Interaction of oppositely charged polyelectrolyte-ionic surfactant mixtures: adsorption of sodium poly(acrylic acid)-dodecyl trimethyl ammonium bromide mixtures at the air-water interface

The Gibbs and Butler Equations and the Surface Activity of Dilute Aqueous Solutions of Strong and Weak Linear Polyelectrolyte-Surfactant Mixtures: The Roles of Surface Composition and Polydispersity

In a previous paper, we applied a combination of direct measurements of both surface tension and surface excess in conjunction with the Gibbs equation to explain features of the adsorption and surface tension of mixtures of surfactants and strong linear polyelectrolytes at the air-water interface. This paper extends that model by including (i) the restrictions of the Butler equation for the behavior of the surface tension of mixed systems and (ii) the surface behavior of surfactant and linear weak polyelectrolyte mixtures, for which the inclusion of measurements of the surface excess and composition is shown to be particularly important. In addition, a closer examination of earlier data at higher concentrations provides evidence that the surface layering that is often observed in polyelectrolyte-surfactant systems is also an average equilibrium phenomenon and is driven by particular aggregation patterns that occur in some systems and not in others. Although the successful application of the Gibbs and Butler equations indicates that strong polyelectrolyte-surfactant systems can be described in terms of an average equilibrium over wide ranges of concentration, we have identified two concentration ranges where polydispersity in either polyelectrolyte molecular weight or composition results in significant time dependence of the surface behavior.

Polyelectrolyte/Surfactant Mixtures: A Pathway to Smart Foams

This review deals with liquid foams stabilized by polyelectrolyte/surfactant (PS) complexes in aqueous solution. It briefly reviews all the important aspects of foam physics at several scales, from interfaces to macroscopic foams, needed to understand the basics of these complex systems, focusing on those particular aspects of foams stabilized by PS mixtures. The final section includes a few examples of smart foams based on PS complexes that have been reported recently in the literature. These PS complexes open an opportunity to develop new intelligent dispersed materials with potential in many fields, such as oil industry, environmental remediation, and pharmaceutical industry, among others. However, there is much work to be done to understand the mechanism involved in the stabilization of foams with PS complexes. Understanding those underlying mechanisms is vital to successfully formulate smart systems. This review is written in the hope of stimulating further work in the physics of PS foams and, particularly, in the search for responsive foams based on polymer-surfactant mixtures.

Interfacial complexation of a neutral amphiphilic 'tardigrade' co-polymer with a cationic surfactant: Transition from synergy to competition

HYPOTHESIS

Neutral amphiphilic PEG-g-PVAc co-polymer (a "tardigrade" polymer consisting of a hydrophilic polyethylene glycol, PEG, backbone with hydrophobic polyvinyl acetate, PVAc, grafts) can form complexes at the air-water interface with cationic dodecyltrimethylammonium bromide (DTAB) via self-assembly. Compared to anionic SDS, cationic DTAB headgroups are expected to interact strongly with the negatively charged OH^- groups from the partial dissociation of the PVAc grafts. We anticipate a transition from synergistic to competitive behaviour, which is expected to be dependent on the surfactant structural characteristics and concentration.

EXPERIMENTS

DTAB/PEG-g-PVAc mixtures were investigated using a combination of dynamic and equilibrium surface tension measurements, neutron reflectivity (NR) at the air-water interface, and foaming tests. We varied the concentrations of both the DTAB (0.05 to 5 critical micelle concentration, cmc) and that of PEG-g-PVAc (0.2 and 2 critical aggregation concentration, cac).

