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Picric acid (PA) blast is a very dangerous for public protection and health. In current times, the design of cheap and competent sensing materials for sensitive and exacting detection of PA is very appealing fields of research due to the continuous increase in terrorist activities. This paper focused on the study of gel-emulsion property and also the picric acid sensing capability of some synthesized nicotinic acid based amphiphiles. All the amphiphiles have capability to form gel-emulsion in various solvent at room temperature by shaking hand.
Rheological study demonstrated that the network structures of all the gel emulsions are mechanically stable and good tolerances to external forces. The presence of network morphology in emulsion state is confirmed by Optical microscopy study.
SEM observation demonstrated that the morphology depends on water: solvent composition. XRD study established the patterns of the morpholgy are different in emulsion state. Spctrophotometrically study established that all the amphiphiles have capability of entrap bio-molecule and anti cancer drug molecule but only one amphiphile has release capacity.
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UV-Vis and fluorescence study shows that all the synthesized compounds have capability to detect the picric acid. Not only had these method but solution and contact mode detection study also confirmed that all amphiphiles have good sensitivity towards PA. 1HNMR and theoretical study reveal that the charge transfer interection was takes place between the electron rich pyridine moiety of synthesized amphiphile and the electron deficient –NO2 group of PA.
Due to availability of numerous types of surfactants from either natural or synthetic sources, they have applicability in many fields: detergents, wetting agents, foaming agents, and dispersants.
They have also been extensively used as emulsifiers, in many fields: foods, cosmetics, painting, pharmaceutics, etc. This matter has been long investigated and a very significant amount of studies can be found in the literature, either as theoretical work or applications in specific fields. For emulsion, the hydrophobic part of the amphiphile place in oil and hydrophilic part in water. In emulsion state on one side of the interface the interaction between the hydrophilic part of the amphiphile and water takes place and in another side between the hydrophobic part and the oil.
The occurrence of surfactant at the interface increases the interactions between oil and water molecules, which tend to reduce the tension across the interface. It seems obvious that by increasing the concentration of surfactant, the interfacial tension decreases. Bhattacharya et al. have reported that N-lauroyl-L-alanine and related amphiphilic molecules act as phase-selective gelators of oil/water mixtures. The gelation of hydrocarbon solvents has been attributed to intermolecular amide hydrogen bonding between amphiphilic molecules. The phase-selective gelation was achieved only by heating and cooling cycles of the mixture.
In fact, most of these organic gelators need heating and cooling cycles to form gels. However, it is well-known that the heating process is a major disadvantage for many industrial applications of organogelators. Amphiphilic molecules can also form two-dimensional network structures at the air/water interface or at the interface of two immiscible liquids. Because of their ability to stabilize interfaces, surfactants were quietly used as emulsifiers, dispersing agents, and also as foam-forming additives. It has been observed that some amphiphilic molecules strongly stabilize emulsions to form viscous gel-like materials in organic solvents. Under certain conditions, the highly concentrated emulsions are formed in either water-rich or oil-rich regions. The concentrated emulsions are called high-internal-phase-ratio (HIPR) emulsions.
The HIPR emulsions contain more than 70% (v/v) of the internal phase. Because of the high viscosity and translucence characteristics, these are often called gel emulsions. The gel emulsions have applicability in cosmetics, aviation fuels, and emulsion explosives. Fang et al. reported that polymerizablecholesteryl derivative have capability to formation of gel emulsion and its utilization in the template preparation. Hanabusa and his co-workers reported that the gelator N-3-hydroxybutylcarbonyl-L-isoleucyl-aminooctadecane was synthesized and used to prepare gel-emulsions.
Since in synthesized compound pyridine moiety is present which acts as a electron donor center so the capability of sensing towards nitro-aromatic groups were examined. The explosive nature of picric acid is stronger than TNT and it was used as an explosive until World War I.2−5PA has great utility indye industries, chemical laboratories and pharmaceuticals.6,7In spite of that PA also shows the health effects: it is very harmful to skin/eye and can cause potential damage of the organs involved in therespiratory system.8−10 PA is highly soluble in water so during explosion it can easily pollutethe soil and ground water. Therefore design of the amphiphile which can detect the very low concentration of the picric acid is a very appealing field of research due to prevent terrorist attack as well as environmental pollution.
