Kinetics of channel formation in bilayer lipid membranes (BLMs) and tethered BLMs: monazomycin and melittin

The kinetics of channel formation by the polyene-like antibiotic monazomycin, both in a bilayer lipid membrane (BLM) and in a tethered BLM (tBLM), and by the peptide melittin in a tBLM, is investigated. Stepping the applied potential from a value at which channels are not formed to one at which they are formed yields current vs time curves that are sigmoidal on a BLM, while they show a maximum on a tBLM; in the latter case, sigmoidal curves are obtained by plotting the charge against time. These curves are interpreted on the basis of a general kinetic model, which accounts for the potential-dependent penetration of adsorbed monomeric molecules into the lipid bilayer, followed by their aggregation with channel formation by a mechanism of nucleation and growth. In the case of monazomycin, which is present in the solution in the form of relatively hydrophilic clusters and is adsorbed as such on top of the lipid bilayer, penetration into the bilayer following a potential jump is assumed to be preceded by a potential-independent disaggregation of the adsorbed clusters into adsorbed monomers.     

New kinetic model for the rapid step of calcium oxide carbonation by carbon dioxide

Carbonation of solid calcium oxide by gaseous carbon dioxide was monitored by thermogravimetry. A kinetic model of CaO carbonation is proposed in order to interpret the first rapid step of the reaction. By taking into account, the existence of large induction period as well as the sigmoidal shape of the kinetic curves in this kinetic-controlled region, a surface nucleation and isotropic growth kinetic model based on a single nucleus per particle is proposed, and the expressions of the fractional conversion and the reaction rate versus time are detailed. The induction period is found to have a linear variation with respect to temperature and to follow a power law with respect to CO₂ partial pressure. The areic reactivity of growth decreases with temperature increase, and increases with CO₂ partial pressure increase. A mechanism of CaCO₃ growth is proposed to account for these results and to determine a dependence of the areic reactivity of growth on the temperature and the CO₂ partial pressure.     

Electrochemical formation and investigation of a self-assembled [60]fullerene monolayer

The formation and characterisation of a C(60) monolayer at the electrode|electrolyte interface has been studied by cyclic voltammetry, potential step chronoamperometry and ac voltammetry. The presence of the monolayer is evidenced by the presence of a very sharp peak P in the voltammogram, attributed to the faradaic phase formation of an ordered monolayer, and of a reduction post peak Q associated with the reduction of adsorbed species. The chronoamperograms exhibit a well-defined maximum, characteristic of a nucleation and growth mechanism. By comparison with existing models of phase transitions, a progressive polynucleation and growth mechanism is demonstrated. The monolayer is proposed to consist of a 2D fulleride salt. It is suggested that the formation of the monolayer can take place for a broad range of solution compositions, but requires an atomically smooth substrate such as mercury.     

Porosity of silica gels by thermal desorption of liquids

Thermal desorption of liquids was measured using a derivatograph. Alcohols and aliphatic hydrocarbons were used as liquid adsorbates. Pore size distribution curves of silica gels were determined on the basis of thermogravimetric curves using the Kelvin equation. Calculated distributions and total pore volumes were compared with those obtained from the nitrogen method and mercury porosimetry. The influence of heating programs on the shape of the distribution curves is discussed.     

Computer simulation of the kinetic behaviour of surface reactions driven by a linear potential sweep: Part III. Monolayer formation by a nucleation and growth mechanism

The kinetic behaviour of a surface process involving the deposition of a two-dimensional surface film by a nucleation and growth mechanism is treated for the case of linear potential sweep control. Features which distinguish nucleation and growth in monolayer formation from random electrodeposition (Langmuir case) treated in previous papers are emphasized. Two main mechanisms are considered: one where the growth occurs from a fixed surface density of nuclei and the other where growth occurs from a potential-dependent density of nuclei. Computer simulations and some analytical derivations of the kinetic behaviour for these two cases are made and the characteristic kinetic features of the process are deduced, enabling the latter to be distinguished in terms of experimentally accessible criteria. The extent of reversibility of the processes can be usefully expressed in terms of a limiting sweep-rate parameter, s₀, which is related to the rate constant for nucleation or the surface density of nuclei and the rate constant for growth.     

