Manipulating interfacial polymer structures through mixed surfactant adsorption and complexation

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.

Kinetics of Polymer Desorption from Colloids Probed by Aggregation-Induced Emission Fluorophore

Polymer adsorption and desorption are fundamental in many industrial and biomedical applications. Here, we introduce a new method to monitor the polymer desorption kinetics in situ based on the behavior of aggregation-induced emission. Poly(ethylene oxide) and colloidal silica (SiO₂) were used as a model system. It was found that the aggregation-induced emission method could be successfully used to determine the polymer desorption kinetics, and the polymer desorption followed the first-order kinetics. It was also found that the polymer desorption rate constant decreased with the increasing molecular weight, which could be described by a power law function k_d ≈ M^-0.28, close to that of the adsorption rate constant.

Synergistic Effects of Bound Micelles and Temperature on the Flexibility of Thermoresponsive Polymer Brushes

The persistence length is a key parameter for the quantitative interpretation of the flexibility of polymers. We have studied complexes composed of a spherical poly(N-isopropylacrylamide) (PNIPAM) brush and a sodium dodecyl sulfate micelle in an effort to characterize the flexibility of tethered PNIPAM below the lower critical solution temperature T_LCST. An analytical mean-field model is used to describe the persistence length L_p in a broad range of ψ, the number of bound micelles per chain. The persistence length of micelle-constrained PNIPAM is quantitatively correlated with the thermal energy k_BT, electrostatic repulsion f_C, and effective excluded-volume parameter ν_eff. The persistence length per ψ, which depends on T and f_C, is found to scale with a synergistic effect f_C/(ψk_BT). The results reveal that the bound-micelle charges affecting the persistence length are analogous to the fixed charges of polyelectrolytes, though the bound micelles are separated by a large number of neutral monomers. The extension ⟨L⟩ of micelle-constrained PNIPAM decreases as ⟨L⟩ ∼ f_C^-β_F with f_C, where β_F ≈ 0.58-0.8 depending on ψ, but as the universal power law ⟨L⟩ ∼ (f_C/k_BT)^-0.6 with the synergistic effect f_C/(k_BT), irrespective of ψ. In spite of the intricate interplay among the multiple components in the system, the extension scales as a function of ν_eff as ⟨L⟩ ∼ (ν_eff/ψL_p)^-β_V, where β_V ≈ 0.35 for the significant monomer interaction and β_V ≈ 0.2 for the weak or negligible monomer interaction.

Relaxation NMR as a tool to study the dispersion and formulation behavior of nanostructured carbon materials

Solvent relaxation NMR has been used to estimate the surface areas and wettability of various types of nanostructured carbon materials in a range of solvents including water, ethanol, and tetrahydrofuran. We illustrate the application of the technique through several short case studies using samples including nanocarbon blacks, graphene oxide, nanographites, and porous graphenes. The technique is shown to give a good measure of surface area, correlating well with conventional surface area estimates obtained by nitrogen adsorption, transmission electron microscopy, or light scattering for the non-porous samples. NMR relaxation has advantages in terms of speed of analysis and being able to use concentrated, wet, and opaque samples. For samples that are porous, two distinct surface areas can be estimated assuming the two environments ('inner' and 'outer') have the same surface chemistry, and that there is a slow exchange of solvent molecules between them. Furthermore, we show that differences in wettability and dispersability between samples dispersed in water, ethanol, and cyclopentanone can be observed, along with changes to the surface chemistry of the interface. Copyright © 2015 John Wiley & Sons, Ltd.

Interpolymer complexation: comparisons of bulk and interfacial structures

The interactions between the strong polyelectrolyte sodium poly(styrenesulfonate), NaPSS, and the neutral polymer poly(vinylpyrrolidone), PVP, were investigated in bulk and at the silica/solution interface using a combination of diffusion nuclear magnetic resonance spectroscopy (NMR), small-angle neutron scattering (SANS), solvent relaxation NMR, and ellipsometry. We show for the first time that complex formation occurs between NaPSS and PVP in solution; the complexes formed were shown not to be influenced by pH variation, whereas increasing the ionic strength increases the complexation of NaPSS but does not influence the PVP directly. The complexes formed contained a large proportion of NaPSS. Study of these interactions at the silica interface demonstrated that complexes also form at the nanoparticle interface where PVP is added in the system prior to NaPSS. For a constant PVP concentration and varying NaPSS concentration, the system remains stable until NaPSS is added in excess, which leads to depletion flocculation. Surface complex formation using the layer-by-layer technique was also reported at a planar silica interface.

Adsorption of charged and neutral polymer chains on silica surfaces: the role of electrostatics, volume exclusion, and hydrogen bonding

We develop an off-lattice (continuum) model to describe the adsorption of neutral polymer chains and polyelectrolytes to surfaces. Our continuum description allows taking excluded volume interactions between polymer chains and ions directly into account. To implement those interactions, we use a modified hard-sphere equation of state, adapted for mixtures of connected beads. Our model is applicable to neutral, charged, and ionizable surfaces and polymer chains alike and accounts for polarizability effects of the adsorbed layer and chemical interactions between polymer chains and the surface. We compare our model predictions to data of a classical system for polymer adsorption: neutral poly(N-vinylpyrrolidone) (PVP) on silica surfaces. The model shows that PVP adsorption on silica is driven by surface hydrogen bonding with an effective maximum binding energy of about 1.3k(B)T per PVP segment at low pH. As the pH increases, the Si-OH groups become increasingly dissociated, leading to a lower capacity for H bonding and simultaneous counterion accumulation and volume exclusion close to the surface. Together these effects result in a characteristic adsorption isotherm, with the adsorbed amount dropping sharply at a critical pH. Using this model for adsorption data on silica surfaces cleaned by either a piranha solution or an O(2) plasma, we find that the former have a significantly higher density of silanol groups.

