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Langmuir 1988,4, 354-359
fluid is therefore losing its viscoelastic and drag reduction permicelles can therefore be formed much easier. The capabilities. Again, the described behavior is observed results shown in Figure 8 are in agreement with this exexperimentally. pectation. Although the SIS model has already been shown to be responsible for drag reduction in turbulent tube f l 0 ~ , ~ ~ 9 ~ 1 ~ ~ Conclusions it is instructive to examine why this drag reduction is It is generally agreed that the SIS model is responsible different a t low and a t high temperatures. At relatively for the behavior of aqueous C16TMA-Sal solutions in both low temperatures (2" = 45 "C), large supermicelles with a viscometric flows and in turbulent tube flow. That the relatively rigid structure are generated, near the wall, at same model can also explain the results in the flow through relatively low velocity gradients, as a result of aggregation. a porous medium has been shown in this study. It is The latticelike organization of the micelles leads in this suggested that the competition between aggregation and case to a pressure drop which is even higher than that dissociation of micelles determines the extent of superencountered in the turbulent flow of the pure solvent. micellization. Both these processes depend on the velocity When the velocity gradient is increased, the supermicelles gradient, temperature, and flow characteristics. At reladecrease in size and become more flexible, and the vistively low temperatures, for the turbulent flow in a pipe, coelasticity which they impart to the fluid leads to drag the aggregation process, which leads to a latticelike orreduction. A further increase of the velocity gradient ganization of the micelles, is dominant, a t relatively low decreases the size of the supermicelles to such an extent velocity gradients. Increasing the rate of shear leads to that the fluid completely loses its viscoelasticity and hence supermicelles with viscoelastic capabilities and further to its drag reduction capabilities. The experiments at low their destruction because the dissociation process becomes temperatures do show the above suggested b e h a v i ~ r . ' ~ ~ ~important. ~ In different conditions, such as flow through At higher temperatures (T = 65 " C ) , supermicelles start a porous medium or higher temperature turbulent pipe to form after the transition to turbulence and only when flow, the aggregation process among micelles is more the velocity gradient becomes sufficiently large. They grow moderate and increases with the rate of shear at low rates in size with increasing velocity gradient, thus imparting of shear. The dissociation process plays, however, an imincreasing viscoelasticity and drag reduction to the fluid. portant part a t high rates of shear. Under the latter When the velocity gradient becomes too large, the dissoconditions the supermicelles disappear. ciation process begins to play an important role and the Registry No. C16TMA-Sal, 61482-44-8. size of the supermicelles becomes increasinglysmaller. The
Micelle Formation of Detergent Molecules in Aqueous Media. 2. Role of Free Salicylate Ions on Viscoelastic Properties of Aqueous Cetyltrimethylammonium Bromide-Sodium Salicylate Solutions Toshiyuki Shikata* and Hirotaka Hirata Department of Physical Chemistry, Niigata College of Pharmacy, Kamishin-ei-cho, Niigata 950-21, Japan
Tadao Kotaka Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan Received August 10, 1987 The factor governing the stability of micelles and hence controlling the rheological behavior of aqueous solutions of cetyltrimethylammoniumbromide (CTAB) containing sodium salicylate (Nasal) was investigated. The solutions containing fully entangling threadlike micelles of CTAB-Nasal complexes may be modeled by a Maxwell model with a single relaxation time. Concentrations CS*of free salicylate ions in the systems were estimated by carrying out 'H NMR measurement on deuterium oxide solutions with varying CTAB (C,) and NaSal (Cs)concentrations. The results of NMR measurement suggested that the threadlike micelles existed in the form of a 1:l complex between CTAB and NaSal. Thus, the relation Cs* = Cs - CD was obtained in the range Ds> CD. On the other hand, we confirmed that the relaxation time 7, is influenced only by Cs* independently of CD,and thus the factor controlling 7, is Cs*. Furthermore, consideration on the basis of a quasi-networkmodel led to an idea that the free salicylate ions are behaving as a catalyst for a disentangling reaction.
Introduction Certain cationic detergents with ammonium or pyridinium head groups often form rodlike or threadlike micelles in aqueous solution and exhibit remarkable viscoelasticity when a salt or an acid with aromatic rings is added.'-4 (1) Gravsholt, S. J. Colloid Interface Sci. 1976, 57, 575. (2) Rehage, H.; Hoffmann, H. Rheol. Acta 1982, 21, 561.
In our previous paper," we reported unique viscoelastic properties of aqueous cetyltrimethylammonium bromide (CTAB) solutions with sodium salicylate (Nasal) as an added salt. The features of their unique viscoelasticity (3) Bunton, C. A. Reaction Kinetics in Micelles; Plenum: New York, 1973. (4) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987,3,1081-1086.
0 1988 American Chemical Society 0143-1463/SS/2404-035~~01.50/0
Micelle Formation of Detergent Molecules
Figure 1. Electron micrograph of an aqueous cetyltrimethylammonium bromidesodium salicylate solution with Co = 1.0 x lWsand CS= 1.0 X lW3 mol L-l. The details were described in our previous paper?
were dassied into three types,which we named the type I, II, and III behavior acmrding to increasing concentration of sodium salicylate. Cs, while that of cetyltrimethylammonium bromide, CD, was kept constant. The viscoelasticity ofthe solutions with small C, relative to CD,which was called the type I behavior, quite resembled that of flexible polymer solutions of low molecular weight, in which the polymer chains were not entangling. This type I behavior was expressed very well by the Rouse theory! The viscoelasticity of the solutions with intermediate Cs (
