Nanoscale colloidal particles display fascinating electronic, optical and reinforcement properties as a consequence of their dimensions. Stable dispersions of nanoscale colloids find applications in drug delivery, biodiagnostics, photonic and electronic devices, and polymer nanocomposites. Most nanoparticles are unstable in dispersions and polymeric surfactants are added generally to improve dispersability and control self-assembly. However, the effect of polymeric modifiers on nanocolloid properties is poorly understood and design of modifiers is guided usually by empirical approaches. Monte Carlo simulations are used to gain a fundamental molecular-level understanding of the effect of modifiers properties on the thermodynamics and interaction forces of nanoscale colloidal particles. A novel method based on the expanded ensemble Monte Carlo technique has been developed for calculation of the chemical potential of colloidal particles in colloid-polymer mixtures (CPM). Using this method, the effect of molecular parameters like colloid diameter, polymer chain length, colloid-polymer interaction strength, and colloid and polymer concentrations, on the colloid chemical potential is investigated for both hard-sphere and attractive Lennard-Jones CPM. The presence of short-chain polymeric modifiers reduces the colloid chemical potential in attractive as well as athermal systems. In attractive CPM, there is a strong correlation between polymer adsorption and colloid chemical potential, as both show a similar dependence on the polymer molecular weight. Based on the simulation results, simple scaling relationships are proposed that capture the functional dependence of the thermodynamic properties on the molecular parameters. The polymer-induced interaction forces between the nanoparticles have been calculated as a function of the above parameters for freely-adsorbing and end-grafted homopolymer modifiers. The polymer-induced force profiles are used to identify design criteria for effective modifiers. Adsorbing modifiers give rise to attractive interactions between the nanoparticles over the whole parameter range explored in this study. Grafted surface modifiers lead to attraction or repulsion based on the polymer chain length and grafting density. The polymer-induced attraction in both adsorbing and grafted modifiers is attributed primarily to polymer intersegmental interactions and bridging. The location of the thermodynamic minimum corresponding to the equilibrium particle spacing in nanoparticle-polymer mixtures can be controlled by tuning the modifier properties.

Thesis (MTech. : Chemical Engineering.)--Tshwane University of Technology, 2010.

Nos quadros de sepse ocorre o aumento da permeabilidade vascular, translocação e perda de albumina para o espaço extravascular, resultando assim em hipoalbuminemia e redução da pressão coloidosmótica plasmática. Desta forma o objetivo deste estudo foi avaliar a relação da pressão coloidosmótica com a evolução clínica de 41 cadelas com sepse grave ou choque séptico decorrente de piometra que foram submetidas a ovariosalpingohisterectomia. Para tanto, os valores da pressão arterial sistólica, do débito urinário, do lactato, do déficit de base venoso e da pressão coloidosmótica foram avaliados a cada três horas ao longo do período de internação. O momento da mensuração da pressão coloidosmótica foi distinto entre os grupos, sendo grupo I (critério clínico) (n= 21) avaliado após o fim do tratamento e no grupo II (critério quantitativo) (n= 20), as amostras foram avaliadas imediatamente após a colheita. As variáveis clínicas utilizadas como guia a administração de coloide no grupo I não apresentaram correlação com os valores de pressão coloidosmótica baixo. A administração de coloide não apresentou impacto sobre os valores de albumina e pressão coloidosmótica, bem como não interferiu na perfusão tecidual. A pressão coloidosmótica apresentou uma correlação não significativa e inversamente proporcional com o SOFA. Desta forma, a partir dos resultados obtidos é possível concluir que os valores da pressão coloidosmótica não apresentaram correlação com os valores das variáveis de perfusão tecidual; o coloide não contribuiu para a melhora da perfusão tecidual e da manutenção da pressão coloidosmótica após a administração de grandes volumes de solução cristaloide. / Sepsis induces increased vascular permeability, translocation and albumin loss to extracellular space, resulting in hypoalbuminemia and lower plasma colloid osmotic pressure. This study correlate colloid osmotic pressure with the clinical progress of 41 female dogs presenting severe sepsis or sepsis shock due to pyometra, submitted to ovario-salpingo-hysterectomy. The parameters evaluated were: systolic arterial pressure, urinary debt, blood lactate, venous base excess and colloid osmotic pressure. Samples were collected every three hours during the inpatient period. Laboratorial analysis of colloid osmotic pressure was performed in different timings: group I (clinic criteria, n=21), samples analyzed only after the end of the treatment; and group II (quantitative criteria, n=20): samples analyzed immediately after collection. The parameters were a guide to evaluate the necessity of colloid administration. We found no correlation between the parameters evaluate and the values of colloid osmotic pressure. The administration of colloid presented a non-significant and inversely proportional correlation with SOFA. We concluded that colloid osmotic pressure is not directly correlated with tissue perfusion and colloid administration does not improve tissue perfusion or colloid osmotic pressure, even after the administration of substantial volumes of crystalloid solution.

