Molecular Insights of Adsorption and Structure of Surfactants and Inorganic Ions at Fluid – Liquid Interfaces Afshin Asadzadeh Shahir B.Sc. Applied Chemistry, M.Sc. Physical Chemistry A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in 2017 School of Chemical Engineering Abstract Traditional approaches to studying fluid – liquid interfaces include the macroscopic measuring of interfacial properties such as surface tension and matching the collected data against adsorption models. This method is capable of producing valuable data about the thermodynamics of adsorption and has been widely used by the community to extract information about the adsorption of thousands of different surface-active molecules. Nonetheless, this methodology cannot produce any molecular-level information about the microscopic structure of adsorption layers and interfaces. As a result, the molecular origins of many interfacial phenomena remained unknown until the advent of surface-sensitive techniques such as computer simulation and non-linear spectroscopy. The new insights provided by these methods have challenged the traditional views about the origins of some interfacial phenomena and promise modification of classical theories which were developed to explain these phenomena. In general, this thesis aims to study the interfacial structure and adsorption of ionic surfactants, some surface-active alcohols as model nonionic surfactants including, n-pentanol, methyl isobutyl carbinol (MIBC) and n-hexanol and inorganic salts including LiCl, NaCl and CsCl at both microscopic and macroscopic levels. The employed methodology involves a combination of traditional adsorption modelling with some macroscopic measurements and sum frequency generation (SFG) spectroscopy, which is capable of distinguishing between bulk and interfacial molecules. Using SFG spectroscopy, the interfacial structure of adsorption layers and the composition of interfaces were investigated in detail and independently of the bulk solution. Specifically, the surface saturation of solutions of MIBC was identified directly using SFG spectroscopy and was then compared to equilibrium surface tension data to find that surface tension surprisingly continued to decrease even after full surface saturation. This is in contradiction to the traditional view that all surfactants adsorb at the outermost adsorption monolayer. Because of the limited surface area of the topmost adsorption monolayer, its full saturation is considered as the end of the adsorption process. This view is based on the notion of Gibbs dividing plane, which is usually taken as being equal to the physical adsorption monolayer in a surfactant solution. The post- saturation surface tension decrease, therefore, cannot be explained by the Gibbs convention. A different adsorption geometry, which considers the under-monolayer adsorption of alcohols, was proposed based on the Guggenheim extended interface model. In Chapter 5, a recent controversy about the applicability of the famous Gibbs adsorption isotherm to the analysis of surface tension data was addressed with regards to the new concept of under-monolayer adsorption. The results showed that the linearity of a surface tension plot is not necessarily indicative of a fully saturated i surface. The same adsorption geometry was also proposed for two other alcohols in Chapter 6. It was observed that adsorption geometry was not distinguishable by equilibrium surface tension plots but by dynamic characteristics of fluid – liquid interfaces. In other words, the under-monolayer adsorption significantly affected the rheological properties of the interface. This behaviour was also discussed as being responsible for the observed self-defoaming property of low-molecular-weight alcohols. In Chapter 7, immersion of the ionic headgroups of surfactants into the subsurface region was incorporated into some of popular adsorption models. The modified models could successfully explain why sodium dodecylsulphate (SDS) surface excess decreases at the oil – water interface compared to the air – water interface as well as with hydrophobicity of the non-aqueous phase. Finally, SFG spectroscopy was complemented with thin film pressure balance measurements in Chapter 8 to show that monovalent cations, in analogy to anions, can create an ion specific effect on the solution surface electric potential and interfacial water structure. The results also revealed the concentration-dependence of the water SFG response in the presence of salts. We explained these phenomena in terms of the polarizabilities of ions, which lead to the creation of an interfacial charge separation, i.e. ionic capacitance, as well as the kosmotropic-chaotropic effect of ions on water structure. From the findings of this thesis, adsorption geometry seems to be a major factor affecting the physicochemical characteristics of fluid – liquid interfaces and adsorption layers. However, it has often been undervalued by the majority of classical adsorption models while making simplifying assumptions. Now that the community has access to a variety of surface-sensitive techniques with molecular resolutions, the traditional approaches to studying interfaces should be reconsidered and complemented by the new advancements. This will open up new opportunities to shed light on the molecular origins of interfacial phenomena and resolve the existing controversies in the field, a few examples of which are covered in this thesis. ii Declaration by author This thesis is composed of my original work and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my research higher degree candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the policy and procedures of The University of Queensland, the thesis be made available for research and study in accordance with the Copyright Act 1968 unless a period of embargo has been approved by the Dean of the Graduate School. I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis. iii Publications during candidature • Peer-reviewed papers 1. Khoi T. Nguyen, Afshin A. Shahir, Anh V. Nguyen, Probing the Molecular Orientation of Methyl Isobutyl Carbinol at the Air–Water Interface, J. Surfactants Deterg., 2017, 20, 969 – 976. 2. Afshin A. Shahir, Dimitrinka Arabadzhieva, Hristina Petkova, Stoyan I. Karakashev, Anh V. Nguyen, Elena Mileva, Effect of Under-Monolayer Adsorption on Foamability, Rheological Characteristics and Dynamic Behaviour of Fluid Interfaces: Experimental Evidence for the Guggenheim Extended Interface Model, J. Phys. Chem. C, 2017, 121, 11472 – 11487. 3. Afshin A. Shahir, Anh V. Nguyen, Stoyan I. Karakashev, A Quantification of Immersion of the Adsorbed Ionic Surfactants at Fluid – Liquid Interfaces, Colloids Surf. A, 2016, 509, 279 – 292. 4. Afshin A. Shahir, Khoi T. Nguyen, Anh V. Nguyen, A Sum-Frequency Generation Spectroscopic Study of the Gibbs Analysis Paradox: Monolayer or Under-Monolayer Adsorption?, Phys. Chem. Chem. Phys., 2016, 18, 8794 – 8805. Publications included in this thesis • Afshin A. Shahir, Khoi T. Nguyen, Anh V. Nguyen, A Sum-Frequency Generation Spectroscopic Study of the Gibbs Analysis Paradox: Monolayer or Under-Monolayer Adsorption?, Phys. Chem. Chem. Phys., 2016, 18, 8794 – 8805, incorporated as Chapter 5. Contributor Statement of contribution Designed/performed experiments (75%) Afshin Asadzadeh Shahir Interpreted/analysed data (80%) Wrote/revised the paper (85%) Designed/performed experiments (15%) Khoi Tan Nguyen Interpreted/analysed data (15%) Wrote/revised the paper (10%) Designed/performed experiments (10%) Anh Van Nguyen Interpreted/analysed data (5%) Wrote/revised the paper (5%) iv • Afshin A. Shahir, Dimitrinka Arabadzhieva, Hristina Petkova, Stoyan I. Karakashev, Anh V. Nguyen, Elena Mileva, Effect of Under-Monolayer Adsorption on Foamability, Rheological Characteristics and Dynamic Behaviour of Fluid Interfaces: Experimental Evidence for the Guggenheim Extended Interface Model, J. Phys. Chem. C, 2017, 121, 11472 – 11487, incorporated as Chapter 6. Contributor Statement of contribution Designed/performed experiments (70%) Afshin Asadzadeh Shahir Interpreted/analysed data (75%) Wrote/revised the paper (85%) Performed experiments (10%) Dimitrinka Arabadzhieva Interpreted/analysed data (10%) Designed/performed experiments (15%) Hristina Petkova Interpreted/analysed data (10%) Interpreted/analysed data (5%) Stoyan Ivanov Karakashev Wrote/revised the paper (5%) Anh Van Nguyen