Suspensions flocculated state

In the flocculated state for all suspensions, immediately after mixing, no transmission of light could be detected through the cuvette used and the sedimentation behaviour was different. Thus, instead of the parameters used for the pyrogenic silica systems we used the following characteristics for the precipitated silica mixtures to describe the flocculation and sedimentation behaviour ... 

VI Flocculation Studies, Critical flocculation temperatures were measured using suspensions of 0.6% v/v latex containing the appropriate amount of sodium chloride. Differing degrees of neutralisation were produced by the addition of NaOH to stock solutions and checked by conductometric titration with base in the absence and presence of added HC1. Flocculation temperatures (CFT) were determined by visual observation at a cooling/heating rate of M).5 C s. The flocculation was generally reversible provided that the latices had not been left in the flocculated state for more than a few minutes or so. 

The processes involved in the formation of suspensions are shown in Fig. 2. The flocculated state (C) may be reached either directly by wetting and dispersing hydrophobic particles (A) with a suitable flocculating surfactant, or indirectly by first wetting and dispersing to produce a dispersed or peptized particle (B) with a suitable surfactant and then flocculating with a suitable agent such as a hydrophilic colloid or polyelectrolyte. 

The most flocculated state of a colloidal suspension composed of a single mineral occurs at the pH equal to the mineraTs PZC. 

Rheological techniques eue most convenient to assess suspension flocculation without the need of any dilution (which in most cases results in breakdown of the floe structure). In steady state measurements the suspension is carefully placed in... 

The degree of flocculation of sulphamerazine suspensions has been determined as a function of surfactant (sodium dod yl polyoxyethylene sulphate) concentration and electrolyte concentration [14]. At low surfactant levels the system is flocculated, with sedimentation volume fractions of about 0.9 at higher levels the sedimentation volume falls to about 0.6. On the addition of 0.24 m NaCl a dramatic decrease in sedimentation volume is observed, indicating a dense sediment with cloudy supernatant. Further addition of NaCl results in reestablishment of a flocculated state perhaps by adsorption of ions on to the particles. 

Just as it is usually more difficult to prepare stable sols than flocculated systems, there is more chance of obtaining a plastic or at least a flocculated system in preparing a concentrated suspension than a stable, dilatant one The agents promoting the flocculated state are the same as those for dilute colloidal systems and have been treated extensively in chapter VIII Wc only draw attention again to the influence of electrolytes in aqueous suspensions according to the rule of Schulze and Hardy (e chapter VIII, 10 a, p 335) ... 

In part II of the present report the nature and molecular characteristics of asphaltene and wax deposits from petroleum crudes are discussed. The field experiences with asphaltene and wax deposition and their related problems are discussed in part III. In order to predict the phenomena of asphaltene deposition one has to consider the use of the molecular thermodynamics of fluid phase equilibria and the theory of colloidal suspensions. In part IV of this report predictive approaches of the behavior of reservoir fluids and asphaltene depositions are reviewed from a fundamental point of view. This includes correlation and prediction of the effects of temperature, pressure, composition and flow characteristics of the miscible gas and crude on (i) Onset of asphaltene deposition (ii) Mechanism of asphaltene flocculation. The in situ precipitation and flocculation of asphaltene is expected to be quite different from the controlled laboratory experiments. This is primarily due to the multiphase flow through the reservoir porous media, streaming potential effects in pipes and conduits, and the interactions of the precipitates and the other in situ material presnet. In part V of the present report the conclusions are stated and the requirements for the development of successful predictive models for the asphaltene deposition and flocculation are discussed. 

The typical viscous behavior for many non-Newtonian fluids (e.g., polymeric fluids, flocculated suspensions, colloids, foams, gels) is illustrated by the curves labeled structural in Figs. 3-5 and 3-6. These fluids exhibit Newtonian behavior at very low and very high shear rates, with shear thinning or pseudoplastic behavior at intermediate shear rates. In some materials this can be attributed to a reversible structure or network that forms in the rest or equilibrium state. When the material is sheared, the structure breaks down, resulting in a shear-dependent (shear thinning) behavior. Some real examples of this type of behavior are shown in Fig. 3-7. These show that structural viscosity behavior is exhibited by fluids as diverse as polymer solutions, blood, latex emulsions, and mud (sediment). Equations (i.e., models) that represent this type of behavior are described below. 