Adsorption and surfactant-mediated desorption of poly(vinylpyrrolidone) on plasma- and piranha-cleaned silica surfaces

Optical flow cell reflectometry was used to study the adsorption of poly(vinylpyrrolidone) (PVP) to a silica surface and the subsequent surfactant adsorption and polymer desorption upon exposure to the anionic surfactant sodium dodecyl sulfate (SDS). We have studied these effects as a function of pH and surfactant concentration, but also for two different methods of silica preparation, O2 plasma and piranha cleaning. As a function of pH, a plateau in the amount adsorbed of ∼0.6 mg/m(2) is observed below a critical pH, above which the adsorption decreases to zero within 2-3 pH units. An increase in pH leads to dissociation of surface OH groups and a decreased potential for hydrogen bonding between the polymer and surface. For the plasma- and piranha-cleaned silica, the critical pH differs by 1-2 pH units, a reflection of the much larger amount of surface OH groups on piranha-cleaned silica (for a given pH). Subsequent rinsing of the adsorbed layer of PVP with an SDS solution leads to total or partial desorption of the PVP layer. Any remaining adsorbed PVP then acts as an adsorption site for SDS. A large difference between plasma- and piranha-cleaned silica is observed, with the PVP layer adsorbed to plasma-cleaned silica being much more susceptible to desorption by SDS. For a plasma-cleaned surface at pH 5.5, only 30% of the originally adsorbed PVP is remaining, while for piranha-cleaned silica, the pH can be increased to 10 before a similar reduction in the amount of adsorbed PVP is seen. For a given pH, piranha-cleaned silica has a higher surface charge, leading to a smaller amount of adsorbed SDS per PVP chain on a piranha-cleaned surface compared to a plasma-cleaned surface under identical conditions. In that way, the high negative surface charge makes desorption by negatively charged SDS more difficult. The high surface charge thus protects the neutral polymer from surfactant-mediated desorption.

Preparation and solidification of redispersible nanosuspensions

To test the feasibility of preparing redispersible powders from nanosuspensions without further addition of drying protectants, Lovastatin was processed into nanosuspensions and subsequently converted into a powder form using a spray-drying process. The effects of spray-drying process parameters and stabilizers on the properties of the spray-dried powders were evaluated. The inlet air temperature was found to have the most pronounced impact; a low-inlet air temperature consistently yielded dried powders with improved redispersibility. This was attributed to the low Peclet number associated with a low-inlet air temperature, making nanoparticles less prone to aggregation and coalescence during spray drying, as evidenced by the well-defined boundary shown between nanoparticles in the SEM photomicrographs of the spray-dried microparticles. The influence of atomization pressure is significant particularly at a low-inlet air temperature. The redispersibility index value of the powder is dependent on the type of stabilizers used in the nanosuspension formulation. Spray-dried powders with acceptable redispersibility were prepared with drug concentration as high as 3%. In conclusion, with optimized process parameters and selected stabilizers, spray drying is a feasible process in the solidification of nanosuspensions with high drug loading and acceptable redispersibility.

Competition between polymers for adsorption on silica: a solvent relaxation NMR and small-angle neutron scattering study

The competition between poly(vinylpyrrolidone) and poly(ethylene oxide) for adsorption at the silica surface was studied by solvent relaxation nuclear magnetic resonance and small-angle neutron scattering. The additive nature of the NMR relaxation rate enhancement was used to observe changes in the train layer when the two polymers were in direct competition for an increasing weight percentage of silica. PVP is shown to displace preadsorbed PEO from the particle surface, and this was observed for a range of PVP molecular weights. SANS measurements were found to give detailed information on the adsorption of the polymers in the separate systems; however, only qualitative information on the adsorption of the polymers could be obtained from the mixed samples. At a total polymer concentration of 0.4% w/v with 1.1% w/v silica, the SANS data were consistent with PVP adsorbing at the surface and dPEO remaining in solution, in agreement with the NMR data.

Colloidal particles in competition for stabilizer: a solvent relaxation NMR study of polymer adsorption and desorption

The competitive adsorption of poly(vinylpyrrolidone) onto silica and alumina-modified silica particles was studied using solvent relaxation nuclear magnetic resonance. The additive nature of the measured relaxation rate enabled predictions to be made of the relaxation rate in different polymer adsorption scenarios. Preferential adsorption of the poly(vinylpyrrolidone) onto the unmodified silica particles occurred when there was insufficient polymer in the system to coat the entire available surface area. Desorption was also found to occur when the polymer was initially adsorbed upon the alumina-modified particle and silica particles were added.