Stable Colloidal Dispersions Research Articles

Searching for the nano effect in Cu-HCF (II) particles to improve Cs sorption efficiency: Highlighting the use of intrinsic magnetism

The silica templating of Cu-HexaCyanoFerrate (HCF) is a classical technique which is often used to control the size and monodispersity thereby increasing the exposed surface area, and therefore the Cs sorption efficiency of what are normally highly aggregated nanoparticle systems. However, here we show that long term magnetic agitation of unfunctionalised aggregated Cu-HCF nanoparticles leads to a uniform and stable colloidal dispersion. This, we believe, is due to the intrinsic magnetic nature of Cu-HCF as seen by low temperature SQUID measurements. In this work we see that the Cs kinetics of magnetically dispersed Cu-HCF show a remarkable increase in the rate Cs sorption due particle disaggregation. Finally, the Cs sorption kinetics of these magnetically disaggregated particles proved similar to a chemically similar monodisperse silica templated sample.

Poly(N-isopropylacrylamide) Response to Salt Concentration and Anion Identity: A Brush-on-Brush Study.

The stability of poly(N-isopropylacrylamide) (PNIPAM) brush-modified colloidal silica particles was compared to asymmetric and symmetric PNIPAM brush direct force measurements in the presence of 1, 10, and 500 mM aqueous salt solution of KCl, KNO3, and KSCN between 10 and 45 °C. Dynamic light scattering measurements highlighted subtle variations in the salt-mediated thermoresponse, while atomic force microscopy (AFM) force curves between a bare silica or PNIPAM brush-modified colloid probe and a planar PNIPAM brush elucidated differences in brush interactions. The AFM force curves in the presence of KCl primarily revealed steric interactions between the surfaces, while KNO3 and KSCN solutions exhibited electrosteric interactions on approach as a function of the chaotropic nature of the ion and the solution concentration. The symmetric PNIPAM brush interaction highlighted significant variations between KCl and KSCN at 1 and 500 mM concentrations, while the approach and retraction force curves were relatively similar at 10 mM concentration. The combination of these techniques enabled the stability of PNIPAM brush-modified colloidal dispersions in the presence of electrolyte to be better understood with specific ion binding and the solution Debye length playing a significant role.

The effect of ceria and zirconia nanoparticles on the corrosion behaviour of cataphoretic epoxy coatings on AA6060 alloy

In this work an attempt was made to produce epoxy coating doped with ceria (CeO2) and/or zirconia (ZrO2) nanoparticles, in single step electrophoretric deposition process. Deposition was carried out on AA6060 cataforetically from epoxy resin suspension containing synthesized stable aqueous colloidal dispersions (sols) of CeO2 and ZrO2. The influence of ceria and zirconia nanoparticles on the physicochemical characteristics of epoxy coatings was evaluated by energy dispersive X-ray fluorescence spectroscopy, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis and adhesion measurements, while their corrosion stability during immersion in 0.5 M NaCl solution was monitored by electrochemical impedance spectroscopy. All ceria and/or zirconia containing coatings showed better corrosion stability and adhesion than pure epoxy coating, while between doped epoxy coatings one with CeO2/ZrO2 exhhibited the greatest values of charge-transfer resistance due to synergetic effect of both types of added nanoparticles.

Easy Formation of Functional Liposomes in Water Using a pH‐Responsive Microbial Glycolipid: Encapsulation of Magnetic and Upconverting Nanoparticles

AbstractThe compartmentalization of colloids into topologically closed, vesicular, microphases offers an attractive mean to concentrate a functional cargo in aqueous solutions for a range of biomedical, cosmetic, and biotechnological applications. In this paper, we develop a simple, phospholipid‐free, phase change method employing a pH‐responsive glycolipid. The method is applied to the encapsulation of a sonicated, metastable, aqueous dispersion of functional colloids in the lumen of lipid vesicles: uncoated magnetic maghemite γ‐Fe2O3 and oleic‐acid coated upconverting NaYF4 : Yb/Ln (Ln=Er or Tm) nanoparticles (NPs). We find a stable liposomal dispersion containing a sub‐population of crowded liposomes with high concentrations of NPs. The encapsulated NPs, formed at nearly neutral pH and room temperature, are stable over time and towards extrusion. The vesicular microphase is entirely composed of pH‐responsive glycolipids, which undergo a pH‐mediated mesoscopic structural transition from an open lamellar (2<pH<4) to topologically closed vesicular state (pH>4). We also show that encapsulation successfully works with a stable colloidal aqueous dispersion of iron clusters stabilized in ferritin cages. This compartmentalization approach combining self‐assembly with an orthogonal nonequilibrium dispersion of nanoparticles shows untapped potential for synthesizing unusual classes of mixed matter.

