Chemist1 u Division, Oak Hidoe National Lahoratoru. Oak R i d g e , l'ennessce. and D e p a r t m n l o/ Chemislrjl. Montana State C'olleg6, 13oteman, Montana (Received Jzdu 106Y) hIiccllar weights of' sodium lauryl sulfate in 0.1 arid 0.4 AI SaCl measured by cqiiilibrium iilt,racciit,rifugation and in 0.4 AT NaC1 by light, scattcriiig are in good agrccment with each other and with values in the literaturc. The micc.llcs are predominantly monodisperse in 0.4 Ji' NaCl, but appcar to bc polydisperse in 0.5 , I ! SaCI. Degrees of aggregation appear to be less in 0.4 M LiCI than in 0.4 ; I ! SaCI. .4 method of estimation of critical micelle concentration by ultracentrifugation is illustrated. Although t,hc partial spccific volume of dodecylt~rimet,hylammonium bromidc (I)T.213) is too near unity for good estimates of molecular 13 eight by equilibrium ultracentrifugation, the tochriiquc dons indicate a monodispcrse distribution in 0.1 M NaHr and polydispcrsit,y in 2.0 AT SaRr. 1,ight scattering values of TITAR dcgrcc of aggregation are presented and compared with literature values. SOF e anomalous observations in the sodimcntatioii of soaps at low centrifugal fields arc prcsont!cd.A number of different, cxpcrimcntd mcthods havc been used in the past to secure information coriceriiiiig the dogrw of aggregation of surfactants in solution. In t,hc case of sodium lauryl Fiulfatt, NaTIS, for cxnmplc, these include light scattering, -I(' self-difrusiori, I sedimentation vclocity,*2 sodirncntatjion velocity combincd wit,h diffiision, I 2 -l 5 centrifugation a t solut,ion boundaries'? (Archibald method), conductivity, 16,17 osmotic prcssiire,18 and X-ray diffraction.1° Sincc different procedures havc led to substantial variations in estimates of aggregation in some cases, it seemed worthwhilo to athcmpt, a study of such systems by equilibrium ultracentrifugation, a method hitherto not applied, to our knowledge.Alt,hough thcrc arc disadvantages to cquilihrium ultracentrifugation, they are in many instances, at least, different from those of other methods. Dust, for example, is a trivial interference hcrc, in contrast to light scattering; the presence of otfher impurities would also be expected to be less important, In X-ray scattering, l9 solutions considerably more concentrated than those in question are involved. Xo assumptions concerning particle shape, which are required in the interpretation of self-diffusion and conductivity, need be made hcrc. Sedimentation velocity requires auxiliary deterniination of diffusiori oocfficicritjs, the mcasurcmcrit, of which is cnmplic,zt,ed by the morinIn(>r-micellar equilit)ria. l 2 Iri the Archibald ~iiotliod, i i i (1) This doc:urrierit is based in part on work performed for the 11. S.Atorriir~ Erirrgy Corrinrissiorl at thc Ouk ltidge Sational 1,ahoratory, operxted by lTnioir C d ) i d e Corporation. Work prcscutcd at, the 141st S s t i o n x l Meetii~g of the Arricricurl Chemical Society. ~'asliirigton. n. ('., Mardl 20-29, 1982.
