Concentration gradients in evaporating binary droplets probed by spatially resolved Raman and NMR spectroscopy.

Understanding the evaporation process of binary sessile droplets is essential for optimizing various technical processes, such as inkjet printing or heat transfer. Liquid mixtures whose evaporation and wetting properties may differ significantly from those of pure liquids are particularly interesting. Concentration gradients may occur in these binary droplets. The challenge is to measure concentration gradients without affecting the evaporation process. Here, spectroscopic methods with spatial resolution can discriminate between the components of a liquid mixture. We show that confocal Raman microscopy and spatially resolved NMR spectroscopy can be used as complementary methods to measure concentration gradients in evaporating 1-butanol/1-hexanol droplets on a hydrophobic surface. Deuterating one of the liquids allows analysis of the local composition through the comparison of the intensities of the C­H and C­D stretching bands in Raman spectra. Thus, a concentration gradient in the evaporating droplet was established. Spatially resolved NMR spectroscopy revealed the composition at different positions of a droplet evaporating in the NMR tube, an environment in which air exchange is less pronounced. While not being perfectly comparable, both methods­confocal Raman and spatially resolved NMR experiments­show the presence of a vertical concentration gradient as 1-butanol/1-hexanol droplets evaporate.

Ice-Water Equilibrium in Nanoscale Confinement.

We show that 2D ^{2}H NMR spectra enable valuable insights into the nature of an ice-water equilibrium in nanoscale confinement, which extends over a broad temperature range. In particular, 2D ^{2}H NMR line-shape analysis allows us to determine the timescale on which the coexisting ice and water phases exchange molecules. For D_{2}O in a silica nanopore with a diameter of 5.4 nm, we find that the residence time of a water molecule in either phase is characterized by an NMR exchange time of τ_{X}=5.7 ms at 220 K. Thus, the ice-water equilibrium is highly dynamic, which is an important aspect for an understanding of deeply cooled confined and, possibly, bulk waters.

Elastin-Like Peptide as a Model for Disordered Proteins: Diffusion Behaviour in Self-Crowding Conditions.

Despite the current high interest, there is limited information on diffusion data for intrinsically disordered proteins (IDPs). This study investigates the effect of crowding on the diffusion behaviour of an elastin-like peptide (ELP), by combined pulse field gradient (PFG) and static field gradient (SFG) NMR techniques. We interpret our findings in terms of highly dynamic chain assemblies with weak interactions, resulting in ELP diffusion that is primarily governed by the viscous flow of the solvent. The diffusion behaviour of the peptide appears to resemble that of globular proteins rather than flexible linear polymers over a wide concentration range.

Complex dynamics of partially freezable confined water revealed by combined experimental and computational studies.

We investigate water dynamics in mesoporous silica across partial crystallization by combining broadband dielectric spectroscopy (BDS), nuclear magnetic resonance (NMR), and molecular dynamics simulations (MDS). Exploiting the fact that not only BDS but also NMR field-cycling relaxometry and stimulated-echo experiments provide access to dynamical susceptibilities in broad frequency and temperature ranges, we study both the fully liquid state above the melting point Tm and the dynamics of coexisting water and ice phases below this temperature. It is found that partial crystallization leads to a change in the temperature dependence of rotational correlation times τ, which occurs in addition to previously reported dynamical crossovers of confined water and depends on the pore diameter. Furthermore, we observe that dynamical susceptibilities of water are strongly asymmetric in the fully liquid state, whereas they are much broader and nearly symmetric in the partially frozen state. Finally, water in the nonfreezable interfacial layer below Tm does not exhibit a much debated dynamical crossover at ∼220 K. We argue that its dynamics is governed by a static energy landscape, which results from the interaction with the bordering silica and ice surfaces and features a Gaussian-like barrier distribution. Consistently, our MDS analysis of the motional mechanism reveals a hopping motion of water in thin interfacial layers. The rotational correlation times of the confined ice phases follow Arrhenius laws. While the values of τ depend on the pore diameter, freezable water in various types of confinements and mixtures shows similar activation energies of Ea ≈ 0.43 eV.

Primary and Revision Hip Arthroscopy in Borderline Hip Dysplasia Shows Comparable Outcomes at a Minimum 5-Year Follow-Up.

