Self-limiting droplet fusion in ionic emulsions.

We make an oil-in-water emulsion, which is initially stabilized using a first ionic surfactant, and mix it with a solution of a second ionic surfactant having the opposite charge, thereby inducing massively parallel droplet fusion. A transient disruption of the screened-charge repulsive barrier between interacting droplets, caused by the second ionic surfactant, arises from significant yet temporary charge neutralization of the first ionic surfactant on the surfaces of the oil droplets while mixing occurs. Interestingly, if a moderate molar excess of one surfactant exists, then the resulting emulsion re-stabilizes after limited droplet fusion. By adjusting the droplet volume fraction, concentrations of first and second surfactants, and volumes of the emulsion and the solution of the second surfactant, we control the degree of droplet coalescence and achieve a self-limiting droplet fusion process. Using optical microscopy, we observe that flat, thin, crystalline films can form between the two oil compartments after fusion of two or more immiscible microscale droplets. However, no such crystalline films are seen on the highly curved oil-oil interfaces inside nanoscale droplets that are composed of two or more immiscible oils and have been fused in the same manner, as revealed by cryogenic transmission electron microscopy.

Corona charge selective micelle degradation catalyzed by P. cepacia lipase isoforms.

Micelles derived from poly(ε-caprolactone) (PCL) poly(ethylene glycol)(PEG) diblock copolymers (HO-PCLn-b-PEG32-RX, R = -O(CH2)3S(CH2)2-; X = -CO2(-), -SO3(-), -NH3(+)) show corona charge selective degradation catalyzed by P. cepacia lipase isoforms.

Cerberus nanoemulsions produced by multidroplet flow-induced fusion.

Through extreme flow-induced fusion and rupturing of microscale droplets within a mixture of two or more oil-in-water emulsions, each having a different type of mutually immiscible oil, we create complex oil-in-water nanoemulsions composed of multicomponent compartmentalized nanodroplets. The extreme flow temporarily overcomes the repulsive barrier between oil droplets, arising from stabilizing surfactant molecules on the droplet interfaces, thereby causing multidroplet fusion as well as droplet fission down to the nanoscale. After the droplets leave the vicinity of extreme flow, they remain stable against subsequent coarsening and coalescence. Using this highly parallel, top-down, nonequilibrium synthetic approach, we create bulk quantities of engulfed-linear Cerberus oil-in-water nanoemulsions. Each Cerberus nanodroplet contains three different immiscible oils that form complex-shaped internal compartments, as revealed by cryogenic transmission electron microscopy (cryo-TEM). Within a given Cerberus nanodroplet, depending upon the interfacial tensions and relative volume fractions of the different oils, the internal oil-oil interfaces can be significantly deformed. Such multicomponent compartmentalized oil nanodroplets have the capacity of holding different types of oil-soluble cargo molecules, including fluorinated drug molecules, which have a wide variety of functional capacities and the potential for local synergistic effects. Their size range is small enough to permit a wide variety of pharmaceutical applications. As such, Cerberus nanoemulsions open up possibilities for simultaneously delivering several different types of oil-soluble drug molecules, each of which is readily soluble in at least one of the different types of immiscible oils, to the same cell or tissue.

Nanoinclusions in cryogenically quenched nanoemulsions.

Nanodroplets containing mixtures of silicone oil and squalene are dispersed in a simple aqueous surfactant solution, quenched in liquid ethane, and examined using cryogenic transmission electron microscopy (CTEM). Depending on the phase of ice that forms around the nanodroplets and on the composition of the oil mixture, nanoinclusions can be observed inside oil nanodroplets, independent of surfactant type. Our observations suggest that these nanoinclusions arise from nucleation of vapor cavities as the water freezes and expands while the oil remains liquid during the quench.

Palladium metal nanoparticle size control through ion paired structures of [PdCl4]2- with protonated PDMAEMA.

The mean diameter of palladium metal particles produced by citrate reduction of the (H(+))(n) PDMAEMA/[PdCl(4)](2-) aqueous system increases from 1.4 nm to 5.0 nm as the pH decreases from 6.3 to 3.3 and the (H(+))(n) PDMAEMA/[PdCl(4)](2-) species undergo conversion from a cross linked polymer network to a single chain structure.

Advanced nanoemulsions.

