Structural transformations of even-numbered n-alkanes confined in mesopores

Instabilities, thermal fluctuations, defects and dislocations in the crystal-RI-RII rotator phase transitions of n-alkanes

The theoretical study of instabilities, thermal fluctuations, and topological defects in the crystal-rotator-I-rotator-II (X-RI-RII) phase transitions of n-alkanes has been conducted. First, we examine the nature of the RI-RII phase transition in nanoconfined alkanes. We propose that under confined conditions, the presence of quenched random orientational disorder makes the RI phase unstable. This disorder-mediated transition falls within the Imry-Ma universality class. Next, we discuss the role of thermal fluctuations in certain rotator phases, as well as the influence of dislocations on the X-RI phase transition. Our findings indicate that the herringbone order in the X-phase and the hexatic order in the RII-phase exhibit quasi-long-range characteristics. Furthermore, we find that in two dimensions, the unbinding of dislocations does not result in a disordered liquid state.

Crystallization of n-Alkanes under Anisotropic Nanoconfinement in Lipid Bilayers

Understanding crystallization behavior is integral to the design of pharmaceutical compounds for which the pharmacological properties depend on the crystal forms achieved. Very often, these crystals are based on hydrophobic molecules. One method for delivering crystal-forming hydrophobic drugs is by means of lipid nanoparticle carriers. However, so far, a characterization of the potential crystallization of fully hydrophobic molecules in a lipid environment has never been reported. In this work we investigate the crystallization behavior of two model hydrophobic chains, n-eicosane (C20) and n-triacontane (C30), in phospholipid bilayers. We combine static 2H nuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC) and show that C30 molecules can indeed crystallize inside DMPC and POPC bilayers. The phase transition temperatures of C30 are slightly reduced inside DMPC, and rotator phase formation becomes a two-step process: Preorganized n-alkane chains assemble in rotator-phase crystallites just as fast as bulk C30, but further addition of molecules is notably slower. Under the same isothermal conditions, different crystal forms can be obtained by crystallization in the membrane and in bulk. In excess water conditions, homogeneous nucleation of C30 is observed. The initial anisotropic molecular arrangement of C30 molecules in the membrane is readily recovered upon reheating, showing reversibility. The shorter C20 molecules on the other hand become trapped in the DMPC membrane gel-phase upon cooling and do not crystallize. This work marks the first observation of the crystallization of hydrophobic chains inside a lipid bilayer environment. As such, it defines a fundamental starting point for studying the crystallization characteristics of various hydrophobic molecules in lipid membranes.

Dissecting the Chain Length Effect on Separation of Alkane-in-Water Emulsions with Superwetting Microchannels

Oil/water separation is an essential process in the petrochemical industry, environmental remediation, and water treatment. Alkanes are the major components of crude oil and are difficult to separate once they form emulsions in water. Much less attention has been focused on the feature of liquid alkanes that could, in turn, influence the separation process. The role of chain length is systematically studied herein by separating the alkane-in-water emulsions with superwetting titanium microchannels of 14-55 μm. The chain length covers the entire liquid alkane spectrum with carbon numbers ranging from 6 to 16. The separation efficiency decreases while the TOC content increases with the chain length of liquid alkanes for a given channel. This is attributed to the small Ostwald ripening rate with the long chains, which stabilize the oil droplets of small sizes that could pass through the zigzag channels. Accordingly, a high separation efficiency of >99.97% and a low TOC content of <5 ppm are achieved with superhydrophilic channels of 14 μm for alkanes with less than 12 carbons. The metallic microchannels surpass the conventional organic membranes and inorganic frameworks over the entire liquid n-alkane spectrum, paving the way for the future development of oil/water separation using porous metals.

