Thermodynamic consideration on single oil in water emulsion film stabilized by cationic surfactant.

The emulsion film has become the focus of the study of emulsion because the film stability and structure play a crucial role on the whole stability and structure of emulsions. In this study, the single emulsion film stabilized by tetradecyltrimethylammonium bromide (C14TAB) in dodecane phase has been investigated thermodynamically. In order to make clear the theoretical treatment, it has been reviewed how the thermodynamic quantities of extremely thin films are defined and how it is related to the experimental variables such as temperature, pressure, and concentrations of solutes. By using the equations demonstrated here, the film tension and film surface tension, which were evaluated from the measurement of contact angle between the emulsion film and the surrounding bulk meniscus, have been inspected thermodynamically from the viewpoint of the influence of added salt (KBr) concentration on the structure of thin emulsion film. In addition, the comparison of the results obtained has also been made between the foam and emulsion films to reveal the effect of an ambient dodecane phase on the properties of the film.

Gold-nanoparticle/organic linker films: self-assembly, electronic and structural characterisation, composition and vapour sensitivity.

Gold-nanoparticle/organic films were prepared via layer-by-layer self-assembly using dodecylamine-stabilised Au-nanoparticles and poly(propyleneimine) (PPI) dendrimers of generation one to five (G1-G5) or hexadecanedithiol (HDT) as linker compounds. TEM and FE-SEM images revealed that the bulk of the films consisted of nanoparticles with diameters of about 4 nm. XPS was used to study the chemical composition of the films. The C 1s and N 1s signals of an AuPPI-G4 film were interpreted qualitatively according to the dendrimer structure. The absence of the nitrogen signal in case of an AuHDT film indicated that the dodecylamine ligands were quantitatively exchanged during film assembly. About 76% of the sulfur atoms were bound to the nanoparticles. the remainder being present as free thiol (S H) groups. All films displayed linear current voltage characteristics and Arrhenius-type activation of charge transport. The conductivities of the AuPPI films decreased exponentially over approximately two orders of magnitude (6.8 x 10(-2) to 1.0 x 10(-3) ohms(-1) cm(-1)) with a five-fold increase of the dendrimer generation number. Dosing the films with solvent vapours caused their resistances to increase. Using different solvent vapours demonstrated that the sensitivity of this response was determined by the solubility properties of the linker compounds. Microgravimetric measurements showed that absorption of analyte was consistent with a Langmuir adsorption model. These measurements also revealed a linear correlation between the electrical response (deltaR/Rini) and the concentration of absorbed analyte. The absorption of d4-methanol from a saturated vapour atmosphere was studied by neutron reflectometry with an AuPPI-G4 film. This measurement indicated condensation of methanol on top of the film and a uniform distribution of the analyte across the film thickness.

Control of the water permeability of polyelectrolyte multilayers by deposition of charged paraffin particles.

We present a new way to protect polyelectrolyte multilayers from water, consisting in the adsorption and subsequent fusing of charged wax particles atop a multilayer. The formation of the wax layer is demonstrated by different techniques such as ellipsometry, contact angle measurements, and atomic force microscopy. The diffusion of water in protected and unprotected multilayers is studied by in situ neutron reflectometry. Whereas a top layer of wax crystals already allows substantial reduction of the diffusion, the fusion of this top layer leads to the dominating exclusion of water from the multilayers when dipped in water. This method opens up new interesting avenues for polyelectrolyte multilayers in practical applications where permeability of water, ions, or hydrophilic drugs is an issue.

Stability of various silicone oil/water emulsion films as a function of surfactant and salt concentration.

There have been reports, originally by the Bristol group, and subsequently by others, of the preparation and properties of emulsions of stable, nearly monodisperse droplets of poly(dimethylsiloxane) (PDMS) in water, where no added surfactant is used. It has been assumed that their stability is due to the high density of surface-ionized hydroxyl groups, similar in fact to the closely related Stöber silica particles. In this study we confirm, from droplet lifetime studies, that droplets, prepared from such synthesized PDMS, are significantly more stable to coalescence than similar-sized droplets prepared from three types of commercially available PDMS, containing HO-, MeO-, or Me3-terminated chains, respectively. It is shown, however, that the zeta potentials of the synthesized PDMS and of the various commercial oils are all very similar (as indeed are their Hamaker constants). So some other explanation must be inferred for the enhanced stability to coalescence of the synthesized PDMS droplets compared to the commercial PDMS droplets. It is shown, for droplets formed from n-hexane and the synthesized oil, that stability to coalescence is conferred at PDMS volume fractions (phiPDMS) around 0.2 in the mixture. The synthesized PDMS is known to consist of mixtures of cyclic PDMS and short-chain linear species, with terminal -OH groups. There is some (indirect) evidence that in the interval 0.25 < phiPDMS < 0.35, the linear PDMS chains may be adsorbed close to a monolayer at the mixed oil/water interface, possibly conferring some enhanced Gibbs elasticity to the interface. This underpins the possibility that, in the synthesized oil droplets themselves, there is also preferential adsorption of the linear chains at the PDMS/water interface, and this leads to a value of the Gibbs elasticity, sufficient to significantly reduce coalescence. Unfortunately, the Gibbs elasticity could not be measured in this case. However, such preferential adsorption is unlikely to occur with the commercial PDMS oils, which are not so heterogeneous. Finally, it is shown that droplets of the three commercial PDMS oils could be stabilized against coalescence, if a sufficient, minimum amount of sodium dodecyl sulfate (SDS) is added. Gibbs elasticity values have been estimated in these cases, from plots of interfacial tension against ln(SDS concentration).