Giuseppe Briganti | Università degli Studi "La Sapienza" di Roma (original) (raw)

Papers by Giuseppe Briganti

Il Nuovo Cimento D, 1994

Summary Small-angle neutron scattering (SANS) measurements on the large 50S ribosomal subunit of... more Summary Small-angle neutron scattering (SANS) measurements on the large 50S ribosomal subunit of the extreme thermophile archaebacterium Sulfolobus solfataricus are presented. The structural effects induced by magnesium ions and temperature are investigated. Supramolecular aggregation was present in all the investigated conditions. Removal of magnesium ions and increase of temperature reduce the intensity of the scattering at Q=0. This reduction is discussed

Journal of Physics-condensed Matter, 2010

The thermal response of three proteins with mesophilic, thermophilic and hyperthermophilic charac... more The thermal response of three proteins with mesophilic, thermophilic and hyperthermophilic character hints at the essential role played in thermostability by the protein-water interface. The formation of spanning water clusters enveloping the macromolecule and their resistance to thermal stress is shown to correlate with the charge distribution at the protein surface; in particular our findings suggest an effective role of

Langmuir, 2004

Hydration of a spherical micelles of C12E6 in solution is studied by molecular dynamics simulatio... more Hydration of a spherical micelles of C12E6 in solution is studied by molecular dynamics simulation. The interface is found to be separated in an inner part composed of water and hydrophobic and hydrophilic moieties and an outer part with hydrophilic moiety and water only. Hydration numbers in the inner and in the outer parts are in excellent agreement with experimental data from various different methods. Temperature dehydration occurs in the inner region only and is related to the presence of water molecules directly in contact with the hydrophobic core at low temperature.

Physical Review Letters, 2004

A model protein and surrounding water have been investigated at different temperatures. We have d... more A model protein and surrounding water have been investigated at different temperatures. We have detected an anomalous compression of the protein near the freezing point of water---a compression not obviously related to the negative thermal expansion of the solvent. Moreover, the physiological protein working temperature (T=300 K) appears to be related to the activation of exchange of vicinal water with

Journal of Physical Chemistry B, 1999

ABSTRACT Aqueous solutions of a nonionic surfactant, n-dodecylhexaoxyethylene glycol [C12H25 (OCH... more ABSTRACT Aqueous solutions of a nonionic surfactant, n-dodecylhexaoxyethylene glycol [C12H25 (OCH2CH2)6 OH], termed C12E6, have been studied in the mole ratio range W0 = 5.6−14.9, where W0 = [D2O]/[C12E6]. There, below a given temperature, a smectic lamellar phase exists. As shown by 1H and 2H NMR spectra, cooling induces the isotropic phase to become an almost completely isooriented lamellar phase when the magnetic field strength is high (14.1 T). In presence of high magnetic fields, the phase diagram is slightly different with respect to the originally reported one, and the coexistence of an isotropic phase and of the isooriented lamellar one can be observed. In a wide temperature range, only the isooriented lamellar phase is present; it is stable out of the magnetic field for several months. A careful study of the 2H splitting as a function of temperature and concentration indicates the onset of fluctuating cylinder inside the lamellar phase and defines their onset and region of existence. Optical polarizing microscopy experiments confirm the above observation, showing the occurrence of fine grained, mosaic optical textures.

The temperature dehydration of a C 12 E 6 spherical micelle is characterized through the study of... more The temperature dehydration of a C 12 E 6 spherical micelle is characterized through the study of the structure and dynamics of the hydrogen bonds formed by water within the micellar interface. Water molecules in proximity of the hydrophilic fragment of the C 12 E 6 surfactants form strong H-bonds with the oxyethilene units E and with the polar alcoholic heads. The activation energies of such H-bonds fall in the range 2-3 Kcal mol -1 . On the exposed oil core, the number of water-water H-bonds decreases as an effect of dehydration. The dynamics of such bonds exhibits a slow relaxation with respect to the bulk, and two time scales can be discerned: the first one, τ = 3-6 ps, is typical of water-water H-bonds around small hydrophobic molecules, whereas the second one, τ = 40-80 ps, is probably due to the confining effect of the long hydrophilic fragments which reduces the probability of a water molecule to leave the hydration layer of the exposed oil core. Water molecules around the core form H-bond clusters whose size and distribution change with temperature. From a cluster analysis, the system appears to be below the percolation threshold, suggesting that the exposed oily surface is formed by disconnected patches of size around 1 nm 2 , close to the estimate of the solvated hydrophobic patches on protein surfaces. The network connectivity is also considered for concentric hydration shells along the interface: it turns out that near the oil core, the cluster size is larger than elsewhere in the interface demonstrating a strong structural effect induced by the exposed hydrocarbon tails. Temperature affects the cluster size only in the innermost shell.

