An AFM study of the interactions between colloidal particles vorgelegt von Liset A. C. Lüderitz M. Sc. aus Havanna - Kuba von der Fakultät II - Mathematik und Naturwissenschaften der Technischen Universität Berlin zur Erlangung des akademischen Grades Doktor der Naturwissenschaften Dr. rer. nat. genehmigte Dissertation Promotionsausschuss: Vorsitzender: Prof. Dr. R. Schomäcker, TU Berlin Berichter: Prof. Dr. R. von Klitzing, TU Berlin Berichter: Prof. Dr. G. Papastavrou, Universität Bayreuth Tag der wissenschaftlichen Aussprache: 14.09.2012 Berlin 2012 D 83 Abstract This research project is focused on the study of the interaction forces between two col- loidal particles. The interaction between colloidal particles may differ from the interaction between macroscopic bodies. The interactions are measured across different electrolytes: CsCl, KCl, NaCl and LiCl using Colloidal Probe Atomic Force Microscope (CP-AFM) techniques. The resulting forces may be different depending on the electrolyte solution used, which is known as ion specificity. In this study no ion specificity effect is observed at long range for the adsorption of counterions to the silica surface but a slight tendency for Cs+ to be more adsorbed at the surface than the other counterions is present at 10−3 M. Deviations from the DLVO theory at small separations (non-DLVO forces) are reported in this work. Short range attractions at 10−4 M ionic strength were measured whereas at 10−3 M short range repulsions are present. An explanation of the non-DLVO forces is given based on the hydration forces. A further chapter studies the interaction forces between silicon oxide surfaces in the presence of surfactant solutions. Based on the qualitative and quantitative analysis of these interaction forces the correlation with the structure of the aggregates on the surfaces is analysed. A colloidal probe atomic force microscope (AFM) was used to measure the forces between two colloidal silica particles and between a colloidal particle and a silicon wafer in the presence of hexade- cyltrimethylammonium bromide (CTAB) at concentrations between 0.005 mM and 1.2 mM. Different interaction forces were obtained for the silica particle–silica particle system when compared to those for the silica particle–silicon wafer system for the same studied concentration. This indicates that the silica particles and the silicon wafer have different aggregate morphologies on their surfaces. The point of zero charge (pzc) was obtained at 0.05 mM CTAB concentration for the silica particles and at 0.3 mM for the silica particle– silicon wafer system. This indicates a higher charge at the silicon wafer than at the silica particles. The observed long range attractions are explained by nanobubbles present at the silicon oxide surfaces and/or by attractive electrostatic interactions between the sur- faces, induced by oppositely charged patches at the opposing Si oxide surfaces. In order to analyze the role of the nanobubbles on the hydrophobic interactions hydrophilic silicon wafers were studied against aqueous solutions of CTAB at concentrations between 0.05 mM and 1 mM (CMC). AFM studies show that nanobubbles are formed at concentra- tions up to 0.4 mM. From 0.5 mM upward, no bubbles are detected. This is interpreted as the formation of hydrophobic domains of surfactant aggregates, becoming hydrophilic at about 0.5 mM. The high contact angle of the nanobubbles (140-150◦ through water) in comparison to the macroscopic contact angle indicates that the nanobubbles are located on the surfactant domains. A combined imaging and colloidal probe AFM study serves to highlight the surfactant patches adsorbed at the surface via nanobubbles. The nanobub- bles have a diameter between 30 and 60 nm (after tip deconvolution), depending on the surfactant concentration. This corresponds to a Laplace pressure of about 30 atm. The presence of the nanobubbles is correlated with force measurements between a silica probe and a silicon wafer surface. The study is a contribution to a better understanding of the short range attraction between hydrophilic surfaces exposed to a surfactant solution. The substrate properties hydrophobicity and roughness influence the morphology and size of the nanobubbles. Nanobubbles with a contact angle through water of 132◦ and a Laplace pressure of 18 atm were visualized at the interface of a hydrophobically modified silicon wafer exposed to water and surfactant solutions. An increase in surfactant concentration has an impact on the morphology of the nanobubbles, they were flattened at the surface with increasing surfactant concentration. 3 To my father and my grandparents Contents List of Figures8 List of Tables 13 List of Symbols 14 Acknowledgments 16 1. Introduction and Literature Review 17 1.1. Colloidal Particles............................... 