Externally Triggered Oscillatory Structural Forces.

This paper addresses triggering of oscillatory structural forces via temperature variation across an aqueous dispersion of thermoresponsive poly( N-isopropylacrylamide) (PNIPAM) nanogels confined between silica surfaces. Oscillatory structural forces are a well-known phenomenon in colloidal science, caused by interactions between molecules or colloids. Modulation of these forces usually requires changing the internal parameters of the dispersion, such as ionic strength, particle concentration, and surface charge, or changing the properties of the confining walls, such as surface roughness, potential, or elasticity. All of these parameters are usually fixed and can only be changed via exchange of the sample or the complete experimental setup. Here, a new approach is presented, combining the characteristics of smart materials with the properties of nanoparticles, using negatively charged PNIPAM nanogels. Aqueous dispersions of these nanogels express no oscillatory structural forces in the initial state (20 °C), below the volume phase transition temperature (32 °C). Heating (60 °C) reduces the nanogel size and leads to a more negative ζ-potential, which triggers the onset of oscillatory structural forces.

Experimental evaluation of additional short ranged repulsion in structural oscillation forces.

The paper addresses additional short ranged repulsion in structural oscillation forces between silica surfaces across a suspension of silica nanoparticles. Fit and prediction of the structural oscillation forces usually involve an exponentially decreasing harmonic as introduced by Israelachvili [Israelachvili, Intermolecular & surface forces, Academic Press, San Diego, USA, 1985]. Recently we demonstrated, for aqueous suspensions of silica nanoparticles at various concentrations, that this fit equation is insufficient to describe the structural oscillation forces in its whole range [Schön et al., Beilstein J. Nanotechnol., 2018, 9, 1095-1107]. An additional force acting on short separations leads to the fit parameters scattering widely as well as being dependent on each other and the starting point of the fit. An additional repulsive term was introduced to solve these problems. The additional repulsive force has also been observed by others, in ionic liquids and polyelectrolyte solutions at high ionic strength. It was attributed to the diffusive double layer forces. The rise of the additional repulsion with increasing particle concentration seems to conflict with this interpretation. In this work, colloidal probe atomic force microscopy is used in aqueous suspensions of silica nanoparticles to investigate other contributing factors such as the increasing hydrodynamic drag in the normal direction to the confining surface with increasing particle concentration. A kinetic component to the structural oscillation forces is observed. Furthermore, sodium chloride is used to adjust the ionic strength of two different concentrated silica nanoparticle suspensions. For these systems the additional decay length is compared to the Debye length in the range from low to high ionic strength. A master curve of the additional decay length over Debye length at different ionic strengths, approach speed and particle concentration is produced. It affirms the link between the two and the connection between the additional force and the diffusive double layer forces. The increasing trend for the additional repulsion with increasing particle concentration reveals a synergistic effect of diffusive double layer forces and structural oscillation forces at low to medium ionic strength, which cannot be observed at high ionic strength.

A simple extension of the commonly used fitting equation for oscillatory structural forces in case of silica nanoparticle suspensions.

Background: The ordering of molecules or particles in the vicinity of a confining surface leads to the formation of an interfacial region with layers of decreasing order normal to the confining surfaces. The overlap of two interfacial regions gives rise to the well-known phenomenon of oscillatory structural forces. These forces are commonly fitted with an exponentially decaying harmonic oscillation as introduced by Israelachvili (Israelachvili, J. N. Intermolecular & surface forces; Academic Press: San Diego, CA, USA, 1985). From the fit three important parameters are obtained, namely wavelength, amplitude and decay length, which are related to the period, the strength and the correlation length of the oscillatory structural forces, respectively. The paper addresses structural forces between a silica microsphere and a silicon wafer across silica nanoparticle suspensions measured with a colloidal probe AFM. Using the simple fitting procedure with three parameters often leads to underestimation of actually measured forces. The deviation of the fit from the experimental data is especially pronounced at small distances of the confining surfaces and at high concentrations of silica nanoparticles. As a consequence, the parameters of the common fit equation vary with the starting point of the fit. Although the wavelength is least affected and seems to be quite robust against the starting point of the fit, all three parameters show distinct oscillations, with a period similar to the wavelength of the oscillatory structural forces themselves. The oscillations of amplitude and decay length, which are of much higher magnitude, show a phase shift of 180° implying not only a dependence on the starting point of the fit but also on each other. The range affected by this systematic deviation of the fit parameters is much larger than the optically perceived mismatch between fit and experimental data, giving a false impression of robustness of the fit. Results: By introducing an additional term of exponentially decaying nature the data can be fitted accurately down to very small separations and even for high silica nanoparticle concentrations (10 wt %). Furthermore wavelength, amplitude and decay length become independent of the starting point of the fit and in case of the latter two of each other. The larger forces at small separations indicate a more pronounced ordering behavior of the particles in the final two layers before the wall. This behavior is described by the proposed extension of the common fit equation. Conclusion: Thus, the extension increases the accessible data range in terms of separation and concentration and strongly increases the accuracy for all fitting parameters in the system studied here.