Two mesophilic/thermophilic variants of the G-domain of the elongation factor Tu were studied via... more Two mesophilic/thermophilic variants of the G-domain of the elongation factor Tu were studied via molecular dynamics simulations. By analyzing the simulation data via the Voronoi space tessellation, we have found that the two proteins have the same macromolecular packing, while the water-exposed surface area is larger for the thermophile. A larger coordination with water is probably due to a peculiar corrugation of the exposed surface of this species. From an enthalpic point of view, the thermophile shows a larger number of intramolecular hydrogen bonds, stronger electrostatic interactions, and a flatter free-energy landscape. Overall, the data suggest that the specific hydration state enhances macromolecular fluctuations but, at the same time, increases thermal stability.

A jointed experimental and computational study of binary and ternary blends of syndiotactic polys... more A jointed experimental and computational study of binary and ternary blends of syndiotactic polystyrene (sPS) and high density polyethylene (HDPE) in presence of a compatibilizant (SEBS) is reported. Small angle neutron scattering and Monte Carlo simulations have revealed that the structure factor of the blends is strongly dependent on the confinement and the degree of crystallinity of the single components. The interpretation of the known mechanical properties of the systems are given in terms of a structural arrangement where localized crystallites are distributed all over the blends and SEBS is located at the interface of the two immiscible components. This picture envisages a rapid growth of the crystalline phases that is even more efficient in the ternary rather than in the binary blends due to the elimination of discontinuities at the sPS-HDPE interface.

Journal of Physical Chemistry A, 2000

Density measurements have been performed on water solutions of nonionic surfactants oligooxyethyl... more Density measurements have been performed on water solutions of nonionic surfactants oligooxyethylene glycol-monoether (C 12 E j with j ) 5, 6, 7, 8) in a wide range of temperatures and concentrations. The densities of the pure surfactants in their liquid state were measured too. The observed values are almost a linear combination of the densities of an oil and of an oxyethylene (EO) bulk liquid phases. The deviation from ideality reduces as j increases and may be reflects an entropic contribution due to a partial mixing of the oil and EO termination. A temperature (T cross ), at which the C 12 E j aqueous solution density coincides with the solvent one up to 50 wt % has been found in all the E j species. The T cross of the different E j results to be scaling temperatures for the excess density of these surfactants. The sphere-to-rod transition temperatures are scaling temperatures too; thus it is expected that the differences between these two temperatures be constant for each of the E j species. In the case of j ) 6, 7, and 8 the values of the sphere-to-rod transition temperatures are known and these differences are all about 20°C. For C 12 E 5 only rods were experimentally observed and, coherently, the previous consideration led to a sphere-to-rod transition temperature of -7°C. The difference between the critical and the crossing temperatures is constant too, at about 15°C for j < 8, whereas for j ) 8 it is 7°C. In this case the densities of the separated phases, two degrees above the critical temperature, result to be the same, within 10 -6 g/cm 3 . It implies that, when slightly above the critical temperature, C 12 E 8water solution undergoes phase separate as it was in microgravitational condition. In our analysis, the C 12 E j solution is considered a three-component system: the oil core, the bulk water, and the region containing the aggregate interface, a mixture of water molecules, and EO units. Fixing the density of the oil core and of the bulk water phase respectively equal to those of pure dodecane oil and water at the same temperature, we obtain the density and the molar volume of the micellar interface. These data indicate that the amount of water molecules per EO segment decreases with the number of hydrophilic unit and with temperature. The same conclusion follows looking at the temperatures at which the maximum densities as a function of surfactant species and concentrations occur .

Colloids and Surfaces A-physicochemical and Engineering Aspects, 2005

In this article we present a combined analysis of volumetric [1] and molecular dynamics (MD) [2] ... more In this article we present a combined analysis of volumetric [1] and molecular dynamics (MD) [2] determinations of the interfacial properties of non-ionic surfactant aggregates belonging to the family of the oligooxyethylene glycol (CiEj). The volumetric experiments were made on surfactant having i=12 and j ranging from 5 to 8 EO unit and concern the density, and the sound velocity

By means of density, dielectric spectroscopy and sound velocity measurements we perform a systema... more By means of density, dielectric spectroscopy and sound velocity measurements we perform a systematic study on the polyoxyethylene C12E6C_{12}E_{6}C12E6 nonionic surfactant solutions as a function of temperature and concentration. Both density and sound velocity data, at about 34circC34^{\circ}C34circC, coincide with the value obtained for pure water. Above this temperature the density is lower than the water density whereas below it

