Colloid Interface Research Articles

Detection of Lipase Activity in Cells by a Fluorescent Probe Based on Formation of Self-Assembled Micelles.

SummaryReliable and sensitive detection of lipase activity is essential for the early diagnosis and monitoring of acute pancreatitis or progression of digestive diseases. However, the available fluorescent probes for detection of lipase activity are only implemented in a hexane-water two-phase system due to the nature of heterogeneous catalysis of lipase, thus limiting their applications in direct imaging of lipase activity in living cells and tissues. Here we designed and synthesized a “turn on” fluorescent probe CPP based on self-assembled micelles for hydrolysis of lipase. The CPP probe exhibits high selectivity and excellent sensitivity for the detection of lipase in such a homogeneous system and is successfully applied for monitoring lipase activity in pancreatic AR42J cells, tissues, and serums. Taken together, the fluorescent CPP probe not only provides a tool for diagnostic potential in pancreatic disease but also demonstrates an application potential for micelle self-assembly-based development of biological probes.

Evaluation of Kynurenine Behaviour on the Glassy Carbon Electrode Modified By Nafion Layer

Kynurenine is the first stable metabolite of tryptophan degradation formed via kynurenine pathway. Inflammation accompanying several disorders can activate kynurenine pathway, therefore increased level of kynurenine is associated with cancer, depression, schizophrenia, encephalopathy, HIV, dementia, amyotrophic lateral sclerosis, malaria, Alzheimer’s, and Huntington’s disease [1]. Kynurenine is an immuneregulatory molecule, which is further converted to other compounds showing biological activity, i.e. neuroprotective or neurotoxic effects on human cells. Studying properties of kynurenine is currently the important topic and might bring new information in context of biological processes [2].The presented work examines the potential of the cation exchange membrane Nafion® (used i.e. in voltammetric analytical approaches) to interact with kynurenine. Nafion is the polymer (ionomer) consisted of tetrafluorethylene or Teflon® backbone with perfluorovinyl ether groups terminated with sulphonate groups. The Nafion allows positively charged species (molecules or cations) to pass through, and at the same time excludes negatively charged compounds [3]. No studies have been conducted on the electrochemical behavior of kynurenine at Nafion covered glassy carbon electrode. Our preliminary studies have shown that kynurenine exhibits efficient adsorption on the Nafion layer, previously deposited by drop coating on the glassy carbon substrate.Voltammetric response of kynurenine strongly depends on the thickness of the Nafion layer. Thus, we attempted to evaluate the relationship between voltammetric signal of the molecule and morphology of the Nafion film deposited in different ways on the glassy carbon substrate (flat and rod electrode). For comparison several different approaches have been utilized for Nafion deposition, i.e. layer of varied thickness of Nafion were deposited on the rod carbon glassy electrode (CGE) or different withdrawal speed of the electrode from the solution of varied concentration of polymer was applied. The great benefit of this technique is possibility to control the thickness at a very high precision level on the whole electrode surface. The morphology surface electrode (roughness and waviness) both with flat and rod shape before and after Nafion deposition were evaluated by optical profilometer, mechanical profilometer, and atomic force microscopy. Moreover, Nation layer thickness on the rod GCE was determined by laser scan micrometer. Morphology characteristic of flat GCE was combined with chemical analysis at atomic and molecular level using complementary methods, such as: TOF-SIMS (time of flight secondary ion mass spectrometry) and XPS (X-ray photoelectron spectrometry). In addition, we propose new methodology for in situ molecular analysis of the rod CGE/Nafion after kynureine adsorption. It combines TOF-SIMS with cyclic voltammetry (CV). In this fashion CGE/Nafion was withdrawn at constant speed from electrolyte contained kynurenine, whilst CV measurements were conducted. When CV experiment was finished, samples were transferred to TOF-SIMS instruments and chemical analysis of different segments with area 100x100 um2 along length of rod were conducted. In that way molecular analysis of every segment surface CGE/Nafion corresponding to applied potential was performed.The results presented here provide new insights in analytical electrochemistry in case of kynurenine behavior on Nafion layer.[1] Y. Chen, G.J. Guillemin, Kynurenine Pathway Metabolites in Humans: Disease and Healthy States, Int J Tryptophan Res. 2009; 2: 1-19.[2] I. Sadok, A. Gamian, M. Staniszewska, Chromatographic analysis of tryptophan metabolites, J Sep Sci. 2017; 40(15):3020-3045.[3] R.K. Nagarale, G.S. Gohil, Vinod K. Shahi, Recent developments on ion-exchange membranes and electro-membrane processes, Adv. Colloid Interface Sci. 2006; 119: 97-130.

