Confinement or the nature of the interface? Dynamics of nanoscopic water

Interplay between anion-receptor and anion-solvent interactions in halide receptor complexes characterized with ultrafast infrared spectroscopies

The competition between host-guest binding and solvent interactions is a crucial factor in determining the binding affinities and selectivity of molecular receptor species. The interplay between these competing interactions, however, have been difficult to disentangle. In particular, the development of molecular-level descriptions of solute-solvent interactions remains a grand experimental challenge. Herein, we investigate the prototypical halide receptor meso-octamethylcalix[4]pyrrole (OMCP) complexed with either chloride or bromide anions in both dichloromethane (DCM) and chloroform (trichloromethane, TCM) solvent using ultrafast infrared transient absorption and 2D IR spectroscopies. OMCP·Br- complexes in both solvents display slower vibrational relaxation dynamics of the OMCP pyrrole NH stretches, consistent with weaker H-bonding interactions with OMCP compared to chloride and less efficient intermolecular relaxation to the solvent. Further, OMCP·Br- complexes show nearly static spectral diffusion dynamics compared to OMCP·Cl-, indicating larger structural fluctuations occur within chloride complexes. Importantly, distinct differences in the vibrational spectra and dynamics are observed between DCM and TCM solutions. The data are consistent with stronger and more perturbative solvent effects in TCM compared to DCM, despite DCM's larger dielectric constant and smaller reported OMCP·X- binding affinities. These differences are attributed to the presence of weak H-bond interactions between halides and TCM, in addition to competing interactions from the bulky tetrabutylammonium countercation. The data provide important experimental benchmarks for quantifying the role of solvent and countercation interactions in anion host-guest complexes.

Modulating Interfacial Properties in Pseudoternary Microemulsions via Urea Addition: Impact of Cosurfactant on the Reverse Micellar Structure and Interactions

We have studied the structural and interfacial properties of CTAB/isooctane/alcohol/aqueous urea reverse micelles (RMs) for the first time using time-resolved fluorescence and small-angle X-ray scattering techniques. The chain length of alcohol, used as cosurfactant, has been varied to design three microemulsion systems: CTAB/1-butanol, CTAB/1-hexanol, and CTAB/1-octanol/isooctane/water, at a fixed water loading ratio, w0 = 12. Time-resolved fluorescence anisotropy studies indicate that urea induces micellar aggregation in CTAB/1-butanol and CTAB/1-hexanol RMs but breaks down RM aggregates in CTAB/1-octanol RMs. Urea addition slows down solvation dynamics inside RMs at higher urea concentrations, evident from the longer lifetimes of solvent correlation decay. The underlying changes in microemulsion structure and intermicellar interactions are studied using small-angle X-ray scattering studies. The significant intermicellar interactions were modeled using the sticky hard sphere (SHS) for the CTAB/1-butanol and CTAB/1-hexanol RMs and by using the Macroion model for the CTAB/1-octanol RMs. The two different structural factors highlight the dominance of attractive and repulsive forces, respectively. Although there is no change in RM shape, the combination of urea addition and chain length variation in cosurfactants significantly alters the size and interface in these pseudoternary RMs.

Shape of AOT Reverse Micelles: The Mesoscopic Assembly Is More Than the Sum of the Parts

AOT reverse micelles are a common and convenient model system for studying the effects of nanoconfinement on aqueous solutions. The reverse micelle shape is important to understanding how the constituent components come together to form the coherent whole and the unique properties observed there. The shape of reverse micelles impacts the amount of interface present and the distance of the solute from the interface and is therefore vital to understanding interfacial properties and the behavior of solutes in the polar core. In this work, we use previously introduced measures of shape, the coordinate-pair eccentricity (CPE) and convexity, and apply them to a series of simulations of AOT reverse micelles. We simulate the most commonly used force field for AOT reverse micelles, the CHARMM force field, but we also adapt the OPLS force field for use with AOT, the first work to do so, in addition to using both 3- and 4-site water models. Altogether, these simulations are designed to examine the impact of the force field on the shape of the reverse micelles in detail. We also study the time autocorrelation of shape, the water rotational anisotropy decay, and how the CPE changes between the water pool and AOT tail groups. We find that although the force field changes the shape noticeably, AOT reverse micelles are always amorphous particles. The shape of the micelles changes on the order of 10 ns. The water rotational dynamics observed match the experiment and demonstrate slower dynamics relative to bulk water, suggesting a two-population model that fits a core/shell hypothesis. Taken together, our results indicate that it is likely not possible to create a perfect force field that can reproduce every aspect of the AOT reverse micelle accurately. However, the magnitude of the differences between simulations appears relatively small, suggesting that any reasonably derived force field should provide an acceptable model for most work on AOT reverse micelles.

Anomalous proton transfer of a photoacid HPTS in nonaqueous reverse micelles

The proton transfer reaction is one of the fundamental chemical reactions where the reaction dynamics strongly depend on solvent properties such as acidity or basicity. A photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) shows a sharp decrease of pKa (7.7 → 0.5) upon photoexcitation, and the excited-state proton transfer (ESPT) occurs with ultrafast time constants of 2.5 and 89 ps in bulk aqueous solution. However, the two-step proton transfers via the contact ion pair formation and the proton diffusion are strongly limited inside the nanopools of reverse micelles (RMs). The confinement in small RMs strongly impeded the proton transfer reactions. In this work, we report the ESPT of HPTS confined in methanol-in-oil RMs by steady-state and time-resolved electronic spectroscopy. Interestingly, HPTS shows substantial deprotonation in the excited state only in small RMs, while the ESPT of HPTS does not occur in bulk methanol solution due to the low basicity of aliphatic alcohols. The kinetic analysis of time-resolved fluorescence and transient absorption measurements will compare the proton transfer dynamics of HPTS in the water-in-oil and methanol-in-oil RMs. The ESPT of photoacids, especially in the nonaqueous RMs, can be crucial in understanding many important chemical reactions involving proton transfer in the confined environments of cells and membranes.

