Single microgel particle studies demonstrate the influence of hydrophobic interactions between charged micelles and oppositely charged polyions

The binding of two cationic surfactants, dodecyltrimethylammonium bromide (DoTAB) and N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium bromide (HFDePB), to covalently cross-linked sodium poly(styrenesulfonate) (PSS) microgels has been investigated by means of micromanipulator-assisted time-resolved light microscopy on single gels. It is demonstrated that repeated measurements on the same microgel under conditions of controlled liquid flow give highly reproducible results. The two surfactants are found to behave very differently with respect to degree of swelling, surfactant distribution in the gels, both during shrinking and at equilibrium, and kinetics of volume changes induced by them. The main difference is attributed to the presence of a hydrophobic interaction between PSS and the DoTAB micelles, absent in the case of HFDePB. Kinetic shrinking curves are recorded and analyzed using a model for steady-state transport of surfactant between the solution and the gels. Aggregation numbers for DoTAB in PSS solutions obtained from fluorescence quenching measurements are presented. A strong dependence on the surfactant-to-polyion concentration ratio is observed. Relations between surfactant binding isotherms, phase diagrams for linear polyelectrolyte/surfactant/water systems, and the binding to gels are discussed.     

Determination of the latex particle size in emulsion polymerization of methyl methacrylate with low emulsifier concentrations

The mean size of the latex particles formed in emulsion polymerization of methyl methacrylate under definite conditions (water: monomer volume ratio 15: 1, 80°C, potassium persulfate concentration 0.07 wt %) decreases from 200 to 9–10 nm as the concentration of an ionic surfactant (anionic Disponil AES 60, SDS, cationic C₁₉H₄₂BrN) is increased from 0.0 to 1.0 wt %. The nonionic surfactants studied influence the size of the latex particles formed differently: with ALM-10, the particle size decreases from 200 to 150–190 nm, whereas with ALM-7 and ALM-2 it increases from 200 to 320 nm as the surfactant concentration is increased from 0.0 to 1.0 wt %. An increase in the concentration of F127 amphiphilic ternary block copolymer from 0.0 to 1.0 wt % leads to a monotonic decrease in the size of the poly(methyl methacrylate) latex particles formed from 200 to 53 nm.     

Role of emulsifier on the particle size and polymer particle size distribution using multiwavelength spectroscopy measurements

Latex emulsions depend strongly on the polymer composition, and particle size distribution, which in turn, is a function of the preparation of the latex and on the formulation and composition variables. This study reports measurements of particle size and particle size distribution of latex emulsions as function of the reaction time and the type and concentration of emulsifier by using the multiwavelength spectroscopy technique. Results show changes in the particle size of latex emulsions with the reaction time, obtaining larger particles and broader distributions with increasing of Tween 80 ratio. The steric stabilization provides the sole nonionic emulsifier is not enough to protect the polymer particle, causing the flocculation among the interactive particles, resulting in unstable latex. However, latex emulsions prepared with Tween 80 ratio <70 wt.% can stabilize efficiently the nucleated particles, probably due to the effects provided by both, the electrostatic and steric stabilization mechanisms. The same effect is shown in the curves of conversion (%) as a function of reaction time, resulting in slower polymerization rate for Tween 80 ratio >70 wt.%. On the other hand, smaller polymer particles, in all range of emulsifier mixture, have been obtained to higher emulsifier concentration.     

Dynamic interaction between oppositely charged vesicles: aggregation, lipid mixing, and disaggregation

We investigated dynamic interactions between oppositely charged small unilamellar vesicles using positively charged vesicles containing 1,2-dioleoyl-3-trimethylammonium-propane or 3beta-[N-(N('),N(')-dimethylaminoethane)-carbamoyl] cholesterol and negatively charged vesicles containing L-alpha-phosphatidyl-DL-glycerol. Aggregation, lipid bilayer mixing, contents mixing and contents leakage were systematically examined using optical density measurements, fluorescence resonance energy transfer assays, fluorescence quenching assays, light-scattering analyses, and freeze-fracture transmission electron microscopy. The oppositely charged vesicles aggregated immediately. Lipid mixing was observed, but there was no mixing of the contents. The vesicle aggregates disaggregated spontaneously after several minutes. The surface potential of the disaggregated vesicles was neutralized. From these results, we infer that the lipids in the external monolayers were exchanged between the oppositely charged vesicles while the internal monolayers remained intact. The two types of cationic lipids used exhibited different speeds of disaggregation.     