Analytic Expression for Electrophoretic Mobility of Soft Particles with a Hydrophobic Inner Core at Different Electrostatic Conditions.

This paper presents a simplified model for the electrophoresis of a soft particle with a nonwettable rigid core with charged polyelectrolyte corona under a weak-field and low-charge density consideration. We have derived a closed form solution for the mobility, which reduces to the well-known expressions for mobility as derived by Ohshima for limiting cases such as a hydrophilic charged core coated with an uncharged polymer (Ohshima, H. J. Colloid Interface Sci. 2002, 252, 119-125) or an uncharged no-slip core coated with a polyelectrolyte layer (Ohshima, H. Electrophoresis 2006, 27, 526-533). The generalized mobility expression reduces to the existing expression for mobility of a rigid hydrophobic colloid as the soft layer shrinks to zero. The general form of the mobility expression involves elliptic integrals, which can be computed easily through a software like Mathematica. We have derived analytical solutions for mobility pertaining to several particular cases. The occurrence of mobility reversal when the core and polyelectrolyte layer has a charge of opposite polarity is demonstrated in this paper.

Response to comment on “Visible-light-driven, hierarchically heterostructured, and flexible silver/bismuth oxyiodide/titania nanofibrous membranes for highly efficient water disinfection” by Song et al

A reply to the comment of Kim et al. on “Visible-light-driven, hierarchically heterostructured, and flexible silver/bismuth oxyiodide/titania nanofibrous membranes for highly efficient water disinfection” is presented. We would like to thank Kim et al. for being interested in our recent work [J. Song, J. Yu, G. Sun, Y. Si, and B. Ding, J. Colloid Interface Sci. 555 (2019) 636]. In response to the comment from Kim et al., we studied and discussed the effects of silver ions under both dark and visible light irradiation conditions on the inactivation of E. coli, and verified that Ag ions played quite minor roles in E. coli disinfection for Ag/BiOI/TiO2 under our experimental conditions.

Brownian Disks Lab: Simulating time-lapse microscopy experiments for exploring microrheology techniques and colloidal interactions

Brownian Disks Lab (BDL) is a Java-based application for the real-time generation and visualization of the motion of two-dimensional Brownian disks using Brownian Dynamics (BD) simulations. This software is designed to emulate time-lapse microscopy experiments of colloidal fluids in quasi-2D situations, such as sedimented layers of particles, optical trap confinement, or fluid interfaces. Microrheology of bio-inspired fluids through optical-based techniques such as videomicroscopy is a classic tool for obtaining the mechanical properties and molecular behavior of these materials. The results obtained by microrheology notably depend of the time-lapse value of the videomicroscopy setup, therefore, a tool to test the influence of the lack of statistics by simulating Brownian objects in experimental-like situations is needed. We simulate a colloidal fluid by using Brownian Dynamics (BD) simulations, where the particles are subjected to different external applied forces and inter-particle interactions. This software has been tested for the analysis of the microrheological consequences of attractive forces between particles [1], the influence of image analysis on experimental microrheological results [2], and to explore experimental diffusion with optical tweezers [3]. The output results of BDL are directly compatible with the format used by standard microrheological algorithms [4]. In a context of microrheology of complex bio-inspired fluids, we use this tool here to study if the lack of statistics may influence the observed potential of a bead trapped by optical tweezers. Program summaryProgram Title: Brownian Disks Lab (BDL)Program Files doi:http://dx.doi.org/10.17632/dbwzdkttkb.1Licensing provisions: GPLv3Programming language: Java (JDK 7 and above)Supplementary material: We provide a detailed user manual which describes how to use BDL, the theoretical basis of Brownian dynamics simulations, the particle–particle interactions implemented in this software, and additional details and explanations regarding the developed code.Nature of problem: By using time-lapse microscopy experiments (video-microscopy), we can observe the Brownian motion of colloidal particles under different particle–particle interactions and external forces. Sedimented quasi-two-dimensional layers, fluid interfaces, or optically trapped particles can be considered as two-dimensional colloidal systems. The centers of mass of the colloidal objects are subsequently obtained by the image analysis of the time-lapsed images stored. We need an application to generate an ideal real-time motion of colloidal objects in a simple fluid, providing the position of the particles without the need of image analysis. This software should allow to inspect, before the experiments, the general behavior of a colloidal fluid, including visual inspection of the particles’ movement. Using this application, we should be able to analyze the collective motion of the Brownian objects, the influence of different inter-particle forces, and the limitations of the experimental setup, e.g., the effect of image analysis in the results obtained.Solution method: BDL performs the simulation of Brownian 2D disks contained in a transparent medium with a constant viscosity value. The theoretical scheme allows us to introduce external forces in the disks for testing different experimental conditions, and the interactions observed in experiments on colloidal physics (Einstein, 1905; Xin et al., 2016). We use a computational multi-platform environment without high computing requirements since a small number of particles (n≤500) and small concentrations are typical of time-lapse microscopy experiments. The output data is analogous to a video-microscopy setup: the required statistical quantities can be later calculated from the particles’ positions using microrheology standard algorithms (Bevan and Eichmann, 2011). BDL has been developed using Easy Java/JavaScript Simulations (EjsS) (Dinsmore et al., 2002), a software which allows to simplify the code generation and the visualization of simulated objects. Java allows to run the software in any SO without compilation or installation as long as Java Runtime Environment (JRE) is previously installed in the computer.Additional comments including restrictions and unusual features: Requires SO with Java Runtime Environment (JRE) installed.[1] P. Domínguez-García, Europhys. J. E. Soft. Matter. 35, p. 73, 2012.[2] P. Domínguez-García and M. A. Rubio, Appl. Phys. Lett. 102, p. 074101, 2013.[3] M. Pancorbo, M. A. Rubio, P. Domínguez-García, Procedia Comp. Sci.. 108, p. 166–174, 2017.[4] J. C. Crocker and D. G. Grier, J. Colloid Interface Sci. 179, p. 298–310, 1996.

