Md. Mufazzal Hossain

Phase transition in adsorbed monolayers of 2-hydroxyethyl laurate at the air-water interface

Abstract Kinetics of adsorption of a sparingly water soluble amphiphile, 2-hydroxyethyl laurate (2-HEL), at the air-water interface is followed by the measurement of surface pressure ( π ) with time ( t ). Simultaneous change in the surface morphology of the adsorbed monolayers is investigated with Brewster angle microscopy (BAM). A cusp point in each of the π – t curves indicates the start of a phase transition. Highly ordered condensed phase domains having stripes in their structures are observed to grow during the phase transition. One end of the separate lines of stripes gather to a point on the circular peripheral line of the domain like a vertex of the boojum. However, another end of the separate lines disappear at the peripheral line keeping the stripe width almost constant. After the completion of the phase transition, surface pressure starts to increase again and reaches a final equilibrium value after a long time. Spread monolayers of the same amphiphile can be obtained by the usual solvent spreading technique and compared with the adsorbed monolayers. The critical surface pressures necessary to cause the phase transition are almost the same for both adsorbed and spread monolayers. Although the shapes of the domains are circular for both cases, the size of those in the spread monolayers is considerably smaller exhibiting no internal stripes than those in adsorbed monolayers.

Orthorhombic structure of cadmium behenate monolayers on the water surface of a Langmuir trough detected by polarization modulation infrared spectroscopy

Abstract Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was applied to investigate the structure of behenic acid monolayers on the surface of aqueous Cd 2+ sub-phase in a Langmuir trough at the room temperature of 293 K. The PM-IRRAS spectra were recorded at representative points throughout 0.401∼0.100 nm 2 and the surface-pressure area isotherms were recorded during compression to these points. It is found that compressed cadmium behenate (CdB) monolayers possess an orthorhombic unit cell. On the other hand, single monolayers of CdB deposited on CaF 2 substrates at 20 mN m −1 possessed a triclinic parallel packing.

Temperature dependent dendritic domain shapes in Langmuir monolayers of tetradecanoyl N-ethanolamide at the air-water interface

The effect of temperature on the surface phase behavior of tetradecanoyl N-ethanolamide (NHEA-14) in Langmuir monolayers at the air-water interface has been investigated by film balance and Brewster angle microscopy (BAM). It has been observed that dendritic domains are formed in the coexistence region between liquid-expanded (LE) and liquid-condensed (LC) phases at different temperatures. At 10 and 15°C, the domains are four-armed dendrites having wide arms which have a tendency to be fractal while growing in size. At 20°C, five-armed dendritic domains are formed. At a temperature higher than 20°C, the domains are mainly six-armed dendrites having very narrow and sharp arms. The formation of dendritic domains should be due to the presence of interfacial hydrogen bonding among the head groups of the amphiphile. Increased dehydration of the head groups with an increase in the temperature should be responsible for the temperature dependency of the dendritic domain shapes in the monolayers of NHEA-14.

Effect of head groups on the phase transitions in Gibbs adsorption layers at the air–water interface

The adsorption kinetics and the surface phase behavior of four different amphiphiles, which are 2-hydroxyethyl laurate (2-HEL), dodecanoyl N-ethanolamide (NHEA-12), dodecanoyl N-methylethanolamide (NMEA-12) and tetradecanoyl N-methylethanolamide (NMEA-14), have been investigated at the air-water interface by film balance, surface tensiometer and Brewster angle microscopy (BAM). The former two amphiphiles show a first-order phase transition from a lower density liquid like phase to a higher density condensed phase in Gibbs adsorption layers. On the other hand, the latter two amphiphiles are unable to show such characteristics under any experimental conditions. The presence of a methyl group in the head group of NMEA-12 sterically hinders the molecules and resists the formation of any condensed phases. This steric hindrance is so high that even an increase in the chain length by two CH(2) groups in NMEA-14 does not allow the formation of condensed domains. Although, both 2-HEL and NHEA-12 are able to form the condensed phase, the domain morphology formed in these monolayers is different from each other. The domains of 2-HEL at lower temperatures are circular having a stripe texture, while those at higher temperatures show fingering patterns having uniform brightness. On the other hand, the domains of NHEA-12 are dendritic in shape. The presence of hydrogen bonding sites close to the interface should be responsible for the formation of such domains in NHEA-12.

Phase transition in Gibbs monolayers of mixed surfactants

Abstract Phase transitions in mixed monolayers of 2-hydroxyethyl laurate (2-HEL) and Na-salt of 3,6,9,12-tetra oxa octacosanoic acid (TOOCNa) formed by co-adsorption from their mixed bulk solutions have been studied. The presence of cusp points followed by plateau regions in the π-t curves, which is accompanied by two phase coexistent state indicates a first-order phase transition. The domains have circular shape with internal segments whereas those of pure 2-HEL at the same temperature are of fingering pattern with uniform brightness all over the domains. With increasing the fraction of TOOCNa, the domains show more and more expanded behavior which favor easy fusion of them.

Effect of Some Metal Ions on the Photocatalytic Oxidation of Remazol Black B in Aqueous Solution under UV Irradiation

Abstract This study investigates the advance oxidation process of common azo dye, Remazol Black B (RBB). Photocatalytic degradation of RBB was carried out using metal ion (Fe3+, Cu2+, Sn2+, Hg2+, and Co2+) as photocatalyst. Fe3+, Cu2+, and Sn2+ were used in varying all the necessary parameters in the photodegradation experiment while all five metal ions were used for the comparison of photodegradation efficiency of different metal ions in the optimal conditions that were obtained for the treatment of RBB using first three metal ions. Fe3+ ion was widely used to demonstrate the photodegradation pattern. The degradation efficiency was observed above 90% after 90 minutes irradiation of UV light (intensity=12.83 Wm-2). Different kinetic models have been used to interpret the results and found that the kinetics of the degradation follow pseudo first-order model. Photodegradadation was carried out by varying the concentration of metal ion, initial concentration of RBB, the pH of the medium and light sources using each of the three metal ions (Fe3+, Cu2+, and Sn2+). A tentative mechanism has been proposed to demonstrate this photocatalytic activity. More efficient degradation for all metal ions was observed at pH=1.0. Treatment performance of the investigated advanced oxidation systems was comparatively assessed in terms of bleaching and evaluated by spectrophotometric measurements.