Polymer vs. surfactant : competitive adsorption at the solid-liquid interface

B.R. Postmus

Research output: Thesisinternal PhD, WU

Abstract

The research described in this thesis focuses on the competitive adsorption of nonionic polymer and nonionic surfactant on a silica surface. These type of systems are interesting from both an academical and a technological viewpoint. Our academic interest stems simply from the observation that we had a hard time predicting the (adsorption) behaviour of the system beforehand. The technological relevance of our study can be attributed to the observation that technological applications are often complex mixtures containing a large variety of additives. The interactions between all these different components, such as the formation of mixed aggregates or co-adsorption, are quite complex. For applications, these interactions are very important since the properties of a mixture on a microscopical scale can be used to manipulate the macroscopical behaviour. Or, in the case of undesirable macroscopic behaviour, a detailed knowledge about the microscopic interactions can be used to improve on the situation.

We have restricted ourselves to relatively simple complex mixtures, i.e. we have chosen a well-defined model system consisting of homodisperse components. This model system is an aqueous mixture of the nonionic polymer PEO with the nonionic surfactant CnEm. To study the adsorption behaviour of this mixture, we have chosen to use a flat silica surface as a model surface. The CnEm surfactants adsorb (on a hydrophilic surface such as silica) with their head groups. Because the head groups consist entirely of EO segments, the binding mechanism of the surfactants to the silica is exactly the same as the PEO binding mechanism, namely H-bonding. By evaluating the competitive adsorption of the system, we are effectively investigating the subtle effects of layer structure. By making small changes to the choice of surfactant architecture, polymer length or solvent quality, large changes in layer structure can be induced.

Reflectometry was used to look at the competitive adsorption from mixtures containing PEO and CnEm. There are several methods to test this competitive adsorption. In the case of simultaneous adsorption, the polymer and surfactant are allowed to adsorb from a mixture. It is also possible to study adsorption sequentially, i.e. first adsorb component A, and then sequentially try to displace component A with component B. We decided to start by doing sequential adsorption experiments, because these are easier to control. In such an experiment, the PEO is allowed to adsorb onto the surface from a solution with 5 mg PEO/L. Care was taken to insure that the layer was in its steady state. Next, the flow of PEO solution was replaced by the background solution, and subsequently by a solution containing only surfactants. The concentration of the surfactant solution was 110-4 mol/L for all surfactants except for C12E3, where solubility problems demanded the use of a lower concentration c = 610-5. Still, all surfactant solutions had a concentration higher than the CMC. The results of these experiments can basically be grouped in two categories. Upon changing to the surfactant solution, the adsorbed amount would either increase sharply, or the adsorbed amount would remain constant. In the first case where the adsorbed amount would increase until the amount that the surfactant would also reach from a single component solution. Furthermore, subsequent rinsing of this layer would result in a total dissolution of the layer, and hence, the adsorbed amount would go to zero. Since this is typical surfactant behaviour, we can conclude that the surfactant displaces the polymer as it adsorbs.

To better understand the experimental observations, we have developed an SCF model. In this model, it is possible to calculate the charge on the silica surface as a function of the pH and the ionic strength. This yields titration curves that can be compared with experimental titration curves. Our calculated results correspond quite well with literature data.

One can also use the model to make predictions about the adsorption of PEO on our silica surface. It is possible to go to concentrations much lower than those that are experimentally accessible. We have made predictions about the response of the adsorbed polymer layer upon changes in ionic strength and pH. The results show that PEO adsorption is relatively insensitive for the ionic strength at pH ≈ 7, but at pH ≈ 10, the ions can displace the polymer quite well. This type of behaviour is also found experimentally. Every time that we perform a calculation (and we do find a solution), we obtain the mean field free energy and the most likely conformation of the system. By looking at the profiles of the most likely conformation, i.e. plotting the volume fraction of a species versus the distance from the surface, we can see that the adsorbed polymer inhibits the adsorption of salt. Hence, the polymer and the salt are in competition for adsorption.

The behaviour of CnEm surfactants can also be evaluated with the model. Here we use exactly the same parameters that we used for the PEO. Again, we started by evaluating the surfactant bulk behaviour. Instead of investigating the first occurrence of a micelle, we have defined a more experimentally relevant CMC. We have evaluated that concentration where the volume fraction of micelles is approximately equal to the volume fraction of unimers. Based upon this criterion we have calculated the CMC and the corresponding micellar size for a number of surfactant architectures and for a number of ionic strengths. We have also evaluated surfactant adsorption isotherms. These calculated adsorption isotherms feature a first order transition at the CSAC. By evaluating the behaviour of the CSAC, we have found that the CSAC shifts to a higher concentration when the pH or the ionic strength is increased. We identified conditions for which the CSAC > CMC, which effectively implies that the surfactant does not adsorb anymore. We compared these predicted results to data measured using a reflectometer, and we find that the model predicts the experimental results quite well.

The next step is to use the model to try and reproduce the displacement results. We have defined systems that include both PEO and CnEm, at some pH and ionic strength. To determine which component adsorbs from a mixture, we evaluate the response of the CnEm to the competing polymer. The surfactant starts adsorbing at some concentration (CSAC). If the surfactant concentration is lower than the CSAC, then the PEO will adsorb (we assume that the pH and ionic strength are such that the PEO is capable of adsorbing). For surfactant concentrations higher than the CSAC but lower than the CMC, the surfactant will preferentially adsorb. In the case of CSAC > CMC, the surfactant will not adsorb. Typically, the polymer will adsorb in this case, however, one can think of situations (high pH and high ionic strength) where the polymer will also stay in solution.

Using the method described above, we can model the competitive adsorption of PEO and CnEm. We can evaluate the response of the surfactant to competing species, such as PEO of length N. By identifying for every surfactant architecture that polymer length N where CSAC = CMC, we can make predictions about the adsorption from mixtures.


Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Cohen Stuart, Martien, Promotor
  • Leermakers, Frans, Promotor
Award date10 Jun 2008
Place of Publication[S.l.
Print ISBNs9789085049289
Publication statusPublished - 2008

Keywords

  • polymers
  • surfactants
  • adsorption
  • interface

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