Excited-State Dynamics of a Photoacid: A Potential Probe for Recognizing Transition from Lamellar to Nonlamellar Inverted Structures of Liposome based Nanocarriers

Liposomes of a cationic lipid dioctadecyldimethylammonium bromide (DODAB) are efficient nanocarriers of nucleic acids. Incorporation of a neutral lipid monoolein (MO) in excess (xMO >0.5) changes the lamellar organization of DODAB liposomes into non-lamellar inverted structures of DODAB/MO liposomes facilitating nucleic acid delivery to cells. Photoexcitation of 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS), a photoacid, initiates an excited state proton transfer (ESPT) reaction in its protonated form (ROH*) generating the deprotonated anionic form (RO- *). The fluorescence intensity ratio (IROH* /IRO-* ) of these two forms is governed by the ESPT dynamics, and increases with increasing MO content (xMO ) in the cationic liposomes of DODAB. Transition from lamellar organization of DODAB liposomes into non-lamellar inverted structures of DODAB/MO liposomes, due to incorporation of MO (xMO ~0.7), is manifested by a significant increase of ESPT time (τPT ) and the time constant of wobbling motion (τW ) of HPTS. Thus, the lamellar organizations of DODAB or DODAB-rich (xMO 0.2) liposomes and the non-lamellar organizations of MO-rich (xMO ~0.7) liposomes are recognized by significantly different excited state dynamics of the photoacid.

Hidden Beneath the Layers: Extending the Core/Shell Model of Reverse Micelles

Reverse micelles (RMs) provide a unique and highly tunable model system to study water in confined environments. The complex properties of water within RMs arise from the disruption of extended hydrogen bond (H-bond) networks that mediate local and long-range dynamics in bulk aqueous systems. Modulating the water pool size influences its H-bond dynamics, with smaller RMs increasingly restricting the H-bond network rearrangements leading to slower dynamics; however, within small confined systems, the dynamics of the surfactants also influence the water dynamics. Using ultrafast two-dimensional infrared spectroscopy, we investigate the effects of RM size on the surfactant headgroup rotamer populations and picosecond interfacial H-bond dynamics of aerosol-OT surfactants. We find that the increased water penetration accelerates H-bond dynamics, with larger RMs showing faster dynamics. These results imply that the changes in the RM structure alter the physical structure of the RM interface and thus alter the solvation dynamics. The findings in this study can be used for developing models for structure-specific solvation dynamics that account for the surfactant packing and hydration at the interface.

Probing nearby molecular vibrations with lanthanide-doped nanocrystals

The photoluminescence (PL) of lanthanide-doped nanocrystals can be quenched by energy transfer to vibrations of molecules located within a few nanometers from the dopants. Such short-range electronic-to-vibrational energy transfer (EVET) is often undesired as it reduces the photoluminescence efficiency. On the other hand, EVET may be exploited to extract information about molecular vibrations in the local environment of the nanocrystals. Here, we investigate the influence of solvent and gas environments on the PL properties of NaYF4:Er3+,Yb3+ upconversion nanocrystals. We relate changes in the PL spectrum and excited-state lifetimes in different solvents and their deuterated analogues to quenching of specific lanthanide levels by EVET to molecular vibrations. Similar but weaker changes are induced when we expose a film of nanocrystals to a gas environment with different amounts of H2O or D2O vapor. Quenching of green- and red-emitting levels of Er3+ can be explained in terms of EVET-mediated quenching that involves molecular vibrations with energies resonant with the gap between the energy levels of the lanthanide. Quenching of the near-infrared-emitting level is more complex and may involve EVET to combination-vibrations or defect-mediated quenching. EVET-mediated quenching holds promise as a mechanism to probe the local chemical environment-both for nanocrystals dispersed in a liquid and for nanocrystals exposed to gaseous molecules that adsorb onto the nanocrystal surface.

Highly Altered State of Proton Transport in Acid Pools in Charged Reverse Micelles

Transport mechanisms of solvated protons of 1 M HCl acid pools, confined within reverse micelles (RMs) containing the negatively charged surfactant sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) or the positively charged cetyltrimethylammonium bromide (CTABr), are analyzed with reactive force field simulations to interpret dynamical signatures from TeraHertz absorption and dielectric relaxation spectroscopy. We find that the forward proton hopping events for NaAOT are further suppressed compared to a nonionic RM, while the Grotthuss mechanism ceases altogether for CTABr. We attribute the sluggish proton dynamics for both charged RMs as due to headgroup and counterion charges that expel hydronium and chloride ions from the interface and into the bulk interior, thereby increasing the pH of the acid pools relative to the nonionic RM. For charged NaAOT and CTABr RMs, the localization of hydronium near a counterion or conjugate base reduces the Eigen and Zundel configurations that enable forward hopping. Thus, localized oscillatory hopping dominates, an effect that is most extreme for CTABr in which the proton residence time increases dramatically such that even oscillatory hopping is slow.

The Journey of 1-Keto-1,2,3,4-Tetrahydrocarbazole Based Fluorophores: From Inception to Implementation

Carbazole is a unique template associated with several biological activities. It is due to the diverse and versatile biological properties of carbazole derivatives that they are of immense interest to the research community. 1-keto-1,2,3,4-tetrahydrocarbazoles are important synthetic intermediates to obtain carbazole derivatives. Several members of this family emit fluorescence on photoexcitation. In the context of biochemical and biophysical research, designing and characterising small molecule environment sensitive fluorophores is extremely significant. This article aims to be a state of the art review with synthetic and photophysical details of a variety of fluorophores based on 1-keto-1,2,3,4-tetrahydrocarbazole skeleton.

