Polymer-surfactant interaction at liquid interface

There is considerable industrial interest in understanding the nature of surface and bulk interactions between polymers and surfactants because such mixtures are used extensively in commercial products such as shampoos and fabric conditioners.

Considerable effort over the last two decades has been devoted to gaining an understanding of the nature of the interaction between polyelectrolytes and oppositely charged surfactants in the bulk phase. Most of the work has been carried out on linear polyelectrolyte systems. These investigations showed that usually a one-phase transparent system forms in the presence of a small amount of surfactant. By increasing the bulk surfactant concentration, the solution becomes turbid, and an associative phase separation occurs (i.e., a concentrated phase enriched in both polymer and surfactant separates from a dilute aqueous phase containing mostly small ions). With further increases in the bulk surfactant concentration, the turbidity may decrease, and redissolution of the polyelectrolyte/surfactant complexes can occur. The surfactant binding in the bulk phase is usually interpreted in terms of a two-part equilibrium process.

Poly(ethylene imine) [PEI] has captured a lot of attention from the academic and industrial research communities in recent years. Factors that in particular make PEI an interesting class of polymer both from a fundamental and practical perspective:
1.    The polymer size as well as molecular architecture can be readily varied from linear to hyperbranched, which affects its morphology in bulk solution and at interfaces.
2.    The polyelectrolyte charge can be set by modifying the solution pH: effectively, PEI has a high positive charge density at low pH but a diminished charge density at high pH.
3.    The physicochemical properties are affected by the ionic strength because the addition of electrolyte increases the surface activity of polymer/surfactant complexes.

A useful optical technique for the characterization of polymer/surfactant adsorption layers at the air/liquid interface is ellipsometry, where the reflection of elliptically polarized light depends on the dielectric (refractive index) profile normal to the interface. The phase change of light upon reflection at the surface of a polymer/surfactant solution, relative to that of a clean interface, is determined by the structure and amount of adsorbed material. We exploit ellipsometry in the present project by analyzing fluctations in the optical signal, features that would most likely not be detected in the relatively slow and macroscopic measurements made using for instance neutron reflectometry (NR). Furthermore, the comparison of ellipsometry measurements, both with changing bulk composition and evolving time, allows us to track the relative amount of adsorbed material.

We have exploited the spatial and kinetic resolution of ellipsometry to monitor the lateral movement of inhomogeneous patches of material in mixed adsorption layers of poly(ethylene imine) and sodium dodecyl sulfate at the air/liquid interface. We show that the choice of sample preparation methods can have a profound effect on the state of the interface for chemically equivalent samples. The extent of aggregation in the bulk solution on relevant time scales is affected by specific details of the polymer/surfactant mixing process, which produces varying numbers of aggregates that can become trapped in the interfacial layer, resulting in an enhanced and fluctuating ellipsometry signal. It can be beneficial to apply the surface-cleaning method of aspiration prior to physical measurements to remove trapped aggregates through the creation of a fresh interface. At low pH, the ellipsometry signal of samples with surface cleaning is remarkably constant over a factor of >500 in the bulk composition below charge equivalence, which is discussed in terms of possible adsorption mechanisms. At high pH, through observing temporal fluctuations in the ellipsometry signal of samples with surface cleaning, we reveal two important processes: there is the spontaneous adsorption of aggregates >0.2 μm in diameter into the interfacial layer, and with time there is the fusion of smaller aggregates to generate new large surface aggregates. We attribute the favorability of the adsorption and fusion processes at high pH to reduced electrostatic barriers resulting from the low surface charge density of the aggregates. It is inappropriate in this case to consider the interface to comprise a homogeneous adsorption layer that is in dynamic equilibrium with the bulk solution. Our work shows that it can be helpful to consider whether there are macroscopic particles embedded in molecular layers at the air/liquid interface for systems where there is prior knowledge of aggregation in the bulk phase.

People: Richard Campbell,  Katrin Tonigold, Marianna Yanez, Tommy Nylander, Lennart Piculell

Collaboration: Imre Varga and Róbert Mészáros at Eötvös Loránd University, Budapest; Hungary; LSS Group, Institut Laue-Langevin, Grenoble, France (present affiliation of Richard Campbell).

OMM support: Post Doc project

Contact person: Tommy Nylander