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

Effect of Added Surfactant on Poly(Ethylenimine)-Assisted Gold Nanoparticle Formation

In a variety of applications, functionalization of gold nanoparticles is needed to ensure adequate surface charge and hydrophobicity for their biodistribution, interparticle interactions, or self-organization. In the present paper, we provide an economic way for the synthesis of hydrophobized poly(ethylenimine) (PEI) capped gold nanoparticles at room temperature using sodium dodecyl sulfate (SDS). The approach is based on the controlled competition between the nucleation of gold nanophases within the PEI molecules and the SDS binding onto their amine groups. This can be achieved via utilizing the strongly irreversible nature of the association between the oppositely charged polymer and that of the surfactant molecules. Specifically, by varying the order and timing of SDS addition during the process of gold nanoassembly formation, the size distribution, the morphology, and the local hydrophobic environment of the produced Au-PEI/SDS nanohybrids can be tuned even at one composition of the system. The results may be further exploited for the preparation of noble metal nanoassemblies with controlled hydrophobicity and charge.

Fine-tuning the nonequilibrium behavior of oppositely charged macromolecule/surfactant mixtures via the addition of nonionic amphiphiles

The various commercial applications of oppositely charged polyelectrolytes (P) and ionic surfactants (S) with added nonionic amphiphiles initiated intensive research on the polyion/mixed surfactant interaction. A large group of earlier studies revealed that one of the major effects of the nonionic cosurfactants is the suppression of the associative phase separation of P/S systems. In contrast, recent studies indicated that in the dilute surfactant concentration range the added uncharged amphiphile enhances the precipitation concentration range. In order to rationalize these observations, the mixtures of poly(diallyldimethylammonium chloride) (PDADMAC), sodium dodecyl sulfate (SDS), and dodecyl maltoside (C12G2) are investigated using a variety of experimental methods. It is shown that the nonionic cosurfactant has two distinct and competing impacts on the mixed surfactant binding onto the polyions. The composition dependent variation of the chemical potentials of the amphiphiles determines which of these effects is the dominant one, explaining the seemingly diverse earlier observations and their interpretations. We also demonstrate that the nonionic amphiphile affects considerably the nonequilibrium features of polyion/ionic surfactant complexation. Namely, the presence of the uncharged surfactant can destabilize the colloidal dispersion of P/S nanoparticles formed in the two-phase composition range. However, at the same concentration range highly stable dispersions of polyion/mixed surfactant nanoparticles can be produced through the application of a new two-step solution preparation technique. This method is based on the order of addition effect of the two surfactants which can be utilized in future scientific and industrial applications.

Effects of ionic strength on the surface tension and nonequilibrium interfacial characteristics of poly(sodium styrenesulfonate)/dodecyltrimethylammonium bromide mixtures

We rationalize the surface tension behavior and nonequilibrium interfacial characteristics of high molecular weight poly(sodium styrenesulfonate)/dodecyltrimethylammonium bromide (NaPSS/DTAB) mixtures with respect to the ionic strength. Excellent agreement is achieved between experimental data and our recent empirical model [Langmuir 2013, 29, 11554], which is based on the lack of colloidal stability of bulk aggregates in the phase separation region and has no free fitting parameters. We show that the size of a surface tension peak positioned at the edge of the phase separation region can be suppressed by the addition of inert electrolyte, which lowers the critical micelle concentration in relation to the phase separation region. Such manipulation of the peak is possible for the 100 ppm NaPSS/DTAB system because there is a high free surfactant concentration in the phase separation region. The close agreement of our model with the experimental data of samples in the phase separation region with respect to the ionic strength indicates that the surface tension behavior can be rationalized in terms of comprehensive precipitation regardless of whether there is a peak or not. The time scale of precipitation for the investigated system is on the order of one month, which emphasizes the need to understand the dynamic changes in the state of bulk aggregation in order to rationalize the surface properties of strongly interacting mixtures; steady state surface properties measured in the interim period will represent samples far from equilibrium. We show also that the surface properties of samples of low ionic strength outside the equilibrium phase separation region can be extreme opposites depending on the sample history, which is attributed to the generation of trapped nonequilibrium states. This work highlights the need to validate the underlying nature of oppositely charged polyelectrolyte/surfactant systems prior to the interpretation of experimental data within an equilibrium framework.

