Development of Heteroatomic Constant Potential Method with Application to MXene-Based Supercapacitors.

We describe a method for modeling constant-potential charges in heteroatomic electrodes, keeping pace with the increasing complexity of electrode composition and nanostructure in electrochemical research. The proposed "heteroatomic constant potential method" (HCPM) uses minimal added parameters to handle differing electronegativities and chemical hardnesses of different elements, which we fit to density functional theory (DFT) partial charge predictions in this paper by using derivative-free optimization. To demonstrate the model, we performed molecular dynamics simulations using both HCPM and conventional constant potential method (CPM) for MXene electrodes with Li-TFSI/AN (lithium bis(trifluoromethane sulfonyl)imide/acetonitrile)-based solvent-in-salt electrolytes. Although the two methods show similar accumulated charge storage on the electrodes, the results indicated that HCPM provides a more reliable depiction of electrode atom charge distribution and charge response compared with CPM, accompanied by increased cationic attraction to the MXene surface. These results highlight the influence of elemental composition on electrode performance, and the flexibility of our HCPM opens up new avenues for studying the performance of diverse heteroatomic electrodes including other types of MXenes, two-dimensional materials, metal-organic frameworks (MOFs), and doped carbonaceous electrodes.

Constant Potential and Constrained Charge Ensembles for Simulations of Conductive Electrodes.

Constant potential method molecular dynamics simulation (CPM MD) enables the accurate modeling of atomistic electrode charges when studying the electrode-electrolyte interface at the nanoscale. Here, we extend the theoretical framework of CPM MD to the case in which the total charge of each conductive electrode is controlled, instead of their potential difference. We show that the resulting thermodynamic ensemble is distinct from that sampled with a fixed potential difference but they are rigorously related as conjugate ensembles. This theoretical correspondence, which we demonstrate using simulations of an ionic liquid supercapacitor, underpins the success of recent studies with fixed total charges on the electrodes. We show that equilibration is usefully sped up in this ensemble and outline some potential applications of these simulations in the future.

ELECTRODE: An electrochemistry package for atomistic simulations.

Constant potential methods (CPMs) enable computationally efficient simulations of the solid-liquid interface at conducting electrodes in molecular dynamics. They have been successfully used, for example, to realistically model the behavior of ionic liquids or water-in-salt electrolytes in supercapacitors and batteries. CPMs model conductive electrodes by updating charges of individual electrode atoms according to the applied electric potential and the (time-dependent) local electrolyte structure. Here, we present a feature-rich CPM implementation, called ELECTRODE, for the Large-scale Atomic/Molecular Massively Parallel Simulator, which includes a constrained charge method and a thermo-potentiostat. The ELECTRODE package also contains a finite-field approach, multiple corrections for nonperiodic boundary conditions of the particle-particle particle-mesh solver, and a Thomas-Fermi model for using nonideal metals as electrodes. We demonstrate the capabilities of this implementation for a parallel-plate electrical double-layer capacitor, for which we have investigated the charging times with the different implemented methods and found an interesting relationship between water and ionic dipole relaxations. To prove the validity of the one-dimensional correction for the long-range electrostatics, we estimated the vacuum capacitance of two coaxial carbon nanotubes and compared it to structureless cylinders, for which an analytical expression exists. In summary, the ELECTRODE package enables efficient electrochemical simulations using state-of-the-art methods, allowing one to simulate even heterogeneous electrodes. Moreover, it allows unveiling more rigorously how electrode curvature affects the capacitance with the one-dimensional correction.

Fully periodic, computationally efficient constant potential molecular dynamics simulations of ionic liquid supercapacitors.

Molecular dynamics (MD) simulations of complex electrochemical systems, such as ionic liquid supercapacitors, are increasingly including the constant potential method (CPM) to model conductive electrodes at a specified potential difference, but the inclusion of CPM can be computationally expensive. We demonstrate the computational savings available in CPM MD simulations of ionic liquid supercapacitors when the usual non-periodic slab geometry is replaced with fully periodic boundary conditions. We show how a doubled cell approach, previously used in non-CPM MD simulations of charged interfaces, can be used to enable fully periodic CPM MD simulations. Using either a doubled cell approach or a finite field approach previously reported by others, fully periodic CPM MD simulations produce comparable results to the traditional slab geometry simulations with a nearly double speedup in computational time. Indeed, these savings can offset the additional cost of the CPM algorithm, resulting in periodic CPM MD simulations that are computationally competitive with the non-periodic, fixed charge equivalent simulations for the ionic liquid supercapacitors studied here.

