Mesoscopic and microscopic investigation on poly(vinyl alcohol) hydrogels in the presence of sodium decylsulfate

The structure of poly(vinyl alcohol) (PVA) hydrogels formed as a result of freeze/thaw treatments of aqueous solutions of the polymer (11 wt % PVA) in the freshly prepared state is analyzed through the combined use of small (SANS) and ultrasmall (USANS) angle neutron scattering techniques. The structure of these hydrogels may be described in terms of polymer rich regions, with dimensions of the order of 1-2 microm, dispersed in a water rich phase, forming two bicontinuous phases. The PVA chains in the polymer rich phase form a network where the cross-linking points are mainly crystalline aggregates of PVA having average dimensions of approximately 45 A. The structural organization of freeze/thaw PVA hydrogel membranes does not change either after rehydration of dried gels or in the presence of a tensile force. Finally, addition of surfactant micelles inside the gel provides a formulation with both hydrophobic and hydrophilic regions, which demonstrates the potential of the system for drug delivery. Both SANS and EPR measurements show that sodium decylsulfate (C10OS) micelles do not significantly interact with the PVA gel. Variation of the gel structure by the number of freeze/thaw cycles should modulate the rate of release of an active constituent, for example, in a dermal patch.     

Physicochemical properties of mixed micellar aggregates containing CCK peptides and Gd complexes designed as tumor specific contrast agents in MRI

New amphiphilic molecules containing a bioactive peptide or a claw moiety have been prepared in order to obtain mixed micelles as target-specific contrast agents in magnetic resonance imaging. The first molecule, C(18)H(37)CONH(AdOO)(2)-G-CCK8 (C18CCK8), contains a C18 hydrophobic moiety bound to the C-terminal cholecystokinin octapeptide amide (CCK 26-33 or CCK8). The second amphiphilic compound, C(18)H(37)CONHLys(DTPAGlu)CONH(2) (C18DTPAGlu) or its gadolinium complex, (C18DTPAGlu(Gd)), contains the same C18 hydrophobic moiety bound, through a lysine residue, to the DTPAGlu chelating agent. The mixed aggregates as well as the pure C18DTPAGlu aggregate, in the presence and absence of Gd, have been fully characterized by surface tension measurements, FT-PGSE-NMR, fluorescence quenching, and small-angle neutron scattering measurements. The structural characterization of the mixed aggregates C18DTPAGlu(Gd)-C18CCK8 indicates a spherical arrangement of the micelles with an external shell of approximately 21 A and an inner core of approximately 20 A. Both the DTPAGlu(Gd) complexes and the CCK8 peptides point toward the external surface. The measured values for relaxivity in saline medium at 20 MHz proton Larmor frequency and 25 degrees C are 18.7 mM(-)(1) s(-)(1). These values show a large enhancement in comparison with the isolated DTPAGlu(Gd) complex.     

Equilibrium and Transport Properties of Sodium n-Octyl Sulfonate Aqueous Solutions

Measurements of osmotic coefficients, mutual diffusion coefficients, and conductivity were performed on the binary system sodium n-octyl sulfonate (C₈SO₃Na)–water at 25°C both below and above the micellar composition range. The osmotic coefficient data were obtained through vapor-pressure osmometry, while the Taylor dispersion method was used to measure diffusion coefficients. The mass equilibrium model was applied to this self-aggregating system, taking into account the deviation of the activity coefficients from the Debye–Hückel limiting law by using the Guggenheim corrective terms for mixed electrolyte solutions. The expressions derived from the model fit the experimental osmotic and diffusion coefficient data well, when the same values of aggregation number, fraction of condensed counterions, and equilibrium constant are used. Osmotic coefficients were also used to determine the thermodynamic factor required to compute the solute mobility from diffusion data. Conductivity data were used to test two theoretical models, namely, the Onsager–Fuoss and the Mean Spherical Approximation theories. Both models have been found to yield unsatisfactory fits to our experimental data and some arbitrary terms had to be applied to the theoretical expressions to obtain good agreement between experiment and theory.     

