Charging Mechanism for Polymer Particles in Nonpolar Surfactant Solutions: Influence of Polymer Type and Surface Functionality.

Surface charging phenomena in nonpolar dispersions are exploited in a wide range of industrial applications, but their mechanistic understanding lags far behind. We investigate the surface charging of a variety of polymer particles with different surface functionality in alkane solutions of a custom-synthesized and purified polyisobutylene succinimide (PIBS) polyamine surfactant and a related commercial surfactant mixture commonly used to control particle charge. We find that the observed electrophoretic particle mobility cannot be explained exclusively by donor-acceptor interactions between surface functional groups and surfactant polar moieties. Our results instead suggest an interplay of multiple charging pathways, which likely include the competitive adsorption of ions generated among inverse micelles in the solution bulk. We discuss possible factors affecting the competitive adsorption of micellar ions, such as the chemical nature of the particle bulk material and the size asymmetry between inverse micelles of opposite charge.

Mechanisms of Particle Charging by Surfactants in Nonpolar Dispersions.

Electric charging of colloidal particles in nonpolar solvents plays a crucial role for many industrial applications and products, including rubbers, engine oils, toners, or electronic displays. Although disfavored by the low solvent permittivity, particle charging can be induced by added surfactants, even nonionic ones, but the underlying mechanism is poorly understood, and neither the magnitude nor the sign of charge can generally be predicted from the particle and surfactant properties. The conclusiveness of scientific studies has been limited partly by a traditional focus on few surfactant types with many differences in their chemical structure and often poorly defined composition. Here we investigate the surface charging of poly(methyl methacrylate) particles dispersed in hexane-based solutions of three purified polyisobutylene succinimide polyamine surfactants with "subtle" structural variations. We precisely vary the surfactant chemistry by replacing only a single electronegative atom located at a fixed position within the polar headgroup. Electrophoresis reveals that these small differences between the surfactants lead to qualitatively different particle charging. In the respective particle-free surfactant solutions we also find potentially telling differences in the size of the surfactant aggregates (inverse micelles), the residual water content, and the electric solution conductivity as well as indications for a significant size difference between oppositely charged inverse micelles of the most hygroscopic surfactant. An analysis that accounts for the acid/base properties of all constituents suggests that the observed particle charging is better described by asymmetric adsorption of charged inverse micelles from the liquid bulk than by charge creation at the particle surface. Intramicellar acid-base interaction and intermicellar surfactant exchange help rationalize the formation of micellar ions pairs with size asymmetry.

Influence of surfactant structure on reverse micelle size and charge for nonpolar electrophoretic inks.

Electrophoretic inks, which are suspensions of colorant particles that are controllably concentrated and dispersed by applied electric fields, are the leading commercial technology for high-quality reflective displays. Extending the state of the art for high-fidelity color in these displays requires improved understanding and control of the colloidal systems. In these inks, reverse micelles in nonpolar media play key roles in media and particle charging. Here we investigate the effect of surfactant structure on reverse micelle size and charging properties by synthesizing different surfactants with variations in polyamine polar head groups. Small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) were used to determine the micelle core plus shell size and micelle hydrodynamic radius, respectively. The results from SAXS agreed with DLS and showed that increasing polyamines in the surfactant head increased the micelle size. The hydrodynamic radius was also calculated on the basis of transient current measurements and agreed well with the DLS results. The transient current technique further determined that increasing polyamines increased the charge stabilization capability of the micelles and that an analogous commercial surfactant OLOA 11000 made for a lower concentration of charge-generating ions in solution. Formulating magenta inks with the various surfactants showed that the absence of amine in the surfactant head was detrimental to particle stabilization and device performance.

Interfaces Charged by a Nonionic Surfactant.

Highly hydrophobic, water-insoluble nonionic surfactants are often considered irrelevant to the ionization of interfaces at which they adsorb, despite observations that suggest otherwise. In the present study, we provide unambiguous evidence for the participation of a water-insoluble surfactant in interfacial ionization by conducting electrophoresis experiments for surfactant-stabilized nonpolar oil droplets in aqueous continuous phase. It was found that the surfactant with amine headgroup positively charged the surface of oil suspended in aqueous continuous phase (oil/water interface), which is consistent with its basic nature. In nonpolar oil continuous phase, the same surfactant positively charged the surface of solid silica (solid/oil interface) which is often considered acidic. The latter observation is exactly opposite to what the traditional acid-base mechanism of surface charging would predict, most clearly suggesting the possibility for another charging mechanism.

Characterizing the acid/base behavior of oil-soluble surfactants at the interface of nonpolar solvents with a polar phase.

