Effect of Inorganic Additives on Solutions of Nonionic Surfactants

Molecular Dynamic Simulations of Aqueous Micellar Organometallic Catalysis: Methane Functionalization as a Case Study

Molecular Dynamics (MD) simulations constitute a powerful tool that provides a 3D perspective of the dynamical behavior of chemical systems. Herein the first MD study of the dynamics of a catalytic organometallic system, in micellar media, is presented. The challenging methane catalytic functionalization into ethyl propionate through a silver-catalyzed process has been targeted as the case study. The intimate nature of the micelles formed with the surfactants sodium dodecylsulfate (SDS) and potassium perfluorooctane sulfonate (PFOS) has been ascertained, as well as the relative distribution of the main actors in this transformation, namely methane, the diazo reagent and the silver catalyst, the latter in two different forms: the initial compound and a silver-carbene intermediate. Catalyst deactivation occurs with halide containing surfactants dodecyltrimethylammonium chloride (DTAC) and Triton X-100. Computed simulations allow explaining the experimental results, indicating that micelles behave differently regarding the degree of accumulation and the local distribution of the reactants and their effect in the molecular collisions leading to net reaction.

Physicochemical perspectives (aggregation, structure and dynamics) of interaction between pluronic (L31) and surfactant (SDS)

The influence of the water soluble non-ionic tri-block copolymer L31 on the microstructure and self-aggregation dynamics of the anionic surfactant sodium dodecylsulfate (SDS) in aqueous solution was investigated.

Krafft Points and Cloud Points of Polyoxyethylated Nonionic Surfactants

The solubility of polyoxyethylated nonionic surfactants (POSs) in water is limited by an upper and a lower critical temperature. The former, called cloud point (CP), is the temperature at which POSs precipitate from solution on heating because of excessive dehydration of their polyoxyethylene (POE) moieties. The latter, called Krafft point (KP), is the temperature at which POSs crystallize from solution on cooling because of increased alignment and attraction between their hydrocarbon chains. It occurs less frequently: Even if the POSs were capable of crystallizing, ice often crystallizes first.

Aqueous solutions of over 100 POSs were examined by obtaining their CPs and ascertaining whether they exhibit KPs. KPs were found in polyoxyethylated primary alcohols and fatty acid amides and in polyethylene glycol esters of fatty acids. Only surfactants with strictly linear hydrocarbon chains crystallize, exhibiting KPs. Any branching, including that in polyoxyethylated linear secondary alcohols, prevents crystallization. Crystallization only occurs when the number mof carbon atoms in the hydrocarbon chains equals or exceeds 12. The KPs are then proportional to m. In homologous series, the surfactants most prone to crystallizing are those with borderline solubility, which possess just enough oxyethylene groups to promote solubility in cold water. Large POE moieties depress or eliminate KPs.

Relation between the lower consolute boundary and the structure of mesoporous silica materials

In this study, we have shed some light on the relation between the position of the lower consolute boundary of various nonionic surfactants in water and the structure of the mesoporous silica materials synthesized from these surfactants-based systems. In the first part, the lower consolute boundary was shifted by adding salts. Depending on the features of the phase diagram, we have looked for either a salting out or a salting in effect. Mesoporous materials were prepared from a micellar solution of the investigated surfactants. Results clearly evidenced that the cooperative self-assembly mechanism is not favored if the lower consolute boundary is not shifted toward high temperatures. Moreover, the higher the difference between the phase separation temperature and the temperature at which the silica precursor is added to the surfactant solution, the better the mesopore ordering is. In the second part, this tendency has been confirmed by using a hydrogenated surfactant as additive.