No description available.

In Sweden the encapsulated nuclear waste will be surrounded by compacted bentonite in the granitic host rock. In contact with water-bearing fractures the bentonite barrier may release montmorillonite colloids that may be further transported in groundwater. If large amounts of material are eroded from the barrier, the buffer functionality can be compromised. Furthermore, in the scenario of a leaking canister, strongly sorbing radionuclides, can be transported by montmorillonite colloids towards the biosphere. This thesis addresses the effects of groundwater chemistry on the generation, stability, sorption and transport of montmorillonite colloids in water bearing rock fractures. To be able to predict quantities of montmorillonite colloids released from the bentonite barrier in contact with groundwater of varying salinity, generation and sedimentation test were performed. The aim is first to gain understanding on the processes involved in colloid generation from the bentonite barrier. Secondly it is to test if concentration gradients of montmorillonite colloids outside the barrier determined by simple sedimentation experiments are comparable to generation tests. Identical final concentrations and colloid size distributions were achieved in both types of tests. Colloid stability is strongly correlated to the groundwater chemistry. The impact of pH, ionic strength and temperature was studied. Aggregation kinetics experiments revealed that for colloid aggregation rate increased with increasing ionic strength. The aggregation rate decreased with increasing pH. The temperature effect on montmorillonite colloid stability is pH-dependent. At pH≤4, the rate constant for colloid aggregation increased with increasing temperature, regardless of ionic strength. At pH≥10, the aggregation rate constant decreased with increasing temperature. In the intermediate pH interval, the aggregation rate constant decreased with increasing temperature except at the highest ionic strength, where it increased. The relationship between the rate constant and the ionic strength allowed the critical coagulation concentration (CCC) for Na- and Ca-montmorillonite to be determined. In order to distinguish the contribution of physical filtration and sorption to colloid retention in transport, the different retention mechanisms were quantified. Sorption on different representative minerals in granite fractures was measured for latex colloids (50, 100, 200 nm) and montmorillonite colloids as a function of ionic strength and pH. Despite of the negative charge in mineral surfaces and colloids, sorption was detected. The sorption is correlated to the mineral point of zero charge and the zeta potential of the colloids, and increases with increasing ionic strength and decreasing pH. In transport experiments with latex colloids in columns packed with fracture filling material, the retention by sorption could clearly be seen. In particular at low flow rates, when the contact time for colloids with the mineral surfaces were the longest, sorption contributed to retention of the transport significantly. The retention of latex colloids appeared to be irreversible in contrary to the reversible montmorillonite colloid retention. Generation, stability and sorption of the montmorillonite colloids are controlled by electrostatic forces; hence, the results were in qualitative agreement with DLVO.

