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Title: Effect of Counterions on Surface Properties of Ionic Surfactants and Their Optical and Catalytic Applications
Authors: Rehman, Attiq-ur-
Keywords: Physical Sciences
Issue Date: 2022
Publisher: COMSATS University, Islamabad
Abstract: Effects of Counterions on Surface Properties of Ionic Surfactants and their Optical and Catalytic Applications This work explores the ionic surfactants-based systems with emphasis on the role of “counterions” in inducing customized responses in materials while understanding the variations in surface responses. Furthermore, an effort was made to exploit the micelle formation ability of surfactants and hence their modified forms, as means to the synthesis of different solid materials. Well-known ionic amphiphiles cetyltrimethylammonium bromide (CTAB) and oleic acid (OA), with established roles in the syntheses of different materials were chosen as model surfactants. Transition metal ions of Cu2+, Ni2+, Fe3+, Zn2+ and lanthanide ions i.e., Gd3+, Eu3+, Tb3+, Ce3 were evaluated as part of the counterions in CTAB and OA respectively. Pre-established methods were adopted for the modifications of bromide counter-anion of CTAB with transition metals chlorides of Cu2+, Ni2+, Fe3+, Zn2+ which were subsequently called as CTAC, CTAN, CTAF, CTAZ and rare earth chloride salts of Gd3+, Eu3+, Tb3+, Ce3+ ions and were named CTAG, CTAE, CTAT and CTACe respectively. OA was modified using chlorides of Gd3+, Eu3+, Tb3+, Ce3+ and Mn2+ and was named OAG, OAE, OAT, OAC and OAM, respectively. In this way, the micelle properties were tuned to get the desired nanomaterials. The characterizations shown that the modified micelles provide high uniformity, dispersity, and doping of the required metal ions in the host matrix. Surface properties i.e., critical micelle concentration (CMC), surface tension (g), surface excess (G), adsorption efficiency (pC20), surface pressure (p), surface area (Amin) have been studied using conductometric method and tensiometery. The measured CMC values were later used to ensure micellization and thus formation of micelles as soft templates for the synthesis of catalytic and optically active materials. Soft templates evolved during last many years and used as a method of choice for the syntheses of hierarchical nanomaterials. Surfactants based soft templates (SSTs) are not uncommon and are frequently used by chemists working on nanomaterials. SSTs provide control over size, shape and structures which is a requirement for controlling properties of nanomaterials. Due to the well-known role of quaternary ammonium surfactants in the synthesis of silica-based nanostructures, CTAB with modified counterions i.e., CTAC, CTAN, CTAF and CTAZ and their combinations were used for the synthesis of catalytically active materials such as NiO/SiO2, CuO/SiO2, ZnO/SiO2 or a bimetal oxide such as CuO-NiO/SiO2. The silica supported oxides of transition metals were checked for the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). A very minute amount of the catalyst showed best catalytic activities. The catalytic activities exhibited by these materials were compared to the reported values of the catalysts in literature. Similarly, CTAG, CTAE, and CTACe were used to synthesize optically active rare earth oxide and fluorides materials by sol-gel / hydrothermal methods. All the ions were doped successfully and retained their corresponding luminescence peaks. The use of SiO2 as matrix have many advantages such thermal and mechanical stability, structural variety and size control obtained due to the presence of quaternary ammonium based cationic surfactants and possibility of customization of properties due to the use of various types of metal ions with required properties. Due to favorable surface and internal features of the silica matrix in addition to its stability, dispersing a required catalytic (NiO/SiO2, CuO/SiO2, ZnO/SiO2, CuO-NiO/SiO2) or optical material in the SiO2 matrix provide a cost-effective solution due to minimization of the use of the active material while keeping the efficiency high or comparable. Similarly, small amount of the luminescent material is enough to glow the bulk silica matrix. OAG, OAT, OAC, OAE, and OAM based modified oleate micelles offered more control over size in addition to the response induced in the materials. Eu3+, Tb3+ and Mn2+ doped NaGdF4 matrix was synthesized by modifying oleate micelles. To the best of our knowledge, no reports have been found on the charge transfer mechanism of Ce3+ ions with Eu3+ ions along with other lanthanides /Mn2+ ions using MOM organometals method in down conversion materials. Soft templating provides the easiest route to accommodate the Mn2+ ions in the host fluoride matrix. The analysis and characterizations were studied by conductivity meter, surface tensiometer, x-ray diffraction (XRD), field emission electron microscopy (FESEM), transmission electron microscope (TEM), electron dispersive x-ray (EDX), x-ray photoelectron spectroscopy (XPS), and UV-Visible spectroscopy etc.
Gov't Doc #: 26971
Appears in Collections:PhD Thesis of All Public / Private Sector Universities / DAIs.

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