Control over colloidal aggregation in monolayers of latex particles at the oil-water interface

The controlled generation of 2D aggregate networks is studied experimentally using micrometer-sized polystyrene latex particles attached to the oil-water interface. Starting from an initially crystalline monolayer, appropriate combinations of carefully added electrolyte and surfactant enable control over both the fractal dimension and the kinetics of aggregation. Remarkably, the colloidal crystals formed upon first spreading remain stable, even for days, when substantial amounts of electrolyte are added to the aqueous phase. Pressure-area isotherms reveal a slow time evolution of the electrostatic dipole-dipole interaction. When the electrostatic interaction has been sufficiently weakened, aggregation proceeds in well-defined, reproducible manner. The aggregation process is analyzed using quantitative video microscopy. The evolution of the cluster size distribution and its moments is characterized, and static and dynamic scaling exponents are derived to identify the nature of the aggregation process. In the range of concentrations studied here, the kinetics all agree with a "fast", diffusion-limited cluster type of aggregation. However, the fractal dimension strongly depends on the composition of the aqueous subphase. Rather dense structures are found when only electrolyte is used, whereas more open structures are obtained when even small amounts of surfactant are added. It is suggested that this structural dependency is related to the effect of surfactant on the contact angle and its consequences for the anisotropic nature of the capillary interactions.     

Packing conditions of ionic surfactant molecules in ellipsoidal micelles

Relations between the main parameters characterizing the structure of ellipsoidal micelles of ionic surfactants in aqueous solutions and the geometric sizes of micelles are derived and analyzed within the framework of the model of the two-axial ellipsoid of revolution. Conditions of the existence of “continuous” and hollow ellipsoidal micelles are discussed using the generalized Gibbs-Curie principle. Shape factors of spherical, ellipsoidal, and cylindrical micelles are compared. Results are verified using micelles of sodium decyl sulfate in aqueous solutions as examples.     

Vibrational spectroscopy of mechanically compressed monolayers

Sum-frequency spectroscopy has been used to investigate the behavior of self-assembled monolayers in a solid-solid contact. Various alkanethiol layers on gold were observed before, during, and after compression to 660 MPa against a sapphire counterface. Well-ordered layers that differ only in the length of their alkane tails (C(8) versus C(18)) behave similarly. In contrast, defective and partly melted monolayers are more sensitive to stress than are their well-ordered analogues, and they are more prone to irreversible changes. In all cases, the intensity of methyl C-H stretching modes decreases with applied pressure, indicating a loss of net orientational order among the terminal methyl groups. The magnitude of this effect in well-ordered layers can be compared with the theoretical sensitivity of the resonant sum-frequency signal to molecular orientation. On these grounds, an increased population of terminal gauche conformers is identified as the disordering mechanism under pressure.     

Shape and buckling transitions in solid-stabilized drops

We study shape and buckling transitions of particle-laden sessile and pendant droplets that are forced to shrink in size. Monodisperse polystyrene particles were placed at the interface between water and decane at conditions that are known to produce hexagonal, crystalline arrangements on flat interfaces. As the volumes of the drops are reduced, the surface areas are likewise diminished. This effectively compresses the particle monolayer coating and induces a transition from a fluid film to a solid film. Since the particles are firmly attached to the interface by capillary forces, the shape transitions are reversible and shape/volume curves are the same for drainage and inflation. Measurements of the internal pressure of the drops reveal a strong transition in this variable as the buckling transition is approached.     

Preparation of monodisperse ellipsoidal polystyrene particles

A method is described for the preparation of monodisperse ellipsoidal particles of polystyrene in the colloidal size range. Monodisperse polystyrene particles were dispersed in a solution of polyvinyl alcohol. This dispersion was then allowed to form, by evaporation, a thin film of polyvinyl alcohol containing spherical polystyrene particles. Strips of this film were clamped into a metal frame, heated rapidly in an oil bath to 200°C and stretched to a predetermined extent in order to convert the spherical particles into ellipsoids; the film was then cooled. A wide range of axial ratios for a variety of initial particle sizes was obtained by this method.     

Microscopic folds and macroscopic jerks in compressed lipid monolayers

We report hitherto unrecognized cooperative behavior in the stochastic collapse of certain compressed lipid monolayers implicated in pulmonary function. The cooperativity emerges from a statistical analysis of the collapse events captured using fluorescence microscopy and digital image analysis. The collapse events involve folding of the monolayer on a micron scale, yet each event produces a macroscopic jerk of the layer. The cooperative collapse is striking for its temporal sharpness and large spatial extent.     

