Dehydration Determines Hydrotropic Ion Affinity for Zwitterionic Micelles

Short-chain branched sulfosuccinate as a missing link between surfactants and hydrotropes

Surfactants aggregate in water into micelles, and these micelles incorporate organic substances to solubilize them. Hydrotropes are compounds that increase the solubility of hydrophobic substances in water without this form of aggregation. Decreasing the chain length of the classical surfactant Aerosol OT (AOT) from C8 to C5 results in a molecule with intermediate properties. Molecular dynamics simulations and surface tension measurements are performed on this short chain derivative of AOT. This compound shows high solubility and at the same time progressive weak aggregation. The hydration of head groups hinders significant plunging into a hydrophobic core, which leads to well defined liquid chain nanodomains. The transition to bicontinuous aggregates is in the concentration range of 1 mol L^-1. The sulfonate group of the head groups (placed at the water interface of worm-like aggregates) rather than the aggregate-aggregate interaction is responsible for the unusual small angle X-ray scattering pattern.

Osmotic Method for Calculating Surface Pressure of Monolayers in Molecular Dynamics Simulations

Surface pressure is a fundamental thermodynamic property related to the activity of molecules at interfaces. In molecular simulations, it is typically calculated from its definition: the difference between the surface tension of the air-water and air-surfactant interfaces. In this Letter, we show how to connect the surface pressure with a two-dimensional osmotic pressure and how to take advantage of this analogy to obtain a practical method of calculating surface pressure-area isotherms in molecular simulation. As a proof-of-concept, compression curves of zwitterionic and ionic surfactant monolayers were obtained using the osmotic approach and the curves were compared with the ones from the traditional pressure tensor-based scheme. The results shown an excellent agreement between both alternatives. Advantageously, the osmotic approach is simple to use and allows to obtain the surface pressure-area isotherm on the fly with a single simulation using equilibration stages.

Surfactant aggregate size distributions above and below the critical micelle concentration

Aggregate size distributions in an aqueous solution containing either charged or neutral surfactants are investigated using Raman multivariate curve resolution (Raman-MCR) spectroscopy and analyzed with the aid of a multi-aggregation chemical potential surface (MCPS) modeling strategy. Total least squares decompositions of the concentration-dependent Raman-MCR spectra are used to quantify the free and micelle surfactant populations, and the surfactant's C-H stretch frequency is used as a measure of its average aggregation state. MCPS predictions relate the experimental measurements to the underlying surfactant aggregate size distribution by fitting either the component concentrations or the average C-H frequency to MCPS predictions, and thus determine the critical micelle concentration (CMC) and estimate the corresponding micelle size and polydispersity. The Raman-MCR spectra of aqueous 1,2-hexanediol, sodium octanoate, and sodium dodecyl sulfate, measured both below and above CMC, provide critical tests of the assumed functional form of the MCPS and the presence of low-order premicellar aggregates. Our results indicate that the low-order aggregate population gradually emerges as the CMC is approached and then remains nearly concentration-independent after the appearance of micelles.

Thermodynamics of anion binding to zwitterionic sulfobetaine micelles

One of the most intriguing aspects of zwitterionic surfactant micelles is their propensity to exhibit selectivity in the binding of the anions of added salts. In this work we examine the thermodynamics of the interaction of the strongly bound perchlorate ion and the more weakly bound bromide ion with micelles of N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB3-14) in aqueous solution employing enthalpies derived from isothermal titration calorimetry combined with Gibbs free energies derived from literature data for the binding equilibria. In both cases, the binding is exothermic and enthalpy driven, but entropically unfavorable, with only modest changes in the Gibbs free energy as a function of the extent of anion binding. Likewise, perchlorate ion binding was found to have little or no effect on the aggregation numbers of SB3-14 micelles determined by time-resolved fluorescence quenching of pyrene by the N-hexadecylpyridinium cation. The results are interpreted within the context of the factors involved in the ion-pairing between the anions and the positive charge center of the zwitterion headgroup and the interplay between electrostatics, solvent reorganization and a net loss of translational degrees of freedom that accompany anion binding.

Naphthalimide-Containing BP100 Leads to Higher Model Membranes Interactions and Antimicrobial Activity

In a large variety of organisms, antimicrobial peptides (AMPs) are primary defenses against pathogens. BP100 (KKLFKKILKYL-NH₂), a short, synthetic, cationic AMP, is active against bacteria and displays low toxicity towards eukaryotic cells. BP100 acquires a α-helical conformation upon interaction with membranes and increases membrane permeability. Despite the volume of information available, the action mechanism of BP100, the selectivity of its biological effects, and possible applications are far from consensual. Our group synthesized a fluorescent BP100 analogue containing naphthalimide linked to its N-terminal end, NAPHT-BP100 (Naphthalimide-AAKKLFKKILKYL-NH₂). The fluorescence properties of naphthalimides, especially their spectral sensitivity to microenvironment changes, are well established, and their biological activities against transformed cells and bacteria are known. Naphthalimide derived compounds are known to interact with DNA disturbing related processes as replication and transcription, and used as anticancer agents due to this property. A wide variety of techniques were used to demonstrate that NAPHT-BP100 bound to and permeabilized zwitterionic POPC and negatively charged POPC:POPG liposomes and, upon interaction, acquired a α-helical structure. Membrane surface high peptide/lipid ratios triggered complete permeabilization of the liposomes in a detergent-like manner. Membrane disruption was driven by charge neutralization, lipid aggregation, and bilayer destabilization. NAPHT-BP100 also interacted with double-stranded DNA, indicating that this peptide could also affect other cellular processes besides causing membrane destabilization. NAPHT-BP100 showed increased antibacterial and hemolytic activities, compared to BP100, and may constitute an efficient antimicrobial agent for dermatological use. By conjugating BP100 and naphthalimide DNA binding properties, NAPHT-BP100 bound to a large extent to the bacterial membrane and could more efficiently destabilize it. We also speculate that peptide could enter the bacteria cell and interact with its DNA in the cytoplasm.