PURPOSE: To compare patient-reported outcomes (PROs), achievement of clinically significant outcomes, and reoperation-free survivorship between primary and revision hip arthroscopy (HA) for femoroacetabular impingement syndrome (FAIS) in propensity-matched borderline hip dysplasia (BHD) patients at a minimum 5-year follow-up. METHODS: Patients with BHD, characterized by a lateral center-edge angle 18° to 25°, who underwent HA for FAIS with capsular repair by a single surgeon between January 2012 and June 2018 with a minimum 5-year follow-up were identified. Cases of revision HA were propensity-matched 1:2 to cases of primary HA, controlling for age, sex, and body mass index. A 1:2 ratio was chosen to maximize the number of included patients. Collected PROs included Hip Outcome Score-Activities of Daily Living and Sport Subscales, International Hip Outcome Score 12, modified Harris Hip Score, and Visual Analog Scale for Pain. Achievement of minimal clinically important difference, patient acceptable symptom state, and substantial clinical benefit for any measured PRO was compared between groups along with reoperation-free survivorship using Kaplan-Meier analysis. RESULTS: Thirty-six revision HA hips (34 patients) were propensity-matched to 72 primary HA hips (70 patients). The groups were similar in age (31.5 ± 10.3 years vs 30.5 ± 11.2, P = .669), sex (69.4% female vs 70.8%, P = .656), and body mass index (25.7 ± 4.0 vs 25.5 ± 3.7, P = .849). The revision group showed a greater prevalence of prolonged preoperative pain (50.0% vs 27.8%, P = .032) compared with the primary group. A significant improvement in all PROs was observed for both groups with comparable PROs preoperatively and at the 5-year follow-up between groups (P ≥ .086). The revision and primary groups showed comparable minimal clinically important difference (95.0% vs 95.7%, P ≥ .999), patient acceptable symptom state (80.0% vs 83.6%, P = .757), and substantial clinical benefit (62.5% vs 70.7%, P = .603) achievement for any PRO. Comparable reoperation-free survivorship was observed (P = .151). CONCLUSIONS: Propensity-matched patients with BHD undergoing primary and revision hip arthroscopy for FAIS achieved similar minimum 5-year PROs, clinically significant outcomes, and reoperation-free survivorship. LEVEL OF EVIDENCE: Level III, retrospective comparative case series.

Cartilage Defects Are Negatively Associated With Long-Term Hip Survivorship Following Contemporary Hip Arthroscopy for Femoroacetabular Impingement Syndrome: A Propensity-Matched Analysis at Minimum 10-Year Follow-Up.

PURPOSE: To identify the timing and risk factors associated with secondary surgery following primary hip arthroscopy for femoroacetabular impingement syndrome (FAIS) at 10-year minimum follow-up. METHODS: A prospectively collected clinical repository was evaluated for cases of primary hip arthroscopy for FAIS between January 2012 and February 2013 with minimum 10-year follow-up. Patients who underwent secondary surgery were propensity matched 1:4 to patients who did not undergo secondary surgery, controlling for age, sex, and body mass index (BMI). The groups were compared on demographics, radiographs, intraoperative findings, operative procedures, and patient-reported outcomes. A Kaplan-Meier survivorship curve was generated. Among the reoperation-free survivors, minimal clinically important difference (MCID) and patient acceptable symptom state (PASS) achievement were recorded for Hip Outcome Score-Activities of Daily Living (HOS-ADL), Hip Outcome Score-Sports Specific (HOS-SS) subscales, modified Harris Hip Score (mHHS), 12-item international Hip Outcome Tool (iHOT-12), and Visual Analog Scale for Pain (VAS Pain). RESULTS: Twenty-four reoperation patients (67% female; age 40.1 ± 14.3 years; BMI 27.2 ± 5.5) were matched to 96 reoperation-free patients (62% female; age 37.0 ± 10.8 years; BMI 25.2 ± 4.7, P ≥ .111). Mean follow-up was 10.3 ± 0.2 years. No preoperative demographic differences were found between groups. The reoperation group showed more high-grade cartilage defects on the acetabulum and femoral head (33% vs 8%, P = .004; 29% vs 7%, P = .007). A bimodal distribution of time to reoperation was evidenced independent of the secondary surgery performed. Among the reoperation-free survivors, MCID and PASS achievement was as follows: HOS-ADL (69.1%, 62.1%), HOS-SS (69.9%, 74.4%), mHHS (73.3%, 58.1%), iHOT-12 (n/a, 63.8%), and VAS-Pain (80.2%, 62.6%). CONCLUSIONS: Patients requiring reoperation following primary hip arthroscopy for FAIS demonstrated more severe cartilage defects and a bimodal distribution of time to reoperation. LEVEL OF EVIDENCE: Level III, retrospective comparative case series.