Aggregation of stable colloidal dispersion under short high-shear microfluidic conditions

We have designed and carried out intense shear-driven aggregation of charge-stabilized polystyrene nanoparticles (NPs) to big fractal clusters, in a single and a double Z-shape microchannels (SZ-MC and DZ-MC). Both the MCs have the same cross-section, but the length of DZ-MC is double of SZ-MC. It is found that when the NP dispersion repeatedly passes through each of the two MCs, the conversion of the NPs to big clusters can only reach a plateau value that is far smaller than full conversion. The plateau conversion at a given mean velocity (shear rate) is larger for the DZ-MC than for the SZ-MC, i.e., it increases as the MC length (or the residence time of one pass) increases. This surprising phenomenon can be explained upon considering that, as the conversion increases, the average NP separation distance increases, i.e., the time for the specific shear-driven bi-particle aggregation to occur increases. When the aggregation time becomes larger than the NP residence time in a MC, the NP dispersion passing through the MC does not lead to aggregation. The situation does not change if we let the NP dispersion repeatedly pass through the MC many times (i.e., by increasing the cumulative residence time), because if the residence time of one pass is not enough for the NPs to overcome the interaction energy barrier, when they come out from the MC, the intense shear force disappears and the NPs return to their original interaction state.

Stable colloidal dispersion of octylated Ti3C2-MXenes in a nonpolar solvent

Two-dimensional (2D) early transition metal carbides and/or nitrides (MXenes) have shown significant potential in a wide variety of applications. The majority of MXene research has utilized aqueous colloidal dispersions of MXenes because of the unique hydrophilicity of the MXene surface. In some cases, however, aqueous dispersions have various disadvantages such as the degradation of MXenes in long-term storage, incompatibility with water-insoluble polymers, and dewetting on low-surface-energy substrates. Herein, we demonstrated a stable nonpolar colloidal dispersion of MXenes chemically grafted with lipophilic octyltriethoxysilanes (MX@OTS). A high grafting density of OTS on the MXene surface was achieved by employing a strong acid polyelectrolyte, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS), as an OTS (water insoluble)-in-water emulsifier while the reaction was carried out in the aqueous media in which the hydrophilic MXenes showed the best dispersion. The remarkable hydrophobicity of these octylated MXenes was demonstrated by a high water-contact angle on the macroscopic MX@OTS surface as well as the high interfacial tension between the MX@OTS-in-hexane dispersion and water, which was almost identical to that for pure hexane/water. Despite the negligible surface charging, the hydrophobic MX@OTS showed excellent colloidal stability in very nonpolar hexanes over four weeks.

Comparison of Formulation Methods to Produce Nano-Chitosan as Inhibitor Agent for Bacterial Growth

Chitosan is known as an antibacterial agent. The effective surface area ratio of chitosan can be increased by modification into nanoparticles. Nano-chitosan can be prepared with several simple methods, i.e. precipitation, ionic gelation, or the polyelectrolyte complex method. This study compared these three methods in terms of the targeted product characteristics, i.e. stability of the average nanoparticle size as well as the colloidal dispersion, and the antibacterial characteristics. All three methods resulted in nanoparticle formation, but in the precipitation method significant zeta potential reduction was observed due to the presence of negative ions from the alkali that neutralized the chitosan amine group. The ionic gelation method yielded higher zeta potential and higher inhibition of bacterial growth than those yielded by the polyelectrolyte complex method. Ionic gelation and the polyelectrolyte complex method resulted in much better colloidal dispersion stability than the precipitation method, where a significant particle size increase was observed after one week of storage. This result indicates that both ionic gelation and the polyelectrolyte complex method can be used for forming nano-chitosan for the purpose of food preservation. However, for fishery products it is advisable to use the polyelectrolyte complex method because the TPP usually used in ionic gelation is not allowed to be applied to fish.

