Stimuli-responsive surface localized ionic cluster (SLICs) formation from nonspherical colloidal particles

Structural features of phospholipids provide a unique opportunity for utilizing these amphiphilic species to stabilize the synthesis of colloidal dispersion particles by controlling concentration levels relative to dispersion synthesis components. 1,2-Bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DCPC) phospholipid was utilized as cosurfactant in the synthesis of sodium dioctyl sulfosuccinate (SDOSS) stabilized methyl methacrylate/n-butyl acrylate (MMA/nBA) colloidal dispersions. Aqueous dispersions containing various concentration levels of DCPC result in the formation of cocklebur particle morphologies, and when prepared in the presence of Ca2+ and annealed at various temperatures, stimuli-responsive behaviors of coalesced films were elucidated. The formation of surface localized ionic clusters (SLICs) at the film-air (F-A) and film-substrate (F-S) interfaces is shown to be responsive to concentration levels of DCPC, Ca2+/DCPC ratios, and temperature. These studies show that it is possible to control stratification and mobility to the F-A and F-S interfaces during and after coalescence. Using attenuated total reflectance Fourier transform infrared (ATR-FTIR) and internal reflection infrared imaging (IRIRI) spectroscopies, molecular entities responsible for SLIC formation were determined. These studies also show that stimuli-responsive behaviors during film formation can be controlled by colloidal solution morphologies and synergistic interactions of individual components.     

Release and formation of surface-localized ionic clusters (SLICs) into phospholipid rafts from colloidal solutions during coalescence

Stimuli-responsive behavior of phospholipids in the presence of ionic surfactants utilized in synthesis of MMA/nBA colloidal particles was investigated. Utilizing 1-myristoyl-2-hydroxy-sn-glycero-phosphocholine (MHPC) phospholipid, and sodium dioctyl sulfosuccinate (SDOSS) surfactant as dispersing media in H(2)O, narrow unimodal particle size distributions of methyl methacrylate (MMA)/n-butyl acrylate (nBA) copolymers were synthesized. The particle diameters were 154 nm when a SDOSS/MHPC mixture was used and 161 nm using MHPC as the only surface-stabilizing species. When such colloidal dispersions are exposed to 1.7, 3.3, and 6.7 mM aqueous CaCl(2) and KCl electrolyte solutions, surface-localized ionic clusters are generated at the film-air interface that may serve as lipid rafts composed of crystalline phases of MHPC deposited on poly(MMA)/nBA films. These studies illustrate that it is possible to control release and morphology developments of surface phospholipid rafts on artificial surfaces.     

Particle morphology as a control of permeation in polymer films obtained from MMA/nBA colloidal dispersions

The combination of precision-controlled weight loss measurements and spectroscopic surface FT-IR analysis allowed us to identify unique behaviors of poly(methyl methacrylate) (p-MMA). When MMA and n-butyl acrylate (nBA) are polymerized into p-MMA and p-nBA homopolymer blends, MMA/nBA random copolymers, and p-MMA/p-nBA core-shell morphologies, a controlled mobility and stratification of low molecular weight components occurs in films formed from coalesced colloidal dispersions. Due to different affinities toward water, p-MMA and p-nBA are capable of releasing water at different rates, depending upon particle morphological features of initial dispersions. As coalescence progresses, water molecules are released from the high free volume p-nBA particles, whereas p-MMA retains water molecules for the longest time due to its hydrophilic nature. As a result, water losses at extended coalescence times are relatively small for p-MMA. MMA/nBA copolymer and p-MMA/p-nBA blends follow the same trends, although the magnitudes of changes are not as pronounced. The p-MMA/p-nBA core-shell behavior resembles that of p-nBA homopolymer, which is attributed to significantly lower content of the p-MMA component in particles. Annealing of coalesced colloidal films at elevated temperatures causes migration of SDOSS to the F-A interface, but for films containing primarily p-nBA, reverse diffusion back into the bulk is observed. These studies illustrate that the combination of different particle morphologies and temperatures leads to controllable permeation processes through polymeric films.     

