John C. Berg

The collapse of surfactant monolayers at the air—water interface

Abstract The collapse pressures and collapse rates of nine different surfactant monolayers: oleic, myristic and stearic acid, methyl stearate, heptadecanol, octadecanol, nonadecanol, eicosanol, and dipalmitoyl lecithin (DPL), are determined experimentally using a Langmuir trough-Wilhelmy balance apparatus. All solid surfactants but DPL are found to collapse at rates consistent with a model of homogeneous nucleation and subsequent growth of bulk surfactant fragments. DPL films appear to collapse only by a fracture process. Oleic acid, the only liquid studied, collapses instantaneously at its equilibrium spreading pressure.

Evidence of a hairy layer at the surface of polystyrene latex particles

Abstract Polystyrene latexes are treated as model colloids, yet the observed electrokinetic and size behavior do not conform to the predictions of theory. Upon the addition of low concentrations of electrolyte, the electrophoretic mobility and zeta potential of the latex increase, while the hydrodynamic size decreases. One explanation for the anomalous behavior is the presence of a hairy layer at the latex surface. According to the hairy layer model, flexible polymer chains, or hairs, at the surface extend into solution, owing to electrostatic repulsion between the ionic groups which terminate the hairs and the ionic groups which are anchored at the surface. The thickness of the hairy layer is altered by the presence of electrolyte owing to charge shielding. As such, the size and electrokinetic parameters are functions of ionic strength. This model is frequently invoked in the literature; however, to the authors knowledge, no previous studies have attempted to verify the presence of a hairy layer via simultaneous measurement of both the size and electrokinetic characteristics of a given latex over a range of conditions. Evidence from other investigators indicates that heating a latex to its glass transition temperature yields particles whose electrokinetic behavior is more nearly ideal. Interpreted in terms of the hairy layer model, such results suggest that heat treatment smoothes or removes the hairs. In the current study, the presence of a hairy layer is investigated by comparing size, mobility, critical coagulation concentration, and surface charge density measurements for three different surfactant-free latexes before and after heat treatment. The new results strongly support the existence of a hairy layer. Light scattering measurements indicate that the thickness of the layer may be as great as 7 nm.

The aggregation of colloidal particles at the air—water interface

Abstract Previous investigators have noted that small particles at a fluid interface may exhibit a special tendency to aggregate. Colloidal dispersions which are stable in the bulk often form a coagulum at the air—liquid surface. Such surface aggregation is an important factor in operations that require clean or controlled interfaces. The present work seeks to describe particle adsorption and aggregation at the air—water interface in quiescent systems. A model is proposed for two-dimensional rapid aggregation as well as for slow surface aggregation dependent upon modified van der Waals and electrostatic interactions between particles at the surface. Experimental results from direct observation by darkfield microscopy of particle adsorption and surface aggregation are reported and found to be in qualitative agreement with the model predictions.

Antifoam action by solid particles

Abstract Inert hydrophobic solid particles have been found to exhibit antifoam action for many systems. A mechanism for particle-induced film rupture is proposed, and a hydrodynamic analysis is performed to determine criteria for effective action. A comparison of predictions from the analysis with experimental data indicates that the particle shape, the receding contact angle, q rec , and the adsorption rate for the surfactant solution are the critical parameters.

The effect of surfactants on wicking flow in fiber networks

Abstract Wicking flow in random fiber networks is examined for various pure liquids and surfactant solutions. Although the pore geometry of the fiber structures is extremely complex, it is found that all liquids used obey the rate law of the simple Lucas—Washburn theory. Imbibition of surfactant solutions is found to differ significantly from that of pure liquids of equivalent surface tension, indicating depletion of surfactant from the wetting front by adsorption onto the fiber surfaces during penetration. For fiber networks which are wet out by the imbibing liquid, wicking rate is directly proportional to an effective surface tension at the meniscus. The difference between this wicking-equivalent tension and the equilibrium surface tension varies between different surfactants. Fiber structures which are partially wet exhibit penetration rates proportional instead to the adhesion tension, and for surfactant solutions, there are differences between the wicking-equivalent and equilibrium values. Differences in wicking performance among the surfactants are attributed to differences in both the extent of their adsorption at the fiber surfaces and their diffusivities.