Spatiotemporal control of l-phenyl-alanine crystallization in microemulsion: the role of water in mediating molecular self-assembly

Water confined or constrained in a cellular environment can exhibit a diverse structural and dynamical role and hence will affect the self-assembly behavior of biomolecules. Herein, the role of water in the formation of l-phenyl-alanine crystals and amyloid fibrils was investigated. A microemulsion biomimetic system with controllable water pool size was employed to provide a microenvironment with different types of water, which was characterized by small-angle X-ray scattering, attenuated total reflectance-Fourier transform infrared spectroscopy and differential scanning calorimetry. In a bound water environment, only plate-like l-phenyl-alanine crystals and their aggregates were formed, all of which are anhydrous crystal form I. However, when free water dominated, amyloid fibrils were observed. Free water not only stabilizes new oligomers in the initial nucleation stage but also forms bridged hydrogen bonds to induce vertical stacking to form a fibrous structure. The conformational changes of l-phenyl-alanine in different environments were detected by NMR. Different types of water trigger different nucleation and growth pathways, providing a new perspective for understanding molecular self-assembly in nanoconfinement.

Structure and dynamics of a water/methanol mixture confined in zeolitic imidazolate framework ZIF-8 from atomistic simulations

A classical atomistic simulation study is reported for the microscopic structure and dynamics of a water/methanol mixture confined in flexible nanoporous zeolitic imidazolate framework ZIF-8. Both the radial density distribution and vivid two-dimensional density profile demonstrate that methanol molecules can roughly be viewed as "embedded" between two layers of water molecules to form a "sandwich" structure. The reason for the formation of such a specific structure is explained based on the hydrogen-bonding state and the strength of various hydrogen bonds. The investigation of guest molecular diffusion shows that the self-diffusion coefficient of confined water is generally one to two orders of magnitude smaller than that of bulk water. In addition, the dependence of the self-diffusion coefficient on loading is non-monotonic: the self-diffusion coefficient firstly shows a significant increase and then decreases at higher loading. Moreover, both the structure and dynamics of the hydrogen bond (HB) network of confined water molecules are investigated in a spatially resolved manner. The results indicate that both the HB structure and dynamics of water molecules near the ZIF-8 surface deviate significantly from those of bulk water. However, while water molecules located at the pore center are relatively similar to bulk water molecules with respect to the HB structure, they exhibit strong slowdown in HB dynamics when compared with bulk water. This simulation study elucidates in detail the structural and dynamical properties of a water/methanol mixture in nanoscopic ZIF-8 confinement, which is expected to provide a deep insight into the role of porous fillers, such as ZIF-8, in improving the performance of the dehydration of alcohols via pervaporation and other related processes.

Water reorientation dynamics in colloidal water-oil emulsions

We study the molecular-scale properties of colloidal water-oil emulsions consisting of 120-290 nm oil droplets embedded in water. This type of emulsion can be prepared with low concentrations of surfactants and is usually kinetically stable. Even though colloidal water-oil emulsions are used ubiquitously, their molecular properties are still poorly understood. Here we study the orientational dynamics of water molecules in these emulsions using polarization resolved pump-probe infrared spectroscopy, for varying surfactant concentrations, droplet sizes, and temperatures. We find that the majority of the water molecules reorients with the same time constant as in bulk water, while a small fraction of the water molecules reorients on a much longer time scale. These slowly reorienting water molecules are interacting with the surface of the oil droplets. The fraction of slowly orienting water molecules is proportional to the oil volume fraction, and shows a negligible dependence on the average droplet size. This finding indicates that the total surface area of the oil droplets is quite independent of the average droplet size, which indicates that the larger oil droplets are quite corrugated, showing large protrusions into the water phase.

New State-Diagram of Aqueous Solutions Unveiling Ionic Hydration, Antiplasticization, and Structural Heterogeneities in LiTFSI-H2O

Here, we report a new state-diagram for aqueous solutions based on concentration-dependent glass-transition temperatures of concentrated and ice freeze-concentrated solutions. Different from the equilibrium phase diagram, this new state-diagram can provide comprehensive information about the hydration numbers of solutes, nonequilibrium vitrification/cold-crystallization, and vitrification/devitrification processes of aqueous solutions in three distinct concentration zones separated by two critical water-content points of only functions of the hydration number. Based on this new state-diagram, we observe the comparable hydration ability of LiTFSI to LiCl and an atypical concentration-dependent cold-crystallization behavior of the LiTFSI-H₂O system. These results unveil the negligible hydration ability of TFSI^- in a water-rich solution, characterize the antiplasticizing effect of water induced by the strengthened Li⁺-TFSI^--H₂O interaction when only hydration water and confined water are present, and confirm the increasing fraction of water-rich domains with the decrease in water content when the cation and anion become incompletely hydrated on average. These results highlight the novel water-content-mediated interactions among the anion, cation, and H₂O for LiTFSI-H₂O.

