Nondilute Solutions Research Articles

Kinetic effects during the plane-front and dendritic solidification of multicomponent alloys

A generalized model for the kinetics of a dendrite tip with a non-equilibrium interface is presented for multicomponent alloys. The model includes full coupling with thermodynamic databases to account for non-dilute non-ideal solutions, in addition to a full diffusion matrix in the liquid. The consequences of the computed non-equilibrium phase diagram boundaries on the dendrite tip kinetics are considered, and substantial deviations from existing theories are observed, especially in the case of zero solute drag. The model is applied to the rapid solidification of Inconel 718 (a nickel-based superalloy) and 316L (a stainless steel), which contain seven and five solute species, respectively. From the model, the phase diagram properties (i.e., partition coefficients and liquidus slopes) are able to be directly visualized as a function of velocity and observed to vary non-linearly and, in some cases, non-monotonically due to the combination of non-linear phase diagrams and kinetic effects. Finally, recent reports in the literature on the competition between the growth of ferrite and austenite from the melt in 316L are revisited with these new developments.

Numerical model supplemented by thin-layer analysis for diffusiophoresis of a particle incorporating finite ion size effects

The impact of finite-sized ions on the diffusiophoresis of a charged colloid subjected to a concentration gradient of electrolyte solution consisting monovalent or multivalent ionic species, is studied. In diffusiophoresis, the ion concentration is of O(1M). In this non-dilute electrolyte solutions, the ion–ion steric interaction is important. We have adopted the Boublik–Mansoori–Carnahan–Starling–Leland (BMCSL) model to account for the ion steric interactions and the Batchelor–Green expression for the relative viscosity of suspension. We have solved the standard model numerically considering ions as point charge (PNP-model), the modified Nernst–Planck equations incorporating the ion steric interaction with constant viscosity (MNP-model), and modification of the MNP-model by incorporating the viscosity variation with the ionic volume fraction (MNPV-model). Semi-analytical expressions for mobility based on a linear perturbation technique under a thinner Debye length is presented for PNP- and MNP-models. In the MNP-model, counterion saturation in the Debye layer due to the ion steric interaction enhances the surface potential by attenuating the shielding effect, diminishes the surface conduction, and magnifies the induced electric field. These in combination create a larger mobility at a thinner Debye length compared with the PNP-model. This increment in mobility attenuates when the MNPV-model is considered. The MNPV-model is more appropriate to analyze the finite ion size effects, and it is found to yield the mobility values more close to the experimental data compared with the MNP- and PNP-model. The semi-analytical expressions for mobility based on the PNP- and MNP-models agree with the corresponding exact numerical solutions when the surface potential is in the order of thermal potential. However, a large discrepancy between the simplified expression and the exact numerical results is found for a concentrated electrolyte in which the induced electric field is large.

The Algorithm to Predict the Grain Boundary Diffusion in Non-Dilute Metallic Systems

The analysis of grain boundary (GB) diffusion in metallic systems based on Cu, Ni, Ag and Al was made to set demonstrate the common behavior. It was shown that the slow penetration for 11 systems can be connected with negative segregation or specific interatomic interaction. Two energetic parameters such as energy of interaction with GB and energy of interatomic interaction are proposed as main characteristics. The analysis of a tendency toward segregation and tendency of intermediate phase formation in these terms allows us to divide the systems on four groups and formulate a qualitative way to predict the behavior of the diffusing elements in a non-dilute solution. Mathematical formulation of GB diffusion problem and typical solutions are presented.

Impact of Ion-Ion Correlated Motion on Salt Transport in Solvated Ion Exchange Membranes.

The influence of dynamical ion-ion correlations and ion pairing on salt transport in ion exchange membranes remain poorly understood. In this study, we use the framework of Onsager transport coefficients within atomistic molecular dynamics simulations to study the impact of ion-ion correlated motion on salt transport in hydrated polystyrene sulfonate membranes and compare with the results from aqueous salt solutions. At sufficiently high salt concentrations, cation-anion dynamical correlations exert a significant influence on both salt diffusivities and conductivities. Anion-anion distinct correlations, arising from the imbalance between the concentration of free (mobile) cations and anions, and the retarding effect of the fixed charge groups on cations, proves to be an additional important feature for polymer membranes. Our results demonstrate that dynamical correlations should become an important consideration in experimental measurements of salt diffusivities and conductivities for non-dilute salt solutions in polymer membranes.

