Surface heterogeneity of passively oxidized silicon carbide particles: vapor adsorption isotherms.

The surfaces of silicon carbide particles subjected to two different passive oxidation treatments have been characterized by immersion calorimetry and vapor adsorption techniques. Surface enthalpies and surface free energies have been computed using semiempirical models and are compared to theoretical estimations. The surface entropy term appears higher than in the case of other solids studied with the same analysis. The definition of the surface entropy term is discussed in order to explain the discrepancy between calculation and experiment. An explanation of results is proposed, which is related to the constitution of silicon oxide layers at the surface of silicon carbide, a fact demonstrated by previous XPS measurements.

Adsorption of a cationic gemini surfactant from aqueous solution onto aluminosilicate powders of the MCM-41 type: effect of pore size and co-adsorption of phenol.

The adsorption of cationic gemini dodecanediyl-alpha,omega-bis(dodecyldimethylammonium) bromide (i.e., C12C12C12) from aqueous solution onto aluminosilicate powders of the MCM-41 type (referred to as SiAl32dx, where x is the mean pore diameter in A) has been studied at 298 K under the conditions of free pH of the aqueous phase. Macroporous silica gel XO15M was used for comparative purposes. The discussion was based on the interpretation of experimental adsorption isotherms and differential enthalpies of displacement obtained on various solid samples. For the hydrogen-exchanged SiAl32d28 sample (i.e., H+-SiAl32d28), the adsorption of bromide counterions Br- at the solid-solution interface, the isotherm of the pH evolution in the equilibrated supernatant liquid, and the electrophoretic mobility of the solid particles coated with the adsorbed C12C12C12 were additionally measured. The uptake of phenol (PhOH) by a surfactant-solid system from a 1.5 mmolkg(-1) PhOH solution and the related thermal effect of displacement onto SiAl32d50 were quantified with the use of the solution depletion technique combined with UV spectrophotometry and the titration calorimetry technique. Titration calorimetry was also used to determine the molar enthalpy change accompanying micellization of C12C12C12 in pure deionized water and in a 1.5 mmolkg(-1) PhOH solution at 298 K. The adsorption of C12C12C12 occurs simultaneously on the external surface and on the pore walls and it is a strongly co-operative phenomenon. Surfactant aggregates forming at adsorption saturation are thought to be composed of the adsorbed surfactant units having their cationic head groups mostly oriented outward with respect to the solid surface. Therefore, they can provide co-adsorption sites for polarisable phenol molecules. On average, there is one phenol molecule retained for one gemini cation adsorbed. Transfer of phenol from the aqueous phase to either the bulk micelles or to the interfacial aggregates is enthalpically favourable.

Alkanediyl-alpha,omega-bis(dimethylalkylammonium bromide) surfactants 9. Effect of the spacer carbon number and temperature on the enthalpy of micellization.

The enthalpies of dilution of micellar solutions of several 12-s-12 dimeric surfactants of the alkanediyl-alpha,omega-bis(dodecyldi-methylammonium bromide) type, differing by the carbon number s of the alkanediyl spacer, and of dodecyltrimethylammonium bromide (DTAB) have been measured calorimetrically, in a range of concentrations extending from well below to well above the critical micelle concentration (cmc). The results permitted the determination of the enthalpy of micellization, DeltaH degrees (M), of the investigated surfactants at 25 and 35 degrees C. The values of DeltaH degrees (M) were always negative and became more negative as the temperature was increased. The plot of -DeltaH degrees (M) against s showed a shallow minimum at about s=5 and a large decrease of -DeltaH degrees (M) going from 12-2-12 to 12- 4-12. This effect has been attributed to the contribution to DeltaH degrees (M) of the hindered rotation of the dodecyl chains around the spacer C-C bond for 12-2-12. This hindrance is shown to rapidly disappear when s is increased from 2 to above 4. The specific heats of micellization, the free energies of micellization, and the entropies of micellization (DeltaS degrees (M)) have been calculated using the DeltaH degrees (M) values and the reported cmc and micelle ionization degree data for 12-s-12 surfactants and DTAB. For all surfactants the results show that TDeltaS degrees (M)>-DeltaH degrees (M), indicating an entropy-driven micellization.