Herein, the gelation property of three sodium 2-decylpyridylcarboxylates having two different amino acids on the head group (abbreviated as SDPC, SDPCG and SDPCA respectively) has been investigated. Therefore, the aim of this work is to investigate the effect of amide hydrogen bonding interactions and chirality on the gel-emulsion properties. The gelation properties of the surfactants have been compared. Rheological measurement was studied to scrutinize the effect of amide hydrogen bonding interactions on the mechanical strength, elasticity and tolerance of outer forces of the gel-emulsions.
The actual morphology in the gel-emulsions state were observed by optical (normal and fluorescence) and field emission scanning electron microscopy techniques. In order to examine the applicability of the gel-emulsion in pharmaceutical industry drug releasing capacity were performed using UV-Vis spectrophotometer. In spite of that the sensing capability of synthesized amphiphiles towards picric acid was also carried out in solution and contact mode detection.
In presence of 50 μl water all the synthesized amphiphilesarecapable to form gel-emulsion in a variety of organic solvent and mineral oils. They form gel-emulsion only by shaking hand and no heating-colling cycle not required. Gel-formation is verified by turn upside down the test tube and no down ward flow is occurred. The minimum gelation concentration (MGC) and gelation number (Ngel) of the amphiphiles are tabulated 1. This two value confirmed the amphiphiles have good gelation ability [44]. The gelation ability and hence the Minimum Gelation Concentration (MGC) values in most of the organic solvents increases in the order SDPCA< SDPCG < SDPC which indicates that SDPCA is the best gelator among all the amphiphiles. The result indicates that amide hydrogen bonding and also the hydrophobicity of the head group modulate the gelation ability of the amphiphiles.
To get the information about the thermal stability and spontaneity of the gelation processgel to sol transition temperature (Tgel) can be investigated by inverted-tube method. All the Tgel values of the three amphiphile are listed in table 2. Table shows that the thermal stability of the amphiphiles are decreases in the order SDPCA> SDPCG > SDPC.All the thermodynamic parameter was obtained by ploting1/T (K) with ln [gelator], given in SI. The equations which give the value of ∆H,∆S, ∆Gare also given in SI.The ∆H gel →sol and ∆S gel →sol values are highly negative which indicate that the enthalpic contribution compensates for the undesirable entropic change in the gelation.
The negative values of ∆H gel →sol suggest that intermolecular hydrogen bonds are formed when the gel is built up in different solvents. Hanabusa et al. in 1996 have claimed the large negative value of ∆H gel →sol and ∆S gel →sol is due to the formation of two intramolecular hydrogen bonds of alkylamides derived from trans-1,2-diaminocyclohexane [51]. Also the very high negative ∆G gel →sol values indicate the spontaneity of the gelation process. The lowest ∆G gel →sol and ∆H gel →sol values of SDPCG suggest gelation process and hence hydrogen bonding interaction among the amphiphilic molecules is most favorable in case of SDPCG.
The measure of mechanical strength of a gel-emulsion using rheology study is of importance for applicative purposes.Shear stress and a frequency creep measurements were performed to give the idea about the stability of 3D network structure and tolerences of the outer forces.To look at the effect of amide hydrogen bonding and the hydrophobicity of the head group upon the mechanical properties of the gel-emulsions at 298K, G' (storage modulus) and G'' (loss modulus) of the gel emulsions were measured as a function of shear stress.
The results of stress sweep experiments are shown in Figure 6.1.7. From the Figure, it can be seen that above a critical stress value referred to as yield stress (σy), both G’andG''abruptly fall to a very low value;which proposed partial dissolution of the 3D network structure.The yield stress increases accordingly to the following way SDPC < SDPCG < SDPCA, suggesting that both the stability of the gel network and the elastic property of the gel-emulsions are well depended upon the amide hydrogen bonding as well as the hydrphobicity of the head group.
