Adsorption and micellization of surfactants : comparison of theory and experiment

<p>The purpose of this thesis is to extend the knowledge of micellization and adsorption of surfactants in aqueous solutions or the solid /electrolyte interface. To this end experimental results for well defined systems have been compared with theoretical calculations.<p>The theoretical calculations have been performed with the selfconsistent field lattice theory for adsorption and association developed in this laboratory by Scheutjens, Fleer, and Leermakers. The theory uses a lattice and a mean field approximation is applied to each lattice layer. The calculations start from molecular properties of all molecules present in the system. For surfactants these include the sizes of the head and tail groups, the fact that the chains are flexible, the charge and the intra- and intermolecular interactions. Using these properties together with the amounts of the different molecules present, equilibrium volume fraction profiles are calculated, for instance, perpendicular to an adsorbing surface. From the volume fraction profiles, experimentally easily accessible quantities such as the adsorbed amount and layer thickness can be derived.<p>In chapter 2 the micellization and adsorption of nonionic surfactants is calculated. Due to steric repulsion between the head groups the cmc increases and the aggregation number decreases as the polar chain becomes longer. Similarly the steric repulsion also causes a decrease in the maximum adsorbed amount as a function of the length of the head groups both on hydrophilic and hydrophobic surfaces, which agrees with experimental work. On hydrophobic surfaces surfactant monolayers are formed, on hydrophilic surfaces bilayers are formed if the head group is not too long relative to the tail. For nonionic surfactants with long head groups only weak association of the tails was found.<p>In chapter 3 the effect of polar chain length on the adsorption of nonionic surfactants is investigated in more detail. Experimental data are obtained for adsorbed amounts and hydrodynamic layer thicknesses. Moreover, with neutron reflection, the structure of the adsorbed layer is measured more directly. The experiments show that even for surfactants with very long head groups strong association between the tails occurs. The theory used in chapter 2 did not predict this behavior. However, with an extension of the theory allowing for inhomogeneities not only perpendicular but also parallel to the surface, a much better agreement between experiment and theory is reached. On hydrophilic surfaces nonionic surfactants with long polar head groups are found to form aggregates with a hydrophobic core. Nonionic surfactants with short polar head groups form bilayers on these surfaces.<p>Chapter 4 deals with micelle formation of ionic surfactants. The critical micelle concentration decreases and the micelle aggregation number increases with rising salt concentration and tail length. Transitions to other than spherical shapes at high salt concentrations and for branched chains were predicted. The calculated profiles show that the head groups are not located on a single shell but have a rather wide distribution. Micelles behave as spheres with a low but constant net charge density. The potential in the head group region is adjusted by attracting as many counterions to maintain this charge density. The agreement with experimental results on aggregation numbers, cmc-values and electrical potentials in the head group region is good.<p>In chapter 5 calculations on the adsorption of ionic surfactants on surfaces with a constant charge are performed. The most typical effect is that "two- step" adsorption isotherms are obtained, especially at low csalt. After the first step the surface charge is compensated and at a solution concentration of about 10% of the cmc the adsorbed amount starts to increase again to form a bilayer. At high salt concentrations, the two steps cannot be distinguished so clearly. The adsorption isotherms have a common intersection point which coincides with the isoelectric point. The head group distribution of the bilayer at the solution side is rather wide and the charge profile at the solution side of the bilayer does not depend on the salt concentration. Just as was the case with micelles, the potentials at the solution side of the bilayer depend strongly on the salt concentration. Experimental results from literature indeed show the calculated isotherm shapes and changes in structure of the adsorbed layer from a "single layer" to a bilayer.<p>In chapter 6 the adsorption of ionics on variable charge surfaces is studied. If the experimental data for sodium nonylbenzene sulfonate on rutile are plotted on a double logarithmic scale the characteristic shape of the adsorption isotherm of anionic surfactants on metaloxides appears. The adsorption increases steeply at very low adsorbed amounts (region II), due to the interaction between adsorbed molecules. At a few percent of the maximum coverage the slope in the log-log plot decreases again (region III) and at the cmc a plateau is reached (region IV). A common intersection point in the adsorption isotherms at different salt concentrations is found in region III. The common intersection point coincides with the isoelectric point.<p>The measured isotherms do not have a "two-step" shape as found for constant charge surfaces. This due to the adjustment of the surface charge upon surfactant adsorption. From the measured surface charge, the amount of surfactant adsorbed with their head group on the surface can be estimated. These data show that after the common intersection point bilayer formation starts. The bilayer structures that have been formed can be asymmetrical: the amount at the surface is dictated by the surface charge while the amount at the solution side depends mainly on the repulsion between the head groups and the attraction between the tails.<p>The calculated results show the characteristic regimes in the adsorption isotherm. For the initial part and the upper part of the isotherm the results agree qualitatively with the experimental ones and corroborate that first head-on adsorption and later bilayer formation takes place. The part of the isotherm where lateral interaction dominates the behavior agrees less satisfactorily with experimental data. At the calculated II/III transition a monolayer of surfactants, with an area per head group equal to that in bilayer membranes, is on the surface. In reality the II/III transition occurs at a lower coverage and local aggregates are present on the surface. The mean field approximation turns out to be an oversimplification for the description of region II.<p>In chapter 7 the effects of chain length and branching are investigated. Experimentally it is shown that the adsorption increases with chain length and decreases with degree of branching. These trends are also calculated. Just as the aggregation number of micelles increases with chain length and decreases with the degree of branching, the size of primary aggregates (hemi- micelles) at the II/III transition increases with chain length and decreases with degree of branching. This indicates that similar packing constraints apply to micellization and adsorption in region 11. At the cmc the adsorbed layer is more like a bilayer membrane.<p>Most of the observations do not need a complex theory to be understood qualitatively. If the repulsion between the head groups increases, either by reducing the ionic strength or by making the head group longer, one can expect the cmc to increase, the aggregation number to decrease and the adsorption to decrease as well, all of this because the molecules have less tendency to get together. The benefit of the theory is however twofold. Firstly, it allows for an approximate quantification of these effects. A nice example is the variation of the plateau adsorption with salt concentration. The salt concentration affects the repulsion between charged head groups. In general this will lead to an increase in adsorption and a decrease of the cmc. Because of the lowering of the cmc the concentration range in which the adsorption can increase decreases. The net result is that a priori the effect of salt on the plateau adsorption is not clear. However, using the theory it appeared that both effects almost cancel, which was also shown to be the case experimentally.<p>The other advantage of using the theory is that information on the structure of the adsorbed layer or the micelle can be deduced. A typical example of a new feature emerging from the calculations is the charge regulation mechanism in micelles and adsorbed layers. Other examples are the wide distribution of the head groups, and the change of the structure of the layer with equilibrium concentration.<p>Qualitatively, the many different isotherm shapes observed in surfactant adsorption studies can all be described with one single theory. The theoretical calculations predict the structure of micelles and adsorbed layers and therefore this theory can be used to obtain insight in the behavior of surfactants at interfaces and in solution. It can be concluded that surfactant adsorption on hydrophilic surfaces and micellization have many aspects in common, because aggregation in adsorbed layers is very important