17 1.2. Non-DLVO Forces............................... 23 1.2.1. Hydration Forces............................ 23 1.2.2. Hydrophobic Interactions....................... 26 1.2.3. Structural Forces............................ 27 1.3. Surfactants................................... 28 1.3.1. Classification.............................. 28 1.3.2. Surfactants at Interfaces........................ 31 1.4. Nanobubbles.................................. 32 Bibliography 34 2. Techniques 36 2.1. Atomic Force Microscopy........................... 36 2.2. Scanning Electron Microscopy......................... 40 2.3. Zeta Potential.................................. 41 Bibliography 45 3. Force Measurements between Colloidal Particles across Aqueous Electrolytes 46 3.1. Introduction................................... 46 3.2. Experimental Section.............................. 47 3.2.1. Materials................................ 47 3.2.2. Preparation and Methods....................... 48 3.2.3. Simulations............................... 48 3.3. Results...................................... 49 3.3.1. Effect of Ionic Strength: 10−4 M and 10−3 M............ 49 3.3.2. Effect of pH............................... 55 5 Contents 3.3.3. Interactions through Water...................... 56 3.4. Discussion.................................... 58 3.4.1. Effect of Ionic Strength........................ 58 3.4.2. Effect of pH............................... 62 3.5. Conclusions................................... 63 Bibliography 64 4. Interaction Forces between Silica Surfaces in Cationic Surfactant Solutions 66 4.1. Introduction................................... 66 4.2. Experimental Section.............................. 68 4.2.1. Materials................................ 68 4.2.2. Preparation and Methods....................... 68 4.2.3. Simulations............................... 69 4.3. Results...................................... 69 4.3.1. Interaction forces between two silica particles (system I)...... 69 4.3.2. Interaction forces between a silica particle and a silicon wafer (sys- tem II).................................. 71 4.3.3. Point of zero charge.......................... 74 4.4. Discussion.................................... 76 4.4.1. Interaction between two silica particles (system I).......... 76 4.4.2. Interaction between a silica particle and a silicon wafer (system II) 78 4.4.3. Comparison between the system silica particle–silica particle (I) and the system silica particle–silicon wafer (II)........... 80 4.4.4. Non DLVO forces............................ 81 4.5. Conclusions................................... 82 Bibliography 84 5. Scanning of Silicon Wafers in Contact with Aqueous CTAB Solutions below the CMC 86 5.1. Introduction................................... 86 5.2. Experimental Section.............................. 88 5.2.1. Materials................................ 88 5.2.2. Preparation and Methods....................... 88 5.2.3. Simulations............................... 89 5.3. Results...................................... 89 5.4. Discussion.................................... 95 5.4.1. Nanobubbles.............................. 95 5.4.2. Correlation with Force Curves..................... 98 5.5. Conclusions................................... 101 Bibliography 102 6. Nanobubbles at the Surface of a Divinyl Disilazan modified Silicon Wafer 104 6 Contents 6.1. Introduction................................... 104 6.2. Experimental Section.............................. 105 6.2.1. Materials................................ 105 6.2.2. Preparation and Methods....................... 105 6.2.3. Simulations............................... 106 6.3. Results...................................... 106 6.4. Discussion.................................... 111 6.5. Conclusions................................... 114 Bibliography 115 7. Conclusions and Future Work 117 7.1. General Conclusions.............................. 117 7.2. Future Work.................................. 119 Bibliography 121 A. Appendix 122 7 List of Figures 1.1. Scheme of the DLVO theory: (a) Surfaces repel strongly, small colloidal particles remain stable; (b) Surfaces are at equilibrium at secondary min- imum if it is deep enough, colloids remain kinetically stable; (c) Surfaces come into secondary minimum, colloids coagulate slowly; (d) The critical coagulation concentration, surfaces may remain in secondary minimum or adhere, colloids coagulate rapidly; (e) Surfaces and colloids coalesce rapidly [6]. ........................................ 19 1.2. Helmholtz double layer model ......................... 20 1.3. Gouy-Chapmann double layer model ..................... 20 1.4. Stern model of the double layer .......................