A novel but simple cloud-point extraction (CPE) process is developed to preconcentrate the trace of selected polycyclic aromatic hydrocarbons (PAHs) with the use of the readily biodegradable nonionic surfactant Tergitol 15-S-7 as extractant. The concentrations of PAHs, mixtures of naphthalene and phenanthrene as well as pyrene, in the spiked samples were determined with the new CPE process at ambient temperature (23 degrees C) followed by high performance liquid chromatography(HPLC) with fluorescence detection. More than 80% of phenanthrene and pyrene, respectively, and 96% of naphthalene initially present in the aqueous solutions with concentrations near or below their aqueous solubilities were recovered using this new CPE process. Importantly Tergitol 15-S-7 does not give any fluorometric signal to interfere with fluorescence detection of PAHs in the UV range. No special washing step is, thus, required to remove surfactant before HPLC analyses. Different experimental conditions were studied. The optimum conditions for the preconcentration and determination of these selected PAHs at ambient temperature have been established as the following: (1) 3 wt% surfactant; (2) addition of 0.5 M Na2SO4; (3) 10 min for equilibration time; and (4) 3000 rpm for centrifugal speed with duration of 10 min.

Interaction between the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the anionic polyelectrolyte poly(acrylic acid) (PAA), having molecular weight by viscosity of 750 kDa, is investigated by rheological measurements. Upon addition of 1 or 0.5 wt % CTAB to the semidilute PAA solutions with concentration ranging from 0.04 to 0.1 wt %, the insoluble complex salts (the surfactant ion + the polyion) precipitate. In contrast, with addition of 2 or 3 wt % CTAB to the aforementioned PAA solutions, evidence of transition from shear-thinning to nearly Newtonian behavior is observed. The origin of this observation appears to lie in the fact of formation of more compact PAA/CTAB aggregates, because the radii of gyration, R g , of PAA coils have been reduced by about 60% with addition of 2 wt % CTAB by static light-scattering measurements. Furthermore, pH measurements also reveal the possible formation of "ion pairs" between PAA and CTAB as the pH values of the PAA solutions decrease with addition of CTAB. That is, the pH values of the solutions containing PAA and CTAB together are less than those of neat PAA or CTAB solutions. This implies that addition of CTAB leads to more acids dissociated from the acrylic acid monomers on the poly(acrylic acid) macromolecules.

Polyimide nanoparticles are fabricated using a combined liquid-liquid phase separation and solvent/ nonsolvent mixing technology. This technology allows us to produce stable polyimide nanoparticles with tunable size without any surfactants. Selective solvation and electron pair donor/electron pair acceptor interaction are employed to stabilize nanoparticles. The formation of polyimide nanoparticles is governed by a nucleation-dominated process and therefore the particle size is controlled by the nucleation rate. A very high level of supersaturation can be attained under the intensive local motions induced by ultrasound, resulting in a very high nucleation rate. This effect is found extremely useful in the fabrication of sub-50-nm polyimide nanoparticles.

It is generally agreed that, in most cases, surfactants are required to obtain stable polymeric nanoparticle dispersions. Here, we report a method which can be used to produce surfactant free yet stable polymeric nanoparticle dispersions. This method is based on explored mechanism of selective solvation of nanoparticles and EPD (electron pair donor)/EPA (electron pair acceptor) complexes formed among solvent and nonsolvent molecules. Using polyimide P84 (copolyimide 3,3(') 4,4(')-benzophenone tetracarboxylic dianhydride and 80% methylphenylene diamine+20% methylene dianiline) as the model polymer, this mechanism was realized through a combined liquid-liquid phase separation and solvent/nonsolvent mixing technology. Surfactant-free polyimide nanoparticles (<100 nm) were produced. Experimental details and principles of this technology were given based on the ternary diffusion, the liquid-liquid phase separation and the advanced nucleation and growth theory. Two types of methods [denoted as the forward titration method and the backward titration (BT) method] were examined. It was found that the BT method is extremely helpful to prepare polyimide nanoparticles (<100 nm). As another important aspect, explored stabilization mechanism of the resultant nanoparticle dispersions was supported by the comparative experiments, implying that selective solvation of nanoparticles and EPD/EPA complexes may play key roles in stabilization.

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