Surface self-assembly of fluorosurfactants during film formation of MMA/nBA colloidal dispersions

These studies focus on the behavior of fluorosurfactants (FS) containing hydrophobic and ionic entities in the presence of methyl methacrylate/n-butyl acrylate (MMA/nBA) colloidal dispersions stabilized by sodium dodecyl sulfate (SDS). The presence of FS significantly not only alters the mobility of SDS in MMA/nBA films, but their hydrophobic and ionic nature results in self-assembly near the film-air (F-A) interface leading to different surface morphologies. Spherical islands and rodlike morphologies are formed which diminish the kinetic coefficient of friction of films by at least 3 orders of magnitude, and the presence of dual hydrophobic tails and an anionic head appears to have the largest effect on the surface friction. Using internal reflection IR imaging, these studies show that structural and chemical features of FS are directly related to their ability to migrate to the F-A interface and self-assemble to form specific morphological features. While the anionic nature of FS allows for SDS migration to the F-A interface and the formation of stable domains across the surface, intermolecular cohesion of nonionic FS allows for the formation of rodlike structures due to inability to form mixed micelles with SDS. These studies also establish the relationship between surface morphologies, kinetic coefficient of friction, and structural features of surfactants in the complex environments.     

Monolayer behavior of silica particles at air/water interface: a comparison between chemical and physical modifications of surface

Silica particles are hydrophobized either by chemical graft of alkyl chains or by physical adsorption of cationic surfactants, alkyltrimethylammonium bromide. The effects of the two modification methods on the monolayer behavior of silica particles at the air/water interface are studied, as well as the packing structure of the particulate films. The results show that the hydrophobicity of particles chemically modified by octanol (SiO2-C8) and dodecanol (SiO2-C12) are similar and higher than that modified by butanol (SiO2-C4). The monolayer composed of particles with higher hydrophobicity shows a large lift-off area, higher compressibility, and significant hysteresis due to the higher particle-particle interaction. As a result, the particulate films exhibit 2-dimensional (2D) aggregative domains of closely-packed structure, but with particle free regions presenting among the domains. The monolayer prepared by SiO2-C4 shows a contrary behavior resulted from the higher particle-water interaction. The particles modified by adsorption of cationic surfactants have an amphiphilic property at the air/water interface. Such monolayer exhibits lower compressibility and hysteresis, higher re-spreading characteristic, and a lower collapse pressure compared with those of the chemically modified particles. A particulate film with high uniformity and closely-packed structure can be obtained by using the octyltrimethylammonium bromide (OTAB) modified particles. When the alkyl chain of surfactant increases, the packing of the particles becomes looser. Such phenomenon is probably caused from the higher probability for the long-chain surfactants to stay at the air/water interface which obstructs the intimate contact of particles.     

固体粒子稳定的乳状液研究进展

综述了固体粒子对乳状液稳定性影响的有关研究进展。微细不溶的固体粒子构成重要的一类乳化剂,被水相和油相部分润湿的固体粒子能够有效地稳定乳状液。固体粒子稳定乳状液的效果取决于以下因素:粒子大小、粒子间相互作用和粒子的润湿性质。固体粒子存在的油-水界面表现出粘弹行为,这种粘弹界面膜可大大地提高空间位阻,减缓乳状液液珠间液膜变薄的速率,从而提高乳状液地稳定性。原油中的粘土、胶质、沥青质和石蜡等胶体粒子被证明对乳状液的稳定性起很大的作用。     

Cocklebur-shaped colloidal dispersions

Unique cocklebur-shaped colloidal dispersions were prepared using a combination of a nanoextruder applied to the aqueous solution containing methyl methacrylate (MMA) and n-butyl acrylate (n-BA) with azo-bis-isobutyronitrile (AIBN) or potassium persulfate (KPS) initiators and stabilized by a mixture of sodium dioctyl sulfosuccinate (SDOSS) and 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DCPC) phospholipid. Upon extrusion and heating to 75 degrees C, methyl methacrylate/n-butyl acrylate (MMA/nBA) colloidal particles containing tubules pointing outward were obtained as a result of DCPC phospholipids present at the particle surfaces. The same cocklebur-shaped particles were obtained when classical polymerization was used without a nanoextruder under similar compositional and thermal conditions, giving a particle size of 159 nm. However, when Ca(2+) ions are present during polymerization, cocklebur morphologies are disrupted. Because DCPC tubules undergo a transition at 38 degrees C, such cocklebur morphologies may offer numerous opportunities for devices with stimuli-responsive characteristics.     

