Incipient agglomerate-creaming in silicone emulsions: Prediction and detection

Incipient Agglomerate-Creaming in Silicone Emulsions: Prediction and Detection exhibit reversible shear degradation) is a nonNewtonian, Shear Thinning fluid whereas one that does not contain agglomerates is a Newtonian fluid. The existence of reversible shear-dependent structure can be used to indicate the presence of agglomerates and hence, a tendency for creaming by this mechanism. In the emulsions studied, the breakdown of structural viscosity followed the Power Law (4) and the reciprocal of the Flow Behavior Index was used to characterize the system. This parameter, d In ~/d In r (where ~ is angular velocity and r is shear stress) was called the Shear Thinning Index, or STI. Rheological measurements were made with a Brookfield Synchro-Lectric Viscometer and a coaxial cylinder attachment. S T I values were easily determined using a special Template (5) and log-log plots of rpm versus dial reading. Agglomerated emulsions had S T I values greater than 1.0; nonagglomerated systems had S T I values equal to 1.0. Shear Thinning Index was determined immediately after emulsion preparation. The samples were then aged at 25°C and one-gravity for up to two months. To determine the extent of separation, a rapid, nondestructive technique based oll electrical conductivity was employed. Whereas the conductivity of an emulsion is a complex function of the concentration and conductivity of the dispersed and continuous phases, a homogeneous system will have constant conductivity at all points within the emulsion column, but a creamed emulsion will not. Thus, a Conductivity Profile, or plot of conductivity versus distance from the emulsion surface, can be used to determine the degree of homogeneity. If the conductivity probe is small and moves slowly through the emulsion, the system is relatively undisturbed and several Profiles can be made while creaming is in progress. In a typical system (see Fig. 1) no visible separation was evident after 4 days but creaming had occurred, as noted from the lower conductivity near the top (higher concentration) and the higher conductivity near the bottom (lower concentration). The central portion of the column retained a constant composition and, therefore, constant conductivity. After 25 days, conductivity in the upper portion decreased further indicating greater compacting of agglomerates. Visible

Instability in oil/water emulsions is commonly manifested by a reversible partitioning of dispersed phase into two regions: one of higher concentration and one of lower concentration than the original emulsion. This phenomenon is known as creaming and results from the sedimentation of large primary particles and/or the movement of agglomerates. Agglomerates are groups of individual primary particles, held together by weak cohesive forces and associated with some immobilized continuous phase. If present in sufficient quantity, their movement produces a high oil concentration gradient at the b o t t o m of an emulsion, forming a cream " l i n e " which moves upward with time (1). In some cases, a significant amount of dispersed phase partitioning can occur without visible evidence of separation. In characterizing such "invisible creaming" simple observation is inadequate. In a recent study (2) it was necessary to predict the creaming tendency of an emulsion shortly after its preparation and to measure the extent of separation when it occurred. For convenience, emulsion stability was defined on a macroscopic scMe as: the ability of an emulsion to retain a homogeneous distribution of dispersed phase for a prolonged period of time. The oil/water emulsions of interest consisted of high viscosity polydimethyl siloxane (silicone oil) emulsified with a mixture of nonionie ethoxylated f a t t y alcohols. The ultimate particle size ranged between 0.03 and 0.3 t~ and, therefore, was too small for primary particle creaming. The systems were identical in composition, apparently noncoalescing over a period of several months, and contained agglomerates which creamed within several hours or days. To determine the tendency for agglomeratecreaming, a method based on emulsion rheological properties was employed. The apparent viscosity of a dispersed system may be viewed as the sum of a shear-independent and a shear-dependent viscosity (3). The former is a result of hydrodynamic effects whereas the latter results from the work necessary to rupture links between agglomerated particles, which are continually reforming under shear. As the shear rate on such a system increases, average agglomerate size decreases and apparent viscosity decreases. Thus, a dispersed system containing agglomerates (which

Journal of Colloid and Interface Science, •ol, 36, No. 1, May lg7L

155

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Fro. 1. Conductivity Profile. separation was noted at the position which corresponded to the break in the Profile. As expected, emulsions with an S T I of 1.0 exhibited little, if any, redistribution of dispersed phase whereas all those with an S T I greater than 1.0 + did separate. The degree of separation, measured by the difference in concentration between the agglomerated and nonagglomerated region below, increased with increasing STI. Systems with an S T I between 1.0+ to about 1.2 exhibited "invisible creaming." In those with S T I values from 1.2 to 2.5 (the highest observed), the appearance of the visible boundary became more well defined with increasing STI. Although all emulsions were of approximately the same weight concentration, apparent viscosity at an arbitrarily chosen shear rate of 0.5 sec-~ increased with S T I from about 10 cP (STI = 1.0) to about 400 cP (STI = 2.5). This was attributed to an increase in the effective volume fraction occupied by the agglomerates. The viscosity increase was accompanied by increased hindered settling, and emulsions with an S T I greater than about 2.0 took much longer for a visible boundary to appear than those with lower S T I values. With further in-

Journal of Colloid and Interface Science, Vol. 36, No. 1, May 1971

creases in STI, agglomerate crowding became sufficiently great to form a solid structure at low shear (i.e., the emulsion had a yield stress). Such systems have a great tendency for agglomeratecreaming but the rate of creaming approaches zero. In these cases, the addition of a small amount of continuous phase reduced the crowding and agglomerate creaming quickly resulted. REFERENCES 1. COCKBAIN, E. G., Trans. Faraday Soc. 48, 185-196 (1952). 2. ROSEN,M. R., To be published. 3. CRoss, M. M., Y. Colloid Sci. 20,417-437 (1965). 4. CRAMER, S. D . , " M o m e n t u m Transfer in Simple Fluids for Vise ometric Flows," Ph.D. Thesis, University of Maryland, College Park, Maryland, (1968). 5. R o s ~ , M. R., J. Colloid Interface Sci., In press. MEYER R . ROSEN

Union Carbide Corporation, Tarrytown Technical Center, Tarrytown, New York Received February 1, 1971; accepted February 2, 1971