Thermodynamic Barrier Research Articles

Concentrating photo-thermo-organized single-atom and 2D-raft Cu catalyst for full-spectrum solar harmonic conversion of aqueous urea and urine into hydrogen

Solar-driven hydrogen evolution from urea-rich wastewater is a highly promising energy-environment bi-functional synthetical solution, while the intrinsic contradict between spectral energy quantity and quality of sunlight limits the existing wavelength-dependent technologies. Herein, we report the first demonstration of concentrated photo-thermo-catalysis (CPTC) technology that regulates the spectral energy quantity and quality and creates the synergy between photon and thermal effects to achieve full-spectrum solar harmonic conversion of aqueous urea and urine into hydrogen. The 50% enhanced CPTC performance compared with sole photo-/thermo-catalysis is achieved due to: (1) Catalyst structure optimization. Single Cu atom and 2D Cu raft are acquired on defect-rich TiO2 support by photo-thermo-organized metal-support interaction; (2) Reaction pathway expansion. The breakthrough against thermodynamic barrier of urea to hydrogen is realized by the novel cascade reactions of thermally-triggered urea hydrolysis and thermally-assisted ammonia photo-decomposition. The technological viability is convincingly presented through outdoor test with artificial urine under real sunlight.

Secondary Nucleation by Interparticle Energies. III. Nucleation Rate Model.

A nucleation rate model for describing the kinetics of secondary nucleation caused by interparticle energies (SNIPEs) is derived theoretically, verified numerically, and validated experimentally. The theoretical derivation reveals that the SNIPE mechanism can be viewed as enhanced primary nucleation, i.e., primary nucleation with a lower thermodynamic energy barrier (for nucleation) and a smaller critical nucleus size, both caused by the interparticle interactions and the associated energy between the surface of a seed crystal and a molecular cluster in solution, as shown in part I of this series. In the case of a sufficiently agitated suspension, the model depends on four parameters: two reflecting primary nucleation kinetics and the other two accounting for the intensity and effective spatial range of the interparticle interactions. As a numerical verification of the model, we show that the nucleation kinetics described by the SNIPE rate model is in quantitative agreement with those given by the kinetic rate equation model developed in part II of this series. A sensitivity analysis of the SNIPE rate model is conducted to present the effect of key model parameters on the nucleation kinetics. Moreover, the SNIPE rate model is validated by fitting the model to the time-resolved data of secondary nucleation experiments as well as to two other, well-known secondary nucleation rate models. Importantly, all of the estimated parameter values for the SNIPE model were consistent with the theoretical estimates, while some of the estimated parameter values for one of the well-known secondary nucleation models deviated from the corresponding theoretical values significantly.

The Graphitic Carbon Nitride (g-C3N4): A Promising Material for Photocatalytic CO2 Reduction

The excess use of fossil fuel and rapid industrialization increase of CO2 level in the atmosphere. Efficient conversion of CO2 to value added chemicals and fuels are the best method to reduce CO2 level as well as to maintain sustainability. To overcome the kinetic and thermodynamic barrier in CO2, a suitable heterostructure composite is necessary. Among different types of materials carbon based materials and their composites are shown their suitability electrochemical, thermal and photo-assisted method of CO2 conversion. Among different carbon based materials, graphitic carbon nitrite (g-C3N4) is a sustainable, inexpensive photocatalyst for CO2 conversion. This present article tries to summarize recent works based on conversion of CO2 in presence of solar light using g-C3N4 based composites. This provides an outlook for the future development of new catalysts to reduce CO2 effectively and save our earth from CO2 pollution.

Ion-pair reorganization regulates reactivity in photoredox catalysts.

Cyclometalated and polypyridyl complexes of d6 metals are promising photoredox catalysts, using light to drive reactions with high kinetic or thermodynamic barriers via the generation of reactive radical intermediates. However, while tuning of their redox potentials, absorption energy, excited-state lifetime and quantum yield are well-known criteria for modifying activity, other factors could be important. Here we show that dynamic ion-pair reorganization controls the reactivity of a photoredox catalyst, [Ir[dF(CF3)ppy]2(dtbpy)]X. Time-resolved dielectric-loss experiments show how counter-ion identity influences excited-state charge distribution, evincing large differences in both the ground- and excited-state dipole moment depending on whether X is a small associating anion (PF6-) that forms a contact-ion pair versus a large one that either dissociates or forms a solvent-separated pair (BArF4-). These differences correlate with the reactivity of the photocatalyst toward both reductive and oxidative electron transfer, amounting to a 4-fold change in selectivity toward oxidation versus reduction. These results suggest that ion pairing could be an underappreciated factor that modulates reactivity in ionic photoredox catalysts.

