Polymer and particle adsorption at the PDMS droplet-water interface

Uptake of silica particulate drug carriers in an intestine-on-a-chip: towards a better in vitro model of nanoparticulate carrier and mucus interactions

Micro and nano-particulate carriers have potential to increase bioavailability of oral drugs, but must first overcome the mucus barrier of the intestinal epithelium to facilitate absorption and entry to systemic circulation. We report on mucus-silica nanoparticulate carrier interactions in an in vitro intestine-on-a-chip (IOAC) microfluidic model. Caco-2 cells cultured within the IOAC model recapitulate the morphology of the human intestinal epithelium that is currently lacking in traditional static Transwell models. Fine control over the cell culture conditions produced a mucus layer, previously problematic to achieve without employing cell co-culture. The microdevice design also allowed for direct imaging of silica particulate carrier (40-700 nm) uptake through the mucus and cellular monolayer. PEGylated particulate carriers penetrated more readily through the mucus layer compared to non-PEGylated particulate carriers while larger particulate carriers tended to retard particulate carrier penetration through a dense mucus mesh. This was confirmed via imaging flow cytometry and UV-fluorescence spectroscopy. The IOAC also demonstrated the ability to mimic intestinal peristaltic fluidic conditions, which in turn affects the particulate carrier uptake. This in vitro IOAC model has potential to directly elucidate mucus interactions and uptake mechanisms for a range of drug carrier systems.

Comparing Surfactant Structures at "Soft" and "Hard" Hydrophobic Materials: Not All Interfaces Are Equivalent

The interfacial structures of a range of amphiphilic molecules are studied with both "soft" and "hard" hydrophobic substrates. Neutron reflection and quartz crystal microbalance with dissipation measurements highlight the differences between the adsorbed structures adopted by sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (C₁₆TAB), and the "AM1" surface active peptide. At the soft siloxane/water interface, small molecular surfactants form loosely packed layers, with the hydrophobic tails penetrating into the oily layer, and an area per surfactant molecule that is significantly less than previously reported for the air/water interface. Neutron reflection measurements, supported by quartz crystal microbalance studies, indicate that for C₁₆TAB, approximately 30 ± 8% of the alkyl tail penetrates into the poly(dimethylsiloxane) (PDMS) layer, whereas 20 ± 5% of the alkyl tail of SDS is located in the PDMS. For the engineered peptide surfactant AM1 (21 residues), it was found that one face of the α helix penetrated into the PDMS film. In contrast, penetration of the surfactant tails was not observed against hard solidlike hydrophobic surfaces made from octadecyltrichlorosilane (OTS) for any of the molecular species studied. At the OTS/water interface, C₁₆TAB and SDS were seen to adsorb as larger aggregates and not as monolayers. Amphiphilic adsorption (amount, structural conformation) at the PDMS/water interface is shown to be different from that at both the air/water interface and the hard OTS/water interface, illustrating that interfacial structures cannot be predicted by the surfactant packing parameter alone. The bound PDMS layer is shown to be a useful proxy for the oil/water interface in surface and stabilization studies, with hydrophobic components of the molecules able to penetrate into the oily PDMS.

Highly stable, electrostatically attractive silicone nanoemulsions produced by interfacial assembly of amphiphilic triblock copolymers

This article presents a useful and promising approach for fabricating extremely stable silicone oil nanoemulsions, whose liquid-liquid interface is structured with a thin film of amphiphilic triblock copolymers. For this, two types of amphiphilic triblock polymer, poly(2-methacryloyloxy ethyl phosphorylcholine)-block-poly(ε-caprolactone)-block-poly(2-methacryloyloxy ethyl phosphorylcholine) (PMPC-PCL-PMPC) and poly(2-aminoethyl methacrylate)-block-poly(ε-caprolactone)-block-poly(2-aminoethyl methacrylate) (PAMA-PCL-PAMA), were synthesized by atom transfer radical polymerization. Employing the phase separation technique was critical for the formation of thin polymer interfaces, of less than 10 nm, thus eventually producing structurally stable silicone oil nanoemulsions. The co-assembly of PAMA-PCL-PAMA with PMPC-PCL-PMPC enabled the patching of positive charges on the surface of the emulsion drops. We show that these charged silicone oil nanoemulsions could be used to form a multilayer emulsion thin film by layer-by-layer deposition. Finally, we experimentally demonstrate that the silicone oil nanoemulsions fabricated in this way were highly stable and had the ability to electrostatically interact with hair, which enabled complete coating of the hair surface with a layer of silicone oil.