Determination of diethylhexyladipate and acetyltributylcitrate in aqueous extracts after cloud point extraction coupled with microwave assisted back extraction and gas chromatographic separation

The determination of commercial plasticizers (di-(2-ethylhexyl)adipate (DEHA) and acetyl tributyl citrate (ATBC)) in aqueous solutions is described. The newly proposed technique of applying microwaves to cloud point extracts in order to enable combination with gas chromatographic analysis has been used for this purpose. Both plasticizers were entrapped in the micelles of the non-ionic surfactant Triton X-114 and removed from the bulk phase by centrifugation. Micellization was enhanced by increasing the ionic strength of the solution with concentrated NaCl. Extraction recoveries of the proposed method were over 95% for water and 3% (w/v) aqueous acetic acid and over 85% for 10% (v/v) aqueous ethanol, respectively. The calibration curves obtained, following the proposed methodology have a linear range between 50 and 2000 microg/L for each analyte while the detection limits were as low as 15 and 19 microg/L for DEHA and ATBC, respectively, with an RSD below 5% even for low concentrations. As an analytical demonstration the proposed methodology was applied for the determination of the migration levels of the selected plasticizers from a PVC food packaging film into aqueous simulants.

Cationic surfactant-based polyfluorate salts: phase separation and analytical applications in the extraction and preconcentration of ionic species prior to liquid chromatography

The liquid-solid phase separation originating from the formation of cationic surfactant-based polyfluorate salts (CSBPS) has been explored for extracting and preconcentrating ionic species. Two cationic surfactants were tested; one with aliphatic hydrocarbon tail [Cetyltrimethylammonium bromide (CTAB)]and the other containing a heterocyclic ring [Hexadecylpyridinium bromide (HPyBr)]. Phase separation possibility was investigated with the use of hexafluorophosphates (PF6-) and tetrafluoroborates (BF4-). The effect of added acid, base and salt on the phase separation and analyte extraction was also investigated. In all cases the obtained phase diagrams consisted of two regions: a homogeneous liquid region and a solid-liquid region. Analytes of hydrophilic and hydrophobic nature such as amines, amino acids and organic chromophores were used as test compounds in both their anionic and cationic forms. The respective recoveries ranged from over 90% for anionic species and in the proximity of 50% for cationic species, remaining below 20% for neutral species. Extracts from alkaline aqueous and plasma samples spiked with tyrosine and phenylalanine were also subjected to HPLC separation with UV detection with satisfactory results. On line application was also enabled using a flow through-solid phase extraction-HPLC hyphenated apparatus, thus adding the element of automatization and increased reproducibility.

Cloud Point Extraction Coupled with Microwave or Ultrasonic Assisted Back Extraction as a Preconcentration Step Prior to Gas Chromatography

Cloud point extraction of nonionic and anionic surfactants was applied as a preconcentration step prior to gas chromatography. No cleanup step preceded chromatographic analysis. The obtained surfactant-rich phase was treated with water-immiscible solvents, and the target analytes were back extracted by short-term microwave application or ultrasonication. A mixture of six PAHs (naphthalene, acenaphthene, fluorene, anthracene, fluoranthene, pyrene) was used as test compounds. The obtained detection limits were in the microgram per liter area. Recoveries of spiked water and soil samples ranged between 92 and 105% while analysis of certified reference materials gave results in good agreement with the certified values. Under the optimum experimental conditions, there was no interference or blocking of the column. According to our results, this approach presents a convenient solution to the up-to-date problem of combining gas chromatography with micellar cloud point extraction.

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

New functional thermoreversible metal complexing surfactants consisting of a chelating amino acid residue grafted to the tip of a nonionic surfactant [alkyl poly(oxyethylene) CiEj] or in a branched position are studied. Nonionic surfactants are thermoreversible and exhibit a clouding phenomenon associated with phase separation of micelles. The functional molecules retain both the surface-active properties and the characteristic thermoreversible behavior. Because of the hydrophilic contribution of the chelating group (acetyl lysine), the cloud point and the area at the air-water interface are higher for functional surfactants than for nonionic precursors. These new surfactants have efficient complexing properties toward metal ions and are more efficient than the mixture of the corresponding nonionic surfactant and the acetyl lysine ligand solubilized in micelles. This reveals the synergistic effect obtained by the covalent link between the two functions. Addition of a bulky group on classical amphiphilic structures modifies markedly the packing constraints at the origin ofmicellar structures. Small-angle X-ray or neutron scattering results, modeled jointly on the absolute scale, demonstrate the influence of unrecognized lithium nitrate (LiNO3) as well as specifically recognized uranyl nitrate [UO2(NO3)2] salts on micellar structure and phase boundaries. The determination of the micellar shape variations induced by a recognized salt, that is, a decrease of the polar headgroup, allows the rationalization of uncommon synergistic effects on the cloud point variation: increase with lithium nitrate, no decrease in the presence of uranyl nitrate, and a very large decrease when these two salts are present together.

