EFFECTS OF THE METHOD OF PREPARATION ON THE OPTICAL PROPERTIES AND STABILIZATION OF SUSPENSIONS AGAINST SEDIMENTATION OF AQUEOUS DISPERSIONS OF A DOUBLE-CHAIN CATIONIC SURFACTANT
In the practical applications of colloidal dispersions and suspensions, such as inks, paints, and food industry, the suspended particles must be stabilized, and remain well-dispersed for long times. Particles which are more dense than the suspending media may sediment rapidly, even with no agglomeration occurring, under many conditions of size and density difference. Then, a dispersant would be necessary for stabilization of particle suspensions against both agglomeration and sedimentation, while the suspensions should remain flowable in many applications. Moreover, when many aqueous suspension media may contain salts, the dispersant also needs to be an effective stabilizer against sedimentation under the specific salinity conditions of that application.
DDAB (didodecyldimethylammonium bromide) , a cationic double-chain surfactant, forms lamellar liquid crystal phases when dispersed in water. It also easily forms aqueous vesicle dispersions (unilamellar closed particles with an internal solvent compartment) and liposomes (multilamellar vesicles, MLVs, or lamellar liquid crystallites) at relatively low DDAB weight fractions, wD. To better understand the phase/dispersion behavior of DDAB and the corresponding optical properties, new analytical solutions of the spherical particles have been obtained for the light scattering theory in the Rayleigh (R) and the Rayleigh-Debye-Gans (RDG) regimes, for single and independent scattering. Moreover, the specific Rayleigh ratio Rq** and the specific turbidity t** were derived analytically for both scattering regimes. Spectroturbidimetry (ST) data at 25 °C for DDAB were compared to the t** predictions. t** data for DDAB vesicles are consistent with the RDG predictions, which are also used to estimate the vesicle sizes.
For a better understanding of the effect of the preparation method and salinity on the formation of DDAB vesicles, spectroturbidimetry was used to measure the average radius of the unilamellar DDAB vesicles, which were prepared via two different methods in water and in NaBr salt solutions. The radius was ~24 nm after sonication (SS method) and ~74 nm after extrusion/ultrafiltration (SE method). The radii were larger when the vesicles were produced in 10 mM NaBr, ~65 nm for the SS method and ~280 nm for the SE method. The t** values of these vesicular dispersions increased with decreasing wD values, until a constant value was reached at wD*, which depends on the preparation method and the dispersion medium. The constant values of t** are indicative of single and independent scattering, and were used to estimate vesicle radii by solving the t** equations derived for the RDG regime. Estimates of the average distances between the vesicles and their corresponding Debye lengths were obtained to evaluate the importance of inter-vesicle electrostatic interactions, which could lead to dependent scattering at higher weight fractions.
DDAB prepared with magnetic stirring of multilamellar liposomes, followed by ultrasonication to generate unilamellar vesicles, were found to have very high viscosities at very low shear stresses at DDAB weight fractions wD from 0.025 to 0.027. The vesicles had average diameters ranging from 68 to 80 nm, as previously determined from spectroturbidimetry. These vesicle dispersions stabilized suspensions of monodisperse spherical amorphous silica particles with diameters of dsed = 454 nm, 691 nm, and 826 nm against sedimentation, at least for several weeks. Similar results were obtained for suspensions, in DDAB vesicle dispersions, of polydisperse, nonspherical, crystalline titania particles with sizes ranging from ca. 96 nm to 156 nm. At the relatively low values of wD = 0.009 and 0.018, the effective viscosities, heff, of the DDAB dispersions, determined from the sedimentation velocities, ranged from 1.35 to 1.87 cP and from 4.34 to 5.57 cP, respectively. At wD = 0.027 for the silica particles with dsed = 454 nm, or at wD = 0.025 for all other particles considered, heff was essentially infinite, and each vesicle dispersion behaved as a gel at low shear stresses. At higher shear stresses, however, the dispersions were highly shear-thinning, and flowable in a capillary tube under gravity. This behavior is critical for the practical applications of such dispersions for paints and inkjet printing. To further understand the feasibility of the vesicle stabilization mechanism at various NaBr concentrations, wNaBr, the salinity effects on the stabilization of silica particles against sedimentation were also examined. It was found that at wNaBr < 0.0020 and at wD > 0.060, the DDAB dispersion could stabilize silica particles against sedimentation for at least two weeks. The relationship of the phase and dispersion behavior of DDAB/aqueous NaBr solutions to their stabilizing effectiveness will be further studied.
A first discovery of iridescent liquid-like aqueous vesicle dispersions formed from the DDAB is also reported. Although iridescence arises from some solid crystallites, thin films, and colloidal crystals, it had never been observed in systems that are liquid-like. Visual observations and ST at wavelengths of 350 nm to 700 nm were used to determine vesicle sizes and microstructure formation in dispersions for DDAB weight fractions wD between 0.020 to 0.030. The DDAB vesicle dispersions exhibited iridescent colors for wD = 0.023 to 0.027, due to the formation of “soft” crystallites formed by self-assembled vesicles. Effective vesicle radii from 30 to 60 nm were inferred from the ST measurements. The volume fractions of the vesicles fv and their effective volume fractions fv*, which account for the electrostatic double layers around a vesicle, were also estimated. The high values of fv* for the iridescent dispersions indicate that they contain neighboring vesicles with highly overlapping electrostatic double layers, even though their values of fv remain relatively low. Hence, strong electrostatic repulsive interactions arise between the vesicles. These interactions probably drive the formation of the “soft” crystallites, and thus the observed iridescence. Nevertheless, these “soft” crystallites, which could be easily broken up but were quick to reform, remain suspended. Consequently, these vesicle dispersions still flowed as a bulk dispersion with a high viscosity; the dispersion as a whole remained liquid-like or as a “liquid gem”, in contrast to what occurs to the other colloidal crystals made of rigid colloids. Beside their beautiful appearances, these DDAB vesicle dispersions also act as effective stabilizers of dense silica suspensions against sedimentation even at relatively low values of wD.
- Doctor of Philosophy
- Chemical Engineering
- West Lafayette