Primary Hip Arthroscopy is Associated with Earlier Achievement of Substantial Clinical Benefit Compared to Revision Hip Arthroscopy for Femoroacetabular Impingement Syndrome.

PURPOSE: To compare time to achievement of clinically significant outcomes (CSOs) between patients undergoing primary and revision hip arthroscopy (HA) for femoroacetabular impingement syndrome (FAIS). METHODS: Patients undergoing primary and revision HA for FAIS with complete 6-month, 1-year, and 2-year Hip Outcome Score - Activities of Daily Living (HOS-ADL) and Sport Subscale (HOS-SS) were identified. Revision patients were propensity matched 1:4 to primary HA patients, controlling for age, sex, and body mass index (BMI). Time to achievement of minimal clinically important difference (MCID) and substantial clinical benefit (SCB) were compared alongside cumulative CSO achievement at 6, 12, and 24 months. Hazard ratios (HR) for predictors of earlier CSO achievement were identified with multivariate Cox regressions. RESULTS: Fifty revision HA patients were propensity-matched to 200 primary HA patients of similar age, sex, and BMI. Primary HA patients demonstrated a greater prevalence of regular preoperative physical activity (87% vs. 59%, p<0.001). Primary HA patients showed significantly greater SCB achievement for HOS-ADL at 6, 12, and 24 months (p<0.001) and significantly greater SCB achievement for HOS-SS at 12 and 24 months (p≤0.001) compared to revision HA patients. Primary HA patients achieved SCB for HOS-ADL (p<0.001) and HOS-SS (p=0.015) quicker than revision HA patients. Predictors of earlier CSO achievement included preoperative PRO score (HR: 0.98-1.02, p≤0.007), lower BMI (HR: 0.97, p=0.038), presence of physical activity (HR: 1.51, p=0.038), and absence of revision status (HR: 0.52-0.56, p≤0.019). CONCLUSIONS: Primary HA patients showed a quicker time to SCB achievement for HOS-ADL and HOS-SS compared to revision HA patients. Preoperative PRO score, lower BMI, regular physical activity, and primary HA status predicted earlier CSO achievement. LEVEL OF EVIDENCE: Level III, retrospective comparative series.

Gluteus Repair Yields Satisfactory Clinically Significant Outcome Achievement by 1 Year in Mostly Partial-Thickness Tears With Preoperative Hip Abduction Weakness Associated With Delayed Achievement.

PURPOSE: To define the time to achievement of clinically significant outcomes (CSOs) after primary gluteus medius and/or minimus (GM) repair and to identify factors associated with delayed CSO achievement. METHODS: Patients who underwent primary GM repair between January 2012 and June 2021 with complete preoperative, 6-month, 1-year, and 2-year Hip Outcome Score-Activities of Daily Living (HOS-ADL) were retrospectively identified. Cohort-specific minimal clinically important difference (MCID) and patient acceptable symptom state (PASS) were calculated. The time to achievement of MCID and PASS was analyzed using Kaplan-Meier survival analysis. Median time to MCID and PASS achievement was recorded. Multivariate stepwise Cox regressions were used to identify factors associated with delayed CSO achievement. RESULTS: Fifty GM repairs were identified (age 59.4 ± 9.7 years, body mass index 27.9 ± 6.2, 94% female). Tears were grade 1 in 39 cases, grade 2 in 7 cases, and grade 3 in 4 cases. Endoscopic repair was performed in 35 cases, and open repair was performed in 15 cases. Labral debridement and repair were each performed in 15 cases. Median time to CSO achievement was 5.7 months for MCID and 11.0 months for PASS. The 2-year cumulative probability of MCID and PASS achievement was 92.7% and 66.7%, respectively. Preoperative hip abduction weakness on physical examination was associated with delayed achievement of MCID (hazard ratio 2.27, confidence interval 1.067-7.41, P = .039) and PASS (hazard ratio 3.89, confidence interval 1.341-11.283, P = .012). CONCLUSIONS: This study demonstrated that in patients undergoing repair of primarily grade 1 GM tears, most achieved MCID by 6 months, and more than one half achieved PASS by 12 months. Preoperative hip abduction weakness on physical examination was associated with delayed CSO achievement. LEVEL OF EVIDENCE: Level IV, retrospective case series.

