Richard L. Zollars

Comparison of different methods to measure contact angles of soil colloids

We compared five different methods, static sessile drop, dynamic sessile drop, Wilhelmy plate, thin-layer wicking, and column wicking, to determine the contact angle of colloids typical for soils and sediments. The colloids (smectite, kaolinite, illite, goethite, hematite) were chosen to represent 1:1 and 2:1 layered aluminosilicate clays and sesquioxides, and were either obtained in pure form or synthesized in our laboratory. Colloids were deposited as thin films on glass slides, and then used for contact angle measurements using three different test liquids (water, formamide, diiodomethane). The colloidal films could be categorized into three types: (1) films without pores and with polar-liquid interactions (smectite), (2) films with pores and with polar-liquid interactions (kaolinite, illite, goethite), and (3) films without pores and no polar-liquid interactions (hematite). The static and dynamic sessile drop methods yielded the most consistent contact angles. For porous films, the contact angles decreased with time, and we consider the initial contact angle to be the most accurate. The differences in contact angles among the different methods were large and varied considerably: the most consistent contact angles were obtained for kaolinite with water, and illite with diiodomethane (contact angles were within 3 degrees); but mostly the differences ranged from 10 degrees to 40 degrees among the different methods. The thin-layer and column wicking methods were the least consistent methods.

Does colloid shape affect detachment of colloids by a moving air-water interface?

Air-water interfaces interact strongly with colloidal particles by capillary forces. The magnitude of the interaction force depends on, among other things, the particle shape. Here, we investigate the effects of particle shape on colloid detachment by a moving air-water interface. We used hydrophilic polystyrene colloids with four different shapes (spheres, barrels, rods, and oblong disks), but otherwise identical surface properties. The nonspherical shapes were created by stretching spherical microspheres on a film of polyvinyl alcohol (PVA). The colloids were then deposited onto the inner surface of a glass channel. An air bubble was introduced into the channel and passed through, thereby generating a receding followed by an advancing air-water interface. The detachment of colloids by the air-water interfaces was visualized with a confocal microscope, quantified by image analysis, and analyzed statistically to determine significant differences. For all colloid shapes, the advancing air-water interface caused pronounced colloid detachment (>63%), whereas the receding interface was ineffective in colloid detachment (<1.5%). Among the different colloid shapes, the barrels were most readily removed (94%) by the advancing interface, followed by the spheres and oblong disks (80%) and the rods (63%). Colloid detachment was significantly affected by colloid shape. The presence of an edge, as it occurs in a barrel-shaped colloid, promoted colloid detachment because the air-water interface is being pinned at the edge of the colloid. This suggests that the magnitude of colloid mobilization and transport in porous media is underestimated for edged particles and overestimated for rodlike particles when a sphere is used as a model colloid.

Shear coagulation in the presence of repulsive interparticle forces

Abstract This investigation sought to study the repulsive forces within a colloidal dispersion by monitoring the progress of a shear-induced coagulation. Two monodisperse polystyrene latices were used whose only difference was the presence of chemically bound ionizable surface groups on one. Both latices were tested in the absence and presence of a variety of sodium alkyl sulfate and octaphenol ethoxylate surfactants. The latices exhibited excellent resistance to Brownian coagulation for as long as 4 years. When exposed to shearing at rates of the order of 20,000 s−1 substantial coagulation occurred in 30 min or less. The particles possessing the chemically attached ionizable groups exhibited higher stability at all shear rates, even in the presence of large amounts of either anionic or nonionic surfactants. The addition of surfactant destabilized either latex especially if (a) the particles were originally stabilized by chemically attached surface groups or (b) the surfactant had a long hydrophobic group and a weakly ionized hydrophilic group. All of these effects suggest the formation of hydrophobic associations between surfactant molecules on neighboring particles. As a result, the current trajectory analysis models are incomplete and cannot be used to analyze the results from shear coagulation tests conducted under conditions of significant repulsive forces.

Effects of ionizable groups on the adsorption of surfactants onto latex particle surfaces

Abstract Soap titration of polymeric latices can be used both for the determination of an average latex particle size and as a method for investigating the adsorption of surfactants onto hydrophobic surfaces. This investigation has studied the adsorption of anionic and nonionic surfactants onto polystyrene latex particles in order to develop an understanding of the relationship between surfactant structural characteristics (chain length and end-group) and the area occupied by an adsorbed molecule. As expected the area occupied by a surfactant molecule increased as the ethylene oxide chain length increased for a series of octaphenoxy ethoxylate nonionic surfactants. The results also indicated that in addition to structural factors the adsorption area of a surfactant molecule is also dependent upon the surface charges present on the particle surface. For the nonionic surfactants studied the adsorption area per molecule increased as the number of charges chemically attached to the particle surface increased (increasing surface polarity). For sodium dodecyl sulfate, however, just the opposite trend was observed. The increase in the amount of sodium dodecyl sulfate adsorbed with increasing surface charge may be due to a more extended configuration for the adsorbed surfactant molecules which gives rise to more surfactant—surfactant interactions. These surface charges, which arise from the initiator, may vary widely from one latex to another depending upon the polymerization method and recipe and may be responsible for some of the scatter in previous soap titration results.