Multicomponent bionanocomposites based on clay nanoarchitectures for electrochemical devices.

Based on the unique ability of defibrillated sepiolite (SEP) to form stable and homogeneous colloidal dispersions of diverse types of nanoparticles in aqueous media under ultrasonication, multicomponent conductive nanoarchitectured materials integrating halloysite nanotubes (HNTs), graphene nanoplatelets (GNPs) and chitosan (CHI) have been developed. The resulting nanohybrid suspensions could be easily formed into films or foams, where each individual component plays a critical role in the biocomposite: HNTs act as nanocontainers for bioactive species, GNPs provide electrical conductivity (enhanced by doping with MWCNTs) and, the CHI polymer matrix introduces mechanical and membrane properties that are of key significance for the development of electrochemical devices. The resulting characteristics allow for a possible application of these active elements as integrated multicomponent materials for advanced electrochemical devices such as biosensors and enzymatic biofuel cells. This strategy can be regarded as an “a la carte” menu, where the selection of the nanocomponents exhibiting different properties will determine a functional set of predetermined utility with SEP maintaining stable colloidal dispersions of different nanoparticles and polymers in water.

Inversion of Dispersion: Colloidal Stability of Calixarene-Modified Metal-Organic Framework Nanoparticles in Nonpolar Media.

Making metal-organic frameworks (MOFs) that are stabilized in nonpolar media is not as straightforward as making their inorganic nanoparticle counterparts, since surfactants penetrate through the porous structures or dissolve the secondary building units (SBUs) through ligand-exchange linker modulator mechanisms. Herein, we report that calixarenes stabilize UIO-66 nanoparticles effectively by remaining outside the grains through size exclusion, without pores becoming blocked, all the while providing amphiphilicity that permits the formation of stable colloidal dispersions with much narrower size distributions. Using the UIO-66 dispersed solutions, we show that smooth films from an otherwise immiscible polystyrene can be made feasibly.

Top-down synthesis of S-doped graphene nanosheets by electrochemical exfoliation of graphite: Metal-free bifunctional catalysts for oxygen reduction and evolution reactions

In this study, top-down synthesis of sulfur-doped graphene nanosheets (SDGNs) by simple electrochemical exfoliation was explored as a means of producing metal-free electrocatalysts for oxygen reduction and oxygen evolution reactions (ORR and OER, respectively). In a typical procedure, graphite foils were used to obtain bulk quantities of SDGN catalysts in the presence of thiosulfate as a sulfur source. Highly stable colloidal dispersions of SDGNs were obtained by applying a voltage of 15 V at an optimized Na2S2O3:H2SO4 molar ratio of 5 : 1 (denoted SDGN(5)). Physicochemical characterizations by Raman spectroscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy confirmed the existence of sulfur and its electronic/structural properties in graphene nanosheets. In alkaline media, SDGN(5)-modified electrodes were comparable or superior to pristine graphene and a benchmark commercial platinum-based electrodes in terms of stability, methanol tolerance, n values, and onset potential for ORR and OER. The specific capacitance (149.9 F g−1) of SDGN(5) supported its excellent ORR/OER performance and enhanced surface area.

Impact of local inhomogeneities on the complexation between poly(diallyldimethylammoniumchloride) and sodium dodecyl sulfate

The nonequilibrium behavior of oppositely charged polyelectrolyte(PE)/surfactant(S) mixtures has been the focus of recent investigations due to its industrial relevance. The studies indicated that the initial local inhomogeneities may significantly affect the characteristics of the formed PE/S nanoassemblies. However, this phenomenon is not clearly understood and it has not been investigated systematically. In the present work, the effects of polyelectrolyte and salt concentration gradients were studied in poly(diallyldimethylammoniumchloride) (PDADMAC)/ sodium dodecyl sulfate (SDS)/ sodium chloride (NaCl) mixtures. The results revealed two distinct nonequilibrium phenomena at low ionic strengths. If the SDS concentration is significantly larger than that of the polyelectrolyte monomers (i.e. at surfactant excess), then the system can be instantly arrested in a stable colloidal dispersion state, where the size of the trapped overcharged aggregates is primarily dependent on the local PDADMAC concentration gradients. However, if the polyelectrolyte charges largely exceed the amount of surfactant molecules (i.e. at polyelectrolyte excess), then another nonequilibrium feature, e.g. a highly reproducible non-monotonic variation of the turbidity with time is observable. This is attributed to the extremely slow approach towards the equilibrium state due to the heterogeneous local distribution of the surfactant ions and their hindered repartitioning process between the PDADMAC/SDS complexes. The results of the present work may be exploited to tune the time dependent surface and bulk features of PE/S systems, which are of crucial importance for their commercial applications.

