2,580 research outputs found
Dynamic Surface Tension of Aqueous Solutions of Ionic Surfactants: Role of Electrostatics
The adsorption kinetics of the cationic surfactant dodecyltrimethylammonium
bromide at the air-water interface has been studied by the maximum bubble
pressure method at concentrations below the critical micellar concentration. At
short times, the adsorption is diffusion-limited. At longer times, the surface
tension shows an intermediate plateau and can no longer be accounted for by a
diffusion limited process. Instead, adsorption appears kinetically controlled
and slowed down by an adsorption barrier. A Poisson-Boltzmann theory for the
electrostatic repulsion from the surface does not fully account for the
observed potential barrier. The possibility of a surface phase transition is
expected from the fitted isotherms but has not been observed by Brewster angle
microscopy.Comment: 13 pages, 5 figure
Contribution of Cross-Correlations to the 21cm Angular Power Spectrum in the Epoch of Reionization
Measurement of the 21cm hyperfine transition of neutral hydrogen provides a
unique probe of the epoch of reionization and the Dark Ages. Three major
mechanisms are believed to dominate the radiation process: emission from
neutral hydrogen surrounding the ionized bubbles of first galaxies and/or
quasars, emission from neutral hydrogen inside minihalos, and absorption of
diffuse neutral hydrogen against the cosmic microwave background. In the
present work, by simply combining the existing analytic models for the three
mechanisms, we investigate the contribution of cross-correlation between these
three components to the total 21cm angular power spectrum, in the sense that
neutral hydrogen associated with different radiation processes traces the
large-scale structures of underlying density perturbations. While the overall
21cm power spectrum remains almost unchanged with the inclusion of the
cross-correlations, the cross-correlation may play a key role in the
determination of the 21cm power spectrum during the transition of 21cm
radiation from emission-dominated phase to absorption-dominated phase at
redshift z~20. A significant suppression in the 21cm angular power spectrum
during this transition is anticipated as the result of negative contribution of
the cross-correlation between the absorption of diffuse neutral hydrogen and
the emission components. Therefore, an accurate prediction of the cosmic 21cm
power spectrum should take the cross-correlation into account especially at the
transition phase.Comment: 10 pages, 4 figures, accepted for publication in MNRA
Self-Assembly in Mixtures of Polymers and Small Associating Molecules
The interaction between a flexible polymer in good solvent and smaller
associating solute molecules such as amphiphiles (surfactants) is considered
theoretically. Attractive correlations, induced in the polymer because of the
interaction, compete with intra-chain repulsion and eventually drive a joint
self-assembly of the two species, accompanied by partial collapse of the chain.
Results of the analysis are found to be in good agreement with experiments on
the onset of self-assembly in diverse polymer-surfactant systems. The threshold
concentration for self-assembly in the mixed system (critical aggregation
concentration, cac) is always lower than the one in the polymer-free solution
(critical micelle concentration, cmc). Several self-assembly regimes are
distinguished, depending on the effective interaction between the two species.
For strong interaction, corresponding experimentally to oppositely charged
species, the cac is much lower than the cmc. It increases with ionic strength
and depends only weakly on polymer charge. For weak interaction, the cac is
lower but comparable to the cmc, and the two are roughly proportional over a
wide range of cmc values. Association of small molecules with amphiphilic
polymers exhibiting intra-chain aggregation (polysoaps) is gradual, having no
sharp onset.Comment: 21 pages, 5 figures, RevTex, the published version, see also
cond-mat/990305
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