FTIR-ATR spectroscopic determination of the distribution of surfactants in latex films

FTIR-ATR (Fourier Transform Infra-Red-Attenuated Total Reflection) has been used to analyze the surface composition of coalesced acrylic latex films. The behavior of two anionic surfactants has been characterized. It has been found that surfactant distribution depends on the nature of the surfactant. A comparison between the normalized absorbance in transmission and in reflection has shown an enrichment of surfactants at the surfaces of films with a coalescence time of 3 days. The surfactant concentration at the film-air interface is higher than at the film substrate interface. A concentration gradient exists through the film thickness. In addition, the incompatible surfactant migrates towards the interface as coalescence proceeds.     

Phospholipid-assisted synthesis of stable F-containing colloidal particles and their film formation

This letter illustrates for the first time the preparation of p-methyl methacrylate/n-butyl acrylate/heptadecafluorodecyl methacrylate (p-MMA/nBA/FMA) colloidal dispersions containing up to 15% w/w FMA, which is accomplished by the utilization of biologically active phospholipids (PLs) and ionic surfactants. The use of monomer-starved conditions during emulsion polymerization and the utilization of 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), sodium dodecyl sulfate (SDS), and phosphoric acid bis(tridecafluoro-octyl) ester ammonium salt (FSP) as surfactants, which function as transfer and dispersing agents, facilitate a suitable environment for the polymerization of p-MMA/nBA/FMA colloidal dispersions that exhibit nonspherical particle morphologies. Such nonspherical particles upon coalescence form phase-separated films with unique surface properties.     

Biocompatible and pH-Responsive Colloidal Surfactants with Tunable Shape for Controlled Interfacial Curvature

Molecular-surfactant-stabilized emulsions are susceptible to coalescence and Ostwald ripening. Amphiphilic particles, which have a much stronger anchoring strength at the interface, could effectively alleviate these problems to form stable Pickering emulsions. Herein, we describe a versatile method to fabricate biocompatible amphiphilic dimer particles through controlled coprecipitation and phase separation. The dimer particles consist of a hydrophobic PLA bulb and a hydrophilic shellac–PEG bulb, thus resembling nonionic molecular surfactants. The size and diameter ratio of the dimer particles are readily tunable, providing flexible control over the water/oil interfacial curvature and thus the type of emulsion. The particle-stabilized emulsions were stable for a long period of time and could be destabilized through a pH-triggered response. The biocompatible amphiphilic dimer particles with tunable morphology and functionality are thus ideal colloidal surfactants for various applications.     

单一非离子表面活性剂制备胶质气体泡沫的稳定性

用单一的十二烷基醇聚氧乙烯醚(C12EOn)非离子表面活性剂制备了稳定的胶质气体泡沫(CGA). 采用偏光显微镜和流变仪对其表面活性剂溶液相态和泡沫体系的微观结构及流变行为进行研究, 以探索CGA的稳定化机理. 实验结果表明, 分别由C12EO3和C12EO5制备的CGA分散体系中均存在层状液晶相, 层状液晶吸附在气泡的界面上. CGA稳定性可达20 h以上, 没有明显的相分离发生. 而分别由C12EO7和C12EO9制备的CGA呈现由胶束组成的连续相, 不存在液晶相结构, 因而其稳定性较差, 仅能维持数分钟. 实验结果表明, 层状液晶相结构可以显著提高CGA的稳定性. 其稳定作用的机理是通过影响泡沫排液过程, 增强Gibbs-Marangoni效应, 从而提高气泡液膜强度和减缓气相扩散速率.     

Biocompatible and pH‐Responsive Colloidal Surfactants with Tunable Shape for Controlled Interfacial Curvature

Molecular‐surfactant‐stabilized emulsions are susceptible to coalescence and Ostwald ripening. Amphiphilic particles, which have a much stronger anchoring strength at the interface, could effectively alleviate these problems to form stable Pickering emulsions. Herein, we describe a versatile method to fabricate biocompatible amphiphilic dimer particles through controlled coprecipitation and phase separation. The dimer particles consist of a hydrophobic PLA bulb and a hydrophilic shellac–PEG bulb, thus resembling nonionic molecular surfactants. The size and diameter ratio of the dimer particles are readily tunable, providing flexible control over the water/oil interfacial curvature and thus the type of emulsion. The particle‐stabilized emulsions were stable for a long period of time and could be destabilized through a pH‐triggered response. The biocompatible amphiphilic dimer particles with tunable morphology and functionality are thus ideal colloidal surfactants for various applications.     

