The interactions between the polymer and SDS naturally
The aggregation of sodium dodecyl sulfate (SDS) in aqueous depend on their mutual concentration. At constant polymer solution containing a constant amount of poly(ethylene oxide) concentrations three regions appear when SDS is added. At (PEO) at 2000 ppm has been investigated by several experimental low concentrations the SDS molecules remain as dissolved techniques at three temperatures, 298.1, 288.1, and 283.1 K. The monomers. In a medium range the surfactant interact with techniques include conductivity, viscosity, NMR self-diffusion, the polymer and form aggregates on the polymer strand. The NMR chemical shift, and NMR relaxation. The critical aggregasurfactant binds to the polymer at a critical concentration, tion concentration of SDS on the polymer strand (cac) as well as normally termed cac. At the cac the surfactant forms micelle-
the concentration where ordinary micelles start forming (c 2 ) has been determined. There are some inconsistencies in the data due to like aggregates or clusters on the polymer (1, 6, 8). The cac different measuring techniques. However, techniques that basically appears to be well defined and independent of the polymer monitor the SDS molecules, for instance conductivity and SDSmolar mass (4, 5), as long as it exceeds 5000. At cac the SDS sensitive NMR techniques, provide cac values that are consistent. micellar aggregation number is lower than the corresponding Above the cac the concentration of free SDS monomers will inmicellar aggregation number of ordinary micelles in a solucrease in parallel to the aggregation process. At c 2 ordinary mition without polymer (10, 17).
celles will form, and in the concentration range of about 15-25
At sufficiently high surfactant concentrations ordinary mimmolal the SDS molecules will aggregate as normal micelles or celles start forming in the solution (4-6, 8). At this point in aggregates on the polymer, and these processes run in parallel.
the SDS molecules can either be free monomers in water,
A crude calculation of the Gibbs' energy of the two aggregation bound to the polymer as surfactant aggregates, or reside in processes suggests that the energetics of the two processes are of similar magnitude, and thus consistent with observation. ᭧ 1998 normal micelles. The surfactant concentration where ordi-
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nary micelles are formed in the solutions is normally termed Key Words: sodium dodecyl sulfate; poly(ethylene oxide); critic 2 and appears to be less distinct compared to the cac and cal aggregation concentration; saturation of the polymer; micellithe cmc. When the surfactant concentration has reached c 2 zation; viscosity; NMR; self-diffusion; chemical shift; relaxation; it has been anticipated that the polymer has been saturated conductivity.
with surfactant molecules (8). The most frequently used methods to measure c 2 are surface tension (4, 5, 8) and conductivity 5,. Both methods register 1 To whom correspondence should be addressed.
penetrates the aggregated micelles (28).
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