Hydration of Dimethyldodecylamine-N-oxide:  Enthalpy and Entropy Driven Processes

Abstract

Dimethyldodecylamine-N-oxide (DDAO) has only one polar atom that is able to interact with water. Still, this surfactant shows very hydrophilic properties: in mixtures with water, it forms normal liquid crystalline phases and micelles. Moreover, there is data in the literature indicating that the hydration of this surfactant is driven by enthalpy while other studies show that hydration of surfactants and lipids typically is driven by entropy. Sorption calorimetry allows resolving enthalpic and entropic contributions to the free energy of hydration at constant temperature and thus directly determines the driving forces of hydration. The results of the present sorption calorimetric study show that the hydration of liquid crystalline phases of DDAO is driven by entropy, except for the hydration of the liquid crystalline lamellar phase which is co-driven by enthalpy. The exothermic heat effect of the hydration of the lamellar phase arises from formation of strong hydrogen bonds between DDAO and water. Another issue is the driving forces of the phase transitions caused by the hydration. The sorption calorimetric results show that the transitions from the lamellar to cubic and from the cubic to the hexagonal phase are driven by enthalpy. Transitions from solid phases to the liquid crystalline lamellar phase are entropically driven, while the formation of the monohydrate from the dry surfactant is driven by enthalpy. The driving forces of the transition from the hexagonal phase to the isotropic solution are close to zero. These sorption calorimetric results are in good agreement with the analysis of the binary phase diagram based on the van der Waals differential equation. The phase diagram of the DDAO-water system determined using DSC and sorption calorimetry is presented.

The interaction of hybrid nanoparticles inserted within surfactant bilayers

“…The membranes were composed of dimethyldodecylamine-N-oxide (DDAO), a singlechain zwitterionic surfactant. The DDAO (purchased from Sigma-Aldrich) was first dried in vacuum for 20 h to remove any residual water 26 and then dissolved in isopropanol.…”

“…The two stock solutions were then mixed to yield the desired BuSn12/DDAO ratio and the mixtures dried in vacuum, yielding a final mass of about 200 mg for each sample. Varying amounts of water were then added so that the mixtures were in the fluid lamellar L α phase 26 . For DDAO, the molecular weight is 229.40, the density is 0.84 g/cm 3 and the bilayer thickness is 25 ± 1 Å27,28 , resulting in an area per surfactant molecule A DDAO = 37.8…”

Acid–Base Equilibrium of Dodecyldimethyl‐Amine‐N‐Oxide Micelles in Water–Butanol Binary at 298 K

“…The change of medium [58], even if the introduced quantity of butanol remains small, will affect the polar heads, because the alcohol can partly replace the water molecules of hydration [59,60]; this will have the effect of diminishing the value of the dielectric constant at the micelle, and also affect the structure of the interfacial middle layer. Fig.…”

Repulsion Between Inorganic Particles Inserted Within Surfactant Bilayers

“…The zwitterionic surfactant, dimethyldodecylamine-Noxide (DDAO) was purchased from Sigma-Aldrich and dried in vacuum (using a liquid nitrogen-cooled solvent trap) for 20 h. No weight loss was observed after this step, so we conclude that the surfactant was dry as supplied (see [9] for a detailed discussion). DDAO was then dissolved in isopropanol at a concentration of 23.02 wt.%.…”

“…We mixed the BuSn12 and DDAO solutions to yield the desired particle concentrations, and then dried the mixtures in vacuum; the final (dry) mass was about 200 mg for each sample. We then added water at a concentration of 15-27 wt.% of the final hydrated mixtures, which are thus in the fluid lamellar L α phase (see the phase diagram of the undoped system in [9]). The molecular weight of DDAO is 229.40 (Sigma-Aldrich), its density is 0.84g/cm 3 and the thickness of the bilayer is 25 ± 1 Å [10], yielding an area per surfactant molecule A DDAO = 37.8 Å2 .…”