Electrostratic stabilization of suspensions in non-aqueous media

Concentrated suspensions of detergent powder solids in a liquid nonionic surfactant are considered for practical application as liquid detergent products. If no precautions are taken, upon storage the viscosity of such suspensions increases and the pourability drops because the suspensions are colloidally unstable. It has been found that after the addition of a small amount of dodecylbenzene sulphonic acid (DoBS-acid or HDoBS) good pourability is maintained on storage. All the phenomena observed with such suspensions suggest that the addition of DoBS-acid reduces coagulation and improves colloidal stability. It was hypothesized that the colloidal stability obtained is of an electrostatic nature. In a liquid non-aqueous medium this is unexpected. A study of the mechanism of stabilization is described in this thesis.After a general introduction to the topic in Chapter 1, in Chapter 2 we discuss the character of the interactions which play a role in nonionic suspensions. The used nonionics are condensates of long chain alcohols and 3 to 9 alkylene oxide units. The dispersed solids are sodium salts as are usually present in current detergent powders, or oxides. They are aggregates or agglomerates of smaller crystalline primary particles and consist of irregular spheroids. The solids, the liquid nonionics and the anionic acid have been characterized with respect to a number of properties, including the molecular and crystalline structure, specific density, specific surface area, porosity, axial ratio and water content. The refractive indices and dielectric constants of the liquids and solids are also measured. Elemental analysis of the supernatants of our suspensions is carried out by Atomic Absorption, by Plasma Emission and by X-ray Fluorescence Spectroscopy. Since analysis of the supernatants indicated only very limited dissolution of the solids, it is concluded that the suspensions are lyophobic. It is demonstrated that, when DoBS- acid is added to a suspension of sodium salts in nonionic, it is converted quantitatively into anionic NaDoBS.Sedimentation rates, sediment volumes and viscosities are important physical characteristics of concentrated nonionic suspensions; they reflect the interactions between the suspended particles. The interactions follow the DLVO-theory, meaning that they are governed by the balance between attractive and repulsive or 'stabilizing' forces.The literature on van der Waals attraction (energy and forces) between particles in suspension is discussed in Chapter 3. It shows that for particles in the micron-size range, geometrical parameters (differences in particle size, interparticle distance), retardation and surface roughness are of more importance than in colloidal systems, having smaller particles. This means that the van der Waals bonding energy obtained on approach is larger, but, as a function of increasing interparticle distances, it decays more rapidly.In the van der Waals attraction, material properties are reflected in the Hamaker constant. Hamaker constants for the inorganic crystalline solids considered in this study are not available in the literature. Therefore it was necessary to evaluate them theoretically. Two approaches have been applied, a macroscopic theory and a microscopic theory. In a comparison they gave identical results within a few tens of percent. For the crystalline detergent solids the constants have been evaluated from their dielectric constants and refractive indices. The results showed the Hamaker constants for the detergent solids (except activated Zeolite 4A) to exceed those of the nonionics, but to be lower than those of the metal oxides. The differences between the constants of crystalline detergent solids and those of nonionics are relatively small, implying that suspensions of detergent solid particles in nonionics can be made to relatively high volume fractions and can be stabilized easily.In Chapter 4 the electrostatic theory for interactions of particle pairs in suspension is evaluated for its applicability in non-aqueous media, using models of plates and spheres. For both models the conclusion is that, for the calculation of the repulsive energies and forces, approximated equations can be used. They result in repulsive energies, pressures and forces, which are in good agreement with those of exact computations at distances>10 nm, but underestimate the repulsions at shorter distances.DLVO energy and force curves have been constructed and demonstrate the dependence of the repulsion on five parameters that govern the behaviour, viz. the dielectric constant, the ionic strength, the electric surface potential, the Hamaker constant and the particle size. For our suspensions with surface potentials ≥20 mV, significant repulsions already develop at distances between 2 and 40 nm. The theoretical repulsions are much higher than the van der Waals attractions and cause much larger repulsive barriers than those usually reported for non- polar, nonaqueous media. They are expected to play a role in the colloidal stabilization of nonionic suspensions and to influence the resistance against coagulation under pressures at the bottom of sediments. Secondary minima are only a few kT at most and coagulation is only expected at the protrusion points of contact and at relatively high ionic strengths.Ionic strengths in HDoBS-stabilized suspensions in the nonionics Plurafac LFRA30 and Imbentin C91/35 are evaluated from the conductivity in the supernatants and from their respective limiting molar conductivities. The methodology is described in Chapter 5. It was found that in both nonionics the