Surface Conductivity and Disjoining Pressure of Common Black Films Stabilized with Sodium Dodecyl Sulfate
✍ Scribed by E.N. Swayne; John Newman; C.J. Radke
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 437 KB
- Volume
- 203
- Category
- Article
- ISSN
- 0021-9797
No coin nor oath required. For personal study only.
✦ Synopsis
ducted for multiple applications, as outlined by Mysels (3). We have conducted simultaneous measurements of the dis-Recent efforts (4-13) have focused on obtaining informajoining pressure and the film conductance for sodium dodecyl tion about the molecular structure of soap films and the sulfate-(SDS-) stabilized foam films in aqueous electrolyte soluforces that stabilize them, in an effort to understand interfations and over a range of film thicknesses. The disjoining pressure cial and colloidal phenomena better.
ascertains the repulsive forces within the film while the conduc-Disjoining-pressure-isotherm measurements by several tance gauges the possible changes in mobilities and concentrations authors (4, 6, 7, 9, 10) show that with the aqueous surfactant of the ions within the thin-film region, relative to their bulk values.
sodium dodecyl sulfate (SDS) addition of salt to the surfac-
The disjoining pressure is measured using a modified version of tant solution results in thinner, and yet more robust films.
the porous-glass-plate technique. To permit electrical measurements in the same apparatus, a novel film holder was designed in Classical DLVO theory (14,15) explains the behavior of which the film is formed in the annular region between two concenthese common black films (CBF) only qualitatively. Above tric, porous stainless steel disks. The inner and outer disks serve a critical salt concentration, the Newton black film (NBF) as electrodes, and they constrain the current to flow radially emerges, for which the equilibrium film thickness is essenthrough the film. To avoid electroosmosis, the film conductance tially independent of pressure and additional salt. The origin is obtained using AC impedance spectroscopy. New data are reof the forces in NBF films remains unresolved. ported for the disjoining pressure, conductance, and film thickness X-ray reflectivity experiments (16) and molecular-dyof 0.002-M SDS foam films with and without added sodium nitrate. namics (MD) simulations (17) of NBF suggest two high-Film conductances exhibit a decrease with decreasing film thickcoverage adsorbed surfactant layers, with hydrated headness, which suggests a decrease in the mobility of the film ions groups, and neutralized by tightly knit, partially hydrated relative to their bulk values, even for films as thick as 40 nm.
counterions. No co-ions or fully hydrated counterions exist
Surface-charge densities are estimated from the conductance data and are consistent with available surface tension data. However, in the film, and very little space remains for unbound water the surface-charge densities estimated from the disjoining-pressure molecules. Hence, the interior of the film has a solid-like data are more than an order of magnitude smaller than the values structure. The X-ray reflectivity experiments imply that the estimated from conductance and surface-tension data. Improvesurfactant organization is quite similar for the CBF and the ments in both experimental precision and theory are needed to NBF, with the transition between these two film states ocovercome this discrepancy. ᭧ 1998 Academic Press curring when the aqueous layer in the film abruptly changes