Effect of surfactant types and their concentration on the structural characteristics of nanoclay.

A series of organo-modified nanoclays was synthesized using three different surfactants having different alkyl chain lengths and concentrations [0.5-5.0 cation exchange capacity (CEC)]. These surfactants were Ethanolamine (EA), Cetyltrimethylammoniumbromide (CTAB) and Tetraoctadecylammoniumbromide (TO). The obtained modified nanoclays were characterized by X-ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM) and compared with unmodified nanoclay. The results of XRD analysis indicated that the basal d-spacing has increased with increasing alkyl chain length and surfactant concentration. From the obtained microstructures of these organo-modified nanoclays, the mechanism of surfactant adsorption was proposed. At relatively low loading of surfactant, most of surfactant entered the spacing by an ion-exchange mechanism and is adsorbed onto the interlayer cation sites. When the concentration of the surfactant exceeds the CEC of clay, the surfactant molecules then adhere to the surface adsorbed surfactant. Some surfactants entered the interlayers, whereas the others were attached to the clay surface. When the concentration of surfactant increased further beyond 2.0 CEC, the surfactants might occupy the inter-particle space within the house-of-cards aggregate structure.

A novel luminescent terbium-3-carboxycoumarin probe for time-resolved fluorescence sensing of pesticides methomyl, aldicarb and prometryne.

The luminescence arising from lanthanide cations offers several advantages over organic fluorescent molecules: sharp, distinctive emission bands allow for easy resolution between multiple lanthanide signals; long emission lifetimes (µs-ms) make them excellent candidates for time-resolved measurements; and high resistance to photo bleaching allow for long or repeated experiments. A time-resolved (gated) luminescence-based method for determination of pesticides methomyl, aldicarb and prometryne in microtiterplate format using the long-lived terbium-3-carboxycoumarin in 1:3 metal:ligand ratio has been developed. The limit of detection is 1.20×10(6), 5.19×10(5) and 2.74×10(6)ng L(-1) for methomyl, prometryne and aldicarb, respectively. The quantum yield (QY=0.08) of Tb(III)-3-carboxycoumarin was determined using 3-(2-benzothiazolyl)-7-diethylamino-coumarin (coumarin 6). Stern-volmer studies at different temperatures indicate that collisional quenching dominates for methomyl, aldicarb and prometryne. Binding constants were determined at 303, 308 and 313 K by using Lineweaver-Burk equation. A thermodynamic analysis showed that the reaction is spontaneous with negative ΔG. Effect of some relevant interferents on the detection of pesticides has been investigated.

Sorption profile and chromatographic separation of uranium (VI) ions from aqueous solutions onto date pits solid sorbent.

The retention profile of uranium (VI) as uranyl ions (UO(2)(2+)) from the aqueous media onto the solid sorbent date pits has been investigated. The sorption of UO(2)(2+) ions onto the date pits was achieved quantitatively (98+/-3.4%, n=5) after 15 min of shaking at pH 6-7. The sorption of UO(2)(2+) onto the used sorbent was found fast, followed by a first order rate equation with an overall rate constant, k of 4.8+/-0.05 s(-1). The sorption data were explained in a manner consistent with a "solvent extraction" mechanism. The sorption data were also subjected to Freundlich isotherm model over a wide range of equilibrium concentration (1-20 microgmL(-1)) of UO(2)(2+). The results revealed that, a "dual-mode" of sorption mechanism involving absorption related to "solvent extraction" and an added component for "surface adsorption" is most likely operated simultaneously for uranyl ions uptaking the solid sorbent. The thermodynamic parameters (-DeltaH, DeltaS and DeltaG) of the uranyl ions uptake onto the date pits indicated that, the process is endothermic and proceeds spontaneously. The interference of some diverse ions on the sorption UO(2)(2+) from the aqueous media onto the date pits packed column was critically investigated and the data revealed quantitative collection of UO(2)(2+) at 5 mLmin(-1) flow rate. The retained UO(2)(2+) was recovered quantitatively with HCl (3.0 molL(-1)) from the column at 5 mLmin(-1) flow rate. The mode of binding of the date pits with UO(2)(2+) was determined from the IR spectral date bits before and after extraction of uranium (VI). The height equivalent (HETP) and the number (N) of theoretical plates of the date pits packed column were determined from the chromatograms. Complete retention and recovery of UO(2)(2+) spiked to wastewater samples by the date pits packed column was successfully achieved. The capacity of the used sorbent towards retention of uranium (VI) from aqueous solutions was much better than the most common sorbents.

Platelet aggregometry can accurately predict failure of externally supported knitted Dacron femoropopliteal bypass grafts.

PURPOSE: Our purpose was to evaluate whether a method for quantification of platelet aggregability will predict failure of knitted Dacron femoropopliteal bypass grafts. METHODS: A numerically derived platelet aggregation (PA) score, based on the aggregation pattern and platelet count, was determined in the 40 patients available for platelet analysis who underwent 53 femoropopliteal bypass grafts with preclotted, 6 mm, externally supported knitted Dacron grafts from 1981 to 1991 (mean follow-up 50 months). The preoperative score was found to remain stable after surgery, enabling the use of postoperative values when preoperative values were not available. The PA score was available in 19 patients (23 grafts) before surgery and 23 patients (30 grafts) after surgery. The following factors were analyzed for predicting graft failure by the Cox proportional hazards regression model: PA score, age, gender, history of smoking, coronary artery disease, hypertension, hyperlipidemia, cerebrovascular disease, diabetes, claudication versus limb salvage, site of the distal anastomosis, previous ipsilateral bypass, and state of the runoff as determined by preoperative angiography. RESULTS: Of the studied risk factors, the value of the PA score was the most significant predictor of graft closure (p < 0.0001). An increase of 10 units was associated with an increased relative risk of 2.02. Throughout the follow-up period, 15 of 16 grafts remained patent in patients with a PA score of 15 or less, but only 2 grafts out of 17 remained patent in patients with a PA score of 30 or greater. CONCLUSIONS: These data suggest that the PA score is a potential risk factor for failure of femoropopliteal bypass with externally supported knitted Dacron grafts.