V. Turco Liveri

Study of the growth of ZnS nanoparticles in water/AOT/n-heptane microemulsions by UV-absorption spectroscopy

Abstract ZnS nanoparticles were synthesized at 25°C using water-containing AOT reversed micelles as nanoreactors and characterized by UV–vis spectroscopy. The time dependence of the spectra emphasizes a slow growing process of the ZnS nanoparticles coupled with a change of their photophysical properties. Both processes are well described by power laws. The nanoparticle size can be controlled by the molar ratio R ( R =[water]/[AOT]), i.e. by the micellar size. The deposits obtained by evaporation of the volatile components of the microemulsions are found to be composed of a surfactant matrix containing ZnS nanoparticles smaller and more stable than that in the corresponding microemulsions.

Mass action model for solute distribution between water and micelles. Partial molar volumes of butanol and pentanol in dodecyl surfactant solutions

The densities of 1-butanol and 1-pentanol were measured in aqueous solutions of dodecyltrimethylammonium bromide and dodecyldimethylamine oxide and the partial molar volumes at infinite dilution of the alcohols in aqueous surfactants solutions were obtained. The observed trends of this quantity as a function of the surfactant concentration were rationalized using a mass-action model for the alcohol distribution between the aqueous and the micellar phase. At the same time, the model was revised to account for the alcohol effect on the surfactant micellization equilibrium. The partial molar volume of alcohols in the aqueous and in the micellar phases and the ratios between the binding constant and the aggregation number were calculated. These thermodynamic quantities are nearly the same in the two surfactants analyzed in this paper but differ appreciably from those in sodium dodecylsulfate. The apparent molar volume of surfactants in some hydroalcoholic solutions at fixed alcohol concentration were also calculated. In the micellization region the trend of this quantity as a function of the surfactant concentration shows a hump, which depends on the alcohol concentration and on the alcohol alkyl chain length. The alcohol extraction from the aqueous to the micellar phase due to the addition of the surfactant can account for the observed trends.

Water structure in water/AOT/n-heptane microemulsions by FT-IR spectroscopy

Abstract FT-IR spectra in the OH stretching region of water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-heptane microemulsions as a function of the water/AOT molar ratio (R) and of the weight fraction (φ) of the micellar phase have been recorded at 25°C. After elimination of the unwanted CH contribution and correction for the water concentration, the shape of the normalized OH bands has been found to be dependent on R and practically independent of φ. The analysis of these bands in terms of “bound” and “bulk” water (representing, as a first approximation, the two water domains within the water pool) allowed the evaluation of the fraction (α) of the “bound” water as a function of R. At each R, α was found to be nearly the same for percolated and unpercolated microemulsions. This suggests that the water/AOT/n-heptane microemulsions well above the percolation threshold are still formed by water-containing AOT reversed micelles dispersed in the oil phase.

Synthesis, size control, and passivation of CdS nanoparticles in water/AOT/n-heptane microemulsions

Abstract CdS nanoparticles have been synthesised by adding tetrabutylammonium hydrogen sulphide to water/sodium bis(2-ethylhexyl) sulfosuccinate/ n -heptane microemulsions containing CdSO 4 . Analysis of UV–Vis absorption spectra recorded at various times indicates that an initial rapid formation of CdS nanoparticles is followed by a very slow growth process which can be well described by a power law. The growth process is totally inhibited by the addition of an appropriate amount of bis(2-ethylhexyl)amine (BEA) leading to the formation of stable nanosized CdS particles coated by an oriented monolayer of chemically bonded BEA molecules. Depending on the BEA addition time, the growth inhibition can be achieved at any moment of the process leading to an easy and fine size control. The main advantage of this procedure is that the BEA-coated CdS nanoparticles can be easily separated from the reaction medium and dispersed in nonpolar media such as heptane and in polar media such as an aqueous surfactant solution.

AFM investigation of gold nanoparticles synthesized in water/AOT/n-heptane microemulsions

Abstract Gold nanoparticles, synthesized within AOT reversed micelles, have been imaged by an atomic force microscope (AFM) operating in the contact mode in air at room temperature. The AFM images show an attractive interaction among gold nanoparticles leading to the formation of well distinct aggregates where, however, each nanoparticle retains its individuality. Moreover, the AFM images show that the mean size of gold nanoparticles is a function of the size of the AOT reverse micelles and offer evidence of a stabilization mechanism of the nanoparticles due, most probably, to the formation of an adsorbed layer of surfactant molecules at the nanoparticle surface. This stabilization mechanism results in being active even at high nanoparticle concentration.

