Correlating Linactant Efficiency and Self-Assembly: Structural Basis of Line Activity in Molecular Monolayers

Abstract

Surfactants exhibit characteristic phenomena, including the reduction of interfacial free energy, self-assembly into aggregates, and even the formation of lyotropic liquid crystalline phases at high concentrations. Our research has shown that a semifluorinated phosphonic acid can act as the two-dimensional analogue of a surfactant-a linactant-by reducing the line tension between hydrocarbon-rich and fluorocarbon-rich phases in a Langmuir monolayer. This linactant can also self-assemble into nanoscale clusters in a monolayer. Here, we explore the dependence of linactant behavior on molecular structure. Members of a homologous series of partially fluorinated phosphonic acids were synthesized and tested as linactants: CF(3)(CF(2))(n-1)(CH(2))(m)PO(3)H (abbreviated as FnHmPO(3)). The tests revealed that linactants with longer hydrophobic chains were most efficient in lowering line tension. For linactants with the same overall chain length, the length of the fluorocarbon block was correlated with efficiency. Thus, the linactant efficiency was ranked in the order F8H8PO(3) < F10H6PO(3) < F8H11PO(3) < F10H9PO(3). In all cases, linactant-containing Langmuir-Blodgett monolayers exhibited nanoscale molecular clusters with characteristic dimensions of 20-30 nm; enhanced linactant efficiency was systematically correlated with larger clusters.

Size, Shape, and Lateral Correlation of Highly Uniform, Mesoscopic, Self‐Assembled Domains of Fluorocarbon–Hydrocarbon Diblocks at the Air/Water Interface: A GISAXS Study

“…[12][13][14] However, an antiparallel conformation for FnHm diblocks [15] and the formation of as mecticb ilayer [16] have been proposed, too. Disk-like and striped surface domains were also reportedf or partially fluorinated carboxylic acids [17] and partially fluorinated diether glycerols, [18] respectively.S urfaced omains of variouss hapes were also published for combinations of fluorinated amphiphiles (phosphates, diethyl glycerols) with hydrocarbon and fluorocarbon oils [19] or phospholipids. [20][21] Since FnHm domains can be transferred onto solid substrates even at av ery low surface pressure p % 0mn m À1 , [22] it was suggested that the self-assembly of the FnHm is dependent on surface concentration but not on surface pressure.…”

Stacking of Self‐Assembled Surface Micelles in Ultrathin Films

“…[7,9,10] Similarly sized surface micelles have been observed in monolayers of amphiphilic perfluoroalkylated carboxylic acids, when contacted with a multivalent cation-loaded subphase, [6] and of partially fluorinated phosphates. [26] We have recently reported the formation of vertically stratified films made from FnHm diblocks that comprise a lower monolayer of surface micelles surmounted by a bilayer of the same diblocks. [40] To the best of our knowledge, there is no report of layered stacks of discrete self-assembled nano-objects such as surface micelles.…”

“…These features endow FnHm diblocks with distinctive behavior in both the solid (e.g. [26] We have recently reported the formation of vertically stratified films made from FnHm diblocks that comprise a lower monolayer of surface micelles surmounted by a bilayer of the same diblocks. surface freezing), as well as in solutions (micelle formation in both hydrocarbons and fluorocarbons) and at interfaces.…”

Nucleation of Surfactant–Alkane Mixed Solid Monolayer and Bilayer Domains at the Air–Water Interface

“…In order to create such a small domain, it is necessary to reduce the line tension term by a considerable extent. According to this line, similar to surfactants in 3D systems, it has been proposed that some type of molecule acts as a line-active agent (linactant) which stabilizes 2D domains [26,27]. Studying the effect of linactants on the domain morphology of surfactant-alkane mixed monolayers is therefore an interesting future extension of the present work.…”