Moving contact lines and Langevin formalism

HypothesisIn previous work [J.-C. Fernández-Toledano, T. D. Blake, J. De Coninck, J. Colloid Interface Sci. 540 (2019) 322–329], **we used molecular dynamics (MD) to show that the thermal oscillations of a contact line formed between a liquid and a solid at equilibrium may be interpreted in terms of an overdamped 1-D Langevin harmonic oscillator. The variance of the contact-line position and the rate of damping of its self-correlation function enabled us to determine the coefficient of contact-line friction ζ and so predict the dynamics of wetting. We now propose that the same approach may be applied to a moving contact line. MethodsWe use the same MD system as before, a liquid bridge formed between two solid plates, but now we move the plates at a steady velocity Uplate in opposite directions to generate advancing and receding contact lines and their associated dynamic contact angles θd. The fluctuations of the contact-line positions and the dynamic contact angles are then recorded and analyzed for a range of plate velocities and solid-liquid interaction. FindingsWe confirm that the fluctuations of a moving contact line may also be interpreted in terms of a 1-D harmonic oscillator and derive a Langevin expression analogous to that obtained for the equilibrium case, but with the harmonic term centered about the mean location of the dynamic contact line xd, rather than its equilibrium position x0, and a fluctuating capillary force arising from the fluctuations of the dynamic contact angle around θd, rather than the equilibrium angle θ0. We also confirm a direct relationship between the variance of the fluctuations over the length of contact line considered Ly, the time decay of the oscillations, and the friction ζ. In addition, we demonstrate a new relationship for our systems between the distance to equilibrium xd-x0 and the out of equilibrium capillary force γLcosθ0-cosθd, where γL is the surface tension of the liquid, and show that neither the variance of the fluctuations nor their time decay depend on Uplate. Our analysis yields values of ζ nearly identical to those obtained for simulations of spreading drops confirming the common nature of the dissipation mechanism at the contact line.

Interplay between Wetting and Filling of Argon Adsorption in Slit Pores with Different Surface Energies Transition from Filling in Micropores to Capillary Condensation in Mesopores

