SELECTION OF SUSPENDING AGENTS 129 MEASUREMENT OF SUSPENSION QUALITY In the process of screening potential vehicles for suspensions, we found that yield stress was largely responsible for the suspension qualities of a vehicle. Because of this, we found it necessary to develop methods which could provide reliable measures of yield stress. These methods will not be discussed at this time, but they will be presented in a forthcoming paper (5). Nevertheless, I would like to present to you the relationship which we found useful for determining the yield stress of non-Newtonian vehicles. I. Forpseudoplastic flow: r =AtD q- Be-I•D (4) II. For Bingham plastic and thixotropic flow (downcurve): r = (2UfD) TM +f (5) where r is the shearing stress, D the rate of shear, U the plastic viscosity, f the yield stress, and z/, B, and k are constants. By using these relation- ships, we obtained reliable measures of yield stress which showed good correlations with actual performance tests using glass and nylon beads of known sizes and densities. PHYSlcO-CHEMICAL FACTORS Having dealt with the rheological aspects of suspension formulation, let us consider some factors which may influence the performance of suspension vehicles. These factors are: 1. Concentration 2. Ionic charge 3. pH 4. Temperature 5. Compatibility with substituents normally used in suspensions. CONCENTRATION Quite often the suspension properties of a vehicle can be enhanced significantly by slightly increasing the concentration of the suspending agent. This effect is shown in Fig. 4 for a typical Bingham plastic vehicle. It can be seen that as much as a twofold increase in yield stress can-be achieved when only an additional 15 % of the suspending agent is used. This small increase in the suspending agent concentration may be the difference between a good and a poor suspension vehicle. The data shown in Fig. 4 could facilitate the selection of suitable con- centrations of the suspending agent, if the effective settling force of the particles is known. Also, since economics, to a large degree, governs the marketability of a suspension product, this information could aid in deter- mining the minimum amount of the agent required for effective suspending action.
130 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 4O 20- IO o c• i. i i i i l 0 .• o• 0.• 0.4 0.3 0.• 0.7 Concentration (%) Figure 4.--Effect of concentration up. on the yield stress of a Bingham plastic suspension vehicle. IoNIc CHARGE It is important that the ionic charge of the suspending agent is known. Very often the use of a suspending agent and a drug of opposite charges results in unwanted effects. These effects are usually evidenced as a suspension which has settled rapidly, has decreased in drug potency or is altered in color, taste or over-all effectiveness. For this reason, the suspending agent should be one with the same charge as the drug or, prefer- ably, one which is nonionic. PH In Fig. 5 is shown the effects of pH upon the yield stress of a suspension vehicle with Bingham plastic flow properties. It is evident that maximum suspension performance is achieved between pH 6 and 10. If it is neces- sary that the final pH of the suspension be less than 6 (or greater than 10) for reasons of drug stability, solubility or compatibility, then this vehicle should not be used. Instead, a vehicle which is not pH sensitive or one which is most effective at the desired pH should be selected.
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