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This collection is exclusively made up of theses, dissertations, and project reports submitted as a requirement for completing a graduate degree at UPR-Mayagüez.
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Publication Molecular simulations of model Langmuir monolayers(2005) Villalobos Rivera, Leslie V.; López Quiñones, Gustavo E.; University of Puerto Rico, Mayagüez Campus; College of Arts and Sciencies - Sciences; Cruz Pol, Astrid J.; Patrón, Francis; Department of Chemistry; López Quiñones, Gustavo E.The study of Langmuir monolayers has generated the attention of researchers because of their unique properties and their not well understood phase equilibrium. These monolayers exhibit interesting phase diagrams where the unusual liquid-liquid equilibrium can be observed for a single component monolayer submitted to an external applied pressure. In this study, the thermodynamic properties of a model Langmuir monolayer were determined using two types of computer simulations. First, Monte Carlo simulations in the Isothermal-Isobaric ensemble were used to obtain adsorption isotherms of Langmuir monolayers. The results clearly show coexistence of two liquid phases, denominated as liquid expanded state (LES) and liquid condensed state (LCS). Radial distribution function and distribution functions of enthalpies for the monolayer were also computed to clearly identify each liquid phase and the coexistence region. A second model was used to obtain the critical properties for this model system. The phase equilibrium between the liquid phases and the LES-Vapor (V) phases has been considered using Monte Carlo computer simulations in the Standard Virtual Gibbs Ensemble. The Caillete- Mathias phase diagrams were constructed and two models were implemented in order to determine the critical parameters of the system. Specifically, the Ising Model and the rectilinear approximation were used to identify the critical temperature (Tc*) and the critical density (ρc*), respectively. These critical parameters were identified by varying the interaction between the surfactant molecules and the aqueous phase. Finally, we have identified the coexistence between the LES and LCS states, in agreement with experimental and theoretical evidence in the literature. Furthermore, we have successfully identified the critical parameters Tc* and ρc* for the LES-LCS and the LES-V equilibrium of the monolayer.Publication Manejo de fertilizante nitrogenado para la optimización del rendimiento de una línea pura de maíz (𝘡𝘦𝘢 𝘮𝘢𝘺𝘴 𝘓.)(2015) Rivera Zayas, Johanie; Sotomayor Ramírez, David; College of Agricultural Sciences; Beaver, James S.; Román Paoli, Elvin; Department of Agro-Environmental Sciences; Cafaro, Matías J.Nitrogen (N) is one of the most important nutrients needed to maximize inbred maize (𝘡𝘦𝘢 𝘮𝘢𝘺𝘴 𝘓.) yield. An integrated management of the different contributing N sources to the agroecosystem can help improve N use efficiency. This study evaluated the effect of six fertilizer N levels (0, 34, 68, 102, 135 and 203 kg N ha⁻¹) and of a maize-fallow and maize-cowpea (𝘝𝘪𝘨𝘯𝘢 𝘶𝘯𝘨𝘶𝘪𝘤𝘶𝘭𝘢𝘵𝘢) rotation on inbred maize seed yield. The experiment was performed in a Jacaguas soil series (𝘓𝘰𝘢𝘮𝘺-𝘴𝘬𝘦𝘭𝘦𝘵𝘢𝘭, 𝘮𝘪𝘹𝘦𝘥, 𝘴𝘶𝘱𝘦𝘳𝘢𝘤𝘵𝘪𝘷𝘦, 𝘪𝘴𝘰𝘩𝘺𝘱𝘦𝘳𝘵𝘩𝘦𝘳𝘮𝘪𝘤 𝘍𝘭𝘶𝘷𝘦𝘯𝘵𝘪𝘤 𝘏𝘢𝘱𝘭𝘶𝘴𝘵𝘰𝘭𝘭𝘴) in the DowAgrosciences® experimental farm in Santa Isabel, Puerto Rico during the 2013-2014 winter season. Crop development was evaluated using plant height and flag leaf area as indicators. The parameters GreenSeeker®, SPAD-502® and N concentration in the indicator leaf were evaluated as indicators of N sufficiency. The cropping rotation maize-fallow and maize-cowpea did not affect seed yield performance (p>0.05). The fertilizer N level influenced seed yield (p<0.05) with an optimum fertilizer N level of 68 kg N ha⁻¹ for a seed yield of 2,918 kg ha⁻¹. The agronomic measurements of plant height, SPAD-502®, GreenSeeker® and N concentration in leaf tissue were adequate indicators of crop N sufficiency during the vegetative stages V6 to V12. In this study, the inbred maize line D065125VH requires 68 kg N ha⁻¹ to produce an optimum seed yield of 2,918 kg ha⁻¹ either with a rotation with cowpea or a fallow-maize cropping system.Publication Presence, quantification and analysis of ironstone in a Guanajibo soil profile(2012) Pérez Lizasuain, Bianca; Muñoz, Miguel A.; College of Agricultural Sciences; Lugo, Wanda; O’Hallorans, Julia M.; Department of Agro-Environmental Sciences; Gilbes, FernandoPlinthite is defined as a weakly cemented material rich in iron, poor in humus –a mixture of clay and other diluents, such as quartz. It commonly occurs as dark red redox concentrations that can change irreversibly to ironstone hardpans or irregular aggregates on exposure to repeated wetting and drying. The presence of this material may affect plant root growth, soil fertility as well as soil drainage, acidity, and management. The studied soil belongs to the Guanajibo series and is located in the municipality of Cabo Rojo, Puerto Rico. The objectives of this study were: (i) determine soil layers color and ground ironstone colors and relate these to mineral components; (ii) quantify on volume and mass bases the presence of ironstone in each soil profile; (iii) determine mineral components in ironstone and soil by x-ray diffraction (XRD) and determine the content of iron and aluminum from each horizon by extracting the soil and grounded ironstone by two distinct methods: citrate-bicarbonate dithionite (CBD) and ammonium oxalate (OX). The CBD method extracts the crystalline fraction of the iron while the OX method removes the iron complexes with organic matter and amorphous iron. We suggest that reddish 10R and 5YR are associated to the presence of hematite and reddish yellow (2.5YR to 7.5YR) are associated with the presence of goethite. The highest content of ironstone was found in the upper horizon Ap, with a reduction in the next two lower horizons Bt and Btv1. An increase was observed in horizon Btv2 indicating that the soil was formed by alluvial deposits. The content of iron oxides and the effective cation exchange capacity (ECEC) were related to the clay content of each horizon. Higher clay content coincided with higher iron oxides content and higher ECEC. The opposite was observed for aluminum oxides content. The ECEC was low in all horizons, indicative of the presence of highly weathered minerals. This finding was confirmed by XRD analysis, showing minerals such as kaolinite, goethite, quartz, hematite and gibbsite. The main component of ironstone was goethite and the main component of the soil was kaolinite. Each horizon was classified as plinthic or not plinthic following the requirements established by the USDA- Natural Resources Conservation Service: the soil fine-fraction must have more than 2.5% of iron in the CBD extraction, and a ratio between OX and CBD extractable iron (OX/CBD) of less than 0.10. Of the five horizons only Btv1 was classified as plinthic, showing 2.8% CBD extractable iron and a ratio less than 0.10. The other horizons met with the requirement of a OX/CBD ratio less than 0.10, but all had less than 2.5% of iron extracted by the CBD method.Publication Evaluación agronómica y morfológica de variedades de arroz (𝘖𝘳𝘺𝘻𝘢 𝘴𝘢𝘵𝘪𝘷𝘢 𝘓.) en secano bajo condiciones climáticas de Puerto Rico(2013) Orélien, Eléonore; Beaver, James S.; College of Agricultural Sciences; Avilés Rodríguez, Lucas N.; Estevez De Jensen, Consuelo; Department of Agro-Environmental Sciences; Santos Flores, Carlos J.Rice (Oryza sativa L.) is one of the most important food crops in the world, and specifically in Puerto Rico. However, water availability is a limiting factor for rice production that can be overcome through the development of drought resistant genotypes. In order to determine their adaptability to the climatic conditions of Puerto Rico, 30 upland rice varieties were grown at the Lajas Substation of the Agricultural Experiment Station of the University of Puerto Rico. The experiment was conducted under two planting systems, rainfed and flood irrigation, using a randomized complete block with a split plot arrangement and four replications. The planting systems constituted the main plots and the 30 rice varieties the subplots. The variables evaluated were flowering date, days to maturity, growth habit, plant height, grain yield and its components, biomass, incidence of Helminthosporium oryzae, resistance to shattering, lodging, drought index and harvest index. The results showed variability among genotypes under both planting systems. Cultivars ‘Chenire’ (4948 and 1875 kg/ha, flood irrigation and dryland, respectively) and ‘Pirogue’ (4074 and 1863 kg/ha) had higher grain yield under both conditions and were identified as the most drought resistant cultivars (STI>1.4), with high mean relative performance (MRP>3.6), higher harvest index and higher geometric mean productivity (GMP>2700). The cultivar ‘PR-358’ (13,830 and 6,510 kg/ha) produced the greatest amount of biomass among all cultivars. Under irrigated conditions, 1000 seed weight was 21.5 to 32.2 g, panicle length varied from 13.3 to 30.0 cm. The average height of the plants ranged from 0.80 to 1.77 m, the mean values of days to flowering and days to maturity were respectively 57-106 and 74-123 days, and the density of the panicle varied from compact to open. Under rainfed conditions, 1000 seeds weight ranged from 18.6 to 34.4 g, panicle length varied from 13.3 to 28.1 cm, the average height of the plants ranged from 0.55 to 1.14 m. The mean values of days to flowering and days to maturity were, respectively, 61-109 and 81-129 days, and the density of the panicle varied from compact to intermediate. Under rainfed conditions, the variables days to flowering, growth habit and maturity were not correlated with any of the other variables. Furthermore, the results of this research suggest that cultivars ‘Chenire’, ‘Pirogue’, ‘IR-64’ can be planted under both systems and can be used for the development of rice with greater drought tolerance.Publication Evaluation of blend homogeneity with large differences in particle sizes using Near Infrared Spectroscopy (NIRS)(2005) Villafañe García, Sachel M.; Romañach, Rodolfo J.; College of Arts and Sciencies - Sciences; Patrón, Francis; Velázquez, Carlos; Department of Chemistry; Perales, OscarSolid dosage forms are the most commonly prescribed pharmaceutical formulations. They consist of a homogeneous mixture of ingredients that provide desired characteristics. The process of blending during manufacturing processes is important to assure the quality of the product and it may be assessed by blend uniformity analysis. The sampling process is, also, very important and has been widely studied since it has been demonstrated that the sample thieves, the most common sampling method used in the pharmaceutical industry, introduce errors in the process. Some of these sampling errors are segregation, weight variability and product adulteration. Sample thieves do not follow Allen “golden rules” of sampling that state that a powder should always be sampled when in motion. Near Infrared Spectroscopy (NIRS) and stream sampling are alternative sampling techniques to evaluate blend homogeneity. In this work, lactose granules were prepared to obtain large differences in particle size, creating a blend difficult to sample and evaluate. Three sampling techniques: a pocket thief, Bellamy’s thief and stream sampling were used with a 20% (m/m) ibuprofen – granules blend. Sample weight variation and Relative Standard Deviation (RSD), for ibuprofen content, between 10 and 13 % (m/m) were observed. Also, the distribution of ibuprofen by particle size of granules was determined and its concentration was found to vary with particle sizes: as the particle size increases the ibuprofen concentration decreases. Also, a NIR Blend Monitor with wireless connection was evaluated as a tool for the in-line determination of blend homogeneity. Granules of different particle sizes were sieved and placed in a V-blender to observe the blend homogeneity from a physical point of view. The spectra showed differences in baseline and absorption due to the differences in particle sizes when multiplicative scattering correction (MSC) was applied to original data. Small particles showed less absorption and baseline than larger particles. Different approaches such as mean square of differences, moving block standard deviation and absorbance at 1542 nm were evaluated to analyze changes in spectra.Publication Microscopic mass transfer of TNT on Ottawa sand(2006) Rizo Vivas, Olga L.; Castro-Rosario, Miguel E.; College of Arts and Sciencies - Sciences; Hernández-Rivera, Samuel P.; Briano-Peralta, Julio G.; Department of Chemistry; Mehta, Narinder K.Understanding the transport mechanism of buried explosives in soil is crucial in order to implement an effective system for their detection. A major recurring problem is the representation of diffusion processes of these compounds on soils due their different and complex properties. The aim of this work was to describe the transport of 2, 4, 6 –Trinitrotoluene (TNT) on Ottawa sand using a mathematical model. The microscopic mass transfer of TNT on sand particles was studied using Scanning Electron Microscopy (SEM) coupled to Energy Dispersed X-ray Fluorescence Spectroscopy (EDAX). Samples of TNT on sand were analyzed varying the mass of the explosive and modifying pH conditions of sand in function of time. The images obtained from SEM showed smooth differences in the morphology and spatial distribution of the explosive on sand a few hours after sample preparation. This change in morphology and spatial distribution was found to be dependent both on the mass ratio of TNT in the sample and the pH of the sand. On the other hand, EDAX measurements showed a decrease in the nitrogen fluorescence signal intensity demonstrating that movement of TNT into the sand occurs after about eight hours; carbon measurements showed a similar behavior during this time, that is called dead time. In addition, the kinetics of surface sublimation for TNT was studied. On the basis to the experimental results, a theoretical model incorporates parametric equations to explain a nonlinear sublimation rate of a solid TNT thin film in a very close proximity to the sand surface. A dependency of the diffusion constants and sublimation on the initial amount of explosive was found.Publication Non-destructive quantitative analysis of moving tablets by Near Infrared Spectroscopy (NIRS)(2007) Suárez Cabán, Ivelisse; Romañach, Rodolfo J.; College of Arts and Sciencies - Sciences; De Jesús, Marco A.; González, David; Department of Chemistry; Baigés, IvanIn recent years, Near Infrared Spectroscopy (NIRS) has gained wide acceptance in pharmaceutical industry. The growing interest in NIR is due to all the advantages that this powerful analytical technique presents. NIR spectroscopy is non-destructive, requires little or no sample preparation, does not require chemicals, solvents and generate little or no waste. The technique is also, simple, fast, and allows for multivariate analysis. NIRS have become a useful analytical tool to develop a system capable of performing non-destructive analysis of drug content for an on-line testing of compressed tablets from a pharmaceutical batch. Analyzing just a fraction of the sample, thus eliminating the need to analyze instead of the entire sample, as in case of HPLC or UV analysis is it best attractive. To explore the merits of NIRS for process control, an on-line drug system was constructed and NIR diffuse reflectance spectra were obtained with a diode array spectrometer that monitors the transport of up to 200 tablets per minute, carried in a stabilized conveyor belt. Ninety tablets with prepared at levels of 1% - 30% (w/w) of Active Pharmaceutical Ingredient (API). The wavelength used in the calibration models was 1645-1863 nm, corresponding to the API of the tablets, although others spectral regions associated with the majority excipients ingredients were also evaluated. Calibration models were constructed using several types of pretreatments to the data for the determination of drug content of the moving tablets, obtaining better results with PLS / SNV using two factors describing a 98.5% of variation with a RSEC of 7.3% and a RSEP of 6.5%.Publication Removal of cadmium, copper, and lead ions from aqueous solutions using waste crumb rubber as sorbent(2007) Sánchez-Rivera, Diana; Román, Félix R.; College of Arts and Sciencies - Sciences; Perales-Pérez, Oscar; Vera, Marisol; Department of Chemistry; Rivera-Santos, JorgeThe removal of toxic heavy metals ions from aqueous solutions using waste tire crumb rubber (WTCR) as a low cost sorbent was studied. Nowadays, a variety of techniques exist to remove heavy metals from water such as, chemical precipitation, electrochemical techniques, and ionic exchange, among others. However, due to their high cost or low efficiency of removal at trace levels, adsorption is seen as a more cost efficient and realistic solution. In our study, parameters such as pH, metal concentration, competition with alkaline metals, and size of the sorbent used, were evaluated for each metal ion in order to optimize the sorption uptake conditions. In addition, mixtures of the metals and their desorption with nitric acid were also studied. The results showed that the metal uptake increased when the WTCR area exposed also increased. The equilibrium between the metal and WTCR surface is reached faster when the initial concentrations were lower (1, 5 ppm). The optimum pH for the three cations was pH 6.0. This is favorable because it is near the pH of most natural waters. Langmuir isotherms were constructed for each metal ion and the constant Qo, that represents the maximum sorbent capacity, was calculated. Lead was the better sorbed metal with a calculated Qo of 1.744 mg/g, followed by copper with a Langmuir constant value of 1.245 mg/g, and finally cadmium with a Qo value of 0.532 mg/g. The mechanism of sorption is facilitated by interactions with the oxygen groups present in the surface of carbon black, a main component of the rubber and ionic exchange with zinc.Publication Synthesis and characterization of ruthenium nanoparticles(2007) Santiago-Torres, Priscila; Castro-Rosario, Miguel E.; College of Arts and Sciencies - Sciences; Irizarry, Roberto; Cádiz-García, Mayra E.; Department of Chemistry; Benítez, JaimeWe report work on the synthesis of ruthenium nanoparticles in a polymer support and, for the first time, characterization of the particles in this support is performed using the scanning tunneling microscope (STM). Ruthenium nanoparticles are prepared from the alcoholic reduction of Ru⁺³, in the presence of 10 nm polyvinyl pyrrolidone nanoparitcles (PVP). UV visible absorption measurements are used to track the stepwise reduction of Ru⁺³ into Ru⁺², Ru⁺¹ and Ru⁰. Ru⁺³ is characterized by a broad absorption spectra between 300 and 470 nm. The maximum absorption wavelength changes from 357 nm to 346 nm upon the reduction of Ru⁺² to Ru⁺¹. The spectrum of the ruthenium nanoparticles is characterized by a high baseline, associated with light scattering by the nanoparticles, and a broad absorption band below 350 nm. The PVP nanoparticles are changed into rods in the synthesis process. The PVP derived rods are about 46.1 nm long and 21.5 nm in diameter. Ruthenium nanoparticles are detected on the rods surfaces in scanning tunneling electron microscopy measurements. Electron tunneling current measurements as a function of bias voltage of the particles on the PVP surface reveal three independent bands associated with the stepwise ionization of Ru into Ru⁺, Ru⁺² and Ru⁺³. No significant results about the mechanism by which PVP particles prevent the agglomeration of ruthenium nanoparticles was found in FTIR measurements.Publication Cyclic voltammetry of DNA bases adsorbed onto polycrystalline gold surface; effect of the adsorbates on the carboplatin oxidation signal(2005) Seda-Barrios, Mariana I.; Vega Olivencia, Carmen A.; College of Arts and Sciencies - Sciences; Román, Félix A.; Meléndez, Enrique; Department of Chemistry; Santos, JeannetteThere is an increasing interest in the creation of modified surfaces with properties that differ from the corresponding bare surfaces. A variety of modified metal surfaces can be developed for several purposes, including surface protection and corrosion inhibition. In order to study the effect of a modified gold surface over the electroactivity of a particular species, carboplatin was chosen as a model electroactive species. The present work was aimed to determine the effect of adsorbed DNA bases on the electrode-mediated oxidation of aqueous carboplatin at physiological conditions. Hydrolyzed carboplatin is an electroactive species capable of undergoing electrocatalytic oxidation over a clean polycrystalline gold surface. Cyclic voltammetry experiments revealed that adsorbed purine bases on gold are good inhibitors of the carboplatin electrocatalytic oxidation. Results will be presented for the interfacial behavior of the four DNA bases over polycrystalline gold surfaces. Recommendations will be given in order to obtain new insights for future work in the field of surface electrochemistry.Publication The role of ultrafast events of hemoglobin I and hemoglobin II ligand complexes from Lucina pectinata(2005) Rodríguez Guilbe, María M.; López Garriga, Juan; College of Arts and Sciencies - Sciences; Melénde, Enrique; López, Gustavo E.; Department of Chemistry; Torres, MadelineIn hemeproteins, the nature of the distal iron ligand, the distal heme environment and the heme iron electronic structure are essential factors in ligand binding dynamics. Ultrafast time resolved absorption studies were performed in HbI(Fe III)CN, HbII(Fe III)CN, HbI(Fe II)CN and HbII(Fe II)CN to evaluate the effect of distal heme environment and the iron electronic structure. The time constants for the cyanide (CN) relaxation of each compound were 3.12 ps, 3.57 ps, 5.78 ps and 5.68 ps respectively. According the results, HbI and HbII ferric CN derivatives did not photodissociate and were dominated by the relaxation of an unphotolyzed derivative through vibrational cooling. In contrast, the HbI and HbII ferrous CN complexes suggested photodissociation of the ligand with no geminate recombination and the ultrafast spectra of both species were dominated by the relaxation of the deoxy species. The results suggested also that the Phe/B10/Tyr substitution did not affect CN dynamics. However, difference in the relaxation time constants was observed between HbICN and HbIICN iron oxidation state. This was interpreted in terms of differences in the relaxation pathway of both species due to their iron electronic nature. Time resolved ultrafast absorption studies were also performed in HbICO, HbIICO and HbI mutants; HbI(Phe/B10/Tyr)CO, HbI(Phe/B10/Val)CO and HbI(Gln/E7/Val)CO. The relaxation time constants of CO derivatives were 9.19 ps, 2.87 ps, 6.23 ps, 4.29 ps and 5.80 ps for HbI, HbII, HbI(Phe/B10/Tyr), HbI(Phe/B10/Val) and HbI(Gln/E7/Val), respectively. Ultrafast results indicated that the dynamics of the HbI mutants resembled that of HbI, suggesting that they both have the similar structural conformational arrangement. The differences between HbII and the HbI(Phe/B10/Tyr) in signal relaxation support the suggestion that the orientation of these distal environment may vary in both adopting two different conformation (open and closed). Similarly, in HbI the docking site process may help to establish a barrier to the reverse rebinding process and thereby inhibits ultrafast geminate ligand rebinding in the closed conformation.Publication Structural changes of ¹³C‐labelled 𝘊𝘩𝘭𝘢𝘮𝘺𝘥𝘰𝘮𝘰𝘯𝘢𝘴 centrin and melittin upon complex formation in the presence of calcium(2006) Del Valle Sosa, Liliana; Pastrana-Ríos, Belinda; College of Arts and Sciencies - Sciences; Ramírez, Doris; Ríos Steiner, Jorge; Department of Chemistry; Ríos Velázquez, CarlosChlamydomonas centrin is an acidic, low molecular weight protein that belongs to the EF-hand superfamily of calcium binding proteins. It is an essential component of the centrosomal structure in a wide range of organisms. ¹³C-labeled Chlamydomonas centrin (¹³C-Cen) and a model peptide from bee venom, known as melittin (MLT) were studied upon complex formation. The main objective of this thesis was to study the level of protein/peptide interaction, to explore structural changes and the relative stability of centrin, mellitin, and the complex. Two sets of experiments were performed H→D exchange via attenuated total reflectance (ATR) and thermal dependence via transmission Fourier transform-infrared (FT-IR) spectroscopy. The spectral analysis was carried-out by two-dimensional correlation analysis (2D-COS). In addition, differential scanning calorimetry (DSC) experiments were performed to corroborate the existence of pre-transitions and the thermal denaturation temperature for these proteins and the complex. For the FT-IR studies, the two protein components were studied simultaneously as: the amide I’ band (1650 cm⁻¹) for MLT and the amide I*’ band (1600 cm⁻¹) for ¹³C-Cen in order to establish the structural changes during the interaction. Furthermore, the amide II band (1550 cm⁻¹) comprised of side chains and the N-H deformation mode, and the amide II’ (1450 cm⁻¹) band comprised of the N-D deformation mode for both proteins are studied to determine side chain interaction and the extent of solvation (80% for ¹³C-Cen, 100% for MLT and 100% ¹³C-Cen/MLT). The interaction within the protein complex, was driven by MLT, which was suggested from the effects observed in the α-helical motifs of centrin to a greater extent than its interaction with calcium. This effect was observed for the calorimetric studies whereby centrin pre-transition was shifted by 13 °C from 81 °C to 95 °C. Finally, these results provide the framework to study other centrin target peptides to further understand the biophysical and biochemical changes driving their interaction.Publication In situ studies of sulfamerazine and carbamazepine crystallization(2005) Ramos Rivera, Zainette; Romañach, Rodolfo J.; College of Arts and Sciencies - Sciences; López, Gustavo E.; Ríos-Steiner, Jorge L.; Department of Chemistry; Hernández, ArturoThe phenomenon of polymorphism in active pharmaceutical ingredients (API) has become an important area of study in the pharmaceutical industry. Polymorphs of a given substance can exhibit different properties. These properties can affect both the bulk chemical properties and pharmaceutical performance. As a result the control and monitoring of crystallization is one of the most important stages in the pharmaceutical manufacturing process. This research project investigated the influence of solvents in the crystallization of two active pharmaceutical ingredients: sulfamerazine (Form I) and carbamazepine (Form III). For sulfamerazine, the experimental approaches to achieve the supersaturated solutions involve two methods: the use of temperature dependence on solubility and the introduction of some other solvent to reduce the solubility of the solution. Seeding a saturated solution was also employed to observe the interaction of solvent in the crystallization process. For carbamazepine only the temperature dependence of solubility was employed. Fiber optics near infrared (NIR) and Raman spectroscopy were used for in situ monitoring of crystallization process. The crystallized solid phase was identified using XRD, SEM and Raman microscopy. The results show that solvents affect the crystallization process of the polymorph. However, the studies did not establish prior to precipitation, any preference for a particular crystal form, when using the band shift of solvent as criteria.Publication Design and characterization of non-invasive thermal sensors for Laser Induced Fluorescence Thermometry (LIFT)(2005) Rivera Guzmán, Leslie A.; Rivera, Luis A.; College of Arts and Sciencies - Sciences; Mina, Nairmen; Robledo, Cynthia; Vega, Carmen; Department of Chemistry; Díaz, RicardoLaser Induced Fluorescence Thermometry (LIFT) is a technique based on imaging to measure the temperature of a substance. The substance is doped with small concentrations of a natural fluorescent dye. LIFT takes advantage of the fluorescence properties of a substance in response to excitation from a laser source. It is selective to a specific wavelength range of emission, it reacts quickly and the temperature can be measured for highly dynamic systems. The objective of the present work is to apply LIFT to measure the thermal history of micron size in-flight droplets and for microscale nucleate boiling processes using water as the main working fluid. For these purposes two LIFT sensors have been developed. Pyrene, which is one of the most widely studied sensitive organic dyes, was one of the sensors evaluated. To perform the experiments in water it was necessary to add surfactants to enhance Pyrene’s solubility in water. A protocol, which considers several parameters, was designed and evaluated to prepare the solutions for the fluorescence studies. The second set of water based temperature sensors were Rhodamine B and the mixture Rhodamine B/Rhodamine 110. Rhodamine B and Rhodamine 110, are very soluble in water and therefore did not required additives. Even though the addition of surfactants was not required for solubilization, they were used to enhance their fluorescence intensities. Rhodamine 110 was used as a non-temperature sensitive reference dye. A mixture of dyes was used because, in principle, the variation in exciting light intensity due to refraction of the light passing through the thermal field was avoided. In general, the fluorescent dyes studied showed fluorescence emission intensities in the range of 1.0 x 10⁷ to 1.5 x 10⁷, which depend directly upon temperature. The requirements to develop a robust sensor for LIFT were fulfilled. The water-based sensors studied were selective to a specific wavelength range, sensitive to temperature from ambient to near boiling point range, reversible to temperature changes, and reproducible within the experimental error.Publication TNT adsorption on Jobos oil and Jobos clay fractions(2006) Rivera Vargas, Rosángela; Mina, Nairmen; College of Arts and Sciencies - Sciences; Hernández, Samuel P.; Muñoz, Miguel A.; Department of Chemistry; Benítez, JaimeThe use and manufacture of nitroaromatic compounds have resulted in contamination of soil, sediments and aquifers. TNT is the most common nitroaromatic explosive contaminant. The adsorption behavior of TNT was evaluated in soil and clay samples from Jobos Ap and A horizons. The clay fraction was separated from the other soil components by centrifugation. Physical and chemical characterization studies such as cation exchange capacity (CEC), percent of organic matter, pH, surface area and texture were performed on soil and clay samples. X-ray diffraction analysis of the clay fraction revealed the presence of kaolinite, goethite, hematite, gibbsite and quartz. High performance liquid chromatography (HPLC) was used to determine TNT adsorption. TNT adsorption on the soil was described by the Langmuir model. A higher adsorption was observed on the Ap horizon (83 ± 6 mg/kg) than on the A horizon (44 ± 4 mg/kg). The higher organic matter content of the Ap horizon may have contributed to the higher TNT adsorption observed. TNT adsorption on the clay fraction was described by the Freundlich model. The adsorption coefficient (K𝘥) for the clay fraction of the A horizon was 109 ± 13 L/Kg, whereas for the Ap clay fraction it was 73 ± 9 L/kg. The higher adsorption on the clay fraction of the A horizon may be the result of a larger surface area (189.71 m² /g). Higher TNT adsorption resulted in less TNT recovery. The recovery of TNT adsorbed on Ap soil samples was 26%, whereas 53% was recovered from the A horizon samples. A similar situation was observed for the clay fractions. The recovery of adsorbed TNT from the Ap clay fraction, where less adsorption was observed, was 84%, whereas for the A clay fraction it was 78%.Publication Synthesis and characterization of calcium sulfide nanoparticles(2014) Rivera Vázquez, Daniel; Castro-Rosario, Miguel E.; College of Arts and Sciencies - Sciences; Cádiz, Mayra