Characterization of Vesicles by Classical Light Scattering JOHN H. VAN ZANTEN AND HAROLD G. MONBOUQUETTE ~ Chemical Engineering Department, University of California, Los Angeles, Los Angeles, California 90024-1592 Received March 12, 1991; accepted March 20, 1991 Classical, or static, light scattering techniques can be used to determine the geometric size, shape, apparent molecularweight,and radius of gyration of phosphatidyleholinevesicles in aqueous suspension. A Rayleigh-Gans-Debye (RGD) analysisof multiangle scattered light intensity data yieldsthe size and degree of polydispersityof the vesiclesin solution, while the Zimm plot technique provides the radius of gyration and apparent weight~average molecular weight. Together RGD theory and Zimm plots can be used to confirm the geometricshape of vesicles. Vesiclesvaryingfrom 65 to 90 nm in diameter have been characterized effectively. The static light scatteringmeasurementsindicate that, as expected, phos- phatidylcholine vesiclesin this size range scatter light as isotropic hollow spheres. This dual treatment of the static light scattering data also provides an internal, self-consistentcheck of the vesiclesize deter- mination. The values for geometric radii determined by classical light scattering typically agree with those estimated by dynamic light scatteringto within a few percent. © 1991 Academic Press, Inc. INTRODUCTION Vesicles are closed membrane capsules consisting of single or multiple bilayers of non- covalently assembled surfactants. When com- posed of natural phospholipids, vesicles are called liposomes. Liposomes have been used extensively as models for the study of biolog- ical membrane structure and function ( 1 ). In addition, both natural and synthetic surfactant vesicles have been investigated for use in drug delivery and targeting (2), medical imaging (3), catalysis (4), energy conversion (4), and separations (5). Publications describing the study and applications of vesicles have mush- roomed in number to the tens of thousands as of 1986 (4). Thus, there is a strong driving force for the development of noninvasive, ac- curate, and economical methods for the char- acterization of these species. For the analysis of much experimental data involving tailored vesicle systems, a minimum of the size, shape, and degree of polydispersity is needed to cal- culate the total surface area and encapsulated volume of the vesicular dispersion. To whom correspondence should be addressed. Vesicle suspensions can be size character- ized by a variety of techniques including elec- tron microscopy, analytical ultracentrifuga- tion, sedimentation flow field fractionation, NMR spectroscopy, gel chromatography, and light scattering (7). Electron microscopy offers the advantage of visualization and therefore is of greatest value when it is suspected that the suspension consists of vesicles of unusual shape and of widely varying size. This tech- nique, however, requires vesicles to be ana- lyzed outside of their true aqueous environ- ment and sample preparation protocols may lead to artifacts (7). Other characterization methods, including those based on light scat- tering, are best applied when the dispersion is fairly homogeneous and consists of vesicles which have a well-defined shape. Dynamic, or quasielastic, light scattering (6) has emerged as the standard technique for an- alyzing a vesicle suspension for mean geo- metric size and for the size distribution. Dy- namic light scattering equipment can be rel- atively inexpensive, and since vesicles are analyzed in aqueous solution using a low en- ergy laser, they are practically undisturbed by 330 0021-9797/91 $3.00 Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved. Journal of Colloid and Interface Science, Vol. 146, No. 2, October 15, 1991