Steady-state and dynamic fluorescence measurements were used to investigate the interactions and structures of complexes formed between bovine serum albumin (BSA) and anionic, cationic, and nonionic surfactants:  sodium dodecyl sulfate (SDS), N-cetyl-N,N,N-trimethylammonium bromide (CTAB), and octaoxyethylene glycol n-dodecyl ether (C12E8), respectively. The lysozyme (Lys)−SDS complex was also studied. The measurements were carried out at different pH's and ionic strengths. In all systems, micelle-like clusters adsorbed on protein were evidenced. The average aggregate numbers are smaller than those of free micelles and are not strongly influenced by pH and ionic strength. The fluorescence lifetime of pyrene in BSA/surfactant complexes was constant at low surfactant concentrations, started to decrease at approximately the same protein−surfactant ratio (0.15 mM BSA/1 mM surfactant) regardless of the surfactant type or pH buffer, and, at high surfactant concentrations, merged to the lifetime values corresponding to free micelles. The results of the fluorescence techniques support a “necklace and bead” model of the complex for BSA/surfactant systems, with protein wrapped around the micelles. For the Lys/SDS complex, the model essentially is the same; however, some differences, due to their different sizes, appear. Lysozyme is smaller and more rigid and does not wrap up well around the micelle.

The interaction between bovine serum albumin (BSA) and several surfactants has been investigated by light scattering. Anionic (sodium dodecyl sulfate, SDS), cationic (dodecyl trimethylammonium bromide, DTAB), and nonionic (polyoxyethylene 8 lauryl ether, C12E8) surfactants, all containing a C12 alkyl chain, were used to study the effect of different headgroups on the complex formation. The hydrodynamic radii of the complexes obtained by dynamic light scattering indicate that cooperative binding of DTAB occurs at higher surfactant concentrations than in comparative solutions of SDS and C12E8. The effect of chain length is shown for the cationic surfactants DTAB and cetyl trimethylammonium bromide (CTAB, C16 alkyl chain). The higher surface activity of CTAB results in complex formation at a lower surfactant concentration compared to DTAB. The hydrodynamic radii of the BSA−SDS and BSA−DTAB complexes at saturation were determined as ∼5.9 nm and ∼4.8 nm, respectively. The hydrodynamic radius of the reduced BSA−SDS complex is somewhat smaller than the corresponding native BSA−SDS complex. Static light scattering (SLS) measurements were performed on BSA−SDS systems to determine the number of BSA molecules in the complex. Prior to SLS measurements the BSA−SDS solutions were dialyzed against a large volume of SDS solution in order to determine the refractive index increment ∂n/∂c BSA at constant chemical potential. It was observed that a very long dialysis time (several weeks) was needed to reach equilibrium. Measurements on solutions that had not reached equilibrium resulted in improbably high values of the number of BSA molecules in the complex.

The interaction between the nonionic surfactant C12E5 and a high molar mass (M ) 5.94 × 10 5 ) poly(ethylene oxide) (PEO) in aqueous solution has been examined as a function of temperature by dynamic light scattering and fluorescence methods over a broad concentration range. Clusters of small surfactant micelles form within the PEO coil, leading to its extension. The hydrodynamic radius of the complex increases strongly with temperature as well as with the concentrations of surfactant and polymer. At high concentrations of the surfactant, the coil/micellar cluster complex coexists with free C12E5 micelles in the solution. Fluorescence quenching measurements show a moderate micellar growth from 155 to 203 monomers in PEO-free solutions of C12E5 over a wide concentration range (0.02-2.5%) at 8°C. Below 0.25% C12E5, the average aggregation number (N) of the micelles is smaller in the presence of PEO than in its absence. However, N increases with increasing surfactant concentration up to a plateau value of about 270 at about 1.2% (ca. 30 mM) C12E5. At high surfactant concentrations, N is larger in the presence of polymer than in its absence, a finding which is connected to a significant lowering of the clouding temperature due to the PEO at these compositions. Similar results of increasing aggregation number followed by a plateau were also found at a fixed concentration of surfactant (2.5%) and varied PEO.

The application of the time-resolved fluorescence quenching method (TRFQ) to study the structure and aggregation behavior of nonionic surfactant polyoxyethylene(4)lauryl ether (C12E4) in reverse micellar systems is presented. Solutions of different concentrations in three nonpolar solvents (cyclohexane, n-decane, and n-dodecane), at different water content and temperature, employing Ru(bpy )a2+-methylviologen, solubilized in the polar core of micelles, as probe-quencher pair, were studied, tiie aggregation number and fluorescence quenching rates were determined and the size and shape of micelles estimated; the dependence of their values on medium was evidenced. Dynamic light scattering was utilized as a complementary method in checking the hypotheses regarding micellar size/shape. It was found that in cyclohexane, at the compositions studied, the micelles can be regarded as spherical, growing with the water content, with a constant area per head group of about 50 Á2. Only at the lowest water content (3 water molecules per surfactant or less) is there a deviation with a larger area per surfactant. The radius of the polar core is then smaller than the length of the EO tail, and there is no free water that could allow the micelle to swell beyond this limit. The micelles in decane and dodecane appear to grow in a nonspherical way and become much larger with water addition.

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