Silanes at Interfaces

Michael A. Brook 
Professor and Consulting Chemist 

Performing Chemistry on Silica Surfaces

Inorganic materials are often used as reinforcing agents in organic polymers. To be useful, it is frequently necessary to modify the inorganic surface with hydrophobic organic groups. This is usually done using coupling agents in which an alkoxysilane covalently bonds to the inorganic material. We have used this technology to modify model colloidal silica particles (100-250 nm diameter). By controlling organic groups on the surface, we can control the stability of the particles - normally in the absence of stabilizers, the particles aggregate. In doing so, we are learning about the colloidal stability of polar and not so polar silica particles in polar and non-polar solvents.21,46,49,72 Understanding these phenomena is important in industries, such as the paint industry, for whom control of latex particle aggregation is extremely important.

Scheme 1                                                        A                                            B

Reversible Coupling Agents

Coupling agents ideally modify mineral surfaces using covalent bonds that are resistant to hydrolysis (hopefully forever).  They convert polar surfaces to organic surfaces (Scheme 1A) that frequently will be able to chemically graft to an organic matrix (Scheme 1B), reinforcing a composite.  However, it would be sometimes convenient to be able to thermally reprocess these, essentially thermoset, materials.  We have designed a silane coupling agent that can graft through stable siloxane bonds with the surface, but which, in the presence of cyclopentadiene, can undergo reversible thermal cleavage (Scheme 2, submitted).

Scheme 2

Immobilized Hydrosilylation Catalysts

Once silica is modified with Si-H functional groups, one can form well-defined Pt colloids (2 nm diameter particles) that are attached to a silica surface (Scheme 3, Figure 1).  These are heterogeneous hydrosilylation catalysts (Scheme 3).39, 50, 62
 

Pt-colloids are suspended/grafted in the gel. The external gel is accessible to small molecules, but not polymers

Scheme 3

a)      b) 

Figure 1: a) TEM of Pt-nanoparticles (silica particle diameter 180 nm) on a silica surface; b) zoom of a.

Protein-Silicone Emulsions (see also Protein-Silicone Copolymers)

Proteins can be used as both emulsifiers and protecting groups for other proteins.  We have seen that human serum albumin sits at the interface of D4 (octamethylcyclotetrasiloxane) and water when a silicone co-surfactant is used (Figure 2).  If enzymes such as alkaline phosphatase are incorporated in the emulsion they remain stable for several months at room temperature, but will react with substrate under shear.  Other functional silicones such as (EtO)3Si(CH2)3SiMe2(SiMe2)n(CH2)3Si(OEt)3 similarly stabilize D4/water emulsions ONLY in combination with human serum albumin and, perhaps, other proteins.91

Figure 2: Confocal microscope picture of fluorescein-labelled human serum albumin sitting at the D4/water interface.

References

93. F  Bartzoka, V.; McDermott, M. R.; Brook, M. A., Protein-Silicone Interactions at Liquid/Liquid Interfaces, In Emulsions, Foams and Thin Films, Mittal, K. L.; Kumar, P., Eds., Dekker, New York, 2000, Chap. 21, pp. 371-380, Invited manuscript.

 

92. F  Vasiliki Bartzoka, Gladys Chan and Michael A. Brook, Protein-Silicone Synergism at Liquid/Liquid Interfaces, Langmuir 2000, 16, 4589-4593.

72. F Howard A. Ketelson, Robert Pelton, and Michael A. Brook, Surface and Colloidal Properties of Hydrosilane Modified Stöber Silica, Colloids and Surfaces A 1998, 132,229-239.

60 F Michael A. Brook, Howard A. M. Ketelson, F. LaRonde and Robert H. Pelton, Pt0 compounds bound in a silsesquioxane layer: active hydrosilation catalysts protected by the gel, Inorg. Chim. Acta 1997, 264, 125-135.

52. C Michael A. Brook, Howard A. M. Ketelson, Robert H. Pelton and Yew. M. Heng, Surface Nucleation of Silica-Supported Platinum Nanoparticles, Chem. Mater. 1996, 8, 2195-2199.

49. F Howard A. Ketelson, M. A. Brook and R. H. Pelton, Colloidal Stability of Stöber Silica in Acetone-Water Mixtures, J. Colloid Interface Sci. 1996, 179, 600-607.

46. Howard A. Ketelson, M. A. Brook and R. H. Pelton, Colloidal Stability of Stöber Silica in Acetone, Langmuir, 1996, 12, 1134-1140.

39. H. A. M. Ketelson, Michael A. Brook, and Robert H. Pelton,Colloidal Silica Bearing Hydrosilane Groups, Chem. Mater. 1995, 7, 1376-1383.

38. Howard A. M. Ketelson, Michael A. Brook and Robert H. Pelton, Sterically Stabilized Silica Colloids: Radical Grafting of Poly(methyl methacrylate) and Hydrosilylative Grafting of Silicones to Functionalized Silica, Polym. Adv. Technol. 1995, 6, 335-344.

25. Robert H. Pelton, Andrea Osterroth and Michael A. Brook, Silicone Stabilized Poly(methyl methacrylate) Nonaqueous Latex. 2 Flocculation By Degradation of the Steric Layer, J. Colloid Interface Sci., 147, 523-530 (1991).

21. Robert H. Pelton Andrea Osterroth and Michael A. Brook, Silicone Stabilized Poly(methyl methacrylate) Nonaqueous Latexes, J. Colloid Interface Sci., 137, 120-127 (1990).

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Last revision: 2001-01-16; mab © 1998-2001, M. Brook