The aggregation in dodecyltrimethylammonium hydroxide aqueous solutions

The aggregation of dodecyltrimethylammonium hydroxide (DTAOH) aqueous solutions has been studied by several methods. It is stepwise and four critical points were found. AtC _T=(2.51±0.10)×10^–4 mol · dm^–3 the surface excess becomes zero, atC _T=(1.300±0.041)×10^–3 mol · dm^–3 small aggregates from, which grow with concentration. AtC _T=(1.108±0.010)×10^–2 mol · dm^–3 true micelles form (CMC) and at (3.02±0.28)×10^–2 mol · dm^–3 the structure of micelles probably changes affecting their properties. The DTAOH micelles are highly ionized (=0.8) at the CMC, and decreases to reach very small values when the total concentration increases.     

Micellization of decyl- and dodecyldimethylbenzylammonium bromides at various temperatures in aqueous solutions

Electrical conductivity and density measurements of aqueous solutions of decyl- and dodecyldimethylbenzylammonium bromides have been determined at various temperatures. From these data the critical micelle concentration (cmc) and the degree of ionization of the micelles were determined. The standard free energy of the micellization process and the minima of the temperature dependent values of the cmc were estimated. From the density data the apparent molal volumes of the monomeric and micellized surfactants as well as the contributions of the methylene and phenylene groups were estimated.     

Evaporation and aggregation in aqueous monododecyltrimethylammonium n-dodecanephosphonate solutions

 Electrobalance evaporation rate measurements were used to measure solute weights in the aqueous catanionic system monododecyltri-methylammonium-n-dodecanephos-phonate. At very low concentration premicelles composed of ion pairs between 3.6 and 7 were found, which increased with concentration. Above the cmc the aggregates increased in size with concentration much more rapidly. Aggregates had 54 ion pairs at the higher studied concentration (6×10^-3 mol dm^-3). This value agrees with literature data of other similar systems. Since the system is probably polydisperse and the evaporation rate method gives number average weights, the true aggregation numbers are probably higher than those found. In this system the cmc did not indicate the starting point of aggregation, but a change in the aggregates structure and growing regime. Received: 23 June 1997 Accepted: 13 August 1997     

Premicellar aggregation of fatty acid N-methylethanolamides in aqueous solutions

The aggregation behaviors of an excellent nonionic foam booster, namely, fatty acid N-methylethanolamide (NMEA-X; X indicates the carbon number of the acyl group), in aqueous solutions have been studied by equilibrium surface tension (gamma), solubilization of oil-soluble dye, and steady-state fluorescence techniques. NMEA, having a longer alkyl chain than NMEA-08, clearly had two break points on the gamma versus log C (where C is concentration) curves. The solubilization of the oil-soluble dye for NMEA aqueous solutions began at the break point of higher concentration in the gamma versus log C curves, so this concentration was confirmed to be the critical micellization concentration (cmc). Above the cmc, however, a separate oil phase of NMEA was observed instead of micelles of limited size. Another break point at lower concentration was also observed in plots of the fluorescence intensity ratio of pyrene, I1/I3, versus log C of NMEA. The gradual decrease of I1/I3 and the appearance of excimer emission of pyrene in the concentration region between the two break points suggest the existence and growth of premicellar aggregates and the solubilization ability of pyrene. Consequently, this break point at lower concentration was assumed to be the critical premicellization concentration (cac). The surface tension reduction in the premicellar region decreased with increasing alkyl chain length of NMEA.     

Salting-in of alcohols in aqueous solutions by tetraalkylammonium bromides at the freezing temperature

The freezing-point depression of the ternary systems tetraalkylammonium bromides-t-butanol-water for the first five homologs of R₄NBr was measured. In the case of Bu₄NBr, the effect of size of the alcohol (methahol ton-butanol) was also investigated. From the corresponding freezing-point data for the binary systems the apparent salting constants were calculated. The true salting constantsk _s were obtained by extrapolation to infinite dilution. These are all very close to zero at the freezing temperature. From the corresponding thermochemical data the temperature dependence ofk _s was calculated, and above 5°C all the R₄NBr salts int-butanol; the salting-in increases with temperature and with the size of the hydrophobic cations. The scaled-particle theory is at present the only one which can account semiquantitatively for the temperature dependence of the salting-in effect. On leave of absence from Chemistry Department, The University, Sheffield S3 7HF, England To whom correspondence should be addressed.     

Investigation of the aggregation capacity of gossypin in aqueous solutions

Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, FAX (3712) 89 14 75. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 572–573, July–August, 1994.     