The impact of nonionic surfactant additives on the nonequilibrium association between oppositely charged polyelectrolytes and ionic surfactants

The effect of uncharged surfactant additives on the oppositely charged polyion/ionic surfactant complexation is usually described as a direct equilibrium association between the polyelectrolyte molecules and free mixed micelles analogous to the polyion/colloidal particle interactions. This approach predicts that the binding of the ionic surfactant to the polyelectrolyte molecules can be completely suppressed by increasing the nonionic-to-ionic surfactant ratio. In the present work, it is shown that the addition of nonionic surfactants to poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate mixtures considerably enhances the binding of the anionic surfactant to the polycation in the dilute surfactant concentration regime. The dynamic light scattering, turbidity, electrophoretic mobility and fluorescence spectroscopic measurements are consistent with the synergic binding of the ionic and nonionic surfactants to the polyelectrolyte molecules. The enhanced surfactant binding could be utilized for the preparation of stable colloidal dispersions of novel polyion/mixed surfactant nanoparticles over a wide composition range provided that adequate mixing protocols are used. These results clearly indicate that the nonionic surfactant additives can be successfully used to tune the nonequilibrium association of oppositely charged macromolecules and amphiphiles.

Effect of linear nonionic polymer additives on the kinetic stability of dispersions of poly(diallyldimethylammonium chloride)/sodium dodecylsulfate nanoparticles

In this article, the impact of different neutral polymers on the kinetic stability of charge-stabilized poly(diallyldimethylammonium chloride) (PDADMAC)/sodium dodecylsulfate (SDS) colloidal dispersions is analyzed using dynamic light scattering, electrophoretic mobility, turbidity, and coagulation kinetics measurements. Poly(ethyleneoxide) (PEO), poly(vinylpyrrolidone) (PVP), and dextran of comparable molecular masses as well as a higher-molecular-weight dextran sample were tested as nonionic additives. The light scattering and mobility data indicate that the PEO and PVP molecules may adsorb on the surface of the PDADMAC/SDS nanoparticles formed in the presence of excess surfactant. The primary effect of these additives is manifested in enhanced coagulation of the PDADMAC/SDS nanoparticles due to bridging at lower polymer concentrations and depletion flocculation at higher polymer concentrations. These findings are in sharp contrast to the earlier published effect of the same nonionic polymers on the poly(ethyleneimine) (PEI)/SDS colloidal dispersions, which can be sterically stabilized at appropriate PEO or PVP concentrations. However, the adsorption of the investigated dextran samples is negligible on the PDADMAC/SDS nanoparticles. Therefore, dextran molecules may cause only depletion flocculation in the PDADMAC/SDS system in the vicinity of the critical overlap concentration.

Control of Rheological Behaviour with Oppositely Charged Polyelectrolyte Surfactant Mixtures

Mixtures of the cationic, cellulose based polyelectrolyte JR 400 and the anionic surfactant sodium dodecylbenzene sulfonate (LAS) have been used to control the rheological behaviour of aqueous solutions. Around charge equilibrium precipitation takes place, but both for the surfactant-rich and the polymer-rich side homogeneous solutions are formed. In these monophasic regions the rheology depends strongly on the mixing ratio between the two components and highly viscous systems can be obtained on the polyelectrolyte rich side upon the addition of relatively small amounts of surfactant. Here the viscosity increases by more than four orders of magnitude before reaching the phase boundary of precipitation. Small-angle neutron scattering (SANS) showed the formation of an interconnected network of rodlike aggregates composed of a polyelectrolyte/surfactant complex, which explains the observed high viscosity due to the high degree of interconnection by the polyelectrolyte.

