Network Formation of Catanionic Vesicles and Oppositely Charged Polyelectrolytes. Effect of Polymer Charge Density and Hydrophobic Modification

In nonequimolar solutions of a cationic and an anionic surfactant, vesicles bearing a net charge can be spontaneously formed and apparently exist as thermodynamically stable aggregates. These vesicles can associate strongly with polymers in solution by means of hydrophobic and/or electrostatic interactions. In the current work, we have investigated the rheological and microstructural properties of mixtures of cationic polyelectrolytes and net anionic sodium dodecyl sulfate/didodecyldimethylammonium bromide vesicles. The polyelectrolytes consist of two cationic cellulose derivatives with different charge densities; the lowest charge density polymer contains also hydrophobic grafts, with the number of charges equal to the number of grafts. For both systems, polymer−vesicle association leads to a major increase in viscosity and to gel-like behavior, but the viscosity effects are more pronounced for the less charged, hydrophobically modified polymer. Evaluation of the frequency dependence of the storage and loss moduli for the two systems shows further differences in behavior: while the more long-lived cross-links occur for the more highly charged hydrophilic polymer, the number of cross-links is higher for the hydrophobically modified polymer. Microstructure studies by cryogenic transmission electron microscopy indicate that the two polymers affect the vesicle stability in different ways. With the hydrophobically modified polymer, the aggregates remain largely in the form of globular vesicles and faceted vesicles (polygon-shaped vesicles with largely planar regions). For the hydrophilic polycation, on the other hand, the surfactant aggregate structure is more extensively modified: first, the vesicles change from a globular to a faceted shape; second, there is opening of the bilayers leading to holey vesicles and ultimately to considerable vesicle disruption leading to planar bilayer, disklike aggregates. The faceted shape is tentatively attributed to a crystallization of the surfactant film in the vesicles. It is inferred that a hydrophobically modified polyion with relatively low charge density can better stabilize vesicles due to formation of molecularly mixed aggregates, while a hydrophilic polyion with relatively high charge density associates so strongly to the surfactant films, due to strong electrostatic interactions, that the vesicles are more perturbed and even disrupted

Solution properties of a hydrophobically modified polymer

The influence of hydrophobic modification on solution properties of water-soluble polymers in the semi-dilute regime have been studied. The nonionic ethyl(hydroxyethyl)cellulose (EHEC) and its hydrophobically modified analogue (HM-EHEC) are used as model substances. Interactions between the polymer molecules in solution and a third component are investigated, and in particular the influence of the surfactant sodium dodecylsulfate (SDS). The polymer/SDS complex formation is investigated by using several techniques (phase behaviour, rheology, calorimetry, DS- sensitive electrode, NMR, and light scattering and fluorescence techniques). This combination of methods gives thermodynamic information on the complex formation, as well as information about the dynamics of the polymer/surfactant complex. In a binary, semi-dilute, HM-EHEC solution the hydrophobic tails associate in clusters with an average of ten tails. These 'pre-formed' micelles act as nucleation sites for other molecules that contain hydrophobic moieties. The number of mixed micelles, and the strengths of the inter polymeric bonds which they give rise to, control both microscopic properties (as chain motions) and macroscopic behaviour (as rheology and phase behaviour). Experimental phase studies with hexanol as a third component were compared with model calculations in a modified Flory-Huggins approach. By assuming that hydrophobic modification causes enhanced association with the polymer semi-quantitative agreement was found. The associative phase separation that is usually observed in mixtures of two oppositely charged polyelectrolytes is effectively prevented by hydrophobic modification of both polyelectrolytes; a highly viscous phase is formed

Effect of Hydrophobic Modification of a Nonionic Cellulose Derivative on the Interaction with Surfactants. Phase Behaviour and Association.K. Thuresson, B. Lindman,The Journal of Physical Chemistry, submitted.