Confined Crystallization and Melting Behaviors of 3-Pentadecylphenol in Anodic Alumina Oxide Nanopores

To explore the effects of end groups on the confined crystallization of an alkyl chain, 3-pentadecylphenol (PDP) was infiltrated into the anodic aluminum oxide template (AAO) to investigate the melting and crystallization behaviors of PDP in a nanoconfined environment. Wide-angle X-ray diffraction (WAXD) found that the solid-solid phase transition of PDP occurred under confined conditions, and the absence of the (00L) reflections indicated that the stacking of the end groups of the alkyl chain layered structure was seriously disturbed. Thermal analysis (TG) showed that the thermal stability of the confined samples decreased due to the confinement effect, and the introduction of end groups made the confinement effect more obvious. Differential scanning calorimeter (DSC) results well reflected the space-time equivalence in the PDP crystallization processes, i.e., the solid-solid phase transition can be achieved by reducing the cooling rate or confining PDP in the nanometer space. Compared with C15, the introduction of the end groups with a phenol ring led to the disappearance of the solid-solid phase transition of an alkyl chain at high cooling rates. In the confined environment, the introduction of the end groups with a phenol ring caused the melting double peaks of the alkyl chain to become a single melting peak, and it also caused the disappearance of the surface freezing monolayer for alkyl chains. Through the analysis of crystallinity, it was found that AAO-PDP was more sensitive to AAO pore size changes than AAO-C15, the X c of AAO-PDP had a good linear relationship with the pore size d, but the X c of the AAO-C15 had a nonlinear relationship with the pore size d. Attenuated total reflection (ATR)-IR proved that in the confined environment, the order of the alkyl chain decreased and the degree of chain distortion increased.

Nanocrystallite-liquid phase transition in porous matrices with chemically functionalized surfaces

Nanocrystallite-liquid phase transitions are studied for 1-octadecene confined in the pores of chemically functionalized silica gels. These silica gels possess similar fractal geometries of the pore system but differ in chemical termination of the surface, specific surface area (F) and pore volume (V). Linear dependencies of the melting temperature and specific melting heat on the F/V ratio are found for a series of silica gels with identical surface termination. A thermodynamic model based on experimental data is established, which explains the observed shift of the phase transition parameters for porous matrices with different surface chemistries. In addition, this model allows evaluation of actual changes in nanocrystallite density, surface tension and entropy upon melting.

Rotator phases in alkane systems: In bulk, surface layers and micro/nano-confinements

Medium- and long-chain alkanes and their mixtures possess a remarkable physical property - they form intermediate structured phases between their isotropic liquid phase and their fully ordered crystal phase. These intermediate phases are called "rotator phases" or "plastic phases" (soft solids) because the incorporated alkane molecules possess a long-range positional order while preserving certain mobility to rotate, which results in complex visco-plastic rheological behaviour. The current article presents a brief overview of our current understanding of the main phenomena involved in the formation of rotator phases from single alkanes and their mixtures. In bulk, five rotator phases with different structures were identified and studied in detail. Along with the thermodynamically stable rotator phases, metastable and transient (short living) rotator phases were observed. Bulk rotator phases provided important information about several interfacial phenomena of high scientific interest, such as the energy of crystal nucleation, entropy and enthalpy of alkane freezing, interfacial energy between a crystal and its melt, etc. In alkane mixtures, the region of existence of rotator phases increases significantly, reflecting the disturbed packing of different molecules. All these phenomena are very important in the context of alkane applications as lubricants, in cosmetics, as phase-change materials for energy storage, etc. Significant expansion of the domain of rotator phases was observed also in confinements - in the pores of solid materials impregnated with alkanes, in polymeric microcapsules containing alkanes, and in micrometer sized emulsion droplets. The rotator phases were invoked to explain the mechanisms of two recently discovered phenomena in cooled alkane-in-water emulsions - the spontaneous "self-shaping" and the spontaneous "self-bursting" (fragmentation) of emulsion drops. The so-called "α-phases" formed by fatty acids and alcohols, and the "gel phase" formed in phospholipid and soap systems exhibit structural characteristics similar to those in the alkane rotator phases. The subtle connections between all these diverse systems are outlined, providing a unified outlook of the main phenomena related to the formation of such soft solid materials. The occurrence of alkane rotator phases in natural materials and in several technological applications is also reviewed to illustrate the general importance of these unique materials and the related phenomena.

