Miguel A. Valdez

The zeta potential and surface properties of asphaltenes obtained with different crude oil/n-heptane proportions☆

Abstract We have investigated some surface properties of asphaltenes precipitated from crude oil with different volumes of n-heptane. According to the crude oil/n-heptane proportions used, asphaltenes are identified as 1:5, 1:15 and 1:40. Zeta potential results indicate that the amount of n-heptane determines the electrokintetic behaviour of asphaltenes in aqueous suspensions. Asphaltene 1:5 exhibits an isoelectric point (IEP) at pH 4.5 whereas asphaltenes 1:15 and 1:40 show an IEP at about pH 3. Surface charge on asphaltenes arises from the dissociation of acid functionalities and the protonation of basic functional groups. The presence of resins remaining on the asphaltene molecules may be responsible for the different IEP of asphaltene 1:5. Both sodium dodecyl sulfate (an anionic surfactant) and cetylpyridinium chloride (a cationic surfactant) were found to adsorb specifically onto asphaltenes. They reverse the sign of the zeta potential under certain conditions. These surfactants may be potential candidates to aid in controlling the stability of crude oil dispersions. Critical micelle concentration, interfacial tension measurements, and Langmuir isotherms at the air–water interface confirm the different nature of asphaltene 1:5, which also showed more solubility and a larger molecular size.

Linseed pectin: gelling properties and performance as an encapsulation matrix for shark liver oil

Abstract Fully de-esterified Na-pectate with excellent gelling properties was isolated from linseed ( Linum usitatissimum ) using an alkaline extraction procedure aided with a sequestering agent. The gelling behavior of this material was studied as a function of amount of Ca 2+ added and temperature by dynamic oscillatory rheology. Calcium cross-linking adjusted at varying stoichiometric ratios, ( R =2[Ca 2+ ]/[COO − ]), resulted in unequivocal gel network formation with mechanical spectra characteristic of permanent gel structure. Crude shark liver oil (SLO) was encapsulated into a composite matrix comprising linseed Na-pectate, alginate and chitosan. To this end, SLO was loaded into a linseed pectin–alginate solution at 2% (w/v), and gel beads were formed by dropwise addition into a CaCl 2 bath and further coated by chitosan. Three different formulations were evaluated by varying the ratio of pectin to alginate (in % of the total equivalent concentration), while keeping constant the total polymer concentration at ∼4% (w/w). As the proportion of linseed pectin in the formulae increased, the gel beads lost mechanical strength, sphericity and swelled more rapidly in water by a transport mechanism controlled only by diffusion of the solvent. Loading efficiency was greater in the capsules in which pectin contributed to 28% of the negatively charged polymer species (P28), which retained 73% of SLO, while capsules of pure alginate (P0) and those with a higher amount of pectin (P71) retained 65 and 67% of SLO, respectively. The observed differences between the three types of capsules were a direct consequence of the strength of linseed pectin gels, weaker than those of alginate.

Characterization and antiproliferative activity of nobiletin-loaded chitosan nanoparticles

Nobiletin is a polymethoxyflavonoid with a remarkable antiproliferative effect. In order to overcome its low aqueous solubility and chemical instability, the use of nanoparticles as carriers has been proposed. This study explores the possibility of binding nobiletin to chitosan nanoparticles, as well as to evaluate their antiproliferative activity. The association and loading efficiencies are 69.1% and 7.0%, respectively. The formation of an imine bond between chitosan amine groups and the carbonyl group of nobiletin, via Schiff-base, is proposed. Nobiletin-loaded chitosan nanoparticles exhibit considerable inhibition (IC50 = 8 µg/mL) of cancerous cells, revealing their great potential for applications in cancer chemotherapy.

Structural Characterization of Mesquite (Prosopis velutina) Gum and its Fractions

Structural and physicochemical characteristics of mesquite gum (from Prosopis velutina) were investigated using FT-IR spectroscopic, mass spectrometric and chromatographic methods. Four fractions (F-I, F-IIa, F-IIb and F-III) were isolated by hydrophobic interaction chromatography. The samples were characterized and analyzed for their monosaccharide and oligomers composition by high performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). L-Arabinose (L-Ara) and D-galactose (D-Gal) were found as the main carbohydrate constituent residues in the polysaccharides from mesquite gum and their ratio (L-Ara/D-Gal) varied within the range 2.54 to 3.06 among the various fractions. Small amounts of D-glucose (D-Glc), D-mannose (D-Man) and D-xylose (D-Xyl) were also detected, particularly in Fractions IIa, IIb and III. Infrared spectroscopy identified polysaccharides and protein in all the samples. Data from mass spectrometry (MALDI-TOF MS) was consistent with the idea that the structure corresponding to the periphereal chains of Fraction I is predominantly a chain of pentoses attached to uronic acid.