Frequency sweep is conducted to ascertain the tolerance capability of gel-emulsion to outer forces.The results gained from frequency sweep measurements are exposed in Figure 6.1.8.G' remains larger than G'', and there is no crossover between the storage modulus and the loss modulus within a range of frequency 0.01-50 Hz (Figure 6.1.8 show the frequency plot in the range of frequency from 0.01 to 10 Hz for clarity of the plot).It is observed that in every case, both G' and G'' are independent of frequency which suggest that the gel-emulsions under study show good tolerance to external forces at frequency range examined.
In order to determine the gel liquefying temperature, temperature sweep experiment was carried out for all the amphiphiles in gel-emulsion phase.The gel liquefying temperature was examined by evaluating the G' and G'' as a role of temperature at a frequency of 1 Hz.The gel liquefying temperature is that temperature at which G' = G'which means the transform from a solid-like state to a viscoelastic liquid-like state. In temperature sweep experiment for the gel-emulsions of the amphiphiles, no intersection point at which G' = G' was observed which indicate that there is no gel-sol transition for all the gel-emulsions (Figure 6.1.9). Also on heating, the gel-emulsion persists up to 348 K (with G'> G') for all the gel emulsions. In temperature sweep experiment for all the amphiphilesG' and G'' decreases very slowly similar to that of stress sweep experiment.
In order to discern the nature of the microstructures that may be present in such gel-emulsions with such varying gelation capacity, the gel-emulsion samples were examined by optical and FESEM techniques.
The actual morphology of the gel-emulsions wasrecognized by optical microscopy method. Figure 6.1.10 shows the network structures of gel in solutions of all the amphiphiles in different solvents. The networks are connected through different types of polygons such as spherical, square, pentagonal, hexagonal etc. with meanbreadth of the compartments ~ 20-200 µm. Similar types of network structures were found in other solvents also. The similar types of morphology were also showed under fluorescence microscope using Rhodamin B and Fluoroscein as fluorescence probe molecules.
The systems of amphiphiles/water/solvent having different solvent content were prepared for investigation of the microstructures of the gel-emulsions. The compositions of three typical gel-emulsions were water:solvent = 1:50 and 1:80 respectively at the amphiphile concentration 1.9×10-3 M for SDPC in mesitylene, water: solvent = 1:165 and 1:310 at the amphiphile concentration 5.32 ×10-4 M for SDPCG in cyclohexane and water: solvent = 1:130 and 1:240 at the amphiphile concentration 6.46 ×10-4 M for SDPCA in xylene respectively. At the compositions water:mesitylene = 1:50 and 1:80 of SDPC, emulsified as well as fibrous network structures were observed in FESEM pictures.
The thicknesses of the fibers are 0.5-2 µm and are of micron in length. On the other hand, the FESEM micrographs of SDPCG demonstrate presence of emulsified structure, fibrils and vesicles at both the composition of water:solvent. Closer observation of the vesicles at higher solvent content (1:310) revealed some of them are porous in nature. The 3-D network and sheet like morphology was obtained in the FESEM pictures of SDPCA xerogel at composition water:solvent = 1:130. But the freeze dried gel-emulsion of SDPCA at higher solvent content (1:240) showed emulsion as well as fibrous textures.
The XRD pattern of the air-dried gel cast film of the amphiphiles showed periodical diffraction peaks, demonstrating an ordered layer structure.The XRD data were records in the range 2θ = 1º to 10º. The peak positions (2θ) values, corresponding planes and inter-planar distances (d) have been summarized in table 4. The gel-emulsions formed by SDPC and SDPCA showed predominantly single intense peak (100%) at 36.61 Å and 36.18 Å respectively, which confirmed the presence of single periodicity in the morphologyformed by the amphiphile. On the other hand, the cast film of SDPCG shows two different types of repeated peaks corresponding to the two sets of planes.
Bhattacharya et al. have recognized these two sets of planes corresponding to (001) plane of the two morphologies I (cisoid) and II (transoid) [53]. It is also interesting to observe that the interlayer distances of the lamellar structure, corresponding to first ‘d’ value of SDPC and SDPCG is higher than the twice of the length of the hydrophobic chain length (lc) of the corresponding amphiphiles suggesting that in the lamellar structure the alkyl chains of SDPC and SDPCG are oriented in non interdigited pattern. However, the‘d’ spacing of the lamellar structure of SDPCA is slightly less than twice of the lcvalue of the hydrophobic chain which is indicative of formation of slightly interdigited bilayer structure.