The concept of was first introduced by Dubinin (Dubinin, A. M. M. Q. Rev., Chem. Soc. 1955, 9, 101; Bering, B. P.; Dubinin, A. M. M.; Serpinsky, V. V. J. Colloid Interface Sci. 1966, 21, 378) to describe the adsorption in pores having widths of less than 1.5 nm at temperatures less than the bulk critical point temperature. In mesopores, another phenomenon, capillary condensation to a liquidlike adsorbate, or condensate, occurs (Do, D. D. Adsorption Analysis: Equilibria and Kinetics; World Scientific: London, 1983). The distinction between micropore filling and capillary condensation is not unambiguous. It is implicitly assumed in the literature that there is a strong attraction between the adsorbent and a given adsorbate, resulting in the formation of an adsorbed film which thickens as pressure increases. As the pressure approaches the bulk coexistence pressure, the adsorbed film on a planar surface becomes infinitely thick, a feature known as (complete) wetting. In this work, we provide a clear distinction between filling and condensation and also show how wetting behavior (nonwetting/partial wetting/continuous wetting) affects filling and condensation in slit pores and determine conditions under which filling occurs even when the surfaces are nonwetting or partial wetting. We have carried out extensive simulations of argon adsorption in slit pores to investigate the effects of substrate strength, pore size, and temperature on the transition from nonfilling to filling and condensation. From the analysis of the simulation results, we define filling as the phenomenon whereby a pore is filled before the formation of an adsorbate layer on the surface and capillary condensation as one in which adsorbate layers are formed on the pore walls prior to condensation. A parametric map or phase diagram of wetting/filling is constructed to show the interplay between the adsorbent strength, temperature, and pore size, which delineates the various regions of filling and capillary condensation.

Self-Consistent Field Theory Coupled with Square Gradient Theory of Free Surfaces of Molten Polymers and Compared to Atomistic Simulations and Experiment

A self-consistent field (SCF) theoretic approach, using a general excess Helmholtz energy density functional that includes a square gradient term, is derived for polymer melt surfaces and implemented for linear polyethylene films over a variety of temperatures and chain lengths. The formulation of the SCF plus square gradient approximation (SGA) developed is generic and can be applied with any equation of state (EoS) suitable for the estimation of the excess Helmholtz energy. As a case study, the approach is combined with the Sanchez–Lacombe (SL) EoS to predict reduced density profiles, chain conformational properties, and interfacial free energies, yielding very favorable agreement with atomistic simulation results and noticeable improvement relative to simpler SCF and SGA approaches. The reduced influence parameter invoked in the SGA to achieve accurate density profiles and interfacial free energies is consistent with the definition of Poser and Sanchez, J. Colloid Interface Sci. 1979, 69, 539–548. The ...

Slipping moving contact lines: critical roles of de Gennes’s ‘foot’ in dynamic wetting

In the context of dynamic wetting, wall slip is often treated as a microscopic effect for removing viscous stress singularity at a moving contact line. In most drop spreading experiments, however, a considerable amount of slip may occur due to the use of polymer liquids such as silicone oils, which may cause significant deviations from the classical Tanner–de Gennes theory. Here we show that many classical results for complete wetting fluids may no longer hold due to wall slip, depending crucially on the extent of de Gennes’s slipping ‘foot’ to the relevant length scales at both the macroscopic and microscopic levels. At the macroscopic level, we find that for given liquid height $h$ and slip length $\unicode[STIX]{x1D706}$, the apparent dynamic contact angle $\unicode[STIX]{x1D703}_{d}$ can change from Tanner’s law $\unicode[STIX]{x1D703}_{d}\sim Ca^{1/3}$ for $h\gg \unicode[STIX]{x1D706}$ to the strong-slip law $\unicode[STIX]{x1D703}_{d}\sim Ca^{1/2}\,(L/\unicode[STIX]{x1D706})^{1/2}$ for $h\ll \unicode[STIX]{x1D706}$, where $Ca$ is the capillary number and $L$ is the macroscopic length scale. Such a no-slip-to-slip transition occurs at the critical capillary number $Ca^{\ast }\sim (\unicode[STIX]{x1D706}/L)^{3}$, accompanied by the switch of the ‘foot’ of size $\ell _{F}\sim \unicode[STIX]{x1D706}Ca^{-1/3}$ from the inner scale to the outer scale with respect to $L$. A more generalized dynamic contact angle relationship is also derived, capable of unifying Tanner’s law and the strong-slip law under $\unicode[STIX]{x1D706}\ll L/\unicode[STIX]{x1D703}_{d}$. We not only confirm the two distinct wetting laws using many-body dissipative particle dynamics simulations, but also provide a rational account for anomalous departures from Tanner’s law seen in experiments (Chen, J. Colloid Interface Sci., vol. 122, 1988, pp. 60–72; Albrecht et al., Phys. Rev. Lett., vol. 68, 1992, pp. 3192–3195). We also show that even for a common spreading drop with small macroscopic slip, slip effects can still be microscopically strong enough to change the microstructure of the contact line. The structure is identified to consist of a strongly slipping precursor film of length $\ell \sim (a\unicode[STIX]{x1D706})^{1/2}Ca^{-1/2}$ followed by a mesoscopic ‘foot’ of width $\ell _{F}\sim \unicode[STIX]{x1D706}Ca^{-1/3}$ ahead of the macroscopic wedge, where $a$ is the molecular length. It thus turns out that it is the ‘foot’, rather than the film, contributing to the microscopic length in Tanner’s law, in accordance with the experimental data reported by Kavehpour et al. (Phys. Rev. Lett., vol. 91, 2003, 196104) and Ueno et al. (Trans. ASME J. Heat Transfer, vol. 134, 2012, 051008). The advancement of the microscopic contact line is still led by the film whose length can grow as the $1/3$ power of time due to $\ell$, as supported by the experiments of Ueno et al. and Mate (Langmuir, vol. 28, 2012, pp. 16821–16827). The present work demonstrates that the behaviour of a moving contact line can be strongly influenced by wall slip. Such slip-mediated dynamic wetting might also provide an alternative means for probing slippery surfaces.