Tetragonal transition metal selenide for hydrogen evolution

Hydrogen evolution reaction (HER) plays an important role in energy conversion through water splitting to produce hydrogen. Considering the large surface/volume ratio, exploration of two-dimensional materials with high-efficient basal plane has attracted a great enthusiasm in designing electrocatalysts. Herein, tetragonal copper selenide (CuSe) was identified as a promising candidate toward HER by density functional theory calculations. The low exfoliation energy Eexf of 41.47 meV/Å2 ensures the exfoliation of CuSe monolayer along [001] direction, which allows the exposure of the basal plane. More importantly, the pristine CuSe monolayer without any treatment delivers relatively good activity, as reflected by the small thermodynamic barrier of 0.3 eV. Furthermore, CuSe possesses good electron conductivity, which makes it beneficial to the electron transfer during electrocatalysis. Therefore, these results indicate that CuSe acts as a promising material for the next-generation HER catalysis with the high basal utilization.

Molybdenum bound nitrogen‐doped graphene catalyst for reduction of N2 to NH3 and NH2NH2, using FLP as a co‐catalyst: A DFT study

Chemists have long been pursuing the aim of synthesising practically viable low temperature catalysts with high stability, activity and selectivity towards the generation of ammonia under less expensive reaction conditions by using H2 and N2. In this regard, we have used molybdenum bound nitrogen‐doped graphene as a catalyst in the present computational investigation. We developed a new concept of the Frustrated Lewis Pair [AH]+[BH]− (FLP‐H2), which can be used as a source of hydrogen to functionalise the coordinated dinitrogen into ammonia. Furthermore, the utilisation of FLP‐H2 can prevent the competition in binding of N2 and H2 with Mo centre and enable the product formation. Our computed thermodynamic and kinetic barriers clearly reveal that the molybdenum bound nitrogen‐doped graphene can be used as a catalyst to activate and functionalise the dinitrogen. In the proposed catalytic cycle, H+ and H− can be added in an alternate manner using the FLP‐H2 source to yield NH3 and NH2NH2. Some thermodynamically viable penta‐coordinated [M]H intermediates have also been identified in the current study.

A universal chemical-induced tensile strain tuning strategy to boost oxygen-evolving electrocatalysis on perovskite oxides

Exploring effective, facile, and universal tuning strategies to optimize material physicochemical properties and catalysis processes is critical for many sustainable energy systems, but still challenging. Herein, we succeed to introduce tensile strain into various perovskites via a facile thermochemical reduction method, which can greatly improve material performance for the bottleneck oxygen-evolving reaction in water electrolysis. As an ideal proof-of-concept, such a chemical-induced tensile strain turns hydrophobic Ba5Co4.17Fe0.83O14-δ perovskite into the hydrophilic one by modulating its solid–liquid tension, contributing to its beneficial adsorption of important hydroxyl reactants as evidenced by fast operando spectroscopy. Both surface-sensitive and bulk-sensitive absorption spectra show that this strategy introduces oxygen vacancies into the saturated face-sharing Co-O motifs of Ba5Co4.17Fe0.83O14-δ and transforms such local structures into the unsaturated edge-sharing units with positive charges and enlarged electrochemical active areas, creating a molecular-level hydroxyl pool. Theoretical computations reveal that this strategy well reduces the thermodynamic energy barrier for hydroxyl adsorption, lowers the electronic work function, and optimizes the charge/electrostatic potential distribution to facilitate the electron transport between active sites and hydroxyl reactants. Also, this strategy is reliable for other single, double, and Ruddlesden–Popper perovskites. We believe that this finding will enlighten rational material design and in-depth understanding for many potential applications.