Nanometer Confinement-Driven Promotion and Stabilization of a Hexatic Phase Intervening between Ordered Rotator Phases

Bulk phase binary mixture of two rotator phase forming alkanes, n-tricosane (C₂₃H₄₈) and n-octacosane (C₂₈H₅₈), has been previously studied. C₂₃H₄₈ exists in the R_II and R_I phases, whereas C₂₈H₅₈ exists in the R_III and R_IV phases. Over a certain range of composition, this binary mixture was found to exist in R_II, R_I and an intervening mesophase was reported to be the hexatic phase, wherein the long-range two-dimensional in-plane hexagonal lattice order of the R_II is lost and what remains is molecules present in hexagonal geometry without long-range positional correlation between individual hexagons. Upon confinement in cylindrical anodized alumina pores 200 nm wide, on the one hand, the temperature range of the hexatic phase was found to extend, and on the other hand, it underwent increased molecular ordering compared to the hexatic phase in bulk, exhibiting two counter-reacting behaviors in confinement. We provide here a temperature-dependent X-ray diffraction study and a theoretical approach combining the Landau and Flory-Huggins theories to, first, understand the underlying mechanism leading to emergence of the hexatic phase and then to explain the effect of confinement on it in the light of finite size and interfacial interaction between the alkanes and alumina pores.

Confinement-driven radical change in a sequence of rotator phases: a study on n-octacosane

Rotator-phase forming n-alkanes have been studied extensively in both their bulk state and in nanoconfinement. While some alkanes maintain their bulk-state rotator phases in nanoconfinement albeit with increased disorder, there are others exhibiting new rotator phases upon confinement. We present here a temperature dependent X-ray diffraction (XRD) and differential scanning calorimetric (DSC) study on n-octacosane (C₂₈H₅₈), which almost completely loses its bulk state R_IV phase and undergoes complete disappearance of its R_III phase. In their place, when confined in cylindrical anodized alumina nanopores, a phase highly resembling the hexatic mesophase is formed at a higher temperature and the R_I rotator phase at a lower temperature. The effects of finite size, interfacial interactions with the host matrix and alkyl chain flexibility are used to provide an explanation for such unexpected behaviour.

Temperature-Tuned Faceting and Shape Changes in Liquid Alkane Droplets

Recent extensive studies reveal that surfactant-stabilized spherical alkane emulsion droplets spontaneously adopt polyhedral shapes upon cooling below a temperature T_d while remaining liquid. Further cooling induces the growth of tails and spontaneous droplet splitting. Two mechanisms were offered to account for these intriguing effects. One assigns the effects to the formation of an intradroplet frame of tubules consisting of crystalline rotator phases with cylindrically curved lattice planes. The second assigns the sphere-to-polyhedron transition to the buckling of defects in a crystalline interfacial monolayer, known to form in these systems at some T_s > T_d. The buckling reduces the extensional energy of the crystalline monolayer's defects, unavoidably formed when wrapping a spherical droplet by a hexagonally packed interfacial monolayer. The tail growth, shape changes, and droplet splitting were assigned to the decrease and vanishing of surface tension, γ. Here we present temperature-dependent γ(T), optical microscopy measurements, and interfacial entropy determinations for several alkane/surfactant combinations. We demonstrate the advantages and accuracy of the in situ γ(T) measurements made simultaneously with the microscopy measurements on the same droplet. The in situ and coinciding ex situ Wilhelmy plate γ(T) measurements confirm the low interfacial tension, ≲0.1 mN/m, observed at T_d. Our results provide strong quantitative support validating the crystalline monolayer buckling mechanism.

Polymorphism of a hexadecane–heptadecane binary system in nanopores

Phase behaviors of a hexadecane–heptadecane (n-C₁₆H₃₄–C₁₇H₃₆, C₁₆–C₁₇) binary system in the bulk and in nanopores of controlled porous glasses (CPGs) are investigated using differential scanning calorimetry (DSC) and temperature-dependent powder X-ray diffraction (XRD).

