K eywoRds
4. El cambio climático como ejemplo de securitización exitosa del medio ambiente La securitización del medio ambiente ha logrado poner de relieve determinados riesgos
4.2. Actores: ¿Quiénes tienen capacidad de securitizar un tema medioambiental?
18 3.1. Materials:
Human hair of Caucasian origin was obtained from Indian volunteers for conducting the study. The natural surfactants used for wetting, were extracted from reetha (Sapindus mukorossi), shikakai (Acacia concinna) and acacia (Acacia auriculiformis). Extraction of saponin from reetha (MW-966) was done according to methodology followed by Kommalapati and co-workers (Kommalapati et al., 1997, Kommalapati et al., 1998). While saponin extraction from shikakai (MW-1056) was done according to the methods of Tezuka et al., 2000 and that of acacia (MW-1765) was done according to the works of Garai and Mahato, 1997, Pal et al., 2009, respectively. The only exception was that the extraction of saponin from acacia was done by di-ethyl ether (sd fiNE-cHEM LiMiTEd, 99%) and methanol (MERCK Chemicals, 99%) mixture (5:1), instead of chloroform-methanol (80: 20) followed by Garai and Mahato, 1997. Certified analytical grade ethanol (Hong Yang Chemicals Corp., China, 99.9%), sodium borohydride (Acros Organics, 98+%) and silver nitrate (MERCK Chemicals) were used. Ultrapure water (Sartorius, Göttingen, Germany) of 18.20 MΩ cm resistivity, 72.16 mN/m surface tension, and 6.5-7.0 pH was used for all the experiments.
Figure-3.1: Fruits of (A) acacia [Acacia auriculiformis] (B) shikakai [Acacia concinna] (C) reetha [Sapindus mukorossi].
19 3.2. Hair Sample Preparation
These studies were conducted on Indian black virgin hair (both male and female), without any chemical treatment. Nevertheless, prior to being used for the experimental work, each strand of hair was rinsed thoroughly with water, followed by ethanol and finally with water again, for reducing the amount of unwanted deposits which accumulate over the cuticle over a period of time. Then after each wash, individual hair strand was dried for 3 minutes, using an ordinary hot air blower. This procedure was repeated before each use for the dual purpose of curbing cuticular deposits as well as to maintain the dryness of hair, since due to its porosity it has the ability to retain some moisture from previous washes.
3.3. Measurement of surface tension and calculation of CMC
The solution preparation was done moments before the actual measurement. The solutions of the desired concentration (0.001-1 mmol/L) were prepared from 5 mmol/L stock solution, using 50 mL volumetric flask. For measuring surface tension a surface tensiometer, (DataPhysics, Filderstadt, Germany, DCAT-11EC) was used. Platinum sheet is used for surface tension measurement in Wilhelmy plate technique.
Figure 3.2: Photograph of surface tensiometer.
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To avoid the adsorption of surfactant on the plate it was cleaned properly with ethyl alcohol and water and was also burned in alcohol flame to ensure a clean surface, after each measurement. During the experiment, the temperature was maintained constant at 25 ± 0.5 °C with the help of a circulator. The immersion depth of the platinum plate in the surfactant solution was maintained at 3 mm. The CMCs of reetha, shikakai and acacia were found to be 0.52, 0.5 and 0.53 mM/l respectively while that of TritonX-100 being 0.125 mM/l.
3.4. Measurement of Contact Angle
For measuring contact angle, contact angle meter, Data Physics, Germany, Contact Angle System (OCA-20) was used. A pump (Tarson Rockyvac 320) was employed to develop pressure (1 bar) for the purpose of nano-level surfactant solution dosing on a single strand of human hair.
Figure 3.3: Photograph of optical contact angle meter.
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To avoid surfactant solution retention by the hair strand (adsorption on the surface and absorption in pores of each strand), it was washed with water and methanol after each set of reading. Surfactants are compounds which act in both lowering the surface tension of fluids as well as improving the stability of the nanofluid. Since, nanoparticles are prone to sedimentation after a few hours, so ultra-sonication was carried out each time before contact angle measurement. The ultra-sonication provides surface charge to the particle, which in turn creates a repulsive force between the nanoparticles, thus preventing particle agglomeration. The experiments were carried out each time with freshly prepared sample solutions within a few hours after nanofluid preparation.
3.5. Synthesis of silver nanoparticle
In this study, silver nanoparticle was formed by reducing aqueous silver nitrate using a strong reducing agent, sodium borohydride (chemical synthesis) on one hand and acacia (green synthesis), as mild reducing agent on the other. In the chemical synthesis route, freshly prepared NaBH4 solution (100 mM) was added to 10 mL of solution containing shikakai or reetha (1 mM) and AgNO3 (1mM), under continuous stirring to reach a constant NaBH4 / AgNO3 molar ratio of 2:1. Although the colour of the sample instantly changes from a pale yellowish (due to presence of shikakai) to dark reddish-brown indicating the formation of Ag nanoparticle, the reaction is allowed to proceed further for 1 hour, under continuous stirring for complete reduction at room temperature (37 ± 3 °C). Similarly, the colourless solution of reetha turns dark reddish brown, when Ag nanoparticle is produced from AgNO3 on addition of NaBH4 in the surfactant medium containing AgNO3. While in the green synthesis route, AgNO3 (150 mM) was added drop-by-drop in 10mL of acacia (1mM) and reetha solution (1mM) separately, until the acacia / AgNO3 and reetha / AgNO3 molar ratio of 4:1 and 11:1 was attained respectively. These ratios were found to be optimum leaving no unreacted AgNO3 in the surfactant solutions. The colour of the sample turned from yellow-ochre (due to presence of acacia) to deep orange upon completion of nanoparticle synthesis. At room temperature (37 ± 3 °C), the complete reduction of AgNO3 by acacia surfactant solution was completed in 6 hours while for reetha the same reduction took nearly 90 hours. So, the chemical route of nanoparticle synthesis was preferred over the green synthesis for reetha, mainly due to the longer time needed for the completion of reaction, for the latter procedure.
It is interesting to note the colour change of reetha on addition of AgNO3 at different time intervals. The solution is colourless initially, but it turns to light pink after 48 hours and then
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to pale brownish colour after 90 hours and then it remains steady, indicating completion of reaction.
Figure 3.4: The colour change in reetha after addition of AgNO3 at (A) t=0 mins, (B) t=48 hours and (C) t=72 hours.
3.6. Characterization techniques for silver nanoparticles
For measuring absorbance of silver nanoparticle in surfactant solution, UV 3600 (Shimadzu) was employed. The spectra were recorded at room temperature (37 ± 3 °C) using one-centimetre quartz cuvettes. While the absorption spectra provided solid evidence of nanoparticle formation and their growth kinetics, the size and shape of the resultant particles were elucidated with the help of dynamic light scattering (DLS), Nano ZS (Malvern Instruments) and from the images obtained in SEM analysis, respectively. Samples for scanning electron microscopy (SEM) operated at 20 KV, was prepared by drop-coating the Ag nanoparticle solutions onto glass slides. Samples were dried and coated with platinum before SEM analysis.
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