Mixture Of Trifluoroacetic Acid Research Articles

Development of anticorrosive self-adhesives from post-consumer poly(ethylene terephthalate) bottles for industrial applications

Corrosion is a spontaneous process that can result in both aesthetic and structural damage, incurring significant costs. One of the most commonly employed methods for mitigating the effects of corrosion is the application of organic coatings. However, no coating is precluded from potential damage, whether due to factors such as end-of-life degradation, inadequate application, poor quality, or impact-related incidents. Within this context, the primary objective of this study is to develop a novel and easy to apply anticorrosive adhesive for carbon steel surfaces from post-consumer poly(ethylene terephthalate) (PET) bottles as a temporary coating. PET films were produced by phase inversion technique, which involves the dissolution of PET bottles in a mixture of trifluoroacetic acid (TFA) and dichloromethane (DCM) solvents, followed by precipitation in a non-solvent bath. The resulting films were utilized as backing materials in the production of self-adhesive tapes with anticorrosive properties. Two different solvent proportions were selected to assess the feasibility of this technique, which are 70/30 and 50/50 (TFA/DCM v/v). For each condition, scanning electron microscopy, Fourier transform infrared spectroscopy, thermal analysis, immersion tests in NaCl aqueous solution 3.5 wt%, and electrochemical impedance spectroscopy were performed. Experimental results revealed that the utilization of a solvent system with a higher concentration of DCM produces a more cohesive and denser film, leading to improved outcomes in both total immersion and electrochemical impedance spectroscopy tests. The impedance modulus value obtained after 168 h of immersion in an aqueous solution of NaCl 3.5 wt% for 50/50 TFA/DCM film was 106 Ω·cm2, with no corrosion product observed on the protected area. Furthermore, thermal and infrared analyzes demonstrated that there is no change in PET chemical and thermal properties after inversion phase process.

Plasticized, greaseproof chitin bioplastics with high transparency and biodegradability

A mixture of trifluoroacetic acid:trifluoroacetic anhydride (TFA:TFAA) was used to dissolve chitin from shrimp shells. Free-standing films were prepared by blending the chitin solution and glycerol at different percentages, followed by drop-casting, and the complete evaporation of the solvents. After this process, the chitin matrix showed an amorphous molecular structure, as determined by X-ray diffraction. Optical, mechanical, thermal, and antioxidant properties were also thoroughly investigated. The incorporation of glycerol induced a plasticizing effect on the mechanical response of films and improved their transparency. In addition, hydrodynamic and barrier properties were determined by contact angle and water vapor/oxygen transmission rates, respectively, and revealed typical values of other polysaccharides. These bioplastics also presented an excellent greaseproof behavior with the highest degree of oil repellency as determined by the Kit test. Moreover, the overall migration was evaluated by using Tenax® as a dry food simulant and levels were compliant with European regulations. Their antifungal properties were tested using Botrytis cinerea as a model. Biodegradability was also determined by measuring the biological oxygen demand in seawater. Degradation rates were high and similar to those of other fully-degradable materials.

Total Synthesis of a Reversed-Bacicyclin Using a Combination of Solid- and Solution-Phase Methods

Bacicyclin is a cyclic hexapeptide with antibacterial activity against Enterococcus faecalis and Staphylococcus aureus with minimum inhibition concentration (MIC) values of 8 and 12 µM, respectively. Studies on a reversed sequence of bacicyclin were conducted to investigate how the reversed peptide sequence affects its biological properties. A reversed-bacicylin, cyclo-(Gly-Leu-Val-Ile-Ala-Phe), was successfully synthesized by constructing the linear precursor on 2-chlorotrityl chloride resin using a Fmoc-based strategy. The HATU/HOAt reagent was applied in all peptidic bond formations, and the desired linear hexapeptide (82% yield) was cleaved off the resin using a mixture of trifluoroacetic acid:dichloromethane (2:8). The linear peptide was cyclized using 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium-3-oxidehexa-fluorophosphate (HATU) as a coupling agent and diisopropylethylamine (DIPEA) as the base in a very dilute solution (0.001 M) in dichloromethane, then purified by octadecyl silica gel (ODS) column chromatography to obtain the reversed-bacicyclin (43.7% yield). The purity of the cyclic product was analyzed using analytical RP-HPLC (tR = 20.01 min), and characterized by HR-TOF-MS, 1H-NMR, and 13C-NMR. The biological activity of the reversed-bacicyclin is much lower compared to bacicyclin, indicating that the amino acid sequence of the cyclopeptide dictates the antibacterial activity. This finding gives additional information on the relationship between peptide sequence and biological properties.

