(298f) Effects of the Method of Preparation and Dispersion Media on the Optical Properties and Particle Sizes of Aqueous Dispersions of Didodecyldimethylammonium Bromide (DDAB)

The general equations for the specific light scattering intensities and specific turbidities, or turbiditiesper unit pathlength and divided by the weight fraction w, of dilute aqueous dispersions of unilamellar vesicles and multilamellar liposomes of double-chain surfactants have been derived for the Rayleigh (R) and Rayleigh-Debye-Gans (RDG) light scattering regimes (Hsieh, Corti, and Franses, J. Colloid Interf. Sci., 2020, 578, 471-483). A new turbidity equation was also derived, to account for absorbance errors due to some low-angle scattered light reaching the detector. These equations are valid only for single and independent scattering, for which the specific intensities and turbidities are independent of w, and which is reached at sufficiently low values of w. For vesicles and liposomes, the intensities and turbidities depend on the bilayer thickness, the vesicle or liposome radius, and the refractive indices of the bilayer and the dispersion media. For liposomes, they also depend on the water layer thickness between the various bilayers. For R scattering, the intensities and turbidities are proportional to the square of the vesicle radius and to the cube of the liposome radius. The RDG size dependence is weaker for both vesicles and liposomes because of substantial intraparticle interference. For wavelengths from 350 to 700 nm, the R regime is valid for vesicle radii smaller than 20 nm. For surfactant vesicles or liposomes, the RDG regime is valid for larger radii, up to 800 nm for liposomes and over 1000 nm for vesicles.

The equations were used for estimating vesicle sizes of DDAB, for which the vesicle thickness is about 2.4 nm. Vesicular dispersions of DDAB at weight fractions of 0.02 to 0.03 have been found to prevent suspended particles from sedimentation because they can increase the low-shear-stress viscosity of the dispersion (Yang, Corti, and Franses, Langmuir, 2015, 31, 8802-8808 and Yang, Franses, and Corti, J. Phys. Chem., 2019, 123, 922-935). These dispersions were produced either with vigorous stirring (S method), which yields both vesicles and liposomes, or with stirring followed by sonication (SS method) which produces only vesicles. It is important to determine how to control the vesicles sizes, which affect the resulting volume fractions of the vesicles and the inter-vesicle interactions.

Since the formation of vesicles is sometimes considered “spontaneous,” their size is assumed to be determined from thermodynamic and curvature considerations, the effect of the method of vesicle preparation on the vesicle sizes was examined by using the SS method and another commonly used method, the extrusion/ultrafiltration method (SE method). Moreover, the vesicles were produced either in water or in aqueous solutions of 10 mM NaBr. The specific turbidity for each of the four types of vesicular dispersions increased with a decrease in w and reached a constant value at a certain value of w, w^*, which was specific for each dispersion type. The lower turbidities at the higher mass fractions imply that the scattering is multiple, or dependent, or both, and cannot be used to determine the vesicle sizes. The specific turbidities at the lower weight fractions were used with the RDG equations. The average vesicle radii in water were estimated to be about 24 nm for the SS method and 74 nm for the SE method. The vesicle radii in the salt solution were larger than in water and, again, different for the two methods, about 65 nm and 280 nm, respectively. It is concluded that the DDAB vesicle formation is not spontaneous, and appears to depend on the hydrodynamic conditions during the breakdown of the liposomes to form vesicles. The vesicle formation also depends on the ionic strength of the aqueous medium. From the vesicle sizes the actual volume fractions of the vesicles were determined to be much larger than w. The average distances between the vesicles were also calculated, to estimate the extent of the electrostatic interactions among the vesicles. Such interactions may lead to dependent scattering. It was estimated that the changes of the specific turbidity on w, for w less than 0.01, were primarily due to multiple scattering in the case of the salt solution. For water, they were due to dependent and some multiple scattering.