Kinetics of N-glutaryl-L-phenylalanine p-nitroanilide hydrolysis catalyzed by alpha-chymotrypsin in aqueous solutions of alkyltrimethylammonium bromides

The rate of N-glutaryl-L-phenylalanine p-nitroanilide hydrolysis catalyzed by alpha-chymotripsin has been measured in aqueous solutions of cetyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and dodecyltrimethylammonium bromide at concentrations below and above their critical micellar concentrations (CMC). For the three surfactants considered superactivity was observed, with maximum catalytic efficiencies taking place near the corresponding CMCs. The effect of the surfactants after the CMCs is mostly due to a decreased thermodynamic activity of the substrate due to its incorporation into the micelles. After addition of the surfactants, the Michaelis constant values (corrected to take into account the free substrate concentration) tend to decrease, passing through an ill defined minimum, afterwards reaching a constant value. The catalytic rate constants show the same profiles that the catalytic efficiency, being maxima near the surfactants CMCs. This maximum is more important for the surfactant having the shorter tail. This result is explained by considering that the hydrophobicity of the surfactant influences more the CMC than its association to the enzyme.     

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.     

Phthalocyanine conjugates with carbohydrates: synthesis and aggregation in aqueous solutions

The review is focused on the current state of the synthesis of glycosylated phthalocyanines and their precursors, mono-, di-, and tetraglycosyl phthalonitriles. Features of the synthesis of phthalogen conjugates with carbohydrates and the formation of related metal phthalocyanine complexes are discussed. The formation of aggregates of phthalocyanines in various media exerts a significant effect on the physicochemical and photochemical properties of phthalocyanine macrocycles, which is especially important in the practical use of phthalocyanines as photosensitizes. An aggregation of phthalocyanine conjugates in aqueous solutions strongly decreases the quantum yield of singlet oxygen, thus diminishing the biological activity of these compounds. Specific features of phthalocyanine conjugate aggregation with carbohydrates in aqueous solutions, organic solvents, and some their mixtures are analyzed.     

Femtosecond midinfrared study of aggregation behavior in aqueous solutions of amphiphilic molecules

We study the spectral and orientational dynamics of HDO molecules in aqueous solutions of different concentrations of tertiary butyl alcohol (TBA) and trimethylamine-N-oxide (TMAO). The spectral dynamics is investigated with femtosecond two-dimensional infrared spectroscopy of the O-H stretch vibration of HDO:D(2)O, and the orientational dynamics is studied with femtosecond polarization-resolved pump-probe spectroscopy of the O-D stretch vibration of HDO:H(2)O. Both the spectral and orientational dynamics are observed to show bimodal behavior: part of the water molecules shows spectral and orientational dynamics similar to bulk liquid water and part of the water molecules displays a much slower dynamics. For low solute concentrations, the latter fraction of slow water increases linearly as a function of solute molality, indicating that the slow water is contained in the solvation shells of TBA and TMAO. At higher concentrations, the fraction of slow water saturates. The saturation behavior is much stronger for TBA solutions than for TMAO solutions, indicating the aggregation of the TBA molecules.     

Concentration-dependent aggregation patterns of conjugated bile salts in aqueous sodium chloride solutions

Based on light scattering intensity measurements, a critical concentration for micelle formation can be assigned to sodium taurodeoxycholate in aqueous electrolyte solutions. For sodium taurocholate a progressive aggregation even at very low concentrations of bile salt is indicated. Surface tension and diffusion coefficients are also reported.     

Slow aggregation and disaggregation of TiO2 nanocrystals in aqueous HCl solutions

The kinetics of slow aggregation and disaggregation of the plate-shaped TiO₂ nanocrystals in aqueous HCl solutions at room temperature was studied using turbidimetry and small-angle X-ray scattering. The rate of slow aggregation of the diluted TiO₂ hydrosols with 0.3 ≤ pH ≤ 3 was compared. The sols with pH about 1 were the most stable and the aggregation rate increased at the decrease of pH from 1 to 0.3 and at the increase of pH from 1 to 3. The reversible loose aggregates slowly formed at the low pH, while the more stable aggregates grew at higher pH.     

Studies on the aggregation behaviour of some direct dyes in aqueous solutions

Summary Physicochemical properties of the two direct dyes ChrysophenineG and Chlorazol Sky Blue FF in aqueous solutions, have been studied using solubility and electrical conductivity methods. The results indicate that in aqueous solutions, both dyes in the presence of added sodium chloride start forming aggregates at a critical dye concentration. This critical concentration is a function of the temperature and the concentration of added sodium chloride. An analysis of the conductivity data shows that in all solutions containing dye aggregates the concentration of single dye ions is constant and equal to the critical cocentration. Since the solubility of ChrysophenineG unlike that of Chlorazol Sky Blue FF, is very small in the presence of added sodium chloride, the solubility product principle has been used to experimentally determine mean ionic activity coefficients for the dye ions. These values have been compared with activity coefficients calculated using a suitable form of theDebye-Hückel expression. The results indicate that ChrysophenineG in aqueous solutions behaves as a strong unaggregated 12 valence type electrolyte in dilute solutions below the critical concentration for aggregate formation.Finally, a close correlation has been shown to exist between the critical dye concentrations for aggregate formation and the equilibrium dye bath concentrations at which the aqueous solution adsorption isotherms for these dyes on cotton, viscose and cuprammonium rayon fibres show a marked change in slope leading to the formation of a flat plateau region.     