Adsorption of poly(ethylene oxide)-containing amphiphilic polymers on solid-liquid interfaces: Fundamentals and applications

The adsorption of amphiphilic molecules of varying size on solid-liquid interfaces modulates the properties of colloidal systems. Nonionic, poly(ethylene oxide) (PEO)-based amphiphilic molecules are particularly useful because of their graded hydrophobic-hydrophilic nature, which allows for adsorption on a wide array of solid surfaces. Their adsorption also results in other useful properties, such as responsiveness to external stimuli and solubilization of hydrophobic compounds. This review focuses on the adsorption properties of PEO-based amphiphiles, beginning with a discussion of fundamental concepts pertaining to the adsorption of macromolecules on solid-liquid interfaces, and more specifically the adsorption of PEO homopolymers. The main portion of the review highlights studies on factors affecting the adsorption and surface self-assembly of PEO-PPO-PEO block copolymers, where PPO is poly(propylene oxide). Block copolymers of this type are commercially available and of interest in several fields, due to their low toxicity and compatibility in aqueous systems. Examples of applications relevant to the interfacial behavior of PEO-PPO-PEO block copolymers are paints and coatings, detergents, filtration, and drug delivery. The methods discussed herein for manipulating the adsorption properties of PEO-PPO-PEO are emphasized for their ability to shed light on molecular interactions at interfaces. Knowledge of these interactions guides the formulation of novel materials with useful mesoscale organization and micro- and macrophase properties.

Removal of radioactive contaminants by polymeric microspheres

Radionuclide removal from radioactive liquid waste by adsorption on polymeric microspheres is the latest application of polymers in waste management. Polymeric microspheres have significant immobilization capacity for ionic substances. A laboratory study was carried out by using poly(N-isopropylacrylamide) for encapsulation of radionuclide in the liquid radioactive waste. There are numbers of advantages to use an encapsulation technology in radioactive waste management. Results show that polymerization step of radionuclide increases integrity of solidified waste form. Test results showed that adding the appropriate polymer into the liquid waste at an appropriate pH and temperature level, radionuclide was encapsulated into polymer. This technology may provide barriers between hazardous radioactive ions and the environment. By this method, solidification techniques became easier and safer in nuclear waste management. By using polymer microspheres as dust form, contamination risks were decreased in the nuclear industry and radioactive waste operations.

Investigation of Four Different Laponite Clays as Stabilizers in Pickering Emulsion Polymerization

Clay-armored polymer particles were prepared by emulsion polymerization in the presence of Laponite platelets that adsorb at the surface of latex particles and act as stabilizers during the course of the polymerization. While Laponite RDS clay platelets are most often used, the choice of the type of clay still remains an open issue that is addressed in the present article. Four different grades of Laponite were investigated as stabilizers in the emulsion polymerization of styrene. First, the adsorption isotherms of the clays, on preformed polystyrene particles, were determined by ICP-AES analysis of the residual clay in the aqueous phase. Adsorption of clay depended on the type of clay at low concentrations corresponding to adsorption as a monolayer. Adsorption of clay particles as multilayers was observed for all the grades above a certain concentration under the considered ionic strength (mainly due to the initiator ionic species). The stabilization efficiency of these clays was investigated during the polymerization reaction (free of any other stabilizer). The clays did not have the same effect on stabilization, which was related to differences in their compositions and in their adsorption isotherms. The different grades led to different polymer particles sizes and therefore to different polymerization reaction rates. Laponite RDS and S482 gave similar results, ensuring the best stabilization efficiency and the fastest reaction rate; the number of particles increased as the clay concentration increased. Stabilization with Laponite XLS gave the same particles size and number as the latter two clays at low clay concentrations, but it reached an upper limit in the number of nucleated polymer particles at higher concentrations indicating a decrease of stabilization efficiency at high concentrations. Laponite JS did not ensure a sufficient stability of the polymer particles, as the polymerization results were comparable to a stabilizer-free polymerization system.

