Correlation between specific ion adsorption at the air/water interface and long-range interactions in colloidal systems

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Specific ion effects are of high impact in colloid science and dominate processes in aqueous systems from protein folding or precipitation to ordering of particles or macromolecules in bulk solutions. Due to the large internal interface of colloidal systems especially interfacial ion effects are of importance. This paper presents a new insight into the specific ion effects at the air/water interface of monovalent electrolyte solutions and their consequences for long-range interactions in colloidal systems. Solely, in an asymmetric film (i.e. wetting film) one can determine the sign and precise value of the surface potential of the free air/water surface. It is shown that the all over charges of the interfacial region, which are affected by the type of ion, dominate the interfacial forces even over several tens of nm. This is of interest for tailoring the stability of colloidal systems. It is clearly shown that the air/water interface is negatively charged and that both anions and cations affect the surface potential even at very low electrolyte concentrations (10−4 M).DFG, SPP 1273, Kolloidverfahrenstechni

Untangling superposed double layer and structural forces across confined nanoparticle suspensions

The description of forces across confined complex fluids still holds many challenges due to the possible overlap of different contributions. Here, an attempt is made to untangle the interaction between charged surfaces across nanoparticle suspensions. Interaction forces are measured using colloidal-probe atomic force microscopy. The experimental force profiles are considered as a superposition of double layer and structural forces. In order to independently describe the decay of the double layer force, the ionic strength of the suspension is determined by electrolytic conductivity measurements. Jellium approximation is used to define the impact of the fluid on screening the surface potential. There, the nanoparticles are considered homogeneously distributed across the fluid and screening is only carried out via the particles counterions and added salt. The structural force follows a damped oscillatory profile due to the layer-wise expulsion of the nanoparticles upon approach of both surfaces. The description of the oscillatory structural force is extended by a depletion layer next to the confining surfaces, with no nanoparticles present. The thickness of the depletion layer is related to the electrostatic repulsion of the charged nanoparticles from the like-charged surfaces. The results show that the total force profile is a superposition of independent force contributions without any mutual effects. Using this rather simple model describes the complete experimentally determined interaction force profiles very well from surface separations of a few hundred nanometres down to the surfaces being almost in contact

Surface adsorption of sulfonated poly(phenylene sulfone)/C(14)TAB mixtures and its correlation with foam film stability

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Polyelectrolyte/surfactant mixtures of rigid monosulfonated poly(phenylene sulfone) (sPSO(2)-220) and tetradecyl trimethylammonium bromide (C(14)TAB) were investigated by surface tension, surface elasticity and foam film stability measurements. The results were compared to former measurements of polyelectrolyte/surfactant mixtures containing more flexible polyelectrolytes (PAMPS or PSS and C(14)TAB). For all polyelectrolyte/surfactant mixtures an increased surface adsorption in comparison to the pure surfactant was detected. Moreover, sPSO(2)-220/C(14)TAB mixtures showed a much higher surface activity and foam film stability than mixtures with more flexible polyelectrolytes. The results presented give insight into the surface adsorption and foam film formation of rigid polyelectrolyte/surfactant mixtures. Therefore, this study helps to understand the role of polyelectrolyte backbone rigidity in the formation and stabilization of foam films made from polyelectrolyte/surfactant mixtures.DFG, SPP 1273, Kolloidverfahrenstechni

Impact of Ultrasound on the Motion of Compact Particles and Acousto-responsive Microgels

In this study, we investigate dynamic light scattering (DLS) from both randomly diffusing silica particles and acousto-responsive microgels in aqueous dispersions under ultrasonic vibration. Employing high-frequency ultrasound (US) with low amplitude ensures that the polymers remain intact without damage. We derive theoretical expressions for the homodyne autocorrelation function, incorporating the US term alongside the diffusion term. Subsequently, we successfully combine US with a conventional DLS system to experimentally characterize compact silica particles and microgels under the influence of US. Our model allows us to extract essential parameters, including particle size, frequency, and amplitude of particle vibration, based on the correlation function of the scattered light intensity. The studies involving non-responsive silica particles demonstrate that US does not disrupt size determination, establishing them as suitable reference systems. Microgels show the same swelling/shrinking behavior as that induced by temperature, but with significantly faster kinetics. The findings of this study have potential applications in various industrial and biomedical fields that benefit from the characterization of macromolecules subjected to US

Influence of Dopamine Methacrylamide on swelling behaviour and nanomechanical properties of PNIPAM microgels

The combination of the catechol-containing co-monomer dopamine methacrylamide (DMA) with stimuli-responsive microgels such as poly(N-isopropylacrylamide) (PNIPAM) bears a huge potential in research and for applications due to the versatile properties of catechols. This research gives first detailed insights into the influence of DMA on the swelling of PNIPAM microgels and the correlation with their nanomechanical properties. Dynamic light scattering (DLS) was used to analyse the swelling behaviour of microgels in bulk solution. The incorporation of DMA decreases the volume phase transition temperature (VPTT) and completion temperature (VPT CT) due to its higher hydrophobicity when compared to NIPAM, while sharpening the transition. The cross-linking ability of DMA decreases swelling ratios and mesh sizes of the microgels. Microgels adsorbed at the solid surface are characterised by atomic force microscopy (AFM): Scanning provides information about the microgel's shape on the surface and force spectroscopy measurements determines their nanomechanical properties ($E$ modulus). As the DMA content increases, microgels protrude more from the surface, correlating with an increase of $E$ modulus and a stiffening of the microgels - confirming the cross-linking ability of DMA. Force spectroscopy measurements below and above the VPTT display a stiffening of the microgels with the incorporation of DMA and upon heating across it's entire cross-section. The affine network factor $\beta$, derived from the Flory-Rehner theory describing the elasticity and swelling of the microgel network, is linearly correlated with the $E$ moduli of the microgels for both - pure PNIPAM and P(NIPAM-co-DMA) microgels. However, for large amounts of DMA, DMA appears to hinder the microgel shrinking, while still ensuring mechanical stiffness, possibly due to catechol interactions within the microgel network

