| 1. |
- Campbell, Richard A., et al.
(author)
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Direct Impact of Nonequilibrium Aggregates on the Structure and Morphology of Pdadmac/SDS Layers at the Air/Water Interface
- 2014
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 30:29, s. 8664-8674
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Journal article (peer-reviewed)abstract
- We discuss different nonequilibrium mechanisms by which bulk aggregates directly modify, and can even control, the interfacial structure and morphology of an oppositely charged polyelectrolyte/surfactant (P/S) mixture. Samples are categorized at the air/water interface with respect to the dynamic changes in the bulk phase behavior, the bulk composition, and the sample history using complementary surface-sensitive techniques. First, we show that bulk aggregates can spontaneously interact with the adsorption layer and are retained in it and that this process occurs most readily for positively charged aggregates with an expanded structure. In this case, key nonequilibrium issues of aggregate dissociation and spreading of surface-active material at the interface have a marked influence on the macroscopic interfacial properties. In a second distinct mechanism, aggregates inherently become trapped at the interface during its creation and lateral flocculation occurs. This irreversible process is most pronounced for aggregates with the lowest charge. A third mechanism involves the deposition of aggregates at interfaces due to their transport under gravity. The specificity of this process at an interface depends on its location and is mediated by density effects in the bulk. The prevalence of each mechanism critically depends on a number of different factors, which are outlined systematically here for the first time. This study highlights the sheer complexity by which aggregates can directly impact the interfacial properties of a P/S mixture. Our findings offer scope for understanding seemingly mysterious irreproducible effects which can compromise the performance of formulations in wide-ranging applications from foams to emulsions and lubricants.
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| 2. |
- Campbell, Richard A., et al.
(author)
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Effects of Bulk Colloidal Stability on Adsorption Layers of Poly(diallyldimethylammonium Chloride)/Sodium Dodecyl Sulfate at the Air-Water Interface Studied by Neutron Reflectometry
- 2011
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In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 115:51, s. 15202-15213
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Journal article (peer-reviewed)abstract
- We show for the oppositely charged system poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate that the cliff edge peak in its surface tension isotherm results from the comprehensive precipitation of bulk complexes into sediment, leaving a supernatant that is virtually transparent and a depleted adsorption layer at the air/water interface. The aggregation and settling processes take about 3 days to reach completion and occur at bulk compositions around charge neutrality of the complexes which lack long-term colloidal stability. We demonstrate excellent quantitative agreement between the measured surface tension values and a peak calculated from the surface excess of surfactant in the precipitation region measured by neutron reflectometry, using the approximation that there is no polymer left in the liquid phase. The nonequilibrium nature of the system is emphasized by the production of very different interfacial properties from equivalent aged samples that are handled differently. We go on to outline our perspective on the "true equilibrium" state of this intriguing system and conclude with a comment on its practical relevance given that the interfacial properties can be so readily influenced by the handling of kinetically trapped bulk aggregates.
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| 3. |
- Campbell, Richard A., et al.
(author)
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Multilayers at Interfaces of an Oppositely Charged Polyelectrolyte/Surfactant System Resulting from the Transport of Bulk Aggregates under Gravity
- 2012
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In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 116:27, s. 7981-7990
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Journal article (peer-reviewed)abstract
- We show conclusively that multilayers at interfaces of an oppositely charged polyelectrolyte/surfactant system can result from the transport under gravity of bulk aggregates with internal molecular structure. This process was demonstrated by measurements of poly-(diallyldimethylammonium chloride)/sodium dodecyl sulfate solutions at the air/liquid and solid/liquid interfaces using neutron reflectometry. In the latter case a novel approach involving the comparison of reflection up versus down measurements provided key evidence. Interfacial multilayers indicated by a strong Bragg peak and clear off-specular scattering are exhibited under three conditions: (1) only for samples in the phase separation region, (2) only for fresh samples where a suspension of bulk aggregates remains in solution, and (3) only when the creaming or sedimentation process occurs in the direction of the interface under examination. This bulk transport mechanism is an alternative route of formation of interfacial multilayers to surface induced self assembly The two processes evidently give rise to interfaces with very different structural and rheological properties. Such directionality effects in the formation of nanostructured liquid interfaces may have implications for a broad range of soft matter and biophysical systems containing macromolecules such as synthetic polymers, proteins, or DNA.
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| 4. |
- Campbell, Richard A., et al.
(author)
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New Perspective on the Cliff Edge Peak in the Surface Tension of Oppositely Charged Polyelectrolyte/Surfactant Mixtures
- 2010
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In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 1:20, s. 3021-3026
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Journal article (peer-reviewed)abstract
- We present how dramatically the nonequilibrium nature of an oppsitely charged polyelectrolyte/surfactant mixture can affect the interfacial properties. We show for the first time that the cliff edge peak in the surface tension of the poly(diallydimethylammonium chloride)/sodium dodecyl surfate system is produced as a direct result of depletion of surface-active material from the bulk solution due to a show precipitation process in the phase separation region. Simple illustrations are given of how to control the production of the peak, to eliminate the feature for equivalent aged solutions through the use of different sample handling methods, and even to change its characteristics at short surface ages. The potential to tune nonequilibrium, steady-state interfacial properties for such strongly associating system is clearly demonstrated. We propose that our findings in general may be applicable to a broad range of mixture containing surfactants and oppositely charged macromolecules such as polymers proteins and DNA.
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