Exercise intensity does not effect body composition change in untrained, moderately overfat women.

OBJECTIVE: To determine whether exercise intensity effects a change in body composition. DESIGN/SUBJECTS: Twelve untrained, moderately overfat, weight-stable women were randomly assigned to a high-intensity (80% VO2max) or low-intensity (50% VO2max) exercise group. Subjects trained four times per week for 12 weeks in monitored sessions, with a duration sufficient to expend 300 kcal. During this time, subjects were instructed to maintain their normal diet and activity patterns. OUTCOME MEASURES: Pretesting and posttesting included measurement of height, weight, body fat (via hydrostatic weighing), seven skinfold sites, seven circumference sites, and VO2max. STATISTICAL ANALYSES: Results were analyzed using the Student's t test and paired samples t test. RESULTS: Posttesting revealed no significant between-group differences for change in weight, percent body fat, fat mass, fat-free mass, sum of skinfold measurements, or sum of circumference measurements. Mean weight loss was 0.7 lb for the high-intensity group (P = .55) and 3.3 lb for the low-intensity group (P = .03). Hydrostatic data revealed that each group lost an identical amount of fat (5.0 lb), but the high-intensity group gained more than twice as much fat-free mass (4.3 vs 1.8 lb). The greater increase in fat-free mass by the high-intensity group explains why the low-intensity group had a greater absolute weight loss. APPLICATIONS/CONCLUSIONS: This study suggests that fat loss is a function of energy expended rather than exercise intensity. Therefore, if fat loss is the goal and time is limited, persons should exercise safely at as high an intensity as tolerable to expend as much energy as possible during their allotted time.

Effects on whole saliva of chewing gums containing calcium phosphates.

To evaluate chewing gums as a vehicle to increase salivary mineral saturation levels and enhance salivation, monocalcium phosphate monohydrate (MCPM) and an equimolar mixture of tetracalcium phosphate (TTCP) with dicalcium phosphate anhydrous (DCPA) were chosen as experimental chewing gum additives. Each of eight subjects chewed a commercial sugarless bubble gum (control) for 16 min or the same gum to which 5 wt% of MCPM or the TTCP-DCPM mixture had been added. The saliva samples collected every 2 min were analyzed for weight, pH, and total calcium (Ca) and phosphate (P) concentrations. Both experimental gums were found to increase significantly the Ca and P concentrations of saliva during the 16-minute period even more than with a previously evaluated gum that contained dicalcium phosphate dihydrate. The degree of saturation of tooth mineral was significantly increased by both experimental gums, with the greater increase being produced by the TTCP-DCPA gum. The MCPM gum produced a significantly greater saliva flow and a lower salivary pH than did the control and TTCP-DCPA gums. The results suggest that the experimental gums may be useful for promoting remineralization in general and for inducing salivation in xerostomic patients.

Influence of age on the secretory rates of the human minor salivary glands and whole saliva.

This investigation comprised two studies of healthy, unmedicated individuals. The first measured the effect of collection time on the volume of secretions of the minor salivary glands at four specified areas of the lower lip of 13 individuals before and after a mild gustatory stimulus. The second (n = 51) monitored the influence of age and gender on the secretory rates of unstimulated labial, buccal and palatal salivary glands. Also, unstimulated and stimulated flow rates of whole saliva were monitored to provide a point of reference. Volumes of minor gland secretions were measured with a Periotron unit. Results of the first study indicated a linear increase in volumes with collection time (15, 30, 45 and 60 s). Flow rates were similar among the four labial sites, approx. 1 microliter/cm2/min, and were not influenced by mild citric acid stimulation. Results of the second study indicated that flow rates differed significantly (p = 0.0001) among the anatomical sites, with similar rates on the right- and left-hand sides. Gender exerted no influence on flow from the minor salivary glands. Similarly, age exerted no influence on flow from the buccal or labial glands. However, the secretory rate for the palatal glands decreased significantly with age (r = -0.44; p < 0.005). As for unstimulated whole saliva, secretory rates were not influenced by age nor gender; rates for stimulated whole saliva increased with age (r = 0.31; p < 0.05). No association was detected between the flow rates of the whole saliva and that of the minor salivary glands.

A method for measuring secretions from the palatal minor salivary glands.