Manganese Catalyzed Partial Oxidation of Light Alkanes

The catalytic partial oxidation of methane is achieved at low temperatures (<200 °C) using manganese oxides and manganese salts in mixtures of trifluoroacetic acid and trifluoroacetic anhydride. Dioxygen is used as the in situ terminal oxidant. For Mn oxides (e.g., MnO2, Mn2O3, and Mn3O4), we studied stoichiometric methane partial oxidation in HTFA (TFA = trifluoroacetate). Using a Mn trifluoroacetate salt, at 180 °C and under 25 psig of methane, product selectivity for the mono-oxidized product methyl trifluoroacetate (MeTFA) is observed to be >90% at ∼35% methane conversion at approximately 6 turnovers. Under our catalytic methane oxidation reaction conditions, MeTFA is stable against overoxidation, which explains the likely high selectivity at conversions >15%. Using combined experimental studies and DFT calculations, a mechanism involving soluble and molecular Mn species in the catalytic cycle is proposed. The proposed reaction pathway involves initial activation of MnII by dioxygen, cleavage of a methane C–H bond by a MnIII hydroxo intermediate, rebound of the methyl radical to generate MeTFA, and finally regeneration of the starting MnII complex. Also, this process is shown to be applicable to the oxidation of ethane, favoring the mono-oxidized product ethyl trifluoroacetate (EtTFA) and reaching ∼46% conversion.

A Novel Stability Indicating RP-HPLC Method for Simultaneous Quantification of Serdexmethylphenidate and Dexmethylphenidate in Fixed Dosage Form.

A simple, robust stability indicating RP-HPLC method was developed for simultaneous quantification of serdexmethyl phenidate and dexmethyl phenidate in a fixed capsule dosage form. This is the first method to be reported for simultaneous estimation and quantification of degradation products produced from forced stressing of the dosage form as per ICH guidelines. The chromatographic separation was attained on Waters X-terra C18 column using a mixture of trifluoro acetic acid and acetonitrile (70:30v/v) as mobile phase with a flow rate of 1mL, monitored at 265nm over a run time of 10min. Serdexmethyl phenidate and dexmethyl phenidate were eluted with retention times of 2.71min and 7.33min, respectively. The method displayed linear responses in the range of 4.2-63μg/mL (0.9994) for serdexmethyl phenidate and 0.9 to 3.5μg/mL (0.9998) for dexmethyl phenidate, respectively. The percentage recoveries of the two drugs were found within the acceptable limits. Forced degradation was conducted and showed considerable degradation in various stress conditions. It also confirms the specificity of the method as no interference peaks were observed concerning for to stress products. This method can be routinely used in quality control labs for simultaneous determination of respective drugs in marketed dosage form.

Patterning, Transfer, and Tensile Testing of Covalent Organic Framework Films with Nanoscale Thickness

Covalent organic frameworks (COFs) are promising materials for a variety of applications, including membrane-based separations, thin-film electronics, and as separators for electrochemical devices. Robust mechanical properties are critical to these applications, but there are no reliable methods for patterning COFs or producing free-standing thin films for direct mechanical testing. Mechanical testing of COFs has only been performed on films supported by a rigid substrate. Here, we present a method for patterning, transferring, and measuring the tensile properties of free-floating nanoscale COF films. We synthesized COF powders by condensation of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthalaldehyde (PDA) and prepared uniform thin films by spin casting from a mixture of trifluoroacetic acid and water. The COF films were then reactivated to recover crystallinity and patterned by plasma etching through a poly(dimethylsiloxane) (PDMS) mask. The films were transferred to the surface of water, and we performed direct tensile tests. We measured a modulus of approximately 1.4 GPa for TAPB-PDA COF and a fracture strain of 2.5%, which is promising for many applications. This work advances the development of COFs for thin-film applications by demonstrating a simple and generally applicable approach to cast, pattern, and transfer COF thin films and to perform direct mechanical analysis.

