Langmuir 1987,3,821-830 fluorocarbon-rich ones slightly increased. The existence of a critical demicellization concentration has been predicted by My~e1s.l~ The necessary conditions are that two micelles of limited solubility form and that the mole fraction of one surfactant be higher in both micelles than in the system as a whole. Such a phenomena was not observed in this LiFOS-LiTS system because the first condition was only satisfied. That is, the concentration of both kinds of mixed micelles monotonously increased as the total concentration increased (Figure 8). The gel filtration was found to be a useful method to prove the coexistence of two kinds of mixed micelles. The region of phase separation could be accurately determined in the terms of the appearance of the second type of mixed
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micelles. The group contribution method could apply the simulation of gel filtration. Moreover, this method predicted not only the variation of monomer concentration but also the demixing micelle compositions with reasonable accuracy, which were failed by the regular solution theory. The simple method was established to indicate the micelle demixing by a direct separation of two kinds of mixed micelles. The next problem will be to estimate the chain length of surfactant necessary to cause the phase separation.
Acknowledgment. We are grateful to Dainippon Ink Chemical Industry Co., Ltd., for providing the fluorocarbon surfactant.
Experimental Observation of Chemically Modulated Admittance of Supported Phospholipid Membranes Ken-ichi Hongyo,t Jose Joseph, Robert J. Huber, and Jiri Janata* Center for Sensor Technology, University of Utah, Salt Lake City, Utah 84112 Received January 31, 1987 Phase-resolved changes of admittance of supported black lipid membranes in response to addition of various chemicals are described. It is shown that at a 1-kHz excitation frequency either the capacitance or the conductance can be modulated by addition of phloretin or valinomycin, respectively. Changes of conductance (at 1 kHz) of several hundred percent are produced by reactions involving specific binding of concanavalin A and yeast mannan or by the enzymatic hydrolysis of urea.
Introduction There are numerous reports in the literature about the effect of chemicals on the ionic conductivity of phospholipid-based membranes. These membranes were either freely suspended in a pinhole between two aqueous compartments, in the form of a so-called “black lipid membrane” (BLM), or found as phospholipid vesicles. The reasons for studying these structures range from fundamental electrochemical and electrophysiological interest in their properties to sensor applications.’ In their early study, DelCastillo et a1.2 observed marked changes in the membrane conductance in the presence of various specific binding reactions, e.g., antibody-antigen or enzymatic reactions. Their measurements were done by using an ac sinusoidal applied voltage in the frequency range between 0.3 and 1 kHz. Later, DeLevie et al.3 described modulation of conductance of phospholipid BLM with phloretin and derived the expression for the adsorption isotherm of that compound on the membrane. However, in that paper they did not state whether their measurements were done in the ac or in the dc mode. Later still, Thompson and his group undertook an extensive investigation of various forms of phospholipid membranes and the modulation of their dc conductance by these and other compounds and proposed a mechanism for the observed p h e n ~ m e n a . ~ For practical sensor applications it is mandatory to provide a rugged mechanical support for the phospholipid bilayer. This implies an asymmetrical situation in which * T o whom correspondence should be addressed. UBE Industries, Ltd., Tokyo, Japan.
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the internal volume of the aqueous electrolyte is much smaller than that of the external sample. Recently, Thompson et al. reported measurements of dc conductivity5 done on polyacrylamide gel-supported planar phospholipid-based membranes formed by the Langmuir-Blodgett technique (LB). Although the same chemical stimulants under similar conditions were used, the response of this system was markedly different from the one in which the membrane was freely s u ~ p e n d e d . ~For ?~ the freely suspended membrane a quasi-steady-state time response was obtained which reached its stable value after 10-15 min, while in the supported membrane the response was a transient peak with a half-width of
