Marilyn E. Karaman

The production of stable hydrophobic surfaces by the adsorption of hydrocarbon and fluorocarbon carboxylic acids onto alumina substrates

Abstract The adsorption of various carboxylic acids onto alumina substrates has been studied as a model system for the study of the long range hydrophobic interaction. The adsorption was studied using classical adsorption isotherms, contact angle measurements and colloid probe atomic force microscopy. Carboxylic acids form esters with the hydroxyl groups present on the alumina surface and produce surfaces with a range of hydrophobicities and stabilities depending on the acids functionality. Hydrocarbon carboxylic acid adsorption was found to be adequately described by the Langmuir adsorption model and produced stable surfaces of varying hydrophobicities which resisted dissolution by water. By comparison, hydrophobic surfaces produced with fluorinated acids adsorbed from aqueous solution were not always stable, although their stability was greatly improved when the chemisorption was carried out in a non-aqueous solvent at elevated temperatures. Adsorption of these fluorinated carboxylic acids produced the most hydrophobic surfaces. Interaction forces between alumina surfaces coated with adsorbed propionic acid were as expected from DLVO theory with no evidence of hydrophobic forces.

Interaction between Cryptosporidium oocysts and water treatment coagulants.

The electrokinetic properties of gamma-irradiated Cryptosporidium oocysts in the presence of coagulants (ferric chloride and alum) and coagulant aids (DADMAC based cationic polyelectrolytes) have been studied. The zeta potential of the oocysts was unaffected by the addition of ferric chloride at all pH values (3-10) studied. Addition of alum resulted in reversal of the oocysts charge, which suggests that the initial stage in the coagulation process leading to floc formation proceeds via the adsorption of hydrolysed aluminium species. The cationic polyelectrolyte Magnafloc LT35 was adsorbed onto iron flocs at doses of 0.1 mg/L even against an electrostatic barrier. The cationic polyelectrolyte only adsorbed and caused charge reversal at the oocyst surface at around 0.4 mg/L, suggesting a lower affinity for this surface. These results indicate that the oocysts, unlike inorganic colloidal materials such as metal oxides, appear to possess a lower surface density of active or charged sites. The lower density of sites, combined with the rapid precipitation of iron salts, may be responsible for the lack of specific adsorption of either hydroxylated ferric species or primary iron hydroxide particles on the oocysts. Further, this suggests that a process of sweep flocculation, where oocysts are engulfed in flocs during coagulation and floc formation, is the more likely mechanism involved. By comparison, it is likely that the specific interaction of hydrolysed aluminium species with the oocysts surface would result in a stronger link at the oocyst-floc interface and that the flocculation process may initially proceed via charge neutralisation.

A comparison of the interaction forces between model alumina surfaces and their colloidal properties

Abstract Previous work has demonstrated that alumina dispersions are only destabilized by monovalent electrolytes such as KCl at concentrations

Microelectrophoresis of Cryptosporidium parvum oocysts in aqueous solutions of inorganic and surfactant cations

Abstract Cryptosporidium parvum is a protozoan parasite associated with waterborne outbreaks of diarrhoeal disease. The life cycle of this parasite includes the production of a spheroidal oocyst that is of 4–6 microns in diameter. The thickness of the oocyst wall and its capacity to strongly adhere to both organic and inorganic surfaces are features of the oocysts which could be attributed to its survival in the environment for extended periods. Hence, the need to study their surface chemistry in the aqueous environment. The surface charging properties of the intact C. parvum oocysts were derived from microelectrophoresis measurements on these robust biological species. The ζ potentials of Cryptosporidium oocysts were measured in a range of inorganic electrolyte solutions and in solutions of a multivalent cationic surfactant. The surface potential of the oocyst was found to be pH dependent, with an isoelectric point in mM NaCl of ∼2, suggesting the presence of surface carboxylate groups associated with glycoproteins or phosphate groups. The area/charge for the fully ionised oocysts was found to be ∼80 nm 2 , corresponding to a total maximum charge of 1.6×10 −13 C per oocyst. The effect of a highly charged novel cage surfactant known as CS12 on the Cryptosporidium oocyst surface potential provided valuable insight into its uptake and possible surface activity. Uptake of CS12 was detected at concentrations as low as 2×10 −8 M. At ∼2×10 −5 M CS12 the oocyst surface was uncharged and became positively charged at higher concentrations. These findings suggest that there could be improvements to current concentration methods by manipulation of the surface charge.

Use of the light-lever technique for the measurement of colloidal forces

Abstract Force measurements in a variety of colloidal systems have been conducted using a new device that utilises the light-lever technique employed in Atomic Force Microscopy. The systems studied include electrostatic double-layer forces, hydrophobic interactions and liquid crystal structures. The new device has some clear advantages over adaptation of an imaging AFM for colloidal force measurements.

The role of the meniscus in the drying of latex films

Abstract In this study we have examined the fundamental thermodynamic role of water in the formation of polymer films from latex suspensions. We also report some simple density measurements, taken during the controlled (wet) osmotic drying of bulk latex samples, which show that there is no significant formation of internal air voids during drying. These results demonstrate that capillary forces are not necessary for the formation of continuous polymer films and suggest that it is unlikely that internal void spaces will be formed during the drying process.

A new approach to the measurement of the minimum film formation temperature of latex dispersions

A new approach to the determination of the minimum film formation temperature (MFFT) of latex dispersions is reported here. The procedure is based on the measurement of the change in electrical conductivity with temperature when a bulk sample of a latex dispersion is partially dried, under an applied osmotic stress, to the stage where the latex particles are in close proximity. The variation in conductivity with temperature, under these conditions, was found to have a marked change or transition at the expected minimum film formation temperature for the latex samples studied. This new method has the potential for applications involving latex dispersions containing pigments, where the current optical techniques are less applicable.