The chemical composition of polyphenolic compounds Achillea micranthoides Klok. herbs

For obtain modern herbal remedies, medicinal plants containing flavonoids and hydroxycinnamic acids are promising. These polyphenolic compounds are characterized by structural diversity, non-toxicity, a wide range of pharmacological activity. An actual problem is the development of new modern instrumental research methods. In scientific and folk medicine the genus Achillea L. of the Asteraceae L. family are widely used to accelerate wound healing, stop bleeding of various etiologies, as anti-inflammatory and anthelmintic agents. The herb of Achillea micranthoides Klok. has theoretical and practical interest. The plant is distributed in the southern regions of the Black Sea and Mediterranean countries, Europe, Ukraine. A long growing season contributes to the accumulation of biologically active polyphenolic compounds from groups of flavonoids and hydroxycinnamic acids.&#x0D; The aim of the work was to study the composition of polyphenolic compounds (flavonoids and the hydroxycinnamic acids) in the herbs of the Achillea micranthoides Klok. during flowering (june-september).&#x0D; The object of research was herb of Achillea micranthoides Klok. Harvesting was carried out in south and oust regions of Ukraine during flowering (June–September, 2017). The investigation of polyphenolic compounds were performed by TLC and HPLC methods. HPLC analyzes were performed using an Agilent Technogies (Model 1100) (Japan). The chromatographic column ZORBAX-SB C-18 measuring 2.1×150 mm, filled with octadecylsilyl sorbent, with a grain diameter of 3.5 microns were used. The systems as the mobile phase: trifluoroacetic acid 0,2%, methanol anhydrous and a mixture of trifluoroacetic acid 0,2% with methanol 70% were used; wave length 313 nm, 350 nm.&#x0D; During the flowering period, the presence and quantitative content were established 8 flavonoids (up to 1,176 ± 0,122%) and 6 hydroxycinnamic acids (up to 0,807 ± 0,071%). The identified flavonoids are dominated by compounds: apigenin-7,4'-di-O-glucoside, quercetin-3-O-rutinoside, luteolin-6-C-glucoside, luteolin-7-O-β-D-glucopyranoside, apigenin-7-O-β-D-glucopyranoside. The highest content in the composition of hydroxycinnamic acids was found for chlorogenic and caffeic acids.&#x0D; The obtained results to be used for perspective modern herbal remedies from Achillea micranthoides Klok. herb with anti-inflammatory, antioxidative, haemostatic and hepatoprotective activities. The qualitative and quantitative determination methods of polyphenolic compounds in Achillea micranthoides Klok. herb may be used to the State Ukraine Pharmacopoeia monographs.

Development and validation of a simple method for the extraction and quantification of crisaborole in skin layers.

Crisaborole is a boron compound recently approved by the US Food and Drug Administration as a 2% ointment for the treatment of mild to moderate atopic dermatitis. This work describes a simple method for the quantification of the drug in the skin layers at the end of in-vitro permeation experiments. Chromatographic separation was carried out on a reverse-phase C18 column using a mixture of trifluoroacetic acid 0.05%-acetonitrile (55:45, v/v) as mobile phase, pumped at 1 ml/min. Column temperature was 35°C and UV detection was performed at 250 nm. The method was linear in the range of concentration from 0.06 to 6 μg/ml (R2 = 1) and was selective, precise and accurate. Depending on the solvent used, the LOQ ranged from 0.014 to 0.030 μg/ml and the LOD from 0.005 to 0.010 μg/ml. The extraction from all the skin layers was quantitative. The developed method was successfully tested in an in-vitro permeation study, proving to be an effective tool in the development of new formulations containing crisaborole.

Exploring Next-Generation Engineering Bioplastics: Poly(alkylene furanoate)/Poly(alkylene terephthalate) (PAF/PAT) Blends.