Altered relaxation dynamics of excited state reactions by confinement in reverse micelles probed by ultrafast fluorescence up-conversion

Chemical reactions in confined environments are important in areas as diverse as heterogenous catalysis, environmental chemistry and biochemistry, yet they are much less well understood than the equivalent reactions in either the gas phase or in free solution. The understanding of chemical reactions in solution was greatly enhanced by real time studies of model reactions, through ultrafast spectroscopy (especially when supported by molecular dynamics simulation). Here we review some of the efforts that have been made to adapt this approach to the investigation of reactions in confined media. Specifically, we review the application of ultrafast fluorescence spectroscopy to measure reaction dynamics in the nanoconfined water phase of reverse micelles, as a function of the droplet radius and the charge on the interface. Methods of measurement and modelling of the reactions are outlined. In all of the cases studied (which are focused on ultrafast intramolecular reactions) the effect of confinement was to suppress the reaction. Even in the largest micelles the result in the bulk aqueous phase was not usually recovered, suggesting an important role for specific interactions between reactant and environment, for example at the interface. There was no simple one-to-one correspondence with direct measures of the dynamics of the confined phase. Thus, understanding the effect of confinement on reaction rate appears to require not only knowledge of the dynamics of the reaction in solutions and the effect of confinement on the medium, but also of the interaction between reactant and confining medium.

Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism

The properties of the water network in concentrated HCl acid pools in nanometer-sized reverse non-ionic micelles were probed with TeraHertz absorption, dielectric relaxation spectroscopy, and reactive force field simulations capable of describing proton hopping mechanisms. We identify that only at a critical micelle size of W0=9 do solvated proton complexes form in the water pool, accompanied by a change in mechanism from Grotthuss forward shuttling to one that favors local oscillatory hopping. This is due to a preference for H+ and Cl- ions to adsorb to the micelle interface, together with an acid concentration effect that causes a "traffic jam" in which the short-circuiting of the hydrogen-bonding motif of the hydronium ion decreases the forward hopping rate throughout the water interior even as the micelle size increases. These findings have implications for atmospheric chemistry, biochemical and biophysical environments, and energy materials, as transport of protons vital to these processes can be suppressed due to confinement, aggregation, and/or concentration.

Unraveling Depth-Specific Ionic Conduction and Stiffness Behavior across Ionomer Thin Films and Bulk Membranes

Interfacial behavior of submicron thick polymer films critically controls the performance of electrochemical devices. We developed a robust, everyday-accessible, fluorescence confocal laser scanning microscopy (CLSM)-based strategy that can probe the distribution of mobility, ion conduction, and other properties across ionomer samples. When fluorescent photoacid probe 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) was incorporated into <1 μm thick Nafion films on substrates, the depth-profile images showed thickness- and interface-dependent proton conduction behavior. In these films, proton conduction was weak over a region next to substrate interface, then gradually increased until air interface at 88% RH. Conversely, consistently high proton conduction with no interface dependence was observed across 35-50 μm thick bulk, free-standing Nafion membranes. A hump-like mobility/stiffness distribution was observed across Nafion films containing mobility-sensitive probe (9-(2-carboxy-2-cyanovinyl)julolidine) (CCVJ). The proton conduction and mobility distribution were rationalized as a combinatorial effect of interfacial interaction, ionomer chain orientation, chain density, and ionic domain characteristics.

Structural Characterization of Protonated Water Clusters Confined in HZSM-5 Zeolites

A molecular description of the structure and behavior of water confined in aluminosilicate zeolite pores is a crucial component for understanding zeolite acid chemistry under hydrous conditions. In this study, we use a combination of ultrafast two-dimensional infrared (2D IR) spectroscopy and ab initio molecular dynamics (AIMD) to study H₂O confined in the pores of highly hydrated zeolite HZSM-5 (∼13 and ∼6 equivalents of H₂O per Al atom). The 2D IR spectrum reveals correlations between the vibrations of both terminal and H-bonded O-H groups and the continuum absorption of the excess proton. These data are used to characterize the hydrogen-bonding network within the cluster by quantifying single-, double-, and non-hydrogen-bond donor water molecules. These results are found to be in good agreement with the statistics calculated from an AIMD simulation of an H⁺(H₂O)₈ cluster in HZSM-5. Furthermore, IR spectral assignments to local O-H environments are validated with DFT calculations on clusters drawn from AIMD simulations. The simulations reveal that the excess charge is detached from the zeolite and resides near the more highly coordinated water molecules in the cluster. When they are taken together, these results unambiguously assign the complex IR spectrum of highly hydrated HZSM-5, providing quantitative information on the molecular environments and hydrogen-bonding topology of protonated water clusters under extreme confinement.

Bursting the bubble: A molecular understanding of surfactant-water interfaces

Surfactant science has historically emphasized bulk, thermodynamic measurements to understand the microemulsion properties of greatest industrial significance, such as interfacial tensions, phase behavior, and thermal stability. Recently, interest in the molecular properties of surfactants has grown among the physical chemistry community. This has led to the application of cutting-edge spectroscopic methods and advanced simulations to understand the specific interactions that give rise to the previously studied bulk characteristics. In this Perspective, we catalog key findings that describe the surfactant-oil and surfactant-water interfaces in molecular detail. We emphasize the role of ultrafast spectroscopic methods, including two-dimensional infrared spectroscopy and sum-frequency-generation spectroscopy, in conjunction with molecular dynamics simulations, and the role these techniques have played in advancing our understanding of interfacial properties in surfactant microemulsions.

