chapter  8
40 Pages

8

I. Introduction ................................................................ 380 II. Air-Water Interfaces.................................................. 381

A. Dynamic Surface Excess and Surface Tension . 381 B. Experimental Design........................................... 382

1. Selection of Surfactants ................................ 382 2. Experimental Techniques.............................. 383

C. Dynamic Surface Properties of CmTAB Solutions............................................................... 387

D. Adsorption Mechanism for CmTAB at the Air-Water Surface ............................................... 392 1. Surfactant Solutions below the cmc............. 392 2. Surfactant Solutions above the cmc............. 394

E. General Observations for Other Ionic Surfactants........................................................... 396

III. Solid-Liquid Interfaces.............................................. 397 A. Optical Reflectometry and the Wall-Jet Cell..... 397 B. Kinetics of Cationic Surfactant Adsorption....... 400

1. Surfactant Chain Length Effects ................. 400 2. Influence of Electrolytes................................ 403 3. The Slow Adsorption Region (SAR) ............. 405 4. Evidence for Kinetic Trapping...................... 407

C. Mechanism of Adsorption ................................... 409 1. The Electrostatic Concentration Span......... 409 2. The Electrostatic and Hydrophobic

Concentration Span....................................... 411 3. The Hydrophobic Concentration Span......... 411 4. Adsorption Kinetics and the Adsorption

Isotherm ......................................................... 411 IV. Conclusions and Outlook ........................................... 412

A. Air-Water Interfaces........................................... 412 B. Solid-Liquid Interfaces ....................................... 412

V. Acknowledgments....................................................... 413 References............................................................................ 413

I. INTRODUCTION

The aim of this chapter is to compare and contrast adsorption kinetics of model cationic surfactants at air-water and solidliquid interfaces, so as to draw general conclusions and identify dominant processes. Recently, strides have been made in understanding surfactant adsorption kinetics, and in this area development and application of new surface selective techniques has been key. Methods of relevance in this chapter are neutron reflectivity (NR), ellipsometry, and optical reflectometry (OR). These techniques are based on scattering and/or interference of neutron radiation or polarized laser light, and hence the principal advantages are that they directly probe surface layer structures and adsorption densities. In the text the terms surface excess, adsorbed amount, and surface density are used interchangeably to express two-dimensional concentrations, either at air-water or solid-liquid surfaces. The main surfactants considered are the family of n-alkyltrimethylammonium bromides: CmTAB, of alkyl chain carbon number m.