Evaluation of Surface Enthalpy of Porous Aluminosilicates of the MCM-41 Type Using Immersional Calorimetry: Effect of the Pore Size and Framework Si:Al Ratio.

Surface properties of porous aluminosilicates of the MCM-41 type have been tested by immersional calorimetry. Two series of materials, referred to as SiAlxCn, where x is the Si : Al mole ratio and n the chain length of the surfactant template, having (1) x=32 and n=8, 12, 14, 16, 18 and (2) n=14 and x=8, 32, infinity, were used. The results of thermogravimetric analysis on these samples served to evaluate the surface density of hydroxyl groups. This parameter is rather sensitive to the pore size than to the aluminium content in the solid matrix. Based on the experimentally measured enthalpies of immersion in n-heptane, water, and formamide per unit BET specific surface area, estimates could be made of the apolar, Lewis acid, and Lewis base contributions to the total surface enthalpy of MCM-41 materials. The samples studied have a predominant surface acidic character, which is markedly enhanced by incorporating aluminium into the silica matrix. Surface acidity is also modified by changes in the porous structure, although the trends are less noticeable here. Nevertheless, the total surface enthalpy of MCM-41 aluminosilicates appears to be small in comparison with typical inorganic oxides, such as silica or alumina. Copyright 2001 Academic Press.

Assessment of the Surface Heterogeneity of Talc Materials.

The hydrophobic and hydrophilic components of the surface of talc materials in aqueous solution were determined using ionic surfactants and their polar headgroup adsorption isotherms. The hydrophilic and hydrophobic surface areas are inferred from the amount of probe molecule adsorbed and the structure of the adsorbed layer. Natural dispersion of talc shows at 298 K a pH of 9.4 and the electrophoretic measurements indicate that the particles are negatively charged. The hydrophilic surface area is estimated from the adsorption of benzyltrimethylammonium ions (BTMA(+)) through electrostatic interactions as supported by the increase of divalent ions in the bulk phase and the decrease in the exothermic displacement enthalpy. It was also observed from the adsorption isotherm of benzene sulfonate anions that the density of positive surface sites is very low and is thus neglected. The adsorption of an anionic surfactant essentially occurs through dispersive interactions between the nonpolar organic tail of the molecule and the hydrophobic surface. Furthermore, some assumptions on the structure of dodecyl sulfate surfactant aggregates at the interface allow the hydrophobic part of the talc particles to be estimated. The cationic surfactant adsorption has been investigated and found to corroborate the hydrophilic and hydrophobic area values first obtained. Copyright 2001 Academic Press.

On the Behavior of Nonionic Surfactants at the N-Heptane/Silica Gel Interface: Influence of the Presence of Interfacial Water Inferred from Adsorption Isotherms and Calorimetric Data.

The behavior of two polydisperse nonionic surfactants, poly (oxyethylene) glycol alkylphenyl ether TX-35 and TX-100, at the prewetted silica gel/n-heptane and dried silica gel/n-heptane interfaces has been compared by the determination of the average adsorption isotherms of the polydisperse surfactants and of displacement enthalpies. From HPLC experiments, we could also separately quantify the adsorption of each ethyleneoxide (EO) fractions for silica gel from the polydisperse surfactant solution. The adsorption isotherms clearly indicate an incomplete preferential adsorption of the large (EO) chains over the small ones, as well on dried silica gel as on a prehydrated sample. This preferential adsorption and its driving force follow the solubility rules of the poly(oxyethylene) glycol alkylphenyl ether in an apolar solvent and support the idea of a solubility-limited adsorption: solubility in organic solvents of the smaller (EO) chains is much more significant than that of the longer ones and hence prevents adsorption of the smaller species. Consequently, it is observed that the presence of interfacial water decreases the affinity of TX-35 molecules for the hydrophilic silica surface due to the hydration of (EO) chains. In contrast, for TX-100 adsorption after the prewetting treatment the clearest trend is a drastic increase of the adsorption ascribed to the additional solubilization (and micellization) of the TX-100 molecules in the interfacial aqueous phase. The differential molar enthalpies of displacement show a change in the adsorption mechanism, depending on the presence of molecular water on the surface. In the initial part of the adsorption isotherm, a prevailing exothermic process is obtained with prehydrated silica and suggests that hydration of the polar heads of TX-35 and the solubilization of the TX-35 in interfacial water are occurring. For higher equilibrium concentrations, the enthalpies of displacement observed with the prehydrated adsorbent become slightly lower than those obtained with dry silica gel. It may be that this difference is due to the micellization phenomenon of the surfactant species with longer EO chains in interfacial water. These features emphasize the influence of interfacial water on the adsorption of EO fractions from organic solvent. Copyright 2000 Academic Press.

