Hydrophobically modified polyelectrolytes : synthesis, properties and interactions with surfactants

A.C. Nieuwkerk

Research output: Thesisinternal PhD, WU

Abstract

Hydrophobically modified polyelectrolytes can form micelle-like aggregates, so-called microdomains, in aqueous solution. The hydrophobic side chains constitute the apolar inner part of these microdomains and the hydrophilic groups on the polyelectrolyte backbone are at the surface of the microdomains. The microdomain formation is mainly determined by the polyelectrolyte charge density, which can be varied by changing the pH of the solution, and the length of the hydrophobic side chains. The flexibility of the backbone and the structure of the side chains are also important.<p>From literature it is known that the interaction between hydrophobically modified polyelectrolytes, also called polysoaps, and surfactants strongly depends on the charge density on the polyelectrolyte and the length of the hydrophobic side chains. These factors also determine the microdomain formation. The interaction between <em></em> surfactants and polyelectrolytes in aqueous solution is thus highly dependent on the presence of microdomains. The research described in this thesis was aimed at determining the effects of charge density and hydrophobicity of polyelectrolytes on the interaction with surfactants.<CENTER><img src="/wda/abstracts/i2394_1.gif" height="220" width="400"/></CENTER>Copolymers from maleic anhydride and alkyl vinyl ethers ( <strong>I-n</strong> ) have been synthesised by radical polyrnerisation. These polymers were hydrolysed to polyelectrolytes <strong>II-n</strong> By reaction of <strong>I-n</strong> with 2- aminoethanesulfonic acid polyelectrolytes <strong>III-n</strong> were obtained. The alkyl vinyl ethers are substituted with rigid aromatic units, (cyanobiphenylyl)oxy units, which can induce the formation of liquid crystalline phases. Polymers <strong>I-n</strong> and <strong>II-n</strong> indeed form these phases. The isotropisation temperature of II-n is higher than for <strong>I-n</strong> due to the more flexible backbone of <strong>II-n</strong> . The enthalpy gain associated with the formation of the liquid crystalline phase increases with increasing spacer length and is larger for <strong>II-n</strong> than for <strong>I-n</strong> at comparable spacer length (Chapter 2).<p>The (cyanobiphenylyl)oxy units can also be used as chromophores. By the use of spectroscopic techniques one can study the microdomain formation of the polyelectrolytes and the interaction with surfactants. The UV spectra of the polyelectrolytes depend on the pH of the solution, thus on the charge density of the polyelectrolyte, and on the hydrophobicity of the side chain. When the chromophores aggregate a blue shift of the absorption maximum of the (cyanobiphenylyl)oxy chromophores as compared to the monomeric absorption maximum is observed. So-called π <strong>-π</strong> stacking interactions between parallel oriented aggregated chromophores are responsible for the blue shift. The extent of the blue shift is indicative for the conformation of the polyelectrolyte. UV measurements show that microdomains are formed by the polyelectrolytes <strong>II-n</strong> and <strong>III-n</strong> at 2 &lt; pH &lt; 13, and that more compact microdomains. are formed at lower pH.<p>Dynamic light scattering experiments confirm the formation of large aggregates in which the polyelectrolyte chains are highly entangled. Generally, the aggregates swell upon increasing the charge density, due to increased electrostatic repulsion on the polyelectrolyte backbone, and upon decreasing the side chain length (Chapter 3).<p>Oppositely charged surfactant molecules bind strongly to polyelectrolytes <strong>II-n</strong> and <strong>III-n</strong> in aqueous solution. Measurements using a dodecyltrimethylammonium (DTA +) selective electrode shows that DTA+ binds strongly and noncooperatively to both labelled and nonlabelled polyelectrolytes. The surfactant molecules bind individually to the microdomains formed by a polyelectrolyte, indicating the formation of mixed micellar aggregates.<p>Surface tension measurements also show the very strong binding between the polyelectrolytes and DTAB which results in a synergistic lowering of the surface tension. The hydrophilicity of the polyelectrolytes is lowered in the presence of DTAB which results in a decrease in water solubility and an increase in the amount of polyelectrolyte-DTA+ complex at the airsolution interface (Chapter 4).<p>By UV spectroscopy it is shown that within the microdomains the (cyanobiphenylyl)oxy chromophores are aggregated which results in a blue shift of the absorption maximum. Upon addition of surfactant molecules the blue shift decreases because the surfactant molecules penetrate between the hydrophobic side chains of the polyelectrolyte. The decrease in blue shift depends on the charge of the surfactant headgroup and on its hydrophobicity. The negatively charged sodium dodecyl sulfate (SDS) shows no influence on the wavelength of the absorption maximum and does not penetrate the microdomains. A non-ionic surfactant like polyoxyethylene(4)lauryl ether (Brij 30) interacts with the polyelectrolytes by purely hydrophobic interactions which results in a small decrease of the blue shift of the (cyanobiphenylyl)oxy chromophores. In addition to the results from the DTA+ selective electrode and the surface tension measurements, the UV measurements also show the very strong interaction between polyelectrolytes and oppositely charged surfactants. Addition of a cationically charged surfactant causes the largest decrease in blue shift, thus the largest destacking of the (cvanobiphenylyl)oxy chromophores. The hydrophobicity of the surfactant is also of importance as is clear from the larger decrease in blue shift observed for the addition of hexadecyltrimethyl ammonium bromide as compared to dodecyltrimethylammonium bromide. Upon increasing the surfactant concentration the blue shifts decrease gradually indicating noncooperative binding between the surfactants and hydrophobically modified polyelectrolytes. This agrees with the results obtained from measurements with the DTA+ selective electrode (Chapter 4).