Precipitation and coarsening kinetics of H-phase in NiTiHf high temperature shape memory alloy

Precipitate hardening is the most easiest and effective way to enhance strain recovery properties in NiTiHf high-temperature shape memory alloys. This paper discusses the precipitation, coarsening and age hardening of H-phase precipitates in Ni50Ti30Hf20 alloy during isothermal aging at temperatures between 450 °C and 650 °C for time to 75 h. The H-phase mean size and volume fraction were determined using transmission electron microscopy. Precipitation kinetics was analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a Time-Temperature-Transformation diagram was constructed. The evolution of H-phase size suggests classical matrix diffusion limited Lifshitz-Slyozov-Wagner coarsening for all considered temperatures. The coarsening rate constants of H-phase precipitation have been determined using a modified coarsening rate equation for non-dilute solutions. Critical size of H-phase precipitates for breaking down the precipitate/matrix interface coherency was estimated through a combination of age hardening and precipitate size evolution data. Moreover, time-temperature-hardness diagram was constructed from the precipitation and coarsening kinetics and age hardening of H-phase precipitates in Ni50Ti30Hf20 alloy.

Thermodynamics of liquid Pb–In–Sn alloys determined by vapour pressure measurements

Application of Knudsen method for studies of liquid Pb–In and Pb–In–Sn alloys in temperatures 868–1200 and 848–1219 K, respectively, provided experimental data which made it possible to characterize thermodynamic properties of liquid phase of Pb–In and Pb–In–Sn systems. The produced experimental data were used in the optimization procedure aimed at determination of parameters of thermodynamic models of the Pb–In and Pb–In–Sn systems, linking the measured mass change rates with the thermodynamic properties of these systems. The model applied in this procedure uses equations proposed by Pelton and Bale, modified in the scope of this study. They provide an extension of the standard “dilute interaction parameter formalism” to non-dilute solutions. Parameters of the equations were calculated during optimization procedure using the least square method where the author’s experimental database was supplemented with the data which characterize molar enthalpies of mixing of both examined liquid alloys.

Study of Segregation in Non-Dilute Solutions of Linear Diblock Copolymers and Symmetric Miktoarm (or Janus Star) Polymers Using Monte Carlo Simulations with the Bond Fluctuation Model

The bond fluctuation model was employed to characterize the approach to the mesophase separation transition of pure linear AB copolymers and symmetric miktoarms, also called Janus, star polymers, Af/2Bf/2, where f = 6 or 12 is the total number of arms, in a common good solvent. We consider a concentration sufficiently high to mimic the melting behavior and also a lower concentration. The segregation between A and B units is represented by a repulsive interaction parameter, ε. Different total numbers of units are also considered. Results for different properties, such as the molecular size, the asphericity and orientational correlation of blocks, or arms, of different compositions are obtained as a function of the segregation parameter. We also calculate scattering structure factors. The initial effect of segregation on the scattering with opposite contrast factors between the A and B blocks can be explained with a common description based on the random phase approximation for both the linear copolymers and the f = 6 miktoarms, once the numerical form factors of the different molecules in their particular systems are considered. However, the results for f = 12 clearly deviate from this description probably due to some degree of ordering in the position of highly armed molecules.

Dynamics and rheology of ring-linear blend semidilute solutions in extensional flow: Single molecule experiments

Ring polymers exhibit unique flow properties due to their closed chain topology. Despite recent progress, we have not yet achieved a full understanding of the nonequilibrium flow behavior of rings in nondilute solutions where intermolecular interactions greatly influence chain dynamics. In this work, we directly observe the dynamics of DNA rings in semidilute ring-linear polymer blends using single molecule techniques. We systematically investigate ring polymer relaxation dynamics from high extension and transient and steady-state stretching dynamics in a planar extensional flow for a series of ring-linear blends with varying ring fraction. Our results show multiple molecular subpopulations for ring relaxation in ring-linear blends, as well as large conformational fluctuations for rings in a steady extensional flow, even long after the initial transient stretching process has subsided. We further quantify the magnitude and characteristic time scales of ring conformational fluctuations as a function of blend composition. Interestingly, we find that the magnitude of ring conformational fluctuations follows a nonmonotonic response with increasing ring fraction, first increasing at low ring fraction and then substantially decreasing at large ring fraction in ring-linear blends. A unique set of ring polymer conformations are observed during the transient stretching process, which highlights the prevalence of molecular individualism and supports the notion of complex intermolecular interactions in ring-linear polymer blends. In particular, our results suggest that transient intermolecular structures form in ring-linear blends due to a combination of direct forces due to linear chains threading through open rings and indirect forces due to hydrodynamic interactions; these combined effects lead to large conformational fluctuations of rings over distributed time scales. Taken together, our results provide a new molecular understanding of ring polymer dynamics in ring-linear blends in the nonequilibrium flow.