Significantly enhanced charge transfer efficiency and surface reaction on NiP2/g-C3N4 heterojunction for photocatalytic hydrogen evolution

• g-C 3 N 4 with NiP 2 can achieved superb charge transfer efficiency and surface reaction. • The thermodynamic energy barrier of g-C 3 N 4 in the water splitting was decreased by loading NiP 2 . • The mechanism of charge transfer and surface reaction are proposed by DFT calculation and experimental measurement. In this work, a novel NiP 2 /g-C 3 N 4 heterojunction via homogeneous precipitation method assisted by thermal phosphorization reaction was designed and constructed, and the optimized sample showed the excellent photocatalytic H 2 evolution activity under visible-light irradiation, which was nearly 112 times higher than that of pristine g-C 3 N 4 sample. Experimental characterizations and DFT calculations demonstrated that the NiP 2 nanoparticles covered on the g-C 3 N 4 surface can form a built-in electric field at the interface to accelerate the transfer of photoexcited electrons from g-C 3 N 4 to NiP 2 , crucial for hindering the recombination of electron-hole pairs. Moreover, the energy barrier of hydrogen evolution reaction can also vastly reduce when combined NiP 2 and g-C 3 N 4 to construct NiP 2 /g-C 3 N 4 heterojunction. This work represents a method through combing experimental and theoretical tools to thoroughly investigate the mechanism of photocatalytic process.

Origin of enhanced thermal atomic layer etching of amorphous HfO2

HfO2 is a high-k material that is used in semiconductor devices. Atomic-level control of material processing is required for the fabrication of thin films of high-k materials at nanoscale device sizes. Thermal atomic layer etching (ALE) of metal oxides, in which up to one monolayer of material can be removed, can be achieved by sequential self-limiting fluorination and ligand-exchange reactions at elevated temperatures. First-principles-based atomic-level simulations using density functional theory can give deep insights into the precursor chemistry and the reactions that drive the etching of metal oxides. A previous study examined the hydrogen fluoride (HF) pulse in the first step in the thermal ALE process of crystalline HfO2 and ZrO2. This study examines the HF pulse on amorphous HfO2 using first-principles simulations. The Natarajan–Elliott analysis, a thermodynamic methodology, is used to compare reaction models representing the self-limiting and spontaneous etch processes taking place during an ALE pulse. For the HF pulse on amorphous HfO2, we found that thermodynamic barriers impeding spontaneous etching are present at ALE relevant temperatures. HF adsorption calculations on the amorphous oxide surface are studied to understand the mechanistic details of the HF pulse. An HF molecule adsorbs dissociatively by forming Hf–F and O–H bonds. HF coverages ranging from 1.1 ± 0.3 to 18.0 ± 0.3 HF/nm2 are investigated, and a mixture of molecularly and dissociatively adsorbed HF molecules is present at higher coverages. A theoretical etch rate of −0.82 ± 0.02 Å/cycle for amorphous HfO2 was calculated using a maximum coverage of 9.0 ± 0.3 Hf–F/nm2. This theoretical etch rate is greater than the theoretical etch rate for crystalline HfO2 that we previously calculated at −0.61 ± 0.02 Å/cycle. Undercoordinated atoms and void regions in amorphous HfO2 allow for more binding sites during fluorination, whereas crystalline HfO2 has a limited number of adsorption sites.

Methane transformation by photocatalysis

Methane hydrate and shale gas are predicted to have substantial reserves, far beyond the sum of other fossil fuels. Using methane instead of crude oil as a building block is, thus, a very attractive strategy for synthesizing valuable chemicals. Because methane is so inert, its direct conversion needs a high activation energy and typically requires harsh reaction conditions or strong oxidants. Photocatalysis, which employs photons operated under very mild conditions, is a promising technology to reduce the thermodynamic barrier in direct methane conversion and to avoid the common issues of overoxidation and catalyst deactivation. In this Review, we cover the development of photocatalysts and co-catalysts, including the use of inorganic materials and polymeric semiconductors, and explain how the use of batch or flow reaction systems affects the reaction kinetics and product selectivity. We also discuss efforts to understand the underlying reaction mechanisms from both a photophysical and a chemical perspective. Finally, we present our view of the challenges facing this field and suggest potential solutions. Methane, as a replacement for crude oil, can be converted into valuable chemicals through photocatalysis. This Review discusses the design of promising photocatalytic systems and strategies to understand their reaction mechanisms and outlines the most pressing questions still facing the field.

Accelerating CO2 Electroreduction to Multicarbon Products via Synergistic Electric-Thermal Field on Copper Nanoneedles.