Film formation from aqueous polyurethane dispersions of reactive hydrophobic and hydrophilic components; spectroscopic studies and Monte Carlo simulations

Film formation of waterborne two-component polyurethanes is exceedingly complex due to the heterogeneous nature along with simultaneous progression of several parallel physicochemical processes which include water evaporation, cross-linking reactions, phase separation, and droplet coalescence, to name a few. While internal reflection infrared imaging (IRIRI) spectroscopy clearly facilitates analysis of chemical changes resulting from film formation, the complexity of processes leading to formation of specific surface/interfacial entities is a major experimental challenge. For this reason, we combined a spectrum of surface/interfacial analytical approaches including IRIRI, atomic force microscopy, and attenuated total reflectance Fourier transform infrared spectroscopy with Monte Carlo computer simulations to advance the limited knowledge of how temperature, stoichiometry, concentration levels, and reactivities of individual components affect the development of surface morphologies and compositional gradients across the film thickness. These studies show that in heterogeneous systems having both hydrophobic and hydrophilic components stratification of individual components to the film-air (F-A) interface is ultimately responsible for formation of rough surface topographies. These studies show that simultaneous stratification of hydrophobic components along with water evaporation to the F-A interface results in metastable interfacial layers, leading to surface dewetting. Subsequently, surface roughness is enhanced by higher concentrations of water in the cross-linking film.     

Coalescence of bubbles covered by particles

The interaction between two bubbles coated with glass particles in the presence of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) was studied experimentally. The time taken for two bubbles to coalesce was determined as a function of the fractional coverage of the surface by particles. The results suggested that the coalescence time increases with the bubble surface coverage. Interestingly, it was found that although the particles did not have any physical role in film rupture at low surface coverage, they still added resistance to film drainage. For particle-loaded bubbles, the initial resistance was due to the lateral capillary interactions between particles on the interface, which hold the particles firmly together. The coalescence dynamics of bubbles was also observed to be affected by the presence of attached particles.     

Hindered coarsening of a phase-separating microemulsion due to dispersed colloidal particles

The addition of sterically stabilized colloidal particles to a phase-separating microemulsion leads to dramatic changes in its demixing behavior, especially during the later stages. Our microemulsion is composed of reverse micelles of sodium dodecyl sulfate, pentanol, and water in a dodecane continuous phase which separates into micelle-rich and micelle-poor phases above a lower critical solution temperature. The poly(methyl methacrylate) particles preferentially partition into the less structured, micelle-poor phase. Nucleation of the minority phase or spinodal decomposition close to criticality continue to occur in the presence of particles, albeit with pronounced pretransitional clustering of particles when the micelle-poor phase is in the minority. The coalescence of micelle-poor droplets and the coarsening of micelle-rich domains are both strongly modified due to the presence of colloidal particles. We use our observations of the early stages of phase separation to understand these late stage changes.     

The effect of surfactant on the efficiency of shear-induced drop coalescence

The volume-averaged shear-induced drop-coalescence efficiency epsilonv is measured by in situ videomicroscopy of blends of poly(propylene glycol) and poly(ethylene glycol), emulsified with poly(ethyleneglycol-b-propyleneoxide-b-ethyleneglycol) block copolymer surfactant. Adsorption of copolymer to the immiscible blend interface is indicated by a reduction in the interfacial tension, measured by the drop retraction method. The effects of temperature, copolymer molecular weight, copolymer concentration, and capillary number Ca are explored. At small Ca, epsilonv is essentially independent of shear rate and drop size, and depends mainly on the solubility, diffusivity, and surface pressure of the surfactant, indicating that drop trajectories during flow are perturbed by surfactant Marangoni stresses that are controlled by the diffusion-limited sorption of surfactant. At larger Ca, epsilonv approaches zero. This arrest of coalescence is associated with the onset of slight deformation of the drops during their collision, and drainage of a film of continuous fluid between them. The effect of the surfactant, though significant, saturates even while the amount of surfactant adsorbed to the interface is quite small. Governing dimensionless parameters, associated material parameters and the behavior of more insoluble surfactants are discussed.     

