Chapter 18
Shear Effects in Surfactant Solutions
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Rhyta S. Rounds Becton Dickinson, Vacutainer Systems, 1 Becton Drive, Franklin Lakes, NJ 07417-1885
Surfactants exhibit a broad spectrum of rheological properties dependent upon the internal structure of the surfactant micelles within the fluid media. Newtonian or highly non-linear viscoelastic behavior can occur as a function of concentration, temperature and solution environment, reflective of the complex phase behavior of these materials. Even dilute isotropic solutions can manifest very significant time-dependent non-linear behavior in simple shearingflowsas the applied external flow field induces change in the morphology of the discrete surfactant units. Examples are provided of nonionic, cationic, anionic and mixed surfactant systems in various aqueous environments and phases exhibiting shear rate and time dependent rheological behavior in both steady shear and oscillatory measurements. Surfactants are remarkable in the complex rheological behavior they manifest throughout the various regions of the phase diagram and within each phase as well. Even within the isotropic phase, Newtonian and non-Newtonian behavior can occur with dramatic shear effects such as rheopexy, thixotropy, dilatancy and pseudoplasticity. This unusual phenomenon has been well documented for specific cationic surfactant systems (7-5). For surfactants exhibiting shear effects, rheological behavior within critical shear rate ranges can become strong functions of the applied shear rate, duration of the shearing interval and surfactant/solvent composition. Behavior of concentrated mesomorphic surfactant systems is also frequently punctuated with dramatic time and shear dependent rheological functions within regions of the liquid crystalline phase with shear effects occurring in both steady shear rotational and oscillatory measurements. Rheological characterization of time and shear rate dependent material functions for surfactant systems can be a difficult and time intensive process. However, shear effects are readily observed in simple shear stress growth measurements, a =f(7,t), spanning an appropriate shear rate range. Under these experimental conditions, a fluid is subjected to a step change in shear rate at t=0, as shown in Figure 1. In response to the applied shear rate, the shear stress, σ, is +
0097-6156/94/0578-0260$08.00/0 © 1994 American Chemical Society In Structure and Flow in Surfactant Solutions; Herb, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
Downloaded by UNIV OF TENNESSEE KNOXVILLE on December 19, 2014 | http://pubs.acs.org Publication Date: December 9, 1994 | doi: 10.1021/bk-1994-0578.ch018
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Shear Effects in Surfactant Solutions
measured as a function of time. These measurements can directly identify and quantify any time dependent viscometric behavior such as rheopexy or thixotropy and strain dependent functions such as pseuoplasticity or dilatancy. Such measurements at constant shear rates can also provide insight into the kinetics of any time dependent shear induced structural transitions occurring within the surfactant matrix. Several examples stress growth characteristics are provided in Figure 2. "Thixotropic loop" measurements are commonly used for qualitative evaluations of shear effects. During a thixotropic loop evaluation, a time dependent ramped shear rate is applied to the experimental material in acceleration and deceleration as shown in Figure 3. As in stress growth measurements, the shear stress is commonly the measured response variable. Such measurements are difficult to interpret, however, due to the coupled time and shear rate independent variables. As such, stress growth measurements are preferred to thixotropic loop measurements to quantify and isolate the independent effects of shear rate and the duration of the applied flow field. Ramped shear rate measurements do provide, however, a preliminary assessment of shear effects although the absence of a "thixotropic loop" does not preclude the existence of time and shear dependent shear effects since the fluid "memory" or kinetic processes can coincide with the time scale of the shear rate ramp. Figure 4 is a graphical presentation of several common flow curves that can be obtained in these types of qualitative measurements. Dilute Solutions Cationic Surfactants. Cationic surfactants are singular in the complex rheological properties they can exhibit. This is especially true of a select subset of this class of surfactants containing mainly pyridinium or trimethylammonium headgroups and strongly binding counterions. While most surfactant solutions in the isotropic solution phase are Newtonian fluids, cationic detergents at dilute concentrations near the cmc can exhibit dramatic non-linear viscoelastic behavior and shear effects. It has been documented that changes in micellar assembly occurring in simple shearing flow produce corresponding changes in rheological behavior. Research attributes the unusual viscometric behavior of these cationic systems to the shear induced formation of rod-like micellar chains, micellar "polymerization" (7), resulting in a dense three dimensional network structure (8-10). When the length of the surfactant polymer-like segment, L, equals the mean separation distance, D, viscoelastic and rheopectic effects can be observed. The supramicellar structure formed from individual rods, entangled and connected by temporal linkages, result in systems exhibiting rheological behavior typical of both linear and crosslinked polymers. There are, however, noteworthy differences exhibited by these specific cationic systems and polymers due primarily to the life-times or temporary nature of the "crosslinkages" or entanglement modes of micellar systems (11-12). In this respect, the entangled rod-like cationic surfactant network is truly unique. Cationic surfactants are also unusual in the breadth of rheological behavior that can occur within a relatively narrow concentration and shear rate range in the dilute regime. For example, within the concentration range of 3-5 mmol/dm of N-cetylΝ,Ν,Ν-trimethylammonium bromide and sodium salicylate, rheopectic, thixotropic, non-Newtonian Power Law and Newtonian behavior has been documented (13,14). 3
In Structure and Flow in Surfactant Solutions; Herb, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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STRUCTURE AND FLOW IN SURFACTANT SOLUTIONS
Φ « OC λ. CO φ CO t=0
Time Figure 1. Step change in shear rate in stress growth measurements.
CO
rheopexy
(0