Hydration of bromide at reverse micelle interfaces studied by X-ray absorption fine structure

Nanoconfined water exhibits various interesting properties, which are not only of fundamental importance but also of practical use. Because reverse micelles (RMs) provide versatile ways to prepare nanoconfined water, the understanding of their physicochemical properties is essential for developing efficient applications. Although the water properties in the RMs could be affected by its interaction with the RM interface, the details have not been well understood. This study focuses on the local structures of Br^- in hexadecyltrimethylammonium bromide (HTAB) RMs formed in chloroform and 10% hexanol/heptane. The dependence in Br^- hydration on the molar ratio of water to HTAB (w) is investigated using X-ray absorption fine structure (XAFS). These systems cover a wide range of w values (0-30) and allow us to study the impact of this parameter on the local structure of Br^- at the RM interface, which comprises water, surfactant headgroups, and organic solvent components. The presence of multiple scattering paths complicates the XAFS spectra and makes it difficult to analyze them using standard fitting methods. The linear combination of the spectra corresponding to the individual scattering paths captures the molecular processes that occur at the RM interface upon increasing w. The maximum hydration number of Br^- is found to be 4.5 at w > 15, suggesting that although most of the ions remain at the interface as partly hydrated ions, some of them dissociate as completely hydrated ones.

Characterization of the Features of Water Inside the SecY Translocon

Despite extended experimental and computational studies, the mechanism regulating membrane protein folding and stability in cell membranes is not fully understood. In this review, I will provide a personal and partial account of the scientific efforts undertaken by Dr. Stephen White to shed light on this topic. After briefly describing the role of water and the hydrophobic effect on cellular processes, I will discuss the physical chemistry of water confined inside the SecY translocon pore. I conclude with a review of recent literature that attempts to answer fundamental questions on the pathway and energetics of translocon-guided membrane protein insertion.

Water Mobility in the Interfacial Liquid Layer of Ice/Clay Nanocomposites

At solid/ice interfaces, a premelting layer is formed at temperatures below the melting point of bulk water. However, the structural and dynamic properties within the premelting layer have been a topic of intense debate. Herein, we determined the translational diffusion coefficient Dt of water in ice/clay nanocomposites serving as model systems for permafrost by quasi-elastic neutron scattering. Below the bulk melting point, a rapid decrease of Dt is found for charged hydrophilic vermiculite, uncharged hydrophilic kaolin, and more hydrophobic talc, reaching plateau values below -4 °C. At this temperature, Dt in the premelting layer is reduced up to a factor of two compared to supercooled bulk water. Adjacent to charged vermiculite the lowest water mobility was observed, followed by kaolin and the more hydrophobic talc. Results are explained by the intermolecular water interactions with different clay surfaces and interfacial segregation of the low-density liquid water (LDL) component.

Photosensitization Dynamics of Stable Copper Nanoclusters inside the Aqueous Core of Reverse Micelles with Different Pool Sizes

The perennial problem of instability of fluorescent copper nanoclusters (Cu NCs), stemming principally from aerial oxidation, has prevented their vivid usage in energy harvesting compared to the other metal NCs. However, replacement of the much expensive metal NCs with the cheaper Cu NCs is desirable if the functions are met with. Although thiolate protection of Cu NCs could bring some stability to them, appreciably decentlystable Cu NCs were produced inside the aqueous core of reverse micelles (RMs). However, this recent development has not been further explored on the photosensitization of the Cu NCs inside the RMs and their controlled modulation as energy antenna. Here we have synthesized stable Cu NCs inside the aqueous core of RMs with three different pool sizes and established photoinduced electron transfer (PET) to an electron acceptor. Considering the bulk quencher concentration, it appears that the extent of PET increases with decrease in the size of the aqueous core of RMs. However, calculating the effective concentration of the electron acceptor inside the RMs and considering the polarity of the microheterogeneous systems, it becomes clear that the extent of PET actually decreases with decrease in the size of the aqueous pool (w₀, i.e., [H₂O]/[AOT]) = 5-20) in the RMs. This proof of concept and the results are promising toward applications in PET-driven phenomena such as solar cells or batteries.

Versatility of Reverse Micelles: From Biomimetic Models to Nano (Bio)Sensor Design

This paper presents an overview of the principal structural and dynamics characteristics of reverse micelles (RMs) in order to highlight their structural flexibility and versatility, along with the possibility to modulate their parameters in a controlled manner. The multifunctionality in a large range of different scientific fields is exemplified in two distinct directions: a theoretical model for mimicry of the biological microenvironment and practical application in the field of nanotechnology and nano-based sensors. RMs represent a convenient experimental approach that limits the drawbacks of the conventionally biological studies in vitro, while the particular structure confers them the status of simplified mimics of cells by reproducing a complex supramolecular organization in an artificial system. The biological relevance of RMs is discussed in some particular cases referring to confinement and a crowded environment, as well as the molecular dynamics of water and a cell membrane structure. The use of RMs in a range of applications seems to be more promising due to their structural and compositional flexibility, high efficiency, and selectivity. Advances in nanotechnology are based on developing new methods of nanomaterial synthesis and deposition. This review highlights the advantages of using RMs in the synthesis of nanoparticles with specific properties and in nano (bio)sensor design.

Interactions of water confined in a metal-organic framework as studied by a combined approach of Raman, FTIR, and IR electroabsorption spectroscopies and multivariate curve resolution analysis

Water in nanoconfinement shows distinct properties that are markedly different from those of bulk water. These unique properties stem not only from the water-water interaction but also from the interactions between water and the surrounding confining environment. Here we used a combined approach of vibrational spectroscopies (Raman, FTIR, and IR electroabsorption) and a multivariate curve resolution technique to study the interactions of water in a heterogeneous confining environment within a prototype of pillared layer-type metal-organic frameworks (MOFs), CPL-1 ([Cu2(pzdc)2(pyz)]n, where pzdc = 2,3-pyrazinedicarboxylate, pyz = pyrazine). The OH stretching Raman spectrum of hydrated CPL-1 microcrystals revealed that the adsorbed water molecules resemble the subpopulation of bulk water whose hydrogen bond is weak. Multivariate curve resolution analysis of FTIR spectra monitoring water desorption from CPL-1 allowed for accurate assignments of the framework's carboxylate vibrational modes associated with water-filled and empty nanopores of the MOF, and for quantitative determination of the number fraction of these pores. Furthermore, building on the assignments so made, IR electroabsorption measurements showed that the hydrogen-bonding interaction with water adsorbed in CPL-1 has little impact on the response to electric fields of the framework vibrational modes. The present findings altogether provide a solid basis of elucidating water confined in CPL-1 and demonstrate the potential of the combined vibrational spectroscopic method for interrogating the interactions within MOFs.