Nonmonotonic dependence of comb polymer relaxation on branch density in semidilute solutions of linear polymers

The relaxation of comb polymers in semidilute solutions is studied using single-molecule fluorescence microscopy and Brownian dynamics simulations with long-range hydrodynamic interactions (HI). Unexpectedly, comb polymer relaxation is found to depend nonmonotonically on branching density, such that lightly branched chains relax faster than linear chains in semidilute solutions. These results challenge commonly held notions in the field and show that the dynamics of branched polymers in nondilute solutions is governed by a coupling between polymer architecture, long-range HI, and excluded volume interactions.

Simultaneous Optical Transmission Relaxation and Electrical Conductivity Relaxation Measurements of Oxygen Surface Exchange Kinetics during Crystallization

Oxygen exchange at the surface of mixed ionic and electronic conductors (MIECs) plays an integral role in the efficiency and lifetime of many electrochemical devices and is often the efficiency-limiting process at intermediate temperatures (300-600 °C). Despite development of many MIEC perovskite compositions for this purpose, there is still a need to deeply understand processing-structure-property relationships in relation to optimizing the surface exchange coefficient. In recent work, we observed by a new optical transmission relaxation (OTR) approach that perovskite SrTi0.65Fe0.35O3-d (STF35) films deposited by pulsed laser deposition at room temperature in the amorphous state, and subsequently annealed at the crystallization temperature, show immeasurably fast oxygen surface exchange kinetics in the intermediate temperature range [1,2]. In order to better understand this effect, our ongoing work has three aims: 1) to characterize the structural changes occurring in STF35 thin films upon crystallization using X-ray absorption spectroscopy 2) to relate the structural evolution to emergent functional properties, and 3) to evaluate the accuracy of the OTR technique for the measurement of oxygen surface exchange kinetics during crystallization. Towards this last goal, surface exchange coefficients (kchem) were analyzed using simultaneous electrical conductivity relaxation (ECR) and optical transmission relaxation (OTR) techniques on amorphous and crystalline films at various temperature points from 300-600 °C and as a function of oxygen activity in the films by varying the oxygen partial pressure (pO2). Our prior work has shown that the change in optical absorption of the films is directly proportional to the change in their oxygen concentration and change in hole concentration (via Fe valence changes). Therefore, we hypothesized that as the films oxidize or reduce, the amount of light absorbed increases or decreases in direct correspondence to the increase or decrease in p-type electronic conductivity in the film, assuming minimal changes in the electronic mobility with pO2 [3]. Across the different measurement conditions, our results showed excellent agreement in relaxation profiles and fitted kchem values between these two techniques (OTR and ECR) for crystalline films (within a factor of 2), demonstrating the applicability of the optical technique to the STF35 system. Further, both ECR and OTR showed the emergence of rapid oxygen exchange upon in situ crystallization, validating the earlier optically-based observation of this effect. This novel OTR technique provides a continuous, contact-free, and in situ measurement of the native surface kinetics in realistic operating conditions that has the potential to be applied to many other systems. [1] T. Chen, G.F. Harrington, K. Sasaki, and N.H. Perry, “Impact of microstructure and crystallinity on surface exchange kinetics of strontium titanium iron oxide perovskite by in situ optical transmission relaxation approach” Journal of Materials Chemistry A, 5, 23006-23019 (2017). [2] T. Chen, G.F. Harrington, J. Masood, K. Sasaki, and N.H. Perry, “Emergence of Rapid Oxygen Surface Exchange Kinetics During In Situ Crystallization of Mixed Conducting Thin Film Oxides” ACS Applied Materials and Interfaces (2019). [3] N.H. Perry, N. Kim, E. Ertekin, H.L. Tuller, “Origins and control of optical absorption in a non-dilute oxide solid solution: Sr(Ti,Fe)O3-x perovskite case study” Chemistry of Materials (2019).