Electrochemical CO2 reduction is a promising way to mitigate CO2 emissions and close the anthropogenic carbon cycle. Among products from CO2RR, multicarbon chemicals, such as ethylene and ethanol with high energy density, are more valuable. However, the selectivity and reaction rate of C2 production are unsatisfactory due to the sluggish thermodynamics and kinetics of C-C coupling. The electric field and thermal field have been studied and utilized to promote catalytic reactions, as they can regulate the thermodynamic and kinetic barriers of reactions. Either raising the potential or heating the electrolyte can enhance C-C coupling, but these come at the cost of increasing side reactions, such as the hydrogen evolution reaction. Here, we present a generic strategy to enhance the local electric field and temperature simultaneously and dramatically improve the electric-thermal synergy desired in electrocatalysis. A conformal coating of ∼5 nm of polytetrafluoroethylene significantly improves the catalytic ability of copper nanoneedles (∼7-fold electric field and ∼40 K temperature enhancement at the tips compared with bare copper nanoneedles experimentally), resulting in an improved C2 Faradaic efficiency of over 86% at a partial current density of more than 250 mA cm-2 and a record-high C2 turnover frequency of 11.5 ± 0.3 s-1 Cu site-1. Combined with its low cost and scalability, the electric-thermal strategy for a state-of-the-art catalyst not only offers new insight into improving activity and selectivity of value-added C2 products as we demonstrated but also inspires advances in efficiency and/or selectivity of other valuable electro-/photocatalysis such as hydrogen evolution, nitrogen reduction, and hydrogen peroxide electrosynthesis.

Nitrogen vacancies enriched Ce-doped Ni3N hierarchical nanosheets triggering highly-efficient urea oxidation reaction in urea-assisted energy-saving electrolysis

Urea oxidation reaction (UOR), which has favorable thermodynamic energy barriers compared with oxygen evolution reaction (OER), can provide more cost-effective electrons for the renewable energy systems, but is trapped by its sluggish UOR kinetics and intricate reaction intermediates formation/desorption process. Herein, we report a novel and effective electrocatalyst consisting of carbon cloth supported nitrogen vacancies-enriched Ce-doped Ni3N hierarchical nanosheets (Ce-Ni3N@CC) to optimize the flat-footed UOR kinetics, especially the stiff rate-determine CO2 desorption step of UOR. Upon the introduction of valance state variable Ce, the resultant nitrogen vacancies enriched Ce-Ni3N@CC exhibits an enhanced UOR performance where the operation voltage requires only 1.31 V to deliver the current density of 10 mA cm−2, which is superior to that of Ni3N@CC catalyst (1.36 V) and other counterparts. Density functional theory (DFT) results demonstrate that the incorporation of Ce in Ni3N lowers the formation energy of nitrogen vacancies, resulting in rich nitrogen vacancies in Ce-Ni3N@CC. Moreover, the nitrogen vacancies together with Ce doping optimize the local charge distribution around Ni sites, and balance the adsorption energy of CO2 in the rate-determining step (RDS), as well as affect the initial adsorption structure of urea, leading to the superior UOR catalytic performance of Ce-Ni3N@CC. When integrating the Ce-Ni3N catalyst in UOR//HER and UOR//CO2R flow electrolyzer, both of them perform well with low operation voltage and robust long-term stability, proofing that the thermodynamically favorable UOR can act as a suitable substitute anodic reaction compared with that of OER. Our findings here not only provide a novel UOR catalyst but also offer a promising design strategy for the future development of energy-related devices.

Flow-induced crystallization of polylactides

In this work, the flow-induced crystallization of two polylactides (PLAs) with different microstructures (different l-lactic acid contents) is studied using simple shear, uniaxial extension and capillary flow experiments. In a simple shear and capillary flow, an increase in shear rate and a decrease in temperature were found to enhance the crystallization kinetics particularly for Weissenberg numbers (based on the reptation relaxation time, Wi) greater than 1 (strong flow causing chain stretching). On the other hand, in a uniaxial extensional flow, once a critical Hencky strain is achieved, crystallization starts independently of strain rate and temperature. The amount of mechanical work per unit volume imposed/dissipated onto the polymers during flow to initialize crystallization was also calculated in the simple shear, capillary, and extensional flow. The critical mechanical work for the onset of flow-induced crystallization was found to be independent of temperature and degree of molecular chain stretch (Wi) as Wi becomes greater than 1. The critical mechanical work for the onset of flow-induced crystallization in an extensional flow was found to be much smaller than that in a shear flow. The PLA sample with higher content of PLLA showed slightly higher zero-shear viscosity and a smaller thermodynamic barrier for the onset of crystallization. Finally, the degree of crystallinity increases linearly from 0% at the start of the flow-induced crystallization region and reaches a plateau at Wi equals to around 1.