Slow Proton Transfer in Nanoconfined Water

The transport of protons in nanoconfined environments, such as in nanochannels of biological or artificial proton conductive membranes, is essential to chemistry, biology, and nanotechnology. In water, proton diffusion occurs by hopping of protons between water molecules. This process involves the rearrangement of many hydrogen bonds and as such can be strongly affected by nanoconfinement. We study the vibrational and structural dynamics of hydrated protons in water nanodroplets stabilized by a cationic surfactant using polarization-resolved femtosecond infrared transient absorption spectroscopy. We determine the time scale of proton hopping in the center of the water nanodroplets from the dynamics of the anisotropy of the transient absorption signals. We find that in small nanodroplets with a diameter <4 nm, proton hopping is more than 10 times slower than in bulk water. Even in relatively large nanodroplets with a diameter of ∼7 nm, we find that the rate of proton hopping is slowed by ∼4 times compared with bulk water.

Consequences of Convex Nanopore Chemistry on Confined Water Dynamics

A fundamental understanding of confined water is crucial for developing selective ion transport and water purification membranes, yet the roles of nanopore geometry and functionality on confined water dynamics remain unresolved. We report the synthesis of perdeuterated ionic alkylsulfonate amphiphiles and their water-induced self-assembly into lyotropic liquid crystal (LLC) mesophases with well-defined, convex, sulfonate-lined nanopores. Quasielastic neutron scattering (QENS) measurements demonstrate that the water self-diffusion coefficients within these sulfonate-lined convex nanopores depend on the hydration level and amphiphile counterion identity (H+, K+, NMe4+). The consistency of the observed counterion-dependent water dynamics trends with those of carboxylate LLCs is rationalized on the basis of similarities in the counterion spatial distributions in the water-filled channels, which we deduce from electron density maps derived from small-angle X-ray scattering (SAXS) analyses. These findings indicate that water diffusion is systematically faster in sulfonate-lined nanopores as compared to carboxylate-lined pores due to weaker water interactions with the softer and more hydrophobic-SO3- functionalities. These molecular-level insights into the relationships between convex pore wall chemical functionalities, hydrated counterions, and confined water diffusion may inform future development of new nanoporous media.

Interfacial H-Bond Dynamics in Reverse Micelles: The Role of Surfactant Heterogeneity

Characterizing the hydrogen bond structure and dynamics at surfactant interfaces is essential for understanding how microscopic interactions translate to bulk microemulsion properties. Heterogeneous blends containing tens or hundreds of surfactants are common in the industry, but the most fundamental studies have been carried out on micelles composed of a single surfactant species. Therefore, the effect of surfactant heterogeneity on the interfacial structure and dynamics remains poorly understood. Here, we use ultrafast two-dimensional infrared spectroscopy and molecular dynamics simulations to characterize sub-picosecond solvation dynamics as a function of the surfactant composition in ∼120 nm water-in-oil reverse micelles. We probe the ester carbonyl vibrations of nonionic sorbitan surfactants, which are located precisely at the interface between the polar and nonpolar regions, and as such, report on the interfacial water dynamics. We show a 7% increase in hydrogen bond populations together with a 37% slowdown of interfacial hydrogen bond dynamics in heterogeneous mixtures containing hundreds of species, compared to more uniform compositions. Simulations, which are in semiquantitative agreement with experiments, indicate that structural diversity leads to decreased packing efficiency, which in turn drives water further into the otherwise hydrophobic region. Interestingly, this increase in hydration is accompanied by a slowdown of dynamics, indicating that water molecules solvating surfactants are conformationally constrained. These studies demonstrate that the composition and heterogeneity are key factors in determining interfacial properties.

Mechanistic Insights into the Growth of Anisotropic Nanostructures Inside Reverse Micelles: A Solvation Perspective

Reverse micelles (RMs) as soft templates have been successfully used in tailoring the structural characteristics (size and morphology) of nanomaterials that in turn have been used in various applications. In this work, we have focused on the local perturbations in the different interior domains of the cetyltrimethylammonium bromide-reverse micelle-based soft template en route to nanorod formation by monitoring the solvation response of coumarin-based solvatochromic probes (C343 and C153). We have observed an appreciable retardation of the solvent coordinate during the initial phases of nanorod growth, which we have attributed to the reorientational motion of the water molecules lodged in the interfacial region. Moreover, these rigid nanostructures leave their imprints on the soft interfacial layer as was observed from the direct correlation in the solvation response of RM-containing nanostructures and respective surfactant aggregates in supernatant solution. Supporting data from time-resolved anisotropy studies further reinforced our conclusions from the solvation experiments. Our study proves that the hydration dynamics can be a promising tool in tracking the heterogeneous growth evolution of nanostructure formation in RMs since solvent reorganization provides insights into the intrinsic, molecular-level features of the micellar assemblies.