Journal of Physical Chemistry C, 2010

ABSTRACT A combined analysis of experiments, small-angle neutron and X-ray scattering, transmissi... more ABSTRACT A combined analysis of experiments, small-angle neutron and X-ray scattering, transmission electron microscopy (TEM), mass density, and thermogravimetric measurements (TGA), was applied to gold nanoparticles (NP) grafted with thiol-terminated poly(ethylene glycol). TEM micrographs indicate that the gold core diameter is ca. 20% polydisperse. The associated polydispersion in volume dictates that the nanoparticle extensive properties thus determined, such as the number of gold atoms and polymer chains, have similar errors (ca. 30%). On the other hand, by combining the results of mass density and TGA the following intensive properties of the nanoparticle can be determined with a precision of 1%: (i) the volume fraction occupied by the gold core, (ii) the density of the nanoparticle, and (iii) the polymer terminations at the interface. An accurate characterization of the structure and hydration of NP is then obtained by fitting the scattering data and use of the density and TGA data to impose constraints on the fits. The scattering data can be fitted with a core−two shell model. The inner core contains only gold atoms and the two shells represent the interfacial region containing the polymer and the hydrating water molecules. The two shells differ in the local solvent−polymer composition: the inner shell is composed almost exclusively of polymer, whereas the outer is hydrated (about 60% of the volume occupied by polymers). Interesting differences have been observed with isotopic substitution in the solvent (H2O/D2O ratio).

Colloids and Surfaces B-biointerfaces, 1996

Electrical conductivity and dielectric dispersions have been measured on suspensions of 30S, 50S ... more Electrical conductivity and dielectric dispersions have been measured on suspensions of 30S, 50S subunits and 70S ribosomal particles from E. coli, in the presence and absence of magnesium ions. Conductivity measurements show a net contribution due to the disperzed phase. Ribosomal particles act in general as obstructants to the ionic motions in the solution, but besides this effect, there is

The Journal of Physical Chemistry B, 2004

Density and sound velocity measurements were performed on water solutions of the nonionic surfact... more Density and sound velocity measurements were performed on water solutions of the nonionic surfactant belonging to the family of the oligooxyethylene glycol (C i E j ), with i ) 12 and j ranging from 5 to 8 EO units. Up to a threshold concentration, the density (F) and the compressibility coefficient ( ) F -1 c -2 ) measured as a function of the temperature cross the density and the compressibility coefficient of water at the temperatures (T F and T ) which depend on the surfactant. Such conditions are proper of ideal solutions; thus, our watersurfactant solutions can be analyzed in this context as ideal mixtures of the pure solvent and effective solutes. We microscopically model such effective solutes as hydrated monomers assembled in micellar aggregates. Under some assumptions on the volumetric properties of the hydrophobic micellar core, we experimentally determine the molar volume of the EO terminations at the micellar interface. By comparing these values with those obtained for solutions of water and PEG (a polymer composed by j EO units), we estimated the degree of hydration of the interfacial EO-water mixture. The compressibility K ) V obtained from our model embodies a contribution due to the derivative of the number of the hydrated water molecules with respect to the pressure. Such derivative is characteristic of a solution in osmotic equilibrium with its solvent and, in the case of the micelles, can be rationalized on the bases of the "small system thermodynamic" theory. By comparing the compressibility of the micellar interface with that of an equivalent PEG-water solutions, we estimate the osmotic contributions for each of the investigated surfactant. The threshold concentrations, below which these solutions mix ideally, reflect an effect due to the different sizes of the solute aggregates and the solvent molecules.

The Journal of Physical Chemistry B, 2006

We present an extensive set of measurements of steady shear viscosity (η°s), longitudinal elastic... more We present an extensive set of measurements of steady shear viscosity (η°s), longitudinal elastic modulus (M') and ultrasonic absorption (α) in the one-phase isotropic liquid region of the nonionic surfactant C 12 E 8 aqueous solutions. Within a given temperature interval, this phase extends along the entire surfactant concentration range, that could be fully covered in the experiments. In agreement with previous studies, the overall results support the presence of two separated intervals of concentration corresponding to different structural properties. In the surfactant-rich region the temperature dependence of η°s follows an equation characteristic of glass-like systems. The ultrasonic absorption spectra show unambiguous evidence of viscoelastic behaviour that can be described by a Cole-Cole relaxation formula. In this region, when both the absorption and the frequency are scaled by the static shear viscosity (η°s), the scaled attenuation reduces to a single universal curve for all temperatures and concentrations. In the water-rich region the behaviour of η°s, M' and α are more complex and reflect the presence of dispersed aggregates whose size increases with temperature and concentration. At these concentrations the ultrasonic spectra are characterized by a multiple decay rate. The high frequency tail falls in the same frequency range seen at high surfactant content and exhibits similar behaviors. This contribution is ascribed to the mixture of hydrophilic terminations and water present at the micellar interfaces that resembles the condition of a concentrated polymer solution. An additional low frequency contribution is also observed, which is ascribed to the exchange of water molecules and/or surfactant monomers between the aggregates and the bulk solvent region.