FINDINGS

Our results show that the interfacial interactions between DTAB and PEG-g-PVAc were both synergistic and antagonistic, depending sensitively on the surfactant concentration. At DTAB concentrations below its cmc, a pronounced cooperative adsorption behaviour was likely driven by the hydrophobic interactions between the DTAB tail and the PVAc grafts and the attraction between the DTAB headgroups and the partially dissociated -O^- groups in the partially hydrolysed PVAc grafts, forming a mixed layer. This synergistic adsorption behaviour transitioned to a competitive adsorption behaviour at DTAB concentrations above its cmc, leading to polymer-surfactant partition, forming a "hanging" polymer layer underlying a surfactant monolayer at the interface. We postulate that DTAB/PEG-g-PVAc complexation in the bulk contributed to partial depletion of the mixture from the interface. We therefore consider this polymer/surfactant system to be a moderately interacting system at the air-water interface. No discernible differences in the foaming behaviour were observed between the DTAB/PEG-g-PVAc systems and the pure surfactant. Our results suggest that surfactant headgroup characteristics (particularly charges) were crucial in determining the structure and composition of polymer-surfactant complexes at the air-water interface, as well as the foamability and foam stability, whilst the coexistence of the synergistic and competitive adsorption behaviour is attributed to the unique architecture of the tardigrade polymer with amphiphilicity and partial charge, facilitating different surfactant-polymer interactions at different DTAB concentrations.

Multilayers formed by polyelectrolyte-surfactant and related mixtures at the air-water interface

The structure and occurrence of multilayered adsorption at the air-water interface of surfactants in combination with other oppositely charged species is reviewed. The main species that trigger multilayer formation are multiply charged metal, oligo- and polyions. The structures vary from the attachment of one or two more or less complete surfactant bilayers to the initial surfactant monolayer at the air-water interface to the attachment of a greater number of bilayers with a more defective structure. The majority of the wide range of observations of such structures have been made using neutron reflectometry. The possible mechanisms for the attraction of surfactant bilayers to an air-water interface are discussed and particular attention is given to the question of whether these structures are true equilibrium structures.

Polymers and surfactants at fluid interfaces studied with specular neutron reflectometry

This review addresses the advances made with specular neutron reflectometry in studies of aqueous mixtures of polymers and surfactants at fluid interfaces during the last decade (or so). The increase in neutron flux due to improvements in instrumentation has led to routine measurements at the air/water interface that are faster and involve samples with lower isotopic contrast than in previous experiments. One can now resolve the surface excess of a single deuterated component on the second time scale and the composition of a mixture on the minute time scale, and information about adsorption processes and dynamic rheology can also be accessed. Research areas addressed include the types of formed equilibrium surface structures, the link to foam film stability and the range of non-equilibrium effects that dominate the behavior of oppositely charged polyelectrolyte/surfactant mixtures, macroscopic film formation in like-charged polymer/surfactant mixtures, and the properties of mixtures of bio-polymers with surfactants and lipids.

Multilayering of Surfactant Systems at the Air-Dilute Aqueous Solution Interface

In the last 15 years there have been a number of observations of surfactants adsorbed at the air-water interface with structures more complicated than the expected single monolayer. These observations, mostly made by neutron or X-ray reflectivity, show structures varying from the usual monolayer to monolayer plus one or two additional bilayers to multilayer adsorption at the surface. These observations have been assembled in this article with a view to finding some common features between the very different systems and to relating them to aspects of the bulk solution phase behavior. It is argued that multilayering is primarily associated with wetting or prewetting of the air-water interface by phases in the bulk system, whose structures depend on an overall attractive force between the constituent units. Two such phases, whose formation is assumed to be partially driven by strong specific ion binding, are a concentrated lamellar phase that forms at low concentrations and a swollen lamellar phase that is not space-filling. Multilayering phenomena at the air-water interface then offer a delicate and easy means of studying the finer details of the incompletely understood attraction that leads to these two phases, as well as an interesting new means of self-assembling surface structures. In addition, multilayering is often associated with unusual wetting characteristics. Examples of systems discussed, and in some cases their bulk phase behavior, include surfactants with multivalent metal counterions, surfactants with oligomers and polymers, surfactant with hydrophobin, dichain surfactants, lung surfactant, and the unusual system of ethanolamine and stearic acid. Two situations where the air-water surface is deliberately held out of equilibrium are also assessed for features in common with the steady-state/equilibrium observations.

