RESUMEN
The stability of colloidal slurries is an important parameter in many industries due to problems that can arise as a result of particle settling. Particle settling is often caused by the shielding of surface charges on the particles which otherwise would prevent coagulation and subsequent settling. This is particularly a problem in high ionic strength slurries, where large amounts of ions serve to enhance the charge shielding and compression of the electrical double layer around the particles. This phenomenon has been investigated for industrially significant slurries used for tungsten and copper chemical mechanical polishing (CMP). It has been found that the effects of addition of conventional stabilizing agents (e.g., ionic surfactants, polymers) to these high ionic strength slurries are neutralized by the electrolytes in solution. However, the synergistic combination of a properly chosen ionic and nonionic surfactant has been found to be a suitable stabilizing agent for this type of system. For the CMP slurries investigated, the synergistic effect has been shown to be maximum for combinations of sodium dodecyl sulfate anionic surfactant and a variety of polymeric nonionic surfactants. The stabilization observed for these mixed surfactant systems has been explained in terms of adsorption of ionic surfactant on particle surfaces and nonionic surfactant molecules penetrating the film of the ionic surfactant due to hydrocarbon chain interactions. This brings about the steric stabilization of the slurry. Copyright 2000 Academic Press.
RESUMEN
The aim of this study was to investigate the importance of surface potential in microbial deposition onto modified granular surfaces. Recent experimental and theoretical work has indicated that surfaces coated with metal oxides and hydroxide rich oxide/hydroxide mixtures ((hydr)oxides) have the potential to increase the capture efficiencies of commercial filtration systems. This study quantitatively compared different metal (hydr)oxide coatings in their abilities to enhance bacterial deposition. Specifically, the deposition rates of bacterial strains Streptococcus faecalis, Staphylococcus aureus, Salmonella typhimurium, and Escherichia coli were compared for Ottawa sand and surface coatings consisting of aluminum (hydr)oxide, iron (hydr)oxide, and mixed iron and aluminum (hydr)oxide. The metal-(hydr)oxide-modified granular media enhanced bacterial deposition relative to the noncoated Ottawa sand. The electropositive surfaces, the aluminum and the mixed (hydr)oxides, had similar average kinetic rate constants, five times larger than the rate constants observed for the untreated Ottawa sand. The measured kinetic rate constants for the positively charged systems of aluminum (hydr)oxide and mixed (hydr)oxide collectors suggested that the overall rate of deposition was limited by the transport of bacteria to the granular surface rather than the rate of attachment. For systems where the collector surfaces were negatively charged, as in the cases of Ottawa sand and the iron (hydr)oxide coating, large energy barriers to attachment were predicted from DLVO theory but these barriers did not totally inhibit bacterial deposition. The deposition results could not be fully explained by DLVO theory and suggested the importance of other factors such as collector charge heterogeneity, motility, and bacterial surface appendages in enhanced deposition. Copyright 1998 Academic Press.
RESUMEN
An experimental investigation on the effect of surfactant monolayers on the heat transfer through air/water and oil/water interfaces was carried out by observing the changes of surface temperature with IR Imaging Radiometer (Model 760). The heat transfer resistance of various single component and mixed monolayers at air/water and oil/water systems was studied. The results show that the surfactant monolayers introduce a noticeable heat transfer resistance to the heat transfer process across the interface. The solid monolayers exhibit lower resistance to heat transfer than the liquid monolayers at the oil/water interface. At air/water interface, the presence of monolayer decreases the evaporative cooling process and therefore increases the surface temperature rapidly. However, the presence of a monolayer at oil/water interface increases the heat transfer resistance across the oil/water interface. Heat transfer resistance increases as the chain length of fatty acid increases at the oil/water interface. The effects of phase-transition from a two-dimensional solid to the liquid state in cholesterol-arachidyl mixed monolayers was observed from the change in heat transfer resistance of the monolayers at the oil/water interface. The optimum molecular packing at the 1:3 molecular ratio in mixed surfactant monolayers of oleic acid-cholesterol and stearic acid-stearyl alcohol at the oil/water interface was also observed by this technique. Copyright 1998 Academic Press.
