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

An-Hsuan Hsieh, Purdue University

Abstract

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 R** and the specific turbidity  ʳ** were derived analytically for both scattering regimes. Spectroturbidimetry (ST) data at 25 °C for DDAB were compared to the R** predictions.  ʳ** data for DDAB vesicles are consistent with the RDG predictions, which are also used to estimate the vesicle sizes.5For 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 ʳ** 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 ʳ** are indicative of single and independent scattering, and were used to estimate vesicle radii by solving the  ** 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.

Degree

Ph.D.

Advisors

Franses, Purdue University.

Subject Area

Optics|Sedimentary Geology

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