Some cellulosic derivatives are soluble in cold water, but above a clouding temperature they form a polymer-rich phase which progressively expels water as the temperature is raised. This phase separation may be modified by adding surfactants. Over a certain range of compositions and temperatures, some of these systems form thermoreversible gels. Small angle neutron scattering experiments are reported for the gels made of ethyl hydroxyethyl cellulose (type CP-30) and cationic surfactants. The results show that the surfactants cause a dissociation of the polymer-rich phase into smaller lumps, with sizes on the order of 500 Å. Each lump is formed by the loose association of polymer strands belonging to many macromolecules and is covered by surfactant. The mechanical rigidity of the gel originates from the association of the macromolecules through these lumps.

High-sensitivity differential scanning calorimetry (HSDSC) experiments and phase studies have been combined in order to investigate thermal events, possibly related to thermoreversible gelation, in semidilute aqueous systems of ethyl(hydroxyethyl)cellulose (EHEC) and sodium dodecyl sulfate (SDS) in the temperature range 20-80 °C. The phase behavior of two gelling EHEC samples in 0.50 and 1.00 wt % aqueous solution containing varying amounts of SDS has been characterized. Both samples formed thermoreversible gels in 1.00% solutions at SDS concentrations above 2 mmol kg -1 , while in 0.50 wt % solution only one sample formed a gel and now in a restricted region of temperature and SDS concentration. Large, well-defined endothermic peaks were seen in DSC traces of the gelling EHEC samples in pure water. The peaks appeared at about the clouding temperature (CP) of the solutions. Surfactant-free solution of a more hydrophilic (nongelling) EHEC only gave a small heat capacity shift at CP. The transition peaks observed for the gelling EHEC in pure water are considered to indicate that polymer association through interaction between the hydrophobic segments precedes the liquid-liquid phase separation. The addition of SDS to solutions of the gelling EHEC significantly changed the features of the DSC peaks, which indicates that SDS changes the association behavior of the polymer as the clouding temperature is approached. At certain compositions the phase separation is arrested and instead a gel forms.

The release of timolol maleate, a drug for treatment of glaucoma, from cubic liquid crystals consisting of charged and uncharged lipids was investigated in vitro. It was shown that the release of drug was dependent on the ionic strength of the receptor medium and the relative amount of a charged phospholipid in the cubic phase. In the absence of charged lipids, the release of timolol was complete, irrespective of the medium used, i.e. distilled water or saline solution. In the presence of a negatively charged phospholipid, diacylphosphatidylglycerol, the release of drug from the cubic phase was incomplete when distilled water was the receptor medium. The residual drug was dependent on the amount of charged lipid in the cubic phase. In saline solution, on the other hand, all of the drug was released. The results are interpreted as an effect of electrostatic attraction between the positively charged drug with the negatively charged bilayer surface, which is screened in the presence of saline. The possible influence of the cubic geometry is discussed as well. This work shows that lipids offer many possibilities in the formulation of sustained drug release systems.

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