Journal of Colloid Science最新文献

The relative surface acidity of several alumina powders with a variety of specific surface areas has been measured by a microcalorimetric technique. The alumina surfaces studied here are those for which the catalytic activity for alcohol dehydration has been measured. The technique used was to measure the heats of immersion (ΔH_i) in pyridine and 2,6-dimethyl pyridine. A maximum in ΔH_i for both liquids was observed as a function of a particle size corresponding to a maximum in the catalytic activity. The technique was also applied to NiSO₄·xH₂O samples, and the results were compared to other surface acidity measurements for these samples.

Measurements of the critical micelle concentration and of micellar molecular weight are reported for polyoxyethylene (23) lauryl alcohol in ethanol-water and dioxane-water solvent systems. With increasing concentration of additive, the c.m.c. increases and the micellar weight decreases; at concentrations above about 25%, no micelle formation occurs. The results are explained qualitatively in terms of the free energy requirements for micellization.

The surface charge on silver iodide suspended in a solution has been measured as a function of the potential of a cell consisting of a silver-silver iodide electrode, the solution under consideration, and a reference electrode. The influence of urea, n-propyl-, n-butyl-, isobutyl-, sec-butyl-, tert-butyl-, and n-amyl alcohol was investigated. Thermodynamic analysis shows that the surface excess of adsorbed alcohols if plotted as a function of the cell potential passes through a maximum. This maximum does not coincide with the zero point of charge. The results suggest that nalcohols adsorb with their hydrophobic part towards the silver iodide. These features are also encountered with the mercury system. Differences between the two systems are quantitative rather than qualitative, e.g., the Gibbs free energies of adsorption on silver iodide are about 25% less negative than the corresponding ones on mercury.

Aqueous micelles of phosphatidic acid are prepared by ultrasonic radiation. In the pH range studied, ≧ 7, these dispersions consist of highly charged polyanions and have low turbidities. The interactions of buffered systems of phosphatidic acid with cations can be studied by turbidimetric titrations. At pH 7.2, increased turbidity occurs in 0.2 N LiCl or 0.4 N NaCl, however, KCl and tetramethylammonium chloride produce no change up to concentrations of 0.75 N. In 10⁻⁵ M CaC1₂ the turbidity increases and rises steeply to 2 × 10⁻⁴ M. Somewhat higher concentrations are necessary with MgCl₂ . Additions of univalent cations to phosphatidic acid containing Ca reduces the turbidity by exchange with Ca.

In systems containing sodium citrate, the concentrations of CaCl₂ required to produce given turbidity changes are greater than in comparable systems containing NaCl. This gives a measure of the concentration of CaCit⁻ formed. From the known formation constant of CaCit⁻, the Ca⁺² concentration in equilibrium with the anions of citric and phosphatidic acids is calculated. This permits the evaluation of the formation constant of CaPA. Values obtained at pH 7.2, 8.8, and 9.6 are 1.2 × 10⁴.

The so-called discreteness-of-charge effect (or discrete-ion effect) for adsorbed counter-ions is incorporated into the stability theory of lyophobic colloids of Derjaguin and Landau and Verwey and Overbeek, for a coagulating 1-1 + 2-2 electrolyte mixture in an aqueous medium. The interaction of two parallel negatively charged colloidal plates in a large volume of the dispersion medium is considered and the approximation of linear superposition of the double layer potential distributions is employed. The Stern-Grahame-Devanathan model is used for the inner region and the Gouy-Chapman theory is applied to the diffuse layer. It is assumed that both the univalent and divalent cations (counter-ions) of the electrolyte mixture are adsorbed on the plate walls and, for simplicity, that the two ion types are situated at the same distance from the wall on the inner Helmholtz plane. Corrections due to the self-atmosphere potentials of the adsorbed ions (discrete-ion effect) and to the entropy effect arising from ion size are introduced into the adsorption isotherms of the two types of counter-ions. It is shown that the theory predicts the phenomenon of mutual antagonism when the self-atmosphere potential of the adsorbed divalent cation is sufficiently large. Among the conditions that favor antagonism are strong adsorption of the divalent cations and low integral capacity of the inner region. Plots of the coagulation curves in the (n, p) plane, where n and p are the concentrations of 1-1 and 2-2 electrolytes, respectively, show the usual pattern of antagonism, as obtained experimentally.