Binding constants and partial molar volumes of primary alcohols in sodium dodecylsulfate micelles

The densities of methanol, ethanol, 1-propanol, 1-butanol and 1-hexanol were measured in aqueous solutions of sodium dodecylsulfate at 25°C. The partial molar volumes of the alcohols at infinite dilution in the aqueous surfactants solutions were calculated and discussed using a mass-action model for the alcohol distribution between the aqueous and the micellar phase. The partial molar volumes of the alcohols in the aqueous and in the micellar phases, and the ratios between the binding constant and the aggregation number, were calculated. The partial molar volume for all the alcohols in micellar phase is 10 cm3-mol−1 smaller than that in octane. This can be related to the strong hydrophilic interaction between the head groups of the alcohol and the micellized surfactant. From the extrapolated values of the distribution constant and the partial molar volumes in the aqueous and micellar phases, the standard partial molar volume of heptanol in micellar solutions was found to decrease with increasing surfactant concentration. The standard free energy of transfer of alcohols from water to micelles was rationalized in terms of hydrophilic and hydrophobic contributions. A model is proposed in which the empty space around each solute is assumed to be the same in the gas and liquid phases, and is used to explain the behavior of micelles in the presence of amphiphilic solutes.

Water-soluble copolymers of an antiviral agent: synthesis and their interaction with a biomembrane model

Abstract Amantadine was modified by direct acylation with succinic and glutaric anhydrides and the covalent bond with α,β-poly( N -hydroxyethyl)-DL-aspartamide (PHEA) was realized. The amount of amantadine in the obtained copolymers was evaluated by hydrolysis of the conjugates. The enthalpic effect due to the interaction between the two synthesized polymeric prodrugs and sodium dodecylsulphate micelles as a simple system mimicking the cellular membrane, was measured by the calorimetric technique and compared with that of PHEA. Binding of PHEA-succinylamantadine (PHEA-S-AMA) to surfactant micelles appeared to be stronger than that shown by PHEA-glutarylamantadine (PHEA-G-AMA).

Mass-action model for solute distribution between aqueous and micellar phases: Mixing enthalpies of alcohols and dodecyltrimethylammonium bromide solutions

Abstract The mixing enthalpies of aqueous solutions of normal alkanols (from methanol to heptanol) and of dodecyltrimethylammonium bromide were measured taking the enthalpy of dilution of the surfactant solutions as the baseline of the mixing process. The measurements were generally made at a given alcohol concentration by systematically changing the surfactant concentration. In some cases measurements as a function of the alcohol concentration were also carried out. Below the CMC from the mixing enthalpies the pair and the triplet interaction parameters between alcohol and surfactant molecules were calculated. The trend of these parameters as a function of the number of carbon atoms in the alcohol alkyl chain is peculiar in that it is linear for the h RS pair parameter and of an exponential-type for h RSS , whereas the h RRS triplet parameter shows a minimum for butanol. Above the CMC the mixing enthalpies were rationalized using a previously reported model for the alcohol distribution between the aqueous and the micellar phases. From the resulting equation the distribution constant and the transfer enthalpy (and then the standard free energy and entropy) can be obtained at the same time. In the calculation of these quantities the role of the alcohol—surfactant interactions in the aqueous phase and of the displacement of the micellization equilibrium due to the added alcohol is pointed out. As predicted, the additivity rule always holds for the standard free energy of transfer but only holds up to butanol for enthalpy and entropy.

Study of AOT-stabilized microemulsions of formamide and n-methylformamide dispersed in n-heptane

Abstract A wide investigation of some physicochemical properties (density, viscosity, conductance, IR spectra, permittivity) of AOT-stabilized dispersions of formamide and n -methylformamide in n -heptane has been performed. The experimental data are consistent with the hypothesis that these highly hydrophilic substances are encapsulated within AOT reversed micelles and that this structure is maintained for both systems well above the volume fraction of the dispersed phase where a percolative transition occurs. In addition, the observed properties of these microemulsions reveal the pivotal role of intermicellar attractive interactions in driving the percolative transition. A marked modification of the structural and dynamical properties of formamide and n -methylformamide accompanying their solubilization within AOT reversed micelles has also been observed.

Calorimetric investigation of the complex formation between surfactants and α-, β- and γ-cyclodextrins

Abstract A calorimetric technique has been used to study complex formation between α-, β- and γ-cyclodextrins (αCD, βCD and γCD) and some surfactants (sodium dodecylsulphate (SDS), hexadecyl trimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl) phenoxypoly(oxyethyleneglycol) (Triton X-100)). The experimental data indicate that some complexes (SDS-αCD, SDS-βCD and CTAB-αCD) are very stable and allow direct determination of their stoichiometry and molar enthalpy of complex formation. Those for other complexes closely fit a model based on an equilibrium reaction between surfactant, cyclodextrin and a single complex. According to the model, data analysis allows determination of the stoichiometry, stability constant and molar enthalpy of their complex formation. The thermodynamic parameters indicate that stoichiometry and complex stability are strongly influenced by entropic contributions.