Response time and electrorheology of semidiluted gellan, xanthan and cellulose suspensions

Response times with electrical fields of gellan and xanthan dry powder suspensions of 25, 32 and 53 μm average diameter and concentrations of 1.0, 1.5 and 2.0% (w/w) dispersed in commercial corn oil were optically measured through a specifically designed set up. In all cases, the delay time was proportional to 1/E a , where E is the applied field and a is an adjustable parameter. The values of parameter a were very different from the typical value of some known electrorheolgical fluids. Response time of gellan suspensions was shorter than the one obtained for xanthan and it is comparable to the time found by using silica particles in silicon oil. Response times for cellulose were very large and the fibrillation phenomenon was negligible for E < 1.0 kv/mm. Viscosity measurements of semidiluted xanthan, gellan and cellulose suspensions (1.0 and 1.5% w/w) under the influence of electrical fields, were performed in a parallel plates rheometer. Results in the range of stress <70 Pa showed that viscosity values of gellan suspensions were larger than those obtained with xanthan or cellulose under the same applied electric field at shear rates higher than 10 s -1 . However, cellulose suspensions showed larger viscosity values compared with the ones measured with xanthan and gellan suspensions at very low shear rates. Dielectric measurements of cellulose, xanthan and gellan 1.5% w/w suspensions were performed in the range 10 0 -8 × 10 4 Hz. Results agree with a Maxwell-Wagner type relaxation model.

Self-assembly process of different poly(oxystyrene)-poly(oxyethylene) block copolymers: spontaneous formation of vesicular structures and elongated micelles.

In the present work, we investigated the micellization, gelation, and structure of the aggregates of three poly(ethylene oxide)-polystyrene oxide block copolymers (E12S10, E10S10E10, and E137S18E137, where E denotes ethylene oxide and S styrene oxide and the subscripts the block length) in solution. Two of them have similar block lengths but different structures (E12S10 and E10S10E10) and the other has longer blocks (E137S18E137). For the first time, the spontaneous formation of vesicles by a poly(oxystyrene)-poly(oxyethylene) block copolymer is reported. These vesicular structures are present when copolymer E12S10 self-assembles in aqueous solution in coexistence with spherical micelles, as confirmed by the size distribution obtained by dynamic light scattering and pictures obtained by polarized optical microscopy, and transmission and cryo-scanning electron microscopies. Vesicle sizes vary between 60 and 500 nm. On the other hand, for copolymers E10S10E10 and E137S18E137, only one species is found in solution, which is assigned to elongated and spherical micelles, respectively. If we compare the high aggregation number derived by static light scattering for the triblock block copolymer micelles, with the maximum theoretical micellar dimensions compatible with a spherical geometry, we can see that the micellar geometry cannot be spherical but must be elongated. This is corroborated by transmission electron microscopy images. On the other hand, tube inversion was used to define the mobile-immobile (soft-hard gel) phase boundaries. To refine the phase diagram and observe the existence of additional phases, rheological measurements of copolymer E137S18E137 were done. The results are in good agreement with previous values published for other polystyrene oxide-poly(ethylene oxide) block copolymers. In contrast, copolymers E12S10 and E10S10E10 did not gel in the concentration range analyzed. Thus, only certain concentrations of copolymer E10S10E10 were analyzed by rheometry, for which an upturn in the low-frequency range of the stress moduli was observed, denoting an evidence of an emerging slow process, which we assign to the first stages of formation of an elastic network.