The Journal of Physical Chemistry A, 2000

Density measurements have been performed on water solutions of nonionic surfactants oligooxyethyl... more Density measurements have been performed on water solutions of nonionic surfactants oligooxyethylene glycol-monoether (C 12 E j with j ) 5, 6, 7, 8) in a wide range of temperatures and concentrations. The densities of the pure surfactants in their liquid state were measured too. The observed values are almost a linear combination of the densities of an oil and of an oxyethylene (EO) bulk liquid phases. The deviation from ideality reduces as j increases and may be reflects an entropic contribution due to a partial mixing of the oil and EO termination. A temperature (T cross ), at which the C 12 E j aqueous solution density coincides with the solvent one up to 50 wt % has been found in all the E j species. The T cross of the different E j results to be scaling temperatures for the excess density of these surfactants. The sphere-to-rod transition temperatures are scaling temperatures too; thus it is expected that the differences between these two temperatures be constant for each of the E j species. In the case of j ) 6, 7, and 8 the values of the sphere-to-rod transition temperatures are known and these differences are all about 20°C. For C 12 E 5 only rods were experimentally observed and, coherently, the previous consideration led to a sphere-to-rod transition temperature of -7°C. The difference between the critical and the crossing temperatures is constant too, at about 15°C for j < 8, whereas for j ) 8 it is 7°C. In this case the densities of the separated phases, two degrees above the critical temperature, result to be the same, within 10 -6 g/cm 3 . It implies that, when slightly above the critical temperature, C 12 E 8water solution undergoes phase separate as it was in microgravitational condition. In our analysis, the C 12 E j solution is considered a three-component system: the oil core, the bulk water, and the region containing the aggregate interface, a mixture of water molecules, and EO units. Fixing the density of the oil core and of the bulk water phase respectively equal to those of pure dodecane oil and water at the same temperature, we obtain the density and the molar volume of the micellar interface. These data indicate that the amount of water molecules per EO segment decreases with the number of hydrophilic unit and with temperature. The same conclusion follows looking at the temperatures at which the maximum densities as a function of surfactant species and concentrations occur .

Langmuir, 2001

... Fabio Sterpone, † Giuseppe Briganti, § and Carlo Pierleoni*. ... Methods. Model. We adopt an ... more ... Fabio Sterpone, † Giuseppe Briganti, § and Carlo Pierleoni*. ... Methods. Model. We adopt an all-atom model throughout this work. For the C 12 system, we exploited the force field of Tobias, Tu, and Klein 39-41 for the dodecane chains and the SPC model for the water. ...

Biophysical Journal, 2005

Size distribution of dimyristoylphosphatidylcholine liposome suspensions was investigated by dyna... more Size distribution of dimyristoylphosphatidylcholine liposome suspensions was investigated by dynamic-light scattering (DLS) as a function of the sonication time (t s ). Cumulant expansion (second-and third-order) and regularized Laplace inversion (CONTIN) of dynamic single-angle laser light-scattering data were performed. With both methods, the intensityweighted mean hydrodynamic radius AErae I depended on the investigated lengthscale. The number-weighted mean hydrodynamic radius (AErae N ), obtained from CONTIN by modeling dimyristoylphosphatidylcholine vesicles as thin-walled hollow spheres, resulted as independent on the lengthscale. However, the AErae N value obtained from cumulant expansions remained lengthscaledependent. Therefore, the number-weighted radius distribution function is highly asymmetric. The number-weighted mean radius, the standard deviation, and the number-weighted radius at the peak ðr peak N Þ all decreased to a plateau when increasing sonication time. At t s longer than 1 h, the r peak N compares well with the radius of unilamellar vesicles in equilibrium with monomers predicted on a thermodynamic basis. The reliability of our analysis is proved by the comparison of experimental Rayleigh ratios with simulated ones, using the normalized number-weighted radius distribution function p N (r) determined by DLS data. A perfect agreement was obtained at longer sonication times, and the average aggregation number was determined. At lower t s values, simulations did not match experimental data, and this discrepancy was ascribed to the presence of large and floppy unilamellar vesicles with ellipsoidal shapes. Our investigation shows that, from single-angle DLS data, the radius distribution function of the vesicles can only be obtained if p N (r) is known.

Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression, 1993

Fluorescence intensity measurements of ethidium bromide (EB) bound to ribosomal RNA (rRNA) in sus... more Fluorescence intensity measurements of ethidium bromide (EB) bound to ribosomal RNA (rRNA) in suspensions of 30S and 50S subunits, of 70S ribosomal particles and of protein-flee extracted rRNA are presented. Changes in the intercalation of EB reflect changes in conformation and degree of exposure of rRNA. The effect of removal of magnesium ions on the binding of EB is compared in protein-free rRNA and in ribosomal particles by a Scatchard plot analysis. In free ribosomal RNA the number of bound EBs do not depend on magnesium content, only the association constant is affected. In intact 70S particles and both in the separated 50S and 30S subunits the presence of magnesium greatly reduces binding of EB and no saturation of the fluorescence intensity with rRNA concentration is observed, preventing a Scatchard plot analysis. Removal of magnesium restores a strong EB intercalation. Then magnesium ions induce a conformational change in the 70S particles as well as in the separated subunits. The different behavior of the free-rRNA and of the ribosomal particles indicates that ribosomal proteins are relevant to the structural changes induced by magnesium ions. The comparison of the number of excluded sites and of the association constant in the 30S, 50S subunits and in the 70S particles indicates that even without Mg 2+ ions the two subunits still interact, at variance with the commonly shared opinion that subunits dissociation takes place at low magnesium concentration.