Partitioning of Laponite Clay Platelets in Pickering Emulsion Polymerization

Partitioning of laponite disklike clay platelets between polymer particles and bulk aqueous phase was investigated in Pickering surfactant-free emulsion polymerization of styrene. Adsorption of laponite clay platelets plays an important role in the stabilization of this system, influencing the particle size and the number of particles, and, hence, the reaction rate. Adsorption isotherms show that, while the laponite clay platelets are almost fully exfoliated in water, they form multilayers on the surface of the polymer particles by the end of polymerization, as confirmed by transmission electron microscopy (TEM). This observation is supported by quartz crystal microbalance, conductivity, and TEM measurements, which reveal interactions between the clay and polystyrene, as a function of the ionic strength. The strong adsorption of clay platelets leaves a low residual concentration in the aqueous phase that cannot cause further nucleation of polymer particles, as demonstrated during seeded emulsion polymerization experiments in the presence of a high excess of clay. A Brunauer-Emmett-Teller (BET)-type model for laponite adsorption on polystyrene particles matches the adsorption isotherms.

Layer-by-layer-assembled healable antifouling films

Healable antifouling films are fabricated by the exponential layer-by-layer assembly of PEGylated branched poly(ethylenimine) and hyaluronic acid followed by post-crosslinking. The antifouling function originates from the grafted PEG and the extremely soft nature of the films. The rapid and multiple healing of damaged antifouling functions caused by cuts and scratches can be readily achieved by immersing the films in normal saline solution.

Micellar structures in nanoparticle-multiblock copolymer complexes

Brownian dynamics simulation is employed to examine the structure changes of complexes composed of a hydrophobic nanoparticle and a multiblock copolymer molecule (MCP). The dependence of the structure transitions on the radius of the nanoparticle, on the interactions between the hydrophobic segments of the MCP, and on the interactions between the hydrophobic segments and hydrophobic nanoparticle is examined. It is shown that the multiblock copolymer adsorbed on a nanoparticle can acquire the structure of a micelle.To better characterize the micelle generated and the structure changes in the nanoparticle-MCP complex, the mass dipole moment of the complex [the distance between the center of mass of MCP and the center of the nanoparticle minus the radius of the nanoparticle (DCC)], the density profiles of MCP segments around its center of mass and around the nanoparticle, the radius of gyration of the MCP, and the thickness of the micelle around the nanoparticle are determined. It was found that, when structural transition of the complex occurs, the above quantities change dramatically. The present simulation may provide new insights regarding the drug-loaded micelle interacting with a virus represented by a nanoparticle.

Assembling nanoparticle coatings to improve the drug delivery performance of lipid based colloids

Lipid based colloids (e.g. emulsions and liposomes) are widely used as drug delivery systems, but often suffer from physical instabilities and non-ideal drug encapsulation and delivery performance. We review the application of engineered nanoparticle layers at the interface of lipid colloids to improve their performance as drug delivery systems. In addition we focus on the creation of novel hybrid nanomaterials from nanoparticle-lipid colloid assemblies and their drug delivery applications. Specifically, nanoparticle layers can be engineered to enhance the physical stability of submicron lipid emulsions and liposomes, satbilise encapsulated active ingredients against chemical degradation, control molecular transport and improve the dermal and oral delivery characteristics, i.e. increase absorption, bioavailability and facilitate targeted delivery. It is feasible that hybrid nanomaterials composed of nanoparticles and colloidal lipids are effective encapsulation and delivery systems for both poorly soluble drugs and biological drugs and may form the basis for the next generation of medicines. Additional pre-clinical research including specific animal model studies are required to advance the peptide/protein delivery systems, whereas the silica lipid hybrid systems have now entered human clinical trials for poorly soluble drugs.

Electrostatic and steric interactions in oil-in-water emulsion films from Pluronic surfactants

Stabilization of oil-in-water emulsion films from PEO-PPO-PEO triblock copolymers is described in terms of interaction surface forces. Results on emulsion films from four Pluronic surfactants, namely F108, F68, P104 and P65 obtained with the Thin Film Pressure Balance Technique are summarized. It is found that film stabilization is due to DLVO (electrostatic) and non-DLVO (steric in origin) repulsive forces. The charging of the oil/water film interfaces is related to preferential adsorption of OH(-) ions. This is confirmed by pH-dependent measurements of the equivalent film thickness (h(w)) at both constant capillary pressure and ionic strength. With reducing pH in the acidic region, a critical value (pH(cr,st)) corresponding to an isoelectric state of the oil/water film surfaces is found where the electrostatic interaction in the films is eliminated. At pH≤pH(cr,st), the emulsion films are stabilized only by steric forces due to interaction between the polymer adsorption layers. Disjoining pressure (Π) isotherms measured for emulsion films from all the four Pluronic surfactants used at pH<pH(cr,st) show a transition to a Newton black film with increasing Π. The experimental data before the NBF-transition in the disjoining pressure isotherms are fitted to the Alexander-de Gennes' scaling theory for steric interaction between polymer brushes with the PEO-brush thickness as a free parameter. The NBF observed are stabilized most probably by short-range steric forces that may differ from the brush-to-brush interaction.