Origins and Control of Optical Absorption in a Nondilute Oxide Solid Solution: Sr(Ti,Fe)O3–x Perovskite Case Study

Understanding and rationally tailoring defect-mediated optical absorption of nondilute oxide solid solutions represents a complex challenge. In this work, we investigate compositions in the SrTiO3–SrFeO2.5 solid solution, departing from the simpler dilute Fe-substituted SrTiO3 case. Through ex situ and in situ optical absorption measurements of mixed conducting thin films prepared by pulsed laser deposition and through density functional theory simulations, we demonstrate understanding and rational tailoring of the optical absorption behavior. Experimentally, broad subgap absorption peaks, centered around 2.1 and 2.8–3 eV, increase in intensity with increasing Fe and/or O concentrations and decrease with increasing La donor dopant concentration. Consistent with these observations, the absorption is found to be proportional to the hole concentration and to the Fe concentration under fully oxidized conditions. This behavior is similar to the dilute case; however, the solid solution electronic structure and ...

Thermodynamics of copper-rich liquid Cu-Fe-Bi alloys determined by vapour pressure measurements

The Knudsen method was used for measurements of mass loss rate of dilute solutions of bismuth and iron in liquid copper, in the temperature range from 1378 to 1631 K. The produced experimental data were used in the optimization procedure aimed at determination of parameters of the thermodynamic model of the Cu-Fe-Bi system, linking the measured mass change rates with the thermodynamic properties of this system. The model applied in this procedure uses equations proposed by Pelton and Bale. They are an extension of a standard "dilute interaction parameter formalism" to non-dilute solutions. The obtained results made it possible to determine the value of the first order ternary interaction parameter:ɛBiFe = 19034/T-7.140.

A Streamlined, General Approach for Computing Ligand Binding Free Energies and Its Application to GPCR-Bound Cholesterol.

The theory of receptor-ligand binding equilibria has long been well-established in biochemistry, and was primarily constructed to describe dilute aqueous solutions. Accordingly, few computational approaches have been developed for making quantitative predictions of binding probabilities in environments other than dilute isotropic solution. Existing techniques, ranging from simple automated docking procedures to sophisticated thermodynamics-based methods, have been developed with soluble proteins in mind. Biologically and pharmacologically relevant protein-ligand interactions often occur in complex environments, including lamellar phases like membranes and crowded, nondilute solutions. Here, we revisit the theoretical bases of ligand binding equilibria, avoiding overly specific assumptions that are nearly always made when describing receptor-ligand binding. Building on this formalism, we extend the asymptotically exact Alchemical Free Energy Perturbation technique to quantifying occupancies of sites on proteins in a complex bulk, including phase-separated, anisotropic, or nondilute solutions, using a thermodynamically consistent and easily generalized approach that resolves several ambiguities of current frameworks. To incorporate the complex bulk without overcomplicating the overall thermodynamic cycle, we simplify the common approach for ligand restraints by using a single distance-from-bound-configuration (DBC) ligand restraint during AFEP decoupling from protein. DBC restraints should be generalizable to binding modes of most small molecules, even those with strong orientational dependence. We apply this approach to compute the likelihood that membrane cholesterol binds to known crystallographic sites on three GPCRs (β2-adrenergic, 5HT-2B, and μ-opioid) at a range of concentrations. Nonideality of cholesterol in a binary cholesterol:phosphatidylcholine (POPC) bilayer is characterized and consistently incorporated into the interpretation. We find that the three sites exhibit very different affinities for cholesterol: The site on the adrenergic receptor is predicted to be high affinity, with 50% occupancy for 1:109 CHOL:POPC mixtures. The sites on the 5HT-2B and μ-opioid receptor are predicted to be lower affinity, with 50% occupancy for 1:103 CHOL:POPC and 1:102 CHOL:POPC, respectively. These results could not have been predicted from the crystal structures alone.

