On the driving forces for complexation of methyl orange with polycations.

HYPOTHESIS: Complexation between Methyl orange and polycations involves multiple interactions dictated by molecular structure, composition (D/P), pH and ionic strength. The effect of ionic strength is considered a generic effect. By step-wise construction of complexes, we expect to gain insight in the nature of interactions and whether displacement by competing ions is a generic effect. EXPERIMENTS: We step-wise constructed complexes of methyl orange with two model polycations, whilst recording visible light spectra, size and electrophoretic mobility in buffered solution. MO organization was derived from discrete spectral changes, whereas complexes were described in terms of size and zeta-potential data. Spectral data were used to study the effect of competing ions, both potassium halides and polyanions, using a manual titration method. FINDINGS: Spectral and size data reveal a complex stoichiometry of D/P=2.2 and 4.6 for poly(ethylenimine hydrochloride)(PEI) and poly(di allyldimethyl amine hydrochloride) PDADMAC, respectively. Contrary to PEI-MO, the formation of PDADMAC-MO complexes is driven by hydrophobic rather than electrostatic interactions. Organization of PDADMAC-MO complexes also shows a strong dependency on the order of construction and polycation concentration. Displacement of MO by halides shows no effect of ion size for PEI, whereas Hofmeister series ordering was found for PDADMAC. The displacement by polyanions is shown to be charge-stoichiometric.

Polycation-sodium lauryl ether sulfate-type surfactant complexes: influence of ethylene oxide length.

Polyelectrolyte-surfactant complexes (PESC) are a class of materials which form spontaneously by self-assembly driven by electrostatic and hydrophobic interactions. PESC containing sodium lauryl ether sulfates (SLES) have found wide application in hair care products like shampoo. Typically, SLES with only one or two ethylene oxide (EO) groups are used for this application. We have studied the influence of the size of the EO block (ranging from 0 to 30 EO groups) on complexation with two model polycations: linear polyDADMAC and branched PEI. PESC size and electrostatic properties were determined during stepwise titration of buffered polycation solutions. The critical aggregation concentration (CAC) of PESC was determined by surface tension measurements and fluorescence spectroscopy. For polyDADMAC, there is no influence of the size of the EO block on the complexation behavior; the stiff polycation governs the structure formation. For PEI, it was seen that the EO block size does affect the structure of the complexes. The CAC value of the investigated complexes turns out to be rather independent of the EO block size; however, the CMC/CAC ratio decreases with increasing size of the EO block. This latter observation explains why the Lochhead-Goddard effect is most effective for small EO blocks.

Controlled block copolymer micelle formation for encapsulation of hydrophobic ingredients.

We report on the formation of polymeric micelles in water using triblock copolymers with a polyethylene glycol middle block and various hydrophobic outer blocks prepared with the precipitation method. We form micelles in a reproducible manner with a narrow size distribution. This suggests that during the formation of the micelles the system had time to form micelles under close-to-thermodynamic control. This may explain why it is possible to use an equilibrium self-consistent field theory to predict the hydrodynamic size and the loading capacity of the micelles in accordance with experimental finding. Yet, the micelles are structurally quenched as concluded from the observation of size stability in time. We demonstrate that our approach enables to prepare rather hydrophobic block copolymer micelles with tunable size and loading.