This study investigated the secretory pattern of unstimulated minor salivary glands in 10 contiguous specified areas of the hard palate in 12 healthy unmedicated adult men and women. Each participant was assessed twice at each of two visits in the same specific palatal areas from the distal portion of the second molar to 30 mm anterior. The 10 areas were isolated, dried, and sampled with a paper strip for 60 seconds. The strips were read with a Periotron device. The rates of flow were significantly influenced by position; higher values were noted posteriorly and medially during testing and retesting at both visits. The rates of flow from the palatal glands were greatest in the area between the maxillary second molars. This method proved to be simple, accurate, and reliable for measuring the secretions of the palatal glands. This method may prove useful for investigating how the relationship between the pattern and the secretory rate of minor salivary glands influences retention of the complete maxillary denture.

Effects of an acidic calcium phosphate solution and the intraoral fluoride-releasing device on dental caries and fluoride uptake in rats.

This investigation comprised two studies evaluating the effects of an acidic calcium phosphate solution (CPS) on fluoride uptake in the enamel, glycolysis of dental plaque, the incidence of dental caries and urinary fluoride concentrations of rats wearing an intraoral fluoride-releasing device (IFRD). In the first study, CPS-fluoride treatment preceded the cariogenic challenge. In the second study, the cariogenic challenge preceded the treatments. In the first study, CPS treatments increased the ability of enamel to bind fluoride. However, the enamel-bound fluoride exerted a negligible effect on plaque glycolysis as measured by the pH decrease after sucrose challenge. In the second study CPS augmented the caries inhibition for both the sulcal-morsal and buccal-lingual surfaces. In both studies the IFRD significantly restricted the development of carious enamel on the sulcal-morsal surfaces and caused elevated concentrations of fluoride in the urine independent of CPS treatments.

In vitro evaluation of fluorescein for testing the permeability of white spots on tooth enamel.

This investigation demonstrates the reliability of fluorescein for detecting the permeability of incipient dental caries (white spots). Artificial white spots were created on the buccal surface of 12 human bicuspids by viscous lactic acid (pH 4). Permeability of these lesions was assessed and reassessed before and after 24 and 48 hr of acid challenge using two disclosants: sodium iodide and sodium fluorescein. Estimates obtained from both disclosants showed that the microvoid volume approximately doubled as the decalcification time doubled. The two disclosants exhibited good intraclass reliability and their scores were correlated (r = 0.69 to r = 0.91). However, only fluorescein disclosed the extent of porous white spot lesions. Thus, fluorescein should be considered when the objective is to detect the location and permeability of incipient lesions.

A method for measuring the flow of saliva from the minor salivary glands.

This investigation evaluated a procedure to measure the quantity of secretion by the minor salivary glands from various locations in the mouth before and after citric acid stimulation. Fourteen healthy non-medicated adults were assessed on each of two morning visits at the same specific areas of the mucosa (labial, buccal, and palatal). During each assessment, an absorbent paper strip was placed on each designated location for 30 s, removed, and its moisture content determined with a calibrated Periotron. Calibration procedures indicated that readings were linearly associated with known saliva volumes and were negligibly influenced by differences in salt concentration and viscosity. At each visit, the buccal flow was significantly greater than those from the lip and palate. The flow from glands in each area was unaffected by a single application of the gustatory stimulant. The mean flow rates at each specified site were similar during the two visits.

Fluoride concentrations in whole saliva following use of fluoride tablets and a rinse.

This clinical investigation is comprised of two studies. The first monitored the oral clearance of fluoride following the use of an oral rinse and two types of tablets: one that was chewed, swished, and swallowed, and another that was allowed to dissolve undisturbed at a specific site in the oral vestibule. Fluoride from the rinse and tablets exhibited similar rapid clearance patterns with a mean concentration of 1.2 ppm or less within 1 hr and returned to baseline concentrations within 24 hr. Data from the second study indicated fluoride was distributed unevenly to various areas of the mouth from the slowly dissolving undisturbed tablet. Information concerning oral clearance of fluoride may be used to rationalize various treatment regimens.

Development and in vitro evaluation of an intra-oral controlled-release delivery system for chlorhexidine.