Polymers from renewable resources and especially strong engineering partially aromatic biobased polyesters are of special importance for the evolution of bioeconomy. The fabrication of polymer blends is a creative method for the production of tailor-made materials for advanced applications that are able to combine functionalities from both components. In this study, poly(alkylene furanoate)/poly(alkylene terephthalate) blends with different compositions were prepared by solution blending in a mixture of trifluoroacetic acid and chloroform. Three different types of blends were initially prepared, namely, poly(ethylene furanoate)/poly(ethylene terephthalate) (PEF/PET), poly(propylene furanoate)/poly(propylene terephthalate) (PPF/PPT), and poly(1,4-cyclohenedimethylene furanoate)/poly(1,4-cycloxehane terephthalate) (PCHDMF/PCHDMT). These blends’ miscibility characteristics were evaluated by examining the glass transition temperature of each blend. Moreover, reactive blending was utilized for the enhancement of miscibility and dynamic homogeneity and the formation of copolymers through transesterification reactions at high temperatures. PEF–PET and PPF–PPT blends formed a copolymer at relatively low reactive blending times. Finally, poly(ethylene terephthalate-co-ethylene furanoate) (PETF) random copolymers were successfully introduced as compatibilizers for the PEF/PET immiscible blends, which resulted in enhanced miscibility.

Transparent and Robust All-Cellulose Nanocomposite Packaging Materials Prepared in a Mixture of Trifluoroacetic Acid and Trifluoroacetic Anhydride.

All-cellulose composites with a potential application as food packaging films were prepared by dissolving microcrystalline cellulose in a mixture of trifluoroacetic acid and trifluoroacetic anhydride, adding cellulose nanofibers, and evaporating the solvents. First, the effect of the solvents on the morphology, structure, and thermal properties of the nanofibers was evaluated by atomic force microscopy (AFM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA), respectively. An important reduction in the crystallinity was observed. Then, the optical, morphological, mechanical, and water barrier properties of the nanocomposites were determined. In general, the final properties of the composites depended on the nanocellulose content. Thus, although the transparency decreased with the amount of cellulose nanofibers due to increased light scattering, normalized transmittance values were higher than 80% in all the cases. On the other hand, the best mechanical properties were achieved for concentrations of nanofibers between 5 and 9 wt.%. At higher concentrations, the cellulose nanofibers aggregated and/or folded, decreasing the mechanical parameters as confirmed analytically by modeling of the composite Young’s modulus. Finally, regarding the water barrier properties, water uptake was not affected by the presence of cellulose nanofibers while water permeability was reduced because of the higher tortuosity induced by the nanocelluloses. In view of such properties, these materials are suggested as food packaging films.

Enhancement of molecular-level ordering of isoindigo based organic materials through deprotecting of cleavable carbamate groups with long alkyl chains

A new highly soluble isoindigo based organic material with both cleavable carbamate protecting groups bearing long alkyl chains and thiophene groups was successfully synthesized. The carbamate protecting groups were easily removed by solvent-vapor thermal annealing using a mixture of trifluoroacetic acid and chloroform. Deprotection of the NH functional groups in the isoindigo backbone in situ facilitated H-bonding that dramatically improved the strength of the intermolecular interactions. X-ray diffraction and high-voltage electron microscopy studies of the resulting organic material showed long-range ordered crystalline structure. High crystallinity due to enhanced π-π stacking and hydrogen bonding significantly improved the charge carrier mobility. The hole mobilities of isoindigo based organic material measured by SCLC method improved from 3.46 x 10−6 cm2 V−1 s−1 to 1.13 x 10-3 cm2 V−1 s−1 upon deprotection of the NH functional groups.

Thermoplastic cellulose acetate oleate films with high barrier properties and ductile behaviour

Cellulose acetate oleate ester has been synthesized from oleic acid and cellulose acetate by using a mixed anhydride system based on a mixture of trifluoroacetic acid and trifluoroacetic acid anhydride with chloroform as a co-solvent. In absence of oleic acid, cellulose acetate was deacetylated in this mixed anhydride system. However, when oleic acid was added, hydrolysis was prevented and oleate groups bound to the cellulose acetate chain were detected as confirmed by Nuclear Magnetic Resonance spectroscopy. Compared to cellulose acetate, cellulose acetate oleate ester showed a mechanical behavior closer to ductile materials and a lower glass transition temperature, indicating that oleate groups can act as an internal plasticizer of the polymer chains. The contact angle values increased from 61° for cellulose acetate to 106° for cellulose acetate oleate, which presented a low surface energy value (16 mJ/m2) with no contribution of the polar component. Furthermore, the esterified cellulose acetate oleate material showed a decrease in the water vapor transmission rates by ∼76% compared to those of pure cellulose acetate films. The oxygen permeability was also decreased by ∼90% after grafting the oleate group onto the cellulose acetate repetitive units because of the creation of a densely packed matrix, confirmed by SEM analysis. This behaviour was attributed to the oleate chain length, which creates a chemical interaction between water and oxygen molecules with free hydroxyl and oleate groups of cellulose acetate oleate.

