The value of the surface charge density of polypropylene reported in literature has a three order of magnitude discrepancy. Nauruzbayeva et al report a 0.7nCcm−2 as the surface charge density of polypropylene as measured using the charge of electrified droplets[1]. Meagher and Craig reported result 111nCcm−2 as estimated by electric double layer theory from colloidal probe Atomic force microscopy (AFM) force spectroscopy [2]. We show that oxidation of hydrophobic surfaces as a potential mechanism in origin of these surface charges. Using colloidal probe AFM We measured the surface charge densities of Teflon AF, perfluorodecanethiol, Perfluorodecyltrichlorosilane(FDTS), Octadecyltrichlorosilane, polystyrene, and polypropylene. Also, The pH dependence of the surface charge density for FDTS was studied and it shows the behavior expected of a weak acid in response to pH. We suspect that the origin of the surface charges is mostly likely impurities or surface oxidation. We conclude that the electrometer and dispensed droplets approach cannot detect these charges because of the process of de-wetting all the surface be neutralized to maintain charge neutrality. This explanation supports Nauruzbayeva et al claims about surface bound charges[1].

Since single nanopores were firstly proposed as a potential rapid and low-cost tool for DNA sequencing in 1990s (PNAS, 1996, 93, 13770), extensive studies on both biological and synthetic nanopores and nanochannels have been reported. Nanochannel based stochastic sensing at single molecular level has been widely reported through the detection of transient ionic current changes induced by geometry blockage due to analytes translocation. Novel properties, including ion current rectification (ICR), memristive and memcapacitive behaviors were reported. These fundamental properties of nanochannels arise from the nanoscale dimensions and enables applications not only in single molecule sensing, but also in drug delivery, electrochemical energy conversion, concentration enrichment and separation, nanoprecipitation, nanoelectronics etc. Electrostatic interactions at nanometer-scale between the fixed surface charges and mobile charges in solution play major roles in those applications due to high surface to volume ratio. However, the knowledge of surface charge density (SCD) at nanometer scale is inaccessible within nanoconfinement and often extrapolated from bulk planar values. The determination of SCD at nanometer scale is urgently needed for the interpretation of aforementioned phenomena. This dissertation mainly focuses on the determination of SCD confined at a nanoscale device with known geometry via combined electroanalytical measurements and theoretical simulation. The measured currents through charged nanodevices are different for potentials with the same amplitude but opposite polarities, which deviates away from linear Ohm's behavior, known as ICR. Through theoretical simulation of experiments by solving Poisson and Nernst-Planck equations, the SCD within nanoconfinement is directly quantified for the first time. An exponential gradient SCD is introduced on the interior surface of a conical nanopre based on the gradient distribution of applied electric field. The physical origin is proposed based on the facilitated deprotonation of surface functional groups by the applied electric field. The two parameters that describe the non-uniform SCD distribution: maximum SCD and distribution length are determined by fitting high- and low-conductivity current respectively. The model is validated and applied successfully for quantification and prediction of mass transport behavior in different electrolyte solutions. Furthermore, because the surface charge distribution, the transport behaviors are intrinsicaly heterogeneous at nanometer scale, the concept is extended to noninvasively determine the surface modification efficacy of individual nanopore devices. Preliminary results of single molecule sensing based on streptavidin-iminobiotin are included. The pH dependent binding affinity of streptavidin-iminobiotin binding is confirmed by different current change signals ("steps" and "spikes") observed at different pHs. Qualitative concentration and potential dependence have been established. The chemically modified nanopores are demonstrated to be reusable through regenerating binding surface.