Phase separation studies have been carried out concerning the addition of different surfactants to systems containing either ethyl(hydroxyethyl)cellulose (EHEC) or the hydrophobically modified analogue (HM-EHEC). The polymer concentration was kept constant at 1 g polymer/100 g of water (1 w/w%), while the surfactant concentration was varied. In the polymer/sodium dodecyl sulfate (SDS) systems, which were studied in more detail, phase diagrams were obtained in the presence of various concentrations of inert salt. Moreover, the phase behavior of the HM-EHEC/SDS system showed no marked changes by purification of the surfactant or by contamination with decanol. It was shown that the SDS binding to EHEC was cooperative and could be described in the framework of a closed association model, while for HM-EHEC a two-step binding model had to be used to get a proper description of the binding. In the noncooperative part of the HM-EHEC binding isotherm, the binding showed similarities to the adsorption of SDS on a hydrophobic surface and could be described by a Langmuir adsorption model. From the binding isotherms for SDS, binding isotherms for ionic surfactants with varying chain length could be calculated from simple assumptions. These isotherms give a basis for the interpretation of the phase diagrams. For the ionic surfactants, the phase behavior could be rationalized by considering the polymer/surfactant complex to possess polyelectrolyte characteristics. The observations were discussed in terms of an attractive hydrophobic interaction and a repulsive electrostatic force. Addition of nonionic surfactants was discussed by observing that generally a pair of a nonionic polymer and a nonionic surfactant segregates. However, with HM-EHEC there is an extra attractive hydrophobic interaction due to the presence of polymer hydrophobic tails which can serve as nucleation sites for the surfactants

Diffusion of nutrients molecules and model drug carriers through mucin layer investigated by magnetic resonance imaging with chemical shift resolution

Magnetic resonance imaging (MRI) with chemical shift resolution is a recent extension of MRI and it provides information about species resolved molecular transport on the macroscopic scale in complex systems. In this contribution, we show that by using this novel method, one can predict the behavior of drug and food molecules when they are in contact with the mucosal layer in the gastrointestinal tract. For the first time, the transport properties of a mixture of nutrients (i.e., a solution of ethanol and glucose) and of a model drug carrier (i.e., an equimolar solution of cationic and nonionic surfactants) through a mucin gel have been investigated. This study shows that transport properties of the diffusing molecules through a mucin gel are dependent on their size and physicochemical properties. In addition, we show that mucin gel acts as an efficient selective barrier. It favors the disintegration of mixed micelles of nonionic and cationic surfactants by stopping the diffusion of cationic surfactants with slightly affecting the diffusion of the nonionic surfactants. (c) 2006 Wiley-Liss, Inc

Effect of Hydrophobic Modification on Phase Behaviour and Rheology in Mixtures of Oppositely Charged Polyelectrolytes

Phase behavior and rheology of polymer mixtures comprising aqueous solutions of oppositely charged polyelectrolytes are investigated. Emphasis is put on the effects of hydrophobic modification of the polymers and addition of salt. The associative phase separation usually observed when mixing oppositely charged polyelectrolytes is effectively prevented over a large miscibility region for the hydrophobically modified polymers. Also, in the extended one-phase region, the viscosity is 3-4 orders of magnitude higher for mixed polyelectrolyte systems compared to that observed for either one of the polymers. Addition of ordinary electrolytes to the mixture decreases the viscosity strongly, and at higher electrolyte contents a phase separation is induced. A mechanism explaining the observations is proposed

A rheological investigation of the association between a non-ionic microemulsion and hydrophobically modified PEG. Influence of polymer architecture

Hydrophobically modified polymers (HM-P) typically behave as thickeners in a wide range of systems. The thickening effect in an aqueous solution of this kind of polymer depends on intermolecular hydrophobic associations and also on chain entanglements if the polymer concentration is significantly above the overlap concentration. In the present investigation a rather short end-capped polymer has been investigated at concentrations that are significantly below the overlap concentration. Despite the rather low polymer concentration, polymer chains were connected into a three-dimensional network by using microemulsion droplets as cross-linking points. The simple structure of the solution simplifies interpretations of results since chain entanglements can be expected to be of low importance and only intermolecular hydrophobic associations have to be considered. In particular the rheological response is in most cases well characterized by one single relaxation time and, then, the solution can be rationalized within the framework of the Maxwell model. We have found that the length of the polymer chain's hydrophobic end-groups, as well as the temperature, have a large influence on dynamics of the system, while the length of the hydrophilic mid-block has a relatively small significance. On the other hand, the connectivity in the system depends critically on the microemulsion concentration. Thus, a maximum was found in viscosity as a function of volume fraction, interpreted as being due to a decrease in crosslink lifetime.http://www.sciencedirect.com/science/article/B6TFR-47CBBD1-8/1/f0f736ee348a7db93770b37c030c67f

