Colloids and Surfaces A: Physicochem. Eng. Aspects 289 (2006) 250–253 Brief notes Fatty-acid monolayers and metal ions: First step towards a new look Sarathi Kundu ∗ , Alokmay Datta Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India Received 19 May 2006; received in revised form 14 June 2006; accepted 1 July 2006 Available online 7 July 2006 Abstract We have studied the surface pressure versus specific molecular area (π–A) isotherms of heneicosanoic acid Langmuir monolayers, varying the pH and concentration of Cd-ions in the aqueous subphase. We have measured the discontinuities in A (A) at the L 2 -L ′ 2 phase transition and in κ, the isothermal compressibility, i.e., κ at the L ′ 2 -S phase transition, and have shown that these have distinct zeroes at specific values of pH, which depend on the Cd-ion concentration. From these we have constructed the pCd–pH phase diagram, where pCd = -log 10 [Cd], [Cd] being the molar concentration of Cd-ions. This phase diagram clearly shows that this complex two-dimensional system has three distinct behavioral regions. © 2006 Elsevier B.V. All rights reserved. Keywords: Langmuir monolayer; Fatty acid; Subphase pH; Cd-ion 1. Introduction It is well known that below a certain temperature, which increases with the tail length, hydrocarbon tails in Langmuir monolayers of fatty acid can exist in two tilted phases, L 2h and L ′ 2h . The ‘h’ in the subscript stands for ‘herringbone order’ among the planes containing the tails in all-trans configurations, whereas L 2 and L ′ 2 refers to the tilt being towards a nearest or a next-nearest neighbor, respectively [1]. These two tilted phases arise due to a mismatch in cross-section between the acid head-group and the hydrocarbon tail [2] and above a cer- tain surface pressure π (measured as γ 0 - γ , where γ 0 (γ ) is the surface tension of bare (monolayer-covered) water) a first-order transition takes place from L 2h phase to L ′ 2h phase through a finite discontinuity (A) in the specific molecular area (A) in the monolayer, while the isothermal compressibility (κ), given by κ = -1/A(∂A/∂π) T , remains unchanged [3]. At higher π, there is another phase-transition from L ′ 2h to the S phase with untilted chains and herringbone order among chain-planes. This is a sec- ond order transition, with a finite discontinuity in κ [3]. These phase transitions form a part of the elaborate π–T phase diagram of fatty-acid Langmuir monolayers that has been constructed from π–A isotherms, X-ray diffraction studies and optical inves- tigations on fatty-acids of different tail lengths [1]. ∗ Corresponding author. Tel.: +91 33 2337 5345; fax: +91 33 2337 4637. E-mail address: sarathi.kundu@saha.ac.in (S. Kundu). What happens when other external parameters are varied? The two most common parameters are the pH of the aqueous sub- phase and the concentration of metal ions, in particular, divalent metal ions in this subphase [4]. The conventional way to look at this situation is to say that under increasing pH more head- groups dissociate, i.e., become COO - from COOH and these dissociated headgroups interact with the metal ions to form salts [5]. In other words the entire process is a formation of a Lang- muir monolayer of ‘salt’ from a Langmuir monolayer of ‘acid’. However, there are a number of experimental evidences against this simple picture. To begin with, in almost all cases the metal ion is added as a dilute salt solution underneath the fatty-acid in the mono- layer [6]. On top of that, most of these divalent metals (Cd, Pb, Mn) are considerably electronegative [7]. Under these circum- stances salt formation is nearly impossible in the bulk, whereas bonds between the carboxylate group and metals have actually been observed when Langmuir–Blodgett films are formed [8] from these monolayers on water surface, and each metal ion is seen to bond with two headgroups [9]. On the other hand, in the Langmuir monolayer one finds two-dimensional lattices of metal ions underneath the air–water interface [10] and these lat- tices evolve in structure with pH [11], ion concentration [12] and time [13]. Also, in general each metal ion is associated with one headgroup [14] and considerable molecular self-assembly takes place during formation of LB film from Langmuir monolayer [15]. Again, when an actual amphiphilic salt of a multivalent metal and a long-chain fatty acid is preformed and spread on 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.07.001