Water Structure and Dynamics in the Stern Layer of Micelles: Femtosecond Mid-Infrared Pump-Probe Spectroscopy Study

Molecular-level understanding of the water structure and dynamics in the Stern layer of micelles is important to elucidate the active role of water in biological processes on membrane surfaces. Micelles and reverse micelles are considered to be excellent membrane model systems. Here, to address the question of whether or not the spatial confinement effect on water in reverse micelles and nanometric water pool systems plays a role in modulating water dynamics, we consider four different aqueous micelle solutions and study the water dynamics in the Stern layer of micelles using a femtosecond mid-infrared pump-probe spectroscopy technique. Vibrational energy relaxation and rotational dynamics of the O?D stretch mode of HDO and the azido stretch mode of hydrazoic acid are critically dependent on the charge, polarity, and chemical structure of the surfactant head group. In particular, water molecules in the Stern layer of micelles, which are not in spatially confined environments, are notably different from those in bulk water. This finding clearly indicates that changes in the vibrational and rotational dynamics of water molecules, even in spatially confined systems, are mainly induced by surface effects instead of spatial confinement effects. We believe that the present experimental results are of importance for understanding water-involved biochemical processes on biological membranes.

Soft biomimetic nanoconfinement promotes amorphous water over ice

Water is a ubiquitous liquid with unique physicochemical properties, whose nature has shaped our planet and life as we know it. Water in restricted geometries has different properties than in bulk. Confinement can prevent low-temperature crystallization of the molecules into a hexagonal structure and thus create a state of amorphous water. To understand the survival of life at subzero temperatures, it is essential to elucidate this behaviour in the presence of nanoconfining lipidic membranes. Here we introduce a family of synthetic lipids with designed cyclopropyl modifications in the hydrophobic chains that exhibit unique liquid-crystalline behaviour at low temperature, which enables the maintenance of amorphous water down to ~10 K due to nanoconfinement. The combination of experiments and molecular dynamics simulations unveils a complex lipid-water phase diagram in which bicontinuous cubic and lamellar liquid crystalline phases that contain subzero liquid, glassy or ice water emerge as a competition between the two components, each pushing towards its thermodynamically favoured state.

Counterion Effect on Vibrational Relaxation and the Rotational Dynamics of Interfacial Water and an Anionic Vibrational Probe in the Confined Reverse Micelles Environment

Vibrational relaxation and the rotational dynamics of water molecules encapsulated in reverse micelles (RMs) have been investigated by ultrafast infrared (IR) spectroscopy and two-dimensional IR (2D IR) spectroscopy. By changing the counterion of the hydrophilic headgroup in the RMs formed by Aerosol-OT (AOT) from Na⁺ to K⁺, Cs⁺ and Ca²⁺, we could determine the specific counterion effects on the rotational dynamics of water molecules. The orientational relaxation time constant of water decreases in the order Ca²⁺ > Na⁺ > K⁺ > Cs⁺. The SCN^- anionic probe and counterion can form ion pairs at the interfacial region of the RMs. The rotational dynamics of SCN^- anion significantly decreases because of the synergistic effects of confinement and the surface interactions in the interfacial region of the RMs. The results can provide a new understanding of the cationic Hofmeister effect at the molecular level observed in biological studies.

Structural Relaxation Processes and Collective Dynamics of Water in Biomolecular Environments

In this simulation study, we investigate the influence of biomolecular confinement on dynamical processes in water. We compare water confined in a membrane protein nanopore at room temperature to pure liquid water at low temperatures with respect to structural relaxations, intermolecular vibrations, and the propagation of collective modes. We observe distinct potential energy landscapes experienced by water molecules in the two environments, which nevertheless result in comparable hydrogen bond lifetimes and sound propagation velocities. Hence, we show that a viscoelastic argument that links slow rearrangements of the water-hydrogen bond network to ice-like collective properties applies to both, the pure liquid and biologically confined water, irrespective of differences in the microscopic structure.

Ion-Specific Confined Water Dynamics in Convex Nanopores of Gemini Surfactant Lyotropic Liquid Crystals

The impact of pore geometry and functionality on the dynamics of water nanoconfined in porous media are the subject of some debate. We report the synthesis and small-angle X-ray scattering (SAXS) characterization of a series of perdeuterated gemini surfactant lyotropic liquid crystals (LLCs), in which convex, water-filled nanopores of well-defined dimensions are lined with carboxylate functionalities. Quasielastic neutron scattering (QENS) measurements of the translational water dynamics in these dicarboxylate LLC nanopores as functions of the surfactant hydration state and the charge compensating counterion (Na⁺, K⁺, NMe₄⁺) reveal that the measured dynamics depend primarily on surfactant hydration, with an unexpected counterion dependence that varies with hydration number. We rationalize these trends in terms of a balance between counterion-water attractions and the nanopore volume excluded by the counterions. On the basis of electron density maps derived from SAXS analyses of these LLCs, we directly show that the volume excluded by the counterions depends on both their size and spatial distribution in the water-filled channels. The translational water dynamics in the convex pores of these LLCs are also slower than those reported in the concave pores of AOT reverse micelles, implying that water dynamics also depend on the nanopore curvature.

Toward comprehensive measurement of protein hydration dynamics: Facilitation of NMR-based methods by reverse micelle encapsulation

Protein-water interactions are a fundamental determinant of protein structure and function. Despite their importance, the molecular details of water orientations and dynamics near protein surfaces remain poorly understood, largely due to the difficulty of measuring local water mobility near the protein in a site-resolved fashion. Solution NMR-based measurement of water mobility via the nuclear Overhauser effect was presented as a method for performing comprehensive, site-resolved measurements of water dynamics many years ago. Though this approach yielded extensive insight on the dynamics and locations of waters buried within proteins, its promise for measuring surface hydration dynamics was impeded by various technical barriers. Over the past several years, however, this approach has been pursued anew with the aid of reverse micelle encapsulation of proteins of interest. The confined environment of the reverse micelle resolves many of these barriers and permits site-resolved measurement of relative water dynamics across much of the protein surface. Here, the development of this strategy for measuring hydration dynamics is reviewed with particular focus on the important remaining challenges to its widespread application.