Morphology of core-shell latex particles

The progressive dissolution of carboxylated latex particles with increasing pH was utilized to investigate the internal structure of core-shell latex particles, in comparison with that of copolymeric latex particles formed from the same monomers. The results indicated that in those latex particles which are formed when ethyl acrylate (EA) -methacrylic acid (MAA) or methyl acrylate (MA) -MAA mixture is polymerized in the presence of poly(MA-MAA) or poly(EA-MAA) seeds the shell is composed of the more hydrophilic poly(MA-MAA) molecules relatively high in MAA content and the core is composed of both poly(MA-MAA) and poly(EA-MAA) molecules, regardless of the order of the stage feed, while the copolymeric latex particles are relatively uniform from surface to center in distribution of all components except MAA. Examination of the distribution of the carboxylic groups in all of the latex particles showed their concentration to be highest at the surface and to decrease with proximity to the center in accordance with other findings reported in the literature.     

Calculation of electrostatic interaction forces between ellipsoidal particles

An analytical expression is presented for describing the electrostatic interaction forces between various shaped particles having mutual orientations. The expression is derived by applying the surface integration method, which is a generalization of the Derjaguin summation procedure. Based on previous theoretical considerations it is possible to calculate the electrostatic interaction force between regularly shaped bodies (both convex or concave in the vicinity of their contact point) by multiplying the interaction energy derived for paralled plates with the corresponding geometric factor. The forces acting between two equal shaped ellipsoids are described and discussed, considering three different limiting orientations, the parallel, the perpendicular, and the contact of the edges' orientation.     

Synthesis and characterization of amphoteric latex particles

Stable monodisperse amphoteric latex particles were prepared by the semibatch surfactant-free emulsion copolymerization of methyl methacrylate and methacrylic acid (MAA) initiated by 2,2'-azobis(2-amidinopropane) dihydrochloride (V-50). These submicron particles have a net positive charge, which is attributed to the ionized amino group at low pH. In contrast, they become negatively charged owing to the ionized carboxyl group at high pH. There exists a pH at which these particles exhibit a net charge of zero (pI). At a constant level of V-50, the pI value of these latices decreases with increasing amount of MAA used in the polymerization recipe. The effect of pH on the colloidal stability of these amphoteric latices toward the addition of the negatively charged latex was investigated. The resultant coagulation kinetics was used to study the electrostatic interaction between the amphoteric particles and negatively charged particles.     

The solubility and swelling of latex particles

A vapor pressure method has been used to determine the solubility of vinyl acetate, styrene, and vinyl hexanoate in both the aqueous phase and in the polymer particles. All the systems have been measured at 30° or 60°C. or both.The vapor pressure measurement permits an estimation of the partial molar free energy of the monomer-polymer system. A thermodynamic treatment of the latex system, modified from that of Morton, yields a relationship from which the monomer-polymer interaction parameter, μ, and the interfacial tension, γ, may be obtained. Several tests of the thermodynamic relationship involving variations of the interfacial tension proved successful. Reasonable values are obtained for both the interaction parameter and the interfacial tension, although checks of two interaction parameters with literature values show some differences due to dissimilarities in the conditions of measurement.The solubility figures for the systems studied are high compared to most literature values at other temperatures; however, the difference is readily explained by consideration of the dependence of solubility on the temperature. Increased solvency of the monomers at polymerization temperatures tends to swell the particles more than at ambient temperatures. The increased solubility of monomer in the polymer is reflected in a lowering of the interaction parameter. The values determined at polymerization temperatures are obviously more representative of the polymerizing system.     

Formation and morphology of latex monolayers. Computer simulation studies

The results of computer simulations of monolayers created from monodisperse latex particles are presented and discussed. Layers are characterized by the normalized coverage,NC (the actual coverage of the surface related to its maximum possible coverage with particles), and by the average number of neighbors,ANN, calculated as the number of particles being in contact with a given one and averaged over all the particles on the surface. Variable parameters used in simulations include: the rate of particles deposition, the probability of lateral movements, the probability of desorption of particles adsorbed on the surface, the probability of covalent immobilization of adsorbed particles, and the on-sphere slip parameter, OSS (characterizing the scattering of a falling particle on the particles being already attached to the surface). Morphology of monolayers is qualitatively characterized by relations betweenANN andNC. It is shown that for all monolayers formed without adhesion (and without repulsion) between the particles adsorbed on the surface the dependence ofANN vs.NC is described by a characteristic master curve (regardless of the values of probabilities of desorption and lateral movements of particles). For the monolayers created including adhesive forces between the adsorbed particles the plots ofANN vs.NC lie above the master curve, while similar plots obtained for the layers made of particles showing various types of repulsive interactions are always placed below it. Thus, the dependencies ofANN vs.NC, derived from computer simulations, can be used for the determination of the character of the interparticle interactions in the real systems.     

