Complex Adsorption Behavior of Rodlike Polyelectrolyte−Surfactant Aggregates (original) (raw)
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Langmuir, 2000
The character of adsorbed layers containing both polyelectrolyte and surfactant depends on the type of polyelectrolyte used and the surfactant concentrations, as demonstrated by several recent studies. However, the layer properties also depend on the experimental pathway. This shows that the adsorbed layers can be trapped in quasi-equilibrium states and that the true equilibrium is reached only after experimentally inaccessible time scales. This has to be kept in mind when comparing different results reported in the literature. The present article highlights these effects using a system consisting of a highly charged cationic polyelectrolyte, poly{(propionyloxy)ethyl}trimethylammonium chloride (PCMA), and an anionic surfactant, sodium dodecyl sulfate (SDS). The adsorbed layer properties were determined using mainly surface force measurements and atomic force microscope (AFM) imaging. We also present some small-angle neutron scattering data for bulk PCMA-SDS complexes formed between the polyelectrolyte and the surfactant. They demonstrate the presence of a characteristic correlation length of about 37-39 Å. A similar characteristic length scale is also observed in some of the adsorbed layers, both employing the AFM and the surface force apparatus. It may be interpreted as the distance between surfactant loaded polyelectrolyte chains.
Langmuir, 2006
Adsorption and deposition from turbid solutions are common in many industrial processes but notoriously difficult to investigate using standard optical techniques such as ellipsometry and reflectometry. In this report, we have addressed this problem by employing a quartz crystal microbalance with dissipation monitoring ability, QCM-D. The system under investigation consisted of a cationic polyelectrolyte, poly(vinylamine), PVAm, and an anionic surfactant, sodium dodecyl sulfate, SDS, which were mixed together in 10 mM NaCl solution. The polyelectrolyte and the surfactant readily associate in bulk solution, resulting in increased solution turbidity once large aggregates are formed. The solutions were placed in contact with a polystyrene surface, and the adsorption process was monitored by following the changes in the resonance frequency and dissipation factor. The results obtained can in most cases be evaluated using the Sauerbrey relation, but in some cases a more elaborate analysis is necessary. It is found that PVAm adsorbs to polystyrene in the absence of SDS. In the turbid region, deposition is observed, and the sensed mass exceeds the sum of that obtained for each of the components alone. On the other hand, at high SDS concentrations, the surfactant dominates in the adsorbed layer. Adsorption equilibrium is in most cases established within 1-2 h, the exception being found around the solution composition that results in the formation of charge-neutralized aggregates. In this case, a slow deposition of aggregates persists over prolonged times.
Interfacial Properties of Aggregates Formed by Cationic Polyelectrolyte and Anionic Surfactant
Langmuir, 2000
Interactions between mica surfaces across aqueous solutions containing mixtures of a highly positively charged polyelectrolyte and an anionic surfactant were studied by use of a surface force apparatus. The investigation was carried out with a constant polyelectrolyte concentration (20 ppm) and a wide range of surfactant concentrations [0-1 times the critical micelle concentration (cmc)]. The chemical composition of the adsorbed polyelectrolyte-surfactant layers was analyzed by x-ray photoelectron spectroscopy (XPS). The properties of polyelectrolyte-surfactant aggregates formed in bulk were studied by measurements of turbidity and electrophoretic mobility. The aggregates formed at low surfactant concentrations (<0.04 times cmc) were positively charged, whereas at higher surfactant concentrations the aggregates carried a net negative charge. It was shown that polyelectrolyte-surfactant aggregates rapidly adsorb on negatively charged mica surfaces regardless of the sign of their charge. At surfactant concentrations up to 0.01 times cmc, the polyelectrolytes adsorb on mica surfaces with loops and tails stretching out into solution and repulsive steric forces are generated. The thickness of the layer decreases with time and we suggest that this conformational change is accompanied by some expulsion of surfactant from the adsorbed layers. Thick adsorbed layers were formed in the surfactant concentration range 0.02-0.1 times cmc. In this concentration regime the measurement of equilibrium forces was inaccessible due to a very slow layer relaxation. During compression the forces were repulsive and during separation an attraction developed. We attribute this to formation of interlayer surfactant bridges. Finally, at high surfactant concentrations (g0.4 times cmc) highly negatively charged aggregates adsorb on the surfaces in rather thin layers, resulting in purely repulsive forces of mixed electrostatic and steric origin.