Bis-Porphyrin Structural Motiff for Chirality and Molecular Sensing

Bis-porphyrin structures are widely used as model systems to mimic various natural processes and have numerous practical applications in the research and technoligical fields such as polymer and nano science, light harvesting and nonlinear optics, chemical sensors and catalysis, molecular and chiral recognition, various supramolecular and chiroptical devices, chirogenesis and absolute configuration determination. Several years ago, we have discovered that a simple ethane-bridged bis-porphyrin structural motif (M = Zn, Mg, Cu, Co, 2H), can be effectively served for different application purposes owing to the specific molecular and supramolecular functionalities. In particular, the unique functional property of this bis-porphyrin host is based upon the structural semi-flexibility of covalent linkage between two porphyrin unitsresulting in the environmentally assisted syn-anti conformational switching and tweezer formation depending upon the guest structure, which can be effectively employed in different sensing areas, such as chirogenic probes, chiral recognitions, and chemical sensors .1-13 Further developments and prospects toward new chiral and achiral bis- and multi porphyrin systems on the basis of heterometallic bis- and multi-porphyrin structures will be discussed. Borovkov, Symmetry 2014, 6, 256. Borovkov, Symmetry 2010, 2, 184. Borovkov, Y. Inoue, Eur. J. Org. Chem. 2009, 189. A. Hembury, V. V. Borovkov, Y. Inoue, Chem. Rev. 2008, 108, 1. V. Borovkov, Y. Inoue, Top. Curr. Chem. 2006, 265, 89. V. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 2004, 37, 449.Giancane, V. Borovkov, Y. Inoue, L. Valli, J. Colloid Interface Sci. 2012, 385, 282.Giancane, V. Borovkov, Y. Inoue, S. Conoci, L. Valli,Soft Matter 2013, 9, 2302.Bettini, E. Maglie, R. Pagano, V. Borovkov, Y. Inoue, L. Valli, G. Giancane, Beilstein J. Nanotechnol. 2015, 6, 2154.Colombelli, M.G. Manera, V. Borovkov, G. Giancane, L. Valli, R. Rella, Sensor Actuat. B-Chem. 2017, 246, 1039. Buccolieri, D. Manno, A. Serra, A. Santino, M. Hasan, V. Borovkov, G. Giancane,Sensor Actuat. B-Chem. 2018, 257, 685.Buccolieri, M. Hasan, S. Bettini, V. Bonfrate, L. Salvatore, A.Santino, V. Borovkov, G.,Giancane, Anal. Chem. 2018, 90, 6952.Bettini, R. Pagano, V. Borovkov, G. Giancane , L. Valli, J. Colloid Interface Sci. 2019, 533, 762-770.

Multilayer Formation in Self-Shaping Emulsion Droplets.