Degree of aggregation of indocyanine green in aqueous solutions determined by Mie scattering

Indocyanine green in water and aqueous NaCl solutions forms large J-aggregates. The degree of aggregation is estimated by Rayleigh scattering and Mie scattering analysis. The refractive index of the aggregates responsible for scattering is deduced from an aggregate absorption cross-section spectrum using the Kramers-Kronig relations. The average degree of aggregation for indocyanine green in water is found to be of the order of 10⁷ while it rises to 10⁹ for indocyanine green in 0.01 molar aqueous NaCl solution.     

Slow salt-induced aggregation of citrate-covered silver particles in aqueous solutions of cellulose derivatives

In this work, the salt-induced aggregation of bare and polymer-covered silver particles has been studied with the aid of light scattering and UV-visible spectroscopy. Light scattering on a suspension of bare silver particles at a low salt concentration shows that the cluster fractal dimension d _f changes from 1.6 to 2 in the course of the aggregation process, whereas no restructuring of the clusters is observed at a higher salinity where d _f ≈ 1.6. The growth of the clusters over time can be described by a power law R _h ∝ t ^α , where R _h is the apparent hydrodynamic radius. The UV-visible experiments revealed that increasing the size of the bare silver particles lead to a significant broadening and red-shift of the absorbance band, whereas for salt-induced growth of Ag clusters, a blue-shift and broadening was observed. Addition of salt to a suspension of silver particles and hydroxyethylcellulose divulged a slower broadening of the surface plasmon peak than without polymer.     

Viscosities of aqueous solutions of quaternary phosphonium bromides at 15, 25 and 35°C

The viscosities of aqueous solutions of Bu _4-n Ph _n PBr n=0–4 have been measured over the concentration range 0.01–0.2 M at 15, 25 and 35°C, to examine the differences in ion-water interactions of these quaternary cations. The viscosity data were treated by the extended Jones-Dole equation, including the square term in solute concentration, Dc². A decrease in the B coefficient with increase in the number of phenyl groups n, and with increasing temperature is observed. Nearly linear dependence of the B coefficient on n was found, in contrast with partial molal volumes for which non-linear relations with n have been found. D decreases with increasing temperature but becomes independent of n at higher temperatures.     

Interactions of alkyl triphenyl phosphonium bromides with aqueous solutions of LM200 and JR400 polymers

The interactions of the cationic cellulose ether derivatives JR400 and the hydrophobically modified derivative LM200 with cationic alkyl triphenylphosphonium bromides, were investigated in aqueous media. Conductometric, tensiometric and fluorescence techniques were employed in this study. The presence of polymer induced surfactant aggregation and polymer bound aggregates, were detected for C₁₆ and C₁₄ triphenylphosphonium bromide surfactants with LM200. Gibbs free energy of transfer and the dielectric constant values sensed by the fluorescent probe at the micellar interface were evaluated and discussed in terms of strength of interaction between the polymers and surfactants.     

Thermodynamic studies for aqueous solutions involving 18-crown-6 and alkali bromides at 298.15 K

Osmotic vapor pressure measurements have been carried out for three ternary systems, H₂O + 0.2 m 18-crown-6 + NaBr, H₂O + 0.2 m 18-crown-6 + KBr and H₂O + 0.2 m 18-crown-6 + CsBr at 298.15 K using vapor pressure osmometry. The concentration of salts was varied between 0.04 and 0.6 m. The measured water activities were used to calculate the activity coefficient of water, 18-crown-6 and mean molal activity coefficients of ions. The lowering of activity coefficients of one component in presence of other is attributed to the existence of host–guest type complex equilibria in solution phase. The Gibbs transfer free energies, which have been calculated using the activity data, were used to estimate the McMillan–Mayer pair and triplet interaction parameters and are compared with that of alkali chlorides reported recently by us using similar studies. The pair interaction parameters, g_NE (non-electrolyte–electrolyte interaction coefficient), are used to obtain the thermodynamic equilibrium constant values for 18-crown-6:M⁺ complexes, which on comparison with alkali chlorides indicate that the counter anions plays a definite role in stabilizing such complexes in solution phase. Sign and the magnitude of triplet interaction parameters (g_NNE or g_NEE) show that along with electrostatic interactions hydrophobic effects also play an important role in stabilizing the host–guest type complexes.