Experimental probes of silver metal nanoparticle formation kinetics: Comparing indirect versus more direct methods

The kinetics of noble metal nanoparticle formation in bottom-up syntheses are important for controlling and optimizing these methods. Hence, experimental probes that are easily accessible to most laboratories are also of interest. We collected kinetic curves for the formation of silver nanoparticles in a modified Turkevich method with citrate acting as the reducing and stabilizing agent by (i) measuring the change in silver nanoparticle surface plasmon resonance by UV-visible spectroscopy, a somewhat indirect method, and then also by (ii) measuring the change in silver ion concentration by ion-selective electrode potentiometry and/or atomic absorption spectroscopy, two more direct methods. The resulting sigmoidal kinetic curves were curvefitted with the Finke-Watzky two-step kinetic model of slow, continuous nucleation and fast autocatalytic growth to extract average rate constants. We found that the kinetic curves obtained by following the change in silver ion concentration were apparent mirror images of those constructed by following the change in nanoparticle surface plasmon resonance, and that their respective curvefits displayed the same sigmoidal characteristics. The resultant values of the rate constants for nucleation and growth overlapped within experimental error between the methods and showed similar trends over the range of citrate concentrations studied. The use of multiple probes in this work to follow the kinetics of nanoparticle formation helps fill a need for the comparison and evaluation of different methods available to scientists, particularly those considered easily accessible.     

Kinetics of Ag300 nanoclusters formation: The catalytically effective nucleus via a steady-state approach

The kinetics of formation of silver nanoparticles consisting of nearly 300 metal atoms is investigated, which were prepared by reduction of silver nitrate with hydrazine in ethylene glycol at 25°C without any stabilizer other than the glycol solvent. The resulting sigmoidal kinetic curves are analyzed by using the 1997 Finke–Watzky two-step mechanism of slow continuous nucleation with subsequent fast autocatalytic surface growth. The kinetics of homogeneous nucleation of metal nanoparticles was analyzed using the assumption about the stepwise adjunction of precursor and the quasi steady-state approximation. The equations were proposed to calculate the concentration of the formed metal nanoparticles and their mean size from the experimentally determined values of the Finke–Watzky rate constants. It is shown that a stepwise nucleation process can be described in the terms of the catalytically effective nucleus concept and that the number of atoms in the catalytically effective nucleus can be estimated.     

Formation and dissolution processes of the 6-thioguanine (6TG) self-assembled monolayer. A kinetic study

This is a report on the kinetics of the destruction and formation processes of the 6-thioguanine self-assembled monolayer (6TG SAM) on a mercury electrode from acid solutions by chronoamperometry. The destruction of the 6TG SAM that has been previously formed under open circuit potential conditions is carried out by stepping the potential from an initial value where the chemisorbed layer is stable up to potentials where the molecules are no longer chemisorbed. The destruction of the SAM has been described by a model that involves three types of contributions: (i) a Langmuir-type adsorption process, (ii) a 2D nucleation mechanism followed by a growth controlled by surface diffusion, and (iii) a 2D nucleation mechanism followed by a growth at a constant rate. The nonlinear fit of the experimental transients by using this procedure allows the quantitative determination of the individual contributions to the overall process. The kinetics of the formation process is studied under electrochemical conditions. The chronoamperometric experiment allows us to monitor the early stages of 6TG SAM formation. The implications of the physisorbed state at low potentials in the type of monolayer formation and destruction processes as well as the influence of temperature are also discussed.     