Crystallization Propensity of Amorphous Pharmaceuticals: Kinetics and Thermodynamics.

Four model compounds, nifedipine, indomethacin, felodipine, and ketoconazole, all with nearly identical glass transition temperatures, were chosen to study the effects of thermodynamics and molecular mobility on their crystallization propensities. The time and temperature dependence of the crystallization induction time of each compound was determined by differential scanning calorimetry (DSC) and enabled the generation of their time-temperature-transformation (TTT) diagrams. The relaxation times (τα) were measured by dielectric spectroscopy, and the Gibbs free energy (ΔG) and entropy (ΔS) difference between the crystalline and amorphous states were obtained by DSC. The temperature dependence of the crystallization induction time (τ0(T)) is a function of the thermodynamic activation barrier and the frequency of "attempted jumps" (1/τα(T)) to overcome the barrier. Even though the four model compounds exhibited very similar molecular mobility (relaxation time) over a wide range of temperatures, their crystallization propensities were very different. The observed difference in crystallization propensity was explained in terms of the difference in the thermodynamic barrier, and it is correlated to the empirical relation (TΔS3)/ΔG2.

Modulating the structure of iron-doped titanium nitride deposited on a nitrogen-doped carbon catalyst for the oxygen reduction reaction in Zn–air batteries

The DFT calculations demonstrate that iron doping boosts the charge transfer rate of Ti0.95Fe0.05N/NC/C, reduces the thermodynamic energy barrier of OOH* formation, and thus enhances ORR catalytic activity.

UV-selective organic absorbers for the cosensitization of greenhouse-integrated dye-sensitized solar cells: synthesis and computational study.

Molecular cosensitization is favorable for manipulating solar radiation through the judicious choice of cosensitizers having complementary absorption spectra. For greenhouse-integrated dye-sensitized solar cells (DSCs), the manipulation of solar radiation is crucial in order to maximize the flow of photosynthetically active radiation (PAR) for the effectual photosynthetic activity of plants; meanwhile, non-PAR is utilized in agrivoltaics for generating electricity. In this study, we report the synthesis of novel four UV-selective absorbers, based on the diimide scaffold, functionalized with carboxylate and pyridyl anchoring groups, for adequate adsorption onto the TiO2 electrode in DSC. The UV/Vis absorption spectra of the DMF solution-based free dyes were measured experimentally. Basic photophysical and energetics requirements for operating greenhouse-integrated DSCs were examined at the molecular level via (time-dependent) density functional theory-based calculations. The computational results revealed the outperformance of the biphenyldiimide-structured DI-CA1 dye, especially for maximum charge transferred to its anchor, lower thermodynamic barrier for dissociating the photogenerated exciton, largest Stokes' shift, strong electronic coupling with TiO2 nanoparticles, and higher degree of charge separation at the DI-CA1/TiO2 interface. PDOS showed deeper existence for the LUMO level in the CB of TiO2, which expedites the electron injection process. The chemical and optical compatibility of DI-CA1 were then investigated as a potential cosensitizer of a reference BTD–DTP1, a green light-absorbing dye. Considerable overlap between the fluorescence spectrum of DI-CA1 and absorption spectrum of the reference BTD–DTP1 advocated the opportunity of excitation energy transfer via the radiative trivial reabsorption mechanism, which confirms the cosensitization functionality. Energy decomposition analysis and reduced density gradient maps estimated the chemical compatibility owing to weak dispersion interactions as the dominant stabilizing attractive force. This noncovalent functionalization retains the chemical compatibility without distorting the π–π conjugation and the associated physicochemical properties of the individual dye molecules. Along with the expanded consumption of non-photosynthetically active solar radiation, an improved power conversion efficiency of greenhouse-integrated DSC is accordingly expected.

