Effect of Pentanol and Salt on the Fusion−Scission Kinetics for CTAB Micelles

Abstract

The kinetic properties of aqueous solutions of CTAB with pentanol and KBr are studied with a T-jump device. The transition range from spherical to cylindrical micelles is determined with a microcalorimeter, and the semidilute range with an Oswald viscometer. The kinetic results show that, for elongated micelles, the value of the relaxation time decreases when the salt or pentanol concentration increases. This variation indicates that the decrease of the potential barrier is larger than the increase of the endcap energy. The observed variation of the potential barrier with salt concentration obeys a scale law, as expected by the DLVO theory. Its variation with pentanol obeys to a linear law. At the transition from spherical to cylindrical micelles the variation of the relaxation time is essentially controlled by the growth in the width of the length distribution.It is well-known 1 that surfactant molecules self-assemble into micelles if the concentration is higher than the critical micelle concentration (cmc). The micelles can undergo uniaxial growth upon an increase of surfactant concentration 2 and upon addition of salt 3,4 or alcohol. 5,6 It is also known that aggregates continuously exchange surfactant molecules with the bulk solution, and that the micelles break and merge. These kinetic processes have been widely investigated theoretically. The theoretical treatment of the exchange of surfactant molecules between micelles and solvent has been worked out by Aniansson et al. 7,8 Experimentally, these kinetics are generally studied by the ultrasonic relaxation absorption method. 9 Kahlweit has performed a theoretical study of the fusion-scission process 10,11 for spherical micelles, Cates 12 has theoretically studied wormlike micelles, and the relaxation time associated with the fusion-scission kinetics has been determined for some micellar shapes. 13 Experimentally, these kinetics are investigated by the T-jump technique with light absorption detection in the case of the fusion-scission process of quasi-spherical micelles. Use of one absorbency probe is often necessary in this last technique. The fusion-scission kinetics can be studied without probes by the T-jump technique with light scattering detection, if the weight aggregation number of the micelles varies sufficiently with temperature. This last technique is the best suited for solutions of micelles from the dilute regime to the beginning of the semidilute regime, but it does not allow observation of the fusion-scission process of micelles when the micelles are strongly entangled.The fusion-scission relaxation process has been studied in CTAB 14 solutions or in SDS 15 solutions at different temperatures and ionic strengths. These studies show that for elongated micelles the relaxation frequency increases as the square root of the average length of the micelles. For short micelles, the variation law of the relaxation time is unknown but it strongly depends on the size distribution of micelles. Zana et al. 16 studied the effect of alcohol molecules of diffe...

Effect of monomeric and polymeric co-solutes on cetyltrimethylammonium bromide wormlike micelles: Rheology, Cryo-TEM and Small-angle neutron scattering

“…For ethanol and 2-propanol, s R and G 0 are practically not affected, due to the high affinity of these co-solutes for the continuous aqueous phase. However, short alcohols have been shown to intercalate in the palisade layer, and at higher alcohol/CTAB ratios, changes in the rheology may be detected, as reported elsewhere [30]. The more hydrophobic co-solutes, instead, are more likely to interact with the wormlike micelles, either by being encapsulated in the core or localising at the core/water interface.…”

“…For instance, a process was reported which reversibly thickens a viscoelastic fluid by adding trichloroethylene, which is then removed by distillation to restore viscoelasticity [14]. Solubilisation of small organic molecules (alcohol, amines) can modify the packing of the surfactants and therefore alter micellar curvature, promoting aggregate shape modification such as sphere-to-rod transition [29,30]. For instance, a range of molecules with the same C8 chain structure but differing functionalities (octanol, octanoic acid, octylamine and octane) was added to CTAB micelles [31].…”

Oscillatory phenomena at liquid–liquid interfaces

“…The dynamics at the interface would be quite complicated due to fusion-scission kinetics [17] of CTAB and CTAP and the possibility of formation of liposomes [18] and even nonspherical lamellar micelles [19].…”

Microstructural transition of aqueous CTAB micelles in the presence of long chain alcohols

“…Medium chain alcohols (n > 4) were found to intercalate between micelles (palisade layer) causing a decrease in CMC and thereby promotes micelle growth. [23][24][25][26] Kuperkar and his co-workers 27 examined the effect of linear alcohols (C 2 OH to C 6 OH) on cationic surfactants and emphasised that upon increasing the hydrocarbon chain length of alcohol (n) there is a marked decrease in CMC, resulting in pronounced micellar growth. Similar behaviour was reported for an ionic micellar system with alcohol chain length up to C 8 .…”