limiting molar conductivity was lower than predicted from the values in water assuming Walden's rule applies. The results indicate that solvation interactions of Na +and DoBS -ions in nonionics are stronger than in water and stronger in Imbentin than in Plurafac.In Chapter 6 the results of the electric and dielectric measurements have been given. It is shown that the dielectric constant of nonionic is increased by HDoBS. Taking this increase into account, the ionic strengths found can be satisfactorily explained from theory. Only at high HDoBS concentration and relatively high dielectric constants are the ion concentrations lower than theoretically predicted, a feature that could be due to the formation of 'molecular associates'.From the limiting conductivities, at HDoBS concentrations between 10 and 150 mM, the ionic strengths have been found to range from 0.05 to 4 mM in Plurafac and from 0.08 to 30 mM in Imbentin. These results demonstrate a weak dissociation of the NaDoBS electrolyte. However, the ionic strengths obtained are considerably larger than those in supernatants of unstable suspensions and are higher than ever reported in the 'non-polar' hydrocarbon media, commonly considered in non-aqueous studies. Liquid nonionic media have a dielectric constant between 5 and 12 and are denoted 'low-polar'. At these ionic strengths, and considering the enhancement of the dielectric constant by HDoBS, in the HDoBS concentration regime between 0.5 and 150 mM, Debye lengths range from 33 to I nm in Plurafac and from 13 to 1 nm in Imbentin, i.e. in the same range as in aqueous media.Electrokinetic (ζ-)potentials of particles of detergent solids suspended in nonionics, given in Chapter 6, are found to be a function of the HDoBS concentration. The surface potentials tend to level off at HDoBS concentrations as low as 0.5 % w/w (15 mM dm -3), to a maximum value ranging from +25 to +70 mV, depending on the nature of the solid and the nonionic liquid. Addition of water or of a crown-complexant (15-Crown-5), reduces the ζ-potential. The formation of positive surface charges can be explained from the dissociation of adsorbed HDoBS.Mechanical properties of concentrated non-aqueous suspensions are discussed in Chapter 7, including their relation to the electrostatic repulsion. Rheology is used to monitor the properties under dynamic conditions. The consistency, which quantifies the particle interactions and shear thinning index was derived from the Sisko model.Addition of HDoBS was found to have little or no influence on the high shear rate viscosity of nonionic suspensions. This viscosity is governed by hydrodynamic interactions, which are, in turn, determined by the viscosity of the nonionic phase, the volume fraction and the temperature. The nature of the solid also has an influence on the 'infinite shear' viscosity, probably due to variations in protrusion size, causing their effective volumes to be larger than the actual volume. Measurements of the intrinsic viscosity of sodium tripolyphosphate (STP) indicated that the particles of this substance are almost spherical.Low shear rate viscosities monitor effects of interparticle interactions. The consistency was found to be inversely proportional to the particle volume. Addition of HDoBS reduces the consistency. As with the ~-potentials, the main effect is already obtained from 0.5 % w/w HDoBS. In this respect the behaviour of the viscosity is correlated with that of the ~-potential of the particles. It is further found that the drop in the 'normalized' consistency has a direct relation to the electrostatic force. These results support the conclusion that the nature of the obtained stabilization is electrostatic. The correlation of the viscosity with the Péclet number further supports this conclusion. It shows that under shear HDoBS-stabilized systems can be considered as hard-sphere suspensions.Creep compliance measurements of suspensions of STP in Plurafac at high volume fractions demonstrated that at low shear stresses the interactions are completely elastic. Under those conditions, relaxation of the stress leads to almost complete recovery. The shear moduli derived from creep compliance, drop less steeply as a function of the volume fraction than predicted from the electrostatic repulsive barrier. It is possible that this difference is a result of secondary minimum coagulation by the particle protrusions.In static sediments the volume fractions can be measured as a function of height by γ-ray absorption. Measurements of γ-ray absorption shows that the particle concentration from top to bottom in a stable sediment shows a concentration gradient. For HDoBS-stabilized suspensions this gradient is more continuous, whereas in unstable suspensions, due to coagulation, it is very irregular. From these results the relations between the static pressure or the network modulus and the volume fraction are derived. Pressures show an exponential relation with the interparticle distances. With low levels of DoBS-acid the interparticle distances are larger than for high concentrations of HDoBS. These results are in agreement with the dependency predicted by electrostatic repulsion, although the experimental pressure drop as a function of distance is much smoother than that theoretically predicted. The experimental network moduli derived from the pressure-volume fraction relation also drop much more slowly than theoretically predicted. This may again be a result of secondary minimum coagulation occurring by the protrusions.The overall conclusion is that the suspensions under consideration are electrostatically stabilized with DoBS-acid as the charge-determining electrolyte