Polyampholyte/surfactant complexes at the water-air interface: a surface tension study

The present paper is related to interactions between strongly alternating polyampholytes, i.e., copolymers of N,N'-diallyl-N,N'-dimethylammonium chloride and maleamic acid derivatives, varying in hydrophobicity and excess charges and the oppositely charged anionic surfactant sodium dodecyl sulfate (SDS). Surface tension measurements have revealed a complex behavior with the formation of polyampholyte-SDS complexes at water-air interfaces which depends on both the hydrophobic character of the polyampholyte and electrostatic attractive forces between the polyampholyte and the anionic surfactant in dependence on pH. Hereby, maleamic acid copolymers with additional carboxylic groups in the phenylic side chain show a significant lower surface tension at the critical association concentration (CAC) due to the formation of surface-active SDS complexes and multicomplexes. In the presence of only one carboxylic group in the p-position the CAC can be strongly shifted by varying the pH due to repulsive electrostatic interactions.

Dilational surface visco-elasticity of polyelectrolyte/surfactant solutions: formation of heterogeneous adsorption layers

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.

Not all anionic polyelectrolytes complex with DTAB

The influence of hydroxypropyl guar (HPG), with and without boric acid, on dodecyltrimethyl ammonium bromide (DTAB) micellization was characterized by surface tension measurements, isothermal titration calorimetry, and small-angle neutron scattering. Although HPG is a nonionic water-soluble polymer, borate ions form weak bonds with HPG, transforming it into an anionic polyelectrolyte, HPG-borate. Surprisingly, the three independent measurements showed that HPG-borate does not promote DTAB micellization or phase separation normally seen when mixing oppositely charged polyelectrolytes and surfactants. However, the neutron scattering results suggested that HPG-borate binds to and flocculates existing DTAB micelles. The unusual behavior of HPG-borate with DTAB was underscored by showing that carboxymethyl guar (CMG) formed precipitates with DTAB.

Interaction of a cationic gemini surfactant with DNA and with sodium poly(styrene sulphonate) at the air/water interface: a neutron reflectometry study

The interactions between a dicationic gemini surfactant with a six-hydrocarbon spacer (1,2-bis(dodecyldimethyl-ammonio)hexane dibromide, C12C6C12Br2) and anionic polyelectrolyte DNA or sodium (polystyrene sulfonate) (NaPSS) at the air/solution interface have been studied and compared using neutron reflectometry together with surface tension. In the presence of the dichained cationic gemini surfactant, DNA and NaPSS display very different adsorption behaviors. The DNA/gemini mixtures show adsorption behavior very similar to that of DNA/C12TAB mixtures, with enhanced surfactant adsorption at low concentrations and thick structured layers at higher concentrations. However, for the NaPSS/gemini mixtures the amount of gemini at the surface is reduced relative to that in the absence of NaPSS at concentrations below the cmc. These differences in adsorption behavior are attributed to differences in the molecular structure and flexibility of the two polyanions. NaPSS is relatively hydrophobic and flexible enough to form bulk-phase polymer-micelle complexes with the gemini surfactant at low surfactant concentrations, whereas the adsorption of surface complexes is much less favorable because the dications on the gemini would require adjacent bulky pendant charges on the NaPSS to be oriented toward the surface. This would force the NaPSS to bend significantly whereas it is more favorable for the NaPSS to adopt an extended conformation at the surface. Thus, surfactant is actually removed from the surface to form bulk-phase complexes. In contrast with NaPSS, DNA has a far more rigid structure, and the charges on the backbone are at fixed intervals, factors that make the formation of surface DNA-monomer complexes much more favorable than bulk-phase DNA-micelle complexes. Finally, a short-chain sample of NaPSS consisting of only five to six segments adsorbs very strongly at the surface with the gemini to form more extensive layered structures than have previously been observed, consisting of approximately five sublayers.