In several recent studies, we showed that micrometer-sized oil-in-water emulsion droplets from alkanes, alkenes, alcohols, triglycerides, or mixtures of these components can spontaneously "self-shape" upon cooling into various regular shapes, such as regular polyhedrons, platelets, rods, and fibers ( Denkov , N. , Nature 2015 , 528 , 392 ; Cholakova , D. , Adv. Colloid Interface Sci. 2016 , 235 , 90 ). These drop-shape transformations were explained by assuming that intermediate plastic rotator phase, composed of ordered multilayers of oily molecules, is formed beneath the drop surface around the oil-freezing temperature. An alternative explanation was proposed ( Guttman , S. , Proc. Natl. Acad. Sci. USA 2016 113 , 493 ; Guttman , S. , Langmuir 2017 , 33 , 1305 ), which is based on the assumption that the oil-water interfacial tension decreases to very low values upon emulsion cooling. Here, we present new results, obtained by differential scanning calorimetry (DSC), which quantify the enthalpy effects accompanying the drop-shape transformations. Using optical microscopy, we related the peaks in the DSC thermograms to the specific changes in the drop shape. Furthermore, from the enthalpies measured by DSC, we determined the fraction of the intermediate phase involved in the processes of drop deformation. The obtained results support the explanation that the drop-shape transformations are intimately related to the formation of ordered multilayers of alkane molecules with thickness varying between several and dozens of layers of alkane molecules, depending on the specific system. The new results provide the basis for a rational approach to the mechanistic explanation and to the fine control of this fascinating and industrially relevant phenomenon.

Temperature Regulation in Colored Infrared-Transparent Polyethylene Textiles

Summary Effectively regulating heat flow between the human body and its environment not only increases thermal comfort but also presents a novel and potentially cost-effective approach to reducing building energy consumption. Infrared property-engineered textiles have been shown to passively regulate radiative heat dissipation for effective cooling and warming of the human body. However, a lack of dyes that can tune the textile color without compromising the infrared properties remains a major impediment to textile commercialization. Here, we report a new strategy utilizing inorganic nanoparticles as a coloring component for scalable brightly colored, infrared-transparent textiles. The as-fabricated composite textiles not only show a high infrared transparency of ∼80% and a passive cooling effect of ∼1.6°C–1.8°C but also exhibit intense visible colors with good stability against washing. This facile coloration approach will promote the commercialization of radiative cooling textiles in temperature-regulating wearable applications for effective energy savings.

Corrigendum to “Batch and column study: Sorption of perfluorinated surfactants from water and cosolvent systems by Amberlite XAD resins” [J. Colloid Interface Sci. 368 (2012) 505–511

Corrigendum to “Batch and column study: Sorption of perfluorinated surfactants from water and cosolvent systems by Amberlite XAD resins” [J. Colloid Interface Sci. 368 (2012) 505–511

Electrophoretic mobility of a charged spherical colloidal particle in an uncharged or charged polymer gel medium

A theory is developed of the electrophoretic mobility of a charged spherical colloidal particle in a gel medium due to the long-range hydrodynamic particle-gel interaction by using the Brinkman-Debye-Bueche model and the general mobility expression is derived. On the basis of the obtained general mobility expression, a simple analytic mobility expression for particles with low zeta potential is derived, which is found to be in excellent agreement with the exact mobility equations expressed in terms of exponential integrals derived by Allison et al. for an uncharged gel (J Colloid Interface Sci 313: 328, 2007) and by Li et al. for a charged gel (J Colloid Interface Sci 423: 129, 2014). The relative error is less than 1.6%. An approximate mobility expression for a particle with arbitrary zeta potential in an uncharged gel medium applicable for large κa > 30, where a is the particle radius and κ is the Debye-Huckel parameter, is also derived.

Effect of thiocyanate anions on switching of poly (N-isopropylacrylamide) tethered to nanoparticle surface

