Microstructure and Rheology of Nonionic Trisiloxane Surfactant

Chapter 13

Microstructure and Rheology of Nonionic Trisiloxane Surfactant Solutions 1,3

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M. He , R. M. Hill , H. A. Doumaux , F. S. Bates , H. T. Davis , D. F. Evans , and L. E. Scriven 1

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Downloaded by CORNELL UNIV on October 24, 2016 | http://pubs.acs.org Publication Date: December 9, 1994 | doi: 10.1021/bk-1994-0578.ch013

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Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue Southeast, Minneapolis, MN 55455 Central Research, Dow Corning Corporation, 2200 West Salzburg, Auburn, MI 48611

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The nonionic trisiloxane surfactant, M(D'E12)M (= ((Me)3SiO)2Si(Me)-(CH2)3(OCH2CH2)12OH), forms clear isotropic solutions in water at all concentrations between 10 and 43 °C. Both water-rich and surfactant-rich solutions are low viscosity Newtonian liquids. However, at intermediate concentrations, the solutions are non-Newtonianand viscoelastic. This rheological behavior along with supporting data from small-angle X-ray and neutron scattering, and pulse-gradient NMR measurements points to a progressive change in surfactant microstructure across the concentration range. This change is parallel to the progression of liquid crystal phase behavior at lower temperatures. The results are interpreted in terms of a model in which small spherical micelles formed at low concentrations transform progressively to entangled worm-like micelles, branched interconnected worm-like micelles, and then a random bilayer structure which persists to 100 % surfactant. In a previous study (7 ), we showed that a nonionic trisiloxane surfactant, ((Me) SiO)2Si(Me)(CH2)3(OCH CH2)i20H, which is denoted as M(D'Ei )M, shows unusual phase behavior in forming clear isotropic solutions across the entire concentration range and over a wide temperatures range: see Figure 1. In the temperature range between 10 and 43 °C, this surfactant is completely miscible with water. But at temperatures below 10 °C, a hexagonal liquid crystal phase, H i , was found at about 50 wt% surfactant, and a lamellar liquid crystal phase, L , at about 70 wt%. At temperatures above 43 °C the liquid-liquid immiscibility called the cloud point or the lower consolute temperature (LCT) is found. The LCT varies with concentration; the minimum in the LCT curve is about 5% and 43 °C A few other trisiloxane and alkyl polyoxyethylene surfactants such as M(D'Ei8)M (7 ), C12E8 and C12E6 (2,3 ) also form clear isotropic solutions in water at all concentrations, but only in a much narrower temperature range well above room temperature. 3

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Current address: Unilever Research United States-Edgewater Laboratory, 45 River Road, Edgewater, NJ 07020 0097-6156/94/0578-0192$08.90/0 © 1994 American Chemical Society Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Nonionic Trisiloxane Surfactant Solutions


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80 wt%); (3) midrange solutions (between 40 and 80 wt%). The solutions of the first two categories are Newtonian and their viscoelasticity can be -1

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Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


Figure 5. (a) logG" vs. log(acco) for the solutions at surfactant concentrations of 25, 30, 35, 40, 60,70, 90, 100 wt%; (b) logG" vs. log^co) for the solutions at surfactant concentrations of 45,50 wt%.


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STRUCTURE AND FLOW IN SURFACTANT SOLUTIONS


described by the Maxwell model. Those of the third category are non-Newtonian and are more elastic. Thus the rheology data presented above indicates that the microstructures in the midrange of concentrations are significantly different from those in the surfactant-rich and surfactant-lean concentration ranges. Small-Angle X-ray (SAXS) and Neutron Scattering (SANS). SANS spectra of 1.92 and 10 % M(D'Ei2)M solutions are shown in Figure 7a and b. These SANS spectra can be simulated by the Fourier transformation of the radial distribution function of hard-spheres (25 and references therein), with the size of the hard spheres being the only adjustable parameter. The contrast difference between the sphere and D2O solvent is calculated as that between neat M(D'Ei2)M liquid and D2O, namely (pm - Ps) = 0.0038 χ 1 0 c m / Â . So the effect of D 0 penetration into the hydrophilic headgroup is not included here. Figure 7 shows that the calculated scattering function of hard-spheres fits the experimental SANS data well. The asymptotic oscillatory behavior shown by the calculated curves at q > 0.1 Â can be damped by incorporating a small amount of polydispersity in size (R), i.e, R = 35 ± 2 Â (26 ). Therefore it is reasonable to conclude that up to at least 10 wt% M(D*Ei2)M the solutions are composed of spherical micelles with the size of the micelles almost unvarying. In a previous study (1 ), small spherical micelles were also found in a 5 wt% solution by cryo-TEM. The SAXS spectra of the solutions at midrange concentrations and above show a broad correlation maxima. The Bragg d-spacing (d ) of the correlation maxima derived from SAXS spectra is shown as a function of surfactant concentration in Figure 8. As a general trend, d diminishes as M(D'Ei2)M concentration rises. However, in the concentration range between 35 and 45 wt%, and between 70 and 80 wt% M(D'Ei2)M, d varies little and forms two composition-independent plateaus at about 68 Â and 55 Â respectively. The neat M(D'Ei2)M liquid was vacuum-dried at 25 millitorr at 80 °C for an hour in order to get rid of the residual amount of water absorbed from air, and then flame-sealed in the SAXS capillary within a few minutes. The SAXS spectrum of neat liquid M(D'Ei2)M at 25 °C shown in Figure 9 exhibits a broad correlation maximum at dmax of 49.7 Â, suggesting that the liquid is locally microstructured even with water absent. Wide angle X-ray diffraction of the neat liquid reveals a broad intensity maximum at d-spacing around 4.6 Â, which likely results from the correlation among the neighboring molecules. As the surfactant concentration rises from 10 to 35 wt%, either the size or the number density of micelles must increase. The rheology data presented above and the NMR data discussed below both point toward an increase in size of the microstructure. Here we analyze the implication of the scattering data. If small micelles are present and only the number density of micellesriseswhen the surfactant concentration increases, then the average inter-micellar distance, which is proportional to d x (27 ), should decrease monotonically. However this conflicts with the d plateaus shown in Figure 8. Also, the spherical close-packing limit is about 70 volume % surfactant - if small micelles are present at concentrated concentrations around 70 %, the solution viscosity would be quite high, similar to that of cubic liquid crystal phase Ii (28 ) 2

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Microstructure and Rheology of Nonionic Trisiloxane Surfactant

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