Lennart Piculell

Association and segregation in aqueous polymer/polymer, polymer/surfactant, and surfactant/surfactant mixtures: similarities and differences

Abstract Recent experimental findings on the phase behaviour of aqueous polymer/surfactant mixtures are reviewed and compared with the phase behaviour of “analogous” polymer/polymer or surfactant/surfactant mixtures, which is also reviewed. Polyelectrolyte effects are given special consideration. Attention is drawn to the polymer aspect of a surfactant aggregate, and, also, to the surfactant aspect of an hydrophobe-modified polymer. It is proposed that a consideration of these aspects should be helpful in predicting the phase behaviour of polymer/surfactant mixtures.

Binding of surfactants to hydrophobically modified polymers

Recent progress in the understanding of the binding of surfactants to hydrophobically modified polymers (HMP), and the consequences of such binding, is reviewed. HMP are water-soluble polymers onto which low proportions of hydrophobic sidechains (hydrophobes) have been grafted. In an aqueous environment, the HMP hydrophobes associate among themselves and with added surfactant molecules into micelle-like aggregates. An HMP may therefore be considered as a ‘modified surfactant’, and the binding of surfactants to HMP is analogous to the mixed micellisation in mixed surfactant solutions. The binding isotherm gives the concentration of free (monomeric) surfactant and the stoichiometry of the HMP/surfactant complex at different total compositions. In mixtures involving ionic surfactants, it is found that the free surfactant often dominates, and gives important contributions to the ionic strength. Characteristic properties of HMP/surfactant mixtures may be related to stoichiometries of the mixed complexes. Thus, the maximum in solution viscosity, which is commonly found in HMP/surfactant mixtures, occurs at a similar hydrophobe stoichiometry (ratio of bound surfactant to HMP hydrophobe) for many different systems, although the total concentrations of surfactant at the maximum may vary by orders of magnitude, depending on the surfactant cmc. The solubility of a complex of oppositely charged HMP and surfactant is related to the charge stoichiometry of the complex. The phase separation/redissolution phenomena occurring in the bulk solution influence the HMP adsorption to surfaces and the forces between surfaces with adsorbed HMP.

Characterisation of kappa- and iota-carrageenan coils and helices by MALLS/GPC

Abstract Iota- and kappa-carrageenans were degraded by ultrasonication to obtain samples of different molecular weights. The coil and the helix conformations of the degraded and non-degraded samples were investigated by multi-angle laser light scattering (MALLS) coupled to gel permeation chromatography (GPC), and the weight-average molecular weight, Mw, as well as the z-average radii of gyration, Rg,z, was obtained for all samples in both conformations. The conformational state was controlled by temperature and/or electrolyte content. All measurements were done under non-aggregating conditions, and an approximate doubling of the weight-average molecular weight was seen for both iota- and kappa-carrageenan helices over the entire molecular weight range studied, implying a double-helix formation. Zimm plots for non-degraded kappa-carrageenan coils and helices confirmed the approximate doubling of the molecular weight for the helix conformation.

Understanding and Exploiting the Phase Behavior of Mixtures of Oppositely Charged Polymers and Surfactants in Water

Complexes of oppositely charged polymers and surfactants (OCPS) in water come in many varieties, including liquid-crystalline materials, soluble complexes, structured nanoparticles, and water-insoluble surface layers. The range of available structures and properties increases even further with the addition of other amphiphilic substances that may enter, or even dissolve, the complexes, depending on the nature of the additive. Simple operations may change the properties of OCPS systems dramatically. For instance, dilution with water can induce a phase separation in an initially stable OCPS solution. More complicated processes, involving chemical reactions, can be used to either create or disintegrate OCPS particles or surface layers. The richness of their properties has made OCPS mixtures ubiquitous in everyday household products, such as shampoos and laundry detergents, and also attractive ingredients in the design of new types of responsive particles, surfaces, and delivery agents of potential use in future applications. A challenge for the rational design of an OCPS system is, however, to obtain a good fundamental understanding of how to select molecular shapes and sizes and how to tune the hydrophobic and electrostatic interactions such that the desired properties are obtained. Recent studies of OCPS phase equilibria, using a strategy where the minimum number of components is always used to address a particular question, have brought out general rules and trends that can be used for such a rational design. Those fundamental studies are reviewed here, together with more application-oriented studies where fundamental learning has been put to use.

Effects of small amounts of kappa-carrageenan on the rheology of aqueous iota-carrageenan

Abstract The temperature dependence of the rheology (shear storage and loss moduli) of two ion forms (sodium and potassium) of aqueous iota-carrageenan was investigated in the presence and in the absence of small fractions (≤10%) of added kappa-carrageenan. Both native iota-carrageenan (which already contains kappa-carrageenan as an impurity) and a sample purified by kappa-carrageenase was investigated. In either ion form, pure iota-carrageenan forms a weak gel as a consequence of its conformational transition, and the gel becomes stronger as the helical content increases. No marked ion specificity is seen in the rheology of pure iota-carrageenan. Small fractions of kappa-carrageenan, whether added or present as impurities, have no effect on the rheology of the sodium form but a large effect on the modulus of the potassium form of iota-carrageenan. The storage moduli of the potassium iota/kappa gels fall within the limits expected for a phase-separated gel structure.