Concentration and fluid flow effects on kinetics, dendrite remelting and stress accumulation upon rapid solidification of deeply undercooled alloys

Upon free solidification of a deeply undercooled melt, the solute diffusion in both the interface and the bulk liquid is far from equilibrium, and the concentration and the fluid flow may also play an important role in the solidification. Thus, under such conditions, assumptions about local equilibrium, ideal dilute solution solidification and solidification free of fluid flow can no longer be valid. In the present work, first, in order to reveal the concentration effect, we compared the results of the non-equilibrium dendrite growth models describing the solidification of dilute and non-dilute undercooled melts. It was found that under local non-equilibrium conditions, the predicted results of the non-dilute solution model and the dilute solution model are to a certain extent different from each other at the intermediate undercooling range. It was also found that the concentration effect could substantially decrease the relaxation effect. Second, we considered the effect of fluid flow on the rapid solidification of an undercooled melt. It was found that the fluid flow would affect not only the size of the dendrite tip radius, mainly at the small undercooling range, but also the dendrite growth velocity at the intermediate undercooling range. In particular, fluid flow makes the sizes of the dendrite tip radius at high undercooling close to those predicted by the dilute model. Thus, fluid flow could make the non-dilute solution exhibit dilute solution solidification behaviors. It was also found that fluid flow reduces the relaxation effect to some extent. Considering the fluid flow effect, we used an extended chemical superheating model to predict the dendrite remelting phenomenon of the non-dilute melt. The model predicted that the dendrite remelting phenomenon would abruptly disappear once the dendrite growth velocity exceeded the solute diffusion velocity in a bulk undercooled melt. Third, considering the fluid flow effect, we used the concept of equivalent undercooling and a recently developed physical model to calculate the stress accumulation during rapid solidification. The results of this new model could explain well the stress-induced dendrite breakup mechanism of grain refinement at high undercooling.

Role of Electrolyte Evolution on Damage Distributions and Rates during Atmospheric Corrosion Under Sodium Chloride Droplets

Corrosion under sodium chloride droplets on copper is being studied via electrochemical modeling and experimentally characterized under rigorously controlled laboratory conditions. The objective of this work is to understand the governance of thin alkaline films that emanate from saline droplets upon corrosion initiation on corrosion rates and damage distributions. The radial spreading of thin alkaline films has been reported for both macroscopic and microscopic saline droplets on a variety of alloys that exhibit active corrosion under humid conditions [1-6]. This process is termed secondary spreading. It is resultant from oxygen reduction at cathode sites near the drop edge which produce concentrated hydroxide solution (e.g., NaOH) that subsequently spreads as a film over the metal surface [1,2,7,8]. These films are characteristically comprised of fields of microdroplets on the order of 100 μm diameter or less and can spread several times the original droplet radius from which they emanated. A number of investigators have characterized secondary spreading, demonstrating the potential for these films to serve as sites for oxygen reduction [7]. Unknown is the extent to which these secondary spreading films provide cathodic current to the corroding system and, in turn, their influence on corrosion rates and damage distributions. In this work, we explore the degree to which secondary spreading films influence corrosion that occurs under NaCl droplets on copper for a range of droplet diameters (50 µm to 1 mm). Our approach is to (1) develop a continuum-based model of the system (with non-continuum informed model extensions) to explore the influence of the film under various conditions and (2) define the physicochemical properties of the droplet-film system and experimentally realize the influence of the film on corrosion behavior to inform the model. Specifically, a transient, multiphysics finite element model is being constructed to predict current and potential distributions during the initial stage of corrosion over a range of droplet-film geometries and electrochemical parameters. Initial model assumptions include Nernst-Planck transport kinetics in non-dilute solution and the absence of heterogeneous reactions in the bulk electrolyte; both assumptions will be improved upon in subsequent iterations. Additionally, Butler-Volmer electrochemical kinetics is assumed, with oxygen reduction as the only cathodic reaction and metal oxidation as the only anodic reaction. The behavior of the droplet-film system is being experimentally studied via electrochemical capillary techniques and the physicochemical evolution of the droplet and film characterized to inform and benchmark the model. Key to the experimental efforts is the ability to rigorously control electrolyte geometry. To do so, a novel testing platform has been constructed that will allow production of varying size microdroplets and allow electrochemical probing within them. Experiments are currently under way to characterize the effect of restricted size secondary spreading regions on corrosion kinetics and damage distributions using this system. Together, this integrated computational and experimental approach seeks to deliver new mechanistic insight into atmospheric aerosol-driven corrosion.

Interpretation of fluorescence correlation spectra of biopolymer solutions.