Six-Month Outcomes Correlate With 10-Year Outcomes After Hip Arthroscopy for Femoroacetabular Impingement Syndrome.

PURPOSE: To identify whether 6-month outcomes after hip arthroscopy for femoroacetabular impingement syndrome (FAIS) correlate with outcomes at minimum 10-year follow-up. METHODS: Patients who underwent primary hip arthroscopy for FAIS from 2012 to 2013 were reviewed and included if they had 6-month and minimum 10-year follow-up. Patient-reported outcome (PRO) measures included the Hip Outcome Score Activities of Daily Living (HOS-ADL) subscale, Hip Outcome Score Sports-Specific (HOS-SS) subscale, modified Harris Hip Score (mHHS), visual analog scale (VAS) for pain, and VAS for satisfaction. We compared 6-month and 10-year outcome scores and analyzed the relations between 6-month and 1-, 2-, 5-, and 10-year outcome scores using Pearson correlation coefficients (r). Six-month scores and clinically significant outcome achievement were then compared with 10-year clinically significant outcome achievement and reoperations, including revision hip arthroscopy and conversion to total hip arthroplasty (THA), using logistic regressions and the Fisher exact test. RESULTS: This study included 60 patients (60.0% female sex; mean age, 36.0 ± 12.2 years). The mHHS, VAS pain score, and VAS satisfaction score significantly improved from 6-month to 10-year follow-up (P ≤ .021), whereas the HOS-ADL and HOS-SS did not (P ≥ .072). There were significant correlations between 6-month and 10-year scores for the HOS-ADL (r = 0.505), HOS-SS (r = 0.592), and mHHS (r = 0.362) (P ≤ .022 for all), as well as significant correlations between 6-month and 1-, 2-, and 5-year scores (P ≤ .014 for all). The 6-month HOS-ADL, HOS-SS, and mHHS were all significantly associated with their respective 10-year achievement of the patient acceptable symptom state (PASS) (P ≤ .044). Furthermore, 6-month HOS-ADL and mHHS were significantly associated with THA conversion (P ≤ .041). Comparable 6-month and 10-year achievement of the minimal clinically important difference (96.5% vs 97.8%, P > .999) and PASS (85.2% vs 87.5%, P > .999) for any PRO was observed. CONCLUSIONS: After hip arthroscopy for FAIS, patients' 6-month HOS-ADL and mHHS were significantly associated with their 10-year PROs, PASS achievement, and THA conversion, although correlation strengths decreased with increasing time from surgery. LEVEL OF EVIDENCE: Level IV, case series.

Open and Endoscopic Gluteus Medius and/or Minimus Repair Achieves Clinical Success Regardless of Tear Grade: High-Grade Fatty Infiltration Portends Worse Outcomes.

