Phase-dependent premelting of self-assembled phosphonic acid multilayers

Abstract

Melting and premelting phenomena in self-organized organic systems have been extensively explored in the literature, exploring distinct behaviors of different molecule lengths and morphologies. Nevertheless, the influence of the supramolecular assembly configuration on the occurrence of premelting remains poorly explored. Here we use phosphonic acids as model systems for self-organized molecular assemblies. These molecules exhibit long-range order on different types of substrates. The balance between chain-to-chain and head-to-head interactions leads to distinct types of stackings. Although their structural configurations are well understood, very little is known about their behavior near the melting transition. We show here that premelting occurs in lamellar structures and that its behavior depends directly on the ordered configuration assumed in the studied multilayers. Two molecules with different chain lengths were investigated: octadecyl phosphonic and octyl phosphonic acids. Although almost no dependence on the molecule length was observed, the occurrence of premelting is strongly influenced by their lamellar packing configuration. For tilted packings premelting is unfavored while in straight configurations, where alkyl chain interactions are weakened with respect to head-to-head interactions, strong premelting is observed. We find that the onset of premelting occurs at the domain boundaries with straight lamellar configurations and the domain sizes exhibit power law temperature dependences.

Emergence of Supramolecular Order from Combined Linear Amphiphilic and Diphosphonate Molecules

“…The simulations were carried out for coherent lamellar domains of 7 stacked bilayers (14 molecules), with a relative volume of 1:2 for bilayers with heights of 3.85 and 3.68 nm, respectively. Such a relatively reduced amount of stacked bilayers, if compared to results on phosphonic acids with other alkyl chain lengths, , is responsible for the observed broad peaks, indicating a degree of structural flexibility of TPA multilayers, which will be further explored here.…”

“…One clearly observes 3.7 nm height steps (the lower step is a multiple of this value, with a 7.3 nm height), which are equivalent to stacked bilayers, as also observed in similar molecules with alkyl chains containing 18 and 8 carbon atoms. 18,21,29 The profile extracted from the DdPA AFM images, shown in Figure 2b, suggests an amorphous or poorly organized molecular clustering. Among all DdPA samples, no steplike or terracelike feature was retrieved by AFM analysis.…”

“…Since only the momentum transfer vector along the substrate normal direction is varied in our measurements and the investigated multilayer systems are organized as lamellar stacks, only the z -projection of the molecule electron density profile is necessary to reproduce the observed diffraction features. , If two or more supramolecular configurations coexist for a given molecular mixing/dilution, the final intensity in this model is obtained by the incoherent sum of intensities for all coexisting periodicities, each one with a relative volume fraction. Similar procedures were successfully employed for the description of lamellar molecular systems, explored in refs , , , and .…”

Polymer Lamellae as Reaction Intermediates in the Formation of Copper Nanospheres as Evidenced by In Situ X‐ray Studies

“…First of all, we excluded the formation of supramolecular assemblies of the alkyl phosphonic acids alone [57] by collecting XRD patterns of TDPA in either TOA or octadecene (ODE) at different temperatures ( Figure S11). As the resulting patterns did not match with the lamellar peaks, we concluded that Cu must be involved in the structure, and that the lamellae must be a coordination polymer.…”

Chemical Stabilization and Improved Thermal Resilience of Molecular Arrangements: Possible Formation of a Surface Network of Bonds by Multiple Pulse Atomic Layer Deposition

“…We know from previous studies that OPA bilayers remain ordered up to ∼100 °C. 22,34 From room temperature up to approximately 75 °C, three lamellar configurations coexist, each one with a particular tilt with respect to the surface-normal direction. These configurations were thoroughly studied in ref 22 and are schematically represented in Figure 4a.…”