Colloids and Surfaces最新文献

The contact angles θ for water at methylated quartz surfaces were measured using the sessile-drop technique to determine the advancing contact angle, and using a captive-bubble technique to examine the effect of bubble size on contact angle. No linearity between cos θ and $\text{1}\text{r}$ where r is the drop base radius, was observed for these systems as would be expected for an ideal system. In fact the pseudo-line tension decreased with decreasing bubble size. Also, the degree of quartz methylation effected a change in the pseudo-line tension. The pseudo-line tension increased from 0.4·10⁻⁶ N to 3.3·10⁻⁶ N with an increase in the fractional coverage of trimethylsilyl groups from 0.14 to 0.51 for large bubbles (bubble base diameter d > 0.34 mm), whereas the pseudo-line tension decreased from 2.2·10⁻⁷ N to 0.8·10⁻⁷ N with an increase in fractional coverage for small bubbles (bubble base diameter d = 0.06–0.2 mm).

The flotation limit of fine particles has been re-examined based on the effect of bubble size on contact angle, and a new surface chemistry-limited relationship describing the minimum particle size which can be floated is proposed:

The structure of sonic flocculated dispersions can be changed reversibly by means of shearing. Often the changes are not instantaneous. The resulting shear-history effect gives rise to a complex but interesting rheological behaviour. Using non-aqueous suspensions of fumed silica, the rheological equilibrium properties of such systems are investigated. To change the floc structure, the water content of the particles is altered. As well as the steady-state shear viscosity, the equilibrium modulus and the yield stress are measured. Various techniques are compared. The effect of concentration on the equilibrium properties is used to test some structural models. The concentration dependence is best described by a power-law relation, the power being identical for modulus and yield stress. These results compare well with some theoretical predictions. Contrary to the assumptions used in the modelling, the yield stress is often dominated by kinetic phenomena. This shortcoming also shows up in the predictions for the critical strain.

The adsorption behaviour of different silicon nitride powders in cyclohexane has been studied. Adsorption isotherms of organic probe molecules covering a wide spectrum of Lewis acidity/basicity showed large variations in adsorption behaviour between the different probes. Mathematical analysis showed that the adsorption data could be described by a Langmuir-Freundlich type of isotherm.

Using a previously presented model of the silicon nitride surface, it was possible to relate the variation of the isoclectric point in water between the three powders, to a difference in the relative site density or silanol and amino groups on the surface. It was found that the maximum concentration (T_M) of benzoic acid increased linearly while of pyridine decreased with an increase in the relative amount of amino groups on the surface.

Early work on colloidal gas aphrons (CGAs) and Colloidal liquid aphrons (CLAs) has shown that they have considerable potential in the field of predispersed solvent extraction (PDSE). While their area of application is potentially very broad, their most promising use is in downstream separation in biotechnology where products are very dilute and occur in complex mixtures. Since little work has been done in this area, this preliminary study examined the influence of a range or solvents, varying from non-polar to mildly polar, and a variety of ionic and non-ionic surfactants, on CLA size, stability and phase volume ratio (PVR, volume ratio of the dispersed oil phase to the continuous aqueous phase). In addition, the effect of surfactant type, stirring speed and time, on the formation of CGAs was also studied. The results show that CLAs can be formulated with quite polar solvents (e.g. pentanol), and their stability increases as the HLB (hydrophilic/lipophilic balance) number of the non-ionic surfactant increases. CLAs could be formulated with PVRs as high as 20 without coalescence, which is markedly higher than with microemulsions, and seems to indicate that the liquid aphrons are stabilised by more than a surfactant monolayer. Finally, it was found that CGAs could be formulated as a foam with a half-life of 6 min, and that they could be used to separate dispersed CLAs effectively from a bulk solution.