Il Nuovo Cimento D, 1994

Summary Small-angle neutron scattering (SANS) measurements on the large 50S ribosomal subunit of... more Summary Small-angle neutron scattering (SANS) measurements on the large 50S ribosomal subunit of the extreme thermophile archaebacterium Sulfolobus solfataricus are presented. The structural effects induced by magnesium ions and temperature are investigated. Supramolecular aggregation was present in all the investigated conditions. Removal of magnesium ions and increase of temperature reduce the intensity of the scattering at Q=0. This reduction is discussed

Journal of Physics-condensed Matter, 2010

The thermal response of three proteins with mesophilic, thermophilic and hyperthermophilic charac... more The thermal response of three proteins with mesophilic, thermophilic and hyperthermophilic character hints at the essential role played in thermostability by the protein-water interface. The formation of spanning water clusters enveloping the macromolecule and their resistance to thermal stress is shown to correlate with the charge distribution at the protein surface; in particular our findings suggest an effective role of

Langmuir, 2004

Hydration of a spherical micelles of C12E6 in solution is studied by molecular dynamics simulatio... more Hydration of a spherical micelles of C12E6 in solution is studied by molecular dynamics simulation. The interface is found to be separated in an inner part composed of water and hydrophobic and hydrophilic moieties and an outer part with hydrophilic moiety and water only. Hydration numbers in the inner and in the outer parts are in excellent agreement with experimental data from various different methods. Temperature dehydration occurs in the inner region only and is related to the presence of water molecules directly in contact with the hydrophobic core at low temperature.

Physical Review Letters, 2004

A model protein and surrounding water have been investigated at different temperatures. We have d... more A model protein and surrounding water have been investigated at different temperatures. We have detected an anomalous compression of the protein near the freezing point of water---a compression not obviously related to the negative thermal expansion of the solvent. Moreover, the physiological protein working temperature (T=300 K) appears to be related to the activation of exchange of vicinal water with

Journal of Physical Chemistry B, 1999

ABSTRACT Aqueous solutions of a nonionic surfactant, n-dodecylhexaoxyethylene glycol [C12H25 (OCH... more ABSTRACT Aqueous solutions of a nonionic surfactant, n-dodecylhexaoxyethylene glycol [C12H25 (OCH2CH2)6 OH], termed C12E6, have been studied in the mole ratio range W0 = 5.6−14.9, where W0 = [D2O]/[C12E6]. There, below a given temperature, a smectic lamellar phase exists. As shown by 1H and 2H NMR spectra, cooling induces the isotropic phase to become an almost completely isooriented lamellar phase when the magnetic field strength is high (14.1 T). In presence of high magnetic fields, the phase diagram is slightly different with respect to the originally reported one, and the coexistence of an isotropic phase and of the isooriented lamellar one can be observed. In a wide temperature range, only the isooriented lamellar phase is present; it is stable out of the magnetic field for several months. A careful study of the 2H splitting as a function of temperature and concentration indicates the onset of fluctuating cylinder inside the lamellar phase and defines their onset and region of existence. Optical polarizing microscopy experiments confirm the above observation, showing the occurrence of fine grained, mosaic optical textures.

The temperature dehydration of a C 12 E 6 spherical micelle is characterized through the study of... more The temperature dehydration of a C 12 E 6 spherical micelle is characterized through the study of the structure and dynamics of the hydrogen bonds formed by water within the micellar interface. Water molecules in proximity of the hydrophilic fragment of the C 12 E 6 surfactants form strong H-bonds with the oxyethilene units E and with the polar alcoholic heads. The activation energies of such H-bonds fall in the range 2-3 Kcal mol -1 . On the exposed oil core, the number of water-water H-bonds decreases as an effect of dehydration. The dynamics of such bonds exhibits a slow relaxation with respect to the bulk, and two time scales can be discerned: the first one, τ = 3-6 ps, is typical of water-water H-bonds around small hydrophobic molecules, whereas the second one, τ = 40-80 ps, is probably due to the confining effect of the long hydrophilic fragments which reduces the probability of a water molecule to leave the hydration layer of the exposed oil core. Water molecules around the core form H-bond clusters whose size and distribution change with temperature. From a cluster analysis, the system appears to be below the percolation threshold, suggesting that the exposed oily surface is formed by disconnected patches of size around 1 nm 2 , close to the estimate of the solvated hydrophobic patches on protein surfaces. The network connectivity is also considered for concentric hydration shells along the interface: it turns out that near the oil core, the cluster size is larger than elsewhere in the interface demonstrating a strong structural effect induced by the exposed hydrocarbon tails. Temperature affects the cluster size only in the innermost shell.