<CENTER><img src="/wda/abstracts/i2394_2.gif" height="132" width="400"/></CENTER>The addition of surfactant molecules which are labelled with a chromophore to a polyelectrolyte solution yields extra information on the interactions. Therefore new surfactants carrying azobenzene chromophores, <strong>X-Cn-Br,</strong> were synthesised. The azobenzene units are substituted at the 4'-position with a cyano, methoxy or fluoro groulp Initially, the interaction between these surfactants and nonlabelled polyelectrolytes was studied. The addition of <strong>X-Cn-Br</strong> to poly(maleic acid-co-n-butyl vinyl ether) results immediately in a maximum blue shift of the absorption maximum of the azobenzene groups. This indicates that the surfactants bind cooperatively to poly(maleic acid- <em>co-n</em> -butyl vinyl ether) allowing direct stacking between the azobenzene units. At the pH values used, poly(maleic acid- <em>co-n</em> -butyl vinyl ether) is in its extended conformation. When surfactant is added, the surfactant molecules form micelle-like aggregates surrounded by polyelectrolyte, and the counterions are disposed into solution. Upon increasing the side chain length of the polyelectrolyte the cooperatively decreases as is clear from the fact that the maximum blue shift is not immediately attained. The cooperativity also decreases upon decreasing the charge density on the polyelectrolyte. Both changes induce the formation of microdomains by the polyelectrolyte. When surfactants bind to these microdomains, mixed micelles are formed in which the surfactants sometimes cluster.<p>By labelling both polyelectrolyte and surfactant, the effects of interaction between polyelectrolytes and surfactants on the surfactant aggregation and on the disruption of the polyelectrolyte microdomains can be monitored. The measurements show that both the polyelectrolyte charge density and the length of the side chains influence the amount of interaction. These factors not only influence the electrostatic and hydrophobic attraction of surfactants to the polyelectrolytes, but they also determine the compactness of the microdomains. The spacer length of the surfactants and their terminal group seem to influence mainly the interaction between surfactant molecules (Chapter 4 and 5).<p>By use of the Langmuir technique the behaviour of polyelectrolytes <strong>I-n</strong> and <strong>II-n</strong> at the airwater interface is studied. Immediately after spreading both polymers form homogeneous layers on the subphase. The lift-off area per repeating unit indicates that the cyano groups of the chromophores of <strong>I-11</strong> and <strong>I-12</strong> are immersed into the subphase. For <strong>I-6, I-8</strong> and <strong>I - 10</strong> the side chains are oriented randomly into the air. Polyelectrolytes <strong>II-n</strong> have a more flexible backbone than <strong>I-n</strong> which allows the (cyanobiphenylyl)oxy chromophores of <strong>II-11</strong> and <strong>II-12</strong> to assume a flat orientation on the subphase. This flat orientation is hindered for the shorter chained polyelectrolytes, but the cyano groups of <strong>I-6, I-8</strong> and <strong>I-10</strong> are thought to interact with the subphase. Polymers <strong>II-n</strong> all show a plateau in their π-A isotherms. In this plateau a triple layer is formed by folding a polymeric double layer onto a monolayer. At a molecular area of one third of the onset of the plateau the pressure starts to rise again, after which the triple layer collapses. The triple layer formation is confirmed by the semi -reversibility of compression-expansion experiments and Brewster angle microscopy.<p>The addition of both positively and negatively charged surfactants results in an increase in the lift-off area of <strong>II-n</strong> . This results from electrostatic interactions between the polyelectrolyte backbone and the surfactant headgroups, and hydrophobic interactions between the surfactant tails and polyelectrolyte side chains.<p>Monolayers of polyelectrolytes <strong>II-n</strong> can be transferred from pure water and from a I mM DTAB solution onto hydrophilic quartz or glass in a Z-type fashion. However, both UV spectroscopy and Second Harmonic Generation measurements show there is no overall order within the transferred layers (Chapter 6).<p>Besides the addition of the azobenzene substituted surfactants to solutions containing polyelectrolytes, their thermotropic and lyotropic phase behaviour in the absence and presence of SDS is also studied. The terminal substituent on the azobenzene chromophores and the counterion, bromide or dodecyl sulfate, influence the physical properties. Smectic A phases are found for <strong>CN- Cn-Br, F-C12-Br, CN-Cn-DS, MeO-C10--DS</strong> and <strong>F-Cn-DS.</strong> The formation of myelin structures of <strong>CN-Cn-Br</strong> and <strong>MeO-Cn-Br</strong> in water is seen by optical microscopy. These myelins transform into giant vesicles which are, however, not very stable and crystallise at room temperature. Vesicles formed in the presence of SDS, from so-called ion pair amphiphiles, are more stable due to the electrostatic and hydrophobic interactions. UV spectra of the ion pair amphiphiles display a blue shift of the azobenzene chromophores as compared to the monomeric chromophores, which indicates that the chromophores are parallelly aggregated within these mixed vesicles (Chapter 7).
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Sudhölter, E.J.R., Promotor, External person
  • Marcelis, A.T.M., Promotor
Award date20 Feb 1998
Place of PublicationS.l.
Publisher
Print ISBNs9789054858126
Publication statusPublished - 1998

Keywords

  • detergents
  • surfactants
  • hydrophobicity
  • electrolytes

Fingerprint Dive into the research topics of 'Hydrophobically modified polyelectrolytes : synthesis, properties and interactions with surfactants'. Together they form a unique fingerprint.

Cite this