Reactive blends of thermoplastics and latex particles

Monodisperse homogeneous and core–shell latex particles of various sized between 200 and 600 nm were synthesized by emulsion copolymerization. Some of the core–shell particles were functionalized with epoxy groups at their peripheries upon introduction of glycidyl methacrylate (GMA) during the synthesis. The core consisted of crosslinked polybutylacrylate and the shell polymethylmethacrylate. Synthesis conditions at high and low temperatures were optimized to obtain coreshell particles with a well-defined morphology. The particles were characterized by quasi-elastic light scattering, scanning electron microscopy and transmission electron microscopy. The latex particles functionalized with GMA were then dispersed into a reactive matrix (styrene and maleic anhydride copolymer) using a batch mixer to obtain blends with well-defined and stabilized morphology. 4 Dimethylaminopyridine was used as a catalyst. The reaction between the epoxy groups at the particle surface and the maleic anhydride or diacid groups of the matrix was evaluated by torque and extraction techniques. A small amount of conversion generates sufficient amounts of grafted species at the matrix and particle interfaces to ensure a good interfacial adhesion.     

Synthesis of cationic core-shell latex particles

Surfactant-free seeded (core-shell) polymerization of cationic polymer colloids is presented. Polystyrene core particles with sizes between 200 nm and 500 nm were synthesized. The number average diameter of the colloidal core particles increased with increasing monomer concentration. Cationic shells were introduced by co-polymerizing styrene with the cationic monomers (vinylbenzyl)trimethylammonium chloride (VBTMAC), [(2-methacryloyloxy)ethyl] trimethylammonium chloride (METMAC) and [(2-(acryloyloxy)ethyl] trimethylammonium chloride (AETMAC) onto the polystyrene cores. The cationic monomer AETMAC, undocumented to our knowledge in colloid synthesis, produced the best cationic shells and could be incorporated at much higher concentrations in the shell compared to the commonly used VBTMAC and METMAC, which yielded undesired polyelectrolyte side products already at relatively low cationic monomer concentrations. In shell formation, feed concentrations of AETMAC between 1.3 mol% (2.4 wt%) and 10.7 mol% (20 wt%) in styrene could be employed, allowing us to control colloid surface charge density over a wide range. The influence of various polymerization parameters (initiator concentration, cross-linking agent, and ionic strength) on the co-polymerization process with AETMAC is discussed. Core-shell particles were characterized using HR-SEM, potentiometric titration and zeta potential measurements.     

Equilibrium encapsulation of polystyrene latex particles

We have performed a series of seeded styrene emulsion polymerizations in which the second stage of growth was initiated only after the second-stage monomer charge had achieved equilibrium saturation with the seed particles. The final particles were observed in the electron microscope by using two means of distinguishing between the first- and second-generation polymer: (a) butadiene tagging and osmium tetroxide staining coupled with ultramicrotomy and (b) tritiated-styrene tagging coupled with autoradiographic detection. We find that the first- and second-generation polymer chains are not uniformly mixed throughout the final latex product; rather, the second-generation polymer overcoats the seed polymer in a core–shell fashion. In order to explain these results, we present the viewpoint that monomer actually concentrates at the periphery of the swollen particle to form a monomer-encapsulated structure, rather than swelling the particle uniformly as has always been envisioned. We believe the encapsulation phenomena to be governed by the microscopic thermodynamic environment of the latex particles which has, in turn, a profound effect upon the conformational behavior of the long-chain polymer molecules as they interact with the particle–water interface.     

Preparation of spheroidal and ellipsoidal particles from spherical polymer particles by extension of polymer film

Generation of particles of various sizes and shapes can be of great interest to scientists and engineers. We have developed a new and robust apparatus which can generate oblate spheroids from spherical polymer particles. A sheet of film dispersed with spherical particles was held by an eight-jaw extensional apparatus. The jaws were positioned in the edges of a regular octagon and moved radially to induce biaxial extension in an oil bath above the glass transition temperatures of the film and particles. We have demonstrated that polystyrene particles dispersed in a polyvinyl alcohol film can be stretched to generate various shapes by this method. The microscopic studies show that the oblate spheroids obtained by this method are virtually exact spheroids without showing knife edges. Also depending on positions with regard to holders, ellipsoids and even prolate spheroids can be obtained. The method has been found to be robust in that the deformation is always reproducible regardless of film thickness and very small deformation can be applied for nearly spherical particles. We have confirmed that this method can be applied for particles of submicrons to 10?μm in diameter or even larger ones. It is expected that the spheroids and ellipsoids obtained by this method can be of help in many studies including colloids, suspension rheology, electrophoresis, printed electronics, and pharmaceutical science.     