AC electrophoretic mobility of individual microscale colloidal particles measured using holographic video microscopy

Electrophoretic mobility has been widely used to evaluate the zeta potential of individual colloidal particles, which governs the stability of colloidal dispersions. We demonstrated two experimental methods to measure the AC electrophoretic mobility μ of micron-sized single particles using holographic video microscopy. The three-dimensional position of the particle was estimated by reconstructing the light field from its two-dimensional holographic image, using the Rayleigh-Sommerfeld back-propagation method. In a planar electric field setup, the height dependence of the measured value of μ in the cell enables us to evaluate the actual value of μ, without interference from electroosmotic flow. In a vertical setup, the true value of μ can be directly evaluated by minimizing the influence of the electrode polarization, using a thick cell and a high-frequency electric field. The estimated values of μ obtained using both methods agree with that from conventional electrophoretic light scattering. We also evaluated the distribution of μ values within a colloidal dispersion.

Encapsulated lanthanum strontium manganese oxide in mesoporous silica shell: Potential for cancer treatment by hyperthermia therapy

We have reported the synthesis of core-shell nanostructure of mesoporous silica coated lanthanum strontium manganese oxide nanoparticles (MSLN) as a potential candidate for cancer treatment via magnetic hyperthermia. N2 adsorption-desorption isotherm revealed that mesoporous silica shell coating increased the surface area and pore volume of lanthanum strontium manganese oxide nanoparticles (LSMO). In comparison to LSMO, MSLN showed a stable colloidal dispersion up to 48 h. In-vitro studies using A549 cells demonstrated increased biocompatibility in MSLN as compared to LSMO. Moreover, cells treated with MSLN exhibited high cell survival rate of up to 1 mg/ml concentration. Further, we used different parameters to modulate the heating efficacies of these nanoparticles. The modulation of specific absorption rates (SAR) with different sample concentrations (0.5, 1, 1.5, 2.0 mg/ml) were studied under four different alternating current magnetic field (AMF) amplitudes (10, 11, 12, 13 kA/m) at a constant frequency of 335 KHz for magnetic hyperthermia experiments. Under the application of AMF, MSLN exhibited faster Néel and Brownian relaxation compared to their LSMO counterparts. SAR value of 295 W/g and intrinsic loss power (ILP) of 5.22 nHm2/kg, at a low sample concentration of MSLN (0.5 mg/ml) were obtained, which was within the clinical limit. Interestingly, our results showed 45% improvement in terms of the heating efficiency over the commercially available products (Table 3). Hence, we conclude that MSLN with high SAR and ILP values, good colloidal dispersion ability and low cytotoxicity could be a potential candidate for magnetic hyperthermia therapy in cancer and warrant further testing in both in-vitro and in-vivo studies. Further, due to their mesoporous nature, MSLN could be considered for target-specific drug delivery applications in clinical therapy.

The Study of the Stability of Colloid Dispersions of Copper Nanoparticles Based on Sodium Dodecyl Sulfate

Methods for the synthesis of copper nanoparticles in a micellar solution of an anionic surfactant, sodium dodecyl sulfate, are improved by optimizing the molar ratios of the precursor and the reductant, Cu(II): N2H4 of 1 : 4, 1 : 8, 1 : 20, 1 : 40, 1 : 100, and 1 : 150 at a pH of 11.1 and pH values of the reaction medium of 10.9, 11.0, 11.2, and 11.9 at the optimum Cu(II): N2H4 molar ratio of 1 : 150. The effect of increasing the Cu(II) : N2H4 molar ratio on the growth in the absorbance of suspensions of copper nanoparticles is established. The stability of solutions over time is studied using the spectrophotometric analysis of colloidal dispersions of copper nanoparticles obtained at a Cu(II) : N2H4 molar ratio of 1 : 150 and values of pH of 10.9, 11.0, 11.2, and 11.9. The process of the aggregation of copper particles accompanied by a hypsochromic shift of the absorption band of surface plasmon resonance towards larger wavelengths is studied by dynamic light scattering. Scanning electron microscopy has shown that copper particles obtained at a Cu(II) : N2H4 molar ratio of 1 : 150 and pH values of 10.9, 11.0, and 11.2 have a spherical shape with dimensions in the range from 12 to 176 nm.

Silver nanoparticles coated with dodecanethiol used as fillers in non-cytotoxic and antifungal PBAT surface based on nanocomposites

In the present study, we report the preparation of antifungal and non-cytotoxic polymer nanocomposites with potential application in biomedical materials. Dodecanethiol-protected silver nanoparticles (AgNPs-DDT) were synthesized by a reduction/precipitation method and dispersed in chloroform to obtain stable colloidal dispersions. PBAT-based nanocomposites containing 0.25, 0.5 and 2 wt% AgNPs-DDT were prepared by casting method. The incorporation of AgNPs-DDT in PBAT matrix resulted in nanocomposites which combine improved mechanical performance and antifungal properties with a non-cytotoxic characteristic.

Structural analysis of carbon nanospheres synthesized by CVD: an investigation of surface charges and its effect on the stability of carbon nanostructures

A simple and effective method for mass production of highly homogeneous carbon nanospheres (CNSs) is reported here using xylene and benzene as carbon precursors. Effect of hydrocarbon precursors on the morphology of carbon nanostructures is investigated, keeping deposition temperature, deposition time, and gas flow rate constant. Morphological studies show that homogeneous CNSs are produced on large scale using xylene hydrocarbon as a carbon precursor. Role of ferrocene is also examined as a catalyst for different distinctive morphologies of carbon nanostructures. This study discloses that catalyst plays an important role for formation of carbon nanotubes during growth process. Excitation wavelength-dependent Raman studies reveal enhancement in the oscillator strength and phonon softening for Raman active modes when excitation wavelength is changed from 514 to 785 nm. Surface charges and colloidal stability of aqueous dispersion of CNSs and CNSs after surface modification with surfactants are also studied by zeta-potential measurement. It was found that CNSs modified with surfactants have higher zeta potential than that of as-synthesized CNSs. Roles of anionic and cationic surfactants are also studied here for determining the surface charges, stability, and agglomeration state in CNSs.

Development of highly stable colloidal dispersions of gelled-oil nanoparticles loaded with cuminaldehyde

The main objective of this work was to develop an aqueous dispersion of gelled-oil nanoparticles as a novel carrier for cuminaldehyde. A hot emulsification process followed by cooling to room temperature was used to prepare gelled-oil nanoparticles in the presence of monoglyceride as organogelator. The effects of monoglyceride (10% and 15% w/w of the total lipid phase) and cuminaldehyde (0, 500 and 1000 mg/L) concentrations on different characteristics (e.g. droplet size, zeta potential, physical stability, rheology, encapsulation efficiency, release behavior and morphology) of the colloidal dispersions were studied during 28-d storage. The average droplet size of freshly prepared samples was within the range of 117–138 nm with a relatively narrow distribution. Large negative ζ-potential values (−38 to −55 mV) were a proof for the kinetic stability of the colloidal dispersions. Samples which were prepared with 10% monoglyceride showed a higher physical stability over a period of 90 d. Non-Newtonian (shear-thinning) and solid-like viscoelastic properties were observed due to particle-particle interactions. The results of the temperature ramp test confirmed that the droplets were in the gelled state. An increase in the organogelator concentration decreased the interfacial tension and contact angle. Transmission electron microscopy micrographs confirmed the formation of spherical nanoparticles. The encapsulation efficiency of cuminaldehyde in gelled-oil nanoparticles was 73–87%. In-vitro release profiles exhibited that the release of cuminaldehyde from gelled-oil nanoparticles in the simulated gastric fluid was lower than its release in the simulated intestinal fluid. Moreover, the release was increased by decreasing the concentration of organogelator. The obtained colloidal dispersions have the potential to be utilized as effective encapsulation systems for poorly water-soluble nutraceuticals.

Determining the dispersion stability of black phosphorus colloids by 3D light scattering

Understanding the dispersion state of 2D black phosphorus (BP) nanosheets is of great significance for the application of BP based materials. However, the dispersion state of BP colloids cannot be analyzed by the conventional methods based on the optical signal detecting due to the intensive light absorption of BP in the ultraviolet-visible region and the high turbidity caused by the aggregation of pristine BP nanosheets. In this work, 3D light scattering has been proven to be an effective technique to characterize the dispersion characteristics of nanocolloids with intensive light absorption and limited dispersity by suppressing multiple scattering. The effect of the concentration, the temperature and the solvent on the dispersion stability of BP nanosheets has been systematically studied by modulated 3D cross-correlation. It has been shown that measurements at smaller angles can obtain better autocorrelation functions. The BP colloids exhibit excellent colloidal stability when the concentration is below 250 μgmL−1, above which aggregation tends to form. Increasing the temperature has shown to result in the formation of aggregation in the colloids. BP nanosheets possess better dispersion state in polar solvents than in nonpolar solvents, and the aggregation process in nonpolar solvents can be monitored by the slow relaxation mode of 3D light scattering. This study indicates that 3D light scattering paves way to analyze the dispersion dynamics of various 2D nanoparticle colloids with intensive light absorption and poor dispensability.

Assessing Physical Stability of Colloidal Dispersions Using a Turbiscan Optical Analyzer.

The physical stability of aqueous colloidal dispersions containing highly concentrated droplets of poorly water-soluble drugs has recently been identified as one of the main considerations in developing amorphous solid dispersions (ASDs). The Turbiscan analyzer, an instrument based on multiple light scattering technology, was employed for the first time to assess colloidal dispersions with ritonavir as the model compound. The physical instability of ritonavir-rich droplets was monitored directly with and without the presence of candidate polymer additives at different drug concentrations and temperatures. The mechanism of the observed instability was confirmed to be coalescence of liquid droplets, based on the low glass transition temperature of water-saturated amorphous ritonavir determined using a newly developed experimental procedure. Temperature and solvent composition, within the range studied, have little influence on the kinetics of ritonavir coalescence. On the contrary, a higher concentration of drug, i.e., more droplets per unit volume, greatly accelerates the coalescence process. In addition, polymers with varying degrees of hydrophobicity resulted in different levels of effectiveness in stabilization, which is likely related to the strength of drug-polymer interactions and the corresponding differences in surface adsorption. This work demonstrates that the Turbiscan optical analyzer can be used as a rapid screening tool that provides a first-pass, high-throughput feasibility ranking of different excipients and additives to support the development of ASD formulations.

Studies on Different Support Seeding Conditions Applied in the Formation of NaY Zeolite Membrane

Abstract Exploring the efficient synthesis method is essential in producing zeolite membranes with minimum defects. Seeding prior hydrothermal synthesis (HT) is the major parameters that affecting the quality of zeolite membrane formed. The colloidal stability of the NaY seed solution and its attachment to the support were studied by varying the pH of the seed solutions. Three seeding methods, including dip-coating, spin coating and vacuum coating, were studied in forming NaY zeolite seed layer. Then, the seeded alumina supports were subjected to hydrothermal treatment (HT) in order to form NaY zeolite membranes. The effect of NaY seeds solution colloidal stability, electrostatic attachment between seeds and support and the seeding methods on the properties of the quality of the formed NaY zeolite membranes were investigated. The zeta potential of the seed solution was measured with respect to different pH of the seed solution. The formed NaY zeolite membrane samples were characterized with X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) and Energy dispersive X-Ray spectroscopy (EDX). Spin coating and vacuum coating displayed better seeds coverage onto the alumina support due to the vacuum lock mechanism however excessive seeds accumulation onto the alumina support will inhibit zeolite growth and expansion hence to produce NaY zeolite membranes with cracks. Nonetheless, seeded supports by dip coating method display less coverage on the support however with a proper selection pH of the seeding solution with relates to its zeta potential, stable aqueous colloidal dispersion of the seeds solution and better attachment between the zeolite seeds and the support could be achieved in order to form zeolite NaY membranes with least defects.