E.; Santana, Alberto; Uwakweh, Oswald N.C.; Department of Chemistry; Gutiérrez, GustavoCalcium sulfide (CaS) nanoparticles are cadmium free fluorescent nanostructures with potential applications in nanomedicine and as a sensing material. We employed different methods for the synthesis of CaS nanoparticles, including (1) the reaction of calcium acetate (Ca(CH₃CO₂)₂) and sodium sulfide (Na₂S) in dimethyl sulfoxide (DMSO), (2) the reaction of Ca(CH₃CO₂)₂ and DMSO in a microwave and (3) dissolving bulk quantities of CaS in DMSO. UV-Vis spectroscopy was used to determine the optical properties of the CaS nanostructures. The absorption spectra of CaS prepared from these methods consists of a well-defined peak in the UV and a long wavelength tail that extends above 700 nm. Emission bands centered around 500 nm with a long wavelength tail that extends above 600 nm are observed upon excitation at 405 nm. Scanning tunneling microscopy (STM) was used for determining the average diameter of CaS nanostructures of (3.3 ± 0.7) nm. The direct and indirect band gaps are estimated to be (0.403 ± 0.003) eV and (4.135 ± 0.006) eV. Configuration interaction singles (CIS) calculations were used to determine the electronic transition spectra of small CaS clusters. Unique to CaS nanostructures is the absorption of light at wavelengths longer that in the bulk material instead of the blue shift associated with quantum confinement effects in semiconductors. Indeed, the strong absorption bands in the visible region of the spectra of the CaS nanostructures do not have a counterpart in the gas or solid phases, with implications in the field of sensing using semiconducting nanoparticles. Density functional theory (DFT) calculations on small CaS clusters are used to establish the vibrational properties of calcium sulfide nanoclusters. Naked CaS nanostructures are found to limit the survival rate and inhibit the growth rate of carcinoma cancer cell lines (ATCC CRL-2124). A single dose of a dispersion containing (3.3 ± 0.7) nm CaS nanoparticles (total calcium content of 3.8 x10⁻⁸ moles) results in a sharp decrease in the density of live cells and a sharp increase in the number of dead cells 96 hours following the dose. No changes are observed in adenocarcinoma control cell lines. CaS do not have any effect on the survival and growth rate of normal fibroblasts as compared to control cell cultures of normal fibroblasts, which suggests that CaS nanostructures are cancer specific. The small size of the nanostructures allows them to have access to the normal and newly developed blood vessels-due to angiogenesis and elimination by the human body. The results encourage further research to establish the mechanisms of action of CaS in cancer cell cultures and laboratory animals.Publication Surfactant Protein C analogues and model lipid interactions in lung surfactant(2006) Ramos-Torres, Sindia; Pastrana-Ríos, Belinda; College of Arts and Sciencies - Sciences; Vera-Colón, Marisol; Ríos-Steiner, Jorge; Department of Chemistry; Ríos-Velázquez, CarlosLung surfactant is a mixture of lipids and proteins that stabilizes the respiratory surface of lungs against collapse. Deficiency in its amount or composition is related to severe respiratory disorders. Exogenous-surfactant treatment has proven to be highly effective for improved oxygen index, increased lung compliance and thus, reduce the work of breathing. Fourier transform infrared (FT-IR) spectroscopy and differential scanning calorimetry (DSC) were successfully applied to study ternary mixtures of native Surfactant Protein C (SP-C), modified SP-C (mSP-C) or T₁₁ peptoid in a model lipid mixture of DPPC-𝘥₆₂/DPPG (7:1 mol/mol). The secondary structure of the SP-C analogues, the orientation of α-helical segments relative to the bilayer normal in membrane films and their effect in the thermotropic properties of lipid multillamelar vesicles were determined. Modified SP-C was found to predominantly possess α-helical secondary structure as native SP-C. The helical segments of mSP-C and SP-C exhibited a parallel orientation with respect to the membrane normal. Only ~25 % of exchangeable protons were observed to exchange in D₂O, which reflects the hydrophobic nature of both molecules. However, T₁₁ is comprised of multiple conformations, which require further analysis by FT-IR. In addition, T₁₁ produced smaller effects on the thermotropic properties of the lipidic environment, as measured from acyl chain CH₂ and CD₂ antisymmetric and lipid carbonyl stretching frequencies. The van’t Hoff enthalpies were also determined by both techniques. The obtained results were used to evaluate the synthetic peptide candidates for lung surfactant replacement therapy.Publication Theoretical studies of 2,4-DNT molecular structure and its interaction with siloxane surface of clays(2005) Ramos Reyes, Carmen M.; Mina, Nairmen; College of Arts and Sciencies - Sciences; Hernández-Rivera, Samuel P.; Muñoz, Miguel A.; Department of Chemistry; Mandavilli, SatyaAmong the many different signature compounds emitted from a landmine in the vapor phase, 2,4-DNT is the most common nitroaromatic compound used to detect buried landmines. This study presents ab initio quantum mechanical calculations on the interaction of 2,4-DNT (DNT) with the basal siloxane surface of the clay mineral kaolinite. Theoretical calculations of the low energy conformation of DNT interacting with the siloxane surface of clay minerals were performed to determine obtain their properties once adsorbed as well as the structure of the adsorbed molecule. The calculations also indicate the orientation of DNT adsorbed on the clay surface and the effect of adsorption. This study was performed using DFT, DFT//HF and MP2//HF methods taking into account the contribution of the Coulombic (CE𝘣) and dispersion (DE𝘣) energies, to obtain the binding energies between DNT and siloxane surface. A comparison of the CE𝘣 and DE𝘣 energies shows that the stabilization of DNT at the siloxane sites is mainly provided by dispersion interaction energy. Considering the accuracy and cost of the computation methods the 6-31+G* basis set produced the best representation of the interaction energy (42 kJ/mol) using the MP2//HF level of theory for the interaction. These theoretical calculations give a good prediction of the interaction between the 2,4-DNT molecule with soil clay minerals. The computational results are compared with the experimental results obtained with the FT-IR microspectroscopic technique. A solvation analysis, using water as solvent, was performed to the DNT, the PCM solvation model affects significantly the spectroscopic signature of the explosive molecule.Publication Fiber Optic Coupled Grazing Angle/Fourier Transform Reflection Absorption Infrared Spectroscopy as an analyzer of explosive and pharmaceutical residues on surfaces(2005) Primera-Pedrozo, Oliva M.; Hernandez-Rivera, Samuel P.; College of Arts and Sciencies - Sciences; Briano-Peralta, Julio G.; Castro-Rosario, Miguel E.; Department of Chemistry; Suleiman, DavidFiber Optic Coupled Grazing Angle/Fourier Transform Reflection Absorption Infrared Spectroscopy (FT-RAIRS) has been investigated as a potential technique to develop a new methodology for detection of explosive and pharmaceutical residues on surfaces. The methodology is remote sensed, in situ and can detect nanograms of the compounds. It is solvent free and requires no sample preparation. Smearing technique and thermal inkjet (TIJ) technology were used for transferring the target analyte on the substrates to be used as standards. The sample transfer methods gave good sample distribution, reduced sample loss on transfer and were easy to manipulate giving good reproducible distributions. Samples with surface concentrations ranging from micrograms/cm² to nanograms/cm² of explosives: 2,6-dinitrotuelene (DNT), 2,4,6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), nitroglycerine (NG) and triacetone triperoxide (TATP) and of the active pharmaceutical ingredient (API) ibuprofen were deposited on stainless steel metal surface. Methanol, acetone, dichloromethane, tetrahydrofuran and acetonitrile were used as transfer solvents for smearing. Mixtures of Acetonitrile, methanol and 2-propanol were used for thermal inkjet mass transfers. TATP sublimation properties were analyzed at various temperatures. The data were analyzed using Chemometrics routines. In particular, Partial Least Squares (PLS) multivariate analysis was used. The results of the experiments gave intense absorption bands in the fingerprint region of the infrared spectrum that were used to quantify and calculate the detection limit for each compound.Publication Dynamic of TNT photo dissociation studied by femtosecond laser-mass spectroscopy(2006) Osorio Cantillo, Celia M.; Castro Rosario, Miguel E.; College of Arts and Sciencies - Sciences; Hernández Rivera, Samuel P.; López, Gustavo; Department of Chemistry; Cardona, NelsonThe 2,4,6-trinitrotoluene, (TNT), is a widely used explosive material whose photo-dissociation both as a solid and in solution using light radiation has been previously studied. TNT can be used as suitable molecule to learn about the elementary process involved in photo induced dissociation. Furthermore, results in TNT photochemistry may provide information on sensing schemes for this explosive, which can be extrapolated to the detection of other nitroexplosives. NO₂ and NO are typically associated with the photodegradation of TNT. Thus studies that can establish the kinectic energy distribution of nascent NO₂ and secondary NO have the potential of contributing to selective sensing by adding to a library based on differences in internal energy distribution in photoproducts of nitro explosives. The purpose of this study is to establish the kinetic energy distribution of NO₂ and NO as a product of photo-fragmentation of neat TNT or and TNT on Ottawa sand using femtosecond laser pulses for molecular dissociation and subsequent mass spectrometry measurements as a function of time. Photolysis experiments on TNT deposits irradiated with 100 femtosecond laser pulses at 400 nm and 266 nm wavelengths were performed. NO₂ and NO were analyzed as key fragments. The obtained data showed that there are slight differences in the NO₂ and NO speeds from TNT crystals and TNT deposited on sand and vapor phase of TNT. Those results indicate that the energetic processes that result in fragments ejection are different in all cases. In conclusion, photo-fragmentation using femtosecond lasers followed by mass spectrometry measurement is a sensitive approach for the kinetic energy distribution establishment of NO/NO₂ fragments; besides, its effectiveness could be enhanced coupling it with another spectroscopic method for trace amounts detection of nitrorganic explosives.