Methods and compositions for producing lipid-based cubic phase nanoparticles were first discovered in the 1990s. Since then a number of studies have been presented, but little is known about how to control key properties such as particle size, morphology, and stability of cubic phase dispersions. In the present work we give examples of how these properties can be tuned by composition and processing conditions. Importantly we show that stable particle dispersions with consistent size and structure can be produced by a simple processing scheme comprising a homogenization and heat treatment step.

Cloud point curves and conjugate coexistence curves for the quasi-binary water systems of ethyl(hydroxyethyl)cellulose (EHEC) and its hydrophobically modified analogue (HMEHEC) have been determined. The cloud point curves are remarkably independent of the polymer concentration up to as high as 20 wt % polymer, and they are clearly different from the coexistence curves. This is interpreted as an effect of the multicomponent nature of the polymer. Hydrophobic modification of EHEC lowers the cloud point by approximately 15 °C. On heating above the cloud point, EHEC redistributes to the concentrated phase, but, however, a substantial amount of the polymer remains in the dilute phase even far above the cloud point. For HMEHEC, essentially all polymer is found throughout in the concentrated phase. Addition of the anionic surfactant, sodium octylbenzenesulfonate (SOBS), affects the phase behavior. Small amounts of SOBS cause a swelling of the concentrated phase for both EHEC and HMEHEC, due to an electrostatic repulsion between polymer−surfactant aggregates. On further addition of SOBS, a dissolution of polymer from the concentrated phase is observed for EHEC. For HMEHEC, the surfactant binding swells the concentrated phase until the one-phase region is reached.

The effect of surfactants on the adsorption properties of ethyl(hydroxyethyl)cellulose (EHEC) and its hydrophobically modified analogue (HM-EHEC) at the solid−liquid interface has been studied by ellipsometry. The adsorption characteristics of EHEC and HM-EHEC without the presence of surfactants are also presented. The polar silica surface and a hydrophobized silica surface were used as substrates. On the polar silica surface, a small addition of the anionic surfactant sodium dodecyl sulfate (SDS) caused a 3- to 5-fold expansion of the preadsorbed EHEC or HM-EHEC layers, while the adsorbed amount was less influenced. On the hydrophobized silica surface, SDS could replace EHEC (>10 mM SDS), while some adsorbed HM-EHEC still could be detected well above the critical micelle concentration of SDS in the bulk (14 mM). The nonionic surfactant octa(ethylene oxide) dodecyl ether (C12E8) did not affect the adsorbed layer structure on silica, and the cationic surfactant cetyltriammonium chloride (CTAC) on hydrophobized silica showed similar effects as SDS but with a smaller magnitude. It is proposed that the adsorbed layer structure mainly is governed by polymer−surfactant interfacial interactions.

Semidilute (above the overlap concentration, c*) aqueous solutions of ethyl(hydroxyethyl)cellulose (EHEC) as a function of concentration were investigated with the surface force apparatus (SFA), and by static light scattering (SLS) and osmotic pressure measurements. The anomalous excess in scattering intensity observed in SLS experiments at small angles indicates concentration inhomogeneities in the samples, probably as a result of association of the EHEC chains. The light scattering and osmotic pressure results show that EHEC in aqueous soluttion does not behave as expected for a polymer in a good solvent. The results from the SFA experiments show that EHEC adsorbs strongly on mica. These adsorbed layers give rise to a short-range steric repulsion when the two EHEC-coated mica surfaces are brought into close contact. The most remarkable result, however, is the very long-range attractive force observed at larger surface separations. This attractive force features oscillations with a periodicity that decreases when the polymer concentration increases. The periodicity corresponded to the mesh network size determined by SLS. To our knowledge, this type of long-range attraction has not previously been reported. The origin of the force could not be explained in terms of bridging or classical depletion effects. Rather, this long-range attraction might be induced by segregation of different polymer fractions, with different surface activities, present in the EHEC solutions.

The structure and kinetics of clusters formed in the salt-induced (0.7 M NaCl) aggregation processes of polystyrene latex particles in the presence of various amounts of ethyl(hydroxyethyl)cellulose (EHEC) or the hydrophobically modified analogue (HM-EHEC) have been investigated with the aid of light scattering measurements. Under all conditions, both with and without polymer, the aggregating clusters are well described by a fractal geometry with a fractal dimension of 2.1. Three stages can be discerned in the salt-induced aggregation process of bare latex particles. In the early stage, the time evolution of the hydrodynamic radius R h can be described by a power law R h ∝ t 0.04, followed by a steep transition zone and an exponential growth at long times. In the presence of polymer, the slow cluster size growth can be described by the above power law over the whole considered time domain. The value of the power law exponent is independent of polymer concentration and hydrophobicity of the polymer. The results show that only a small amount of adsorbed polymer is needed to slow the rate of the aggregation process. The results show that both polymers adsorb to the latex particles, but the adsorption is more pronounced for the hydrophobically modified polymer.

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A long-ranged attractive force was recently detected between two mica plates immersed in a quasibinary polymer solution (Freyssingeas et al. Langmuir 1998, 14, 5877-5889). The quasi-binary polymer solution was aqueous ethyl(hydroxyethyl)cellulose (EHEC), where the EHEC had a broad polydispersity. The long-ranged attractive force in the EHEC solution could not be attributed to classical mechanisms such as depletion or bridging. In this study, we investigated if this attractive force can give rise to instability effects in mixed polymer-particle solutions. Accordingly, the effects of added particles on the phase behavior of aqueous EHEC solutions were investigated by cloud point measurements. Aqueous EHEC solutions phase separate on heating. Three different samples of EHEC were investigated, including hydrophobically modified EHEC. As colloidal particles, silica and polystyrene latex were used. The dispersed colloidal particles lowered the cloud point temperature at low polymer concentrations for all EHEC-particle combinations. This particle-induced phase separation is discussed in terms of surface effects.

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