The influence of micelle formation on the stability of colloid surfactant mixtures

The stability of colloidal dispersions can be severely affected by the presence of surfactants. Because surfactants can adsorb at colloidal surfaces as well as form micelles, one can expect an interplay between both phenomena. Using grand-canonical coarse-grained Monte Carlo simulations on surfactant solutions confined between two surfaces, we investigate how adsorption and micelle formation affects the effective interaction between two colloidal particles, and hence, the stability of the colloidal dispersion. For solvophilic colloidal surfaces, we observe a short-ranged oscillatory solvation pressure that is hardly affected by the presence of surfactants in the system. The effective surface-surface interaction, however, reveals a decrease in solvophilic stabilization as a function of surfactant chemical potential. For solvophobic surfaces, we find that the capillary evaporation observed in a confined pure solvent, is counteracted by the addition of surfactants. Around the critical micelle concentration (CMC), the surface-surface interaction even becomes repulsive, enhancing stabilization of the colloidal dispersion. In contrast, the formation of micelles at concentrations above the CMC causes an additional depletion effect, resulting in an effective attraction, which in turn can destabilize a colloidal dispersion.     

Water-in-carbon dioxide emulsions stabilized with hydrophobic silica particles

W/C emulsions were stabilized using hydrophobic silica particles adsorbed at the interface, resulting in average droplet diameters as low as 7.5 microm. A porous cross-linked shell was formed about a hydrophilic (colloidal and fumed) silica core with a trifunctional silylating agent, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethyoxysilane, to render the particles CO(2)-philic. The stability of emulsions comprising equal weights of CO(2) and water was assessed with visual observations of settling fronts and the degree of emulsion coalescence, and the average drop size was measured by optical microscopy. The effect of CO(2) density on both emulsion stability and droplet size was determined quantitatively. The major destabilizing mechanism of the emulsions was settling, whereas Ostwald ripening and coalescence were not visible at any density, even over 7 days. Flocculation of the settling droplets did not occur, although gelation of the emulsions through particle interactions resulted after longer periods of time. CO(2)-philic particles offer a new route to highly stable W/C emulsions, with particle energies of attachment on the order of 10(6)kT, even at CO(2) densities as low as 0.78 g ml(-1). At these low densities, surfactants rarely stabilize emulsions as the result of poor surfactant tail solvation.     

Molecular recognition at methyl methacrylate/n-butyl acrylate (MMA/nBA) monomer unit boundaries of phospholipids at p-MMA/nBA copolymer surfaces

Lipid structural features and their interactions with proteins provide a useful vehicle for further advances in membrane proteins research. To mimic one of potential lipid-protein interactions we synthesized poly(methyl methacrylate/ n-butyl acrylate) (p-MMA/nBA) colloidal particles that were stabilized by phospholipid (PLs). Upon the particle coalescence, PL stratification resulted in the formation of surface localized ionic clusters (SLICs). These entities are capable of recognizing MMA/nBA monomer interfaces along the p-MMA/nBA copolymer backbone and form crystalline SLICs at the monomer interface. By utilizing attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy and selected area electron diffraction (SAD) combined with ab initio calculations, studies were conducted that identified the origin of SLICs as well as their structural features formed on the surface of p-MMA/nBA copolymer films stabilized by 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) PL. Specific entities responsible for SLIC formation are selective noncovalent bonds of anionic phosphate and cationic quaternary ammonium segments of DLPC that interact with two neighboring carbonyl groups of nBA and MMA monomers of the p-MMA/nBA polymer backbone. To the best of our knowledge this is the first example of molecular recognition facilitated by coalescence of copolymer colloidal particles and the ability of PLs to form SLICs at the boundaries of the neighboring MMA and nBA monomer units of the p-MMA/nBA chain. The dominating noncovalent bonds responsible for the molecular recognition is a combination of H-bonding and electrostatic interactions.     

侧链液晶聚硅氧烷langmuir膜的原子力显微镜观察

通过原子力显微镜观测了含手性基团侧链液晶聚硅氧烷在空气/水界面上的单层膜,观测了成膜物在界面形成微畴结构以及表面压力升高时微畴聚集、重排过程,分析了纳米级微畴形成、聚集和重排的机理和规律。