Critical evaluation of models for self-assembly of short and medium chain-length surfactants in aqueous solutions

During numerous visits to our Laboratory professor Johannes (Hans) Lyklema emphasized the importance of a holistic view on thermodynamics. In order to fulfill this aim he assembled the monumental Fundamentals of Interface and Colloid Science series. The basic state functions (internal energy, enthalpy and free energies) are interrelated by Gibbs and Helmholtz relationships. First-order phase transitions are characterized by first-order state variables (temperature, pressure, entropy, volume). Interactions are, however best expressed by second-order partial derivatives (compressibility, heat capacity and expansivity). They are related to the first-order state variables by relaxation contributions quantifying the degree of cooperativity of self-assembly processes leading to phase separation. In particular they exhibit the limit when phase transitions are changed to second-order processes. This was the focus of my first review dedicated to the memory of professor Lyklema, "Characterization of van der Waals type bimodal,- lambda,- meta- and spinodal phase transitions in liquid mixtures, solid suspensions and thin films" (ACIS 253 (2018) 66). In the present review the attention is placed on short and medium chain-length surfactant self-assembly in aqueous solutions without additives (salts or solubilizates). The dependence of state functions described above on concentration, temperature and pressure is compared to corresponding dynamic molecular processes occurring on different time, frequency and length scales including structure analysis. It is convincingly shown that Hartley-Tanford space filled spherical anhydrous micelle core - polar shell model designed for long chain-length surfactants (cmc < 0.01 mol/dm³, N > 50) cannot be enforced on short and medium chain-length surfactant non-sperical micelles (cmc close to unity, N < 20). Moreover, it is shown that a proper validity evaluation of proposed models for micelle formation is seriously undermined by their application to only a narrow concentration range near critical micelle concentration (cmc). When successful each model should characterize all self-assembly processes occurring (also at limiting association concentration, lac, at second critical concentration, 2cc and at third critical concen-tration, 3cc) within the entire concentration range of thermodynamically stable surfactant solutions. All other self-assembly processes except micelle formation are rarely considered. The pre-micelle formation at lac is, for example omitted as deviations from presented models. The reviewed reports are therefore selected on the basis of maximum investigated concentration range and of largest possible number of homologues.

Self-assembly of flat nanodiscs in salt-free catanionic surfactant solutions

Discs of finite size are a very rare form of stable surfactant self-assembly. It is shown that mixing of two oppositely charged single-chain surfactants can produce rigid nanodiscs as well as swollen lamellar liquid crystals with frozen bilayers. The crucial requirement for obtaining nanodisc self-assembly is the use of H+ and OH- as counterions. These counterions then form water and lower the conductivity to 10 microsiemens per centimeter. In the case of cationic component excess, a dilute solution of nanodiscs is in thermodynamic equilibrium with a lamellar phase. The diameter of the cationic nanodiscs is continuously adjustable from a few micrometers to 30 nanometers, with the positive charge located mainly around the edges.

A 31P- and 2H-NMR study on lecithins in liquid crystalline polyoxyethylene detergents

Phosphatidylcholines were incorporated into hexagonal liquid crystalline mixtures of the non-ionic detergents Triton X-100 and octaethyleneglycoldodecylether with D2O. It is shown by nuclear magnetic resonance (NMR) that the phospholipids adopt the hexagonal liquid crystalline structure of the detergent host lattice. The anisotropic motion of the phospholipid headgroups seems to be unaffected, whereas the acyl chains are disordered. Increasing phospholipid concentration leads to separation of a lamellar phase. The lamellar structure is also preferred at elevated temperatures. Phosphatidylcholines with saturated acyl chains undergo a transition from the hexagonal liquid crystalline to an ordered lamellar state. The shape of the 31P-NMR signals suggests that pure gel phase phospholipid separates out. The headgroup region of this gel phase phospholipid becomes immobilized after a few weeks of storage below the transition temperature as judged from 31P-NMR. At the same time 2H-NMR exhibits a new signal from D2O undergoing slow isotropic motion. This behavior bears resemblance to the formation of a coagel in fatty acid-water systems.