Added-Value Surfactants

Abstract

Surfactants are ubiquitous in cellular membranes, detergents or as emulsification agents. Due to their amphiphilic properties, they cannot only mediate between two domains of very different solvent compatibility like water and organic but also show fascinating self-assembly features resulting in micelles, vesicles, or lyotropic liquid crystals. The current review article highlights some approaches towards the next generation surfactants, for example, those with catalytically active heads. Furthermore, it is shown that amphiphilic properties can be obtained beyond the classical hydrophobic-hydrophilic interplay, for instance with surfactants containing one molecular block with a special shape. Whereas, classical surfactants are static, researchers have become more interested in species that are able to change their properties depending on external triggers. The article discusses examples for surfactants sensitive to chemical (e.g., pH value) or physical triggers (temperature, electric and magnetic fields).

Keywords: magnetic surfactants; metallosurfactants; self-assembly; shape amphiphiles; surfactant-combined catalysts.

Figure 1

Example for amphiphilic self‐assembly in the absence of water: LLC‐formation (b) of a modified NDI‐based chromophore (a) in mixtures of chloroform/methylcyclohexane. Reprinted with permission from ref. 25; copyright 2011 American Chemical Society.

Figure 2

Molecular structure and appearance of aggregates formed by a disc‐bent core shape amphiphile presented by Gu et al. and formation mechanism of spiral superstructures.29 Reprinted with permission from ref. 29; copyright 2017 The Royal Society of Chemistry.

Figure 3

Molecular structure and self‐assembly of the Pd‐pincer amphiphile used for the catalytic Miyaura–Michael reaction. Reproduced from with permission ref. 41a; copyright 2011 Wiley‐VCH.

Figure 4

pH value dependent self‐assembly of the surfactant N‐dodecyl‐1,3‐diaminopropane showing the transition in aggregate shape from spherical micelles to rod‐like micelles and vesicles. Schematic representation (a) and TEM images (b, c, d, e). Reprinted with permission from ref. 45; copyright 2010 Wiley.

Figure 5

Variation of the head group charge without changing the surfactant shape.52 (a) Structure and composition of the POM‐based surfactants. Unusual bipolar micelles observed for highly charged compounds, Scheme (b) and TEM image (c). Model for interaction responsible for the unusual structuration processes (d).

Figure 6

(a) Oxidation and reduction of the ferrocene (Fc) unit and (b) Cleavage of disulfide and diselenide moieties upon reduction.

Figure 7

Cyclodextrin/ferrocene based surfactant (a) and oxidation state dependent self‐assembly (b). Reproduced with permission from ref. 53; copyright (2017) Royal Society of Chemistry.

Figure 8

The double thermoresponsive di‐block amphiphile and its aggregates formed above LCST 1 and LCST 2. Reprinted with permission from ref. 69e; copyright (2012) American Chemical Society.

Figure 9

Thermo‐induced opening and closing of capsules formed by self‐assembly of a stimuli‐responsive amphiphile. Reprinted with permission from ref. 70; copyright (2008) Wiley‐VCH.

Figure 10

Optical polarized microscopy images, photographs and in situ rheological measurements showing the LLC→micelle transition on an azobenzene‐containing surfactant triggered by light radiation. Reproduced with permission from ref. 76; copyright (2014) American Chemical Society.

Figure 11

Charging dynamics and deformation of vesicles in a constant electric field. (a) $t<{\tau }_c$ and $\chi >1$ ; (b) $t<{\tau }_c$ and $\chi <1;$ and (c) $t\gg {\tau }_c$ . The subsequent deformation is displayed with the dashed lines. Reproduced with permission from ref. 82; copyright (2014) Royal Society of Chemistry.

Figure 12

Squaring of vesicles in presence of ions. Between the prolate‐oblate transitions, short‐lived cylindrical shapes can show up. Reproduced with permission from ref. 89; copyright (2015) Royal Society of Chemistry.

Figure 13

Morphology diagram of vesicles in ac fields with a magnitude of $0.2kV/cm$ . The conductivity ration $\chi ={\sigma }_{in}/{\sigma }_{ex}$ is denoted on the vertical axis and the ac field frequency on the horizontal axis. Four distinct shape transitions can be identified (1–4). (B) Example of images achieved by phase‐contrast microscopy at $\chi =0.5$ , the scale bar is $10\mu \mathrm{m}$ . Reproduced with permission from ref. 90; copyright (2010) American Chemical Society.

Figure 14

(a) Synthesis of DOTA‐based, magnetic surfactants.96b (b) Structure of the surfactant after metal cation (M²⁺) coordination. (c) Influence of the head's magnetic moment on surface tension and micellar aggregate size. (d) Aggregates formed by a surfactant containing Dy³⁺ oriented in a magnetic field.96a