Polyelectrolyte adsorption at the solid/liquid interface
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1998
The forces between negatively charged surfaces in the presence of an adsorbing cationic copolymer of acrylamide and 2(methacryloyloxy)ethyltrimethylammonium chloride have been investigated using an atomic force microscope. The results were compared with measurements from adsorption isotherm, electrophoretic mobility, stability, and light scattering experiments. The adsorbed amount of polyelectrolyte and adsorbed layer conformation at the solid/liquid interface were found to be strongly dependent on the polymer concentration from which initial adsorption takes place. At low polyelectrolyte concentrations unstable silica suspensions were observed from stability tests; light scattering experiments indicate a large aggregate size under equivalent conditions. The adsorbed amount was also seen to be low, well less than monolayer coverage, and force measurements indicated that the polymer was adsorbed in a flat conformation. At high polyelectrolyte concentrations, an increase in the adsorbed amount was observed which resulted in a higher surface coverage, a higher mobility and a stable suspension. Direct force measurements indicated the presence of an electrosteric barrier.
Advances in Colloid and Interface Science, 2011
Recent application of the methods of surface dilational rheology to solutions of the complexes between synthetic polyelectrolytes and oppositely charged surfactants (PSC) gave a possibility to determine some steps of the adsorption layer formation and to discover an abrupt transition connected with the formation of microaggregates at the liquid surface. The kinetic dependencies of the dynamic surface elasticity are always monotonous at low surfactant concentrations but can have one or two local maxima in the range beyond the critical aggregation concentration. The first maximum is accompanied by the generation of higher harmonics of induced surface tension oscillations and caused by heterogeneities in the adsorption layer. The formation of a multilayered structure at the surface for some systems leads to the second maximum in the dynamic surface elasticity. The hydrophobicity and charge density of a polymer chain influence strongly the surface structure, resulting in a variety of dynamic surface properties of PSC solutions. Optical methods and atomic force microscopy give additional information for the systems under consideration. Experimental results and existing theoretical frameworks are reviewed with emphasis on the general features of all studied PSC systems.
Langmuir, 2002
When complexed PSS/CTAB is coadsorbed onto silica, a long-range, predominantly electrosteric repulsion is measured which decays exponentially with surface separation. Adsorption is driven by locally hydrophobic, surfactant-rich regions along the PSS chain. These anchor the PSS chain to the surface, resulting in a classical loop and tail adsorbed conformation. An increase in pH results in an increase in electrostatic repulsion between the silica surface and PSS, which drives an initially rapid desorption of some of the PSS/CTAB complex. Any remaining PSS/CTAB desorbs via a slow unravelling, as points of attachment to the surface are broken over time, allowing the PSS to extend further into solution. This continues until desorption is complete after 66 h. In contrast, adsorption of the PSS/CTAB complex onto a preadsorbed CTAB layer leads to a less extended, more compact surface layer since now adsorption is driven not only by the hydrophobic surfactant-rich regions along the PSS chain but also by a hydrophobic attraction between the PSS backbone and any adsorbed CTAB. An increase in pH results in rearrangement of the PSS/CTAB complex to a more extended conformation since now there is more segment-segment and surface-segment repulsion. This is evidenced by the increase in the range of the net repulsion and the fact that the repulsion now decays exponentially in a manner similar to that obtained when PSS/CTAB is able to loop and tail into solution. More points of attachment must be broken to achieve complete desorption of the PSS/CTAB complex, and hence complete desorption is kinetically hindered.