Surface Heterogeneity of Passively Oxidized Silicon Carbide Particles: Hydrophobic-Hydrophilic Partition.

The surface of silicon carbide (SiC) particles previously subjected to passive oxidation has been characterized using various techniques such as adsorption of positively and negatively charged surfactants in aqueous solution, immersion microcalorimetry in three probe liquids, and flow microcalorimetry in organic media. Adsorption data show that around one quarter of the surface appears negatively charged and thus hydrophilic, while three quarters appear uncharged and hydrophobic. This is attributed to dissociation of silanols groups. Immersion calorimetry in liquids having well-defined polar and nonpolar components of surface energy shows that the Lifshitz-van der Waals component of SiC is very high and that the acid and basic components are weak. The experimental results appear to be consistent with both computations of surface energy using Lifshitz theory and experimental data previously obtained with other minerals. The three indexes are discussed and it is argued that they represent different terms of the solid surface energy. Copyright 2000 Academic Press.

The usefulness of high dose (7-10mci) gallium (67Ga) scanning to diagnose Richter's transformation.

Gallium (67Ga) scan was performed in 29 CLL patients with chronic lymphocytic leukemia who were suspected on clinical grounds to have Richter's transformation (RT). Of 29 patients, nine had a positive 67Ga scan; seven of these had a subsequent biopsy that verified large-cell lymphoma or Hodgkin's disease. The other two patients underwent biopsies that revealed fungal infections, a known cause of 67Ga uptake. Two patients had biopsies that were consistent with RT but showed no affinity to 67Ga. One false negative resulted five days after chemotherapy, a known cause of diminished 67Ga uptake. The other occurred within a small infraorbital mass, containing only 10% centroblasts, which is below the level of detection for 67Ga scanning. Subsequent 67Ga scans in both patients revealed 67Ga avid lesions, which demonstrated RT upon biopsy. This technique was more strongly predictive of RT than was measurement of serum B-2 microglobulin or serum lactate dehydrogenase levels. 67Ga scanning is very useful in localizing an optimal site for biopsy to document RT; it may also have the potential to help assess response to treatment, predict recurrence, and contribute to long-term follow-up in this subset of patients.

Combination chemotherapy with granulocyte-macrophage-colony stimulating factor in patients with locoregional and metastatic gastric adenocarcinoma.

BACKGROUND: A combination of etoposide, 5-fluorouracil, and folinic acid (ELF) remains popular for the treatment of patients with gastric carcinoma and has been reported to result in a response rate of up to 40% with good patient tolerance. The authors elected to add granulocyte-macrophage-colony stimulating factor (GM-CSF) to ELF to determine whether the response rate could be increased in patients with untreated advanced gastric carcinoma. METHODS: Previously untreated patients with measurable metastatic tumor were studied. Outpatient therapy was comprised of etoposide, 120 mg/m2 intravenously (i.v.), on Days 1-3; 5-fluorouracil, 500 mg/m2 i.v., on Days 1-3; and folinic acid, 300 mg/m2 i.v., on Days 1-3. Courses were repeated every 21 days. GM-CSF (at a dose of 250 microg/m2/day for 14 days from Day 4) was added after the first course of ELF if patients developed Grade 4 neutropenia in a previous course. RESULTS: Thirty patients were enrolled and 29 were evaluable for response. Four patients (14%) achieved a partial response (median duration of response, 6.5 months). The median duration of survival was 7.8 months. Grade 4 neutropenia occurred in 16 patients who then received GM-CSF. A similar rate of neutropenic fever was observed in courses both with or without GM-CSF (15% in courses without GM-CSF and 16% in courses with GM-CSF); however, a higher nadir absolute granulocyte count (1300 cells/microL) occurred in courses with GM-CSF compared with courses without GM-CSF (300 cells/microL). CONCLUSIONS: The ELF regimen resulted in a much lower response rate than reported in the literature. The attempt to improve the efficacy of this regimen by the addition of GM-CSF did not prove successful. The authors believe this regimen cannot be recommended for the treatment of patients with advanced gastric carcinoma outside of a protocol setting.