One of the most important problems in statistical mechanics is the measurement of free energies, these being the quantities that determine the direction of chemical reactions and--the concern of this thesis--the location of phase transitions. While Monte Carlo (MC) computer simulation is a well-established and invaluable aid in statistical mechanical calculations, it is well known that, in its most commonly-practised form (where samples are generated from the Boltzmann distribution), it fails if applied directly to the free energy problem. This failure occurs because the measurement of free energies requires a much more extensive exploration of the system's configuration space than do most statistical mechanical calculations: configurations which have a very low Boltzmann probability make a substantial contribution to the free energy, and the important regions of configuration space may be separated by potential barriers. We begin the thesis with an introduction, and then give a review of the very substantial literature that the problem of the MC measurement of free energy has produced, explaining and classifying the various different approaches that have been adopted. We then proceed to present the results of our own investigations. First, we investigate methods in which the configurations of the system are sampled from a distribution other than the Boltzmann distribution, concentrating in particular on a recently developed technique known as the multicanonical ensemble. The principal difficulty in using the multicanonical ensemble is the difficulty of constructing it: implicit in it is at least partial knowledge of the very free energy that we are trying to measure, and so to produce it requires an iterative process. Therefore we study this iterative process, using Bayesian inference to extend the usual method of MC data analysis, and introducing a new MC method in which inferences are made based not on the macrostates visited by the simulation but on the transitions made between them. We present a detailed comparison between the multicanonical ensemble and the traditional method of free energy measurement, thermodynamic integration, and use the former to make a high-accuracy investigation of the critical magnetisation distribution of the 2d Ising model from the scaling region all the way to saturation. We also make some comments on the possibility of going beyond the multicanonical ensemble to `optimal' MC sampling. Second, we investigate an isostructural solid-solid phase transition in a system consisting of hard spheres with a square-well attractive potential. Recent work, which we have confirmed, suggests that this transition exists when the range of the attraction is very small (width of attractive potential/ hard core diameter ~ 0.01). First we study this system using a method of free energy measurement in which the square-well potential is smoothly transformed into that of the Einstein solid. This enables a direct comparison of a multicanonical-like method with thermodynamic integration. Then we perform extensive simulations using a different, purely multicanonical approach, which enables the direct connection of the two coexisting phases. It is found that the measurement of transition probabilities is again advantageous for the generation of the multicanonical ensemble, and can even be used to produce the final estimators. Some of the work presented in this thesis has been published or accepted for publication: the references are G. R. Smith & A. D. Bruce, A Study of the Multicanonical Monte Carlo Method, J. Phys. A. 28, 6623 (1995). [reference details doi:10.1088/0305-4470/28/23/015] G. R. Smith & A. D. Bruce, Multicanonical Monte Carlo Study of a Structural Phase Transition, to be published in Europhys. Lett. [reference details Europhys. Lett. 34, 91 (1996) doi:10.1209/epl/i1996-00421-1] G. R. Smith & A. D. Bruce, Multicanonical Monte Carlo Study of Solid-Solid Phase Coexistence in a Model Colloid, to be published in Phys. Rev. E [reference details Phys. Rev. E 53, 6530–6543 (1996) doi:10.1103/PhysRevE.53.6530].

