Viscosity enhancing substances

Finally, the stabilizing agents such as antioxidants, bacteriostatics and viscosity enhancing substances are added to the aqueous mobile phases when necessary. 

To increase the density of water sugar syrup can be added. An increase of the viscosity can also be achieved by the addition of viscosity enhancing substances. The character of the sediment may be influenced by electrolytes or surfactants. 

Enhancing the viscosity of an aqueous suspension is often necessary to obtain a reasonable physical stability (see Sect. 18.4.2.2). Apart from the already mentioned viscosity enhancers, also mineral viscosity enhancing substances, such as anhydrous colloidal silica (2—4 %, usually 2 %), and colloidal aluminium magnesium silicate (2—4 %, usually 2 %) or bentonite (1-2 %) are used. All percentages refer to the final amount of the preparation. 

The method of processing viscosity enhancing substances in the aqueous phase depends on the type of substance. The general rule is The viscosity enhancer has to be wetted completely at first. Otherwise lumps may occur that cannot swell anymore because they are insulated by the outer, swollen part. Section 23.7.2 gives elaborate information on different preparation methods. 

Hydrogels consist of a hydrophilic fluid (such as water, glycerol, propylene glycol and alcohol) or a mixture of hydrophilic fluids in which a viscosity enhancing substance has been incorporated. 

Many other excipients and active substances that are mainly used for another purpose, also exhibit surface activity. Examples can be found in the group of viscosity enhancing substances (see Sect. 23.7). In particular the low viscous methylceUulose is used for this purpose. Examples of active substances that have surface activity are the tricyclic antihistaminics and antidepressants, and the local anaesthetics (e.g. Udocaine). 

Another term for surfactants is emulsifiers. This designation is functional but has a limited distinctive character. Viscosity enhancing substances, which also stabilise emulsions and suspensions (see Sect. 18.4) are also associated with this term. The preferred name in this book is the physical qualities characterising surfactants . 

Some gel formers, for instance some of the cellulose derivatives, also exhibit a degree of surfactant activity. Others, particularly mineral viscosity enhancing substances, can also show dispersing (see Sect. 18.4.2) characteristics through the release of ions (aluminium-magnesium silicate) that improve dispersion by influencing the zeta-potential. 

The ionic viscosity enhancing substances are usually much more sensitive to pH-changes than the non-ionic. However, the addition of electrolytes, and also of acids and bases, affect thickeners as a result of increasing ionic strength or influencing the zeta potential (see Sect. 18.4.2). 

Viscosity enhancing substances not only increase the viscosity but also introduce rheological qualities such as (pseudo) plastic, dUatant and thixotropic behaviour (see Sect. 18.2.1). 

Several different gel preparation methods exist, based on specific qualities of viscosity enhancing substances. Six of them that are most used are described in this section dispersing by hand in hot water, dispersing by hand in a viscous fluid, pH change, dispersing mechanically, use of classic hydrogel formers and the corpus emulsi (specific emulsifying) method. 

Apart from the polymers mentioned in Table 10.6 some authorised medicines contain other viscosity enhancing agents such as dextran, hydroxyethylcellulose, hyalmonic acid and hydroxypropylguar gum (HP-guar Systane ) [36, 37,57]. Hyaluronic acid possesses good adhesive properties. In situ-gelling systems, such as gellan gum, are used in order to increase the precorneal residence time of the eye drop and to obtain a sustained active substance release [78, 79]. 

Emulsions consist of hydrophilic and hydrophobic ingredients. Therefore emulsions are not physically stable and the phases may separate into a water layer and a fatty layer. The physical stability of emulsions can be increased by decreasing the size of the droplets of the inner phase, by increasing the viscosity of the outer phase and first of all by decreasing the surface tension between the aqueous and the lipophilic phase. The presence of an active substance can influence the stability of an emulsion negatively. Enhancing the viscosity of the aqueous phase may increase the physical stability of an emulsion. Thereby sometimes the percentage of fatty phase can be decreased. An example is the emulsimi base in Table 12.7, where carbomer has been used for the viscosity enhancement. 

If a viscosity enhancing agent is used, a gel has to be prepared first (see Sect. 23.7). Soluble substances are dissolved in the gel or added as solution to the gel. 

Aqueous pastes, also called hydrophilic pastes, consist of a hydrophilic base with 40-60 % solid substance. This type of paste may consist of water only, made viscous by a viscosity enhancer (see Table 12.41) or by the addition of a hydrophilic cream or emulsion. They are supposed to have a good absorptive capacity and are therefore used in the treatment of wetting skin disorders [59b]. 

The formulation of aqueous pastes strongly resembles the formulation of cutaneous suspensions (see Sect. 12.5.5.2). Aqueous pastes contain, however, a higher percentage of solid substances which makes them semisolid. For typical excipients see Sect. 12.5.1. The formulation of an aqueous paste with colloidal aluminium magnesium silicate as viscosity enhancer is described here. Propylene glycol preserves the aqueous phase. 

Until the beginning of the twenty-first century, in pharmacy, the particles in a colloidal system usually did not consist of active substances but of excipients, such as viscosity enhancers. The number of pubUcations in the pharmaceutical literature on colloidal systems in which the dispersed particles solely consist of a drug substance or consist of carrier systems in which an active substance has been incorporated, however, has increased dramatically in recent years. 

Most corticosteroids nasal sprays (licensed preparations) are suspensions in which croscarmellose sodium is used as viscosity enhancer. The inhalation liquids for nebulisation with the same type of active substances however only contain polysorbate and sorbitan laureate to stabilise the suspension. For atomisation in jet nebulisers, the liquid should not be too viscous, in order to prevent clogging of the nebuliser. 

Active substances Quality Excipients Application Raw material Labelling Pharmacopoeia FRC Solvents Surfactants Viscosity enhancers Preservatives Colouring agents Herbal... 

The dispersion of a liquid into another liquid with which it is immiscible to obtain a sufficiently physically stable product is only possible by emulsifying or solubilising, using surface-active substances (tensides, surfactants). This process is applied in the preparation of creams and solubilisations. In oral emulsions surface-active substances are not used, but instead viscosity enhancers that possess little surface activity. Below the preparation of emulsirMis and solubilisations is briefly discussed. 

In all of these cases, the extensive intermolecular association occurring through clustering of the hydrocarbon groups was shown to result in appreciable viscosity enhancements. This was shown to be consistent with several observations including shear thinning (pseudoplastic) behavior, on addition of substances known to compete with hydrophobic interactions such as organic solvent surfactants, etc. Nevertheless, the nature of these hydrophobic... 

In this chapter we examine the flow behavior of bulk polymers in the liquid state. Such substances are characterized by very high viscosities, a property which is directly traceable to the chain structure of the molecules. All substances are viscous, even low molecular weight gases. The enhancement of this property due to the molecular structure of polymers is one of the most striking features of these materials. 

A variety of factors affect the horizontal and vertical migration of PAHs, including contaminant volume and viscosity, temperature, land contour, plant cover, and soil composition (Morgan Watkinson, 1989)- Vertical movement occurs as a multiphase flow that will be controlled by soil chemistry and structure, pore size, and water content. For example, non-reactive small molecules (i.e., not PAHs) penetrate very rapidly through dry soils and migration is faster in clays than in loams due to the increased porosity of the clays. Once intercalated, however, sorbed PAHs are essentially immobilized. Mobility of oily hydrophobic substances can potentially be enhanced by the biosurfactant-production capability of bacteria (Zajic et al., 1974) but clear demonstrations of this effect are rare. This is discussed below in more detail (see Section 5 5). 

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