The interaction of Li⁺, K⁺, NH₄⁺, Mg⁺⁺, Ca⁺⁺, and tetramethyl ammonium (TMA⁺) lauryl sulfates with lauryl alcohol has been examined by differential thermal analysis, infrared, and x-ray methods. Evidence is obtained that the first four of these long-chain salts form intermolecular complexes with lauryl alcohol in which the stoichiometry is 1:1, i.e., one long chain of the sulfate to one of alcohol. The Ca⁺⁺ and TMA⁺ salts do not form such complexes. The formation of a (1:2) complex of sodium lauryl or myristyl sulfate and tetradecanol is inhibited if the hydroxyl group is changed from the 1 to the 2 position. The enthalpies of formation of the complexes are estimated and have the order Na⁺ > Mg⁺⁺ > NH₄⁺ > K⁺ > Li⁺.

The electrophoretic mobility of rutile has been related to the stability of its dispersions in solutions of Aerosol OT (sodium di-2-ethylhexyl sulfosuccinate) in p-xylene over a range of surfactant concentrations. For comparison purposes similar experiments have been carried out with α-alumina, the carbon black Vulcan R, and a copper phthalocynanine green pigment. Zeta potentials in excess of 50 mv. were found to be necessary for long-term stability, and this is in good agreement with theoretical predictions for the size of aggregate found in the stable dispersions. Addition of water to the rutile system has shown its importance in determining the sign and magnitude of the surface potential.

The critical micelle concentrations (CMC) of long-chain cationic and anionic surfactants in water are lowered by the addition of ordinary salts like NaCl. This is usually ascribed to electrostatic effects. It has been found that the CMC of hexadecyltrimethylammonium bromide in water at 30°C. is at first decreased by the addition of the lower quaternary bromides and then increases markedly at higher salt concentration. The initial CMC decrease is in the order K⁺ < (CH₃)₄N⁺ < (C₂H₅)₄N⁺ < (n − C₃H₇)₄N < (n − C₄H₉)₄N⁺. There is a continually decreasing value of the CMC in increasingly concentrated KBr solutions. However, there is a minimum CMC value for each added quaternary salt. Those for (CH₃)₄N⁺ and (C₂H₅)₄N⁺ are found in approximately 0.09 M and 0.01 M salt, respectively, that for the n-propyl compound is at 10⁻³ M solution. In more concentrated salt solutions the CMC is three to five times greater than in water, with the greatest increase found in (n − C₄H₉)₄ NBr solution, and the smallest increase in (CH₃)₄NBr at equal concentrations. The results are interpreted in terms of the effects of added electrolytes on the hydrogen-bonded structure of water. It is concluded that micelle formation is rendered more difficult by electrolytes which organize the solvent structure, and occurs at lower soap concentrations in the presence of structure-disrupting electrolytes like the alkali halides.

The protective value we propose gives the number of grams of a red gold sol which are just protected by 1 g. of the protective agent against flocculation by 1% NaCl solution. Gelatine has a protective value of 90, i.e., 1 g. of gelatine protects 90 g. of gold sol. Amphions are good protecting colloids. One of the ionic groups attaches the protecting agent to the colloidal particle, the other supplies the electrical charge.

Synthetic protective colloids were prepared by introducing acidic groups into the basic polyethylenimine molecule, and by introducing basic groups into the polyacrylic acid molecule.

With some synthetic protective colloids, the protective value can be increased as much as one hundred times by the action of heat and time. The interaction between the protecting agent and the sol to be protected requires a longer time than the 3 minutes previously recommended. We recommend that the reaction be allowed to go to completion.

Good protecting colloids form stable complexes with the coagulating metal ions.

The protective value changes strongly as the pH of the sol is varied within a narrow range. With some polyamphions the protective value, at a given sol pH, can be increased by shifting the isoelectric point (IEP). The protective values and gold numbers of some protecting agents are compared. A synthetic polyacrylic hydrazide had the highest protective value of 400.

The protecting action of natural gelatine is at a minimum at the IEP, but that of the synthetic hydrazide of polyacrylic acid is greatest at this point. In this context the VW theory of Heller is discussed, and the distance between the ionic groups on the polymer chain is postulated as a further important factor contributing to the formation of the stabilizing layers.

The surface characteristics of mono- and dioctadecylphosphoric acid spread at the air/water interface are presented. The influence of bulk pH, inorganic ions, and temperature were investigated. The values of the surface pK, calculated by the surface potential method, are also given.