Dilational viscoelasticity and relaxation properties of interfacial electrostatic complexes between oppositely charged hydrophobic and hydrophilic polyelectrolytes

Strongly adsorbing hydrophobic cationic polyelectrolyte, Eudragit RS, containing approximately 2.5 mol% of pendent hydrophilic trimethylammonium (TMA) groups irreversibly adsorbs from its methylene chloride (MCl) solution at the MCl/water interface and forms solid-like adsorption layers (ALs). Submitted to periodic dilational deformations with the standard radial frequency omega(0)=0.63 rad/s, these ALs exhibit relatively high dilational storage modulus E' approximately 20 mN/m and practically zero loss modulus E' at the bulk concentration C(Eud)=4 x 10(-3)g/L. The frequency scanning of these ALs in the diapason omega=0.01-0.63rad/s and the approximation of the experimental dependences E'(omega) and E'(omega) by two relaxation times rheological model makes it possible to estimate the crossing frequency of these ALs determined from the condition E'(omega(c))=E'(omega(c)) as omega(c) approximately 5 x 10(-4)rad/s. Upon dissolving the hydrophilic anionic polyelectrolyte, chitosan sulfate (ChS), in the water phase (C(ChS)=3 x 10(-2)g/L) the electrostatic interpolyelectrolyte complexes form at the MCl/water interface. The elasticity moduli E' and E' of these mixed AL did not undergo remarkable variations, but the crossing frequency is sharply increased by approximately 10 times becoming equal to omega(c) congruent with 3 x 10(-3)rad/s. The increase of omega(c) certifies for the liquefaction of mixed Eudragit RS/ChS adsorption layers. A remarkable decrease of the storage modulus down to E'=8 mN/m and simultaneous increase of the crossing frequency up to omega(c) congruent with 10(-2)rad/s occurs upon increasing the concentrations of both components, Eudragit RS and ChS, up to 0.1g/L. The liquefaction effect in the mixed ALs of oppositely charged polyelectrolytes was explained on the basis of the proposed relaxation mechanism. The effect of the liquefaction of adsorption layers of strongly adsorbing hydrophobic polyelectrolytes by formation of interpolyelectrolyte complexes with hydrophilic polyelectrolytes must be taken into account in the production of nano-capsules and nano-fibers.     

Supramolecular systems based on cationic surfactants: an influence of hydrotropic salts and oppositely charged polyelectrolytes

Russian Chemical Bulletin - For cationic surfactant 4-aza-1-dodecyl-1-azoniabicyclo[2.2.2]octane bromide (DABCO-16), a significant decrease in the critical micelle concentration (CMC) is shown in...     

Semiclassical variational study of size effects in neutral and charged jellium droplets

An analytic expression for the total energy of metallic clusters composed ofN identical atoms of valencev and with net chargeQ is obtained by means of a variational solution of the Thomas-Fermi-Weizsäcker energy density functional within the spherical jellium model. The minimum energy is given as an expansion in decreasing powers of the cluster radiusR=r _s Z ^1/3, withZ=vN andr _s the radius per electron of the bulk metal. The coefficients are obtained as functions ofr _s . Terms of volume (R ³), surface (R ²), curvature (R), constant (R ⁰), (1/R) and (1/R ²) are clearly separated in the formula, as well as the different contributions (kinetic, coulombic and exchange-correlation) to each of them. The asymptotic values (R→∞) for the work functions,W(r _s ), and surface energies σ(r _s ), are compared to analogous semiclassical and Kohn-Sham calculations of jellium-like surfaces and to the experimental values. The size dependent behaviour of chemical potentials, μ(R), electron affinities,AF(R), and ionization potentials,IP(R), are easily obtained for any kind of metallic clusters. In particular we discuss the Coulomb and quantum corrections to the coefficients β, δ in the asymptotic formulae:IP?W+β/R andAF?W+δ/R.     

Control of particle size distribution in latices through periodic operation of continuous emulsion polymerization reactors

A method is proposed which allows to obtain latices with prespecified number particle size distributions (NPSD), with the only restriction that the said distributions must be monotonically descendent. To this effect, the forced periodic operation of the total feed flow into a continuous stirred-tank reactor (CSTR) is considered. The method is limited to emulsion polymerizations occurring within intervals 1 and 2 of Harkins' mechanism (1), and verifying kinetics of case II (2,3,4). Furthermore, because the procedure has been developed through a steady state (SS) model, the method is strictly valid in the limit of very low frequencies of oscillation. The technique is illustrated by means of a numerical example.     