We propose a simple method of characterizing the (Lewis) acid/base behavior of oil-soluble nonionic surfactants at the interface of nonpolar solvents with a polar phase. Using interfacial tensiometry, we probe the effective acidic and basic response of nonpolar surfactant solutions to contact with a variety of polar reference liquids. The measured interfacial tensions are used as experimental coefficients in a set of equations borrowed from the thermodynamic "surface energy component model" of van Oss, Chaudhury, and Good (vOCG), but used here in a more heuristic fashion and with a revised interpretation of the parameters extracted to describe the dispersive, acidic, and basic character of the sample. We test the proposed characterization method using alkane solutions of purified polyisobutylene succinimide (PIBS) surfactants with systematic structural variations, and observe that the inferred parameter values are consistent with, and sensitive to, subtle differences in the surfactant chemistry. This suggests the possibility to compare different surfactant solutions semiquantitatively with regard to their acidic and basic character. In a further illustration of the proposed analysis, we characterize a solution of commercial PIBS surfactant in hexane, and find that the parameters obtained by the proposed method correctly predict the solution interfacial tension with a polar liquid not included among the chosen reference liquids.

[Evaluation of the "Jiangsu/WHO 100% condom use programme in China"].

OBJECTIVE: To evaluate the effect and to explore the feasible model of 100% condom use programme (100%CUP) under existing policy and law milieu, so as to provide information for the adjustment of HIV/AIDS policy and intervention strategies. METHODS: Information on knowledge, attitude and behavior of female sex workers (FSWs) in entertainment establishments were collected and followed by qualitative and quantitative analysis including: samples tested for Chlamydia; a repeated cross-investigation and the final evaluation on the effect of the programme. RESULTS: The reported rate of condom use at the last sex activity with clients increased from 74.9% to 92.2% while the rate of sexually transmitted disease (STD) prevalence droped from 25.1% to 14.1% after three year's intervention program. The number of condoms distributed increased from 50,000 per quarter of a year in 2001 to over 100,000 in 2003. The rates of awareness on the knowledge of STD/AIDS prevention increased obviously. CONCLUSION: The 100%CUP was proved to be feasible and effective in the pilot site, and should be expanded in entertainment establishments.

Detection and imaging of zinc secretion from pancreatic beta-cells using a new fluorescent zinc indicator.

A novel Zn2+-selective visible wavelength fluoroionophore (FluoZin-3, 9) was synthesized. The chelating portion of the molecule resembles known EGTA-based Ca2+-selective fluoroionophores, except that one of the N-acetic acid moieties has been deleted in 9. FluoZin-3 is virtually non-fluorescent in the absence of Zn2+, and exhibits a several hundred-fold fluorescence increase upon saturation with Zn2+( approximately 100 nM), with a Kd = 15 +/- 2 nM. A 1:1 binding stoichiometry of 9:Zn2+ was determined, and the fluorescence of the complex is pH-independent at pH > 6. FluoZin-3 was used to monitor Zn2+ that was co-secreted with insulin from pancreatic beta-cells by exocytosis following stimulation with glucose. The total Zn2+ concentration near the cells reached 600 nM, and Zn2+ was detectable at least 15 mum away from secreting cells. Heterogeneity in secretion among cells was indicated in that some cells in a cluster did not release Zn2+. Also, within secreting cells some regions of the cell membrane gave rise to secretion while others did not, suggesting active zones of secretion on the cell surface.

Dipeptidyl aspartyl fluoromethylketones as potent caspase-3 inhibitors: SAR of the P2 amino acid.

This article describes the synthesis and biological evaluation of a series of dipeptidyl aspartyl fluoromethylketones as caspase-3 inhibitors. Structure-activity relationship (SAR) studies showed that for caspase-3 inhibition, Val is the best P(2) amino acid. The SAR studies also showed that the Asp free carboxylic acid in P(1) is important for caspase inhibiting activities, as well as for selectivity over other proteases.

Synthesis and SAR of novel di- and trisubstituted 1,4-dihydroquinoxaline-2,3-diones related to licostinel (Acea 1021) as NMDA/glycine site antagonists.

A series of novel di- and trisubstituted 1,4-dihydroquinoxaline-2,3-diones (QXs) related to licostinel (Acea 1021) was synthesized and evaluated as antagonists for the glycine site of the N-methyl-D-asparate (NMDA) receptor. The in vitro potency of these antagonists was determined by displacement of the glycine site radioligand [(3)H]-5,7-dichlorokynurenic acid ([(3)H]DCKA) in rat brain cortical membranes. Structure-activity relationship studies indicate that a cyano group is a good replacement for the nitro group in the 5-position of licostinel while 5-carboxy, 5-ester, 5-ketone and 5-amide derivatives showed reduced potency. 5,6-Cyclized analogues of licostinel also showed significantly reduced potency. Among the trisubstituted QXs investigated, 5-cyano-6,7-dichloro QX and 5-cyano-7-chloro-6-methyl QX are the most potent with IC(50) values of 32 nM and 26 nM, respectively.