Effect of recognized and unrecognized salt on the self-assembly of new thermosensitive metal-chelating surfactants

Clouding phenomenon in amphiphilic systems: A review of five decades

Phase separation in amphiphilic systems is an important phenomenon. The temperature at which an amphiphilic solution phase separates is known as Cloud Point (CP). This article reviews in detail the process of phase separation in various amphiphiles (surfactants, polymers and drugs) and effect of different classes of additives on the CP of these amphiphilic systems. Ions affect the CP of drugs in a different way: kosmotropes and hard bases decrease while chaotropes and soft bases increase the CP of nonionic and cationic surfactants. Anionic surfactants show CP in presence of quaternary salts only. Thus, depending upon the nature and concentration of additive, the CP of an amphiphilic system gets increased or decreased and, hence, properties of the system may be tuned as per the need and use. A system with CP at high concentration can be made to phase separate at lower concentration by simply introducing an appropriate additive in it. This makes the system cost effective. On the other hand, if not required, a low CP can be enhanced with the help of another type of a suitable additive.

Polyoxyethylene alkyl ether carboxylic acids: An overview of a neglected class of surfactants with multiresponsive properties

In this work, an overview on aqueous solutions of polyoxyethylene alkyl ether carboxylic acids is given. Unique properties arise from the combination of the nonionic, temperature-responsive polyoxyethylene block with the weakly ionic, pH-responsive carboxylic acid termination in a single surfactant headgroup. Accordingly, this class of surfactant finds broad application across very different sectors. Despite their large use on an industrial and a technical scale, the literature lacks a systematic and detailed characterization of their physico-chemical properties which is provided herein. In addition, a comprehensive overview is given of their self-assembly and interfacial behavior, of their use as colloidal building blocks and for large-scale applications.

Extractant mediated nano-aggregate formation in Triton X-114 aided cloud formation: structural insights from TEM and SANS studies

Nanoaggregate formation by self assembly was noticed during the cloud formation of Triton X-114 in the presence of dibenzoylmethane (DBM), thenoyltrifluoroacetone (HTTA) and 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP) and tributyl phosphate (TBP).

Performance of the Biocompatible Surfactant Tween 80, for the Formation of Microemulsions Suitable for New Pharmaceutical Processing

The aim of this work was to investigate the phase behaviour and the structure of the n-hexane/water emulsions based on a nonionic, nontoxic and biocompatible surfactant, Tween 80. This system is of interest for new pharmaceutical techniques based on supercritical fluids to form nano- and encapsulated particles. However, it showed a lack of stability denoted by large areas of macroemulsion. For this reason, the effect of additives (alcohols and brine) and external variables (temperature) were explored. The replacement of water by brine caused negligible impact due to the nonionic character of Tween 80. On the contrary, the presence of an alcohol (ethanol or 1-butanol) enhanced the solubility of the surfactant in the oil phase and decreased the mixture viscosity, resulting in improved surface activity. Similar results were obtained by raising the temperature until the cloud point was reached (60°C). With these modifications, microemulsions at relatively low concentrations of surfactant (around 30%) and within a broad interval of compositions could be obtained, widening their possible use in pharmaceuticals manufacturing (such as controlled drug delivery, enzymatic reactions, or excipient processing). The understanding of the surfactant performance could be further used to substitute the n-hexane by a greener solvent, such as supercritical CO2.

Thermo-responsive Metal-chelating Surfactants: Properties and Use in Cloud Point Extraction of Uranyl Nitrate∗

The properties of new thermo-responsive functional surfactants, capable of forming a metal chelate, synthesized by grafting a diamide group (amino-acid residue) to the tip hydrophilic endgroup or in a branched position to polyoxyethylene nonionic surfactants [CiEj: CiH2i+1(OCH2CH2)jOH)], are studied. Their use in cloud point extraction of uranyl nitrate is tested. The reversible temperature-dependent behavior of classical non-ionic surfactants associated to phase separation of micellar solutions known as clouding behavior is exploited for separation based on cation specific binding to the chelating group. The functional surfactants under investigation combine surface-active properties and characteristic thermoreversible behavior with a capacity to bind uranyl cation. The influence of the complexation on the cloud points of functional surfactants is determined. The chelating surfactants are found efficient for the cloud point extraction of uranyl nitrate at low surfactant-to-uranyl ratio. These new thermoresponsive surfactants with chelating properties hold most promise for the development of new solvent free extraction processes.