The capability of entrap and release of biomolecule and anti cancer drug molecule by gel was studied spectrophotometrically.It has been reported in the literature that the presence of a biodegradable amide linkage is susceptible to enzymatic hydrolysis [61,62] in the amino acid based surfactants.At the cell membrane, the soft matter entrapped with the drugs is expected to be taken up by the cells and at acidic pH, the gel will be disrupted, leading to the release of drug. Therefore, the release of encapsulated vitamin B12 within gel-emulsion network was studied at different pHs.
For kinetic study, the gel-emulsions containing vitamin B12 were prepared and the gel-emulsion was poured in a quartz cuvette of path length 1 mm. 20 µl of respective buffer solution of different pHs was added carefully on top of the gel-emulsions into the cuvette with the help of a micropipette and kinetic measurement was started immediately using spectro-photometry. Six different pHs from 2 to 7.5 were tried and absorbance of the solutions was measured at 550 nm. The decrease of absorption intensities and hence release rates of entrapped material from gel-emulsion of SDPCA is very fast in high acidic pH (pH = 2.0); whereas the absorption intensity remains unchanged even after several weeks and so there is no release in other pHs (pH = 2.5, 3.0, 3.5, 5.5 and 7.5.
Therefore, it is clear that the pKa of the SDPCA in gel-emulsion lies in the range of pH 2.0- 2.5. To find out the actual pKa, the effect of pH was checked in the pH range 2.0-2.5 with an increment of pH 0.1. It was found that the gel network remained intact at pH 2.2 but was broken up at pH = 2.1 for SDPCA gel-emulsion. Hence the pKa value of the gel-emulsion lies in between 2.1-2.2. The plot showing the release rates with time at pH 2 is shown in Figure 6.1.17.The amount released at any time at pH 2.0 was calculated as fraction of the total amount of vitamin B12 dissolved in the gel phase. Assuming that the release of the drug from the gel-emulsion over time follows first-order kinetics and the data sets were fitted to eqn. 6.1.4
f = 1 – e-kt (6.1.4)
where f is the fraction of vitamin B12 at time t in the solution and k is the first-order rate-constant. An excellent good fit can be observed for the amphiphile SDPCA. The calculated value of k and half-time (t½) of the process for SDPCA are 4.58 × 10-3 sec-1 and 151.31 sec respectively. But the gel networks remain intact for both SDPC and SDPCG after addition of requisite amount of buffer solutions of respective pHs. This is due to the fact that when gelation took place at room temperature by the non-covalent interaction of the gelator molecules there may be a possibility of gel dissolution/dilution due to the addition of an aqueous layer on top of the gel-emulsion during the release of the entrapped molecules. Hence, the gel-emulsions of the three amphiphiles exhibit high hydrolytic stability at physiological pH.
UV-Vis Titration Experiment:All three synthesized compounds give yellowish color in presence of picric acid in chloroform medium. This observation primarily suggests that the compound must be act as a sensor. Therefore UV-Vis Titration was carried out. For uv-vis titration experiment the dilute solution of all synthesized compound was titrated by picric acid which was shown in figure. When the picric acid was added to synthesize compound the color of the solution suddenly change from colorless to yellow which suggest formation of either picrate anion or charge transfer complex. During this titration a new low energy band was appear at 400 nm. This new intense band was due to the charge –transfer band.
For fluoremetrictitration a fixed concentration of all synthesized compound was titrated by picric acid. During this titration the intensity of the entire compound was quenched but no any other emission was observed. To give the idea about quenching rate the Stern−Volmer constant was calculated.
The Stern−Volmer equation is expressed as
I0/I = 1 + KSV[Q]
where I0 and I are the initial and final fluorescence intensitiesafter addition of analyte and [Q] is the analyte concentration.
Stern-Volmer constant (6.6 × 102 M−1for SDPC, 5.3× 101 M−1for SDPCG, 7.8 × 102 M-1for SDPCA) suggested that all synthesized compound have high quenching efficiency towards picric acid.
