Solution Behaviour of Polyethylene Oxide, Nonionic Gemini Surfactants

Transcription

1 Solution Behaviour of Polyethylene Oxide, Nonionic Gemini Surfactants A thesis submitted to The University of Sydney in fulfilment of the requirements for the admission to the degree of Doctor of Philosophy Paul Anthony FitzGerald December 2002

2 Abstract In recent years there has been increasing interest in novel forms of surfactants. Of particular interest are gemini surfactants, which consist of two conventional surfactants joined by a spacer at the head groups, as they exhibit lower critical micelle concentrations than can be achieved by conventional surfactants. In this work, the self-assembly behaviour of several nonionic gemini surfactants with polyethylene oxide head groups (Gem n E m, where n (= 20) is the number of carbons per tail and m (= 10, 15, 20 and 30) is the number of ethylene oxides per head group) were investigated. The Critical Micelle Concentrations (CMCs) were measured using a fluorescence probe technique. The CMCs are all ~2 x 10-7 M, with almost no variation with m. The CMCs are several orders of magnitude lower than conventional C 12 E m nonionic surfactants. The mixing behaviour of the gemini surfactants with conventional surfactants was also studied. They obeyed ideal mixing behaviour with both ionic and nonionic surfactants. Micelle morphologies were studied using Small Angle Neutron Scattering. The gemini surfactants with the larger head groups (i.e. Gem 20 E 20 and Gem 20 E 30 ) formed spherical micelles. Gem 20 E 15 showed strong scattering at low Q, characteristic of elongated micelles. As the temperature was increased towards the cloud point, the scattering approached the Q -1 dependence predicted for infinite, straight rods. The existence of anisotropic micelles was supported by the viscosity of Gem 20 E 15, which increases by several orders of magnitude on heating towards its cloud point. Phase behaviour was determined using Diffusive Interfacial Transport coupled to near-infrared spectroscopy. Much of the behaviour of these systems is similar to conventional nonionic surfactants. For example, Gem 20 E 10 forms a dilute liquid isotropic phase (W) coexisting with a concentrated lamellar phase (L α ) at around room temperature and forms a sponge phase at higher temperatures. This is similar to the behaviour of C 12 E 3 and C 12 E 4. The other surfactants studied are all quite soluble in water and form liquid isotropic and hexagonal phases from room temperature. At higher concentrations Gem 20 E 15 formed a cubic and then a lamellar phase while Gem 20 E 20 formed a cubic phase and then an intermediate phase. This is also comparable to the phase behaviour of conventional nonionic surfactants except the intermediate phase, which is often only observed for surfactant systems with long alkyl tails. i

3 Acknowledgments Firstly, thanks to my supervisor, Professor Greg Warr, for guiding me through my project for making my time so enjoyable and for showing me the beauty in every scientific problem. I would also like to thank Professor Raoul Zana and my associate supervisor, Professor Don Napper, for helpful discussion during my project. I also thank Dulux for financial and in-kind support. In particular, special thanks to Algi Serelis for synthesising the surfactants used in this project, and to Matt Carr and Chris Such for driving the project. I would also like to thank Huntsman for their contribution. Thanks to Jamie Schulz for assistance with the collection, reduction and analysis of the SANS data. Thanks also for generating my initial interest in surfactant research and for making life a little more interesting. Thanks to Miles Page for assistance with formatting, for offering to proof read, for introducing me to coffee (and beer) and for keeping me out late when I should have been writing my thesis. To Tim Davey, for assistance in the lab and Annabelle Blom, Kathryn Topp and the other members of the group who, over the years, have made my time so enjoyable. I would also like to thank Naomi Osman for proof reading and her friendship over the years. Finally, I would like to thank my parents for supporting me throughout my project and especially for putting up with me while I wrote my thesis. ii

4 Table of Contents Abstract Acknowledgments Table of Contents Preface i ii iii v Chapter 1: Introduction Conventional surfactant Behaviour The Critical Micelle Concentration Gemini surfactants Organization of this thesis 4 Chapter 2: Solution Behaviour of Surfactants Nonionic Gemini Surfactants Conventional Nonionic Surfactants Nonionic Gemini Surfactants Surfactants for This Project Micelle Geometries and the Surfactant Packing Parameter Micelle Shape Transitions Self-Assembly of Nonionic Surfactants Thermodynamics of Micelle Formation The Micelle Equilibrium Model The Pseudo Phase Model Thermodynamics of Gemini Surfactants (Simple Theory) Thermodynamics of Mixing (Ideal Solutions) Phase Behaviour The Cloud Temperature and Solubility Mesophases Surfactant Mesophases Phase Behaviour of Conventional Nonionic Surfactants 26 Chapter 3: Experimental Methods Critical Micelle Concentrations (CMCs) CMC Measurement using a Fluorescence Probe Small Angle Neutron Scattering (SANS) Basic Scattering Theory Scattering Models for Common Micelle Shapes Experimental Equipment and Data Reduction Measurement of Phase Behaviour Microscopic Identification of Mesophases Phase Boundaries Cloud Temperature Measurements Viscosity Measurements High Performance Liquid Chromatography (HPLC) 55 iii

5 Chapter 4: Critical Micelle Concentrations (CMCs) Introduction Results CMCs of Nonionic Gemini Surfactants Mixed CMCs of Gemini and Conventional Surfactants Discussion 63 Chapter 5: Micelle Morphologies Introduction Methods of Study Surfactants Studied Results Gemini Surfactants at 1 wt% and 25 C Concentrated Solutions of Gemini Surfactant Micelles Gemini Surfactants at Elevated Temperatures Discussion 99 Chapter 6: Phase Behaviour of Nonionic Gemini Surfactants Introduction Results Phase Behaviour of Gem 20 E Phase Behaviour of Gem 20 E Phase Behaviour of Gem 20 E Phase Behaviour of Gem 20 E Discussion Phase Behaviour with Increasing EO Comparison of Phase Behaviour 111 Chapter 7: Conclusions Critical Micelle Concentrations Micelle Morphologies and Phase Behaviour Final Remarks 116 References 117 Appendix 122 A.1 Synthesis for Nonionic Gemini Surfactants (Gem n E m ) 122 A.2 The Classical Chemical Potential 123 A.3 Examples of Microscopic Textures for H 1 and L α Phases 124 A.4 Operation of Capillary Viscometer 126 A.5 Difference Between R = 4.5/Q min and R micelle for Core-Shell Micelles 127 A.6 R sphere Calculated from R G for Core-Shell Micelles 128 A.7 Background Subtraction from SANS data 130 A.8 Radius of Gyration Calculations for Gemini Surfactants 132 A.9 Mean Chord Length Calculations for 1wt% Gemini Surfactants 134 A.10 Data Fitting of 1 wt% Gemini Surfactants at 25 C 135 A.11 Determination of Gem 20 E m Phase Behaviour 138 A.12 SANS Data for 1 wt% Surfactants at Elevated Temperatures 140 iv

6 Preface This thesis is the result of research carried out by the author at the University of Sydney between February 1999 to December 2002 and have not been submitted to any other institution for any other degree. Where reference is made to the results of other authors this is stated in either as a footnote or in the numbered reference list. Paul FitzGerald Sydney December 2002 v