates. The addition of host molecules, such as cyclodextrins, to these systems reduces the effects of precipitation by forming inclusion complexes and also modifies the values of other surfactant properties, like the Krafft temperature and the critical aggregation concentration (CAC). We studied the interfacial synergistic effects promoted by electrostatic interactions, using the Rosen model to calculate an interaction parameter for mixtures of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in the presence of α-cyclodextrin (αCD), in aqueous solutions. We measured the CAC of SDS-DTAB-αCD mixtures using a pendant drop tensiometer, with the αCD concentration fixed at 10 mM and at 283.15 K. We performed rheological measurements on the mixtures where the surfactant total concentration is fixed below the measured CAC, varying the αCD concentration and temperature. We found that the dilatational modulus shows a clear correlation with the interaction parameter. It appears that the attractive interactions within the film are those due to the inclusion complexes formed by two αCD and one surfactant molecule, which according to the previous studies, is the dominant species in both the bulk and liquid/air interface. The synergistic effect observed here for SDS-DTAB surfactant mixtures with αCD can be applied to systems and processes (drop emission, drug delivery methods, stabilization of viral capsids and bacterial membranes, and emulsification) where interfacial processes require specific viscoelastic properties.

Dynamical shapes of droplets of cyclodextrin-surfactant solutions

We present a series of experiments with droplets of aqueous cyclodextrin-surfactant solutions, in which the volume is reduced after the equilibrium spherical shape is reached. The final shape of the drop after this perturbation is found to be dependent on the concentration of inclusion complexes in the bulk of the solution. These inclusion complexes are formed by two cyclodextrin molecules and one surfactat molecule. We propose a model to describe these dynamical processes. Dipole–dipole interactions on the surface of the drop trigger a competition between water surface tension and dipole–dipole interaction energies. The results of the model reproduce the spherical and rod-like shapes found in the experiments.

Molecular Dynamics Simulations of Transmembrane Cyclic Peptide Nanotubes Using Classical Force Fields, Hydrogen Mass Repartitioning, and Hydrogen Isotope Exchange Methods: A Critical Comparison

Self-assembled cyclic peptide nanotubes with alternating D- and L-amino acid residues in the sequence of each subunit have attracted a great deal of attention due to their potential for new nanotechnology and biomedical applications, mainly in the field of antimicrobial peptides. Molecular dynamics simulations can be used to characterize these systems with atomic resolution at different time scales, providing information that is difficult to obtain via wet lab experiments. However, the performance of classical force fields typically employed in the simulation of biomolecules has not yet been extensively tested with this kind of highly constrained peptide. Four different classical force fields (AMBER, CHARMM, OPLS, and GROMOS), using a nanotube formed by eight D,L-α-cyclic peptides inserted into a lipid bilayer as a model system, were employed here to fill this gap. Significant differences in the pseudo-cylindrical cavities formed by the nanotubes were observed, the most important being the diameter of the nanopores, the number and location of confined water molecules, and the density distribution of the solvent molecules. Furthermore, several modifications were performed on GROMOS54a7, aiming to explore acceleration strategies of the MD simulations. The hydrogen mass repartitioning (HMR) and hydrogen isotope exchange (HIE) methods were tested to slow down the fastest degrees of freedom. These approaches allowed a significant increase in the time step employed in the equation of the motion integration algorithm, from 2 fs up to 5–7 fs, with no serious changes in the structural and dynamical properties of the nanopores. Subtle differences with respect to the simulations with the unmodified force fields were observed in the concerted movements of the cyclic peptides, as well as in the lifetime of several H-bonds. All together, these results are expected to contribute to better understanding of the behavior of self-assembled cyclic peptide nanotubes, as well as to support the methods tested to speed up general MD simulations; additionally, they do provide a number of quantitative descriptors that are expected to be used as a reference to design new experiments intended to validate and complement computational studies of antimicrobial cyclic peptides.

