Surface molecular view of colloidal gelation
- *Max Planck Institute for Metals Research, Heisenbergstrasse 3, 70569 Stuttgart, Germany;
- ‡Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA, Leiden, The Netherlands;
- §Institute for Solid State Research, Soft Matter, Research Center Juelich, 52425 Juelich, Germany;
- ¶Soft Condensed Matter, Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA, Utrecht, The Netherlands; and
- ‖FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ, Amsterdam, The Netherlands
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Communicated by Gabor A. Somorjai, University of California, Berkeley, CA, July 21, 2006 (received for review April 6, 2006)
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Fig. 1.Overview of the experiment and key experimental results. (A) Schematic illustration of the reversible transformation from gel into suspension. The molecular-scale structural changes observed in this study are depicted with photographs illustrating the macroscopic change in viscosity. (B) The scattering geometry. k SFG, k VIS, and k IR are the wave vectors of the scattered sum frequency and the VIS and IR fields; q is the scattering wave vector; and θ is the scattering angle. The detector and k vectors lie in one plane. The IR, VIS, and SFG fields are polarized either parallel (p) or perpendicular (s) to the plane of incidence. (C) Three sets of SFG scattering spectra (gray lines), obtained at three different polarization combinations. In each set the upper trace represents a gel (T = 293 K, 1 day after preparation), and the lower trace is a fluid suspension of colloidal particles in deuterated n-hexadecane-d34 (T = 323 K). The three-letter codes denote the polarization states of the SFG, VIS, and IR beams, respectively. Spectra for sps (data not shown) and pss combinations are identical. The spectra were collected at θ = 51° and are offset for clarity. The black lines are fits described in the text. The vibrational modes that appear in the SFG spectra are marked in the upper pair of traces.
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Fig. 2.Nonlinear and linear optical response and thermodynamic behavior of the colloidal suspension vs. temperature. (Upper) Spectrally integrated SFG intensity and turbidity as a function of temperature for 123-nm particles. The line representing a single exponential decay is to guide the eye. (Lower) Calorimetric excess heat flow from a 28.9 vol% dispersion of 36-nm particles in deuterated n-hexadecane (≈30 min after preparation). The onset of aggregation is marked with an arrow in both images.
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Fig. 3.Temporal evolution of the colloidal surface after gelation. (Upper) SFG spectra (gray lines, ppp polarization) of a 24 vol% gel as a function of gel age as indicated. Fits (black lines) were performed as described in ref. 28. (Lower) Amplitude of the symmetric CH3 (squares) and CH2 (circles) stretch modes at the frequencies indicated by the arrows in Upper. The structure of the boundary layer is schematically depicted for the three most relevant time intervals during gel formation and aging.
Footnotes
- †To whom correspondence should be addressed. E-mail: roke{at}mf.mpg.de
- © 2006 by The National Academy of Sciences of the USA