Nucleation and growtn of polythiophene films on gold electrodes

The nucleation and growth of polythiophene films on gold electrodes has been studied using potentiostatic steps. The mechanism has been deduced and estimates made of the kinetic parameters. Dissolution of the gold substrate at potentials where thiophene polymerisation occurs is suppressed by the initial rapid formation of a monolayer of polymer. The data indicate that formation of bulk film occurs by the instantaneous nucleation and three-dimensional growth of polymer on top of this monolayer. Rate constants for growth parallel to the surface on the bare gold substrate and the covering polymer layer are surprisingly very similar. Growth perpendicular to the surface is slightly more rapid, typically by a factor of 1.5–3, although it is less dependent on potential. The high density of nuclei results in their overlap at an early stage, after which growth is only possible perpendicular to the surface. Within a narrow potential range, the observation of maxima and minima in current-time transients is interpreted in terms of the “death” and “rebirth” of growing centres.     

Size of elementary clusters and process period in silver nanoparticle formation

The time dependence of small-angle X-ray scattering (SAXS) curves for silver nanoparticle formation was followed in situ at a time resolution of 0.18 ms, which is 3 orders of magnitude higher than that used in previous reports (ca. 100 ms). The starting materials were silver nitrate solutions that were reacted with reducing solutions containing trisodium citrate. The SAXS analyses showed that silver nanoparticles were formed in three distinct periods from a peak diameter of ca. 0.7 nm (corresponding to the size of a Ag(13) cluster) during the nucleation and the early growth period. The Ag(13) clusters are most likely elementary clusters that agglomerate to form silver nanoparticles.     

Electrocrystallization of rhodium clusters on thiolate-covered polycrystalline gold

This study reports on the electrochemical deposition of rhodium metal clusters on a polycrystalline gold electrode, modified with a monolayer of dodecanethiol through self-assembly from solution. The deposition process was investigated using cyclic voltammetry, chronoamperometry, and electrochemical quartz crystal microbalance. It is shown that the presence of the thiol monolayer drastically alters the nucleation and growth mechanism compared with the mechanism on the bare gold electrode. The small uncovered gold domains, located at the imperfections in the thiolate monolayer which are induced by the gold nanoroughness, act as nucleation sites for small rhodium clusters. At longer times, these clusters can outgrow the organic monolayer. The resulting surface morphology was characterized by scanning electron microscopy. Rhodium electrocrystallization on the bare gold substrate resulted in an ensemble of a very large amount of very small clusters that are difficult to distinguish from the gold roughness. In contrast, in the presence of a self-assembled monolayer (SAM) of dodecanethiol covalently attached to the gold electrode, the resulting deposit consisted of an ensemble of hemispherical particles. The size distribution of the rhodium particles obtained by using double step chronoamperometry was compared to the ones obtained with cyclic voltammetry and "classical" chronoamperometry. It is shown by X-ray photoelectron spectroscopy that the SAM is still present after rhodium deposition on the thiolate-covered gold substrate. Because the rhodium clusters are directly attached to the gold substrate and can thus easily be electrified, the resulting interface could be used as a composite electrode consisting of a random array of gold supported rhodium nano/microparticles separated from each other by an organic phase. On the other hand, it is shown that the SAM is easily removed by electrochemical oxidation without dissolving the rhodium clusters and, thus, leaving a different array of rhodium clusters on the gold surface compared with the topography obtained in the absence of the SAM. From this point of view, substrate modification with such "removable" organic monolayers was found to be an interesting tool to tune the nano- or microtopography of electrochemically deposited rhodium.     

Density measurement of 1-d confined water by small angle neutron scattering method: pore size and hydration level dependences

Small angle neutron scattering (SANS) is used to measure the absolute density of water contained in 1-D cylindrical pores of a silica material MCM-41-S with pore diameters of 19 and 15 A. By being able to suppress the homogeneous nucleation process inside the narrow pore, one can keep water in the liquid state down to at least 160 K. From a combined analysis of SANS data from both H(2)O and D(2)O hydrated samples, we determined the absolute value of the density of 1-D confined water. We found that the average density of water inside the fully hydrated 19 A pore is 8% higher than that of the bulk water at room temperature. The temperature derivative of the density shows a pronounced peak at T(L) = 235 K signaling the crossing of the Widom line at ambient pressure and confirming the existence of a liquid-liquid phase transition at an elevated pressure. Pore size and hydration level dependences of the density are also studied.     