PURPOSE: To evaluate minimum 2-year gluteus medius and/or minimus repair clinical success rates stratified by the 3-grade magnetic resonance imaging (MRI)-based classification (MRI grade) and to evaluate clinical success rates by the surgical approach used at each MRI grade and by the Goutallier-Fuchs (GF) classification. METHODS: A retrospective review identified patients who underwent primary endoscopic or open gluteus medius and/or minimus repair from 2012 to 2021 performed by a single surgeon. Preoperative MRI scans were classified using the MRI grade and GF classification. Patient-reported outcomes were collected preoperatively and at minimum 2-year follow-up. Cohort-specific minimal clinically important difference and patient acceptable symptom state achievement was recorded. Rates of clinical success, defined as achievement of the 2-year minimal clinically important difference or patient acceptable symptom state with avoidance of revision surgery, were compared by MRI grade, by surgical approach at each MRI grade, and by GF classification. RESULTS: A total of 112 patients (71 with MRI grade 1, 19 with grade 2, and 22 with grade 3) were included. MRI grade 1 patients underwent endoscopic repair (P < .001) more often than the other groups. The overall clinical success rate was 90%. Clinical success rates by MRI grade were 93% for grade 1, 95% for grade 2, and 77% for grade 3 (P = .087). Clinical success rates by the endoscopic and open surgical approaches used at each MRI grade were 93% versus 90% for grade 1 (P = .543), 91% versus 100% for grade 2 (P > .999), and 60% versus 92% for grade 3 (P = .135). GF grade 1 tears achieved a higher rate of clinical success than GF grade 4 tears (100% vs 71%, P = .030). CONCLUSIONS: Primary repair of gluteus medius and/or minimus tears resulted in clinical success in most patients irrespective of MRI grade and irrespective of the surgical approach used at each MRI grade, yet GF grade 1 tears showed a significantly higher clinical success rate than GF grade 4 tears. LEVEL OF EVIDENCE: Level IV, prognostic retrospective case series.

Hip Arthroscopy Patients With Lower Back Pain Show Delayed Clinical Improvement and Inferior Time-Dependent Survivorship: A Propensity Matched Study at Mid-Term Follow-Up.

PURPOSE: To evaluate patient-reported outcomes (PROs) and survivorship at mid-term follow-up after hip arthroscopy (HA) for femoroacetabular impingement syndrome (FAIS) in patients with and without preoperative lower back pain (LBP). METHODS: Patients with self-endorsed preoperative LBP who underwent HA for FAIS with mid-term follow-up were identified and propensity matched 1:1 to patients without back pain by age, sex, and body mass index (BMI). PROs collected preoperatively and at postoperative years 1, 2, and 5 included Hip Outcome Score-Activities of Daily Living (HOS-ADL) and Hip Outcome Score-Sports Subscale (HOS-SS), 12-item International Hip Outcome Tool (iHOT-12), modified Harris Hip Score (mHHS), and Visual Analog Scale (VAS) for Pain. Achievement of minimal clinically important difference (MCID) and patient acceptable symptom state (PASS) were compared. Survivorship was compared with Kaplan-Meier analysis. RESULTS: In total, 119 patients with LBP were matched to 119 patients without LBP. Group demographic factors were as follows: age (37.4 ± 11.9 vs 37.6 ± 12.6 years, P = .880), sex (64.4% vs 67.7% female, P = .796), and BMI (25.3 ± 5.1 vs 25.3 ± 5.4, P = .930). Average follow-up duration was 6.0 ± 1.9 years. LBP patients showed similar preoperative PROs, yet lower 1-year scores for all PROs (P ≤ .044). At final follow-up, similar PROs were shown between groups (P ≥ .196). LBP and non-LBP patients had similar MCID achievement for HOS-ADL (59.3% vs 63.1%, P = .640), HOS-SS (73.9% vs 70.8%, P = .710), mHHS (66.7% vs 73.4%, P = .544), iHOT-12 (85.1% vs 79.4%, P = .500), and VAS Pain (75.6% vs 69.9%, P = .490). Groups also had similar PASS achievement for HOS-ADL (63.5% vs 61.3%, P = .777), HOS-SS (57.0% vs 62.5%, P = .461), mHHS (81.9% vs 79.1%, P = .692), iHOT-12 (54.6% vs 61.2%, P = .570), and VAS Pain (51.0% vs 55.4%, P = .570). Additionally, achievement of MCID ≥ 1 PRO (P ≥ .490) and PASS ≥ 1 PRO (P ≥ .370) was similar across groups. Conversion to total hip arthroplasty occurred in 3.4% of hips with LBP and 0.8% of hips without LBP (P = .370). Back pain patients demonstrated inferior time-dependent survivorship compared with patients without back pain on Kaplan-Meier survival analysis (P = .023). CONCLUSIONS: Patients undergoing primary hip arthroscopy for FAIS with LBP achieve comparable PROs and clinically significant outcomes to patients without back pain at mid-term, despite lower 1-year PRO scores. LBP patients show inferior reoperation-free time-dependent survivorship compared with those without LBP. LEVEL OF EVIDENCE: Level III, retrospective comparative case series.