Balance of coordination and hydrophobic interaction in the formation of bilayers in metal-coordinated surfactant mixtures

Metal-ligand coordination and hydrophobic interaction are two significant driving forces in the aggregation of mixtures of M(n+) surfactants and alkyldimethylamine oxide (CnDMAO) in aqueous solutions. The coordinated systems exhibit rich aggregation behavior. This study investigated the effect of M(n+) ions (Zn(2+), Ca(2+), Ba(2+), Al(3+), Fe(3+), La(3+), Eu(3+), and Tb(3+)) and hydrophobic chains (hydrocarbon and fluorocarbon) on the formation of metal-coordinated bilayers. We found that fluorocarbon chains and branched hydrocarbon chains are preferable to the corresponding linear hydrocarbon chains for the formation of an Lα phase. Moreover, Lα phases formed by fluorocarbon chains exhibited higher viscoelasticity than ones formed by the hydrocarbons, and the bilayers formed by branched chains were rather flexible, revealing obvious undulation. The construction of bilayers was also strongly affected by metal ions due to their variable coordination ability with CnDMAO. Our results contribute to the understanding of the formation of metal-coordinated bilayers, which is driven by the interplay of noncovalent forces.

Clouding behaviour in surfactant systems

A study on the phenomenon of clouding and the applications of cloud point technology has been thoroughly discussed. The phase behaviour of clouding and various methods adopted for the determination of cloud point of various surfactant systems have been elucidated. The systems containing anionic, cationic, nonionic surfactants as well as microemulsions have been reviewed with respect to their clouding phenomena and the effects of structural variation in the surfactant systems have been incorporated. Additives of various natures control the clouding of surfactants. Electrolytes, nonelectrolytes, organic substances as well as ionic surfactants, when present in the surfactant solutions, play a major role in the clouding phenomena. The review includes the morphological study of clouds and their applications in the extraction of trace inorganic, organic materials as well as pesticides and protein substrates from different sources.

Fluorescence correlation spectroscopy reveals strong fluorescence quenching of FITC adducts on PEGylated gold nanoparticles in water and the presence of fluorescent aggregates of desorbed thiolate ligands

Colloidal gold particles functionalised with oligoethylene-glycolated disulfide ligands and fluorescent moieties derived from fluorescein isothiocyanate (FITC) have been prepared and studied in aqueous suspension using fluorescence correlation spectroscopy (FCS). FCS probes the dynamics of the particles at the single object level, and reveals the desorption of fluorescent ligands which subsequently aggregate into larger (slower diffusing) objects. Cross-correlation spectroscopy of the FITC fluorescence and the Rayleigh-Mie scattering (RM-FCCS) of the gold cores shows that the only detectable fluorescent objects are free ligands and aggregates not associated with a gold particle. The fluorescence of bound fluorophores is quenched making their fluorescence too weak to be detected. FCS and RM-FCCS are useful tools for characterising functionalised noble metal particles in solution, under conditions similar to those used in optical bio-imaging. Desorption of thiolates from gold nanoparticles needs to be taken into account when working with these materials at low concentration.

XAS study of a metal-induced phase transition by a microbial surfactant

The metal-induced micelle-to-vesicle phase change that the ferric complex of the microbially produced amphiphile, marinobactin E (M(E)), undergoes has been investigated by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Marinobactin E is one member of the suite of siderophores, marinobactins A-E, that are used by the source bacterium to facilitate iron acquisition. Fe(III)-M(E) undergoes a micelle-to-multilamellar vesicle transition in the presence of Cd(II) and Zn(II). XRD measurements indicate the interlamellar repeat distance of the Cd(II)- and Zn(II)-induced multilamellar vesicles is approximately 5.3 nm. XAS spectra of the sedimented Cd(II)- and Zn(II)-induced multilamellar vesicles suggests hexadentate coordination of Cd(II) and Zn(II) consisting of two monodentate carboxylate ligands and four water ligands. This coordination environment supports the hypothesis that Cd(II) and Zn(II) bridge the terminal carboxylate moiety of two Fe(III)-M(E) headgroups, pulling the headgroups together in an arrangement that favors vesicle formation over the formation of micelles. XAS spectra of the Fe(III) center in the sedimented Cd(II)- and Zn(II)-induced vesicles confirm the anticipated six-coordinate geometry of Fe(III) by the M(E) headgroup via the two hydroxamate groups and the alpha-hydroxy amide moiety.

Cloud point extraction for the determination of copper, nickel and cobalt ions in environmental samples by flame atomic absorption spectrometry

A cloud point extraction procedure was presented for the preconcentration of copper, nickel and cobalt ions in various samples. After complexation with methyl-2-pyridylketone oxime (MPKO) in basic medium, analyte ions are quantitatively extracted to the phase rich in Triton X-114 following centrifugation. 1.0 mol L(-1) HNO(3) nitric acid in methanol was added to the surfactant-rich phase prior to its analysis by flame atomic absorption spectrometry (FAAS). The adopted concentrations for MPKO, Triton X-114 and HNO(3), bath temperature, centrifuge rate and time were optimized. Detection limits (3 SDb/m) of 1.6, 2.1 and 1.9 ng mL(-1) for Cu(2+), Co(2+) and Ni(2+) along with preconcentration factors of 30 and for these ions and enrichment factor of 65, 58 and 67 for Cu(2+), Ni(2+) and Co(2+), respectively. The high efficiency of cloud point extraction to carry out the determination of analytes in complex matrices was demonstrated. The proposed procedure was applied to the analysis of biological, natural and wastewater, soil and blood samples.