Collapse and shedding transitions in binary lipid monolayers coating microbubbles

We report on a fluorescence microscopy study of the monolayer collapse and shedding behavior due to shell compression during the dissolution of air-filled, lipid-coated microbubbles in degassed media. The monolayer shell was comprised of saturated diacyl phosphatidylcholine (C12:0 to C22:0) and an emulsifier, poly(ethylene glycol)-40 stearate. The morphologies of monolayer collapse structures and shed particles were monitored as a function of phospholipid acyl chain length (n) and temperature. The two components formed a single miscible phase when the phospholipid was near or above its main phase transition temperature, and collapse occurred via suboptical particles to vesicles (both were shed) and tubes as chain length increased. Conversely, two-phase coexistence was observed when the lipid was below its main phase transition temperature. For these bubbles, a transition from primary collapse to secondary collapse was observed. Primary collapse was observed as a loss of expanded phase due to vesiculation. Secondary collapse involved the rapid propagation of monolayer folds and simultaneous deformation. For very rigid monolayers, we observed substantial surface buckling with simultaneous nucleation and growth of folds. The folds merged at a single point or region, providing a conduit for the entire excess lipid to shed in a single event, and the bubble smoothed and became more spherical. These results are discussed in the context of general binary phospholipid collapse behavior, microbubble dissolution behavior, medical applications, and the dissolution behavior of natural microbubbles.     

Hollow latex particles: synthesis and applications

One of the major developments in emulsion polymerization over the last two decades has been the ability to make hollow latex particles. This has contributed many fundamental insights into the synthesis and the development of structure in particles. Hollow latex particles also enhance the performance of industrial coatings and potentially are useful in other technologies such as microencapsulation and controlled release. Ever since the publication of the initial process patents describing these particles, there has been a global R&D effort to extend the synthetic techniques and applications. One prominent synthetic approach to hollow particles is based on osmotic swelling. This dominates the literature, and usually starts with the synthesis of a structured latex particle containing an ionizable core that is subsequently expanded with the addition of base. Fundamental to this approach are a sophisticated control of transport phenomena, chemical reactivity within the particle, and the thermoplastic properties of the polymer shell. Hydrocarbon encapsulation technology has also been employed to make hollow latex particles. One approach involves a dispersed ternary system that balances transport, conversion kinetics, and phase separation variables to achieve the hollow morphology. Other techniques, including the use of blowing agents, are also present in the literature. The broad range of approaches that affords particles with a hollow structure demonstrates the unique flexibility of the emulsion polymerization process.     

Synthesis of monodisperse,magnetic latex particles with polystyrene core

A novel method for producing monodisperse, submicron-sized magnetic latex particles is described. The method provides coating of polymer particles with surface-modified magnetic particles during soap-free polymerization. Experiments were performed with styrene monomer, potassium persulfate initiator, Fe ₃O ₄ magnetic particles, and silane-coupling reagents of methacryloxypropyltrimethoxysilane (MPTMS) and methacryloxypropyldimethoxysilane (MPDMS). The morphology of the magnetic particles depended on the silane-coupling reagents. Use of the tri-functional coupling reagent MPTMS produced particles having a disk-like or concave-like shape, whereas use of the di-functional coupling reagent MPDMS produced spherical particles that had a coefficient of variation of 4.4%, which was much smaller than the standard criteria of monodispersity, 10%.     

A comprehensive approach to electro-orientation, electrodeformation, dielectrophoresis, and electrorotation of ellipsoidal particles and biological cells

Suspended cells may respond to AC polarization by orienting, deforming, moving or rotating. For modeling of ellipsoidal cells, a new dipole approach is proposed. Along each of the principal axis of the model, three finite elements of arbitrary but equal cross-sectional area for the interior, low conductive membrane shell and exterior are assumed. The length of the external medium elements is defined by influential radii which are related to the depolarizing factors. The model predicts the potential at the ellipsoid's surface leading to the induced dipole moment. The moment obtained is identical to the Laplace approach for homogeneous ellipsoids; in the single-shell case, it is slightly different. The reason is the constant shell thickness which overcomes the confocal thickness necessary for the Laplace solution. Expressions for electro-orientation, deformation, dielectrophoresis, and electrorotation are derived. In linearly and circularly polarized fields, different orientation spectra are predicted to occur. While in linearly polarized AC fields, particles are oriented along their axis of highest polarizability, in circularly polarized fields, the axis of lowest polarizability is oriented perpendicular to the plane of field rotation. Based on this finding, a new electro-orientation method is proposed. In dielectrophoresis and electrorotation, reorientations are predicted which lead to discontinuous spectra.