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.     

Long-ranged electrostatic repulsion and crystallization of emulsion droplets in an ultralow dielectric medium supercritical carbon dioxide

Electrostatic repulsion stabilizes micrometer-sized water droplets with spacings greater than 10 microm in an ultralow dielectric medium, CO2 (epsilon = 1.5), at elevated pressures. The morphology of the water/CO2 emulsion is characterized by optical microscopy and laser diffraction as a function of height. The counterions, stabilized with a nonionic, highly branched, stubby hydrocarbon surfactant, form an extremely thick double layer with a Debye screening length of 8.9 microm. As a result of the balance between electrostatic repulsion and the downward force due to gravity, the droplets formed a hexagonal crystalline lattice at the bottom of the high-pressure cell with spacings of over 10 microm. The osmotic pressure, calculated by solving the Poisson-Boltzmann equation in the framework of the Wigner-Seitz cell model, is in good agreement with that determined from the sedimentation profile measured by laser diffraction. Thus, the long-ranged stabilization of the emulsion may be attributed to electrostatic stabilization. The ability to form new types of colloids in CO2 with electrostatic stabilization is beneficial because steric stabilization is often unsatisfactory because of poor solvation of the stabilizers.     

Phase behaviour of oppositely charged protein and surfactant mixtures: DOTAC-BLG-Water

The interactions between the negatively charged protein, β-lactoglobulin (BLG) and the cationic surfactant dodecyltrimethylammonium chloride (DOTAC) in water have been investigated by determining the phase equilibria of the ternary system within the concentration range of 20 wt.% of both protein and surfactant. Three main regions are formed—an isotropic solution phase, a white precipitation region and a blueish, isotropic, highly viscous gel phase. The protein solution can solubilize 1 mole surfactant, [DOTAC] per mole protein, [BLG] prior to precipitation. The protein-surfactant precipitate complex is neutral and consists of 8 [DOTAC]/[BLG]. The net charge of the protein in water at aqueous pH is −7 and this agrees with the determined composition. The pH is, however, decreased on addition of DOTAC, but this does not seem to affect the composition of the precipitate significantly. The amount of precipitate reaches a maximum at about 8 [DOTAC]/[BLG] and thereafter a plateau region occurs where no more precipitate seems to be formed. On further increasing the surfactant concentration the precipitate redissolves either into a solution phase directly or into a solution phase via a gel phase depending on the protein concentration. On decreasing salt concentration the ternary system shows similar phase behaviour, but the stability of the regions are different. It is also observed that oppositely charged protein-surfactant systems show similar phase behaviour irrespective of nature of the net charge on the protein.     

Unseeded semibatch emulsion polymerization of butyl acrylate: Bimodal particle size distribution

Unseeded semibatch emulsion polymerization of butyl acrylate (BA) using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator was carried out at the conditions where secondary nucleation was probable. This was achieved by using no emulsifier in the initial reactor charge. The effects of changes in monomer emulsion feed rate, initiator concentration and distribution, emulsifier concentration in the feed, and temperature on the evolution of particle size averages and distribution were investigated. Bimodal particle size distributions (PSD) were obtained for most of the latexes. Inhibition effects were found to be important in the development of PSD. Primary particle formation occurred through micellar nucleation, whereas secondary nucleation probably occurred through homogenous nucleation. The polydispersity index (PDI) of the latexes increased with the decreasing monomer emulsion feed rate. The application of a larger amount of initiator to the reactor charge or using a higher temperature, reduced the formation of secondary particles and resulted in a formation of an unimodal PSD. The overall steady‐state rate of polymerization was found to approach the rate of monomer addition (R_p ≈ R_a ), if the emulsifier concentration in the aqueous phase was appreciable. This is different from the correlation 1/R_p = 1/K + 1/R_a obtained for the BA semibatch process with neat monomer feed. This suggests that different rate expressions can be used for BA semibatch emulsion polymerization at different conditions. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 528–545, 2000     

Segregation of lipid and polymer in emulsion droplets captured by confocal laser scanning microscopy