Recent advances in the growing field of nanoemulsions are opening up new applications in many areas such as pharmaceuticals, foods, and cosmetics. Moreover, highly controlled nanoemulsions can also serve as excellent model systems for investigating basic scientific questions about soft matter. Here, we highlight some of the most recent developments in nanoemulsions, focusing on methods of formation, surface modification, material properties, and characterization. These developments provide insight into the substantial advantages that nanoemulsions can offer over their microscale emulsion counterparts.

Hierarchical self-assembly of amphiphilic semiconducting polymers into isolated, bundled, and branched nanofibers.

Herein, we report a high-yield click synthesis and self-assembly of conjugated amphiphilic block copolymers of polythiophene (PHT) and polyethylene glycol (PEG) and their superstructures. A series of different length PHT(m)-b-PEG(n) with well-defined relative block lengths was synthesized by a click-coupling reaction and self-assembled into uniform and stably suspended nanofibers in selective solvents. The length of nanofibers was controllable by varying the relative block lengths while keeping other dimensions and optical properties unaffected for a broad range of f(PHT) (0.41 to 0.82), which indicates that the packing of PHT dominates the self-assembly of PHT(m)-b-PEG(n). Furthermore, superstructures of bundled and branched nanofibers were fabricated through the self-assembly of PHT(m)-b-PEG(n) and preformed PHT nanofibers. The shape, length, and density of the hierarchical assembly structures can be controlled by varying the solvent quality, polymer lengths, and block copolymer/homopolymer ratio. This work demonstrates that complex superstructures of organic semiconductors can be fabricated through the bottom-up approach using preformed nanofibers as building blocks.

Optically probing nanoemulsion compositions.

Many types of colloids, including nanoemulsions, which contain sub-100 nm droplets, are dispersed in molecular and micellar solutions, especially surfactant solutions that confer stability. Since it would be desirable to measure the droplet volume fraction ϕ and surfactant concentration C of a nanoemulsion non-destructively, and since the droplet and surfactant structures are significantly smaller than the shortest wavelengths of visible light, optical refractometry could provide a simple and potentially useful approach. By diluting a silicone oil-in-water nanoemulsion having an unknown ϕ and C with pure water, measuring its refractive index n(ϕ,C) using an Abbé refractometer, and fitting the result using a prediction for n that treats the nanoemulsion as an effective medium, we show that ϕ and C can be deduced accurately over a relatively wide range of compositions. Moreover, we generalize this approach to other types of nanoemulsions in which a molecular constituent partitions in varying degrees between the dispersed and the continuous phases.

Anionically cross linked homopolymer colloids applied in formation of platinum nanoparticles.

Diprotonic sulfuric and succinic acids react efficiently with the tertiary amine sites in polydimethylaminoethylmethacrylate (PDMAEMA) to produce polymer colloid nano-particles held together by dinegatively charged anions that cross link the partially protonated PDMAEMA homopolymer. This procedure is used to encorporate [PtCl(6)](2-) as a cross linker into the framework of well defined polymer network colloid particles that have dual roles as nanoreactors and a source of protective polymer coating. Reduction of the cross linking [PtCl(6)](2-) groups produces platinum metal nano-particles (1.12(.25)nm) that are relatively small and narrowly dispersed. Formation of colloid particles by reaction of diprotic acids with homopolymers that have proton accepting centers provides a convenient intentional route to incorporate a variety of homopolymers into self assembled polymer network materials for applications as nanoreactors and transport systems.

Rotational Fourier tracking of diffusing polygons.

We use optical microscopy to measure the rotational Brownian motion of polygonal platelets that are dispersed in a liquid and confined by depletion attractions near a wall. The depletion attraction inhibits out-of-plane translational and rotational Brownian fluctuations, thereby facilitating in-plane imaging and video analysis. By taking fast Fourier transforms (FFTs) of the images and analyzing the angular position of rays in the FFTs, we determine an isolated particle's rotational trajectory, independent of its position. The measured in-plane rotational diffusion coefficients are significantly smaller than estimates for the bulk; this difference is likely due to the close proximity of the particles to the wall arising from the depletion attraction.

Shear-induced disruption of dense nanoemulsion gels.

The structural evolution and rheology of dense nanoemulsion gels, which have been formed by creating strong attractions between slippery nanodroplets, are explored as a function of steady shear rate using rheological small-angle neutron scattering (rheo-SANS). For applied stresses above the yield stress of the gel, the network yields, fracturing into aggregates that break and reform as they tumble and interact in the shear flow. The average aggregate size decreases with increasing shear rate; meanwhile, droplet rearrangements within the clusters, allowed by the slippery nature of the attractive interaction, increase the local density within the aggregates. At the highest shear rates, all clusters disaggregate completely into individual droplets.