Picosecond Solvation Dynamics in Nanoconfinement: Role of Water and Host-Guest Complexation

The dynamics of solvation of an excited chromophore, 5-(4″-dimethylaminophenyl)-2-(4'-sulfophenyl)oxazole, sodium salt (DMO), has been explored in confined nanoscopic environments of β-cyclodextrin (βCD) and heptakis(2,6-di- O-methyl)-β-cyclodextrin (DIMEB). Solvation occurs on a distinctly slower time scale (τ_S³ ∼ 47 ps, τ_S⁴ ∼ 517 ps) in the host cavity of DIMEB than in that of βCD (τ_S³ ∼ 20 ps, τ_S⁴ ∼ 174 ps). The calculated equilibrium solvation response of DMO was characterized by four relaxation components (τ_S¹ ∼ 0.46-0.48 ps, τ_S² ∼ 3.2-3.4 ps, τ_S³ ∼ 32.3-37.7 ps, and τ_S⁴ ∼ 232-485 ps), of which the longer ones (τ_S³, τ_S⁴) are well-consistent with experiments, whereas the ultrafast components (τ_S¹, τ_S²) are unresolved. The observed time constant (τ_S³) within the ∼20-47 ps range arises from slow water molecules in the primary hydration layers of the host CDs and is slower for DIMEB than for βCD presumably due to longer-lived and stronger hydrogen bonds that water forms with the former host relative to the latter. Decomposition of the calculated solvation response of DMO has revealed that conformational fluctuations of the macrocyclic hosts give rise to the observed long-time relaxation component (τ_S⁴), which is much slower for the inclusion complexes with DIMEB than for those with βCD because of slower conformational dynamics of the former host than that of the latter.

Water at surfaces with tunable surface chemistries

Aqueous interfaces are ubiquitous in natural environments, spanning atmospheric, geological, oceanographic, and biological systems, as well as in technical applications, such as fuel cells and membrane filtration. Where liquid water terminates at a surface, an interfacial region is formed, which exhibits distinct properties from the bulk aqueous phase. The unique properties of water are governed by the hydrogen-bonded network. The chemical and physical properties of the surface dictate the boundary conditions of the bulk hydrogen-bonded network and thus the interfacial properties of the water and any molecules in that region. Understanding the properties of interfacial water requires systematically characterizing the structure and dynamics of interfacial water as a function of the surface chemistry. In this review, we focus on the use of experimental surface-specific spectroscopic methods to understand the properties of interfacial water as a function of surface chemistry. Investigations of the air-water interface, as well as efforts in tuning the properties of the air-water interface by adding solutes or surfactants, are briefly discussed. Buried aqueous interfaces can be accessed with careful selection of spectroscopic technique and sample configuration, further expanding the range of chemical environments that can be probed, including solid inorganic materials, polymers, and water immiscible liquids. Solid substrates can be finely tuned by functionalization with self-assembled monolayers, polymers, or biomolecules. These variables provide a platform for systematically tuning the chemical nature of the interface and examining the resulting water structure. Finally, time-resolved methods to probe the dynamics of interfacial water are briefly summarized before discussing the current status and future directions in studying the structure and dynamics of interfacial water.

Potential Role of Inorganic Confined Environments in Prebiotic Phosphorylation

A concise outlook on the potential role of confinement in phosphorylation and phosphate condensation pertaining to prebiotic chemistry is presented. Inorganic confinement is a relatively uncharted domain in studies concerning prebiotic chemistry, and even more so in terms of experimentation. However, molecular crowding within confined dimensions is central to the functioning of contemporary biology. There are numerous advantages to confined environments and an attempt to highlight this fact, within this article, has been undertaken, keeping in context the limitations of aqueous phase chemistry in phosphorylation and, to a certain extent, traditional approaches in prebiotic chemistry.

Counterion-Regulated Dynamics of Water Confined in Lyotropic Liquid Crystalline Morphologies

The dynamics of confined water is of fundamental and long-standing interest. In technologically important forms of confinement, such as proton-exchange membranes, electrostatic interactions with the confining matrix and counterions play significant roles on the properties of water. There has been recent interest on the dynamics of water confined to the lyotropic liquid crystalline (LLC) morphologies of Gemini dicarboxylate surfactants. These systems are exciting because the nature of confinement, for example, size and curvature of channels and surface functionality is dictated by the chemistry of the self-assembling surfactant molecules. Quasielastic neutron scattering experiments have shown an interesting dependence of the water self-diffusion constant, D_α, on the identity (denoted α) of the counterion: at high hydration, the magnitude of the water self-diffusion constant is in the order D_TMA < D_Na < D_K, where TMA, Na, and K refer to tetramethyl ammonium, sodium, and potassium counterions, respectively. This sequence is similar to what is seen in bulk electrolyte solutions. At low hydrations, however, the order of water self-diffusion is different, that is, D_Na < D_TMA < D_K. In this work, we present molecular dynamics simulations for the dynamics of water in the LLC phases of dicarboxylate Gemini surfactants. The simulations reproduce the trends seen in experiments. From an analysis of the trajectories, we hypothesize that two competing factors play a role: the volume accessible to the water molecules and the correlations between the water and the counterion. The excluded volume effect is the largest with TMA⁺, and the electrostatic correlation is the strongest with Na⁺. The observed trend is a result of which of these two effects is dominant at a given water to surfactant ratio.