A New Computational Approach to Calculate Kinetic and Thermodynamic Barriers for Estimation of Nucleation and Interfacial Energy of Caco3 Prenucleation Nanoclusters at High Temperature Using Tga Data

A New Computational Approach to Calculate Kinetic and Thermodynamic Barriers for Estimation of Nucleation and Interfacial Energy of Caco3 Prenucleation Nanoclusters at High Temperature Using Tga Data

Coupling between nucleation and chemical reactions and its importance in understanding the oxidation of iodine by hydrogen peroxide.

We have investigated the reaction of iodine with hydrogen peroxide coupled to gas nucleation. A step-like increase in the nucleation rate with increasing amounts of dissolved oxygen can act as a trigger for the formation of highly reactive components and complete oxidation of iodine to iodate despite the large thermodynamic barrier for the whole process. Energetic coupling of nucleation with chemical reactions is based on local redistribution of energy by collapsing unstable nuclei. The developed model correctly describes the evolution of the measured reaction parameters. It offers a conceptual improvement of formal-kinetic models by drawing attention to important contributions of physical effects to the reaction mechanism.

Structural investigation of phosphonium-based ionic liquid impregnated mesostructured silica nanoparticles and application towards the adsorption of Pb(II)

Adsorption has been widely used in waste water treatment due to its simplicity and cost-effective process. One of the popular adsorbent was silica due to their advantageous properties. Immobilizing additive onto particular adsorbent is one of the way to enhance adsorption capacity. Immobilizing ionic liquid (IL) onto silica-based support could alleviate the drawbacks of IL and enhance the adsorption capacity. Thus, silica-based materials, silica and mesostructured silica nanoparticle (MSN), were impregnated with a phosphonium-based ionic liquid (IL), trioctyldodecylphosphonium bromide (P8,8,8,12Br), which was designated as IL@SIL and IL@MSN. The structural variation of the support greatly impact the adsorbent performance. The results showed that IL@MSN with superior surface properties exhibit a faster reaction kinetic and lower thermodynamic barrier for the adsorption of Pb(II). Both adsorbents followed a pseudo second-order kinetic and chemically adsorbed Pb(II). IL@MSN showed a higher adsorption capacity in both batch and column adsorption than that of IL@SIL. The equilibrium data of both adsorbents fitted well with the Freundlich isotherm model with maximum adsorption capacity of 256.4 and 142.9 mg g−1 for IL@MSN and IL@SIL, respectively. In agreement with the batch experiment, IL@MSN showed a higher adsorption capacity in column adsorption than that of IL@SIL (325.6 and 242.2, respectively). Column adsorption with IL@MSN was able to achieve 94% removal of Pb(II) while IL@SIL achieved 70%. Superior surface properties of IL@MSN allowed a longer duration of column exhaustion than that of IL@SIL. The column experimental data showed a good fit with the Thomas model. Immobilized IL on silica based materials has potential as an adsorbent for heavy metals and structural variation over the support material played an important role.

The permeability of human red blood cell membranes to hydrogen peroxide is independent of aquaporins

Hydrogen peroxide (H2O2) not only is an oxidant but also is an important signaling molecule in vascular biology, mediating several physiological functions. Red blood cells (RBCs) have been proposed to be the primary sink of H2O2 in the vasculature because they are the main cellular component of blood with a robust antioxidant defense and a high membrane permeability. However, the exact permeability of human RBC to H2O2 is neither known nor is it known if the mechanism of permeation involves the lipid fraction or protein channels. To gain insight into the permeability process, we measured the partition constant of H2O2 between water and octanol or hexadecane using a novel double-partition method. Our results indicated that there is a large thermodynamic barrier to H2O2 permeation. The permeability coefficient of H2O2 through phospholipid membranes containing cholesterol with saturated or unsaturated acyl chains was determined to be 4 × 10−4 and 5 × 10−3 cm s−1, respectively, at 37 °C. The permeability coefficient of human RBC membranes to H2O2 at 37 °C, on the other hand, was 1.6 × 10−3 cm s−1. Different aquaporin-1 and aquaporin-3 inhibitors proved to have no effect on the permeation of H2O2. Moreover, human RBCs devoid of either aquaporin-1 or aquaporin-3 were equally permeable to H2O2 as normal human RBCs. Therefore, these results indicate that H2O2 does not diffuse into RBCs through aquaporins but rather through the lipid fraction or a still unidentified membrane protein.