Two mesophilic/thermophilic variants of the G-domain of the elongation factor Tu were studied via... more Two mesophilic/thermophilic variants of the G-domain of the elongation factor Tu were studied via molecular dynamics simulations. By analyzing the simulation data via the Voronoi space tessellation, we have found that the two proteins have the same macromolecular packing, while the water-exposed surface area is larger for the thermophile. A larger coordination with water is probably due to a peculiar corrugation of the exposed surface of this species. From an enthalpic point of view, the thermophile shows a larger number of intramolecular hydrogen bonds, stronger electrostatic interactions, and a flatter free-energy landscape. Overall, the data suggest that the specific hydration state enhances macromolecular fluctuations but, at the same time, increases thermal stability.

A jointed experimental and computational study of binary and ternary blends of syndiotactic polys... more A jointed experimental and computational study of binary and ternary blends of syndiotactic polystyrene (sPS) and high density polyethylene (HDPE) in presence of a compatibilizant (SEBS) is reported. Small angle neutron scattering and Monte Carlo simulations have revealed that the structure factor of the blends is strongly dependent on the confinement and the degree of crystallinity of the single components. The interpretation of the known mechanical properties of the systems are given in terms of a structural arrangement where localized crystallites are distributed all over the blends and SEBS is located at the interface of the two immiscible components. This picture envisages a rapid growth of the crystalline phases that is even more efficient in the ternary rather than in the binary blends due to the elimination of discontinuities at the sPS-HDPE interface.

Journal of Physical Chemistry A, 2000

Density measurements have been performed on water solutions of nonionic surfactants oligooxyethyl... more Density measurements have been performed on water solutions of nonionic surfactants oligooxyethylene glycol-monoether (C 12 E j with j ) 5, 6, 7, 8) in a wide range of temperatures and concentrations. The densities of the pure surfactants in their liquid state were measured too. The observed values are almost a linear combination of the densities of an oil and of an oxyethylene (EO) bulk liquid phases. The deviation from ideality reduces as j increases and may be reflects an entropic contribution due to a partial mixing of the oil and EO termination. A temperature (T cross ), at which the C 12 E j aqueous solution density coincides with the solvent one up to 50 wt % has been found in all the E j species. The T cross of the different E j results to be scaling temperatures for the excess density of these surfactants. The sphere-to-rod transition temperatures are scaling temperatures too; thus it is expected that the differences between these two temperatures be constant for each of the E j species. In the case of j ) 6, 7, and 8 the values of the sphere-to-rod transition temperatures are known and these differences are all about 20°C. For C 12 E 5 only rods were experimentally observed and, coherently, the previous consideration led to a sphere-to-rod transition temperature of -7°C. The difference between the critical and the crossing temperatures is constant too, at about 15°C for j < 8, whereas for j ) 8 it is 7°C. In this case the densities of the separated phases, two degrees above the critical temperature, result to be the same, within 10 -6 g/cm 3 . It implies that, when slightly above the critical temperature, C 12 E 8water solution undergoes phase separate as it was in microgravitational condition. In our analysis, the C 12 E j solution is considered a three-component system: the oil core, the bulk water, and the region containing the aggregate interface, a mixture of water molecules, and EO units. Fixing the density of the oil core and of the bulk water phase respectively equal to those of pure dodecane oil and water at the same temperature, we obtain the density and the molar volume of the micellar interface. These data indicate that the amount of water molecules per EO segment decreases with the number of hydrophilic unit and with temperature. The same conclusion follows looking at the temperatures at which the maximum densities as a function of surfactant species and concentrations occur .

Colloids and Surfaces A-physicochemical and Engineering Aspects, 2005

In this article we present a combined analysis of volumetric [1] and molecular dynamics (MD) [2] ... more In this article we present a combined analysis of volumetric [1] and molecular dynamics (MD) [2] determinations of the interfacial properties of non-ionic surfactant aggregates belonging to the family of the oligooxyethylene glycol (CiEj). The volumetric experiments were made on surfactant having i=12 and j ranging from 5 to 8 EO unit and concern the density, and the sound velocity

By means of density, dielectric spectroscopy and sound velocity measurements we perform a systema... more By means of density, dielectric spectroscopy and sound velocity measurements we perform a systematic study on the polyoxyethylene C12E6C_{12}E_{6}C12E6 nonionic surfactant solutions as a function of temperature and concentration. Both density and sound velocity data, at about 34circC34^{\circ}C34circC, coincide with the value obtained for pure water. Above this temperature the density is lower than the water density whereas below it

Journal of Physical Chemistry C, 2010

ABSTRACT A combined analysis of experiments, small-angle neutron and X-ray scattering, transmissi... more ABSTRACT A combined analysis of experiments, small-angle neutron and X-ray scattering, transmission electron microscopy (TEM), mass density, and thermogravimetric measurements (TGA), was applied to gold nanoparticles (NP) grafted with thiol-terminated poly(ethylene glycol). TEM micrographs indicate that the gold core diameter is ca. 20% polydisperse. The associated polydispersion in volume dictates that the nanoparticle extensive properties thus determined, such as the number of gold atoms and polymer chains, have similar errors (ca. 30%). On the other hand, by combining the results of mass density and TGA the following intensive properties of the nanoparticle can be determined with a precision of 1%: (i) the volume fraction occupied by the gold core, (ii) the density of the nanoparticle, and (iii) the polymer terminations at the interface. An accurate characterization of the structure and hydration of NP is then obtained by fitting the scattering data and use of the density and TGA data to impose constraints on the fits. The scattering data can be fitted with a core−two shell model. The inner core contains only gold atoms and the two shells represent the interfacial region containing the polymer and the hydrating water molecules. The two shells differ in the local solvent−polymer composition: the inner shell is composed almost exclusively of polymer, whereas the outer is hydrated (about 60% of the volume occupied by polymers). Interesting differences have been observed with isotopic substitution in the solvent (H2O/D2O ratio).

