LANGMUIR MONOLAYERS OF POLYDIMETHYLSILOXANE WITH CONTROLLABLE SURFACE-POTENTIAL

Interaction of a Langmuir monolayer of polydimethylsiloxane (PDMS) wit h different hydrophobic ions added to the aqueous subphase was studied using surface pressure-area and surface potential-area isotherm techn iques. For strongly hydrophobic tetraphenylborate (TPB) anions and tet raphenylphosphonium (TPP) cations, a pronounced shift of the monolayer surface potential accompanied by modest variations in the surface pre ssure isotherms was observed. Effects of TPB and TPP ions were found t o be almost identical except the sign of a surface potential shift: it was positive for TPP and negative for TPB. An absolute value of this shift increased with concentration of hydrophobic ions and the monolay er surface density, whereas it was not affected by moderate variations of the subphase ionic strength. An effect of less hydrophobic ions (p icrate anion and tetraethylammonium cation) was much less pronounced t han that of TPP and TPB. A two-phase distribution model was proposed t o describe the behaviour of hydrophobic ions in PDMS monolayer/subphas e system. In spite of the extreme simplicity, the model appeared to de scribe rather well all experimental findings. It was shown that hydrop hobic ions are located inside the monolayer. Their surface density is low; however, a shift of the surface potential is pronounced due to th e low dielectric constant of monolayer material. (C) 1998 Elsevier Sci ence S.A. All rights reserved.