Synthesis and Characterization of New Thiolated Chitosan Nanoparticles Obtained by Ionic Gelation Method

We derivatized low molecular weight chitosan (LMWC) with 3-mercaptopropanoic acid (3-MPA) by a coupling reaction. The chemical modification of LMWC was characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance, 1HNMR. We researched the influence of 3-MPA on the nanoparticles formation by ionic gelation method using sodium tripolyphosphate (TPP) as cross-linker reagent. In order to optimize the nanoparticles formation, we studied the effect of the pH solution and molar ratio on nanoparticles stability. Analyses of particle size, morphology, and surface charge were determined by dynamic light scattering, Atomic Force Microscopy, and zeta potential, respectively. It was found that formation of semispherical and stable nanoparticles was improved due to the chemical modification of chitosan. Optimized semispherical nanoparticles of thiolated chitosan were synthesized with the parameters (pH 4.7, molar ratios 1 : 106). Additionally, we reported the thermodynamic profile of the nanoparticles formation determined by isothermal titration calorimetry (ITC). The aggregation process achieved to form nanoparticles of thiolated and nonmodified chitosan consisted of two stages, considering one binding site model. Gibbs free energy and binding constant () describe the aggregation process of thiolated chitosan/TPP, which is an initial reaction and followed by an endothermic stage. These results are promising for the possible application of these nanoparticles as nanocarriers and delivery systems.

Interaction of the Cationic Peptide Bactenecin with Phospholipid Monolayers at the Air−Water Interface: I Interaction with 1,2-Dipalmitoyl-sn-Glycero-3-Phosphatidilcholine

In this work we have investigated the influence of NaCl on the adsorption of the antimicrobial cationic peptide bactenecin in the monolayer of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air-water interface, as a function of NaCl concentrations in the subphase. We show that the effect of the salt concentration on DPPC monolayers is a monotonic decrease of the liquid-condensed-liquid-expanded (LC-LE) coexistence region. By contrast, the effect of the bactenecin adsorption at the DPPC monolayer not only removed the LC-LE coexistence region plateau, but also shifted the DPPC isotherms to higher pressures and increased the compressibility of the DPPC/bactenecin monolayers with respect to the pure DPPC monolayer around the LC phase. Analysis of the domain structure, obtained by Brewster angle and atomic force microscopes, indicates that the salt concentration in the subphase builds an electrostatic barrier, increasing the rigidity of DPPC monolayers and limiting the bactenecin adsorption at the LC-LE phase coexistence.

Influence of temperature on the slip velocity of semidilute xanthan gum solutions

We obtain experimental evidence of the influence of temperature in the range 12-32°C on the slip phenomena of two different 0.3% xanthan solutions in a glass capillary rheometer. Enhancement of the slip velocity was observed for both samples around the corresponding thermally induced order-disorder transition temperature. Intrinsic viscosity measurements were performed to find the conformation change of both samples. Concentrations of 0.15% and 0.2% were analyzed for one sample, showing absence of slip at 0.15%.Slip velocity measurements were determined with the traditional Mooney method for a L/D ratio of the capillaries (640) enough to neglect entry head losses. Comparisons were done with the method developed by Piau et al. (1990) and with the one developed by Hatzikiriakos and Dealy (1992). The resulting behavior of the slip velocity with the capillary diameters, calculated with the method of Hatzikiriakos and Dealy, was contrary to the behavior experimentally found by other authors. The observed differences in the slip velocity, measured with the other two methods, were proportional and nearly independent of temperature and diameter of the capillaries.

Oligomers, protofibrils and amyloid fibrils from recombinant human lysozyme (rHL): fibrillation process and cytotoxicity evaluation for ARPE-19 cell line.

Amyloid-associated diseases, such Alzheimers, Huntingtons, Parkinsons, and type II diabetes, are related to protein misfolding and aggregation. Herein, the time evolution of scattered light intensity, hydrophobic properties, and conformational changes during fibrillation processes of rHL solutions at 55 °C and pH 2.0 were used to monitor the aggregation process of recombinant human lysozyme (rHL). Dynamic light scattering (DLS), thioflavin T (ThT) fluorescence, and surface tension (ST) at the air-water interface were used to analyze the hydrophobic properties of pre-amyloid aggregates involved in the fibrillation process of rHL to find a correlation between the hydrophobic character of oligomers, protofibrils and amyloid aggregates with the gain in cross-β-sheet structure, depending on the increase in the incubation periods. The ability of the different aggregates of rHL isolated during the fibrillation process to be adsorbed at the air-water interface can provide important information about the hydrophobic properties of the protein, which can be related to changes in the secondary structure of rHL, resulting in cytotoxic or non-cytotoxic species. Thus, we evaluated the cytotoxic effect of oligomers, protofibrils and amyloid fibrils on the cell line ARPE-19 using the MTT reduction test. The more cytotoxic protein species arose after a 600-min incubation time, suggesting that the hydrophobic character of pre-amyloid fibrils, in addition to the high prevalence of the cross-β-sheet conformation, can become toxic for the cell line ARPE-19.