Colloids and Surfaces B-biointerfaces, 1996

Electrical conductivity and dielectric dispersions have been measured on suspensions of 30S, 50S ... more Electrical conductivity and dielectric dispersions have been measured on suspensions of 30S, 50S subunits and 70S ribosomal particles from E. coli, in the presence and absence of magnesium ions. Conductivity measurements show a net contribution due to the disperzed phase. Ribosomal particles act in general as obstructants to the ionic motions in the solution, but besides this effect, there is

The Journal of Physical Chemistry B, 2004

Density and sound velocity measurements were performed on water solutions of the nonionic surfact... more Density and sound velocity measurements were performed on water solutions of the nonionic surfactant belonging to the family of the oligooxyethylene glycol (C i E j ), with i ) 12 and j ranging from 5 to 8 EO units. Up to a threshold concentration, the density (F) and the compressibility coefficient ( ) F -1 c -2 ) measured as a function of the temperature cross the density and the compressibility coefficient of water at the temperatures (T F and T ) which depend on the surfactant. Such conditions are proper of ideal solutions; thus, our watersurfactant solutions can be analyzed in this context as ideal mixtures of the pure solvent and effective solutes. We microscopically model such effective solutes as hydrated monomers assembled in micellar aggregates. Under some assumptions on the volumetric properties of the hydrophobic micellar core, we experimentally determine the molar volume of the EO terminations at the micellar interface. By comparing these values with those obtained for solutions of water and PEG (a polymer composed by j EO units), we estimated the degree of hydration of the interfacial EO-water mixture. The compressibility K ) V obtained from our model embodies a contribution due to the derivative of the number of the hydrated water molecules with respect to the pressure. Such derivative is characteristic of a solution in osmotic equilibrium with its solvent and, in the case of the micelles, can be rationalized on the bases of the "small system thermodynamic" theory. By comparing the compressibility of the micellar interface with that of an equivalent PEG-water solutions, we estimate the osmotic contributions for each of the investigated surfactant. The threshold concentrations, below which these solutions mix ideally, reflect an effect due to the different sizes of the solute aggregates and the solvent molecules.

The Journal of Physical Chemistry B, 2006

We present an extensive set of measurements of steady shear viscosity (η°s), longitudinal elastic... more We present an extensive set of measurements of steady shear viscosity (η°s), longitudinal elastic modulus (M') and ultrasonic absorption (α) in the one-phase isotropic liquid region of the nonionic surfactant C 12 E 8 aqueous solutions. Within a given temperature interval, this phase extends along the entire surfactant concentration range, that could be fully covered in the experiments. In agreement with previous studies, the overall results support the presence of two separated intervals of concentration corresponding to different structural properties. In the surfactant-rich region the temperature dependence of η°s follows an equation characteristic of glass-like systems. The ultrasonic absorption spectra show unambiguous evidence of viscoelastic behaviour that can be described by a Cole-Cole relaxation formula. In this region, when both the absorption and the frequency are scaled by the static shear viscosity (η°s), the scaled attenuation reduces to a single universal curve for all temperatures and concentrations. In the water-rich region the behaviour of η°s, M' and α are more complex and reflect the presence of dispersed aggregates whose size increases with temperature and concentration. At these concentrations the ultrasonic spectra are characterized by a multiple decay rate. The high frequency tail falls in the same frequency range seen at high surfactant content and exhibits similar behaviors. This contribution is ascribed to the mixture of hydrophilic terminations and water present at the micellar interfaces that resembles the condition of a concentrated polymer solution. An additional low frequency contribution is also observed, which is ascribed to the exchange of water molecules and/or surfactant monomers between the aggregates and the bulk solvent region.