Effects of aggregates on mixed adsorption layers of poly(ethylene imine) and sodium dodecyl sulfate at the air/liquid interface

We have exploited the spatial and kinetic resolution of ellipsometry to monitor the lateral movement of inhomogeneous patches of material in mixed adsorption layers of poly(ethylene imine) and sodium dodecyl sulfate at the air/liquid interface. We show that the choice of sample preparation methods can have a profound effect on the state of the interface for chemically equivalent samples. The extent of aggregation in the bulk solution on relevant time scales is affected by specific details of the polymer/surfactant mixing process, which produces varying numbers of aggregates that can become trapped in the interfacial layer, resulting in an enhanced and fluctuating ellipsometry signal. It can be beneficial to apply the surface-cleaning method of aspiration prior to physical measurements to remove trapped aggregates through the creation of a fresh interface. At low pH, the ellipsometry signal of samples prepared with surface cleaning is remarkably constant over a factor of >500 in the bulk composition below charge equivalence, which is discussed in terms of possible adsorption mechanisms. At high pH, through observing temporal fluctuations in the ellipsometry signal of samples prepared with surface cleaning, we reveal two important processes: there is the spontaneous adsorption of aggregates > 0.2 microm in diameter into the interfacial layer, and with time there is the fusion of smaller aggregates to generate new large surface aggregates. We attribute the favorability of the adsorption and fusion processes at high pH to reduced electrostatic barriers resulting from the low surface charge density of the aggregates. It is inappropriate in this case to consider the interface to comprise a homogeneous adsorption layer that is in dynamic equilibrium with the bulk solution. Our work shows that it can be helpful to consider whether there are macroscopic particles embedded in molecular layers at the air/liquid interface for systems where there is prior knowledge of aggregation in the bulk phase.

Adsorption of DNA and dodecyl trimethylammonium bromide mixtures at the air/water interface: a neutron reflectometry study

The interactions between dodecyl trimethylammonium bromide (C12TAB) and two samples of DNA with widely differing molecular weights have been studied using surface tension and neutron reflectometry. Neutron reflection data show that the surfactant and polymer are adsorbed together in a highly cooperative fashion over a 1000-fold change in surfactant concentration. Furthermore, the shorter DNA fragments adsorb with C12TAB as trilayers at higher surfactant concentrations, with overall layer thicknesses of 65-70 A. The high molecular weight DNA, however, shows only approximate monolayer adsorption with thicknesses varying from 19 to 26 A over the entire range of C12TAB concentrations. The difference in behavior between the different samples is believed to be a result of the rigid double helical structure of DNA which makes the formation of bulk phase polymer/micelle aggregates much less favorable for the short fragments. The resulting increase in the critical aggregation concentration (CAC) then leads to the adsorption of additional surfactant/polymer complex to the underside of the initial stable surface active DNA/C12TAB complex. Comparison with previous results obtained for synthetic polyelectrolytes shows that DNA/C12TAB complexes are not capable of reducing the surface tensions to the extent that other mixtures such as the poly(styrene sulfonate)/C12TAB mixtures do. A possible reason for this is the high rigidity of DNA combined with the fact that its hydrophobic moieties are positioned within the double helix so that the external molecule is largely hydrophilic.

Novel nanocomplexes of hyperbranched poly(ethyleneimine), and dodecyl maltoside

The aqueous complexes of poly(ethyleneimine) (PEI), sodium dodecyl sulfate (SDS) and dodecyl maltoside (C12G2) have been studied under dilute conditions using dynamic light scattering, electrophoretic mobility, surface tension and pH measurements. According to the surface tension data the complexation between PEI and C12G2 can be neglected while a strong interaction was detected between PEI and SDS. The charged nature and size of the PEI-SDS-C12G2 complexes vary in a similar manner with SDS concentration as for the PEI-SDS systems. At large excess of SDS a kinetically stable colloid dispersion of the compact PEI-SDS-C12G2 particles forms. The electrophoretic mobility measurements indicate that the charge reversal of the PEI molecules occurs at lower SDS concentrations in the presence than in the absence of dodecyl maltoside. The enhanced charge inversion of PEI affords a significant extension of the concentration range with kinetically stable dispersion of the polyelectrolyte-surfactant nanoparticles compared with the PEI-SDS system. The pH of the PEI-SDS-C12G2 mixtures also reveals a peculiar dependence on the surfactant concentration. These latter findings are explained by the synergistic binding of the ionic and non-ionic surfactants to both the uncharged and charged amine groups of the PEI. It can be concluded that the addition of sugar surfactants is an efficient way to increase the kinetic stability and manipulate the pH of the mixtures of oppositely charged weak polyelectrolytes and surfactants.

