High Frequency Ultrasonic Absorption Spectroscopy on Aqueous Solutions of Zwitterionic and Nonionic Micelles

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

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

Molecular relaxation properties of sodium dodecylsulfate in aqueous solutions with surfactant concentrations between 0.009 and 0.4 mol/L have been studied using broadband ultrasonic spectrometry in the frequency range 0.1-2000 MHz. Ultrasonic excess attenuation characteristics were found that could be well represented by a sum of three relaxation terms, each one characterized by a discrete relaxation time. The low-frequency term with concentration-dependent relaxation time, tau1, between 0.06 and 3.5 micros is discussed in terms of the surfactant monomer exchange. The noticeable effect from the incomplete dissociation of the surfactant counter ions and the variation of the monomer concentration following thereby is discussed. The second relaxation term (0.9 <or= tau2 <or= 2.5 ns) is assigned to the limited radial diffusion of monomers within the micelles, yielding a mean diffusion length of 0.5 nm, in correspondence to protrusions by four methyl groups. The high-frequency relaxation term (0.1 <or= tau3 <or= 0.2 ns) reflects the structural isomerizations of the hydrocarbon chains in the micellar cores, largely resembling those of liquid alkanes.

Kinetics of adsorption from micellar solutions

Previous studies on surfactant adsorption mostly deal with dilute systems without aggregation in the bulk phase. At the same time, micellar solutions can be more important from the point of view of applications. If one attempts to estimate the equilibrium adsorption, neglecting the influence of micelles can lead to reasonable results. The situation differs for non-equilibrium systems when the adsorption rate can increase by an order of magnitude at the increase of the surfactant concentration beyond the CMC. A critical survey of various models describing the influence of micelles on adsorption kinetics at the liquid-gas interface is given and the theoretical results are compared with existing experimental data. The theories proposed for the case of large deviations from the equilibrium are usually based on some unjustifiable assumptions and can describe the kinetic dependencies of adsorption in only a limited number of situations. Consequently, only rough estimates of the kinetic coefficients of micellization can be obtained from experimental data on dynamic surface tension. More rigorous equations can be derived if the system only deviates slightly from equilibrium. In the latter case, the agreement between theoretical and experimental results is essentially better and measurements of the dynamic surface elasticity of micellar solutions allow us to study the micellization kinetics.

Density fluctuations and rotational isomerization in phospholipid bilayers as studied by ultrasonic absorption spectroscopy

Broadband ultrasonic absorption spectra are discussed for some aqueous solutions of single-walled phospholipid bilayer vesicles. It is shown that the excess absorption found with all samples can be represented by a sum of a Debye-type relaxation term with discrete relaxation time and a Fixman-Kawasaki term. The former term reflects the kinetics of structural isomerization of the hydrocarbon chains. The values of its relaxation time (0.09-0.56 ns) agree with those for pure n-alkanes of comparable length. The latter terms seems to be due to density fluctuations in the hydrocarbon part of the double layer. Fluctuation correlation lengths between 1 and 30 A result from the analysis of the ultrasonic spectra.

High-frequency ultrasonic absorption spectroscopy on aqueous suspensions of phospholipid bilayer vesicles

Between 1 MHz and 3 GHz the ultrasonic absorption coefficient has been precisely measured as a function of frequency for some aqueous suspensions of single-walled phospholipid bilayer vesicles. All solutions of the specially purified phospholipids clearly show excess absorption, reflecting three molecular relaxation processes with discrete relaxation times. Typical values for these times are 50, 3 and 0.5 ns. The attempt is made to relate these relaxation processes to mechanisms of rotational isomerization in the hydrocarbon chains. Some other molecular mechanisms which could also contribute to the ultrasonic excess absorption spectra are also briefly discussed.