Long-chain alkyl thiols in Langmuir monolayers

A series of homologous alkyl thiols C2H2n+1SH (n = 44-18, 20, 22) have been synthesized and Langmuir monolayers have been thoroughly characterized using Surface pressure, electric surface potential measurements and Brewster angle microscopy. Although the -SH group cannot form strong hydrogen bonding with the water subphase, thereby causing weak anchoring, it has been found that alkyl thiols having more than 15 carbon atoms are capable of Langmuir monolayer formation at room temperature. The stability of the investigated films is not very high, however, but quite sufficient for LB transfer at low surface pressures. The increase in aliphatic chain length does not lead to any considerable improvement of the monolayer stability. The influence of subphase temperature and the presence of various cations in the aqueous subphase have also been studied. Contrary to literature, it has been found that the presence of alkaline earth metals cations does not improve the monolayer stability, and although the collapse pressure is higher, the equilibrium surface pressure remains the same as on pure water. (C) 2009 Elsevier Inc. All rights reserved.