Dynamic surface properties of polyelectrolyte/surfactant adsorption films at the air/water interface: poly(diallyldimethylammonium chloride) and sodium dodecylsulfate

The dynamic surface elasticity, dynamic surface tension, and ellipsometric angles of mixed aqueous poly(diallyldimethylammonium chloride)/sodium dodecylsulfate solutions (PDAC/SDS) have been measured as a function of time and surfactant concentration. This system represents a typical example of polyelectrolyte/surfactant complex formation and subsequent aggregation on the nanoscale. The oscillating barrier and oscillating drop methods sometimes led to different results. The surface viscoelasticity of mixed PDAC/SDS solutions are very close to those of mixed solutions of sodium polystyrenesulfonate and dodecyltrimethylammonium bromide but different from the results for some other polyelectrolyte/surfactant mixtures. The abrupt drop in surface elasticity when the surfactant molar concentration approaches the concentration of charged polyelectrolyte monomers is caused by the formation of microparticles in the adsorption layer. Aggregate formation in the solution bulk does not influence the surface properties significantly, except for a narrow concentration range where the aggregates form macroscopic flocks. The mechanism of the observed relaxation process is controlled by the mass exchange between the surface layer and the flocks attached to the liquid surface.

Polymer/surfactant interactions at the air/water interface

The development of neutron reflectometry has transformed the study and understanding of polymer/surfactant mixtures at the air/water interface. A critical assessment of the results from this technique is made by comparing them with the information available from other techniques used to investigate adsorption at this interface. In the last few years, detailed information about the structure and composition of adsorbed layers has been obtained for a wide range of polymer/surfactant mixtures, including neutral polymers and synthetic and naturally occurring polyelectrolytes, with single surfactants or mixtures of surfactants. The use of neutron reflectometry together with surface tensiometry, has allowed the surface behaviour of these mixtures to be related directly to the bulk phase behaviour. We review the broad range of systems that have been studied, from neutral polymers with ionic surfactants to oppositely charged polyelectrolyte/ionic surfactant mixtures. A particular emphasis is placed upon the rich pattern of adsorption behaviour that is seen in oppositely charged polyelectrolyte/surfactant mixtures, much of which had not been reported previously. The strong surface interactions resulting from the electrostatic attractions in these systems have a very pronounced effect on both the surface tension behaviour and on adsorbed layers consisting of polymer/surfactant complexes, often giving rise to significant surface ordering.

The impact of electrolyte on the adsorption of sodium dodecyl sulfate/polyethyleneimine complexes at the air-solution interface

The addition of electrolyte (0.1 M NaCl) is shown to have a significant impact upon the surfactant concentration and solution pH dependence of the adsorption of sodium dodecyl sulfate (SDS)/polyethyleneimine (PEI) complexes at the air-solution interface. Substantial adsorption is observed over a wide surfactant concentration range (from 10(-6) to 10(-)2 M), and over much of that range of concentrations the adsorption is characterized by the formation of surface multilayers. The surface multilayer formation is most pronounced at high pH and for PEI with a lower molecular weight of 2K, compared to the higher molecular weight of 25K. These results, obtained from a combination of neutron reflectivity and surface tension, highlight the substantial enhancement in surfactant adsorption achieved by the addition of a combination of the polyelectrolyte, PEI, and a simple electrolyte. Furthermore the effect of electrolyte on the pH dependence of the adsorption further highlights the importance of the hydrophobic interaction in surface surfactant/polyelectrolyte complex formation.