n-Alkanoic acid monolayers on 316L stainless steel promote the adhesion of electropolymerized polypyrrole films

The formation of a self-assembled monolayer significantly promotes the adhesion of electrodeposited polypyrrole on stainless steel. The monolayer affects the nucleation and growth mechanism of polypyrrole as a result of its hydrophobic nature. This was confirmed by analyzing current-time transients of the initial stages of electropolymerization and was in agreement with AFM images.     

Homogeneous nucleation and growth of melt in copper

Molecular dynamics simulations are conducted to investigate homogeneous nucleation and growth of melt in copper described by an embedded-atom method (EAM) potential. The accuracy of this EAM potential for melting is validated by the equilibrium melting point obtained with the solid-liquid coexistence method and the superheating-supercooling hysteresis method. We characterize the atomistic melting process by following the temperature and time evolution of liquid atoms. The nucleation behavior at the extreme superheating is analyzed with the mean-first-passage-time (MFPT) method, which yields the critical size, steady-state nucleation rate, and the Zeldovich factor. The value of the steady-state nucleation rate obtained from the MFPT method is consistent with the result from direct simulations. The size distribution of subcritical nuclei appears to follow a power law similar to three-dimensional percolation. The diffuse solid-liquid interface has a sigmoidal profile with a 10%-90% width of about 12 A near the critical nucleation. The critical size obtained from our simulations is in reasonable agreement with the prediction of classical nucleation theory if the finite interface width is considered. The growth of melt is coupled with nucleation and can be described qualitatively with the Johnson-Meh-Avrami law. System sizes of 10(3)-10(6) atoms are explored, and negligible size dependence is found for bulk properties and for the critical nucleation.     

Non-activated metal cluster growth during nozzle jet expansion

Based on the classical liquid-drop nucleation theory for small clusters, we show quantitatively the existence of a boundary which separates the nucleation and growth and non-activated (physical spinodal) cluster growth regions for supersaturated vapors. We found that for the expansion geometry such as the one used by Yamada and Takagi in their nozzle jet experiments for producing metal clusters, the non-activated growth process may play an important, if not predominant, role in the growth of clusters.     

Self‐Assembly of Achiral Shape Amphiphiles into Multi‐Walled Nanotubes via Helicity‐Selective Nucleation and Growth

Soft nanotubes are normally constructed from chiral amphiphiles through helical self‐assembly. Yet, how to self‐assemble achiral molecules into nanotubes is still a challenge. Here, we report the nanotube construction with achiral shape amphiphiles through helical self‐assembly and also unravel the formation mechanisms. The amphiphiles have a dumbbell shape and are composed by covalently linking three achiral moieties together: two unlike clusters and an organic tether. The difference in polarity between the unlike clusters drives the amphiphiles to self‐assemble into single‐ and multi‐walled nanotubes as well as intermediates. Analysis of the key intermediates unravels the self‐assembly mechanism of helicity‐selective nucleation and growth. Meanwhile, direct visualization of the individual clusters in the ribbons displays a two‐dimensional deformed hexagonal lattice. Thus, we speculate that it is the lattice deformation that creates anisotropic tension along different directions of the ribbon which further results in the formation of helical ribbons towards nanotubes by amphiphiles.     

Kinetic studies of underpotential deposition of antimony on Se-modified Au electrode

Atomic layers of antimony can be electrodeposited onto the Se monolayer covered Au electrode in the underpotential region. In this paper, the formation and dissolution kinetics of antimony monolayer on the Se monolayer covered Au electrode are investigated using cyclic voltammetry (CV) and chronoamperometry (CA) techniques. Scanning-rate-dependent CV experiments reveal that the peak current of underpotential deposition (UPD) wave of antimony is not a linear function of the scanning rate, υ, but scales as υ ^2/3. Similar behavior is observed when the Antimony monolayer is stripped from the modified substrate. These results indicate the character of monolayer formation and dissolution by a two-dimensional nucleation and growth mechanism. Additionally, current density−time transient obtained through CA experiments also reveal that both the deposition and stripping of the antimony monolayer involve an instantaneous nucleation and two-dimensional growth process.     

