Monomer Exchange Kinetics, Dynamics of Concentration Fluctuations, and Chain Isomerization of Nonionic Surfactant/Water Systems. Evidence from Broadband Ultrasonic Spectra

Abstract

Ultrasonic absorption spectra, measured between 0.1 and 2000 MHz, are discussed for a variety of poly(ethylene glycol) monoalkyl ether/water (CiEj=H2O) mixtures. Depending on the temperature, the surfactant concentration, and on the length of the hydrophobic (Ci) as well as the hydrophilic part (Ej) of the surfactant molecules, the spectra reveal a multitude of shapes. The set of spectra, however, can be consistently described considering (i) a relaxation term representing the monomer exchange of the micellar solutions, (ii) another one that reflects the local fluctuations in the surfactant concentration, and, with several systems, (iii) additional terms due to CiEj associations or conformational isomerizations. The parameters of these terms are discussed in the light of relevant models. Evidence is presented for a more general view of a fluctuation controlled monomer exchange mechanism that combines aspects of both theoretical models, the micelle formation/decay kinetics and dynamics of local concentration fluctuations.

Monomer Exchange Kinetics, Radial Diffusion, and Hydrocarbon Chain Isomerization of Sodium Dodecylsulfate Micelles in Water

“…For an n-dodecyltrimethylammonium bromide solution, τ 3 ) 0.15 ns has been found at c ) 0.5 mol/L and 25 °C, 30 in nice agreement with the τ 3 values ( 0.1 ns e τ 3 e 2.1 ns, Table 3) of the SDS solutions. In addition, the relaxation times of the surfactant systems agree well with those for pure n-alkanes 17,45 at the same temperature (Table 4). These results confirm the assignment of the high-frequency relaxation term to the structural isomerization of the alkyl chains, with this respect resembling a droplet of liquid alkane.…”

“…This assignment is supported by two findings. First, the high-frequency relaxation term with relaxation time on the order of some tenths of a nanosecond has been found with anionic, 30 cationic, 30 zwitterionic, 43 and nonionic 17,43 surfactant systems and also with suspensions of phospholipid bilayer versicles. 44 Hence, the relaxation time does not depend significantly on either the nature of the hydrophilic head group of the surfactant or whether the amphiphiles are aggregated in micelles or in the fluid phase of bilayer structures.…”

“…Recent broadband ultrasonic measurements of ionic and nonionic micelle systems [14][15][16][17] with surfactant concentrations, c, near the cmc, feature characteristic differences to the predictions of the Teubner-Kahlweit model. The fast relaxation process displays a relaxation time distribution instead of a discrete relaxation time.…”