???lems of characterizing foam stability are discussed. Coalescence times for foams of different degrees of complexity ??en defined and determined. Time quantities used in this work are: thinning and bursting times of the film, and ??? coalescence both between a pair of bubbles and between a bubble and the continuous gas phase. Bubble lifetimes ???d strongly on the complexity and structure of the foam and an approximate explanation of this is given. The effects of different factors (surfactant, salt content, temperature, bubble size and foam complexity) on stability are presented for several cases investigated and it is noted that these effects cannot be separately treated in an exact manner because the importance of the individual processes involved in the decomposition may vary considerably when altering any of the parameters. A correct characterization of foam stability is consequently possible only if the foaming in real situations is simulated to the greatest possible extent by the method of investigation.

The dynamic surface properties of three widely different protein systems are investigated using different experimental techniques. These techniques are (i) static drop experiments which allow description of the adsorption of proteins at a clean surface, (ii) experiments at a constant rate of dilatation where the area is continuously expanded and a steady state surface tension is obtained, (iii) modulus experiments where a surface in equilibrium is subjected to small periodic disturbances, (iv) stress relaxation experiments which are theoretically similar to moduli experiments, and (v) to a lesser degree, aspiration and sweeping experiments. The results are described by a process involving two steps: (i) diffusion of proteins to the surface and (ii) unfolding of adsorbed protein molecules at the surface. Only native protein molecules are exchangeable, while the unfolded protein molecules are not. From these results the diffusion relaxation times and reaction relaxation times are obtained.

In practice, foam stability is tested by different methods which measure different quantities, the numerical values of which are sometimes without exact physical sense or even contradictory. In order to understand discrepancies and to obtain reliable data for foam stability, several foam models of differing complexity have been elaborated which provide quantities of a similar nature, i.e. bubble lifetimes. The models correspond to single bubbles, to the one-bubble-thick foam in its steady state and during decay, and to thicker, but not too thick, foams.

The reaction of 3-propylethylenediamine groups covalently bonded to silica gel with pentane-2,4-dione in benzene resulted in a modified surface having 11.4·10⁻⁴ mol g⁻¹ of ligand sites which adsorb the divalent cations Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ from ethanol solution with maximum adsorptions of 5.0·10⁻⁴ mol g⁻¹, 2.0·10⁻⁴ mol g⁻¹, 5.0·10⁻⁴ mol g⁻¹ and 5.9·10⁻⁴ mol g⁻¹, respectively. The adsorption isotherms are very similar in shape for all the cations and conform to the Langmuir model. The complexes formed on the surface have a molar composition or one cation to two anchored groups of Schiff bases and the geometries of the complexes are similar to those obtained in solution.

Spreading monolayers of the ionophore valinomycin in mixtures with methyl stearate with different molar ratios were studied at the air/water interface in the 15–30 C temperature range.

The bidimensional miscibility of the two components was deduced by recording spreading isotherms, which allowed also the evaluation of the thermodynamic parameters ΔG_mix, ΔH_mix and TΔS_mix.

Languir—Blodgett (LB) films of mixtures with various molar ratios were built by transferring spreading monolayers from the air/water interface on to hydrophobic and hydrophilic substrates, and their electrical resistance was determined using KCl and NaCl solutions in the 10⁻³-5·10⁻¹M concentration range.

It was verified that valinomycin-containing mixtures showed K⁻ ion selectivity in LB films, at an opportune molar ratio between the two components.

Adsorption of bovine serum albumin (BSA) on calcium hydroxylapatite (HAP) with different Ca/P molar ratios ranging from 1.55 to 1.70 and on HAP surface-modified by silicate and fluoride ions has been studied at pH 6.0 and 288 K. The adsorption isotherms of BSA on HAP particles are of the Langmuir type and adsorption of BSA generated a negative charge on HAP. Unmodified HAP particles which have a larger Ca/P molar ratio tended to yield a larger electrically positive zeta potential, which in turn gave a larger saturated amount of adsorbed BSA (n_s) as a result of their electrostatic attractive force. The surface modification by silicate ions increased the surface acidity and made the HAP surface more electrically negative so as to yield smaller n_s values. On the contrary, surface modification by fluoride ions increased n_s two-fold over that on the unmodified surface. This fact could be explained by the formation of highly basic CaF₂ on the HAP surface.