Therefore, a systematic examination of the influences of pH and electrolyte concentration on the state of flocculation of both silica types seemed promising. For a quantitative examination of the state of flocculation a Laserphoto Sedimentometer was used to follow the turbidity and the sedimentation of the suspension as a function of time. This method was described in detail in our earlier work (1,10). 

The pH value and the electrolyte concentration show decisive influences on the flocculation and the sedimentation behaviour of precipitated silica. Precipitated silica could not be flocculated at high pH values without the addition of an electrolyte. At pH> 3.5 no change in the state of the suspension was observed, e.g. the sedimentogramms of the suspensions with polymer addition up to saturation (0 1) have the same shapes as that of the sedi-mentogram of the polymer free suspension. At pH 2.3 and even at a very low polymer concentration (c(40,000/PEG) > 3 10-5 mg/ml,... 

Transmission electron microscopy micrographs (Fig. 13.24) also indicate an important characteristic of the supported particles. As in the case of suspensions, they are either aggregated or isolated. Support surface properties may he an important factor governing this aggregation. After deposition on the support, we observed that samples prepared from acidic hydrosols are characterized by the presence of aggregated particles constituting flocculates ranging from 10 to 200 nm, whereas samples prepared via basic hydrosols contain only isolated particles. The opposite was observed when hydrosols were concerned. These final states of the supported particles may be controlled... 

Soluble polymers are widely used to control the state of dispersion of fine-particle suspensions. Depending on the polymer, and how it is applied, they can serve to enhance stability (dispersants) or to promote aggregation of the particles (flocculants). The topics covered in this chapter are intended as an overview of the use of polymers for stability control in mineral-particle suspensions with particular emphasis on flocculation processes. A brief discussion of stabilisation by polymers is included for completeness. 

Sediment in an aged dispersion may be collected and measured in a crudely quantitative test. To distinguish between a deflocculated, a flocculated, and an aggregated suspension, a weighed amount of solid is uniformly suspended in a small quantity of liquid, the suspension is transferred to a graduate cylinder, the volume of sediment during a stated period of time is measured, and the specific sediment volume (milliliters per gram) vs time is plotted. In... 

Now the technique provides the basis for simulating concentrated suspensions at conditions extending from the diffusion-dominated equilibrium state to highly nonequilibrium states produced by shear or external forces. The results to date, e.g., for structure and viscosity, are promising but limited to a relatively small number of particles in two dimensions by the demands of the hydrodynamic calculation. Nonetheless, at least one simplified analytical approximation has emerged [44], As supercomputers increase in power and availability, many important problems—addressing non-Newtonian rheology, consolidation via sedimentation and filtration, phase transitions, and flocculation—should yield to the approach. 

The first step Is to ensure that the macromolecules are bound to the surfaces. For this purpose they can be grafted chemically however this Is an expensive process, requiring the use of reactive polymers or tailor made surface active species such as block copolymers. A less expensive method Is to use macromolecules which spontaneously adsorb to the surfaces. Then experimental wisdom states that surfaces which are each completely saturated with macromolecules will repel each other (stabilisation of the suspension), while surfaces which are unsaturated will attract each other because the macromolecules will bridge them together (flocculation) (1). 

Tingstad reviewed test methods for determining the physical stability of pharmaceutical suspensions. The procedures outlined are designed to determine the state of flocculation of a formulation. Because there is more than one method of preparing stable suspensions, the following test methods and performance criteria were found useful for determining the stability of both flocculated and dispersed systems. 

In the literature, the reported use of zeta potential measurement for non-aqueous suspensions is relatively infrequent because non-aqueous suspensions only represent a small percentage of all medicated suspensions. Su and others evaluated the flocculation-deflocculation behavior of cefazolin sodium in non-aqueous media and the effect of surfactants as measured by zeta potential along with sedimentation and porosity measurements. A significant difference in zeta potential was observed when the particles were dispersed in peanut oil and ethyl oleate. The addition of lecithin reduced the zeta potential of cefazolin sodium, resulting in a deflocculated state accompanied by a decrease in sedimentation volume. The effect of surfactant... 

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