Chromatographic separation of In(III) from Cd(II) in aqueous solutions using commercial resin (Dowex 50W-X8)

This work assesses the potential of an adsorptive material, Dowex 50w-x8, for the separation of indium ions from cadmium ions in aqueous media. The adsorption behavior of Dowex 50 w-x8 for indium and cadmium ions was investigated. The effect of pH, initial concentration of metal ions, the weight of resins, and contact time on the sorption of each of the metal ions were determined. It was found that the adsorption percentage of the indium ions was more than 99% at pH 4.0. The result shows that In (III) was most strongly extracted, while Cd(II) was slightly extracted at this pH value. The recovery of In(III) and Cd(II) ions is around 98% using hydrochloric acid as the best eluent.

Naphthalene sulfonate functionalized dendrimers at the solid-liquid interface: influence of core type, ionic strength, and competitive ionic adsorbates

The adsorption of naphthalene disulfonic acid surface-functionalized dendrimers (generation 4) on to colloidal alumina particles is reported, considering the role of dendrimer core type (ammonia vs benzylhydrylamine-polylysine) and electrolyte addition on the adsorption affinity and interfacial packing and competitive adsorption. Irrespective of the dendrimer core type, the maximum adsorbed amount increased with increasing ionic strength. The adsorption affinity of a benzylhydrylamine-cored SPL-7013 increased with increasing ionic strength, whereas a decrease was observed for the ammonia-cored SPL-2923. At high ionic strengths (>or=10(-1) M NaCl) dendrimers close pack at the interface as an array of equivalent hard spheres, whereas at lower ionic strengths both dendrimers occupy a lower area than theoretically predicted for either cubic or hexagonal close packing, based on double layer repulsion. The additional attraction between dendrimers is attributed to the intercalation of the neighboring dendrons. Adsorption of SPL-2923 is enhanced by the presence of Ca2+ ions and depressed by the presence of HCO3- and HPO4(2-) ions, whereas SPL-7013 adsorption is only depressed by the presence of HPO4(2-) ions, suggesting a dendrimer-specific competitive adsorption process. This work clearly demonstrates the role of dendrimer architecture on adsorption at an interface, a process of fundamental importance to a wide range of dendrimer applications.

Colloidosomes from the controlled interaction of submicrometer triglyceride droplets and hydrophilic silica nanoparticles

The self-assembly of hydrophilic silica nanoparticles at the surface of charged submicrometer triglyceride droplets has been investigated with the aim to optimize the preparation of stable colloidosomes. The droplet charge, oil phase volume fraction, droplet/nanoparticle ratio, and salt concentration play important roles in controlling nanoparticle interactions and are reflected in the colloidosome zeta potential, size, stability, and interfacial structure (visualized by freeze-fracture SEM). Silica nanoparticle interactions with negatively charged droplets are weak, and partially covered droplets are identified. Positively charged droplets are strongly coated by silica nanoparticles and undergo charge reversal at specific droplet to nanoparticle ratios and electrolyte concentrations. Droplets at volume fractions (varphi) <10 (-4) undergo time-dependent limited coalescence until nanoparticle coverage is complete. For varphi in the range 10 (-4) to 2.5 x 10 (-4) and at certain critical droplet to nanoparticle ratios, droplets undergo neutralization or charge reversal coupled with aggregation and precipitation; this occurs in a time-independent manner. Specific conditions have been identified where stable 1-3 mum colloidosomes can be phase separated from heterocoagulates of droplets and nanoparticles.