Rheology of kappa-carrageenan in mixtures of sodium and cesium iodide : two types of gels

Abstract Recent studies on dilute solutions (Borgstrom et al. (1996), Int. J. Biol. Macromol. 18 , 223) have shown that kappa-carrageenan helices associate into superhelical rigid rods in mixed 0.1 M aqueous solutions of NaI and CsI above a critical mole fraction ( x Cs = 0.4) of Cs. This work concerns the temperature-dependent rheology of more concentrated systems in mixed and pure solutions of the same salts. Gels with low moduli were even found in NaI alone, although this salt is known to impede the gelation of kappa-carrageenan, but only above 0.9% (w/w) of carrageenan. These gels were reminiscent of iota-carrageenan gels in two respects: the (low) magnitude of the shear storage modulus ( G ′), and the absence of hysteresis in the sol-gel transition. On the other hand, both the threshold concentration for gelation and the ratio between the loss and storage moduli were substantially higher for the kappa-carrageenan gels in NaI. In mixed solutions of CsI and NaI, two types of kappa-carrageenan gels could be distinguished, depending on the cesium content. The transition occurred at x Cs = 0.4, as in the previous studies on dilute solutions. Below x Cs = 0.4, the gels were similar to those in NaI alone. Above x Cs = 0.4, the gels were similar to ‘conventional’ kappa-carrageenan gels, formed in salts such as KC1: a pronounced thermal hysteresis appeared in the sol-gel transition, the gels showed tendencies for syneresis, and G ′ increased dramatically with increasing cesium content.

Conformational change and aggregation of κ‐carrageenan studied by flow field‐flow fractionation and multiangle light scattering

The relatively novel combination of flow field-flow fractionation (FFF) and multiangle light scattering (MALS) was employed to study a nondegraded κ-carrageenan in different 0.1M salt solutions. The applicability of the technique was tested, and the effects of salt type and salt composition on the molar mass and radius of gyration were studied. A conformational ordering was induced at room temperature by switching the solvent from 0.1M NaCl (coil form) to 0.1M NaI (helix form). An approximate doubling of the average molar mass and an increase in radius of gyration was then observed, in agreement with results obtained previously using size exclusion chromatography–MALS. This increase in size was attributed to conformational ordering and to the formation of double helices. Severe aggregation was observed above 40% CsI in the 0.1M mixed salt solution of CsI and NaI. This was ascribed to the association of helices into large aggregates. For these large associates, having molar masses of several millions, a reversal of the elution order in flow FFF was detected.

Mixed solutions of surfactant and hydrophobically modified polymer

The properties of mixed solutions of surfactants and hydrophobically modified polymers are reviewed, with a special emphasis on molecular interpretations. Aspects covered include self-assembly and mixed aggregation, phase behaviour, rheology, and interfacial behaviour. Copyright

On the structure of aggregated kappa-carrageenan helices. A study by cryo-TEM, optical rotation and viscometry

Cryo-transmission electron microscopy (cryo-TEM), viscometry and optical rotation have been used to study the microstructure of dilute solutions of helical kappa-carrageenan under varied electrolyte composition (0.1 M of NaI, CsI and mixtures of the two salts). Microfibers (300-400 nm in length) were observed above a critical cesium content, while below that no microstructure was seen in the micrographs. A drastic increase in the specific viscosity and an onset of thermal hysteresis in the optical rotation above the same critical cesium content provide clear evidence of aggregate formation. This leads to the conclusion that the observed microfibers are aggregates consisting of several helices. At high proportions of cesium, further aggregation of the microfibers was observed by cryo-TEM.

Surface Deposition and Phase Behavior of Oppositely Charged Polyion/Surfactant Ion Complexes. 1. Cationic Guar versus Cationic Hydroxyethylcellulose in Mixtures with Anionic Surfactants

Mixtures of cationic guar (cat-guar) or cationic hydroxyethylcellulose (cat-HEC) with the anionic surfactants sodium dodecyl sulfate or sodium lauryl ether-3 sulfate have been investigated by a wide range of complementary techniques (phase studies, turbidity measurements, dynamic light scattering, gel-swelling experiments, and in situ null ellipsometry), with the following objectives in mind: (1) to establish the relationship between the bulk phase behavior (precipitation and redissolution) of the polyion/surfactant ion complexes and formation/deposition of such complexes at silica surfaces and (2) to obtain molecular interpretations of the large, previously unresolved, quantitative differences between the various investigated mixtures. There were clear similarities, for each studied system, between the bulk phase behavior, gel swelling, and surface deposition on increasing surfactant concentration. This is because all phenomena reflect the polyion/surfactant ion binding isotherm: an initial binding step at a low critical association concentration (cac) of the surfactant and a second more-or-less cooperative binding step beginning at a second cac, the cac(2). The details of the interactions are system-specific, however, and cat-guar/surfactant mixtures generally had larger precipitation regions and gave rise to larger adsorbed amounts on silica compared to mixtures with cat-HEC of a similar charge density. The observed quantitative differences are attributed to a difference in the hydrophobicity of the polyions. For cat-guar, the comparatively weak hydrophobic polyion/surfactant attraction is seen as a very gradual binding commencing at the cac(2) and continuing past the bulk critical micelle concentration of the surfactant, resulting in an unusually large phase-separation region. For cat-HEC, the dissolution of the precipitate takes place at lower surfactant concentrations because of a stronger hydrophobic interaction between the surfactant and the polyion. The results have implications for the successful design of oppositely charged polyelectrolyte/surfactant formulations for surface deposition applications.