The effects of free polymers on osmotic compression, depletion flocculation and fusion of lamellar liquid-crystalline droplets

Aqueous Phase Behavior of a NaLAS-Polycarboxylate Polymer System

Linear alkylbenzene sulfonate (NaLAS) surfactant is often combined with polycarboxylate polymers in detergent formulations. However, the behavior of these aqueous surfactant-polymer systems in the absence of an added electrolyte is unreported. This work investigates the behavior of such systems using polarized light microscopy, small-angle X-ray scattering (SAXS), centrifugation, and ²H NMR techniques. A phase diagram at 50 °C is reported for 0-50 wt % NaLAS concentrations and 0-10 wt % polycarboxylate concentrations. The NaLAS-water system is micellar at concentrations <35 wt %, and a 2-phase micellar-lamellar system is seen at higher NaLAS levels, consistent with that reported by previous studies. As polymers are added at low surfactant concentrations (∼10 to 20 wt % NaLAS), a second optically isotropic phase is formed; this is thought to be a polymer-rich phase. Further addition of polycarboxylate leads to the formation of a lamellar phase. At high surfactant concentrations (>20 wt % NaLAS), the addition of a polymer induces a second lamellar phase. These observed behaviors are thought to arise as a result of depletion flocculation and salting-out effects. The observed lamellar phases adopt colloidal multilamellar vesicle (MLV) structures, and the average MLV radii were estimated using ²H NMR by probing the diffusion and anisotropy of D₂O within the bilayers of the vesicles. The NMR results show that as the polymer concentration was increased from 0 to 10 wt %, an increase in the average multilamellar vesicle size from ∼200 to ∼500 nm was observed. This increase in the calculated average MLV radius likely results from depletion flocculation-induced MLV fusion.

Interaction of surfactants with thickeners used in waterborne paints: A rheological study

Studies of the phase diagram and linear viscoelasticity of aqueous solutions of hydrophobically modified hydroxyethyl cellulose (HMHEC), a thickener used in water-based paints, and SDS reveal that SDS-HMHEC mixed micelles are formed that increase the number of hydrophobic junctions and enhance interpolymer association up to an [SDS]/[HMHEC] ratio. This fact produces a strong increase of viscoelasticity or a phase separation, depending on the [HMHEC]. At higher ratios the excess of micelles with predominant SDS isolates hydrophobes and disrupts the micellar network. Then, viscoelastic functions decrease and HMHEC behaves as a nonassociative polymer. TTAB and Brij30 also interact with HMHEC, but in a different way. No phase separation is observed with these surfactants. TTAB forms mixed micelles and new junction points in the same way as SDS. However, this surfactant does not stabilize the micelles as SDS does, presumably due to the different interaction between the OH from the cellulose and the charged groups. Results seem to indicate that Brij30 enters into the hydrophobic aggregates of HMHEC and stabilizes them, increasing relaxation time, but it does not form new junction points, since it forms quite big micelles.

Interactions between Quaternary Ammonium Surfactant Oligomers and Water-Soluble Modified Guars

The interaction between hydroxypropylguar (HPG) and its dodecyl-modified derivative (HMHPG) and cationic surfactant oligomers has been investigated by measurements of the solution viscosity at constant shear rate, microviscosity of the aggregates (dipyrenylpropane fluorescence emission spectra), and aggregation number of the polymer hydrophobe and of the surfactant (time-resolved fluorescence quenching). The surfactants are dodecyltrimethylammonium bromide (DTAB, monomeric surfactant) and some of its dimers and trimers which differed by the carbon number s of the polymethylene spacer connecting the surfactant moieties (2 </= s </= 20). Most results refer to a polymer concentration of 1 wt%. Only a weak interaction was evidenced between HPG and these surfactants, whereas strong interactions occurred between HMHPG and the surfactant oligomers. The interaction became stronger as the degree of oligomerization of the surfactant increased. The results led us to distinguish three ranges of concentration of added surfactant. The first range corresponds to surfactant concentrations below the surfactant cmc in water. In this range mixed aggregation occurs between polymer hydrophobes and surfactant ions, and the viscosity of the HMHPG + surfactant systems goes through a maximum, as usually found for associating polymers. The second range extends from the cmc to about 10 cmc. Precipitation of a polymer/surfactant complex occurs in this range with all surfactants forming threadlike micelles. For the other surfactants the viscosity goes through a minimum. In the third range, which corresponds to surfactant concentrations above 10 cmc, resolubilization of the precipitated HMHPG/surfactant complexes occurs and a solution-to-gel transition is observed for the surfactants which form threadlike micelles or vesicles. The concentration corresponding to this transition is about the same as that for pure surfactant solutions. Some polymer hydrophobes may contribute to the formation of additional bridges between surfactant micelles. Copyright 1999 Academic Press.