Effect of particle and surfactant acid–base properties on charging of colloids in apolar media

The present work examines role of particle and surfactant acid-base properties using model spherical silica particles, one type with acidic functionality and the other with basic functionality, dispersed in an apolar solvent containing either a basic or an acidic surfactant. The electrical properties of each of the four systems types are quantified by measuring the particle electrophoretic mobility using phase angle light scattering (PALS). Since the magnitude of the mobility in each case also depends strongly on the concentration of the surfactant, this dependence was investigated concurrently. It was found that at sufficiently high surfactant concentration, i.e., where micelles begin to form in the bulk, the particle surfaces could be electrically charged. Both the polarity and magnitude of the surface charge are found to depend strongly on the acid-base properties of the surface and the surfactant. The basic surfactant produces negatively-charged particles, and the magnitude of the mobility increases with the particle surface acidity. The acidic surfactant, on the other hand, charges the particle positively and produces the highest mobility when used with basic particles. This finding is consistent with the acid-base charging mechanism initially proposed by Fowkes and coworkers. It is also found that further increases in micelle concentration can have the effect of lowering the particle mobility. The decline in electrophoretic mobility (or zeta potential) at high surfactant concentration is caused by the increasing concentration of electrically charged micelles, which may partially neutralize the surface charge or compress the electrical double layer.

Convective instability in liquid pools heated from below

The linear hydrodynamic stability analysis of liquid pools heated from below combining surface tension and buoyancy effects as presented by Nield (1964) is confirmed by experiment for a series of silicone oils. The experimental method used is an adaptation of the Schmidt–Milverton technique, in which the stability limit is located by the change of slope in the plot of heat flux versus temperature drop across the liquid pool.

Distribution equilibria in micellar solutions

Abstract Ultrafiltration is used to study the distributions of methyl orthobenzoate (MOB), 1-fluoro-2,4-dinitrobenzene (FDNB), and aniline between water and micelles of sodium dodecyl sulfate (SDS) and a dinonylphenyl polyoxyethylene surfactant (DNPE). The ratio of solute concentration in the micelles to that in the water is measured for each system and shows significant deviation from ideality for all cases except FDNB in DNPE. Except for the systems containing aniline, activity coefficient data for the micelles are successfully correlated using the Wilson equation.

Dynamic wetting in the low capillary number regime

An understanding of dynamic wetting is necessary when considering processes in which one fluid is forced to displace another fluid from a solid substrate. This work examines the forced wetting behavior exhibited by a series of solid-liquid systems in the low capillary number regime. In particular, the effects of specific chemical (i.e. acid-base) interactions between the liquid and the solid and of solid surface roughness on dynamic wetting are investigated. Measurement of the dynamic contact angle as a function of the interline velocity is performed using the technique of microtensiometry. The data are compared with empirical correlations in order to test the hypothesis that a universal correlation may be used to describe wetting behavior. The results suggest that two correlation having the same functional form but different constants are necessary to adequately describe wetting behavior above and below a capillary number of the order of 10−3. No significant differences are observed between the wetting behavior of systems with only dispersive interactions between the liquid and solid and systems with acid-base interactions, implying that the static contact angle adequately accounts for the effect of acid-base interactions. Surface roughness produces noticeable stick-slip effects in the low capillary number regime when the static contact angle is larger than approximately 15°. While these effects produce a large degree of scatter in the dynamic wetting measurements, they do not alter the constants from those found in the case of smooth interline motion.

Effect of moisture on the surface free energy and acid-base properties of mineral oxides.

Surface energetic properties of mineral oxides are important in many applications. Since oxide surfaces in practice have generally come in contact with water molecules, it is important to know how water coverage affects the surface properties. In this work, five oxide samples, namely MgO, Al2O3, TiO2, SnO2 and SiO2 are heat-treated to various extents, to produce different degrees of hydration, and characterized thereafter by inverse gas chromatography. Water contents of the treated samples are determined independently by Karl Fischer titration, and specific surface areas are measured by the BET method. The results show that in general as water coverage decreases, the Lifshitz-van der Waals component of the specific surface free energy (sigma(S)LW) increases, but the acid-base interaction potential (-deltaG(AB)) decreases. These attributes are more sensitive to changes in water coverage at lower coverages, where the surface is presumed to consist of patches of molecular water and unhydrated hydroxyl groups.