Cloud-point extraction and preconcentration of cyanobacterial toxins (microcystins) from natural waters using a cationic surfactant

A new cloud-point extraction and preconcentration method using a cationic surfactant, Aliquat-336 (tricaprylylmethylammonium chloride), has been developed for the determination of cyanobacterial toxins, microcystins, in natural waters. Sodium sulfate was used to induce phase separation at 25 degrees C. The phase behavior of Aliquat-336 with respect to concentration of Na2SO4 was studied. The cloud-point system revealed a very high phase volume ratio compared to other established systems of nonionic, anionic, and cationic surfactants. At pH 6-7, it showed an outstanding selectivity in analyte extraction for anionic species. Only MC-LR and MC-YR, which are known to be predominantly anionic, were extracted (with averaged recoveries of 113.9 +/- 9% and 87.1 +/- 7%, respectively). MC-RR, which is likely to be amphoteric at the above pH range, was not detectable in the extract. Coupled to HPLC/UV separation and detection, the cloud-point extraction method (with 2.5 mM Aliquat-336 and 75 mM Na2SO4 at 25 degrees C) offered detection limits of 150 +/- 7 and 470 +/- 72 pg/mL for MC-LR and MC-YR, respectively, in 25 mL of deionized water. Repeatability of the method was 7.6% for MC-LR and 7.3% for MC-YR. The cloud-point extraction process can be completed within 10-15 min with no cleanup steps required. Applicability of the new method to the determination of microcystins in real samples was demonstrated using natural surface waters collected from a local river and a local duck pond spiked with realistic concentrations of microcystins. Effects of salinity and organic matter (TOC) content in the water sample on the extraction efficiency were also studied.

Surfactant cloud point extraction and preconcentration of organic compounds prior to chromatography and capillary electrophoresis

The use of preconcentration steps based on phase separation by the cloud point technique offers a convenient alternative to more conventional extraction systems. It has been used successfully for the preconcentration of species of widely differing character and nature, such as metal ions, proteins and other biomaterials, or organic compounds of strongly differing polarity. Here we address the most recent analytical applications of this methodology when used as an isolation and trace enrichment step prior to the analysis of organic compounds (polycyclic aromatic hydrocarbons, polychlorinated compounds, pesticides, phenolic derivatives, aromatic amines, vitamins, etc.) via liquid and gas chromatography or capillary electrophoresis.

Comparing the Surface Chemical Properties and the Effect of Salts on the Cloud Point of a Conventional Nonionic Surfactant, Octoxynol 9 (Triton X-100), and of Its Oligomer, Tyloxapol (Triton WR-1339)

The surface-chemical properties, critical micelle concentrations (CMC), and effect of salts on the cloud points (CP) of octoxynol 9 (Triton X-100) and tyloxapol (Triton WR-1339) were compared. The latter nonionic surfactant is essentially a heptamer of the former. Even though the molecular weight of tyloxapol is 7 times larger than that of octoxynol 9, its area per molecule adsorbed at the air-water interface is only twice as large. This suggests an unusual orientation for molecules of tyloxapol at the surface and is in keeping with a plateau that is less horizontal and has a somewhat higher surface tension than the plateaus of most nonionic surfactants. The CMC of octoxynol 9 was 4.4 times larger than that of tyloxapol. Unexpectedly, the CP of dilute aqueous tyloxapol solutions was 28 degreesC higher than that of octoxynol 9 solutions. The salting-out ions Na+, Cl- and SO2-4 lowered the CP of tyloxapol 29% more than that of octoxynol 9. However, because the blank tyloxapol solution started out with a higher CP value, its CPs in the presence of salts were higher than those of octoxynol 9. Pb2+ and Mg2+ cations salted both surfactants in, raising their CP, Pb2+ more extensively than Mg2+. Copyright 1998 Academic Press.