Sub-wavelength probing and modification of photonic crystal nano-cavities

In this work we present a sizeable and reversible spectral tuning of the resonances of a two-dimensional photonic crystal nano-cavity by exploiting the introduction of a sub-wavelength size glass tip. The comparison between experimental near-field data and results of numerical calculations shows that the spectral shift induced by the tip is proportional to the local electric field intensity of the cavity mode. This observation proves that the electromagnetic local density of states in a microcavity can be directly measured by mapping the tip-induced spectral shift with a scanning near-field optical microscope. Moreover, a non-linear control on the cavity resonance is obtained by exploiting the local heating induced by near-field laser excitation at different excitation powers. The temperature gradient due to the optical absorption results in an index of refraction gradient which modifies the dielectric surroundings of the cavity and shifts the optical modes.     

The self-injected XeCl excimer laser

We present experimental and modelling results of the first self-injected excimer laser. The intracavity losses of a XeCl oscillator are properly modulated by a Pockels cell allowing generation, amplification and extraction of short laser pulses with selectable duration in the range of 1–12 ns, tailored temporal profile and peak power increment up to a factor of three. Longer output laser pulses, up to 100 ns, can be obtained by slicing the intracavity laser radiation without peak power increment. Laser output peak powers in excess of 2 MW have been obtained, with remarkable reproducibility characteristics.ENEA fellow     

Interaction of Poly(vinyl alcohol) with Poly(acrylic acid) and with Sodium Polyacrylate in Aqueous Solutions: A Volumetric Study at 25 °C

The infinite dilution partial molar volumes $\bar{V}^{\infty}$ of poly(acrylic acid), at different degrees of neutralization α _N with sodium hydroxide, have been experimentally determined in aqueous solutions. Analysis of the data indicates that, to a good approximation, the observed decrease of $\bar{V}^{\infty}$ with α _N can be interpreted in terms of the electrostriction of the water molecules involved in the hydration cospheres of the –RCOO^? groups. The presence of PVA does not significantly affect the value of $\bar{V}^{\infty}$ for the undissociated acid, but causes a large decrease for the sodium salt. This aspect has been discussed in terms of direct interaction between the two polymers.     

Interaction between pentaethylene glycol n-octyl ether and poly(acrylic acid): effect of the polymer molecular weight

The effect of the polymer molecular weight on the interaction between pentaethylene glycol n-octyl ether (C(8)E(5)) and poly(acrylic acid) (PAA) has been investigated by a combined experimental strategy including tensiometry, potentiometry, calorimetry, fluorescence quenching and intradiffusion (pulsed gradient spin echo-NMR) measurements. PAA samples with an average molecular weight varying in a wide range (M (w)=2000, 100,000, 250,000, and 450,000) have been considered. The measurements have been performed at constant polymer concentration (0.1% w/w) with varying surfactant molality. In all the considered systems, at low surfactant concentration, adsorption of surfactant monomers onto the polymer chain has been detected. At a C(8)E(5) molality (T(1)) independent of the PAA M (w), surfactant molecules start to aggregate, forming clusters to which the polymer co-participates. Above this concentration, the behavior of the system depends on M (w). In fact, if polymer samples with high molecular weight (M (w)100,000) are employed, all the added surfactant aggregates onto the polymer leading to the polymer saturation and, subsequently, to free micelles formation. Both saturation and free micellization occur at surfactant concentrations which are independent of the polymer molecular weight. C(8)E(5) aqueous mixtures containing PAA with low molecular weight (M (w)=2000) behaves differently, in that, above T(1), only a fraction ( approximately 20%) of the added surfactant molecules interact with the polymer, forming aggregates to which more than one PAA chain participate. In this case, C(8)E(5) free micellization occurs before polymer saturation. The experimental evidences have been interpreted in terms of the subtle balance between the various molecular interactions driving the surfactant-polymer aggregation.