Colloid mobilization and transport during capillary fringe fluctuations.

Capillary fringe fluctuations due to changing water tables lead to displacement of air-water interfaces in soils and sediments. These moving air-water interfaces can mobilize colloids. We visualized colloids interacting with moving air-water interfaces during capillary fringe fluctuations by confocal microscopy. We simulated capillary fringe fluctuations in a glass-bead-filled column. We studied four specific conditions: (1) colloids suspended in the aqueous phase, (2) colloids attached to the glass beads in an initially wet porous medium, (3) colloids attached to the glass beads in an initially dry porous medium, and (4) colloids suspended in the aqueous phase with the presence of a static air bubble. Confocal images confirmed that the capillary fringe fluctuations affect colloid transport behavior. Hydrophilic negatively charged colloids initially suspended in the aqueous phase were deposited at the solid-water interface after a drainage passage, but then were removed by subsequent capillary fringe fluctuations. The colloids that were initially attached to the wet or dry glass bead surface were detached by moving air-water interfaces in the capillary fringe. Hydrophilic negatively charged colloids did not attach to static air-bubbles, but hydrophobic negatively charged and hydrophilic positively charged colloids did. Our results demonstrate that capillary fringe fluctuations are an effective means for colloid mobilization.

Changes in coagulum configuration resulting from shear-induced coagulation

Abstract Coagulation of a colloidal suspension may occur by either Brownian motions or bulk fluid motions. In the analysis of either type of coagulation process it is generally assumed that the coagulum assumes a spherical shape, possibly incorporating empty space within the coagulum if the individual particles do not coalesce. Electron micrographs of the coagulum produced during shear coagulation experiments in this laboratory indicate that this assumption is valid only if Brownian coagulation processes are important. When shear coagulation processes dominate the coagulum appears as a string of particles rather than as a cluster. For particles containing adsorbed materials the coagulum is still elongated but forms more of a two-dimensional structure. This may be due to restricted motions of one particle with respect to another within the coagulum because of hydrophobic associations.

Direct determination of anionic surfactants using ion chromatography

Abstract A method for the simultaneous qualitative and quantitative analysis of surfactants in solution via ion chromatography has been developed. This method has the capability of separating linear alkyl anionic surfactants which differ by as little as two carbon atoms in the hydrocarbon chain. In addition, the method can differentiate between surfactants of the same chain length which differ only in the chemical nature of the anionic head group. The ion chromatographic method is much simpler and more accurate than previous techniques of determining surfactant concentration which relied on detecting changes in secondary properties, such as surface tension. The method is also highly flexible and should be applicable to a wide variety of ionic and nonionic surfactants. Use of ion chromatography will greatly simplify studies of surfactant behavior.

TURBIDIMETRIC METHOD FOR DETERMINATION OF LATEX PARTICLE SIZE DISTRIBUTION

ABSTRACT A technique for the determination of latex particle size distributions, which uses specific turbidity measurements at various wave lengths, has been developed and shown to be viable. The method yields results which are in agreement with other commonly accepted particle sizing techniques and does not exhibit the multiple solutions reported by other investigators. This report of multiple valued solutions is apparently the result of an incomplete numerical search technique.

Generation of self-preserving particle size distributions during shear coagulation

Abstract Colloidal dispersions have been reported to develop self-preserving particle size distributions during both Brownian and shear coagulation. During this investigation the evolution of the particle size distribution during the shear coagulation of polystyrene latices, which exhibited excellent Brownian stability, was examined. The resulting distributions were found to be self-preserving in all cases including those systems where the coagulation apparently occurred through the formation of hydrophobic associations. Recent claims that the shear coagulation of dispersions with good Brownian stability does not yield self-preserving distributions are apparently in error due to a misinterpretation of the characteristics of a self-preserving distribution.

Electrostatic effects during surfactant adsorption

Abstract The adsorption of surface-active materials is often modeled with empirical semiempirical equations which frequently fail to account for key features such as the effect of chain length and electrostatic interactions either between the adsorbing molecules themselves or between the adsorbents and the adsorbing surface. A mean field theory, which incorporates electrostatic effects, has been used in this study to overcome these deficiencies and predict the adsorbed amount as well as the conformation of surface-active species. For simple surfactants (linear alkyl sulfates) the model predictions for adsorption isotherms exhibit the correct behavior, although the amount adsorbed predicted at saturation is lower than that observed experimentally. The adsorbed molecules are predicted to have very flat conformations under most conditions with significant extension occurring only when the surface density of the adsorbing surface becomes large enough that complete exclusion of the surfactant from the surface is imminent. The potential for local maxima and minima in adsorption isotherms is also demonstrated.