The phase behaviors of the subsystems of the ethyl acetate (EtAc)/monoolein/polyethylene glycol-poly(D,L-lactide-co-glycolide) (PLG)/water system have been determined. EtAc simultaneously solves MO and PLG in a liquid phase, denoted L. Lipid/polymer composite particles have here been formed by emulsification of such an L phase into aqueous solutions. Characterization, by means of confocal laser scanning microscopy, revealed that distinctive lipid domains appear inside the particles. In aqueous solutions, these lipid domains swell and finally leave the concentrated polymer matrix. The system exhibits a suitable phase behavior in order to form lipid/polymer composite particles. These composite particles may be interesting for drug delivery applications.     

The influence of emulsion structure and stability on lipid digestion

The digestion and metabolism of lipids continues to generate considerable scientific interest, with food emulsions increasingly being seen as a mechanism by which lipid uptake may be controlled. Scientific advancement in this field is partly being driven by the ongoing need to address the obesity crisis, for which the enhancement of satiety and/or reduction of energy intake is seen as a positive solution in achieving more effective weight management. Yet the ability to regulate lipid uptake is also seen as beneficial in other areas, such as improved nutrition for the young and/or elderly and in cardiovascular protection.Because of the complexity of food digestion, the majority of research in this area has been applied to model or highly controlled systems. Through this approach it is becoming increasingly apparent that food emulsion structure and stability does have a contributing role on lipid digestion and metabolism. There is now a clear indication of how emulsion stability within the stomach affects emptying rates. There have been considerable developments in understanding the relationship between interfacial composition and lipolysis in both the gastric and intestinal regions, and how this relates to lipid uptake/metabolism. There is also an emerging understanding of the contribution of gastrointestinal biophysics to emulsion structure and stability, and how intestinal motility is in turn impacted by structural aspects, such as relative changes in particle size.Understanding of lipid digestion has been progressed through recent advancements in the sophistication of in vitro models. These are now seen as providing a more realistic representation of physiological conditions, both in terms of biochemical environment, and the biophysics of the gastrointestinal tract. Improvements in the validity of such models against in vivo and clinical behaviours is allowing aspects of emulsion digestion to be observed without the immediate need of costly human trials. Accordingly, emulsion systems with increasing structural complexity are now able to be characterised in terms of digestion behaviours. The ability to design food emulsions with specific lipid digestion profiles may allow the development of mainstream foods with particular physiological properties, such as enhanced satiation, or targeted delivery.     

Numerical study of the interplay of monomer-surface electrostatic and non-electrostatic interactions in the adsorption of weak polyelectrolytes on oppositely charged surfaces

The adsorption of weak polybase on oppositely charged planar surfaces has been investigated numerically by using the self-consistent field theory (SCFT). Particular attention was paid to the interplay of monomer-surface electrostatic and non-electrostatic interactions in the adsorption behaviors of weak polybase. In this study, the strength of monomer-surface non-electrostatic interactions was set to be no more than the thermal energy k _B T. It was found from the numerical study that in the regime of low surface charge density of the substrate and low pH or high bulk degree of ionization, both the screening-enhanced and screening-reduced salt effects emerge. On the contrary, in the opposite regime, only the screening-reduced salt effect was observed. Moreover, the overall charge neutrality inside the adsorption layer was analyzed. The underlying mechanism governing the adsorption behaviors of weak polybase on oppositely charged surfaces was elucidated.     

Styrene emulsion polymerization: Kinetics and particle size distributions in highly swollen latex systems

The results are reported of studies on the kinetics and the time evolution of the particle size distribution in seeded styrene emulsion polymerization systems wherein the seed latex particles were highly swollen with monomer as a result of prior swelling by dodecane. Conditions were such that no new latex particles were formed nor was a significant number of monomer droplets present (“Interval III”). The data were fitted to obtain values for the rate coefficients for entry and exit (desorption) of free radicals. It was found that, during the early part of the polymerization (when the polymer:monomer ratio in the latex particles is considerably less then in an equivalent emulsion polymerization system without dodecane), the entry rate coefficient was much smaller than that measured in systems without dodecane. This effect is consistent with an entry mechanism wherein entering free radicals must displace surfactant molecules from the latex particles.     