Two-Step Freezing in Alkane Monolayers on Colloidal Silica Nanoparticles: From a Stretched-Liquid to an Interface-Frozen State

The crystallization behavior of an archetypical soft/hard hybrid nanocomposite, that is, an n-octadecane C18/SiO2-nanoparticle composite, was investigated by a combination of differential scanning calorimetry (DSC) and variable-temperature solid-state (13)C nuclear magnetic resonance (VT solid-state (13)C NMR) as a function of silica nanoparticles loading. Two latent heat peaks prior to bulk freezing, observed for composites with high silica loading, indicate that a sizable fraction of C18 molecules involve two phase transitions unknown from the bulk C18. Combined with the NMR measurements as well as experiments on alkanes and alkanols at planar amorphous silica surfaces reported in the literature, this phase behavior can be attributed to a transition toward a 2D liquid-like monolayer and subsequently a disorder-to-order transition upon cooling. The second transition results in the formation of a interface-frozen monolayer of alkane molecules with their molecular long axis parallel to the nanoparticles' surface normal. Upon heating, the inverse phase sequence was observed, however, with a sizable thermal hysteresis in accord with the characteristics of the first-order phase transition. A thermodynamic model considering a balance of interfacial bonding, chain stretching elasticity, and entropic effects quantitatively accounts for the observed behavior. Complementary synchrotron-based wide-angle X-ray diffraction (WAXD) experiments allow us to document the strong influence of this peculiar interfacial freezing behavior on the surrounding alkane melts and in particular the nucleation of a rotator phase absent in the bulk C18.

Soft matter in hard confinement: phase transition thermodynamics, structure, texture, diffusion and flow in nanoporous media

Spatial confinement in nanoporous media affects the structure, thermodynamics and mobility of molecular soft matter often markedly. This article reviews thermodynamic equilibrium phenomena, such as physisorption, capillary condensation, crystallisation, self-diffusion, and structural phase transitions as well as selected aspects of the emerging field of spatially confined, non-equilibrium physics, i.e. the rheology of liquids, capillarity-driven flow phenomena, and imbibition front broadening in nanoporous materials. The observations in the nanoscale systems are related to the corresponding bulk phenomenologies. The complexity of the confined molecular species is varied from simple building blocks, like noble gas atoms, normal alkanes and alcohols to liquid crystals, polymers, ionic liquids, proteins and water. Mostly, experiments with mesoporous solids of alumina, gold, carbon, silica, and silicon with pore diameters ranging from a few up to 50 nm are presented. The observed peculiarities of nanopore-confined condensed matter are also discussed with regard to applications. A particular emphasis is put on texture formation upon crystallisation in nanoporous media, a topic both of high fundamental interest and of increasing nanotechnological importance, e.g. for the synthesis of organic/inorganic hybrid materials by melt infiltration, the usage of nanoporous solids in crystal nucleation or in template-assisted electrochemical deposition of nano structures.

Crystallization features of normal alkanes in confined geometry

How polymers crystallize can greatly affect their thermal and mechanical properties, which influence the practical applications of these materials. Polymeric materials, such as block copolymers, graft polymers, and polymer blends, have complex molecular structures. Due to the multiple hierarchical structures and different size domains in polymer systems, confined hard environments for polymer crystallization exist widely in these materials. The confined geometry is closely related to both the phase metastability and lifetime of polymer. This affects the phase miscibility, microphase separation, and crystallization behaviors and determines both the performance of polymer materials and how easily these materials can be processed. Furthermore, the size effect of metastable states needs to be clarified in polymers. However, scientists find it difficult to propose a quantitative formula to describe the transition dynamics of metastable states in these complex systems. Normal alkanes [CnH2n+2, n-alkanes], especially linear saturated hydrocarbons, can provide a well-defined model system for studying the complex crystallization behaviors of polymer materials, surfactants, and lipids. Therefore, a deeper investigation of normal alkane phase behavior in confinement will help scientists to understand the crystalline phase transition and ultimate properties of many polymeric materials, especially polyolefins. In this Account, we provide an in-depth look at the research concerning the confined crystallization behavior of n-alkanes and binary mixtures in microcapsules by our laboratory and others. Since 2006, our group has developed a technique for synthesizing nearly monodispersed n-alkane containing microcapsules with controllable size and surface porous morphology. We applied an in situ polymerization method, using melamine-formaldehyde resin as shell material and nonionic surfactants as emulsifiers. The solid shell of microcapsules can provide a stable three-dimensional (3-D) confining environment. We have studied multiple parameters of these microencapsulated n-alkanes, including surface freezing, metastability of the rotator phase, and the phase separation behaviors of n-alkane mixtures using differential scanning calorimetry (DSC), temperature-dependent X-ray diffraction (XRD), and variable-temperature solid-state nuclear magnetic resonance (NMR). Our investigations revealed new direct evidence for the existence of surface freezing in microencapsulated n-alkanes. By examining the differences among chain packing and nucleation kinetics between bulk alkane solid solutions and their microencapsulated counterparts, we also discovered a mechanism responsible for the formation of a new metastable bulk phase. In addition, we found that confinement suppresses lamellar ordering and longitudinal diffusion, which play an important role in stabilizing the binary n-alkane solid solution in microcapsules. Our work also provided new insights into the phase separation of other mixed system, such as waxes, lipids, and polymer blends in confined geometry. These works provide a profound understanding of the relationship between molecular structure and material properties in the context of crystallization and therefore advance our ability to improve applications incorporating polymeric and molecular materials.