Calorimetric, Viscosimetric, and Light Scattering Studies of the Aggregation of, and the Solubilization of Water by, Triton X-35 in n-Heptane.

The behavior of a nonionic surfactant TX-35 in solution in n-heptane in the presence and absence of added water has been examined using the microcalorimetric, viscosimetric, and quasielastic light scattering experimental methods. In this paper, we were interested in the aggregation process of the poly(oxyethylene) glycol alkylphenyl ether in n-heptane and in the solubilization of water in the reverse micelle of the surfactant (micellar solubilization). The analysis of the differential molar enthalpies of dilution of TX-35 in dried n-heptane has shown the occurrence of a gradual exothermic aggregation process on a very wide range of concentration which takes place at particular concentration so-called "operational CMC". This operational CMC value has been confirmed by viscosities measurements. The differential molar enthalpies of hydration of TX-35 were also measured and found to be exothermic. The maximal hydration ratio (w0) was found to be equal to 3.2 mol H2O per mole of TX-35 before the point of phase separation. The measurements of the variation of the amount of water contained in TX-35 solutions at different concentrations in n-heptane also show the occurence of a gradual aggregation process and confirm the value of the maximal hydration ratio already determined by microcalorimetry. In the absence of added water, from the quasielastic light scattering experiments, a mean diameter of the aggregates close to 45 Å has been determine, while in the presence of water, a mean diameter of 61 Å was detected and remained unchanged with increasing the hydration ratio indicating that the size of the aggregate is more influenced by the presence of water than by the amount. In the presence of water, it is relevant to discuss aggregates of lamellar or filament shape. Copyright 1999 Academic Press.

Adsorption Mechanism of Conventional and Dimeric Cationic Surfactants on Silica Surface: Effect of the State of the Surface.

The aim of this study was to investigate the effect of the state of the silica surface and of the surfactant molecular structure on the adsorption of cationic surfactants onto silica. Thus, the adsorption of DTAB (dodecyltrimethylammonium bromide) and of the dimeric surfactant 12-2-12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) on raw silica (SiNa) and on HCl-washed silica (SiH) has been investigated under "free" system conditions. The amount of surfactant adsorbed (adsorption isotherm), the sodium ion and bromide ion concentrations and the pH in the equilibrated supernatant, and the silica particle electrophoretic mobility have been measured along the isotherms. The adsorption mechanisms of the two surfactants on the raw and washed silica are qualitatively similar. Nevertheless, important quantitative differences are observed which are all due to (i) the larger number of surface sites present at the surface of SiNa with respect to SiH and (ii) the larger ionic strength of the supernatant in SiNa/surfactant systems with respect to SiH/surfactant systems, due to the much larger amount of sodium ions released by SiNa upon surfactant binding. Thus, the amounts of surfactant adsorbed at the point of zero charge and at saturation of the silica particles, of sodium ions released by the surface and the decrease of critical micelle concentration (cmc) in the supernatant with respect to pure water are all larger for the raw silica than for the treated silica. For the four silica/surfactant systems investigated, the first adsorption step corresponds to the adsorption of individual surfactant ions on the negative sites of the silica surface. It is driven by electrostatic interactions and strongly dependent on the number of surface sites and ionic strength associated to the released ions. At the end of the first adsorption step, which is clearly seen with SiH/surfactant systems, the second adsorption step starts. This step is driven by hydrophobic interaction between surfactant alkyl chains and results in the formation of surface aggregates. The surfactant adsorption on the surface is shown to continue even after the cmc in the supernatant is reached. Copyright 1999 Academic Press.