Nanoparticle-directed self-assembly of amphiphilic block copolymers.

Nanoparticles can form unique cavity-like structures in core-shell type assemblies of block copolymers through the cooperative self-assembly of nanoparticles and block copolymers. We show that the self-assembly behavior is general for common as-synthesized alkyl-terminated nanoparticles for a range of nanoparticle sizes. We examined various self-assembly conditions such as solvent compositions, nanoparticle coordinating ligands, volume fraction of nanoparticles, and nanoparticle sizes in order to elucidate the mechanism of the radial assembly formation. These experiments along with strong segregation theory calculations indicated that both the enthalpic interaction and the polymer stretching energy are important factors in the coassembly formation. The slightly unfavorable interaction between the hydrophobic segment of polymers and alkyl-terminated nanoparticles causes the accumulation of nanoparticles at the interface between the polymer core and the shell, forming the unique cavity-like structure. The coassemblies were stabilized for a limited range of nanoparticle volume fractions within which the inclusion of nanoparticle layers reduces the polymer stretching. The volume fraction range yielding the well-defined radial coassembly structure was mapped out with varying nanoparticle sizes. The experimental and theoretical phase map provides the guideline for the coassembly formation of as-synthesized alkyl-terminated nanoparticles and amphiphilic block copolymers.

Highly tunable photoluminescent properties of amphiphilic conjugated block copolymers.

We report a novel class of amphiphilic conjugated block copolymers composed of poly(3-octylthiophene) and poly(ethylene oxide) (POT-b-PEO) that exhibit highly tunable photoluminescence colors spanning from blue to red. POT-b-PEO self-assembles into various well-defined core/shell-type nanostructures as a result of its amphiphilicity. The self-assembly structure can be readily controlled by altering the solvent composition or by other external stimuli. The color change was completely reversible, demonstrating that the strategy can be used to manipulate the light-emission properties of conjugated polymers in a highly controllable manner without having to synthesize entirely new sets of molecules.

Exchange of organic radicals with organo-cobalt complexes formed in the living radical polymerization of vinyl acetate.

Exchange of organic radicals between solution and organo-cobalt complexes is experimentally observed and the reaction pathway is probed through DFT calculations. Cyanoisopropyl radicals from AIBN (2,2'-azobisisobutyronitrile) enter solutions of cobalt(II) tetramesityl porphyrin ((TMP)Co(II)*, 1) and vinyl acetate (VAc) in benzene and react to produce transient hydride (TMP)Co-H and radicals (*CH(OAc)CH2C(CH3)2CN (R1*)) that proceed on to form organo-cobalt complexes (TMP)Co-CH(OAc)CH3 (4, Co-R2) and (TMP)Co-CH(OAc)CH2C(CH3)2CN (3, Co-R1), respectively. Rate constants for cyanoisopropyl radical addition with vinyl acetate and hydrogen atom transfer to (TMP)Co(II)* are reported through kinetic studies for the formation and transformation of organo-cobalt species in this system. Rate constants for near-degenerate exchanges of radicals in solution with organo-cobalt complexes are deduced from (1)H NMR studies and kinetic modeling. DFT computations revealed formation of an unsymmetrical adduct of (TMP)Co-CH(OAc)CH3 (4) with *CH(OAc)CH3 (R2*) and support an associative pathway for radical interchange through a three-centered three-electron transition state [R...Co...R]. Associative radical interchange of the latent radical groups in organo-cobalt porphyrin complexes with freely diffusing radicals in solution that is observed in this system provides a pathway for mediation of living radical polymerization of vinyl acetate.

Reversible stimuli-responsive nanostructures assembled from amphiphilic block copolymers.

We present a novel route to assemble perpendicular cylinders by converting an asymmetric diblock copolymer from poly(styrene-b-tert-butyl acrylate) (PS-b-PtBA) to poly(styrene-b-acrylic acid) (PS-b-PAA) using an autocatalytic reaction. Upon exposure of the films of PS-b-PAA to water, PAA cylinders constrained by the continuous, glassy PS phase protrude 10 nm above the surface and swell laterally to form mushroom caps, rendering the entire surface hydrophilic. Upon annealing, the original nanostructures re-form demonstrating reversibility of swelling. Because of their stimuli-responsive behavior, these nanoscale materials are excellent candidates for sensors and microfluidic applications.