<sec>A recent experiment carried by Humphreys et al. (Humphreys B A, Wanless E J, Webber Grant B 2018 <i>J. Colloid Interface Sci</i>. <b>516</b> 153) shows that when poly (N-isopropylacrylamide) (PNIPAM) tethered to nanoparticle surface is immersed in potassium thiocyanate solution, the thiocyanate anions (SCN<sup>–</sup>) can increase the low critical solution temperature (LCST) of the PNIPAM below 500 mmol, though the LCST is reduced when at 1000 mmol. It is unclear why the SCN<sup>–</sup> increases the LCST at low concentration and reduces the LCST at high concentration. In this paper, using a molecular theory, we investigate the effect of SCN<sup>–</sup> on the switching and the structure of PNIPAM tethered to nanoparticle surface. In our model the PNIPAM-SCN<sup>–</sup> bonding (P—S bonds), electrostatic effects and their explicit coupling to the PNIPAM conformations are taken into consideration. We find that under the low SCN<sup>–</sup> concentration, as the SCN<sup>–</sup> concentration increases, the SCN<sup>–</sup> is associated with the PNIPAM chains through the PNIPAM—S bonds, and the PNIPAM segments become negatively charged, which makes electrostatic repulsion stronger and results in an increase in the LCST.</sec><sec>According to our model, the reduction of LCST at high SCN<sup>–</sup> concentration can be explained as follows: with the increase of SCN<sup>–</sup> concentration, more and more PNIPAM-SCN<sup>–</sup> bindings occur between SCN<sup>–</sup> and PNIPAM segments, which will lead the hydrophobicity of PNIPAM chains to increase. On the other hand, the P—S bonds have been filled at the high SCN<sup>–</sup> concentration, and the PNIPAM chains become more negatively charged. The increase of the SCN<sup>–</sup> is accompanied with an increase in the concentration of counterions (K<sup>+</sup>). The increase of counterion concentration will give rise to the counterion-mediated attractive interactions along the chains and electrostatic screening within the negatively charged PNIPAM, thus the LCST can be reduced when further increasing the SCN<sup>–</sup> concentration. The reduction of LCST can be attributed to the increased hydrophobicity of PNIPAM chains, or to the counterion-mediated attractive interaction along the chains and the screening of the electrostatic interactions. </sec><sec>By analyzing the distribution of PNIPAM segments near the critical temperature, we find that the distribution of volume fractions of the PNIPAM tethered to nanoparticle surface shows a maximum when the hydration of PNIPAM and PNIPAM-SCN<sup>–</sup> binding are stronger, which implies that a vertical phase separation may occur. Based on our theoretical model, a vertical phase separation and a two-step phase transition behaviors in the PNIPAM tethered to nanoparticle surface are predicted. We also analyze the height of the PNIPAM, which is a function of temperature at different SCN<sup>–</sup> concentrations, and then obtain the critical temperature of the two-step phase transition. The results show that the vertical phase separation and the two-step phase transition are promoted by competition between hydrophobicity, hydrophilicity and electrostatic effects due to the P—S bonds. Our theoretical results are consistent with the experimental observations, and provide a fundamental understanding of the effects of SCN<sup>–</sup> on the LCST of PNIPAM tethered to nanoparticle surface.</sec>

Molecular simulation of chemical reaction equilibria by Kinetic Monte Carlo

ABSTRACTWe describe a new algorithm for the molecular simulation of chemical reaction equilibria, which we call the Reactive Kinetic Monte Carlo (ReKMC) algorithm. It is based on the use of the equilibrium Kinetic Monte Carlo (eKMC) method (Ustinov et al., J. Colloid Interface Sci., 2012, 366, 216–223) to generate configurations in the underlying nonreacting system and to calculate the species chemical potentials at essentially zero marginal computational cost. We consider in detail the typical case of specified temperature, T and pressure, P, but extensions to other thermodynamic constraints are straightforward in principle. In the course of this work, we also demonstrate an alternative method for calculating simulation box volume changes in NPT ensemble simulations to achieve the specified P. We consider two sets of example reacting systems previously considered in the literature, and compare the ReKMC results and computational efficiencies with those of different implementations of the REMC algorithm (Turner et al., Molec. Simulation, 2008, 34, 119–146).

Marangoni-enhanced capillary wetting in surfactant-driven superspreading

Superspreading is a phenomenon such that a drop of a certain class of surfactant on a substrate can spread with a radius that grows linearly with time much faster than the usual capillary wetting. Its origin, in spite of many efforts, is still not fully understood. Previous modelling and simulation studies (Karapetsas et al. J. Fluid Mech., vol. 670, 2011, pp. 5–37; Theodorakis et al. Langmuir, vol. 31, 2015, pp. 2304–2309) suggest that the transfer of the interfacial surfactant molecules onto the substrate in the vicinity of the contact line plays a crucial role in superspreading. Here, we construct a detailed theory to elaborate on this idea, showing that a rational account for superspreading can be made using a purely hydrodynamic approach without involving a specific surfactant structure or sorption kinetics. Using this theory it can be shown analytically, for both insoluble and soluble surfactants, that the curious linear spreading law can be derived from a new dynamic contact line structure due to a tiny surfactant leakage from the air–liquid interface to the substrate. Such a leak not only establishes a concentrated Marangoni shearing toward the contact line at a rate much faster than the usual viscous stress singularity, but also results in a microscopic surfactant-devoid zone in the vicinity of the contact line. The strong Marangoni shearing then turns into a local capillary force in the zone, making the contact line in effect advance in a surfactant-free manner. This local Marangoni-driven capillary wetting in turn renders a constant wetting speed governed by the de Gennes–Cox–Voinov law and hence the linear spreading law. We also determine the range of surfactant concentration within which superspreading can be sustained by local surfactant leakage without being mitigated by the contact line sweeping, explaining why only limited classes of surfactants can serve as superspreaders. We further show that spreading of surfactant spreaders can exhibit either the $1/6$ or $1/2$ power law, depending on the ability of interfacial surfactant to transfer/leak to the bulk/substrate. All these findings can account for a variety of results seen in experiments (Rafai et al. Langmuir, vol. 18, 2002, pp. 10486–10488; Nikolov & Wasan, Adv. Colloid Interface Sci., vol. 222, 2015, pp. 517–529) and simulations (Karapetsas et al. 2011). Analogy to thermocapillary spreading is also made, reverberating the ubiquitous role of the Marangoni effect in enhancing dynamic wetting driven by non-uniform surface tension.