Chemotherapy of Colorectal Cancer.

Progress has been made in the adjuvant treatment of colorectal carcinoma. The improvement in survival with the use of adjuvant 5-FU and leucovorin in patients with stage III colon carcinoma has been readily established. However, a survival benefit in stage II patients treated with adjuvant therapy remains unproven. Further evaluation using additional/new prognostic factors may identify a high-risk stage II group that would benefit from adjuvant treatment. Adjuvant chemoradiation has become standard therapy for stage II and III patients with rectal carcinoma. Investigations using preoperative combined-modality therapy are being explored to assess sphincter preservation rates and to evaluate any impact on survival. Radiosensitizing chemotherapeutic agents need to be evaluated in this patient population. Recent advances in metastatic disease have occurred. Frontline therapy remains 5-FU and leucovorin. CPT-11 has demonstrated responses in 5-FU relapsed and refractory patients and is the new standard therapy in these patients. New data recently available also show a survival advantage in patients treated with CPT-11 versus supportive care in 5-FU and leucovorin failures. New agents such as UFT and oxaliplatin have demonstrated activity in colorectal carcinomas and in the future these agents will likely aid in the treatment of this disease.

Interactions between Talc Particles and Water and Organic Solvents

Three talc samples have been studied by adsorption and immersion methods after a classical characterization of their properties. The combination of adsorption isotherms and of immersion measurements allows the calculation of enthalpies and entropies of adhesion. The studied talcs are characterized as "middle energy" solids. The differences between the particle shapes of the different samples are shown to be of great importance, indicating a linkage between cristallinity and surface properties. The whole results are explained by the influence of intermolecular forces such as acid-base interactions in the interfacial layer. Copyright 1997 Academic Press. Copyright 1997Academic Press

Simple models of fatality trends revisited seven years later.

The author's original paper presented simple linear models of fatalities based on data through 1982 and appeared in this journal in 1984. The earlier paper reported good fits through 22 years of fatality counts using population and employment data, after adjusting for the short-term effect of the oil shortage (in 1974) and the long-term effect of the 55 mph speed limit (beginning in 1974). Attempts (reported here) to refit the model through seven additional years were less successful. It may be that the relationships originally reported have changed, that important new factors have been introduced, or that the original fits were partly luck. The recent emphasis on behavioral solutions to safety problems (including programs to decrease alcohol use and increase safety belt use) is one important change, so the effect of the benefits of these programs on the model fits is explored. The original model began to overpredict fatalities just as safety benefits were beginning to accrue from increased safety belt use and decreased driver-alcohol involvement, and both the model overprediction and the benefits of these programs increased over the seven years of data added to the original model. Thus, previous government estimates of the number of lives saved by these behavioral programs are supported by the observed change in the statistical relationship between fatality counts and economic factors.

Differences in reported car weight between fatality and registration data files.

Two national-level data sources are commonly used together to estimate and compare fatality rates by car weight. The weight of each car in a fatal crash is available on the automated files of the National Highway Traffic Safety Administration's Fatal Accident Reporting System; weight is derived by interpreting the Vehicle Identification Number of each car using a computer algorithm developed and maintained by R. L. Polk & Co. Counts of cars in use, by weight, are available on R. L. Polk & Co.'s National Vehicle Population Profile files; weights are coded from information in state vehicle registration files. However, it appears that there are systematic differences in car weight coding that complicate the use of these two sources together for calculating fatality rates (fatalities per registered car). Overall, the registration data appear to describe a car (of a particular make, model, and model year) as about one hundred pounds heavier than that car is described in the fatality data. The effect is to bias the comparison of fatalities per registered vehicle against lighter cars. Failure to consider this difference can lead to very misleading results. For example, the uncorrected data produce an estimate that the number of occupant fatalities per registered minicompact car (those under 1,950 pounds) was five times the rate in the largest cars (those weighing at least 3,950 pounds). Correcting for differences in car weight reporting produces estimates that the fatality rate in minicompact cars was twice that in the largest cars. Differences by car weight remain, but they are much less than would be concluded from the biased comparison.