Phase transition behavior of a series of even n-alkane C(n)/C(n+2) mixtures confined in microcapsules: from total miscibility to phase separation determined by confinement geometry and repulsion energy

The phase behaviors of binary consecutive even normal alkane (n-alkane) mixtures (n-C(n)H(2n+2)/n-C(n+2)H(2n+6), with mass ratios of 90/10 and 10/90) with different average carbon numbers n¯ both in the bulk state (abbreviated as C(n)/C(n+2)) and in nearly monodisperse microcapsules (abbreviated as m-C(n)/C(n+2)), have been investigated by the combination of differential scanning calorimetry and temperature-dependent X-ray diffraction. The phase behavior of n-alkane mixtures gradually shifts from complete phase separation, partial miscibility to total miscibility in both bulk and microcapsules with the increase of average carbon numbers n¯. There are critical points for average carbon numbers of C(n)/C(n+2), where the corresponding mixtures exhibit coexistence of a triclinic phase (formed by alkane with a longer chain) and an orthorhombic ordered phase (formed by the two components of mixtures). Due to the confinement from hard shells of microcapsules, the critical points of m-C(n)/C(n+2) are smaller than those of C(n)/C(n+2). Such a phase behavior originates from the delicate combined action of confinement and repulsion energy for the encapsulated n-alkane mixtures with different average carbon numbers n¯. When n¯ is less than the critical point, the repulsion energy between the two kinds of molecules exceeds the suppression effect of confinement, and phase separation occurs in microcapsules. It is believed that the average carbon number is another important factor that exerts strong negative influence on the phase separation of m-C(n)/C(n+2) systems.

Confined crystallization of n-hexadecane located inside microcapsules or outside submicrometer silica nanospheres: a comparison study

Crystallization and phase transition behaviors of n-hexadecane (n-C16H34, abbreviated as C16) confined in microcapsules and n-alkane/SiO2 nanosphere composites have been investigated by the combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). As evident from the DSC measurement, the surface freezing phenomenon of C16 is enhanced in both the microcapsules and SiO2 nanosphere composites because the surface-to-volume ratio is dramatically enlarged in both kinds of confinement. It is revealed from the XRD results that the novel solid-solid phase transition is observed only in the microencapsulated C16, which crystallizes into a stable triclinic phase via a mestastable rotator phase (RI). For the C16/SiO2 composite, however, no novel rotator phase emerges during the cooling process, and C16 crystallizes into a stable triclinic phase directly from the liquid state. Heterogeneous nucleation induced by the surface freezing phase is dominant in the microencapsulated sample and contributes to the emergence of the novel rotator phase, whereas heterogeneous nucleation induced by foreign crystallization nuclei dominates the C16/SiO2 composite, leading to phase transition behaviors similar to those of bulk C16.

Phase Behavior of Undecane-Dodecane Mixtures Confined in SBA-15

Phase behavior of undecane-dodecane mixtures (n-C11H24-C12H26, C11-C12) in bulk and confined in SBA-15 (pore diameters 3.8, 7.8, and 17.2 nm) has been studied by differential scanning calorimetry. Phase diagram of the confined C11-C12system shows dependence on pore size, indicating simple behavior compared with the bulk. In the three systems, melting points of the confined C11, C12and the mixtures (xC12= 0.1–0.9) present a linear relation with the pore diameter from 3.8 to 17.2 nm. Melting behavior of the confined C11-C12mixtures is closely as the confined C11, C12. Melting temperatures of the three systems have been fitted as a function of mole fractionxC12.