Factors Associated with Revision Surgery after Internal Fixation of Hip Fractures

Background: Femoral neck fractures are associated with high rates of revision surgery after management with internal fixation. Using data from the Fixation using Alternative Implants for the Treatment of Hip fractures (FAITH) trial evaluating methods of internal fixation in patients with femoral neck fractures, we investigated associations between baseline and surgical factors and the need for revision surgery to promote healing, relieve pain, treat infection or improve function over 24 months postsurgery. Additionally, we investigated factors associated with (1) hardware removal and (2) implant exchange from cancellous screws (CS) or sliding hip screw (SHS) to total hip arthroplasty, hemiarthroplasty, or another internal fixation device. Methods: We identified 15 potential factors a priori that may be associated with revision surgery, 7 with hardware removal, and 14 with implant exchange. We used multivariable Cox proportional hazards analyses in our investigation. Results: Factors associated with increased risk of revision surgery included: female sex, [hazard ratio (HR) 1.79, 95% confidence interval (CI) 1.25-2.50; P = 0.001], higher body mass index (fo

Effects of viscosity on cerebral blood flow after cardiac arrest

Item does not contain fulltextOBJECTIVES: To determine blood viscosity in adult comatose patients treated with mild therapeutic hypothermia after cardiac arrest and to assess the relation between blood viscosity, cerebral blood flow, and cerebral oxygen extraction. DESIGN: Observational study. SETTING: Tertiary care university hospital. PATIENTS: Ten comatose patients with return of spontaneous circulation after out-of-hospital cardiac arrest. INTERVENTION: Treatment with mild therapeutic hypothermia for 24 hours followed by passive rewarming to normothermia. MEASUREMENTS AND MAIN RESULTS: Median viscosity at shear rate 50/s was 5.27 mPa . s (4.29-5.91 mPa . s) at admission; it remained relatively stable during the first 12 hours and decreased significantly to 3.00 mPa . s (2.72-3.58 mPa . s) at 72 hours (p < 0.001). Median mean flow velocity in the middle cerebral artery was low (27.0 cm/s [23.8-30.5 cm/s]) at admission and significantly increased to 63.0 cm/s (51.0-80.0 cm/s) at 72 hours. Median jugular bulb saturation at the start of the study was 61.5% (55.5-75.3%) and significantly increased to 73.0% (69.0-81.0%) at 72 hours. Median hematocrit was 0.41 L/L (0.36-0.44 L/L) at admission and subsequently decreased significantly to 0.32 L/L (0.27-0.35 L/L) at 72 hours. Median C-reactive protein concentration was low at admission (2.5 mg/L [2.5-6.5 mg/L]) and increased to 101 mg/L (65-113.3 mg/L) in the following hours. Median fibrinogen concentration was increased at admission 2,795 mg/L (2,503-3,565 mg/L) and subsequently further increased to 6,195 mg/L (5,843-7,368 mg/L) at 72 hours. There was a significant negative association between blood viscosity and the mean flow velocity in the middle cerebral artery (p = 0.0008). CONCLUSIONS: Changes in blood viscosity in vivo are associated with changes in flow velocity in the middle cerebral artery. High viscosity early after cardiac arrest may reduce cerebral blood flow and may contribute to secondary brain injury. Further studies are needed to determine the optimal viscosity during the different stages of the postcardiac arrest syndrome

Metabolic Engineering toward Sustainable Production of Nylon-6

Pathogenesis and treatment of chronic pulmonary disease

A control benchmark on the energy management of a plug-in hybrid electric vehicle.

A benchmark control problem was developed for a special session of the IFAC Workshop on Engine and Powertrain Control, Simulation and Modeling (E-COSM 12), held in Rueil-Malmaison, France, in October 2012. The online energy management of a plug-in hybrid-electric vehicle was to be developed by the benchmark participants. The simulator, provided by the benchmark organizers, implements a model of the GM Voltec powertrain. Each solution was evaluated according to several metrics, comprising of energy and fuel economy on two driving profiles unknown to the participants, acceleration and braking performance, computational performance. The nine solutions received are analyzed in terms of the control technique adopted (heuristic rule-based energy management vs. equivalent consumption minimization strategies, ECMS), battery discharge strategy (charge depleting-charge sustaining vs. blended mode), ECMS implementation (vector-based vs. map-based), ways to improve the implementation and improve the computational performance. The solution having achieved the best combined score is compared with a global optimal solution calculated offline using the Pontryagin's minimum principle-derived optimization tool HOT

The effect of initial density on interactions between bryophytes at individual and community levels

1  A glasshouse experiment was conducted with seven bryophyte species to determine the effects of competition. We tested whether competitive hierarchies varied with initial abundance (density and biomass) and between two different experimental approaches. 2  Relative competition intensities were calculated based on proportional growth ( G ) and relative biomass ( R ). The standards for comparison (i.e. no interactions) were species’ performance in monocultures at one of two sowing abundances (the combined monocultures method; CM) and in a low-density mixture of equal proportions of all species (the community density series method; CDS). 3  Proportional growth decreased with increasing initial abundance for all species. Community effects (relative biomass of each species) were generally weaker and more variable than individual effects. R increased linearly with abundance for only one species, while three species showed a quadratic response (of which two were negative). 4  Competitive hierarchies derived by the CM method differed with abundance, and we argue that the CDS method is likely to provide a more reliable comparison. 5  With CDS, competitive hierarchies were similar along the community abundance gradient, implying that non-linear competitive effects are not likely to be a mechanism of coexistence in this community. 6  There were significant competitive effects on community composition, but not on diversity as measured by evenness. At the community level individual species tended to show either competitive or positive effects throughout the gradient of initial community abundance, with decreasing values for most species at high initial community abundance, as were the effects of interactions on community composition.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73305/1/j.1365-2745.2000.00442.x.pd