The role of particles in stabilising foams and emulsions

The use of particles as foam and emulsion stabilising species, with or without surfactants, has received great interest in recent years. The majority of work has studied the effects of particles as stabilisers in emulsion systems, but recent successes has widened consideration into foams, where industries such as flotation and food processing have encountered the effects of particle stabilisation for many years. This review seeks to clarify studies into emulsions, highlighting new research in this area, and relate similarities and differences to foam systems. Past research has focused on defining the interaction mechanisms of stability, such as principles of attachment energies, particle-particle forces at the interface and changes to the interfilm, with a view to ascertain conditions giving optimum stability. Studied conditions include effects of particle contact angle, aggregation formations, concentration, size and interactions of other species (i.e. surfactant). Mechanisms can be complex, but overall the principle of particles creating a steric barrier to coalescence, is a straitforward basis of interaction. Much research in emulsions can be applied to foam systems, however evidence would suggest foam systems are under a number of additional constraints, and the stability 'window' for particles is smaller, in terms of size and contact angle ranges. Also, because of increased density differences and interfilm perturbations in foam systems, retardation of drainage is often as important to stability as inhibiting coalescence.

Analytical investigations on the effects of substrate kinetics on macromolecular transport and hybridization through microfluidic channels

In this paper, a generalized surface-kinetics based model is developed to analytically investigate the influences of the substrate types and the buffer compositions on the macromolecular transport and hybridization in microfluidic channels, under electrokinetic influences. For specific illustration, three typical microchannel substrates, namely silanized glass, polycarbonate and PDMS, are considered, in order to obtain analytical expressions for their zeta potentials as a function of the buffer pH and the substrate compositions. The expressions for the zeta potential are subsequently employed to derive the respective velocity distributions, under the application of electric fields of identical strengths in all cases. It is also taken into consideration that the charged macromolecules introduced into these channels are subjected to electrophoretic influences on account of the applied electric fields. Closed form expressions are derived to predict the transport behaviour of the macromolecules and their subsequent hybridization characteristics. From the analysis presented, it is shown that the modification of the channel surface with silane-treatment becomes useful for enhancing the macromolecular transport and surface hybridization, only if the buffer pH permits a large surface charge density. The analytical solutions are also compared with full-scale numerical solutions of the coupled problem of fluid dynamic and macromolecular transport in presence of the pertinent surface reactions, in order to justify the effectiveness of closed-form expressions derived in this study.

Nanoparticle encapsulation of emulsion droplets

The overall aim of this study is to coat emulsion droplets with nanoparticles using a simple heterocoagulation process in aqueous dispersion and determine: the adsorption behavior and interfacial layer microstructure, droplet physical stability against flocculation and coalescence, and the release profile of a model lipophilic molecule (dibutylphtalate (DBP)) from within the droplets. Polydimethylsiloxane (PDMS) droplets were used as a model emulsion due to their colloidal stability in the absence of added stabilisers. Aerosil type silica nanoparticles with different hydrophobicity levels were used as the model nanoparticles. The adsorption behavior of silica nanoparticles at the droplet-water interface was studied using adsorption isotherms and SEM imaging. Adsorption of hydrophilic nanoparticles is weakly influenced by pH, but significantly influenced by salt addition, whereas for hydrophobically modified nanoparticles a balance of hydrophobic and electrostatic forces controls adsorption over a wide range of pH and salt concentrations. The coalescence kinetics (determined under coagulation conditions at high salt concentration) and the physical structure of coalesced droplets were determined from optical microscopy. Adsorbed layers of hydrophilic nanoparticles introduced a barrier to coalescence of approximately 1kT and form kinetically unstable droplet networks at high salt concentrations. The highly structured and rigid adsorbed layers significantly reduce coalescence kinetics. Significant sustained release of DBP can be achieved using rigid layers of hydrophobic silica nanoparticles at the interface. Activation energies for release are in the range 580-630 kJ mol(-1), 10 times higher than for barriers introduced by Pluronic stabilisers.

Nanoparticle adsorption and stabilisation of surfactant-free emulsions

The formation of particle-stabilised emulsions by adding partially hydrophobised silica particles to surfactant-free oil-in-water emulsions (average drop diameter approximately 700 nm) stabilised by hydroxide ions adsorbed at the oil-water interface has been investigated. Nanoparticles (average particle diameter 18 nm) adsorbed onto the drops under alkaline conditions to produce particle-stabilised emulsions with the same drop size distribution as the surfactant-free emulsions. Unlike the surfactant-free emulsions, the particle-stabilised emulsions were stable even in acidic conditions. Strongly flocculated nanoparticles (average particle diameter 150 nm) adsorbed onto the drop surfaces under acidic conditions where the emulsions were destabilised, forming coarser particle-stabilised emulsions with micron-sized drops.