Mixtures of charged colloid and neutral polymer: influence of electrostatic interactions on demixing and interfacial tension

The equilibrium phase behavior of a binary mixture of charged colloids and neutral, nonadsorbing polymers is studied within free-volume theory. A model mixture of charged hard-sphere macroions and ideal, coarse-grained, effective-sphere polymers is mapped first onto a binary hard-sphere mixture with nonadditive diameters and then onto an effective Asakura-Oosawa model [S. Asakura and F. Oosawa, J. Chem. Phys. 22, 1255 (1954)]. The effective model is defined by a single dimensionless parameter-the ratio of the polymer diameter to the effective colloid diameter. For high salt-to-counterion concentration ratios, a free-volume approximation for the free energy is used to compute the fluid phase diagram, which describes demixing into colloid-rich (liquid) and colloid-poor (vapor) phases. Increasing the range of electrostatic interactions shifts the demixing binodal toward higher polymer concentration, stabilizing the mixture. The enhanced stability is attributed to a weakening of polymer depletion-induced attraction between electrostatically repelling macroions. Comparison with predictions of density-functional theory reveals a corresponding increase in the liquid-vapor interfacial tension. The predicted trends in phase stability are consistent with observed behavior of protein-polysaccharide mixtures in food colloids.     

Aspartate: An interesting model for analyzing dipole-ion and ion pair interactions through its oppositely charged amine and acid groups

Anionic species of aspartic acid, Asp⁻, having a zwitterionic backbone and a deprotonated side chain, appears to be a good example for analyzing dipole-ion and ion pair interactions. Density functional theory calculations were herein performed to investigate the low energy conformers of Asp⁻ embedded in a dielectric continuum modeling an aqueous environment, through a scan of the potential energy as a function of the side chain (χ₁, χ₂) torsion angles. The most energetically favorable conformers having g⁺g⁻ and g⁻g⁺ side chain orientations are found to be stabilized by charge-enhanced intramolecular H-bonding involving the positively charged () and the two negatively charged (COO⁻) groups. These conformers were further used to analyze Asp⁻ + nW clusters (W: water, n = 1 or 3), and Asp⁻/Asp⁻ pair formation. COO⁻ groups were found to be the most attractive sites for hosting a water molecule (binding energy: −6.0 ± 1.5 kcal/mol), compared to groups (binding energy: −4.7 ± 1.1 kcal/mol). Energy separation between g⁺g⁻ and g⁻g⁺ conformers increases upon explicit hydration. Asp⁻/Asp⁻ ion pairs, stabilized by the interaction between the group of a partner and the COO⁻ group of the other, shows a quite constant binding energy (−8.1 ± 0.2 kcal/mol), whatever the pair type, and the relative orientation of the two interacting partners. This study suggests a first step to achieve a more realistic image of intermolecular interactions in aqueous environment, especially upon increasing concentration. It can also be considered as a preliminary attempt to assess the interactions of the Lys⁺…Asp⁻/Glu⁻ ion pairs stabilizing intra- and interchain interactions in proteins.     

Effects of stirring prior to starting emulsion polymerization of styrene with nonionic emulsifier on particle formation and its incorporation

Emulsion polymerization of styrene with a nonionic emulsifier (polyoxyethylene nonylphenyl ether, E911) and potassium persulfate as initiator was carried out at different stirring rates (240–500 rpm) at 70 °C, which was started by the addition of initiator after stirring for 100 min at 70 °C. Resulting polystyrene (PS) particles at 240 rpm were 70-nm-sized, spherical particles and incorporated only 5 wt.% of total E911. On the other hand, particles at 500 rpm were 1-μm-sized, nonspherical particles, which were formed by coagulation of small particles, and incorporated above 70 wt.% of E911 in the inside. Before starting emulsion polymerization, E911 and styrene, respectively, transferred from an aqueous phase to a styrene phase and from the styrene phase to the aqueous phase (water and micelles) faster at 500 rpm than 240 rpm. At 240 rpm, there were a lot of almost empty micelles (ca. 5 nm) in the aqueous phase, on the other hand at 500 rpm, 70 wt.% of total E911 transferred to the styrene phase and the micelles were swollen with much monomer (ca. 40 nm) even if the number was smaller. Stirring prior to starting the emulsion polymerization greatly affected partitionings of monomer to the aqueous phase and the nonionic emulsifier to the styrene phase, resulting in the differences in the particle formation and the incorporation of the nonionic emulsifier inside PS particles.