Publication costs assisted by Deutsche ForschungsgemeinschafiSmall-angle X-ray scattering experiments on aqueous solutions of the nonionic surfactant Triton X-100 at 20 and 30 "C in the concentration range 5-10 mg/mL were performed in order to determine size and degree of hydration. The partial specific volume determined independently by density measurements and by the contrast variation method was found to be 0.9125 mL 8-l at 20 "C, and a molecular weight of 95 700 dalton was assigned to this monodisperse solution of Triton X-100 (pH 8.0). A higher molecular weight of 150000 dalton was found at 30 "C at pH 8.0 in 0.01 M Tris-HCI; this particle had a radius of gyration of 39.2 A, a volume of 4.42 X lo5 A3, and a degree of hydration of 1.21 g of water per gram of Triton X-100. The radius of gyration of the 95700 dalton particle was found to be 29.3 A; the volume was 3.35 X lo5 A3 with a degree of hydration of 1.18 g of water per gram of Triton X-100. The largest dimensions of the Triton X-100 particle were determined from the correlation functions and were 105 A at 20 "C and 119 8, at 30 "C. For both particles the scattering curves are more consistent with an oblate ellipsoid of revolution rather than a prolate equivalent. From small-angle X-ray scattering experiments at various electron densities of the solvent the mean-square electron fluctuation was determined from which a high electron density and a low electron density region within the Triton X-100 particle can be proposed. If a two-level step function is assumed, the radius of gyration of the high electron density region is 18 A for the 95 700 dalton particle and 22.7 A for the 150000 dalton molecule, indicating that the hydrocarbon core of the 95 700 dalton particle has a diameter of 48.0 A, whereas the hydrocarbon core of the 150 000 dalton particle is asymmetric with half axes of a = 23.4 A and b = 46.8 A. The average curvature of the external surface area of the Triton X-100 micelle at 293 K is considerably smaller (28 A) than the average radius obtained from the radius of gyration, indicating that the external surface is convoluted. Laser correlation spectroscopy from aqueous solutions of Triton X-100 at 20 and 30 "C in the concentration range 5-10 mg/mL was carried out in addition to small-angle X-ray scattering experiments. The micellar size at 20 "C corresponds to an effective hydrodynamic radius of 41.8 A, and that at 30 "C of 55.7 A, with translational diffusion coefficients of (4.92 f 0.08) X (20 " C ) and (4.15 f 0.06) X cm2 s-l (30 "C). IntroductionTriton X-100 has been widely used as a nonionic surfactant for solubilizing membrane bound enzymes and for stabilizing this class of enzymes in solution. Triton X-100 is a polydisperse preparation of p-(1,1,3,3-tetramethylbutyl)phenoxypoly(oxyethylene glycol) containing an average size of 9.5 oxyethylene units per molecule. The molecular weight of Triton X-100 micelles in aqueous solution was first determined in 1954l by using light scattering techniques i3nd has since been measured several t i m e~....

This is the first time a comprehensive study has been carried out on n-alkyltrimethylammonium bromide salts using (14)N and (81)Br solid state NMR, X-ray diffraction, and theoretical calculations. The investigation represents a necessary step toward further (14)N and (81)Br NMR characterization of the environment of cationic and anionic groups in materials, accounting for the amphiphilic properties of cationic surfactants. The NMR spectra of five C(x)H(2x+1)(CH(3))(3)N(+)Br(-) polycrystalline samples with different n-alkyl chain lengths (x = 1, 12, 14, 16, 18) were recorded and modeled. The (14)N and (81)Br quadrupolar coupling interaction parameters (C(Q), eta(Q)) were also estimated from spectrum modeling and from computer simulation. The obtained results were discussed in depth making use of the experimental and reoptimized crystal structures. In the study, both (14)N and (81)Br nuclei were found to be sensitive probes for small structural variations. The parameters which influence the NMR properties the most are mobility, deviation of C-N-C bond angles from T(d) angles, and variations in r(N-Br) distances.