Aqueous two-phase systems consisting of polyethylene glycol (PEG), sodium polyacrylate (NaPAA), and a salt have been studied. The effects of the polymer size, salt type (NaCl, Na 2 SO 4 , sodium adipate and sodium azelate) and salt concentrations on the position of the binodal curve were investigated. The investigated PEG molecules had a molar mass of 2,000 to 8,000 g/mol, while that of NaPAA was 8,000 g/mol. Experimental phase diagrams, and tie lines and calculated phase diagrams, based on Flory-Huggins theory of polymer solutions are presented. Due to strong enthalpic and entropic balancing forces, the hydrophobicity of the added salt has a strong influence on the position of the binodal, which could be reproduced by model calculations.

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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.

Dynamic light scattering has been used to investigate sonicated aqueous dispersions of dioctadecyldimethylammonium bromide (DODAB). The hydrodynamic radius (RH) of the scattering particles and the mean scattering intensity (I) have been monitored as functions of the DODAB concentration and temperature (T). In the dilute regime, the relaxation time distribution of the sonicated dispersion of DODAB is bimodal with the slow mode dominating the distribution. The slow and fast modes are respectively characteristic of vesicles and bilayer fragments with R H values of 22 and 8.5 nm (25 °C) and 20 and 6 nm (50 °C), respectively. The total scattered intensity initially decreased with temperature up to 45 °C (Tc), above which it was constant; identical behavior was observed for the slow mode intensity, but the fast mode intensity was constant with temperature change, showing that Tc is a property of the vesicles and not of the bilayer fragments. At Tc the slow vesicle mode becomes narrower whereas the fast fragment mode shows no change. On aging, the dispersion showed a slow transition from bimodal to a rather broad single-modal relaxation time distribution. The corresponding RH was 33.8 nm when measured 10 months after preparation. These results suggest that aqueous sonicated dispersions of DODAB are metastable.

Dynamic light scattering measurements have been made to elucidate changes in the coil conformation of a high molecular weight poly(ethylene oxide) (PEO) fraction when the non-ionic surfactant C12E5 is present in dilute solutions. The measurements were made at 20 °C as functions of (a) the C12E5 concentration at constant PEO concentration, (b) the PEO concentration at constant C12E5 concentration, and (c) the C12E5/PEO concentration ratio. The influence of temperature on the interactions in terms of the relaxation time distributions was also examined up to the cloud point. It was found that when the C12E5/PEO weight ratio was >2 and when the temperature was >14 °C, the correlation functions became bimodal with well-separated components. The fast mode derives from individual surfactant micelles which are present in the solution at high number density. The appearance of the slow mode, which dominates the scattering, is interpreted as resulting from the formation of micellar clusters due to an excluded-volume effect when the high molar mass (M = 6 × 105) PEO is added to the surfactant solution. It is shown that the micellar clusters form within the PEO coils and lead to a progressive swelling of the latter for steric reasons. The dimensions of the PEO/C12E5 complex increase with increasing surfactant concentration to a value of R H ≈ 94 nm (R g ≈ 208 nm) at C C 12 E 5 = 3.5%. Fluorescence quenching measurements show that the average aggregation number of C12E5 increases significantly on addition of the high molar mass PEO. With increasing temperature toward the cloud point the clusters increase in number density and/or become larger. The cloud point is substantially lower than that for C12E5 in water solution and is strongly dependent on the PEO concentration.

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The thermotropic phase behavior of cationic liposomes in mixtures of two of the most investigated liposome-forming double-chain lipids, dioctadecyldimethylammonium bromide (DODAB) and didodecyldimethylammonium bromide (DDAB), was investigated by differential scanning calorimetry (DSC), turbidity, and Nile Red fluorescence. The dispersions were investigated at 1.0 mM total surfactant concentration and varying DODAB and DDAB concentrations. The gel to liquid-crystalline phase transition temperatures (Tm) of neat DDAB and DODAB in aqueous dispersions are around 16 and 43 degrees C, respectively, and we aim to investigate the Tm behavior for mixtures of these cationic lipids. Overall, DDAB reduces the Tm of DODAB, the transition temperature depending on the DDAB content, but the Tm of DDAB is roughly independent of the DODAB concentration. Both DSC and fluorescence measurements show that, within the mixture, at room temperature (ca. 22 degrees C), the DDAB-rich liposomes are in the liquid-crystalline state, whereas the DODAB-rich liposomes are in the gel state. DSC results point to a higher affinity of DDAB for DODAB liposomes than the reverse, resulting in two populations of mixed DDAB/DODAB liposomes with distinctive phase behavior. Fluorescence measurements also show that the presence of a small amount of DODAB in DDAB-rich liposomes causes a pronounced effect in Nile Red emission, due to the increase in liposome size, as inferred from turbidity results.