Interactions between Drug Delivery Particles and Mucin in Solution and at Interfaces

Cubosome particles were produced by fragmenting a cubic crystalline phase of glycerol monooleate and water in the presence of a stabilizing poly(ethylene oxide)-based polymer. The aim of our investigation was to study the interaction between these particles and mucin to gain information on how they would perform as a vehicle for mucosal drug delivery. Particle electrophoresis was used to investigate the interactions between particles and mucin in solution, and ellipsometry was utilized to study the interactions between particles and mucin-coated silica surfaces. The interaction studies were performed at relevant physiological conditions, and the pH and ionic strength were varied to gain more information about the driving forces for the interaction. The results from electrophoretic measurements showed that mucin in solution adsorbed to the particles at pH 4, whereas at pH 6 no clear interaction was detected. From ellipsometric measurements it was evident that the particles adsorb reversibly to a mucin-coated silica surface at pH 4, while no adsorption of particles could be detected at pH 6. The overall conclusion is that the interaction between these particles and mucin is weak and pH-dependent. These findings are in agreement with other investigations of the interactions between mucin and poly(ethylene oxide) chains

A rheological investigation of the complex formation between hydrophobically modified ethyl (hydroxy ethyl) cellulose and cyclodextrin

The thickening effect of a hydrophobically modified polymer in an aqueous solution is dependent on intermolecular hydrophobic associations, and if the polymer concentration is significantly above the overlap concentration also on chain entanglements. In this investigation we have added different cyclodextrins (CD) in order to decouple hydrophobic polymer-polymer associations via inclusion complex formation with the polymer hydrophobic tails. Both size and hydrophobicity of the cavity of the CD-molecules were found to have an effect on the process. In addition, the influence of chemical structure of the polymer hydrophobic tails was investigated. Either a linear C-14-chain or a more bulky nonylphenol group was used. The viscosity as a function of CD-concentration first decreased strongly, and then attained a constant value. At excess CD the viscosity became virtually the same as in a solution of the unmodified parent polymer, provided that complex formation was not sterically bindered. This suggests that all hydrophobic links, originating from the hydrophobic modification process, which influence the theology could be deactivated. On the other hand, with combinations where the complex formation was hindered to a certain degree the initial decrease was less accentuated, and also, the viscosity leveled out at a significantly higher value. In an attempt to rationalize the data a simple model based on the assumption that each complex formed deactivates one theologically active link was used. In combination with the Langmuir adsorption model the number of complexes as a function of CD concentration could be obtained. This model also gave a value of the complex formation constant. Furthermore, in solutions where all hydrophobic links could be deactivated the results from the model suggested that all polymer hydrophobic tails were originally active in forming the network. (C) 2002 Elsevier Science Ltd. All rights reserved

Hydrophobically modified ethyl(hydroxyethyl)cellulose as stabilizer and emulsifying agent in macroemulsions

Stability, droplet diameter and viscosity of o/w macroemulsions of olive oil stabilized with lecithin from soybean and different types of polymers have been investigated. The stabilizing polymers were non-ionic cellulose ethers, both with and without hydrophobic groups grafted onto the polymer backbone. Emulsions without polymers showed essentially no stability against creaming. Unmodified polymers provided limited stability, while hydrophobically modified versions of the polymers improved the stability dramatically. The viscosity of emulsions stabilized by hydrophobically modified polymers was significantly increased compared with aqueous solutions of the polymers alone, suggesting network formation including both polymers and emulsion droplets. It was also found that dilution had virtually no effect on the droplet diameters of emulsions stabilized with hydrophobically modified cellulose ethers and that coalescence was effectively opposed. (c) 2005 Elsevier B.V. All rights reserved