Amyloid-β fibrils assembled on ganglioside-enriched membranes contain both parallel β-sheets and turns

Some protein and peptide aggregates, such as those of amyloid-β protein (Aβ), are neurotoxic and have been implicated in several neurodegenerative diseases. Aβ accumulates at nanoclusters enriched in neuronal lipids called gangliosides in the presynaptic neuronal membrane, and the resulting oligomeric and/or fibrous forms accelerate the development of Alzheimer's disease. Although the presence of Aβ deposits at such nanoclusters is known, the mechanism of their assembly and the relationship between Aβ secondary structure and topography are still unclear. Here, we first confirmed by atomic force microscopy that Aβ40 fibrils can be obtained by incubating seed-free Aβ40 monomers with a membrane composed of sphingomyelin, cholesterol, and the ganglioside GM1. Using Fourier transform infrared (FTIR) reflection-absorption spectroscopy, we then found that these lipid-associated fibrils contained parallel β-sheets, whereas self-assembled Aβ40 molecules formed antiparallel β-sheets. We also found that the fibrils obtained at GM1-rich nanoclusters were generated from turn Aβ40 Our findings indicate that Aβ generally self-assembles into antiparallel β-structures but can also form protofibrils with parallel β-sheets by interacting with ganglioside-bound Aβ. We concluded that by promoting the formation of parallel β-sheets, highly ganglioside-enriched nanoclusters help accelerate the elongation of Aβ fibrils. These results advance our understanding of ganglioside-induced Aβ fibril formation in neuronal membranes and may help inform the development of additional therapies for Alzheimer's disease.

Ion size effect on counterion condensation around a spherical colloidal particle in a salt-free medium containing only counterions

A theory of counterion condensation around a spherical colloidal particle in a salt-free medium containing only counterions developed by Imai and Oosawa as reported by Busseiron Kenkyu 52:42, 1952 ibid. 59:99, 1953) and later by Ohshima (J Colloid Interface Sci 247:18, 2002) is extended to incorporate the ion size effect on the basis of the modified Poisson-Boltzmann equation using the Carnahan-Starling activity coefficient of counterions. The potential distribution depends on the particle surface charge, the particle volume fraction, and the average counterion volume fraction. It is shown that for a dilute particle suspension, the counterion condensation occurs, leading to a decrease in the particle surface potential when the following two conditions are both satisfied: (i) the scaled particle surface charge is greater than the logarithm of the inverse of the particle volume fraction and (ii) the average counterion volume fraction is much smaller than the particle volume fraction.

From Pickering foams to porous carbonate materials: crack-free structuring in drying ceramics

Particle-stabilized foams have attracted considerable research interest, due to their long term stability (months to years) and the possibility to use them as precursors for production of porous materials with hierarchical porosity. In our previous study [Lesov et al., J. Colloid Interface Sci. 504 (2017) 48–57] we clarified the role of the rheological properties of the foamed suspensions and the type of foam film stabilization in the production of porous silica materials with low mass density and excellent insulating properties. In the current study we extend our approach to prepare lightweight carbonate ceramics with controlled density, shrinkage and good mechanical properties. To prepare the wet foam precursors, we tested a series of eight anionic surfactants which were previously reported to provide sufficient hydrophobization of CaCO3 particles and long-term stability of the liquid foams. From those surfactants, the medium-chain fatty acids led to crack-free porous materials with superior mechanical strength, compared to the conventional surfactants. We study the reasons for the formation of cracks in drying Pickering foams and, on this basis, propose optimal conditions for obtaining dry porous carbonate materials with required porosity. Mechanistic explanations are proposed for the main observed effects.