Made available in DSpace on 2014-12-17T14:07:19Z (GMT). No. of bitstreams: 1 DianaCSL.pdf: 800391 bytes, checksum: c85f3fef7e600f91edb93d3096219c80 (MD5) Previous issue date: 2005-11-25 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The addition of active silica potentially improves the quality of concrete due to its high reactivity and pore refinement effect. The reactivity of silica is likely related to its charge density. Variations in surface charge alter the reactivity of the material consequently affecting the properties of concrete. The present study aimed at investigating variations in the charge density of silica as a function of acid treatments using nitric or phosphoric acid and different pH values (2.0, 4.0 and 6.0). Effects on concrete properties including slump, mechanical strength, permeability and chloride corrosion were evaluated. To that end, a statistical analysis was carried out and empirical models that correlate studied parameters (pH, acid and cement) with concrete properties were established. The quality of the models was tested by variance analysis. The results revealed that the addition of silica was efficiency in improving the properties of concrete, especially the electrochemical parameters. The addition of silica treated using nitric acid at pH = 4.0 displayed the best cement performance including highest strength, reduced permeability and lowest corrosion current / A adi??o da s?lica ativa ao concreto melhora a qualidade deste material, amplamente utilizado na Constru??o Civil, devido ? a??o dos efeitos: alta reatividade e refinamento dos poros. Acreditando-se que a reatividade da s?lica est? relacionada ? sua densidade de carga, uma varia??o da carga superficial modifica a reatividade do material, e consequentemente as propriedades do concreto. Assim, este trabalho objetiva estudar o efeito da varia??o da densidade de carga superficial da s?lica tratada com solu??es de ?cidos n?trico e fosf?rico a diferentes pHs: 2,0; 4,0 e 6,0 em propriedades do concreto: abatimento, resist?ncia ? compress?o, permeabilidade e par?metros indicativos do processo de corros?o por cloreto. Para tanto, realizou-se um planejamento experimental com a finalidade de obter modelos emp?ricos que relacionam as propriedades e par?metros avaliados com as vari?veis em estudo: pH, tipo de ?cido e tipo de cimento, bem como a an?lise de vari?ncia indicativa da qualidade do modelo proposto em rela??o ?s observa??es experimentais. Os resultados comprovaram a efic?cia da adi??o de s?lica ativa na mistura de concreto ?s propriedades estudadas. Indicam que uma varia??o na densidade de carga superficial da s?lica usada na confec??o de corpos de prova de concreto modifica as propriedades e par?metros eletroqu?micos estudados e que, o concreto com uso de s?lica tratada com solu??o de ?cido n?trico de pH 4,0, apresentou melhor qualidade: maior resist?ncia ? compress?o, menor permeabilidade e menor corrente de corros?o, quando tomados em rela??o aos demais

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Cellulose nanocrystals (CNCs) are frequently prepared by sulfuric acid hydrolysis of a purified cellulose starting material. CNC yields, however, are generally low, often below 20%. This study employs response surface methodology to optimize the hydrolysis conditions for maximum CNC yield. Two experimental designs were tested and compared: the central composite design (CCD) and the Box–Behnken design (BBD). The three factors for the experimental design were acid concentration, hydrolysis temperature, and hydrolysis time. The responses quantified were CNC yield, sulfate group density, ζ-potential, z-average diameter, and Peak 1 value. The CCD proved suboptimal for this purpose because of the extreme reaction conditions at some of its corners, specifically (1,1,1) and (–1,–1, –1). Both models predicted maximum CNC yields in excess of 65% at similar sulfuric acid concentrations (~59 wt %) and hydrolysis temperatures (~65 °C). With the BBD, the hydrolysis temperature for maximum yield lay slightly outside the design space. All three factors were statistically significant for CNC yield with the CCD, whereas with the BBD, the hydrolysis time in the range 60–150 min was statistically insignificant. With both designs, the sulfate group density was a linear function of the acid concentration and hydrolysis temperature and maximal at the highest acid concentration and hydrolysis temperature of the design space. Both designs showed the hydrolysis time to be statistically insignificant for the ζ-potential of CNCs and yielded potentially data-overfitting regression models. With the BBD, the acid concentration significantly affected both the z-average diameter and Peak 1 value of CNCs. However, whereas the z-average diameter was more strongly affected by the hydrolysis temperature than the hydrolysis time, the Peak 1 value was more strongly affected by the hydrolysis time. The CCD did not yield a valid regression model for the Peak 1 data and a potentially data-overfitting model for the z-average diameter data. A future optimization study should use the BBD but slightly higher hydrolysis temperatures and shorter hydrolysis times than used with the BBD in this study (45–65 °C and 60–150 min, respectively). / Master of Science

Humic acids and humates are principal components of humic substances major organic constituents of soil, peat, coal and water around the world. I was involved in research into molecular dynamics simulations of interactions of quartz surfaces with aqueous solutions of ions and small organic molecules representing basic building blocks of larger biomolecules and functional groups of organic matter. We studied interactions of molecules with surfaces for a set of surface charge densities corresponding to the experimentally or environmentally relevant ranges of pH values employing molecular mechanics, molecular dynamics and ab initio techniques. Simulated quartz surfaces covered the range of surface charge densities 0.00, -0.03, -0.06 and -0.12 C-m-2, approximately corresponding to pH values 4.5, 7.5, 9.5 and 11. As model molecules, benzoic acid, phenol, o-salicylic acid and their conjugated bases were chosen. My task was to prepare topologies and parametric models of selected organic matter basic building blocks organic molecules. I focused on studying interactions of these molecules in an aqueous environment with mineral surface quartz. The aim was to process simulation results and analyse conformations of the adsorption complexes and their thermodynamic properties such as interaction energies, free energies and adsorption geometries.