The phase transformations of the surfactant Triton X-100 were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and dielectric relaxation spectroscopy (DRS). In particular, crystallization was induced at different cooling rates comprised between 13 and 0.5 K min(-1). Vitrification was detected by both DSC and DRS techniques with a glass transition temperature of ∼212 K (measured on heating by DSC) allowing classifying Triton X-100 as a glass former. A fully amorphous material was obtained by cooling at a rate ≥10 K min(-1), while crystallization was observed for lower cooling rates. The temperature of the onset of melt-crystallization was found to be dependent on the cooling scan rate, being higher the lower was the scan rate. In subsequent heating scans, the material undergoes cold-crystallization except if cooled previously at a rate ≤1 K min(-1). None of the different thermal histories led to a 100% crystalline material because always the jump typical of the glass transformation in both heat flux (DSC) and real permittivity (DRS) is observed. It was also observed that the extent/morphology of the crystalline phase depends on the degree of undercooling, with higher spherulites developing for lower undercooling degree (24 K ≤ T(m) - T(cr) ≤ 44 K) in melt-crystallization and a grain-like morphology emerging for T(m) - T(cr) ≈ 57 K either in melt- or cold-crystallization. The isothermal cold- and melt-crystallizations were monitored near above the calorimetric glass transition temperature by POM (221 K) and real-time DRS (T(cr) = 219, 220, and 221 K) to evaluate the phase transformation from an amorphous to a semicrystalline material. By DRS, the α-relaxation associated with the dynamic glass transition was followed, with the observation that it depletes upon both type of crystallizations with no significant changes either in shape or in location. Kinetic parameters were obtained from the time evolution of the normalized permittivity according to a modified Avrami model taking in account the induction time. The reason the isothermal crystallization occurs to a great extent in the vicinity of the glass transition was rationalized as the simultaneous effect of (i) a high dynamic fragile behavior and (ii) the occurrence of catastrophic nucleation/crystal growth probably enabled by a preordering tendency of the surfactant molecules. This is compatible with the estimated low Avrami exponent (1.12 ≤ n ≤ 1.6), suggesting that relative short length scale motions govern the crystal growth in Triton X-100 coherent with the observation of a grainy crystallization by POM.

No abstract

The amorphization of the readily crystallizable therapeutic ingredient and food additive, menthol, was successfully achieved by inclusion of neat menthol in mesoporous silica matrixes of 3.2 and 5.9 nm size pores. Menthol amorphization was confirmed by the calorimetric detection of a glass transition. The respective glass transition temperature, T = -54.3 °C, is in good agreement with the one predicted by the composition dependence of the T values determined for menthol:flurbiprofen therapeutic deep eutectic solvents (THEDESs). Nonisothermal crystallization was never observed for neat menthol loaded into silica hosts, which can indicate that menthol rests as a full amorphous/supercooled material inside the pores of the silica matrixes. Menthol mobility was probed by dielectric relaxation spectroscopy, which allowed to identify two relaxation processes in both pore sizes: a faster one associated with mobility of neat-like menthol molecules (α-process), and a slower, dominant one due to the hindered mobility of menthol molecules adsorbed at the inner pore walls (S-process). The fraction of molecular population governing the α-process is greater in the higher (5.9 nm) pore size matrix, although in both cases the S-process is more intense than the α-process. A dielectric glass transition temperature was estimated for each α (T) and S (T) molecular population from the temperature dependence of the relaxation times to 100 s. While T agrees better with the value obtained from the linearization of the Fox equation assuming ideal behavior of the menthol:flurbiprofen THEDES, T is close to the value determined by calorimetry for the silica composites due to a dominance of the adsorbed population inside the pores. Nevertheless, the greater fraction of more mobile bulk-like molecules in the 5.9 nm pore size matrix seems to determine the faster drug release at initial times relative to the 3.2 nm composite. However, the latter inhibits crystallization inside pores since its dimensions are inferior to menthol critical size for nucleation. This points to a suitability of these composites as drug delivery systems in which the drug release profile can be controlled by tuning the host pore size.

The preparation of transparent nanostructured TiO2 (anatase) membranes is described. Detailed characterization shows these membranes to be 50 μm thick nanoporous-nanocrystalline structures with associated values for porosity and surface roughness of 50% and 5000, respectively. Modification of these membranes by coadsorption of a ruthenium complex, bis[(4,4‘-dicarboxy-2,2‘-bipyridine)(4,4‘-dimethyl-2,2‘-bipyridine)ruthenium(II)] dichloride (I), and of a viologen, 1-ethyl-1‘-[(4-carboxy-3-hydroxyphenyl)methyl]-4,4‘-bipyridinium perchlorate (II), is also described. Detailed studies show that visible-light-induced electron transfer by electronically excited I to the conduction band of the nanostructured TiO2 membrane is followed by membrane mediated electron transfer to coadsorbed II. Detailed studies also show that, as a consequence of the rectifying properties of the semiconducting membrane, charge separation is long-lived. The possible significance of these findings for the development of a practical water splitting device is considered.