Nonequilibrium Association of Oppositely Charged Macromolecules and Amphiphiles

Recently, a novel concept has been proposed to interpret the nonequilibrium character of oppositely charged polyelectrolyte/surfactant mixtures. According to this approach, at low surfactant-to-polyelectrolyte ratios the system is a thermodynamically stable solution. In a given composition range colloidal dispersions of the polyelectrolyte/surfactant nanoparticles are formed. In the presence of surfactant excess, the dispersions can be stabilised via the adsorption of the surfactant ions on the surface of the hydrophobic polyelectrolyte/surfactant nanoparticles. In the presence of polyelectrolyte excess, charge stabilised polyelectrolyte/surfactant dispersions might also be prepared if the charge of the macromolecules is large enough. These latter colloidal dispersions are stabilised by the uncompensated charges of the polyelectrolyte. The colloidal dispersion concept provides several options to control the formation of self-assemblies and the kinetic stability of the mixtures. In this paper, the effect of solution preparation protocols and different additives (including non-ionic polymers or surfactants) on the mentioned properties of oppositely charged macromolecule/surfactant systems is discussed.

On the influence of surfactants on the adsorption of polysaccharide-based polymers on cotton studied by means of fluorescence spectroscopy

In this study, we examined the influence of surfactants on the adsorption of polymers on cotton fibers. The extent of polymer adsorption on cotton was determined directly by means of fluorescence spectroscopy using fluorescently labeled polymers. The investigation of polymer adsorption in the presence of different types of surfactants and for a large range of differently structured polymers allows us to obtain a rather general picture of this important issue. Systematic relationships between the presence of surfactant and the type of polymer can be deduced but cannot be cast in simple terms such as electrostatic interaction but instead depend on the detailed interaction between the surfactant and polymer both in solution and adsorbed on the cotton surface. A particularly complex situation arises for the case of oppositely charged surfactant and polymer because of the possibility of precipitate formation. The study of such complex systems not only is of scientific interest but also is of great commercial interest because both polymers and surfactants are parts of detergent formulations and cotton is one of the most abundantly used materials for fabrics.

Preparation of stable electroneutral nanoparticles of sodium dodecyl sulfate and branched poly(ethylenimine) in the presence of pluronic F108 copolymer

Mixing of polyelectrolyte solutions with solutions of oppositely charged surfactants usually leads to phase separation in a certain concentration range. However, since the charge-neutralized polyelectrolyte/surfactant nanoparticles might be utilized as versatile nanocarriers of different substances, it would be desirable to prevent their aggregation for some applications. As it was revealed in earlier investigations, the complete suppression of precipitation may be achieved only in mixtures of ionic surfactants and appropriate copolymer polyelectrolytes with nonionic and ionic blocks. In this work, we present a method that could prevent phase separation in mixtures of homopolyelectrolytes and oppositely charged surfactants. Specifically, it is shown that nonaggregating electroneutral nanocomplexes of branched poly(ethylenimine) (PEI) and sodium dodecyl sulfate (SDS) can be prepared in the presence of the amphiphilic triblock copolymer Pluronic F108, provided that an adequate mixing protocol is used for preparation of the PEI/SDS/F108 mixtures.

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.

Effect of salt on the equilibrium and nonequilibrium features of polyelectrolyte/surfactant association

The impact of an electrolyte on aqueous mixtures of oppositely charged macromolecules and surfactants is usually explained by assuming an equilibrium association between the components. In this work, it is shown that the nonequilibrium character of polyelectrolyte/surfactant systems plays a crucial role in the interpretation of the effect of salt. Experimental investigations of mixtures of sodium poly(styrenesulfonate) (PSS) and hexadecyltrimethylammonium bromide (CTAB) reveal two distinct effects of added sodium chloride (NaCl). At small and moderate NaCl concentrations, the major impact of the electrolyte is manifested in the reduction of the kinetically stable composition range in which the PSS/CTAB mixtures are trapped in the nonequilibrium colloidal dispersion state. The application of high salt concentrations, however, primarily affects the equilibrium phase properties through considerably decreasing the amount of surfactant bound to the polyelectrolyte.

