Interactions between surfactants and uncharged polymers in aqueous solution studied by microcalorimetry

TLDR: In this article, a titration microcalorimetric measurement of poly(n-vinylpyrrolidone) PEO with nominal molar masses of 4 000,8 000 (two different samples ), 20 000, 100 000, and 1 500 000 were used without further treatment.

Abstract: The interaction between ionic surfactants and uncharged polymers in dilute aqueous solution has been studied by titration microcalorimetry. The resulting enthalpic titration curves give detailed information about polymer - surfactant interactions. While sodium dodecylsulfate interacts strongly with poly(ethy1ene oxide) and poly(viny1-pyrrolidone), the micelle formation of the cationic surfactants dodecyl-, tetradecyl-, and hexadecyl-trimethylammonium bromide is not measurably perturbed by the presence of these polymers. However, there is no significant difference between the interaction of the anionic and cationic surfactants with ethyl(hydroxyethy1)cellulose ethers. The aggregation of surfactants in polymer solutions shows clear similarities to the solubilization of small, polar molecules in ionic micelles. Preaggregation between surfactant monomers and polymers can be significant as indicated by sizeable endothermic enthalpy contributions from hydrophobic pair-wise interaction. Systems containing surfactants and water-soluble polymers are of great interest both for their widespread applications and for their inherently interesting properties. Polymer - surfactant complexes are used for instance in paints and coatings, in laundry detergents, cosmetic products, and in tertiary oil recovery. Systems containig nonionic polymers and ionic surfactants have been studied quite extensively during the last couple of decades so much is known about them. The various studies are summarized and discussed in several review articles ( ref 1-5 ). However, fundamental questions such as why and how surfactants form aggregates in the presence of polymers and how the polymer is involved in the aggregate formation are still unanswered.We became interested to see if results of calorimetric measurements could help to answer these questions and therefore, we started a study of some typical polymer - surfactant systems by titration microcalorimetry. EXPERIMENTAL Materials. Sodium dodecylsulfate SDS (BDH, 99%), dodecyltrimethylammonium bromide DoTAB (Aldrich, 99%), tetradecyltrimethylammonium bromide TTAB ( Sigma,99% ), and hexadecyltrimethylammonium bromide CTAB (Sigma, 99%) were used as received.Samples of poly(ethy1ene oxide) PEO with nominal molar masses of 4 000,8 000 ( two different samples ), 20 000, 100 000, and 1 500 000 were used without further treatment. Poly(N-vinylpyrrolidone) PVP with an average molar mass of 40 OOO ( Aldrich, special grade) and poly(propy1ene oxide) PPO with molar mass of 1 000 (Aldrich) were used as received. Samples of ethyl (hydroxy-ethy1)cellulose EHEC E230G and EHEC CST 103 ( Berol Nobel AB, Sweden) were dialyzed before use. The calorimetric measurements were made using the commercial microcalorimetric measuring channel of the 2277 TAM Thermal Activity Monitor system ( ThermoMetric AB, JMiilla, Sweden ) in a homebuilt high-precision thermostat bath (ref 6). The calorimetric titration experiments consisted of series of consecutive additions of concentrated surfactant solution to the calorimeter vessel initially containing 2.7 g of polymer solution. In some of the experiments concentrated polymer solution was added to surfactant solution in the vessel.The liquid samples were added in portions of 7 - 15 mg from a gas-tight Hamilton syringe through a thin stainless-steel capillary tube. A microprocessor-controlled motor-driven syringe drive was used for the injections. The fast titration procedure was used with six minutes between each injection (ref 7). The dynamic correction method used to deconvolute the potential signals was based on the simple Tian equation.