Polymers and surfactants in solution and at interfaces : a model study on detergency

This thesis deals with detergency-related adsorption phenomena of (mixtures of) polymers and surfactants. Both types of molecules play an important role in the removal and subsequent stabilization of soil from a substrate. Starting with a model detergency system consisting of polymers, surfactants, soil and a substrate, a division is made into a set of sub-systems, each focusing on the interactions of two or more of these model components.The first chapter gives a short introduction on the typical behavior of polymers and surfactants in solution and at interfaces, and touches upon the physicochemical principles of detergency. In a washing process it is important to prevent the redeposition of soil, which in an earlier stage has been removed from a substrate. A way to keep particles dispersed in solution is to cover them with a thick polymer layer providing electrostatic and/or steric stabilization. The adsorption of the uncharged polymer poly(vinylpyrrolidone) (PVP) on Na-kaolinite has been studied in chapter 2. The surface of this clay mineral is patchwise heterogeneous with respect to its charge and chemical composition. In order to reveal these charge characteristics, potentiometric acid-base titrations were performed on samples at different concentrations of sodium chloride. An interpretation of the proton adsorption/desorption in terms of the contributions of the individual surface types, i.e. edges and plates, has been given. At the latter type, protons are strongly favored over sodium ions. Striking similarities were observed between the proton adsorption and the PVP adsorption experiments. PVP readily adsorbs high affinity on at least part of the kaolinite surface. Studying the effect of the pH, the electrolyte concentration, and the presence of multivalent ions on the amount adsorbed at the plateau has given further insight into the adsorption mechanisms. Increasing the pH or the electrolyte concentration leads to a decrease in PVP adsorption. A model is proposed in which PVP adsorbs on edges and basal planes by different mechanisms. The adsorption of PVP on the edges is strongly pH dependent, but that on the plates only weakly. Specifically adsorbed protons at the plates act as anchor sites for PVP segments. Multivalent ions do not influence the proposed adsorption mechanism directly, but primarily change the surface area accessible for PVP.Before studying adsorption of a polymer-surfactant mixture, information on the interaction between the polymer and the surfactant in solution is indispensable. Chapter 3 covers the interaction between the anionic surfactant sodium dodecylbenzenesulphonate (SDBS) and the uncharged polymer poly(vinylpyrrolidone) (PVP) by titration microcalorimetry. Since hydrophobic attractions are typically dependent on temperature, which is in general not the case for other types of interaction, measurements carried out at different temperatures have yielded information on the nature of the associations. The interaction enthalpy of mixed PVP/SDBS systems clearly showed a consecutive endothermic and exothermic region with increasing surfactant concentration. The endothermic part can be looked upon as an incremental binding isotherm and reflects the number of surfactant molecules involved in the process. The exothermic region features the inverse of hydrophobic bonding behaviour. In our opinion, this is due to conformational changes of the polyelectrolyte complexes. With increasing amount of surfactants bound to the chain, electrostatic repulsion of neighbouring surfactant head groups tends to expand the complexes, whereas hydrophobic interactions do the opposite. Beyond a certain coverage, the coulombic repulsion forces the chains to swell. This is accompanied by losing hydrophobic inter- and intrachain linking. Additional surfactants continue to adsorb on the vacant hydrophobic adsorption sites. The influence of the initial amount of polymer and the electrolyte concentration support our proposals.The results and the knowledge obtained with this study has helped to understand the mixed adsorption of PVP and SDBS on kaolinite, which is the subject of chapter 4. Both components adsorb from their mixture on the clay. This process is sensitive to the pH, the electrolyte concentration, and the amounts of polymer and surfactant. In the absence of PVP, SDBS adsorbs on the clay by electrostatic and hydrophobic interactions. When polymers are present, the adsorbed amount of SDBS is at 10 -2 M NaCl mainly determined by charge compensation on the edges.Under different conditions PVP shows similar behaviour as a function of the surfactant concentration. With increasing SDBS concentration three subsequent regions in the PVP adsorption can be distinguished: initially a small increase, followed by a strong decrease, which finally flattens off to a plateau. These regions are related to the surface affinity of the species actually present in solution. They reflect the changing character of the charge of the polymer-surfactant complexes with increasing surfactant concentration. At low surfactant content, the polymer chains are not or hardly charged, and they adsorb on the clay by hydrogen bonding and hydrophobic interactions. At high surfactant concentrations, the adsorption of polymer-surfactant complexes is dominated by coulombic attraction. There is experimental evidence for the presence of mixed surface aggregates at the edges. The composition of these complexes differs from that in solution and is controlled by the surface charge. With increasing electrolyte concentration, this difference becomes smaller.After a detailed look at the solution side of the washing process, we have to focus on the substrate. In order to carry out fundamental studies, a flat and well-defined surface was needed which was a good mimic for cotton. To that end, a cellulose surface was developed which was able to function as a model for cotton. Chapter 5 describes the preparation of thick cellulose films. The method is based on the attachment of hydrophobized cellulose on a wafer and subsequent chemical regeneration to cellulose. With the spincoating technique, reproducible, rapidly prepared, and flat cellulose surfaces can be obtained. These are characterized by their thickness, roughness, swelling behaviour, stability, charge, and wetting and adsorption properties.So far, all studies concerned equilibrium aspects. However, in a washing process, the dynamics of processes, such as adsorption, removal, and stabilization, are very important. Kinetic and equilibrium aspects of nonionic surfactant adsorption on cellulose surfaces just described, are studied in a stagnation point flow cell (chapter 6). Nonionic surfactants readily adsorb on cellulose, thereby showing three distinct regions. At low surface coverages, molecules adsorb more or less in a flat state, with a contribution from both the head group and the tail. At increased concentrations, lateral attraction between surfactant molecules is dominant, leading to the formation of half-micelles at the surface. In line with the results of chapter 5, the adsorption features of cellulose are in between those for a hydrophilic and a hydrophobic surface.The kinetics of nonionic surfactant adsorption depends on the composition of the surfactant. Below the CMC, the initial adsorption rate is determined by monomer diffusion. Above the CMC, the magnitudes of the micellar dissociation rate and the micellar diffusion coefficient, should be compared to that of the monomer diffusion coefficient. If the micellar properties are sufficiently large, micelles acts as monomer-suppliers. This was observed for the most hydrophilic surfactant under study. The desorption rate depends on the surface coverage. Initially, it is controlled by monomer detachment. The desorption rate coefficients of different surfactants scaled with the CMC, suggesting an analogy between the surface aggregates to those formed in solution.The set-up of the cellulose surfaces in a stagnation point flow cell can be used for a variety of adsorbates and serve as a model for (re)deposition studies.