Thermoresponsive PDMAEMA Brushes: Effect of Gold Nanoparticle Deposition.

The paper addresses the effect of gold nanoparticle (Au-NP) deposition on the thermoresponsive volume phase transition of the weak polyelectrolyte poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes. PDMAEMA brushes were synthesized via surface-initiated atom transfer radical polymerization (SI-ATRP). The PDMAEMA/Au-NP composite brushes were fabricated by immersing the brush modified wafer in the Au-NP suspension. Atomic force microscopy (AFM), ellipsometry, and scanning electron microscopy (SEM) have been employed to characterize the neat PDMAEMA brushes and PDMAEMA/Au-NP composite brushes. All neat PDMAEMA brushes swelled below the volume phase transition temperature and collapsed with increasing temperature over a broad temperature range independent of the initial brush thickness. Water uptake of the brushes is also independent of initial brush thickness. The adsorption of the charged Au-NPs significantly affects the degree of swelling and the thermoresponsive properties of the brushes. PDMAEMA/Au-NP composite brushes do not exhibit any noticeable phase transition at the experimental temperature range irrespective of the initial brush thickness. The reason for this behavior is attributed to a combination of the following: the decreased conformational entropy of the Au-NP adsorbed polymer chains, the increased hydrophilicity of the system due to the charged Au-NPs, and the ≈13 nm diameter Au-NPs causing steric hindrance. We have also shown that the AFM full-indentation method can be successfully applied to determine the polymer brush thicknesses.

Silica nanoparticle suspensions under confinement of thin liquid films.

The paper deals with the effect of geometrical confinement on the structuring of Silica nanoparticle suspensions in thin films. The confinement is produced by a Colloidal Probe AFM. Approaching of the two outer surface leads to oscillatory forces. The force profile reflects the pair correlation function and its period the average distance between the nanoparticles under confinement. The nanoparticle structuring is compared to the particle distribution in bulk by small angle X-ray scattering (SAXS). The SAXS structure factor which presents the Fourier transform of the pair correlation function gives the same interparticle distance for the bulk as the oscillation period of the AFM force curves. The distance scales with particle number density ρ as ρ(-1/3) and is very robust against different suspension parameters (nanoparticle size, ionic strength) and parameters of the outer surfaces (surface potential, roughness and elasticity).

Ethylene glycol-based microgels at solid surfaces: swelling behavior and control of particle number density.

The adsorption of ethylene glycol (EG)-based microgel particles at silicon surfaces was investigated. Monodisperse p-MeO2MA-co-OEGMA microgel particles were synthesized by precipitation polymerization. Particle size and the volume phase transition temperature (VPTT) can be tailored by changing the amount of comonomer. The effect of geometrical confinement on the microgel particles was studied at the solid/liquid interface. Therefore, layer formation, particle number density, and swelling/deswelling at the surface were studied in dependence on the spin-coating preparation parameters and characterized by means of AFM against ambient conditions. The deswelling/swelling behavior was investigated by AFM in the water-swollen state.

Intramolecular hydroamination with homogeneous zinc catalysts: evaluation of substituent effects in N,N'-disubstituted aminotroponiminate zinc complexes.

A series of symmetrical and unsymmetrical N,N'-disubstituted aminotroponimines (ATIHs) have been prepared. Substituents ranging from linear to cyclic alkyl groups, chelating ethers, and aryl groups were employed. The corresponding aminotroponiminate zinc complexes were then synthesized and characterized by a number of techniques, including by X-ray crystallography. Herein we report on the investigations into their activity in the intramolecular hydroamination of nonactivated alkenes. We also demonstrate that complexes bearing ligands with cyclic alkyl groups show superior activity in a number of selected reactions with functionalized aminoalkenes.