Eric B. Sirota

Rotator phases of the normal alkanes: An x‐ray scattering study

We present results of a detailed x‐ray scattering study on the rotator phases of normal alkanes: CH3–(CH2)n−2–CH3 (20≤n≤33). We have characterized a new tilted rotator phase and determined the temperature and chain length dependence of the distortion, tilt, and azimuthal order parameters which characterize the time‐space averaged structures of the five rotator phases. We have shown that there is no strong even–odd chain length effect on the phase diagram within the rotator phases and have shown the continuity of that phase diagram in the 26‐27 carbon vicinity.

X‐ray structural studies of freely suspended ordered hydrated DMPC multimembrane films

We have performed x‐ray scattering studies of freely suspended, thick (∼2μ), oriented dimyristoyl–phosphatidyl–choline (DMPC)–water multilamellar films with the emphasis on the ordered phases. These stable films were drawn in situ on a triple axis x‐ray spectrometer at a controlled, continuously variable temperature (T) and relative humidity (RH). The structure was determined at several temperatures and humidities and a T–RH phase diagram was constructed. We have found that the previously labeled ordered Lβ’ phase is in fact three distinct two‐dimensional phases differentiated by the direction of chain tilt with respect to the in‐plane lattice. The line shapes of the scattering data indicate that a lower limit of ∼200 A may be placed on the in‐plane chain–chain correlation length, while the chain positions are uncorrelated across the water layers.

The rotator phases of neat and hydrated 1‐alcohols

We report an x‐ray scattering study of the rotator phases in both dry and hydrated 1‐alcohols CH3–(CH2)n−1–OH(12≤n≤26). Hydration was accomplished by using both fixed composition and controlled relative humidity. A hexagonal phase and two tilted phases are reported in the temperature versus chain length and temperature versus humidity phase diagrams. Hydration tends to drive the system from one tilted rotator state to the other, and then to the untilted hexagonal phase. The hexagonal phase shows a six‐layer periodicity illustrating the crossover between trilayer alkane behavior and bilayer surfactant behavior. Hydration and chain‐length mixing increase the temperature range of the rotator phase. The rotator phase will take in about 1 H2O per 2 alcohols which is twice the saturated water content of the liquid phase. This is shown to explain the unusual rise in the melting temperature with hydration.

STRUCTURAL EFFECTS OF HIGH PRESSURE GAS ON THE ROTATOR PHASES OF NORMAL ALKANES

The effects of high pressure gases (P≤400 bar) on the RII, RI, and RV rotator phases of 21, 23, and 25 carbon normal alkanes were studied via x‐ray scattering. We have measured the pressure and temperature dependence of the rotator structures and present these results in terms of the essential structural parameters: layer spacing, area per molecule, lattice distortion, and tilt. The pressure was generated by one of three gases: helium, nitrogen, or argon. In the rotator phases, argon and nitrogen intercalate between the layers while helium acts mostly as a noninteracting pressurizing medium. The thermal expansion and compressibility are anomalously large in the rotator phases, and this implies that the heat capacity in the rotator phases is dominated by anharmonic effects.

A vanishing nucleation barrier for the n-alkane rotator-to-crystal transformation

Alkanes exhibit intermediate plastic–crystalline rotator phases between the liquid and the low-temperature fully ordered crystal. Here we report measurements of the liquid-to-rotator and rotator-to-crystal homogeneous nucleation kinetics using x-ray scattering on emulsified samples. These are the first reported studies of rotator-to-crystal homogeneous nucleation, and they show an anomalous temperature dependence of nucleation. Through use of mixed alkanes, we probe this over a wide temperature range. For three compositions along the C19H40–C20H42 binary, we find that the liquid-to-rotator interfacial free energy, calculated from the nucleation barrier in classical nucleation theory, is the same for all samples. However, the rotator-to-crystal interfacial free energy is both considerably smaller and strongly dependent on composition and temperature. As the temperature range of the rotator phase widens due to chain mixing, both the thermodynamic transition temperature as well as the interfacial energy for ...

Supercooling and transient phase induced nucleation in n-alkane solutions

The supercooling exhibited by bulk solutions of the n-alkane tricosane (C23) in dodecane (C12) is measured as a function of concentration using adiabatic scanning calorimetry. For small dilutions where crystallization occurs into the rotator phase, negligible supercooling is observed. For larger dilutions where crystallization is into the herringbone crystal phase, nucleation occurs at the temperature where the rotator phase becomes stable with respect to the liquid. The supercooling in bulk n-alkane solutions is thus determined by the metastable phase diagram. This is a novel, but possibly often occurring mechanism where nucleation is induced by a transient metastable phase.

X‐ray Couette shear cell for nonequilibrium structural studies of complex fluids under flow

We have designed and constructed a sealed, temperature‐controlled Couette‐type shear cell which allows us to carry out in situ synchrotron x‐ray scattering studies of the nonequilibrium structures of complex fluids under flow, probing lengths ranging from the nanometer to the semimacroscopic micron scale. The sealed design allows the cell to be tilted, facilitating the scanning of all directions in reciprocal space. In contrast to existing neutron shear cells, we access the shear plane containing the velocity and the velocity gradient. The temperature control of the cell enables us to carry out phase transition studies far from equilibrium. The complex fluid systems which may be studied with this cell include thermotropic and lyotropic liquid crystals, microemulsions, and polymeric fluids. Recent results on the nematic and smectic‐A phases of a liquid crystal, and the disordered bicontinuous L3 phases of surfactant membranes in the vicinity of the ordered lamellar Lα phase under flow, are presented.

An x-ray extensional flow cell

We have designed an x-ray extensional flow cell based on the crossed-slot geometry that is compatible with x-ray diffraction. Extensional flow within the cell was both confirmed and characterized by examining the trajectories of small latex spheres suspended in water via video microscopy. The cell was then used to examine extensional-flow-induced structural changes in a well-known lamellar surfactant system, and the alignment of the surfactant domains was observed to increase with increasing extension rate.

An in situ method for observing wax crystallization under pipe flow

We report on an x-ray diffraction investigation of the nucleation and growth of wax crystals and the evolution of the resulting wax-gel deposit in mixtures of paraffin wax and dodecane under pipe flow. This work is an in situ x-ray diffraction study of wax crystallization in oil-wax mixtures under flow and is completely noninvasive. This technique is shown to be consistent with other methods.

Surface and bulk phase behavior of dry and hydrated tetradecanol:octadecanol alcohol mixtures

Surface freezing was studied in dry and hydrated octadecanol:tetradecanol (C18OH:C14OH) mixtures, using surface tension and synchrotron x-ray surface diffraction techniques. Even small amounts of admixed C18OH were found to induce surface freezing in C14OH, which does not exhibit this effect when pure. The phase diagram of the bulk was measured by calorimetry and bulk x-ray diffraction. Upon increasing the bulk mole fraction of C18OH (φ) a sharp increase in the bulk supercooling occurs at φ≈0.4 in dry mixtures, while no supercooling was observed for the hydrated mixtures. A simple thermodynamical model based on the theory of s-regular mixtures is shown to account well for the dependence of the surface freezing onset temperature of both dry and hydrated mixtures, and the hydrated bulk’s freezing temperature on φ. Only a phenomenological description exists for the dry bulk’s phase diagram. This study is expected to provide a baseline for the general surface and bulk behavior of long-chain alcohol mixtures.