Spontaneous imbibition dynamics of an n-alkane in nanopores: evidence of meniscus freezing and monolayer sticking

Capillary filling dynamics of liquid n-tetracosane (n-C24H50) in a network of cylindrical pores with 7 and 10 nm mean diameter in monolithic silica glass (Vycor) exhibit an abrupt temperature-slope change at Ts = 54 degrees C, approximately 4 degrees C above bulk and approximately 16 degrees C, 8 degrees C, respectively, above pore freezing. It can be traced to a sudden inversion of the surface tension's T slope, and thus to a decrease in surface entropy at the advancing pore menisci, characteristic of the formation of a single solid monolayer of rectified molecules, known as surface freezing from macroscopic, quiescent tetracosane melts. The imbibition speeds, that are the squared prefactors of the observed square-root-of-time Lucas-Washburn invasion kinetics, indicate a conserved bulk fluidity and capillarity of the nanopore-confined liquid, if we assume a flat lying, sticky hydrocarbon backbone monolayer at the silica walls.

Thermodynamic and Structural Investigations of Condensates of Small Molecules in Mesopores

Liquids and solids consisting of small, mainly van-der-Waals interacting building blocks, such as Ar, Kr, N2, O2, and CO, are among the most simple systems of condensed matter imaginable. As we shall demonstrate in this microreview on our work sponsored within the Sonderforschungsbereich 277, these cryoliquids condensed in mesoporous hosts with typical mean pore diameters of 7 to 10nm are also particularly suitable for the investigation of fundamental questions regarding the thermodynamics and structure of spatially mesoscale confined systems. An exploration of phase transitions like the vapour–liquid (capillary condensation), the vapour–solid (capillary sublimation), the liquid–solid (freezing and melting) and some solid–solid transformations of such pore condensates reveals a remarkably rich, sometimes perplexing phenomenology. We will show, however, that by experiments combining sorption isotherm, X-ray and neutron diffraction, calorimetric and optical transmission measurements, and by referring to concepts, intermediate between surface and bulk physics, a deeper understanding of the mesoscale mechanisms ultimatively responsible for this complex behaviour can indeed be accomplished, both on a qualitative and a quantitative level.

Crystallization of medium-length 1-alcohols in mesoporous silicon: an x-ray diffraction study

The linear 1-alcohols n-C(16)H(33)OH, n-C(17)H(35)OH, n-C(19)H(39)OH have been imbibed and solidified in lined up, tubular mesopores of silicon with 10 and 15 nm mean diameters, respectively. X-ray diffraction measurements reveal a set of six discrete orientation states ("domains") characterized by a perpendicular alignment of the molecules with respect to the long axis of the pores and by a fourfold symmetry about this direction, which coincides with the crystalline symmetry of the Si host. A Bragg peak series characteristic of the formation of bilayers indicates a lamellar structure of the spatially confined alcohol crystals in 15 nm pores. By contrast, no layering reflections could be detected for 10 nm pores. The growth mechanism responsible for the peculiar orientation states is attributed to a nanoscale version of the Bridgman technique of single-crystal growth, where the dominant growth direction is aligned parallel to the long pore axes. Our observations are analogous to the growth phenomenology encountered for medium length n -alkanes confined in mesoporous silicon [A. Henschel, T. Hofmann, P. Huber, and K. Knorr, Phys. Rev. E 75, 021607 (2007)] and may further elucidate why porous silicon matrices act as an effective nucleation-inducing material for protein solution crystallization.

Preferred orientations and stability of medium length n-alkanes solidified in mesoporous silicon

The n-alkanes C(16)H(34), C(17)H(36), C(19)H(40), and C(25)H(52) have been imbibed and solidified in mesoporous, crystalline silicon with a mean pore diameter of 10 nm. The structures and phase sequences have been determined by x-ray diffractometry. Apart from a reduction and the hysteresis of the melting-freezing transition, we find a set of six discrete orientation states ("domains") of the confined alkane crystals with respect to the lattice of the silicon host. The growth process responsible for the domain selection is interpreted as a nanoscale version of the Bridgman technique known from single-crystal growth. Oxidation of the pore walls leads to extrusion of the hydrocarbons upon crystallization, whereas the solidified n-alkanes investigated in nonoxidized, porous silicon are thermodynamically stable.