Structures and Dynamics of Complex Guest Molecules in Confinement, Revealed by Solid-State NMR, Molecular Dynamics, and Calorimetry.

This review gives an overview of current trends in the investigation of confined molecules such as water, small and higher alcohols, carbonic acids, ethylene glycol, and non-ionic surfactants, such as polyethylene glycol or Triton-X, as guest molecules in neat and functionalized mesoporous silica materials employing solid-state NMR spectroscopy, supported by calorimetry and molecular dynamics simulations. The combination of steric interactions, hydrogen bonds, and hydrophobic and hydrophilic interactions results in a fascinating phase behavior in the confinement. Combining solid-state NMR and relaxometry, DNP hyperpolarization, molecular dynamics simulations, and general physicochemical techniques, it is possible to monitor these confined molecules and gain deep insights into this phase behavior and the underlying molecular arrangements. In many cases, the competition between hydrogen bonding and electrostatic interactions between polar and non-polar moieties of the guests and the host leads to the formation of ordered structures, despite the cramped surroundings inside the pores.

Molecular Dynamics Study of the Green Solvent Polyethylene Glycol with Water Impurities.

Polyethylene glycol (PEG) is one of the environmentally benign solvent options for green chemistry. It readily absorbs water when exposed to the atmosphere. The Molecular Dynamics (MD) simulations of PEG200, a commercial mixture of low molecular weight polyethyelene glycol oligomers, as well as di-, tetra-, and hexaethylene glycol are presented to study the effect of added water impurities up to a weight fraction of 0.020, which covers the typical range of water impurities due to water absorption from the atmosphere. Each system was simulated a total of four times using different combinations of two force fields for the water (SPC/E and TIP4P/2005) and two force fields for the PEG and oligomer (OPLS-AA and modified OPLS-AA). The observed trends in the effects of water addition were qualitatively quite robust with respect to these force field combinations and showed that the water does not aggregate but forms hydrogen bonds at most between two water molecules. In general, the added water causes overall either no or very small and nuanced effects in the simulation results. Specifically, the obtained water RDFs are mostly identical regardless of the water content. The added water reduces oligomer hydrogen bonding interactions overall as it competes and forms hydrogen bonds with the oligomers. The loss of intramolecular oligomer hydrogen bonding is in part compensated by oligomers switching from inter- to intramolecular hydrogen bonding. The interplay of the competing hydrogen bonding interactions leads to the presence of shallow extrema with respect to the water weight fraction dependencies for densities, viscosities, and self-diffusion coefficients, in contrast to experimental measurements, which show monotonous dependencies. However, these trends are very small in magnitude and thus confirm the experimentally observed insensitivity of these physical properties to the presence of water impurities.

Ultrafast Proton Conduction in an Aqueous Electrolyte Confined in Adamantane-like Micropores of a Sulfonated, Aromatic Framework.

Sulfonated, cross-linked porous polymers are promising frameworks for aqueous high-performance electrolyte-host systems for electrochemical energy storage and conversion. The systems offer high proton conductivities, excellent chemical and mechanical stabilities, and straightforward water management. However, little is known about mass transport mechanisms in such nanostructured hosts. We report on the synthesis and postsynthetic sulfonation of an aromatic framework (SPAF-2) with a 3D-interconnected nanoporosity and varying sulfonation degrees. Water adsorption produces the system SPAF-2H20. It features proton exchange capacities up to 6 mequiv g-1 and exceptional proton conductivities of about 1 S cm-1. Two contributions are essential for the highly efficient transport. First, the nanometer-sized pores link the charge transport to the diffusion of adsorbed water molecules, which is almost as fast as bulk water. Second, continuous exchange between interface-bound and mobile species enhances the conductivities at elevated temperatures. SPAF-2H20 showcases how to tailor nanostructured electrolyte-host systems with liquid-like conductivities.

2H NMR studies on the dynamics of supercooled water in a metal-organic framework.