Nucleation of ordered solid phases of proteins via a disordered high-density state: phenomenological approach

Nucleation of ordered solid phases of proteins triggers numerous phenomena in laboratory, industry, and in healthy and sick organisms. Recent simulations and experiments with protein crystals suggest that the formation of an ordered crystalline nucleus is preceded by a disordered high-density cluster, akin to a droplet of high-density liquid that has been observed with some proteins; this mechanism allowed a qualitative explanation of recorded complex nucleation kinetics curves. Here, we present a simple phenomenological theory that takes into account intermediate high-density metastable states in the nucleation process. Nucleation rate data at varying temperature and protein concentration are reproduced with high fidelity using literature values of the thermodynamic and kinetic parameters of the system. Our calculations show that the growth rate of the near-critical and supercritical ordered clusters within the dense intermediate is a major factor for the overall nucleation rate. This highlights the role of viscosity within the dense intermediate for the formation of the ordered nucleus. The model provides an understanding of the action of additives that delay or accelerate nucleation and presents a framework within which the nucleation of other ordered protein solid phases, e.g., the sickle cell hemoglobin polymers, can be analyzed.     

Monte Carlo simulations of critical cluster sizes and nucleation rates of water

We have calculated the critical cluster sizes and homogeneous nucleation rates of water at temperatures and vapor densities corresponding to experiments by Wolk and Strey [J. Phys. Chem B 105, 11683 (2001)]. The calculations have been done with an expanded version of a Monte Carlo method originally developed by Vehkamaki and Ford [J. Chem. Phys. 112, 4193 (2000)]. Their method calculates the statistical growth and decay probabilities of molecular clusters. We have derived a connection between these probabilities and kinetic condensation and evaporation rates, and introduce a new way for the calculation of the work of formation of clusters. Three different interaction potential models of water have been used in the simulations. These include the unpolarizable SPC/E [J. Phys. Chem. 91, 6269 (1987)] and TIP4P [J. Chem. Phys. 79, 926 (1983)] models and a polarizable model by Guillot and Guissani [J. Chem. Phys. 114, 6720 (2001)]. We show that TIP4P produces critical cluster sizes and a temperature and vapor density dependence for the nucleation rate that agree well with the experimental data, although the magnitude of nucleation rate is constantly overestimated by a factor of 2 x 10(4). Guissani and Guillot's model is somewhat less successful, but both the TIP4P and Guillot and Guissani models are able to reproduce a much better experimental temperature dependency of the nucleation rate than the classical nucleation theory. Using SPC/E results in dramatically too small critical clusters and high nucleation rates. The water models give different average binding energies for clusters. We show that stronger binding between cluster molecules suppresses the decay probability of a cluster, while the growth probability is not affected. This explains the differences in results from different water models.     

Nucleation and growth in supersaturated monolayers

A nucleation-growth collision theory has been surveyed which can describe the transformation kinetics for formation and growth of three-dimensional centers from supersaturated monolayers theoretically. The models are based on the following key features: (i) single-step nucleation and subsequent growth of the nuclei, (ii) the total transformation rate as convolution of nucleation rate and growth rate, and (iii) overlapping of the growing centres. Two models are reviewed, the first one for the limiting cases of nucleation and growth with assumed geometric shape and the second one for nucleation according to the exponential law and the formation of lenticular centers from a supersaturated monolayer. In the more general second model, the effect of the interfacial tensions at the three-phase contact air/water/center is allowed for both the contact angles of the lenticular center. Application on experimental data enables the determination of nucleation rate constants. Manifold experimental evidence is provided for the adequacy of the nucleation-growth collision theory.