Response toward Other NAC’s. To check the sensitivity of synthesized compound 4 NT, 2,6-DNT, 2,4-DNT, 1,2-DNT were used along with PA. Except 4NT all other compounds did not shown any quenching efficiency. The quenching efficiency of 4 NT is 20%.
Many researchers shown that sensor have capability to change the color of the PA solution. Thus solution mode naked eye detection of PA is more important for real time application. Therefore, for visual observation10-2 M PA in CHCl3was gradually added to1.5 mL of 10-4 M solution of synthesized amphiphile in CHCl3.The colorless solution of the sensor changed to yellow color upon addition of the PA in sensor solution and under UV light the fluorescence quenched from blue to nonfluorescent.
For contact mode detection, at first Whatman 42 filter paper was cut into 2 cm2 pieces and then dipped into the concentrated solution of the synthesized amphiphile. Thereafter, the pieces are dried under reduced pressure. Then different concentrations of PA solution were prepared and 10 μL of every solution was drop-casted on each fresh test strip. Under naked eye the color of the strips changes from white to yellow depending upon the concentration of the PA. But under UV light the dark black spot were observed and the intensity of darkness decreases upon dilution of the PA.
The color change of the solution, new low energy band appearance in UV study and the fluorescence quenching observation can be explained by the donor−acceptor electron-transfermechanism between the sensors and the picrate anion. For clarification of the electron transfer mechanisim the HOMO−LUMO energy of the sensor, picric acid and picrate anion was calculated by gas phase DFT analysis. This analysis suggests that LUMO of picrate anion reside below the HOMO energy level of all three sensor, therefore electron transfer is possible from sensors to picrate anion. However the energy gap between LUMO of sensors and HUMO of picric acid is very high, therefore the probability of electron transferfrom sensors to PA is very low. Figure x shows the MO diagram of sensors, PA and picrate anion. The MO diagram confirmed that electron transfer from the HOMO of sensors to LUMO of picrate in theground state is possible leading to static quenching of thefluophore.
The1H NMR titration experiment was carried out to scrutinize the actual binding position of PA. In this study when PA was added to sensor in CDCl3 the entire proton shifted to more deshieldedregion. If sensor withdraws the electron density from the picrate anion then upflied shift of proton is observed. This type of observation is only possible if the electron density was transferred from picrate anion to sensor. The spectral data showed that when PA was added to the sensor solution the proton of the pyridine ring was shifted to the deshielding region. This significant change in NMR spectral data suggested that pyridine moiety acts as a doner site. In CDCl3picric acid does not convert to picrate ion, therefore the electron density was transferred from electron rich pyridine moiety of the sensor to picric acid.
All the amphiphiles are good gelators of a variety of organic solventsin presence of a 50l of water. The amphiphiles containing amide linkages are better gelators than the amphiphiles SDPC due to hydrogen bonding interaction among the amide linkages. The high negetives of ∆Ggel→solsuggest gel to sol transformation is spontaneous in nature. Rheology measurements suggest that gel-emulsions formed by amphiphiles are thermally and mechanically stable and are independent on external forces. Optical microscopy images exhibit formation of 3D network structures in gel-emulsion state formed by the amphiphiles. FESEM images established presence of both a network and ribbon-like fibrous morphology for all amphiphiles forming 3-D networks which are formed through hydrogen bonding interaction between the amide linkages and van der Walls interactions of the hydrocarbon chains of the adjacent molecules. In spite of these two morphologies vesicular aggregates was also observed for SDPCG.
XRD studies indicate formation of non interdigited lamellar structures of the gel-emulsions in which the hydrocarbon layers are oriented in cisoid pattern. The release and entrapment of a bio-molecule in the gel-emulsions show hydrolytic stability at physicologicalpH. However, the release rate of bio-molecule from the gel-emulsions except SDPC and SDPCG at pH = 2 is very fast due to gel dissolution. The sensing study established that the amphiphiles have good sensitivity and selectivity towards picric acid over a number of other nitroaromatic explosives.
Gelation Properties of Nicotinic Acid-Based Amphiphiles for Picric Acid Detection. (2024, Feb 16). Retrieved from https://studymoose.com/document/gelation-properties-of-nicotinic-acid-based-amphiphiles-for-picric-acid-detection
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