We use 2H nuclear magnetic resonance (NMR) to study water (D2O) reorientation and diffusion in the metal-organic framework MFU-4l, which features a regular three-dimensional network of nearly spherical pores with diameters of 1.2 and 1.9 nm. We observe that the rotational correlation times follow Vogel-Fulcher-Tammann and Arrhenius (Ea = 0.48 eV) relations above ∼225 K and below ∼170 K, respectively, whereas the temperature dependence continuously evolves from one to the other behavior in the broad crossover zone in between. In the common temperature range, the present NMR results are fully consistent with previous broadband dielectric spectroscopy (BDS) data on water (H2O) in a very similar framework. Several of our observations, e.g., rotational-translational coupling, indicate that a bulk-like structural (α) relaxation is observed above the crossover region. When cooling through the crossover zone, a quasi-isotropic reorientation mechanism is retained, while 2H spin-lattice relaxation evolves from exponential to nonexponential, implying that the water dynamics probed at low temperatures does no longer fully restore ergodicity on the time scale of this experiment. We discuss that the latter effect may result from bulk-like and/or confinement-imposed spatially heterogeneous water properties. Comparison with previous NMR and BDS results for water in other confinements reveals that, for confinement sizes around 2 nm, water reorientation depends more on the pore diameter than on the pore chemistry, while water diffusion is strongly affected by the connectivity and topology of the pores.

Soil fauna drives vertical redistribution of soil organic carbon in a long-term irrigated dry pine forest.

Summer droughts strongly affect soil organic carbon (SOC) cycling, but net effects on SOC storage are unclear as drought affects both C inputs and outputs from soils. Here, we explored the overlooked role of soil fauna on SOC storage in forests, hypothesizing that soil faunal activity is particularly drought-sensitive, thereby reducing litter incorporation into the mineral soil and, eventually, long-term SOC storage. In a drought-prone pine forest (Switzerland), we performed a large-scale irrigation experiment for 17 years and assessed its impact on vertical SOC distribution and composition. We also examined litter mass loss of dominant tree species using different mesh-size litterbags and determined soil fauna abundance and community composition. The 17-year-long irrigation resulted in a C loss in the organic layers (-1.0 kg C m-2 ) and a comparable C gain in the mineral soil (+0.8 kg C m-2 ) and thus did not affect total SOC stocks. Irrigation increased the mass loss of Quercus pubescens and Viburnum lantana leaf litter, with greater effect sizes when meso- and macrofauna were included (+215%) than when excluded (+44%). The enhanced faunal-mediated litter mass loss was paralleled by a many-fold increase in the abundance of meso- and macrofauna during irrigation. Moreover, Acari and Collembola community composition shifted, with a higher presence of drought-sensitive species in irrigated soils. In comparison, microbial SOC mineralization was less sensitive to soil moisture. Our results suggest that the vertical redistribution of SOC with irrigation was mainly driven by faunal-mediated litter incorporation, together with increased root C inputs. Our study shows that soil fauna is highly sensitive to natural drought, which leads to a reduced C transfer from organic layers to the mineral soil. In the longer term, this potentially affects SOC storage and, therefore, soil fauna plays a key but so far largely overlooked role in shaping SOC responses to drought.

Structural and Dynamical Properties of Liquids in Confinements: A Review of Molecular Dynamics Simulation Studies.

Molecular dynamics (MD) simulations are a powerful tool for detailed studies of altered properties of liquids in confinement, in particular, of changed structures and dynamics. They allow, on one hand, for perfect control and systematic variation of the geometries and interactions inherent in confinement situations and, on the other hand, for type-selective and position-resolved analyses of a huge variety of structural and dynamical parameters. Here, we review MD simulation studies on various types of liquids and confinements. The main focus is confined aqueous systems, but also ionic liquids and polymer and silica melts are discussed. Results for confinements featuring different interactions, sizes, shapes, and rigidity will be presented. Special attention will be given to situations in which the confined liquid and the confining matrix consist of the same type of particles and, hence, disparate liquid-matrix interactions are absent. Findings for the magnitude and the range of wall effects on molecular positions and orientations and on molecular dynamics, including vibrational motion and structural relaxation, are reviewed. Moreover, their dependence on the parameters of the confinement and their relevance to theoretical approaches to the glass transition are addressed.

Slow liquid dynamics near solid surfaces: Insights from site-resolved studies of ionic liquids in silica confinement.