This thesis investigated uranium (U) migration behaviour at the Needle‟s Eye natural analogue site, located close to Southwick Water, South West Scotland. The results of this study are important for the prediction of U behaviour in the far-field environments of nuclear waste repositories over long time-scales. The Needle‟s Eye natural analogue site was selected because the processes involved in U mobilisation, the direction of water flow and the extent of retention of uranium in peaty soils had already been identified. To this end, previous results demonstrated that groundwater passing through the mineralisation oxidized U and transported it to the peaty area, where 80-90% of the released U has been retained. Sequential extraction of the peaty soils indicated that more than 90% of the solid phase U was bound to the organic fraction. However, in-depth characterisation of U associations within the soil porewaters and the peaty soils at this site was lacking. Therefore, the processes controlling the migration of uranium within this organic-rich system were the main focus of this study. There were five sampling trips carried out from 2007-2011, in which cave drip waters, bog waters and surface soil and soil core samples were selectively collected for analysis by a range of methods described below. The cave drip waters emerging from the mineralisation were oxidizing and slightly alkaline (7.6-7.8), U was mainly in truly dissolved (<3 kDa) forms (Ca2UO2(CO3)30, CaUO2(CO3)32- and UO2(CO3)22-). It is known that the formation of the ternary Ca-UVI-CO3 complexes inhibits the reduction of U and so it is likely that it is UVI that is present within the peaty soils and their associated porewaters. Sampling trip 1 quantified the U concentrations in cave waters and soil core porewaters. By 30 m from the cave, U concentrations in the soil porewaters had decreased by a factor of ~10. Ultrafiltration fractionated the colloidal fraction (3 kDa-0.2 μm) into large (100 kDa-0.2 μm), medium (30-100 kDa) and small (3-30 kDa) colloidal fractions. It was found that U was mainly associated with the large colloid (100 kDa-0.2 μm) but, with increasing distance from the mineralisation, the U distribution became bimodal with both large and small fractions being equally important. Iron (Fe) was exclusively associated with the large colloid fraction in the peaty soil porewaters. Gel electrophoresis and gel filtration, applied to study the interactions of U (and other elements) with humic substances (HS), showed that the associations were quite uniform with increasing depth of the cores and increasing distance from the U mineralisation. Uranium (and other elements including Fe) was associated with the largest humic molecules. Sampling trip 2 involved collection of three more soil cores and ultrafiltration again fractionated the total dissolved porewater into large, medium and small colloids. This time, the truly dissolved (<3 kDa) fraction was also analysed. Again, U was mainly associated with the large colloidal (100 kda-0.2 μm) fraction. With increasing distance and increasing depth, U was still predominantly associated with the large colloidal fraction, but the importance of the truly dissolved (<3 kDa) phase could not be neglected. At the same time, Fe was also mainly associated with the large colloidal fraction. The remainder of the experimental work on samples from trip 2 focused on determining the importance of U associations with both Fe and humic components of the solid phase. Sequential extraction of the whole soil mainly targeted different iron phases and found that U was mainly released in the sodium acetate and sodium dithionite solutions, which indicated U was associated with (i) Fe carbonates; and (ii) crystalline Fe oxides (e.g. goethite, hematite, and akaganetite). However, very little Fe was extracted in the “carbonate-bound” fraction and separate experiments showed that U was not associated with Fe carbonates but instead had been released from the surfaces of HS and humic-bound Fe surfaces. XRD spectroscopy showed that mineral compositions were in reasonable agreement with the sequential extraction results and SEM-EDX analysis indicated that U in the soil was generally not present in crystalline form, as only two particles with high U content were found after 4-hour searching. Exhaustive extraction of HS showed that >90% U was associated with organic substances, in agreement with previous work and novel experiments involving gel electrophoresis in conjunction with sequential extraction was used to study the relationships between U, Fe and the HS. It was demonstrated that ~20-25% U was weakly held by the HS or at humic-bound Fe surfaces, ~45% was incorporated into crystalline Fe oxides which were intimately associated with HS and the remainder was in the form of strong U-CO3-humic complexes. In sampling trip 3, U migration behaviour in the soil porewaters was the focus. A 30-m transect line, comprising seven0-5 cm soil samples, starting at the cave and passing through the peaty area towards the Southwick Water, was established. Soil porewaters from these surface soils were fractionated into colloidal (3 kDa-0.2 μm) fraction and truly dissolved (<3 kDa) phase. There was a major change in U speciation, from Ca2UO2(CO3)3 0,CaUO2(CO3)32- and UO2(CO3)22- in the truly dissolved fractions of waters close to the cave to a predominant association with the highly coloured colloidal fractions as soon as the boggy area was reached. With distance through the boggy area, it was clear that the colloidal U was being incorporated into the solid phase since porewater concentrations had decreased ~100-fold by 30 m from the cave. Ultrafiltration in conjunction with acetate extraction was then used to extract U from the porewater colloids isolated from a soil core (20 m from cave). In the organic-rich portion of the core (0-30 cm), ~60-70% U was colloidally associated and ~85-95% of this U was extracted from the colloidal fraction. This indicated that the interactions between U and the porewater colloids were weak. In sampling trip 4, U associations in the porewater colloids were still the main focus. Gel filtration of porewater colloids confirmed that U, Fe and humic colloids were intimately associated. It was concluded that although U in the cave drip water was mainly in truly dissolved forms, weak U----humic/Fe colloids were formed immediately when U entered the peaty area. In sampling trip 5, results for soil core porewaters showed that Fe in the whole core was mainly in the form of FeII. Thus strongly reducing conditions prevailed through the core which was situated within the peaty area. Combining the results from the five sampling trips, three zones within the peaty area were distinguished. Zone I was characterised by extremely high concentrations of dissolved HS and this was where the change in U speciation from dissolved to colloidal forms took place. Zone II contained most of the soil cores collected during this study and was characterised by strongly reducing conditions and moderate concentrations of HS. Colloidal U was removed to the solid phase as waters flow through this area. Zone III marks the transition to the saltmarsh. Focusing on Zone II, a conceptual model of U behaviour was developed: upon entering the peaty area, U is weakly held by very large humic-Fe colloids. These colloids are removed to the solid phase and over time the associations of U are transformed; some becomes incorporated into stable humic-bound crystalline oxides as a result of redox cycling of Fe, some becomes strongly complexed to HS and the remainder is weakly held by the HS and/or humic-bound Fe surfaces. The crystalline Fe oxides were transformed to Fe sulfides below 30 cm depth but the associated U was not transferred to these sulfides. Instead the weak associations became more important. In the wider context, since only UVI forms soluble complexes with acetate, UVI does not appear to be reduced even under the strongly reducing conditions encountered within waterlogged organic-rich soils. Initial interactions between UVI and porewater colloids appear to be weak but stronger interactions such as incorporation into Fe phases and complexation by HS occur once the colloids and associated U are removed to the solid phase. Waterlogged organic-rich soils appear to be a long-term sink for U but changing climatic conditions leading to the drying out of such soils may ultimately release U in association with smaller, more mobile organic-rich colloids.