RESUMEN
The use of surfactants to reduce the number of residual particles following chemical-mechanical planarization during integrated circuit manufacturing is relatively new. Recent results using cetyl pyridinium chloride and other cationic surfactant molecules show that surfactants are very effective in reducing the number density of residual polishing particles. The effectiveness of the surfactants is related to their ability to adsorb on the substrate surface. The contact angle and spectroscopic data in this study show that cetyl pyridinium ions can be readily adsorbed or desorbed from a chemical-vapor-deposited tungsten surface by controlling the concentration of other cations in solution. The mechanism for surfactant desorption is likely a competition between the cationic surfactant ions and other cations in solution. Copyright 1998 Academic Press.
RESUMEN
The effect of long chain alcohols (5 mol% CnOH for n = 8, 10, 12, 14, and 16) on the micellar stability of sodium dodecyl sulfate (SDS) solutions (SDS concentration ranging from 25 to 200 mM) was investigated and related to foaming properties, such as foamability, dynamic and equilibrium surface tension, and surface viscosity. The slow micellar relaxation time tau2, which is directly related to micellar stability, was determined by the pressure-jump technique. It was found that below 150 mM all the long chain alcohols investigated in this study cause an increase in tau2 and, hence, micellar stability, due to the strong ion-dipole interaction between the SDS and the alkyl alcohol. However, above approximately 150 mM SDS, all alcohols except C12OH decrease the micellar stability due to mismatching of the alkyl chains. When the chain length of the alcohol and SDS are not equal, the excess hydrocarbon chain exhibits thermal motion, thereby increasing the area per molecule in micelles as well as at the air/water interface. Foamability was determined by two methods: air blowing through a single capillary submerged in the surfactant solution or vigorous hand shaking. When enough time is allowed for the interface to form (in case of single bubble foam generation), the dynamic surface tension approaches the equilibrium surface tension. Since the equilibrium surface tension for the SDS/C12OH mixture is significantly lower (approximately 7 mN/m) than that for the pure SDS solution, more foam is generated with the mixed surfactant system. However, in very high shear rate processes (e.g., vigorous hand shaking), the break up time of micelles determines the flux of surfactant molecules to the interface and hence the foamability. Since the mixed SDS/C12OH micelles are more stable (longer relaxation time, tau2) than pure SDS micelles, higher dynamic surface tensions are attained and thus less foam is generated with the surfactant/alcohol mixture by the shaking method. In conclusion, we show that the foamability can exhibit opposite behavior depending upon the rate of foam generation (i.e., specific method used for foaming). Copyright 1998 Academic Press.
RESUMEN
A general empirical relationship has been developed for estimating the cloud point of pure nonionic surfactants of the alkyl ethoxylate class. For a set of 62 structures, composed of linear alkyl, branched alkyl, cyclic alkyl, and alkylphenyl ethoxylates, cloud points can be estimated to an accuracy of ±6.3°C (3.7°C median error) using the logarithm of the number of ethylene oxide residues and three topological descriptors that account for hydrophobic domain variation. Copyright 1997Academic Press
RESUMEN
Relationships between the molecular structure and the critical micelle concentration (cmc) of anionic surfactants were investigated using a quantitative structure-property relationship approach. Measured cmc values for 119 anionic structures, representing sodium alkyl sulfates and sodium sulfonates with a wide variety of hydrophobic and hydrophilic structures, were considered. The best multiple linear regression model involved three terms (descriptors) and had a correlation coefficient of R2 = 0.940. Very good correlations (R2 = 0.988) were obtained using three descriptors for a subset of 68 structures, with structural variation only in the hydrophobic domain. From the descriptors used in these regressions, one can conclude that the cmc is primarily dependent on the size (volume or surface area) of the hydrophobic domain and to a lesser extent on the structural complexity of the surfactant molecule.
RESUMEN
The effect of surfactant concentration and film area on film stability was investigated in thin liquid films of sodium dodecyl sulfate (SDS) solutions. Film stability was evaluated at various concentrations of SDS solutions and various film areas. In this study, a maximum film stability was observed for the SDS concentration of 200 mM. Interestingly, this concentration also corresponds to the maximum micelle lifetime or long relaxation time (tau2). The results also show that film stability decreases with increasing film area, and that the larger films appear to be more sensitive to the micelle stability and structuring effect in the film than films with smaller surface areas.