Cellulose ethers interactions with surfactants

Incorporation of long-chain hydrocarbon hydrophobes into a cellulose ether backbone leads to an interesting new class of polymeric surfactants. Their enhanced solution viscosity can be explained in terms of intermolecular associations via the hydrophobe moieties. Entropic forces cause the polymer hydrophobes to cluster to minimize the disruption of water structure. The same thermodynamic principles that are used to explain the micellization of surfactants can be applied to explain the solution behavior of HMHEC. HMHECs interact with surfactants that modify their solution viscosities. The chemical nature and the concentration of the surfactant dictate its effect on HMHEC solution behavior. The unique rheological properties of HMHEC can be exploited to meet industrial demands for specific formulations and applications. 

Hydroxypropylmethylcellulose (HPMC) is one of the many mixed ethers of cellulose. It is prepared by reactions of alkali cellulose with methyl chloride and propylene oxide in a slurry process. Reaction conditions may be varied to control compositions despite the greater reactivity of methyl chloride. HPMC is an extremely effective viscosifier compared to conventional cellulose ethers. Its mi-croheterogeneous nature, phase behavior, and interaction with surfactants allow use in food, pharmaceutical, and coatings applications (72,87). 

Other Vinyl Derivatives. PVP is a nonionic surfactant used in 3% to 5% concentrations to increase viscosity of solutions. Although it exhibits surface-active properties similar to the cellulose ethers, PVP appears to have less abiUty to lower the interfacial tension at a water-oil interface. Nevertheless, in contrast to the cellulose ethers, PVP appears capable of forming hydrophilic coatings in the form of adsorbed layers. Because conjunctival mucin is believed to interact with the ocular surfece to form an adsorbing surface for aqueous tears, the formation by artificial means of a hydrophilic layer that would mimic conjimctival mucin (mucomimetic) appears to be clinically desirable. Both mucin- and aqueous-deficient dry eyes would benefit, because the wetting ability of the corneal surfece would be enhanced. 

Polysaccharides with Surfactant Micelles. Consider a solution of a fairly hydrophobic polysaccharide, such as a cellulose ether. The hydrophobic groups cause a weak attractive interaction, leading to a somewhat increased viscosity at low shear rates. If an anionic small-molecule surfactant is added, say SDS (sodium dodecyl sulfate), at a concentration above the CMC (critical micellization concentration), micelles are formed that interact with the polymer more specifically, one or a few polymer chains can pass through a micelle. In this way, polymer chains can be cross-linked. If now the polymer concentration c is below c (the chain overlap concentration), mainly intramolecular junctions are formed. If c > c, however, a gel results. In this manner, viscoelastic gels can be made with a modulus of the order of 10 Pa. 

The present paper reports a more extensive rheological study of a series of mixed aqueous solutions of a cationic cellulose ether, at a higher fixed concentration, and the anionic surfactant SDS, at varying concentration. To summarize the solubility characteristics of such systems, there are three interaction zones which occur with increasing SDS concentration ... 

Winiuk EM, Regismond STA, Goddard ED. Interactions of an anionic surfactant with a fluorescent-dye-labeled hydrophicaUy-modified cationic cellulose ether. Langmuir 1997 13 111-114. 

In the present study, hydrophobic interaction between hydroxypropylcellulose (HPC) and an ionic surfactant in an aqueous phase was discussed. HPC, as well as EHEC, is a nonionic cellulose ether which contains hydrophobic groups in its molecular structure. Therefore, it might be interesting to compare the complex-formation properties of HPC with that of EHEC. The surfactants used here were an anionic surfactant SDS and a cationic one cetylpyridinium chloride (CPC). HPC formed a complex with these surfactants, of which cloud point changed with the surfactant concentration in the same manner as that observed in the EHEC-surfactant systems [4]. Effects of the complex on stability of dilute and concentrated kaoiinite suspensions were also studied, taking physicochemical properties of the complex into account. 

Rosen, O., Sjbstrbm, J. and Piculell, L. (1998) Responsive polymer gels based on hydropho-bically modified cellulose ethers and their interactions with ionic surfactants. Langmuir, 14 (20), 5795-5801. 

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