Colloid detachment and mobilization can be of significant interest to those studying colloid behavior in the environment. The transport of pathogens such as viruses, bacteria, and protozoa can cause health problems in animals and humans. The transport of organics, radionuclides, and other hydrophobic contaminants can be enhanced by adsorption to mobilized colloid surfaces. Research has been done by others quantifying the detachment of colloids from smooth porous media. Real surfaces in the environment and engineered systems are rough. Glass beads were chemically roughened by procedures similar to those from Shellenberger and Logan (2002) and It et al. (2001) using chromic acid and a citric acid/ammonium fluoride solution. Surface asperities were measured using Atomic Force Microscopy (AFM), and the roughness was defined by three parameters: Root Mean Square (RMS) roughness, peak to valley height (P/V height), and peak to peak distance (λ). Detachment from the chemically etched porous media was measured in column tests. The controlling roughness parameter between the two batches of beads was found to be λ. A theoretical model to predict the effect of roughness on detachment was developed. Using a moment balance around the downstream point of contact, the parameters incorporated into the model were particle diameter, P/V height, and λ. The model predicted the shear required for colloid detachment in column tests. Surface roughness was found to significantly inhibit colloid detachment.

Emulsions are commonly found both in nature and industry. Due to the complex nature of emulsion systems, their interfacial properties and stability are poorly understood, particularly the influence of droplet deformability on the colloid and interfacial behaviour. This study has highlighted the role of emulsion droplet cross-linking (deformability and penetrability) on droplet surface chemistry, droplet colloidal stability and adsorption at the droplet-water interface and provides insight into methods for enhancing the performance of emulsion formulations.

The surface force apparatus (SFA) has been an important technique for making direct force measurements and has contributed enormously to our understanding of colloidal interactions. The conventional SFA has been limited to measuring forces between solid surfaces, until recently when a modified SFA was developed at the Ian Wark Research Institute [1]. A fluid drop (mercury) is introduced into the apparatus which allows a range of deformable surfaces to be studied in the SFA. This project is an extension of this technique. Interactions between a mica sheet and a mercury drop are studied, including the modification of mercury with self-assembled monolayers (SAMs) of thiol surfactants, and the drop deformation due to non-equilibrium adsorption effects and hydrodynamic forces.