Viscosity micelle concentration

Performance Indices Quality Factors Optimum E1LB Critical micelle concentration (CMC) Soil solubilization capacity Krafft point (ionic surfactants only) Cloud point (nonionic surfactants only) Viscosity Calcium binding capacity Surface tension reduction at CMC Dissolution time Material and/or structural attributes... 

The dependence of reduced viscosity on concentration is curved upward, which could be interpreted as entanglement of rod-like micelles and/or a strong micellar interaction [47]. The increase in microviscosity results mainly from the growth in micellar size [48]. 

Solubilization oftropicamide, a poorly water-soluble mydriatic/cycloplegicdrug, by poloxamers or Pluronics was studied (Saettone et al., 1988). The polymers evaluated as solubilizers for the drug included L-64, P-65, F-68, P-75, F-77, P-84, P-85, F-87, F-88, and F-127. The authors measured a range of physicochemical properties, such as solubility oftropicamide in polymer solutions, partition coefLcient of the drug between isopropyl myristate and copolymer solutions, critical micelle concentration of the copolymers, and viscosity of the copolymeric solutions containing tropicamide. 

Figure I Viscosity-concentration (t vs c) profile at 26°C of an aqueous pectin dispersion showing the critical micelle concentration (c ).

Surfactants below their critical micelle concentration (CMC) add to the solvent viscosity according to the Einstein equation... 

Fixation of hydrophilic units as side-groups of a hydrophobic macromolecular chain leads to water-soluble polysoaps [205], exhibiting a similar diversity of self-organized structures like the monomeric analogues. A detailed review can be found in [206]. In contrast to polyelectrolytes and ionomers described above, the association of the amphiphilic groups of polysoaps occurs preferentially intramolecularly. As a consequence the solution viscosity remains low, even for highly concentrated solutions [207] and no critical micelle concentration (CMC) can be found up to extreme dilutions [208,209]. 

Schick and Fowkes (11) studied the effect of alkyl chain length of surfactants on critical micelle concentration (CMC). The maximum lowering of CMC occurred when both the anionic and nonionic surfactants had the same chain length. It was also reported that the coefficient of friction between polymeric surfaces reaches a minimum as the chain length of paraffinic oils approached that of stearic acid (12). In order to delineate the effect of chain length of fatty acids on lubrication, the scuff load was measured by Cameron and Crouch (13). The maximum scuff load was observed when both hydrocarbon oil and fatty acid had the same chain length. Similar results of the effect of chain length compatibility on dielectric absorption, surface viscosity and rust prevention have been reported in the literature (14-16). 

In sufficiently dilute aqueous solutions surfactants are present as monomeric particles or ions. Above critical micellization concentration CMC, monomers are in equilibrium with micelles. In this chapter the term micelle is used to denote spherical aggregates, each containing a few dozens of monomeric units, whose structure is illustrated in Fig. 4.64. The CMC of common surfactants are on the order of 10 " -10 mol dm . The CMC is not sharply defined and different methods (e.g. breakpoints in the curves expressing the conductivity, surface tension, viscosity and turbidity of surfactant solutions as the function of concentration) lead to somewhat different values. Moreover, CMC depends on the experimental conditions (temperature, presence of other solutes), thus the CMC relevant for the expierimental system of interest is not necessarily readily available from the literature. For example, the CMC is depressed in the presence of inert electrolytes and in the presence of apolar solutes, and it increases when the temperature increases. These shifts in the CMC reflect the effect of cosolutes on the activity of monomer species in surfactant solution, and consequently the factors affecting the CMC (e.g. salinity) affect also the surfactant adsorption. 

Polysaccharides with Surfactant Micelles. Consider a solution of a fairly hydrophobic polysaccharide, such as a cellulose ether. The hydrophobic groups cause a weak attractive interaction, leading to a somewhat increased viscosity at low shear rates. If an anionic small-molecule surfactant is added, say SDS (sodium dodecyl sulfate), at a concentration above the CMC (critical micellization concentration), micelles are formed that interact with the polymer more specifically, one or a few polymer chains can pass through a micelle. In this way, polymer chains can be cross-linked. If now the polymer concentration c is below c (the chain overlap concentration), mainly intramolecular junctions are formed. If c > c, however, a gel results. In this manner, viscoelastic gels can be made with a modulus of the order of 10 Pa. 

Still, there is the most interesting phenomenon that the cationic polymer poly(iV-hexadecyl-/V,/V-dimethy-N-vinylbenzyl ammonium chloride) 28 exhibits very low reduced viscosities but does not show polyelectrolyte behaviour in aqueous solution [103, 292] the plot of reduced viscosity vs concentration is strictly linear, and is insensitive to added salt (Fig. 20). Importantly, this head type vinyl polymer without main chain spacer is not water-soluble and thus not a true polysoap, but forms only metastable aqueous solutions (see Sect 2.2.4). Similar results were reported for alkylated poly(vinylimidazoles) such as 26 [347], It may be speculated that such solutions represent rigid molecular latexes rather than flexible polymeric micelles , and further studies on such systems would be most interesting. 

Although it is reasonable to believe that micelles in water are close to spherical at low surfactant concentrations, e.g., within an order of magnitude of the critical micelle concentration, there is evidence for micellar growth with increasing surfactant or electrolyte concentration. Such growth cannot easily be accommodated to a strictly spherical structure for the micelle, but requires it to become spheroidal. Solutions of some surfactants become very viscous on addition of salts. For example, addition of iodide, arenesulfonate or aromatic carboxylate ion to cationic surfactants sharply increases solution viscosity, suggesting that long, rod-like micelles are formed. 

These results imply that the dramatic changes observed in viscosity were not due to specific interactions between polymer and surfactant, but were related to the concentration at which micelles form. We suggest that at a surfactant concentration just below the critical micelle concentration, poly-... 

To control the viscosity of many shampoos, salt is added to the surfactant system. The interaction between salt and the long chain surfactants transforms the small spherical micelles of the surfactants into larger rodlike or lamellar or even Uquid-crystalline-type structures that increase the viscosity of the liquid shampoo. If one plots the salt concentration versus the viscosity in such a system, one typically finds an optimum for the maximum viscosity see Figure 5-1. Above this optimum salt concentration, additional salt decreases the viscosity. In developing such a system in which viscosity is controlled by salt addition, it is recommended that one select the appropriate salt concentration on the ascending part of the viscosity-salt concentration curve. Nevertheless, many light-duty liquid products and some shampoos are formulated on the descending part of the curve. The selection of surfactant, amide, and other components are critical to viscosity-salt concentration control in such a system. Furthermore, impurities such as salt contaminants in surfactants must be carefully controlled to obtain the appropriate viscosity when salt control is employed. 

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