Efficient Direct Halogenation of Unsymmetrical N-Benzyl- and N-Phenylureas with Trihaloisocyanuric Acids

A simple and efficient methodology for the direct halogenation of N-phenylureas was developed using trihaloisocyanuric acids in acetonitrile at room temperature. This protocol proved to be effective for the construction of N-phenylureas with different patterns of substitution. Additionally, less reactive N-benzylureas were halogenated in the presence of a mixture of trifluoroacetic acid and acetonitrile at room temperature.

Simple fabrication of cellulose nanofibers via electrospinning of dissolving pulp and tunicate

This work demonstrates a simple fabrication of cellulose nanofibers by direct electrospinning of dissolved cellulose solutions. The hard- and softwood pulps and the outer mantles of tunicate were dissolved in a mixture of trifluoroacetic acid and dichloroethane by stirring and ultrasonication to give highly viscoelastic, clear solutions. These solutions were electrospun to form continuous nanofibers made of unsubstituted cellulose, which were confirmed by scanning electron microscopy (SEM) and IR spectroscopy. Statistical analysis of the SEM images of the nanofibers suggested that there are positive correlations between diameters of the nanofibers and concentration of the cellulose solution. The mean diameters of the nanofibers obtained from softwood pulp (DP of cellulose ≈ 1200) solutions were larger than those from hardwood pulp (DP of cellulose ≈ 500) at the same concentrations. This indicates that the DP of cellulose is one of the important parameters to control the diameters of the electrospun cellulose nanofibers.

Highly Conductive Semitransparent Graphene Circuits Screen‐Printed from Water‐Based Graphene Oxide Ink

The use of graphene materials as conductive inks for flexible and transparent electronics is promising, but challenged by the need for stabilizers, specialized organic solvents, and/or high temperature annealing, severely limiting performance or compatibility with substrates and printing techniques. Here, the development of a scalable water‐based graphene oxide ink is reported that can be screen‐printed on flexible plastic substrates and subsequently reduced using a 1:1 mixture of trifluoroacetic acid and hydroiodic acid, thereby creating an electric circuit. The reduced prints exhibit low sheet resistance of 327 Ω sq−1 for thin semitransparent layers with 37% transmittance. This methodology with postprinting chemical reduction outperforms high temperature annealing, thereby eliminating the need for such a step, which is incompatible with flexible plastic substrates. The strategy relies on low cost, industrially compatible chemicals and can be scaled up for low cost manufacture of roll‐to‐roll printed electronics.

Development of HPLC-Fluorescence Method for the Determination of Ivermectin Residues in Commercial Milk

Objective: A study aimed at the validation of a high performance liquid chromatography (HPLC) method for detection and quantification of ivermectin (IVM) in commercial milk. Method: After a liquid-liquid extraction with acetonitrile and hexane, a derivatization with a mixture of trifluoroacetic acid and trifluoroacetic anhydride was performed. Chromatography conditions were a C8 column, a fluorescent detector (Ex. 360; Em. 470 nm), a run time of 25 minutes and an isocratic elution at 30°C. The methodology was validated in terms of selectivity, linearity, precision, recovery, limit of quantification (LoQ), limit of detection (LoD), and robustness. Results: The method showed an adequate linearity and selectivity (r2=0.999) with an elution time of 13.909 minutes. However, the repeatability and intermediate precision showed RSD values above of 20% and recovery between 37 to 79%. The calculated LoD and LoQ were 2.50 and 5.00 ng/ml respectively. Robustness showed a significant variation on the analytical method with small changes in heating time during the derivatization and flow rate of chromatography system. Conclusion: The HPLC-fluorescence method showed results partially satisfactory and could be the used for IVM detection in commercial milk samples.

