Use of pyrene spectroscopic probes in the study of colloidal systems

The interfacial behavior of polyethylene oxide (PEO) end-labeled with the o ften-used fluorescent probe pyrene was monitored to assess the effect of us e of such hydrophobic probes on system behavior. Mixtures of labeled and un labeled PEO were adsorbed on silica and the resulting suspension properties noted. The suspension behavior was found to be markedly different, dependi ng on the ratio of labeled-to-unlabeled PEO employed. While it was "safe" t o use pyrene probes in amounts of 3% or less for spectroscopic investigatio n of PEO effects, it was found that ratios equal to, or greater than, 1:3 p roduced anomalous behavior of the silica suspensions. This was manifested i n the form of decreased suspension stability, as evidenced by significantly higher settling rates. Similar experiments using free pyrene revealed that these effects could be attributed to the chemical attachment of the pyrene probe to the polyethylene oxide chain. Fluorescence spectroscopy was used to simultaneously monitor the behavior of the adsorbed polyethylene oxide m olecules. These results were used to explain the observed suspension behavi or as a function of the labeled-to-unlabeled polymer mixing ratio. Bridging effects facilitated by hydrophobic interaction between pyrene end groups i s proposed as the reason for the observed phenomena. (C) 2000 Academic Pres s.