Gerhard Findenegg: A Scientific Life in Soft Matter at Interfaces

In April 2014 Gerhard Findenegg was honored with a colloquium on occasion of his 75th birthday. This colloquium was organized by the international graduate school IRTG 1524 “Self-Assembled Soft Matter Nano-Structures at Interfaces”, that is financed by the German Research Council (DFG). Arising from this event many participants of the workshop and members of the IRTG agreed to contribute to a special issue of Zeitschrift für Physikalische Chemie dedicated to Gerhard Findenegg. The variety of topics addressed in this issue stands for the broadness of science Gerhard Findenegg has been working on since more than fifty years, now

Effect of masker level on overshoot in running‐ and frozen‐noise maskers

Masked thresholds were measured with running‐ and frozen‐noise maskers. The 5‐kHz signal was 2 ms in duration. The masker was low‐pass noise (20 Hz–10 kHz); its total duration was 300 ms. The overall level of the masker was 30, 50, or 70 dB SPL. The onset of the signal was delayed by 0, 3, 8, 18, 198, or 278 ms relative to the onset of the masker. In all frozen‐noise measurements, the signal was added to the same fine structure of the noise. Overshoot in frozen noise was measured for two starting phases of the signal that led to a 10‐dB difference for large signal‐onset delays. In all three configurations (running noise and frozen noise with two different signal phases) masker level had a similar influence on overshoot. At the intermediate masker level (50 dB SPL), a significant amount of overshoot (up to 15 dB) was observed in all three conditions. At the low and the high masker levels, overshoot was very much reduced, and even became negative in most conditions for the 30‐dB‐SPL masker. For the 50‐dB frozen‐noise masker, the total variation of thresholds with signal phase was 8 to 11 dB for long signal‐onset delays, but only 3 to 6 dB for short delays. For the low‐ and high‐level maskers, where only a small overshoot was observed, the threshold variation with phase for a signal at masker onset was the same as that for the long‐delay condition. An explanation for the variation of signal detectability with masker level is proposed that refers explicitly to the compressive input–output characteristic of the basilar membrane at intermediate levels

Pegelabhängigkeit des Overshoots in statistischen und reproduzierbaren Rauschmaskierern

Es wurden Messungen zur zeitlichen Struktur der Simultanverdeckung in statistischen und reproduzierbaren Rauschmaskierern vorgestellt. Bei den Experimenten in frozen noise wurde die Wechselwirkung zwischen Testsignal und Feinstruktur des Maskierers berücksichtigt. Die Mithörschwellen zeigen ein Abhängigkeit von der Anfangsphase des Testtones. Daher wurden die Messungen zum Overshootphänomen in frozen noise für die beiden Testtonphasen durchgeführt, bei denen die Schwellen in den Experimenten zur Phasenabhängigkeit ihren maximalem bzw. minimalen Wert annehmen. Ein Zusammenhang zwischen dem Einfluß der Phase auf die Mithörschwellen und der Größe des Overshoot tritt besonders deutlich bei einem Rauschpegel von 50 dB SPL auf. Aufgrund des großen Signal-Rauschverhältnisses zu Maskiererbeginn ist dort die Phasenabhängigkeit verglichen mit den Resultaten am Maskiererende sehr gering. Die Ergebnisse zeigen, daß der Overshoot in frozen noise sowohl von der Startphase des Testtones als auch vom Rauschpegel abhängt. Die Overshootwerte, die für die beiden Phasen in frozen noise ermittelt wurden, bilden i.a. die obere und untere Begrenzung für die Werte in running noise. Die Möglichkeit zur Auswertung der Maskiererfeinstruktur führt demnach nicht generell zu einer Verringerung des Overshoots

Temperature responsive behavior of polymer brush/polyelectrolyte multilayer composites

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The complex interaction of polyelectrolyte multilayers (PEMs) physisorbed onto end-grafted polymer brushes with focus on the temperature-responsive behavior of the system is addressed in this work. The investigated brush/multilayer composite consists of a poly(styrene sulfonate)/poly(diallyldimethylammonium chloride) (PSS/PDADMAC) multilayer deposited onto the poly(N-isopropylacrylamide-b-dimethylaminoethyl methacrylate) P(NIPAM-b-DMAEMA) brush. Ellipsometry and neutron reflectometry were used to monitor the brush collapse with the thickness decrease as a function of temperature and the change in the monomer distribution perpendicular to the substrate at temperatures below, across and above the phase transition, respectively. It was found that the adsorption of PEMs onto polymer brushes had a hydrophobization effect on PDMAEMA, inducing the shift of its phase transition to lower temperatures, but without suppressing its temperature-responsiveness. Moreover, the diffusion of the free polyelectrolyte chains inside the charged brush was proved by comparing the neutron scattering length density profile of pure and the corresponding PEM-capped brushes, eased by the enhanced contrast between hydrogenated brushes and deuterated PSS chains. The results presented herein demonstrate the possibility of combining a temperature-responsive brush with polyelectrolyte multilayers without quenching the responsive behavior, even though significant interpolyelectrolyte interactions are present. This is of importance for the design of multicompartment coatings, where the brush can be used as a reservoir for the controlled release of substances and the multilayer on the top as a membrane to control the diffusion in/out by applying different stimuli.DFG, GRK 1524, Self-Assembled Soft-Matter Nanostructures at Interface