Langmuir, 2004
The coadsorption of a positively charged polyelectrolyte (with 10% of the segments carrying a permanent positive charge, AM-MAPTAC-10) and an anionic surfactant (sodium dodecyl sulfate, SDS) on silica and glass surfaces has been investigated using optical reflectometry and a noninterferometric surface force technique. This is a selective coadsorption system in the sense that the polyelectrolyte does adsorb to the surface in the absence of surfactant, whereas the surfactant does not adsorb in the absence of polyelectrolyte. It is found that the total adsorbed amount goes through a maximum when the SDS concentration is increased. Maximum adsorption is found when the polyelectrolyte/surfactant complexes formed in bulk solution are close to the charge neutralization point. Some adsorption does occur also when SDS is present in significant excess. The force measured between AM-MAPTAC-10-coated surfaces on approach in the absence of SDS is dominated at long range by an electrostatic double-layer force. Yet, layers formed by coadsorption from solutions containing both polyelectrolyte and surfactant generate long-range forces of an electrosteric nature. On separation, adhesive interactions are found only when the adsorbed amount is low, i.e., in the absence of SDS and in a large excess of SDS. The final state of the adsorbed layer is found to be nonhysteretic, i.e., independent of the history of the system. The conditions for formation of long-lived trapped adsorption states from mixed polymer-surfactant solutions are discussed.
Molecules, 2019
The bulk and interfacial properties of solutions formed by a polycation (i.e., poly(diallyldimethylammonium chloride), PDADMAC) and two different zwitterionic surfactants (i.e., coco-betaine (CB) and cocoamidopropyl-betaine (CAPB)) have been studied. The bulk aggregation of the polyelectrolyte and the two surfactants was analyzed by turbidity and electrophoretic mobility measurements, and the adsorption of the solutions at the fluid interface was studied by surface tension and interfacial dilational rheology measurements. Evidence of polymer-surfactant complex formation in bulk was only found when the number of surfactant molecules was closer to the number of charged monomers in solutions, which suggests that the electrostatic repulsion associated with the presence of a positively charged group in the surfactant hinders the association between PDADMAC and the zwitterionic surfactant for concentrations in which there are no micelles in solution. This lack of interaction in bulk is reflected in the absence of an influence of the polyelectrolyte in the interfacial properties of the mixtures, with the behavior being controlled by the presence of surfactant. This work has evidenced the significant importance of the different interactions involved in the system for controlling the interaction and complexation mechanisms of in polyelectrolyte-surfactant mixtures.
Kinetics of Adsorption Layer Formation in Solutions of Polyacid/Surfactant Complexes
The Journal of Physical Chemistry C, 2009
The dynamic dilational elasticity of adsorption layers of the complexes between polyacids and dodecyltrimethyl ammonium bromide were measured by the oscillating ring method as a function of surface age and surfactant concentration. The concentration dependencies of surface tension and dilational surface elasticity are similar to the corresponding dependencies of other polyelectrolyte/surfactant solutions. At the same time, the kinetic dependencies of the dynamic surface elasticity differ significantly from the results for other systems and exhibit in respective concentration ranges one or two local maxima. The first maximum and the associated distortion of the harmonical surface tension oscillations are caused by an aggregate formation in the surface layer. The second maximum occurs simultaneously with a multilayer formation at the liquid surface, which probably results in a new relaxation mechanism of surface stresses. The hydrophobic interactions influence the height and position of the second elasticity maximum, which is more pronounced for complexes of DTAB with the more hydrophobic polymethacrylic acid as compared to the poly(acrylic acid), and the surface tension values in the plateau region.
Polyelectrolyte adsorption on solid surfaces: theoretical predictions and experimental measurements
Langmuir : the ACS journal of surfaces and colloids, 2013
This work utilizes a combination of theory and experiments to explore the adsorption of two different cationic polyelectrolytes onto oppositely charged silica surfaces at pH 9. Both polymers, poly(diallyldimethylammonium chloride), PDADMAC, and poly(4-vinyl N-methylpyridinium iodide), PVNP, are highly charged and highly soluble in water. Another important aspect is that a silica surface carries a relatively high surface charge density at this pH level. This means that we have specifically chosen to investigate adsorption under conditions where electrostatics can be expected to dominate the interactions. Of specific focus in this work is the response of the adsorption to the addition of simple salt (i.e., a process where electrostatics is gradually screened out). Theoretical predictions from a recently developed correlation-corrected classical density functional theory for polyelectrolytes are evaluated by direct quantitative comparisons with corresponding experimental data, as obtai...