We perform molecular dynamics simulations of ionic liquids that are confined between amorphous silica surfaces and composed of 1-butyl-3-methylimidazolium [C4mim] cations and tetrafluoroborate [BF4], hexafluorophosphate [PF6], or bis(trifluoromethylsulfonyl)imide [TFSI] anions. Near the silica surfaces, we observe strong slowdowns of ion dynamics, which involve mainly two layers of ions and amount to about two orders of magnitude, depending on temperature and anion type. For a detailed investigation of the slow interfacial dynamics, we determine the adsorption sites of the various anion species on the amorphous silica surfaces and ascertain the repopulation dynamics of these sites. The analysis reveals that the mean residence times show a broad distribution, where anions stay longer at sites, which provide better opportunities for hydrogen bonding. Furthermore, the mean residence times follow Arrhenius laws, providing access to site-specific activation energies Ei. The distributions G(Ei) have Gaussian shape with mean values from ∼0.40 eV for TFSI to ∼0.48 eV for PF6 and standard deviations of about 0.31 eV. Thus, the amorphous silica surfaces impose static and disordered energy landscapes to the neighboring liquid, which have considerable ruggedness and, in this way, substantially hinder ion rearrangements. We discuss that qualitatively similar situations are expected for all kinds of confined liquids.

Temperature-dependent dynamics at protein-solvent interfaces.

We perform differential scanning calorimetry, broadband dielectric spectroscopy (BDS), and nuclear magnetic resonance (NMR) studies to understand the molecular dynamics in mixtures of ethylene glycol with elastin or lysozyme over broad temperature ranges. To focus on the protein-solvent interface, we use mixtures with about equal numbers of amino acids and solvent molecules. The elastin and lysozyme mixtures show similar glass transition steps, which extend over a broad temperature range of 157-185 K. The BDS and NMR studies yield fully consistent results for the fastest process P1, which is caused by the structural relaxation of ethylene glycol between the protein molecules and follows an Arrhenius law with an activation energy of Ea = 0.63 eV. It involves quasi-isotropic reorientation and is very similar in the elastin and lysozyme matrices but different from the α and ß relaxations of bulk ethylene glycol. Two slower BDS processes, viz., P2 and P3, have protein-dependent time scales, but they exhibit a similar Arrhenius-like temperature dependence with an activation energy of Ea ∼ 0.81 eV. However, P2 and P3 do not have a clear NMR signature. In particular, the NMR results for the lysozyme mixture reveal that the protein backbone does not show isotropic α-like motion on the P2 and P3 time scales but only restricted ß-like reorientation. The different activation energies of the P1 and P2/P3 processes do not support an intimate coupling of protein and ethylene glycol dynamics. The present results are compared with previous findings for mixtures of proteins with water or glycerol, implying qualitatively different dynamical couplings at various protein-solvent interfaces.

Confinement effects on glass-forming mixtures: Insights from a combined experimental approach to aqueous ethylene glycol solutions in silica pores.

We perform nuclear magnetic resonance, broadband dielectric spectroscopy, and differential scanning calorimetry studies to ascertain the dynamical behaviors of aqueous ethylene glycol (EG) solutions in silica pores over broad temperature ranges. Both translational and rotational motions are analyzed, and the pore diameter (2.4-9.2 nm) and the EG concentration (12-57 mol. %) are varied, leading to fully liquid or partially crystalline systems. It is found that the translational diffusion coefficient strongly decreases when the diameter is reduced, resulting in a slowdown of nearly three orders of magnitude in the narrowest pores, while the confinement effects on the rotational correlation times are moderate. For the fully liquid solutions, we attribute bulk-like and slowed down reorientation processes to the central and interfacial pore regions, respectively. This coexistence is found in all the studied pores, and, hence, the range of the wall effects on the solution dynamics does not exceed ∼1 nm. Compared to the situation in the bulk, the concentration dependence is reduced in confinements, implying that the specific interactions of the molecular species with the silica walls lead to preferential adsorption. On the other hand, bulk-like structural relaxation is not observed in the partially frozen samples, where the liquid is sandwiched between the silica walls and the ice crystallites. Under such circumstances, there is another relaxation process with a weaker temperature dependence, which is observed in various kinds of partially frozen aqueous systems and denoted as the x process.