The interaction between sodium alkyl sulfate surfactants and the oppositely charged polyelectrolyte, polyDMDAAC, at the air-water interface: the role of alkyl chain length and electrolyte and comparison with theoretical predictions

The effect of alkyl chain length and electrolyte on the adsorption of sodium alkyl sulfate surfactants and the oppositely charged polyelectrolyte, polyDMDAAC, at the air-water interface has been investigated by surface tension and neutron reflectivity. The variations in the patterns of adsorption and surface tension behavior with alkyl chain length and electrolyte are discussed in the context of the competition between the formation of surface active surfactant/polyelectrolyte complexes and polyelectrolyte/surfactant micelle complexes in solution. A theoretical approach based on the law of mass action has been used to predict the surface effects arising from the competition between the formation of polyelectrolyte/surfactant surface and solution complexes and the formation of free surfactant micelles. This relatively straightforward model is shown to reproduce the principal features of the experimental results.

Dynamics of adsorption of an oppositely charged polymer-surfactant mixture at the air-water interface: poly(dimethyldiallylammonium chloride) and sodium dodecyl sulfate

The dynamic adsorption behavior of mixtures of the cationic polymer poly(dimethyldiallylammonium chloride) [poly(dmdaac)] and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied at the expanding liquid surface of an overflowing cylinder. A combination of ellipsometry and external reflection Fourier transform infrared spectroscopy was used to measure the adsorbed amounts of poly(dmdaac) and SDS as a function of the bulk surfactant concentration for various polymer concentrations in the range 0-0.2 g dm-3. Laser Doppler velocimetry was used to determine the surface age, which was approximately 1 s for solutions where the polymer adsorbed. The interfacial behavior is rationalized in terms of competition between surface activity and mass transport to the expanding surface. At low surfactant concentrations, adsorption of both poly(dmdaac) and SDS is enhanced as a result of the formation in solution of polymer-surfactant complexes that are more surface active than either component alone. The rate of adsorption of these complexes is diffusion-controlled, and their interfacial composition remains constant at three dmdaac units per SDS molecule over a 5-fold change in the surfactant concentration. For the higher polymer concentrations studied, the complexes saturate the air-water interface: the adsorbed amount is independent of the polymer concentration and remains constant also over a factor of 5 in the surfactant concentration. Once the number of bound surfactant molecules per dmdaac monomer exceeds 0.3, the complexes begin to form large aggregates, which are not surface active due to their slower mass transport. The adsorbed amount decreases rapidly on approach to the equivalence point (one SDS molecule per dmdaac monomer), and when it is reached, only a very small amount of material remains at the interface. At still higher surfactant concentrations, the free SDS adsorbs but there is no adsorbed poly(dmdaac). The dynamic adsorption data are compared with equilibrium measurements of the same system by Staples et al. (Langmuir 2002, 18, 5147), which show very different surface compositions and no significant change in surface coverage at the equivalence point.

Polymer-surfactant complexation in polyelectrolyte multilayer assemblies

Layer-by-layer self-assembly can be used to incorporate amphiphilic molecules into multilayered polyelectrolyte architectures. This review examines equilibrium LbL assemblies constructed by direct adsorption from aqueous solution. LbL systems have not only provided fundamental insight into the nature of polyion-surfactant complexation, but have also yielded functional materials with useful surface, optical, and electronic properties.

Characterization limits of a polymer adsorbed under a monolayer by GIXD measurements

The study of interactions between a polyelectrolyte (sodium polystyrene sulfonate, PSSt) or its water-soluble monomer (SSt) at different concentrations and a monolayer of dioctadecyldimethylammonium bromide (DODA) has been investigated. The monolayer phase behavior and structure at the air-water interface were studied by surface pressure-area isotherms and grazing incidence X-ray diffraction measurements. DODA molecules organize following a rectangular unit cell in all three subphases (pure water, water containing SSt or PSSt). The presence of polyelectrolytes in the subphase decreases, on one hand, the tilt angle and the mean area per molecule in the condensed phase, revealing a higher 2D density in this state, and, on the other hand, the amount of organized matter.