Copolymers of hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA) were prepared and used to fabricate a membrane-controlled reservoir-type controlled-release delivery system for chlorhexidine that should be suitable for intra-oral use. The reservoir of the system was prepared by softening an 80:20 mixture of chlorhexidine diacetate and 50:50 HEMA:MMA copolymer with methyl ethyl ketone (MEK), and pressing standard amounts of the resulting dough-like mixture into silicone rubber molds. A membrane was applied to the reservoirs by rotating them through a solution of 30:70 HEMA:MMA copolymer in MEK. The finished oval-shaped controlled-release pellets were approximately 4.7 mm wide, 3.3 mm high, and 7.4 mm long, and contained 45.0 +/- 3.7 mg of chlorhexidine diacetate. The mean in vitro release rate of chlorhexidine diacetate from the pellets into 37 degrees C water was 608 +/- 55 micrograms/24 h for days 2 through 11, and 389 +/- 50 micrograms/24 h for days 15 to 30 of the test period. The chlorhexidine released on day 30 was biologically active, as determined by a serial dilution assay against Streptococcus mutans. The extended release of biologically active chlorhexidine at a controlled rate from this system suggests that it is worthy of further evaluation for the intra-oral therapy of chlorhexidine-treatable oral infections in non-compliant and physically or mentally compromised individuals.

Effects of rinses with an acidic calcium phosphate solution on fluoride uptake, caries, and in situ plaque pH in rats.

The effects of oral rinses on enamel fluoride uptake, caries, and plaque pH in rats were investigated by use of (1) an acidic calcium phosphate solution (CPS) saturated with dicalcium phosphate dihydrate (DCPD) and (2) a 1% fluoride solution. Osborne-Mendel rats, 19 days of age, were randomly assigned to one of four mouth-rinse treatment groups, with the two consecutively administered rinses being: H2O-H2O; H2O-F; CPS-H2O; and CPS-F. The treatments were administered on days 1 through 4, and the animals were provided with a cariogenic challenge throughout the study. The groups treated with F exhibited significantly higher mean levels of enamel-bound F than did the non-F groups. Furthermore, CPS significantly increased uptake of F by enamel when used as an adjunct to the F treatment. The caries scores of the CPS-F and H2O-F groups were not statistically different; however, a strong negative correlation was observed between enamel F content and caries scores, indicating that as more F was incorporated into the teeth, less caries formation occurred. Despite the significant differences in enamel F and caries scores among some of the treatment groups, post mortem in situ plaque pH drop in response to a sucrose rinse showed no differences among these groups.

Effects of various organic compounds and fluoride on dental plaque and caries in rats.

This investigation measured the effect of several organic compounds on dental caries and plaque when used as oral rinses in rats. These compounds were equimolar with regard to their chloride or fluoride moieties or the added sodium fluoride (0.26 M F). Test agents were dissolved in an aqueous glycerol vehicle and applied daily to the teeth of rats. Significant restriction in the mean incidence of dental caries (greater than or equal to 20%) was imparted by the following chemicals used alone or in combination: alexidine dihydrochloride, alexidine dihydrochloride with sodium fluoride, lysine phosphate fluoride, silane polyamine fluoride, tiodonium chloride, and tiodonium chloride with sodium fluoride. Only the groups treated with alexidine chloride alone or in combination with sodium fluoride exhibited significantly less dental plaque (greater than or equal to 64%) than the rats in the placebo-treated groups.

Effects of octenidine on dental plaque and gingivitis in monkeys.

This investigation monitored the effects of daily oral rinses with octenidine on plaque and gingivitis in five monkeys. Formulations containing 0.5% or 1.0% octenidine or the rinse vehicle placebo were provided daily for 2 weeks. Each week the dentition of each monkey was examined, photographed, and sampled for plaque. All responses exhibited a numerical decrease in mean scores following treatments with each concentration of octenidine, whereas the placebo treatment exerted negligible effects. Decreases in plaque mass were observed after 2 weeks of treatment with 1% octenidine (58%) or 0.5% octenidine (55%) compared with the corresponding baseline values. Similar trends were noted in the extent and thickness of supragingival plaque and its ability to decrease the pH of a sucrose solution. Octenidine treatments reduced the proportions of motile forms in samples of subgingival plaque and also restricted its ability to produce H2S. Slight numerical decreases were seen in the Gingival Index and flow rate of the crevicular fluid. These consistent protective trends suggest that octenidine decreases the pathogenic potential of established plaque.

Evaluation of an intraoral controlled release delivery system for fluoride in primates.

An intraoral delivery system designed to release 0.5 mg of fluoride per day was evaluated in short-term studies in primates. This fluoride-releasing device, bonded to the buccal surface of the maxillary right central incisor of each of six monkeys (Macaca fascicularis), produced marked elevations in saliva and plaque fluoride concentrations without increases in serum fluoride concentrations. No changes were observed in the plaque and gingival scores or the populations of various species of plaque bacteria.