The Driving Force for the Association of Gemini Surfactants

Abstract

The self-assembly of surfactants into lyotropic liquid crystalline phases is interesting from a fundamental and practical perspective. The propensity for self-assembly is particularly interesting in Gemini surfactants which have a very low critical micelle concentration. In this work, we study the effect of headgroup identity on the driving force for the self-assembly of Gemini surfactants, using computer simulations of the potential of mean force (PMF). We find that surfactants with sulfonate headgroups have a greater tendency to assemble than those with carboxylate headgroups. The minimum in the PMF is about a factor of 2 deeper and occurs at shorter distances. Interestingly, the driving force is entropic with the carobxylate and energetic with the sulfonate headgroups. Analysis of different contributions suggests that these differences arise from surfactant headgroup electrostatics and size. The results provide an explanation for why the morphology diagram of the sulfonate surfactants is insensitive to temperature.

Clear distinction between CAC and CMC revealed by high-resolution NMR diffusometry for a series of bis-imidazolium gemini surfactants in aqueous solutions

“…Nonetheless, we should expect that some mixed composition of surfactant molecules exists in the concentration region below CC. 42 By mixed compositions we mean, monomers, dimers, trimers which undergo fast exchange between selected forms. This fast exchange effectively averages out the diffusion process in the NMR time-scale (from milliseconds to seconds).…”

“…The existence of such mixed forms such as dimers, trimers and larger aggregates of gemini surfactants has been predicted theoretically by evaluation of Potential Mean Force (PMF) between two gemini molecules. 42 The interaction between two monomers is both, energy (Δ U ) and entropy driven (− T Δ S ) although it cannot be determined unambiguously which of the contributions prevails since it is highly dependent on the chemical composition. 42 What is interesting the existence of dimeric forms was predicted even for monomeric surfactants.…”

“…42 The interaction between two monomers is both, energy (DU) and entropy driven (ÀTDS) although it cannot be determined unambiguously which of the contributions prevails since it is highly dependent on the chemical composition. 42 What is interesting the existence of dimeric forms was predicted even for monomeric surfactants. 43 Still, in this regime, a single-exponential decay is observed in the FT-PGSE data what is shown in Fig.…”

Synthesis and Properties of a Cationic Gemini Asphalt Emulsifier

“…Compared with conventional surfactants, Gemini surfactants exhibit excellent properties such as lower critical micelle concentrations (CMC), lower Krafft points, unique viscoelasticity, and so on. In recent years, Gemini surfactants have attracted considerable interest as "a new generation of surfactants" and have been widely used in sterilization, tertiary-oil recovery, and other fields (Bhadani & Singh, 2011;Jeong & Yethiraj, 2018;Rosen, 2004;Seredyuk, Alami, & Nyden, 2002;Xu, Chen, & Cui, 2011). In addition, Gemini surfactants also have potential values in the field of asphalt emulsifiers owing to their special structures (Peter & Roland, 1991;Russell, 1987;Schilling, 1992).…”

Thermodynamic Characterization of the Dimerization of an Anionic Perylene Bisimide Dye Using Molecular Simulation

“…Their self-assembly into complex structures is determined by size and shape of the monomeric unit, system composition, and thermodynamic boundary conditions. , Consequently, much attention was given to fundamental studies focusing on NMR, UV/vis, and fluorescence spectroscopy to obtain insight into the dye–dye interactions and how these interactions impact the properties of larger π–π stacked aggregates. , In contrast, the thermodynamics of self-assembly of PBIs was studied less extensively and, until recently, mainly in terms of the free energy of aggregation , but hardly with the focus on the enthalpic and entropic contributions. Recent experimental work in this direction points toward a complex dependence of the thermodynamic signature on the monomeric architecture and solvent composition. , The interplay of attraction (e.g., through electrostatic interactions) and repulsion (e.g., between water and hydrophobic groups) of the different parts of the molecule with the surrounding solvent leads to an association that can be either entropy- or enthalpy-driven depending on the nature of the molecules and the solvent. − Given this diversity of factors that influence the self-assembly and their complex interplay, studies on simplified model systems provide important guidance for the design of monomeric building blocks, whereas a molecular-level understanding of the driving forces for the concrete system under study is considered to be an important step to understand the onset of the self-assembly process. To gain such understanding molecular simulations are a promising approach .…”