The Journal of Physical Chemistry A, 2000

Density measurements have been performed on water solutions of nonionic surfactants oligooxyethyl... more Density measurements have been performed on water solutions of nonionic surfactants oligooxyethylene glycol-monoether (C 12 E j with j ) 5, 6, 7, 8) in a wide range of temperatures and concentrations. The densities of the pure surfactants in their liquid state were measured too. The observed values are almost a linear combination of the densities of an oil and of an oxyethylene (EO) bulk liquid phases. The deviation from ideality reduces as j increases and may be reflects an entropic contribution due to a partial mixing of the oil and EO termination. A temperature (T cross ), at which the C 12 E j aqueous solution density coincides with the solvent one up to 50 wt % has been found in all the E j species. The T cross of the different E j results to be scaling temperatures for the excess density of these surfactants. The sphere-to-rod transition temperatures are scaling temperatures too; thus it is expected that the differences between these two temperatures be constant for each of the E j species. In the case of j ) 6, 7, and 8 the values of the sphere-to-rod transition temperatures are known and these differences are all about 20°C. For C 12 E 5 only rods were experimentally observed and, coherently, the previous consideration led to a sphere-to-rod transition temperature of -7°C. The difference between the critical and the crossing temperatures is constant too, at about 15°C for j < 8, whereas for j ) 8 it is 7°C. In this case the densities of the separated phases, two degrees above the critical temperature, result to be the same, within 10 -6 g/cm 3 . It implies that, when slightly above the critical temperature, C 12 E 8water solution undergoes phase separate as it was in microgravitational condition. In our analysis, the C 12 E j solution is considered a three-component system: the oil core, the bulk water, and the region containing the aggregate interface, a mixture of water molecules, and EO units. Fixing the density of the oil core and of the bulk water phase respectively equal to those of pure dodecane oil and water at the same temperature, we obtain the density and the molar volume of the micellar interface. These data indicate that the amount of water molecules per EO segment decreases with the number of hydrophilic unit and with temperature. The same conclusion follows looking at the temperatures at which the maximum densities as a function of surfactant species and concentrations occur .

Langmuir, 2001

... Fabio Sterpone, † Giuseppe Briganti, § and Carlo Pierleoni*. ... Methods. Model. We adopt an ... more ... Fabio Sterpone, † Giuseppe Briganti, § and Carlo Pierleoni*. ... Methods. Model. We adopt an all-atom model throughout this work. For the C 12 system, we exploited the force field of Tobias, Tu, and Klein 39-41 for the dodecane chains and the SPC model for the water. ...

Biophysical Journal, 2005

Size distribution of dimyristoylphosphatidylcholine liposome suspensions was investigated by dyna... more Size distribution of dimyristoylphosphatidylcholine liposome suspensions was investigated by dynamic-light scattering (DLS) as a function of the sonication time (t s ). Cumulant expansion (second-and third-order) and regularized Laplace inversion (CONTIN) of dynamic single-angle laser light-scattering data were performed. With both methods, the intensityweighted mean hydrodynamic radius AErae I depended on the investigated lengthscale. The number-weighted mean hydrodynamic radius (AErae N ), obtained from CONTIN by modeling dimyristoylphosphatidylcholine vesicles as thin-walled hollow spheres, resulted as independent on the lengthscale. However, the AErae N value obtained from cumulant expansions remained lengthscaledependent. Therefore, the number-weighted radius distribution function is highly asymmetric. The number-weighted mean radius, the standard deviation, and the number-weighted radius at the peak ðr peak N Þ all decreased to a plateau when increasing sonication time. At t s longer than 1 h, the r peak N compares well with the radius of unilamellar vesicles in equilibrium with monomers predicted on a thermodynamic basis. The reliability of our analysis is proved by the comparison of experimental Rayleigh ratios with simulated ones, using the normalized number-weighted radius distribution function p N (r) determined by DLS data. A perfect agreement was obtained at longer sonication times, and the average aggregation number was determined. At lower t s values, simulations did not match experimental data, and this discrepancy was ascribed to the presence of large and floppy unilamellar vesicles with ellipsoidal shapes. Our investigation shows that, from single-angle DLS data, the radius distribution function of the vesicles can only be obtained if p N (r) is known.

Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression, 1993

Fluorescence intensity measurements of ethidium bromide (EB) bound to ribosomal RNA (rRNA) in sus... more Fluorescence intensity measurements of ethidium bromide (EB) bound to ribosomal RNA (rRNA) in suspensions of 30S and 50S subunits, of 70S ribosomal particles and of protein-flee extracted rRNA are presented. Changes in the intercalation of EB reflect changes in conformation and degree of exposure of rRNA. The effect of removal of magnesium ions on the binding of EB is compared in protein-free rRNA and in ribosomal particles by a Scatchard plot analysis. In free ribosomal RNA the number of bound EBs do not depend on magnesium content, only the association constant is affected. In intact 70S particles and both in the separated 50S and 30S subunits the presence of magnesium greatly reduces binding of EB and no saturation of the fluorescence intensity with rRNA concentration is observed, preventing a Scatchard plot analysis. Removal of magnesium restores a strong EB intercalation. Then magnesium ions induce a conformational change in the 70S particles as well as in the separated subunits. The different behavior of the free-rRNA and of the ribosomal particles indicates that ribosomal proteins are relevant to the structural changes induced by magnesium ions. The comparison of the number of excluded sites and of the association constant in the 30S, 50S subunits and in the 70S particles indicates that even without Mg 2+ ions the two subunits still interact, at variance with the commonly shared opinion that subunits dissociation takes place at low magnesium concentration.