Influence of the polyelectrolyte poly(ethyleneimine) on the adsorption of surfactant mixtures of sodium dodecyl sulfate and monododecyl hexaethylene glycol at the air-solution interface

The polyelectrolyte poly(ethylenenimine), PEI, is shown to strongly influence the adsorption of the anionic-nonionic surfactant mixture of sodium dodecyl sulfate, SDS, and monododecyl hexaethylene glycol, C(12)E(6), at the air-solution interface. In the presence of PEI, the partitioning of the mixed surfactants to the interface is highly pH-dependent. The adsorption is more strongly biased to the SDS as the pH increases, as the PEI becomes a weaker polyelectrolyte. At surfactant concentrations >10(-4) M, the strong interaction and adsorption result in multilayer formation at the interface, and this covers a more extensive range of surfactant concentrations at higher pH values. The results are consistent with a strong interaction between SDS and PEI at the surface that is not predominantly electrostatic in origin. It provides an attractive route to selectively manipulate the adsorption and composition of surfactant mixtures at interfaces.

Effect of binding of an oligomeric cationic fluorosurfactant on the dilational rheological properties of gelatin adsorbed at the air-water interface

The effect of binding of an oligomeric cationic fluorooxetane surfactant on the interfacial properties of adsorbed gelatin-fluorooxetane complexes has been studied using dynamic surface tension and dilational rheological measurements. Adsorption kinetics of gelatin-fluorooxetane complexes are reminiscent of a mixed (barrier/diffusion limited) process, while the dilational rheological properties of the interface exhibit a strong dependence on surfactant concentration. At low surfactant concentrations, dilational surface moduli as well as phase angles are relatively insensitive to the presence of the fluorooxetane. However, at the critical aggregation concentration of the polymer-surfactant system, there is a sharp increase in the complex modulus. Further increase in the fluorooxetane concentration does not significantly affect the complex modulus. The phase angle, however, does increase with increasing fluorooxetane concentration due to the transport of bound fluorooxetane from the subsurface to the solution-air interface. These results indicate that, at fluorooxetane concentrations exceeding the critical aggregation concentration, the polymer-surfactant complexes adsorb to form cross-linked multilayers at the solution-air interface.

pH sensitive adsorption of polypeptide/sodium dodecyl sulfate mixtures

Neutron reflectivity and surface tension have been used to investigate the pH sensitivity of the adsorption of poly-L-lysine hydrobromide and sodium dodecyl sulfate mixtures at the air-solution interface. The surface tension variation with surfactant concentration is complex, and between the critical aggregation concentration and critical micellar concentration there is a marked increase in the surface tension. The neutron reflectivity results show that this is associated with a depletion of the surface of polypeptide/surfactant complexes. The variations in the adsorption and surface tension with pH are attributed to changes in the polypeptide conformation at the interface and in solution.

Spectroscopy of Growing and Evaporating Water Droplets: Exploring the Variation in Equilibrium Droplet Size with Relative Humidity

We demonstrate that the thermodynamic properties of a single liquid aerosol droplet can be explored through the combination of a single-beam gradient force optical trap with Raman spectroscopy. A single aqueous droplet, 2-6 microm in radius, can be trapped in air indefinitely and the response of the particle to variations in relative humidity investigated. The Raman spectrum provides a unique fingerprint of droplet composition, temperature, and size. Spontaneous Raman scattering is shown to be consistent with that from a bulk phase sample, with the shape of the OH stretching band dependent on the concentration of sodium chloride in the aqueous phase and on the polarization of the scattered light. Stimulated Raman scattering at wavelengths commensurate with whispering gallery modes is demonstrated to provide a method for determining the size of the trapped droplet with nanometer precision and with a time resolution of 1 s. The polarization dependence of the stimulated scatter is consistent with the dependence observed for the spontaneous scatter from the droplet. By characterizing the spontaneous and stimulated Raman scattering from the droplet, we demonstrate that it is possible to measure the equilibrium size and composition of an aqueous droplet with variation in relative humidity. For this benchmark study we investigate the variation in equilibrium size with relative humidity for a simple binary sodium chloride/aqueous aerosol, a typical representative inorganic/aqueous aerosol that has been studied extensively in the literature. The measured equilibrium sizes are shown to be in excellent agreement with the predictions of Köhler theory. We suggest that this approach could provide an important new strategy for characterizing the thermodynamic properties and kinetics of transformation of aerosol particles.