Although the transition between a bilayer and an interdigitated membrane of a surfactant and lipid has been widely known for long, its mechanism remains unclear. This study reveals the transition mechanism of a cationic surfactant, dioctadecyldimethylammonium chloride (DODAC), through experiments and theoretical calculations. Experimentally, the transition from the interdigitated to bilayer structure in the gel phase of DODAC is found to be induced by adding hydrophobic molecules such as n-alkane and its derivatives. Further addition induces a different transition to another bilayer phase. Our theory, considering the competition of the electrostatic interaction between cationic headgroups and the hydrophobic interaction emerging at the alkyl-chain ends exposed to water, reproduces these two phase transitions. In addition, changes in alkyl-chain packing in the membranes at these transitions are reproduced. The underlying mechanism is that the interdigitated membrane is formed at a small additive content due to electrostatic repulsion. As the energetic disadvantage with respect to the hydrophobic interaction becomes dominant as the content increases, the transition to the bilayer occurs at a specific content. The bilayer-bilayer transition at higher content is induced by the change in the balance of these interactions. Based on a similar concept, we suggest the mechanism of the additive-induced bilayer-interdigitated transition of phospholipids, i.e., neutrally charged surfactants.
(Langmuir, 36, 14699-14709 (2020))
Heat capacities of the binary systems consisting of a non-ionic surfactant C16E8 and water were precisely
measured as a function of temperature by adiabatic calorimetry over the
temperature and the concentration ranges where lyotropic liquid
crystals are formed. The enthalpy and entropy of transitions were
determined for all known phase transitions observed. Comparison
of the present result and the previous one on the C12E6–water
system suggests that the higher-order structure in the liquid
crystalline phases in these systems be mainly constructed by surfactant
molecules with a fixed amount of water. The excess heat
capacities, as estimated by measuring the heat capacity of neat C16E8,
are positive over the whole temperature and concentration ranges.
Properties of lyotropic and thermotropic systems are compared briefly,
while paying attention
on the geometries of surfaces characterizing the aggregation (triply
periodic
minimal surface for cubic phases and flat surfaces for lamellar and
smectic
phases).
(J. Phys. Chem. B, 107,
7854-7860 (2003))
The heat capacities of the binary system consisting of a non-ionic
surfactant C12E6 and water were precisely
measured as a function of temperature by adiabatic calorimetry over the
temperature and the concentration ranges where lyotropic liquid
crystals are formed. The enthalpy and entropy of transitions were
determined for all transitions observed. The enthalpy and entropy of
transition between liquid crystalline phases suggest that the higher
order structure in the liquid crystalline phases in this system is
mainly constructed by C12E6 molecules with a
fixed amount of water. The excess heat capacities, as estimated by
measuring the heat capacity of neat C12E6, is
positive over the whole temperature and concentration ranges. The
excess heat capacities also support the suggestion given above
concerning the role of C12E6 molecules on the
structure building.
(J. Phys. Chem. B, 105, 2987-2992 (2001))