Fluorescence correlation spectroscopy (FCS) is regularly used to study diffusion in non-dilute "crowded" biopolymer solutions, including the interior of living cells. For fluorophores in dilute solution, the relationship between the FCS spectrum G(t) and the diffusion coefficient D is well-established. However, the dilute-solution relationship between G(t) and D has sometimes been used to interpret FCS spectra of fluorophores in non-dilute solutions. Unfortunately, the relationship used to interpret FCS spectra in dilute solutions relies on an assumption that is not always correct in non-dilute solutions. This paper obtains the correct form for interpreting FCS spectra of non-dilute solutions, writing G(t) in terms of the statistical properties of the fluorophore motions. Approaches for applying this form are discussed.

Interaction parameters of oxygen and deoxidants in liquid iron

During decades before the evolution of more powerful computational tools, simplified formalisms such as the Wagner dilute solution formalism, have been successfully used in the study of deoxidation reactions of steel. This formalism relies on the introduction of interaction coefficients to account from deviations from Henry?s Law. With the evolution of thermodynamic modeling and of the CALPHAD method, the fact that thermodynamic descriptions using these parameters were derived to be used at relatively dilute solution has been sometimes overlooked and the formalism has been criticized for deviating from reality in non-dilute solutions. In this work, it is shown that the interaction parameters used in this formalism correlate with properties of the solutes and of the solvent. The work focuses on the interactions in systems Fe-M-O, where M is a deoxidant. Correlations between interaction coefficients and heats of formation of the corresponding oxides and with the atomic number of the deoxidants are demonstrated. This not only helps supporting the physicochemical soundness of the formalism but also provides a way of checking the consistency of data presented in this formalism.

Comparison of the Fitting Validity Between the 2P Model and the Nondilute Solution Model Using Statistical Methods in Modeling Cell Membrane Permeabilities.

In cryopreservation, the two-parameter (2P) model and the nondilute solution model have been developed to study the membrane transport properties of cells. However, to our knowledge, comparison of the fitting validity has never been made between the two models. In this study, to make this comparison, the permeability parameters of porcine adipose-derived stem cells (pADSCs) were first determined with the two models, and then the errors between the predictions and the experimental data were tested using the Lilliefors test. The results indicate that the 2P model is slightly better than the nondilute solution model in predicting the mass transport across cell membrane. The causes for this phenomenon are discussed and suggestions on using these two models are given.

A successful desensitization protocol for horse-derived antithymocyte globulin in severe aplastic anemia.

Horse antithymocyte globulin (h-ATG) (ATGAM(®) ) is the first choice of treatment in very severe patients with aplastic anemia who do not have any HLA matched sibling donor. h-ATG is a heterologous serum that may cause anaphylaxis. Alternative treatment strategies must be planned in case of hypersensitivity. Desensitization must be considered in patients without an alternative treatment of choice. We aimed to present the h-ATG desensitization protocol and consider its effectiveness in patients with aplastic anemia who are hypersensitized with h-ATG and do not have an alternative treatment of choice. Skin prick tests were performed with non-diluted solution in eight very severe patients with aplastic anemia who are followed up in Ege University Children's Hospital. Although skin prick test was found negative in these eight patients, different dilution h-ATG intradermal tests were performed and found positive in all patients. h-ATG desensitization program was started to these hypersensitized patients. Desensitization program was started to six male and two female very severe patients with aplastic anemia whose ages were between seven and 19yr (median: 12.9yr). All of the patients completed the desensitization program. While local reaction was seen in two patients, systemic reaction was seen in one patient and late reaction was seen in one patient during and after desensitization program. A successful desensitization program with h-ATG in children with aplastic anemia is presented. Even though there is not an exposure before to such high allergy potential heterologous serum, skin tests should be performed and desensitization must be started to patients who are hypersensitized to h-ATG. As the expected effectiveness of the treatment is so much, the desensitization protocol can be carried out safely and effectively with trained stuff although allergic reactions can be seen.

Instanton bags, high density holographic QCD, and chiral symmetry restoration

We describe the simplest example of an instanton bag in Euclidean space. It consists of a monopole wall and a Kaluza-Klein monopole wall, lifted to one higher dimension, trapping the instanton charge in the middle. This object has finite instanton density in a three-dimensional volume. Baryon physics in holographic QCD models gets translated into a multi-instanton problem in the bulk, and a state with a high density baryonic charge consists of a non-diluted multi-instanton solution. The instanton bag is a good candidate for this high-density state. We compute its parameters via moduli stabilization. Chiral symmetry restoration is exhibited by this state, and it is a direct consequence of its non-diluted features.