The temperature-induced intramolecular coil-globule transition in poly(N-isopropylacrylamide) has been studied by microcalorimetry to investigate the cooperativity of this transition. Measurements were performed in the presence of sodium dodecyl sulfate (SDS) which prevents the polymer from aggregation both in the coil and in the globule state. It has been shown that the effective (van't Hoff) enthalpy of transition of a cooperative unit is 120 times less than the calorimetric enthalpy of a polymer molecule. This means that a coil-globule transition in this polymer is not an "all-or-none" process and occurs independently in cooperative units ("domains") which are 120 times smaller than the polymer molecule and have a molecular weight about 6 X 10.' .
The phase behavior of a fractionated high molecular weight sample of poly(JV-isopropylacrylamide) in dilute aqueous solution containing a surfactant, sodium dodecyl sulfate (SDS), is studied with temperature and surfactant concentration as independent variables. Static and dynamic light scattering are used as methods of investigation. Without surfactant, the polymer exhibits a lower critical solution temperature (LCST) of 34 °C, above which it precipitates. At SDS concentrations of only 250 mg/L, aggregation is completely prevented (intermolecular solubilization) and the behavior of isolated polymer molecules can be studied in the whole temperature range: Upon heating, the polymer undergoes a coil-to-globule phase transition with a volume reduction by a factor of more than 300. The transition temperature depends on the surfactant concentration. It is first constant and equal to the LCST, but begins to increase above 300 mg/L surfactant. Therefore when increasing the surfactant concentration at constant temperatures above the LCST, we cross the phase boundary and observe intramolecular solubilization, i.e., a surfactant-induced globule-to-coil transition. Below the LCST, the surfactant causes an expansion of the polymer coils also setting on at 300 mg/L.
The deposition of positively charged latex particles with radii between 20 and 100 nm on a negatively charged mica surface is investigated with atomic force microscopy at low ionic strengths. The polystyrene latex spheres with amidine headgroups are characterized as to their size distribution, electrophoretic mobility, and aggregation behavior. Surface coverage measurements take into account the polydispersity of the particles on the surface. Deposition kinetics from a quiescent solution are, for low coverage, in good agreement with diffusion-limited adsorption. At long times, the surface coverage tends toward a maximum surface coverage θmax independent of the particle concentration but depending on ionic strength. This work extends the available data for θmax for small particle size and very low ionic strengths. The values for θmax decrease with decreasing ionic strength; this trend is described with a simple model based on random sequential adsorption and the effect of overlapping double layers. Particle size polydispersity can modify the results for θmax substantially; its influence is investigated in detail.
Time-resolved fluorescence depolarization is applied to investigate the association of sodium n-dodecyl sulfate (SDS) micelles with poly(N-isopropylacrylamide) in aqueous solutions using an amphiphilic fluorescent probe (3-perylenedodecanoic acid) which is incorporated into the SDS micelles. First, the effect of the surfactant concentration was measured: in the presence of the polymer, above the critical aggregation concentration (CAC) of SDS, the rotational relaxation of the probe exhibits a slow and a fast component. The relaxation time of the fast component is the same as in a polymer-free solution above the CMC of SDS where, however, only one component is observed. The slower relaxation time is attributed to polymer-bound micelles which incorporate polymer segments into their core. Second, the effect of the temperature induced coil−globule transition is investigated: in the course of the transition the rotational motion slows down almost 10-fold, indicating that the probe remains firmly associated with the polymer even in its dense globular state.
Dilute water-in-oil microemulsions, such as the ternary system water–AOT–hexane, can be modeled as suspensions of spherical droplets coated with surfactant. There is a general agreement that the radius of the droplets is not uniform but the magnitude of the polydispersity is subject to discussions. The polydispersity influences strongly optical properties of the microemulsions, in particular it determines the characteristic features of scattering data observed in the vicinity of optical matching. We present here analytical expressions and data for the refractive index increment, the average scattering intensity, the apparent hydrodynamic radius, and the second cumulant of dynamic light scattering. Our model reproduces without any free parameters the dependence of the refractive index increment on the water-to-surfactant ratio ω. Only two free parameters are needed to fit the ω dependence of the other three quantities: δ represents approximately the thickness of the surfactant layer and γ=〈r 2〉/〈r〉2−1 reflects the radius polydispersity. Our results suggest a polydispersity index (γ)1/2 of only 12%. This value is much smaller than previously estimated by small angle scattering.
Water keeps puzzling scientists because of its numerous properties which behave oppositely to usual liquids: for instance, water expands upon cooling, and liquid water is denser than ice. To explain this anomalous behaviour, several theories have been proposed, with different predictions for the properties of supercooled water (liquid at conditions where ice is more stable). However, discriminating between those theories with experiments has remained elusive because of spontaneous ice nucleation. Here we measure the sound velocity in liquid water stretched to negative pressure, and derive an experimental equation of state, which reveals compressibility anomalies. We show by rigorous thermodynamic relations how these anomalies are intricately linked with the density anomaly. Some features we observe are necessary conditions for the validity of two theories of water.Liquid water exhibits numerous anomalies and different scenarios have been proposed to explain them (1). In particular, the existence of a line of density maxima along isobars has been related to putative maxima in compressibility (2) and heat capacity (3). These maxima may arise from an intriguing phase separation of water in two distinct liquids (1,4), although they can also be explained without resorting to such a phase separation (2). However, the compressibility and heat capacity maxima, whose existence is predicted by molecular dynamics simulations (3,5), have hitherto not been observed in experiments. Alternative theoretical scenarios, namely 1 arXiv:1708.00063v1 [physics.chem-ph] 31 Jul 2017 the stability limit conjecture (6) and the critical-point-free scenario (7, 8), do not require the existence of compressibility and heat capacity maxima, but rather predict a divergence of these quantities at low temperature.Another type of anomaly, namely a minimum in sound velocity along one isochore, was recently discovered at negative pressure (9,10). At negative pressure, the liquid is mechanically stretched, in a state metastable with respect to vapor. To date, the only method able to reach significantly negative pressures (beyond −100 MPa) uses 3 − 10 µm fluid inclusions (FIs) of water in a quartz crystal, and stretching is obtained by cooling liquid water at nearly constant volume (11,12). In our previous work (9, 10), we could only study two FIs along different isochores, and only one clearly showed a minimum in sound velocity. In the present work, we have measured more FIs showing a sound velocity minimum, and reached more negative pressures. We have thus established a more accurate experimental equation of state (EoS) down to −137 MPa. The new EoS strongly supports the existence of the compressibility maxima predicted by some theories of water. Furthermore, we establish new thermodynamic relations between the line of density maxima and the sound velocity anomalies. In contrast to previous works, this provides a relation between quantities that are directly observable in experiments. The corresponding lines of extrema obtained from our experimenta...
Epidemiologic studies have shown correlations between morbidity and particles < or = 2.5 microm generated from pollution processes and manufactured nanoparticles. Thereby nanoparticles seem to play a specific role. The interaction of particles with the lung, the main pathway of undesired particle uptake, is poorly understood. In most studies investigating these interactions in vitro, particle deposition differs greatly from the in vivo situation, causing controversial results. We present a nanoparticle deposition chamber to expose lung cells mimicking closely the particle deposition conditions in the lung. In this new deposition chamber, particles are deposited very efficiently, reproducibly, and uniformly onto the cell culture, a key aspect if cell responses are quantified in respect to the deposited particle number. In situ analyses of the lung cells, e.g., the ciliary beat frequency, indicative of the defense capability of the cells, are complemented by off-line biochemical, physiological, and morphological cell analyses.
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