Wettability Improvement of Poly (Butylene Terephthalate) Nanofibrous Mats Prepared via Electrospinning by Blending With Regenerated Silk Fibroin

Poly (butylene terephthalate) (PBT)/regenerated silk fibroin (RSF) blend electrospun nanofibrous mats were manufactured to combine the excellent mechanical behavior of PBT with the extraordinary hydrophilic property of RSF. A 1:1 mixture of trifluoroacetic acid (TFA) and dichloromethane (DCM) was adopted as the solvents for PBT and RSF with 20% (w/v) PBT and 16 wt% RSF solutions being mixed in various proportions for electrospinning. The morphology, crystallization, Fourier transform infrared (FTIR) spectra, surface roughness, contact angle, and wetting time of the electrospun blended materials were studied. When the weight ratio of RSF was larger than 50%, a water drop on the surface of the electrospun mat was completely permeated within 300 s or less. Besides the chemical influence of the amino and carboxy groups in RSF, the physical characteristics of the RSF in the blend electrospun mats, such as random coil structure, lower crystallinity, rougher surface than PBT, etc., were a partial reason for the improvement of wettability. The blend nanofibrous mats may be especially applicable in biomedical fields.

Degradation pathways study of the natriuretic and β-adrenoceptor antagonist tienoxolol using liquid chromatography–electrospray ionization multistage mass spectrometry

Tienoxolol is a pharmacologically active molecule designed with the functional groups ketothiophene, alkyl benzoate and arylpropanolamine so as to combine a diuretic and a β-adrenoreceptor antagonist into a single molecule. Its degradation products generated in several stress media have been determined by high-pressure liquid chromatography (HPLC) coupled to a hybrid mass spectrometer with a triple quadrupole-linear trap. A Polaris® column with a C18-A stationary phase and a linear gradient mobile phase composed of a mixture of trifluoroacetic acid 1% (v/v) and acetonitrile allowed for optimal separation. Structural elucidation of the degradation products has been based on MS/MS techniques, by comparing their fragmentation patterns to the precursor's data. Up to seven degradation products of the active ingredient, resulting from hydrolysis, oxidation, dehydration and transamidation have been identified, covering a range of possible degradation pathways for derivatives with such functional groups. Kinetics have been studied to assess the molecule's shelf life and to identify the most important degradation factor.

Chromatographic resolution and isotherm determination of (R,S)‐mandelic acid on Chiralcel‐OD column

Resolution of racemic mandelic acid ((R,S)-MA) and numerical determination of binary competitive isotherm of (R,S)-MA on Chiralcel-OD column have been investigated in this study. The effects of the alcohol modifier and acidic additive in the mobile phase on the retention and enantioseparation of (R,S)-MA were studied at first. The inverse method was then used to determine the competitive isotherm parameters of (R,S)-MA by minimizing the sum of square deviations of the model predictions from the measured elution profiles. The results indicate that the mobile phase with 85% hexane/15% isopropanol/0.3% trifluoroacetic acid mixture gives the best resolution of (R,S)-MA and competitive-modified Langmuir isotherm provides the more accurate sorption mechanism of (R,S)-MA on the cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phase.

Assessment of parameters influencing fiber characteristics of chitosan nanofiber membrane to optimize fiber mat production

AbstractElectrospun chitosan nanofiber mats have been obtained using chitosan solutions in a mixture of trifluoroacetic acid and dichloromethane. The relationship between processing parameters (solvent composition, polymer concentration in the solution, feeding rate, applied voltage, traveling distance between the needle, and the collector) and fiber morphology was studied. Taguchi's methodology was followed to determine which parameters have the strongest influence on mean fiber diameter and fiber homogeneity. Chitosan nanofibers obtained with this procedure were water soluble due to the protonation of amine side groups but were successfully neutralized by immersion in calcium carbonate solutions. It was established that fiber diameter is mainly determined by the solution concentration and the distance from the needle to the collector while other parameters have less influence. The set of parameters that produce the thinnest fibers were a concentration of 8 wt%, a TFA/DCM ratio of 80:20 (v/v%), a voltage of 30 kV, a flow rate of 6.0 mL/h, a gap distance of 10 cm, using a needle diameter of 0.5, allowing to produce randomly oriented mats with a mean fiber thickness of 66 nm. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers