Stepwise Aggregation Research Articles

The Excellent Water-Solubility of N-[3-(dimethyl-amino)propyl] docosanamide Chloride : The Most Eco-friendly Cationic Hair Conditioning Agent

A new hair-conditioning agent, N-[3-(dimethylamino)propyl]docosanamide (APA-22) hydrochloric acid salt (APA-22 HCl), has excellent solubility in water with a solubility limit over 10-times larger than that of other APA hydrochloric salts with shorter alkyl chain lengths. The physicochemical characteristics of APA-22 HCl were studied at 25 degrees C by using equilibrium surface tension (gamma), solubilization of an oil-soluble dye, steady-state fluorescence, and chloride ion selective electrode techniques. The APA-22 HCl salt is considered to form an aggregate at a concentration, C(1), which is about twice the concentration of the solubility limit of APAs with shorter alkyl chains. At a higher concentration, C(2), the aggregates of APA-22 HCl start forming another kind of aggregate, which is able to solubilize oil-soluble materials. That is, C(1) is considered to represent the critical aggregation concentration (cac) and C(2) a morphology transition. In the concentration region between C(1) and C(2) the solutions are seemingly transparent whereas at above C(2) they appear bluish or translucent. Since the Krafft point of APA-22 HCl is 55 degrees C, all the solutions, including the aggregates, are thought to exist in metastable states. Notably, however, these metastable solutions do not change significantly over a few months. The excellent solubility of APA-22 HCl in water is considered to be a result of this unique stepwise aggregation with increasing concentration.

Thermodynamics and kinetics of competing aggregation processes in a simple model system

A simple model system has been used to develop thermodynamics and kinetics for bulk and surface aggregation processes capable of competing with each other. The processes are the stepwise aggregation of monomers in a fluid medium and on an impenetrable solid surface bounding the fluid medium, besides the adsorption and desorption of the same species at the solid-fluid interface. Emphasis is on aggregation processes in the high friction limit. The theoretical model is used to compare the kinetics and thermodynamics of the processes and to infer the conditions in which one process dominates another, in the high friction limit, such as in a liquid. The motivation of this study is obtaining insight into competition between aggregation in solution and on an adjoining surface, such as a cell membrane.

Ecological scenarios analyzed and evaluated by a shallow lake model

We applied the complex ecosystem model EMMO, which was adopted to the shallow lake Müggelsee (Germany), in order to evaluate a large set of ecological scenarios. By means of EMMO, 33 scenarios and 17 indicators were defined to characterize their effects on the lake ecosystem. The indicators were based on model outputs of EMMO and can be separated into biological indicators, such as chlorophyll-a and cyanobacteria, and hydro-chemical indicators, such as phosphorus. The question to be solved was, what is the ranking of the scenarios based on their characterization by these 17 indicators? And how can we handle high quantities of complex data within evaluation procedures? The scenario evaluation was performed by partial order theory which, however, did not provide a clear result. By subsequently applying the hierarchical cluster analysis (complete linkage) it was possible to reduce the data matrix to indicator and scenario representatives. Even though this step implies losses of information, it simplifies the application of partial order theory and the post processing by METEOR. METEOR is derived from partial order theory and allows the stepwise aggregation of indicators, which subsequently leads to a distinct and clear decision. In the final evaluation result the best scenario was the one which defines a minimum nutrient input and no phosphorus release from the sediment while the worst scenario is characterized by a maximum nutrient input and extensive phosphorus release from the sediment. The reasonable and comprehensive results show that the combination of partial order, cluster analysis and METEOR can handle big amounts of data in a very clear and transparent way, and therefore is ideal in the context of complex ecosystem models, like that we applied.

Kinetics of fast relaxation of cylindrical micelles

On the basis of the kinetic description of stepwise aggregation upon the release and absorption of surfactant monomers by micelles, the complete spectrum of relaxation times is found for relatively fast establishment of a quasi-equilibrium distribution of cylindrical micelles. These times also include the time required for the establishment of the corresponding quasi-equilibrium concentration of surfactant monomers. The resultant times are compared to an earlier diffusion estimate for the characteristic time of the establishment of quasi-equilibrium state of cylindrical micelles in a solution with a surfactant concentration much higher than the second critical micellization concentration CMC2.

Clustering and Cooperative Dynamics in a Reactive System

We study the dependence of the dynamics on the size of particle clusters that grow by stepwise aggregation in a reactive epoxy-amine mixture. The data reveal the cluster property involved in the glasslike arrest and its quantitative link with the structural relaxation time. We find that the number-average cluster size xn governs the formation of a glassy phase as distinct from a gel phase, and that xn correlates to the size of the "cooperatively rearranging regions" postulated by the Adam-Gibbs model for glass forming liquids. These results suggest that the step polymerization process generates clusters that behave much like dynamical heterogeneities observed in supercooled liquids.

Micelle formation of sodium chenodeoxycholate and solubilization into the micelles: comparison with other unconjugated bile salts

Micellization of sodium chenodeoxycholate (NaCDC) was studied for the critical micelle concentration (CMC), the micelle aggregation number, and the degree of counterion binding to micelle at 288.2, 298.2, 308.2, and 318.2 K. They were compared with those of three other unconjugated bile salts; sodium cholate (NaC), sodium deoxycholate (NaDC), and sodium ursodeoxycholate (NaUDC). The I 1/ I 3 ratio of pyrene fluorescence and the solubility dependence of solution pH were employed to determine the CMC values. As the results, a certain concentration range for the CMC and a stepwise molecular aggregation for micellization were found reasonable. Using a stepwise association model of the bile salt anions, the mean aggregation number ( n̄) of NaCDC micelles was found to increase with the total anion concentration, while the n̄ values decreased with increasing temperature; 9.1, 8.1, 7.4, and 6.3 at 288.2, 298.2, 308.2, and 318.2 K, respectively, at 50 mmol dm −3. The results from four unconjugated bile salts indicate that the number, location, and orientation of hydroxyl groups in the steroid nucleus are quite important for growth of the micelles. Activity of the counterion (Na +) was determined by a sodium ion selective electrode in order to confirm the low counterion binding to micelles. The solubilized amount of cholesterol into the aqueous bile salt solutions increased in the order of NaUDC<NaC<NaCDC<NaDC. The first stepwise association or solubilization constants ( K̄ 1) between a cholesterol monomer and a vacant micelle were evaluated at different bile salt concentrations. The constants were also determined for polycyclic aromatic compounds (benzene, naphthalene, anthracene, and pyrene). The corresponding Δ G 0 value was most negative for cholesterol among the solubilizates studied, which indicated that cholesterol was thermodynamically stabilized most by solubilization into the bile salt micelles.

Generalized coarse-grained Becker–D ring equations

We present and apply a generalized coarse-graining method of reducing the Becker–Doring model; originally formulated to describe the stepwise aggregation and fragmentation of clusters during nucleation. Previous formulations of the coarse-graining procedure have allowed a temporal rescaling of the coarse-grained reaction rates; this is generalized to allow the rescaling to depend on cluster size. The form of this factor is derived for general reaction rates and general mesh function so that the steady-state solution is preserved; in the case of an even mesh function the kinetics can also be accurately reproduced. With a size-dependent mesh function the equilibrium solution and the form of convergence to this state are matched for a specific example. Finally we consider reaction rates relevant to the classical nucleation theory of spherical cluster growth, and numerically compare solutions of the full system to the generalized coarse-grained system in both constant monomer and constant mass formulations, demonstrating the accuracy of the method.

Micelle formation of sodium deoxycholate and sodium ursodeoxycholate (Part 1)

Micellization of sodium deoxycholate (NaDC) and sodium ursodeoxycholate (NaUDC) was studied for the critical micelle concentration (CMC), the micelle aggregation number, and the degree of counterion binding to micelle, where sodium cholate (NaC) was used as a reference. The fluorescence probe technique of pyrene was employed to determine accurately the CMC values for the bile salts, which indicated that a certain concentration range of CMC and a stepwise aggregation for micellization were reasonable. The temperature dependences of micellization for NaDC and NaUDC were studied at 288.2, 298.2, 308.2, and 318.2 K by aqueous solubility change with solution pH. Aggregations of the bile salt anions were analyzed using the stepwise association model and found to grow in size with increasing concentration, which confirmed that the mass action model worked quite well. The average aggregation number was found to be 2.5 (NaUDC) and 10.5 (NaDC) at the concentration of 20 mM and at 308.2 K. The aggregation number determined by static light scattering also agreed well with those by the solubility method in the order of size: NaUDC<NaC<NaDC at 308.2 K. The results indicated that the location of the OH group at C-7 and its orientation were the most important factors from the viewpoint of chemical structure for the growth of micelles. The activity measurement for sodium ions was made by a sodium ion selective electrode in order to confirm the low counterion binding to micelles and the validity of the present association model of bile salts, but the model did not hold good for NaDC at higher concentrations.

Studies on the size and stability of chlorpromazine hydrochloride nanostructures in aqueous solution

The mean aggregate number (MAN) of the antipsychotic drug chlorpromazine hydrochloride (CPZ) nanostructure was investigated by fluorescence quenching using 9-methylanthracene (9-MA) as the quencher. The method was designed to take advantage of the intrinsic fluorescent properties of CPZ. The validity of this method was supported by the results obtained for the MAN which was determined to be approximately 37 for a solution of 10 mM CPZ in 0.1 M pH 6.5 phosphate buffer. An increase in the aggregate size with increasing drug concentration confirmed the stepwise aggregation theory of CPZ micelle formation. Differential scanning calorimetry was used to examine the effects of concentration on the thermodynamics of micellization. The enthalpy of demicellization increased with increasing CPZ concentration (5–12 mM), suggesting a greater stability of the aggregates at higher concentrations. At amphiphile concentrations higher than 12 mM, a plateau of approximately 10 kJ/mol was observed as the enthalpy of demicellization. Fluorescence lifetime results revealed a two-component system at low CPZ concentration, while data at amphiphile concentrations higher than 12 mM could not be fitted to either single or multi-component lifetime values, suggesting an increase in dispersity in these nanostructures at higher CPZ concentrations. Temperatures higher than 40°C tend to destabilize the larger micelles, and demicellization was observed after approximately 45°C. Changes in osmotic pressure in the presence of dextrose up to 0.3 M had no significant effect on the size of these micellar nanostructures.

Thermodynamic and aggregation properties of sodium dodecyl sulfate in aqueous binary mixtures of isomeric butanediols

The effect of aqueous binary mixtures of isomeric butanediols on the micellization of sodium dodecyl sulfate has been investigated. Conductivity and fluorescence techniques were employed to determine the critical micellar concentration, the degree of dissociation of the counterions and the aggregation numbers of the surfactants in these binary blends. Differential conductivity plots were employed to distinguish between the cooperative and the stepwise aggregation process of the surfactant in each solvent system. The mass-action model was employed to calculate the hydrophobic and the electrostatic contributions to the Gibbs energy of micellization as well as the monomer and the counterion concentrations in the postmicellar region. The thermodynamic parameters calculated for each system indicate that the micellization process occurs more readily in the presence of cosolvent owing to the formation of mixed micelles.

Hydrophobic Self-Association of Sodium Taurochenodeoxycholate and Tauroursodeoxycholate

The aggregation patterns of sodium taurochenodeoxycholate (TCDC) and tauroursodeoxycholate (TUDC) in an isotonic sodium chloride solution are investigated by frontal chromatography on Sephadex G-10 columns at 298.2 K and are quantitatively analyzed on the basis of stepwise aggregation models. The micellar aggregation numbers, critical micelle concentrations (cmc's), stepwise aggregation constants, and micelle size distributions of TCDC and TUDC are determined and compared to those of sodium taurocholate and taurodeoxycholate. The logarithms of gel−water partition coefficients, dimerization constants, and reciprocal cmc values for the four bile salts exhibit linearity with their hydrophobic molecular surface areas, and the slopes are close to that of the logarithms of reciprocal cmc values against the hydrophobic molecular surface areas of lecithins. This fact shows that the gel adsorption, dimerization, and micellization of bile salts are driven mainly by hydrophobic interactions and that the Small model ...

Stepwise Self-Association of Penicillin V As Deduced by Frontal Derivative Chromatography

The aggregation pattern of penicillin V (PCV) in a 0.15 M potassium chloride solution at 298.2 K is investigated by frontal derivative chromatography on Sephadex G-10 columns and is quantitatively analyzed on the basis of a stepwise aggregation model and a trimer−dodecamer model. The aggregation parameters for these models are best fitted to the observed centroid volume data. The derivative chromatogram of PCV is simulated on the basis of plate theory with the aggregation models. The height and position of the peak simulated on the basis of the stepwise aggregation model are in excellent agreement with the observed ones, whereas those simulated on the basis of the trimer−dodecamer model remarkably differ from the observed ones, particularly in dilute solutions. Thus, PCV forms dimer and self-associates stepwise.

Determination of the aggregation constants of bile salts by capillary electrophoresis

Capillary electrophoresis has been investigated as a novel experimental method for determination of the aggregation constants of surfactants. The tendency of sodium cholate and sodium taurodeoxycholate to associate was studied in phosphate buffers of pH 8.0 and pH 7.0, respectively. Stepwise aggregation equilibria of bile salt monomers has been described in terms of massbalance equations. The Offord equation was used to model the electrophoretic mobility of the bile salt associates, and the experimental mobility values could be fitted to the model. Interestingly, only even-membered aggregates-dimers and tetramers-besides the monomers were proposed from the results of the curve-fitting for both bile salts. The aggregation constants calculated were (in molar units): cholate logK A2=1.37, logK A4=4.98 taurodeoxycholate logK A2=1.68, logK A4=6.46. From these values, more pronounced aggregation of taurodeoxycholate starting at lower concentrations has been deduced, supporting the back-to-back association model of bile salts.

Low-concentration aqueous solutions of undecenoic acid and sodium undecenoate

The behaviors of low-concentration aqueous solutions of 10-undecenoic acid and its sodium salt were studied by several techniques. The acid does not have a critical micelle concentration, but gives an emulsion of very small droplets at (0.8–1) × 10−4 mol dm−3. The emulsion was clearly visible by eye at 0.002 mol dm−3. The sodium salt has a stepwise aggregation process, giving premicellar aggregates at 0.023 ± 0.008 mol dm−3, which grow to form micelles at 0.117 ± 0.007 mol dm−3. The compositions of the solution and the micelles were also studied.

Self-Association Patterns of Sodium Taurocholate and Taurodeoxycholate As Studied by Frontal Derivative Chromatography

The aggregation patterns of sodium taurocholate (TC) and taurodeoxycholate (TDC) in an isotonic sodium chloride solution at 298.2 K are investigated by frontal derivative chromatography on Sephadex G-10 columns and are quantitatively analyzed on the basis of several aggregation models. The aggregation parameters are best fitted to the observed centroid volume data. The derivative chromatogram of TC at 6.006 mM simulated on the basis of a stepwise aggregation model is in excellent agreement with the observed one, whereas the simulation on the basis of the pentamerization model remarkably differs in shape from the observed one. The derivative chromatograms of TDC at 1.500 and 2.003 mM simulated on the basis of another stepwise aggregation model are closer to the observed ones than those of the Bottari−Festa model. Thus, TC and TDC form their dimers and polydisperse micelles. These conclusions are supported by the fitness to the centroid volume data. In light of the present results and current reports, the structure and counterion binding of bile salt micelles are discussed.

Aggregation effects on evaporation and the air/water interface of dodecyltrimethylammonium hydroxide solutions

The system dodecyltrimethylammonium hydroxide (DTAOH)–water was studied by surface tension, ion-selective electrodes and evaporation in an electrobalance. Results confirmed earlier conclusions about a stepwise aggregation mechanism in DTAOH solutions. The aggregation process started at a total concentration CT=(2.51±0.10)×10-4 mol dm-3) which probably corresponds to the formation of dimers. At CT= (1.300±0.041)×10-3 mol dm-3 there was a change in the surface and evaporation behavior, corresponding to the formation of small, fully ionized aggregates which grew with increasing concentration. At CT= (1.108±0.010)×10-2 mol dm-3 the formation of true micelles with hydroxide counterions in the Stern layer did not change significantly the evaporation and adsorption behavior. This means that between this concentration and CT=(3.02±0.28)× M28.8n10-2 mol dm-3, the changes in structure were gradual. At the latter concentration there was a sudden change in the monolayer state at the air/water interface, with a strong surfactant desorption, and a major change in evaporation behavior. The changes are compatible with the formation of few, large aggregates reducing the total concentration of kinetically independent solute units, which in turn increased the activity of the solvent. This phenomenon is in agreement with literature information. The reduction in the evaporation rate of water was mainly due to the reduction of the water activity, caused by colligative effects. The reduction of the effective area available for evaporation had only a slight effect in water evaporation.

The effect of NaOH on dodecyltrimethylammonium hydroxide aggregation in aqueous solutions

The aggregation of dodecyltrimethylammonium hydroxide (DTAOH) in aqueous NaOH solutions was studied as a function of NaOH concentration. As in NaOH-free DTAOH aqueous solutions, the surfactant underwent a stepwise aggregation mechanism. Changes in the structure of aggregates produced an increase of the concentration at which premicellar aggregates could solubilize hydrophobic dyes and also in the concentration at which hydroxide inons join the aggregates.

Nonmicellar Self-Association of Propantheline Bromide: Effect of the Column Length on the Chromatogram

The self-association behavior of propantheline bromide in a 154 mmol dm-3 sodium bromide solution has been investigated by frontal chromatography on two Sephadex columns and by the surface tension method. Analysis of centroid volume data by asymptotic theory yields the monomer concentration of propantheline bromide as a function of its total concentration. Further analysis of the concentration dependence of the monomer concentration by a multiple equilibrium model provides weight-average and number-average aggregation numbers. The aggregates of propantheline bromide are small and are formed in a stepwise manner without a cmc. Stepwise aggregation constants for propantheline bromide are estimated and are used to predict the aggregate size distribution. Derivative chromatograms support the above results based on the centroid volume data. For instance, the shape of the derivative chromatogram of propantheline bromide at the trailing boundary is closer to that of penicillin V than to that of the surfactant, suggesting that propantheline bromide self-associates in a nonmicellar mode. The dimerization constant of propantheline bromide estimated from the trailing peak volume is close to that obtained from the centroid volume. The shape of the frontal chromatogram is almost independent of the size of the column. This fact indicates that our chromatographic method for investigating the self-association behavior is reliable. The surface tension of the propantheline bromide solution has a transition, albeit diffuse, when plotted against the logarithm of the total concentration and decreases gradually with increasing concentration above this transition. A reasonable molecular area of propantheline bromide on the aqueous surface is obtained by using the monomer concentration, determined by the chromatographic data, in the Gibbs adsorption equation.

Self-Association of Penicillins in Aqueous Solution as Revealed by Gel Filtration Chromatography.

The self-association of penicillin G (PCG) and penicillin V (PCV) in 0.15 M potassium chloride solution has been investigated by frontal gel filtration chromatography (GFC) on Sephadex G-10 columns at 25°C. Analysis of the centroid volume, Vc, of a GFC profile yields the monomer concentration of a penicillin and the weight-average aggregation number, nw. No critical micelle concentration is observed in the Vc vs. concentration profile for PCV. The nw value for PCV is 2 at low concentrations and gradually increases with increasing concentration. PCG forms dimers only below 0.06 M, and provides useful GFC data for the investigation of dimerization. The dimerization constants of PCG and PCV are also evaluated from the concentration dependence of the derivative peak position at the trailing boundary. Stepwise aggregation constants, obtained from non-linear least-squares fitting of the centroid volume data, satisfactorily reproduce observed derivative chromatograms, and are used for calculations of the aggregate size distribution of PCV. The structure-association relationship and the effect of self-association on the bactericidal activity and chemical stability of the penicillins are discussed.

Adsorption of aliphatic species as a stepwise aggregation process like micellization

Adsorption of aliphatic species as a stepwise aggregation process like micellization