The Effect of Poly(oxyethylene) Alkyl Ethers, Alkanediols, and Alkanols on the Critical Micelle Concentration of Sodium Dodecyl Sulfate

The effect of the addition of the homologues of poly(oxyethylene) alkyl ethers (CmH2m+1(OCH2CH2)nOH: m=3–6, n=1–4), α,ω-alkanediols (m=4–6,8), and 1-alkanols (m=2–7) on the critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) was examined. The homologous additives are appropriate for studying the effect of polar groups as well as an alkyl group. The CMC decreases with an increase in the concentration of additive. The rate of CMC-decrease estimated at the infinite dilution of the additive increases with the length of alkyl chain in each homologue. The dependence of the rate on the alkyl chain length is the same for all the homologues with longer chain (m≥4 for ethers and alkanols, and m>6 for alkanediols). This is ascribed to the complete transfer of the long alkyl chain into the interior of micelles. The dependence of rate on the number of polar groups (OH, OCH2CH2) is as follows. The rate for alkanediol is lower than that for alkanol with a given alkyl chain. The rate for ethers increases asymptotically with the length of oxyethylene chain. The result for alkanediol is ascribed to the pronounced hydrophilic nature, and that for ethers to the reduction of electrostatic repulsive force between ionic head groups of the surfactant in micelles.