Cyclodextrin-based Pickering emulsions: functional properties and drug delivery applications

Cyclodextrins (CDs) are biocompatible, cyclic oligosaccharides that are widely used in various industrial applications and have intriguing interfacial science properties. While CD molecules typically have low surface activity, they are capable of stabilizing emulsions by inclusion complexation of oil-phase components at the oil/water interface, which results in Pickering emulsion formation. Such surfactant-free formulations have gained considerable attention in recent years, owing to their enhanced physical stability, improved tolerability, and superior environmental compatibility compared to conventional, surfactant-based emulsions. In this review, we critically describe the latest insights into the molecular mechanisms involved in CD stabilization of Pickering emulsions, including covering practical aspects such as methods to prepare CD-based Pickering emulsions, lipid encapsulation, and relevant stability issues. In addition, the rheological and textural features of CD-based Pickering emulsions are discussed and particular attention is focused on promising examples for drug delivery, cosmetic, and nutraceutical applications. The functionality of currently developed CD-based Pickering emulsions is also summarised, including examples such as antifungal uses, and we close by discussing emerging possibilities to utilize the molecular encapsulation of CD-based emulsions for translational medicine applications in the antiviral and antibacterial spaces.

Dipole-dipole interactions control the interfacial rheological response of cyclodextrin/surfactant solutions

A recent surface rheological study has shown that aqueous solutions of α-cyclodextrin (αCD) with anionic surfactants (S) display a remarkable viscoelasticity at the liquid/air interface, which has not been observed in similar systems. The dilatational modulus is various orders of magnitude larger than those for the binary mixtures αCD + water and S + water. The rheological response has been qualitatively related to the bulk distribution of species, the 2 : 1 inclusion complexes (αCD2 : S) playing a fundamental role. In this work, we have developed a model that considers dipole-dipole interactions between 2 : 1 inclusion complexes ordered on the liquid/air interface. When the model is applied to the specific experimental conditions, the dependencies on concentration and temperature of the dilatational modulus and the surface tension were found to be in excellent agreement with the data, indicating clearly that dipole-dipole interactions determine and control the rheological behavior of the interface.

Cyclodextrin/surfactant inclusion complexes: An integrated view of their thermodynamic and structural properties

Cyclodextrins (CDs) play an important role in self-assembly systems of amphiphiles. The structure of CDs provides distinguished physicochemical properties, including the ability to form host-guest complexes. The complexation affects the properties of guest molecules and can produce supramolecular aggregates with desirable characteristics for fundamental and practical applications. Surfactants are particularly attractive host molecules due to their wide variety, availability, responsiveness to different stimuli, and high relevance in different fields, e.g. medical, cosmetic, pharmaceutical, and food industries. The tendency of organization in higher-order supramolecular aggregates arises the interest in applying such versatile complexes in the development of novel materials. In this review, we provide a comprehensive overview of the thermodynamics aspects of surfactants and CDs inclusion complexes formation in aqueous environment, emphasizing the assessment of the interactions, thermodynamic driving forces, and structural aspects. Also, the most common analytical techniques used to gather deep insight into the aspects of CDs complexes are discussed and the perspectives for the surfactant-cyclodextrin complexes are pointed out.

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

The basket-like geometry of cyclodextrins (CDs), with a cavity able to host hydrophobic groups, makes these molecules well suited for a large number of fundamental and industrial applications. Most of the established CD-based applications rely on trial and error studies, often ignoring key information at the atomic level that could be employed to design new products and to optimize their use. Computational simulations are well suited to fill this gap, especially in the case of CD systems due to their low number of degrees of freedom compared with typical macromolecular systems. Thus, the design and validation of solid and efficient methods to simulate and analyze CD-based systems is key to contribute to this field. The behavior of supramolecular complexes critically depends on the media where they are embedded, so the detailed characterization of the solvent is required to fully understand these systems. In the present work, we use the inclusion complex formed by two α-CDs and one sodium dodecyl sulfate molecule to test eight different parameterizations of the GROMOS and AMBER force fields, including several methods aimed to increase the conformational sampling in computational molecular dynamics simulation trajectories. The system proved to be extremely sensitive to the employed force field, as well as to the presence of a water/air interface. In agreement with previous experiments and in contrast to the results obtained with AMBER, the analysis of the simulations using GROMOS showed a quick adsorption of the complex to the interface as well as an extremely exotic behavior of the water molecules surrounding the structure both in the bulk aqueous solution and at the water surface. The chirality of the CD molecule seems to play an important role in this behavior. All together, these results are expected to be useful to better understand the behavior of CD-based supramolecular complexes such as adsorption or aggregation driving forces, as well as to introduce new methods able to speed up general MD simulations.