N.
1810 @m
-
= constant
w. TAYLOR, J. E. CLUSKEYAND F. R. SENTI
+ kT - In a ze
(4)
This drfference in galvanic potential normally is divided into two parts,IZthe x potential connected with the polarization and orientation of neutral molecules at the interface, and the diffuse ionic potential, D, from the distribution of free charges in the solution around the particle. If the x potential is assumed to be independent of the distribution of the ionic charges near the interface then the diffuse potential must vary with the ionic concentration in a manner similar to that of the galvanic potential difference
D
=
dm - @s
-x
=
constant
RT +In a ZF
(5)
R and F axe the gas and faraday constants. If it were possible to plot the diffuse ionic potential as a function of the gegenion activity a semilog plot according to equation 5 would give a slope of -0.059. Although diffuse ionic potentials cannot be rreasured it is possible to calculate zeta potentials according to Booth’s theory. The values obtained for these quaternary ammonium chloride micelles are tabulated in Table I1 and plotted in Fig. 5 . The slope of the best straight line, as found by the least squares method, is -0.061. The activ ty of the chloride ion was taken to be the (12) See, for example. J T h . G. Overbeek and J. Lyklema, “Electrical Potentitls in Colloidal Systems” in “Electrophoresis,” edited by M. Bier, 4carlemic Press, New York. N. Y , 1959.
Vol. 65
same as that of the mean activity coefficient in sodium chloride solutions of the same concentration. While this is an unproven assumption it may be pointed out that the use of concentrations rather than activities results in only a small change in the slope. The close agreement between the experimental slope for the zeta potentials and the theoretical slope for the diffuse potential suggests that, within the limits of present day experimental technique, the zeta potential in systems of thermodynamicaily stable colloidal electrolytes is essentially equal to the total diffuse ionic potential. Physically this is equivalent to asserting that the micelle is surrounded by a compact portion of the double layer which includes a fixed fraction, 74% for the systems studied, of the gegenions necessary to “neutralize” the micelle. The remainder of the gegenions are distributed in a diffuse double layer. The compact inner portion of the double layer is relatively unresponsive to changes in equilibrium electrolyte concentration, that is, to changed critical micelle concentrations. Such effects are reflected by changes in the zeta potential of the outer ddfuse layer. Acknowledgment.-The authors gratefully acknowledge financial support furnished for this work by the National Science Foundation. Drs. Karol and Estella Mysels offered constructive criticism during the writing of this paper.
WATER SORPTION BY DEXTRANS AND WHEAT STARCH A T HIGH HUMIDITIES BY N. W. TAYLOR, J. E . CLUSEEYAND F. R. SENTI Northern Regional Research Laboratory,’ Peoria, Illinois Reckved October 22, 1980
Water absorption and desorption isotherms of amorphous dextran, partially crystallized dextran, and wheat starch were determined at relative humidities between 74 and 99.7%. Water sorption of NRRL B-512 dextran waa followed before. during and after crystallization. Results, interpreted according t o the Flory-Huggins polymer solution theory, indirate t h a t at water contents of 33yoand higher, the amorphous dextrans behave as randomly coiled polymers in solution, apparently free of complications due t o physical structure. The water absorptions of the partially crystallized dextran, and also of the starch, were interpreted by comparison with amorphous dextrans in terms of current concepts of effects of crystallinity and elastic network.
It, was shown previously that water absorpt,ioii by dextrans is influenced by the presence of crystallinity and that in amorphous dextrans the water adsorption is oniy slightly dependent upon chemical ~ t ’ r u c t u r eif, at ~ all. High wEter ahsorption a t high relative humidity (RH) and disappearance of hysteresis suggest that in t]his region water a,bsorpt>ionis essentially a solut.ion process. The mecha.nism of water absorption at high RK hy such amorphous dext]rans appears to be relatively simple compared to that by sta,rch, fnr example. Fw t,hisreason dextrans may serve as model subst~anceswith which to compare absorp(1) Thin is j laboratory of the Northern Utilization Rasearch and Development Division, Agricultural Research Service, U. S. Department of Agricrilturn. (2) N. W. Taylor, €1. F. Zobcl. N. N. Hellman and F. R. Senti, J. Phus. Chem.. € 8 ,599 11959).
tion by starch. T n this work, water absorpt