Self-aggregation of mixtures of oppositely charged polyelectrolytes and surfactants studied by rheology, dynamic light scattering and small-angle neutron scattering

In this study, the phase behavior, structure and properties of systems composed of the cationic, cellulose-based polycation JR 400 and the anionic surfactants sodium dodecylbenzenesulfonate (SDBS) or sodium dodecylethoxysulfate (SDES), mainly in the semidilute regime, were examined. This system shows the interesting feature of a very large viscosity increase by nearly 4 orders of magnitude as compared to the pure polymer solution already at very low concentrations of 1 wt%. By using rheology, dynamic light scattering (DLS), and small-angle neutron scattering (SANS), we are able to deduce systematic correlations between the molecular composition of the systems (characterized by the charge ratio Z=[+(polymer)]/[−(surfactant)]), their structural organization and the resulting macroscopic flow behavior. Mixtures in the semidilute regime with an excess of polycation charge form highly viscous network structures containing rodlike aggregates composed of surfactant and polyelectrolyte that are interconnected by the long JR 400 chains. Viscosity and storage modulus follow scaling laws as a function of surfactant concentration (η~c(s)(4); G(0)~c(s)(1.5)) and the very pronounced viscosity increase mainly arises from the strongly enhanced structural relaxation time of the systems. In contrast, mixtures with excess surfactant charges form solutions with viscosities even below those of the pure polymer solution. The combination of SANS, DLS, and rheology shows that the structural, dynamical, and rheological properties of these oppositely charged polyelectrolyte/surfactant systems can be controlled in a systematic fashion by appropriately choosing the systems composition.

Complexes of surfactants with oppositely charged polymers at surfaces and in bulk

Addition of surfactants to aqueous solutions of polyelectrolytes carrying an opposite charge causes the spontaneous formation of complexes in the bulk phase in certain concentration ranges. Under some conditions, compact monodisperse multichain complexes are obtained in the bulk. The size of these complexes depends on the mixing procedure and it can be varied in a controlled way from nanometers up to micrometers. The complexes exhibit microstructures analogous to those of the precipitates formed at higher concentrations. In other cases, however, the bulk complexes are large, soft and polydisperse. In most cases, the dispersions are only kinetically stable and exhibit pronounced non-equilibrium features. Association at air-water interfaces readily occurs, even at very small concentrations. When the surfactant concentration is small, the surface complexes are usually made of a surfactant monolayer to which the polymer binds and adsorbs in a flat-like configuration. However, under some conditions, thicker layers can be found, with bulk complexes sticking to the surface. The association at solid-water interfaces is more complex and depends on the specific interactions between surfactants, polymers and the surface. However, the behaviour can be understood if distinctions between hydrophilic surfaces and hydrophobic surfaces are made. Note that the behaviour at air-water interfaces is closer to that of hydrophobic than that of hydrophilic solid surfaces. The relation between bulk and surface complexation will be discussed in this review. The emphasis will be given to the results obtained by the teams of the EC-funded Marie Curie RTN "SOCON".

Novel self-assemblies of oppositely charged polyelectrolytes and surfactants in the presence of neutral polymer

In this Letter, the effect of addition of poly(ethylene oxide) (PEO) on the nature of the self-assemblies of sodium dodecyl sulfate (SDS) and branched poly(ethyleneimine) (PEI) is investigated. We demonstrate that the neutral polymer adsorbs onto the surface of the polyelectrolyte/surfactant nanoparticles, which may result in sterically stabilized colloidal dispersions of the nanoparticles with hydrophobic core and hydrophilic corona. The kinetic stability is maintained even at high ionic strengths, where the charge stabilization of the PEI/SDS dispersions is inefficient. These results might be exploited to improve the efficiency of those formulations, which contain oppositely charged macromolecules and amphiphiles.

The impact of electrolyte on the aggregation of the complexes of hyperbranched poly(ethyleneimine) and sodium dodecyl sulfate

The aggregation of the negatively charged complexes of hyperbranched poly(ethylenimine) (PEI) and sodium dodecyl sulfate (SDS) has been investigated at different sodium chloride (NaCl) concentrations using coagulation kinetics, electrophoretic mobility and dynamic light scattering measurements. The observed variation of the initial rate of coagulation with NaCl concentration indicates the formation of kinetically stable colloid dispersions in the investigated composition and pH range. These dispersions are electrostatically stabilized due to the adsorption of excess dodecyl sulfate ions on the surface of the polyelectrolyte/surfactant particles. Because of the enhanced adsorption of the anionic surfactant, the kinetic stability of the PEI/SDS dispersions increases with increasing SDS concentration and decreasing pH. Finally, we rationalize the effect of salt on the phase behavior and surface properties of polyelectrolyte/surfactant mixtures in terms of the salt-induced aggregation features of polyelectrolyte/surfactant particles.

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.