430 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 15-- c o • I ._ 0 I 0 20 5[0 40 50 60 Glycerin, % w/w Figure g The effect of glycerine concentration on the rigidity of gelafine/glycerine gels (25øC__ O.IøC) Gelatine (99 Bloom) % w/w b •o% c shear (17.59 s-l) was recorded on the X-Y recorder with a time from 0-10 s recorded on the Y axis. RESULTS The effect of glycerin concentration on the rigidity of glycerin/gelatin gels is shown in Figs. oe and $. For gels containing less than 40}/0 w/w glycerin the expression: G=Go+m C . . . (II) held where G=rigidity, Go=rigidity at zero glycerin concentration, m=slope, C=% concentration of glycerin. Above 40% glycerin the non linear increase in the viscosity of the gel interlattice fluid was reflected by a similar non-linear increase in rigidity. The values of Go and m in equation {2) have been found to obey the straight line relationships given under {1). G O = oc q- ip Z 2 . . . (III) m _- I• q- % Z ............ (IV) where Z=% gelatin concentration and oc and [• are intercepts on their respective axes and 9 and % the slopes. Substituting these values equation (II) becomes

SOME APPLICATIONS OF RIGIDITY AND YIELD VALUES 431 Figure 3 20-- e/ 5 io 15 20 Gelafin, % W/W The effect of gelatine and glycerine concentrations on the rigidity of gelatine/ glycerine gels (25øC +_ 0.1 øC) Glycerine • a 0% b lO% o 20% d 40% e 60% G C (v} This empirical equation was valid for gels containing 4% to 15% gelatin and up to 40% w/w glycerin. The constants in this equation showed no simple relationship to the average molecular weight of the material. How- ever for each % w/w gelatin/glycerin ratio it was possible to calculate a linear relationship between rigidity and Bloom number of the gelatin J•"--= r q- kN (VI) where F=intercept on rigidity axis, k=slope and N =Bloom number. The elastic properties of dilute Laponite gels also show an approximately straight line relationship between the rigidity modulus and the concen- tration of Laponite. The presence of electrolytes affects the rigidity modulus of both gelatin/ glycerin and Laponite gels. For the former the relationship