Langmuir 1988,4,486-487
486
E b
5
o-
__-__+__.__ --.-
L
+-+---A 3 50
~
500
I 50
2 50
4 5c
ACTIVITY, mM Figure 4. Graphical evaluation of Gibbs excess surface concentration of sodium dodecyl sulfate, at 2.80 mM, from the data of M y ~ e l s . ~
elasticity is therefore always greater than Gibbs elasticity. The same graphical method can be used to evaluate the Gibbs excess surface concentration of a solute from a rectilinear plot of surface tension versus activity, by means of the Gibbs adsorption theorem: rz= -da/(vaRT d In a,) (3) where F2 is the Gibbs excess surface concentration of so-
lute, v is the number of ions per molecule of solute, a is the degree of dissociation, and a2 is the activity of the solute. A plot of surface tension versus activity (or concentration c2 if the solution is ideal) can be treated in the same way to obtain r2as the T-A curve was treated to obtain E. This application of Maxwell’s method has already been suggested by Couper.2 We take the recently published data of Mysels3 for surface tensions of dilute aqueous solutions of sodium dodecyl sulfate to illustrate Maxwell’s graphical method. In Figure 4 the value of da/d In a2 at a2 = 2.80 mM is measured by FG in mN/m. For a2 = 2.80 mM, da/d In a2 = 14.2 mN/m. The remainder of the calculation to evaluate P2 (a = 1, v = 2) is r,(in mol/cm2) = 2.02
X
10-”FG
(4)
where FG is in mN/m, which gives r2 = 2.86 X mol/cm2 at a, = 2.80 mM. By plotting cr/(vaRT) versus the activity of the solute, the distance FG then gives the value of rz directly. Maxwell’s method is equally as accurate and more illuminating than the usual procedures. (2)Couper, A. in IUPAC Commission 1.6 Physical Chemistry: Enriching Topics from Colloid and Surface Science: van Olphen, H.; Mysels, K. J., Eds.; Theorex: La Jolla, CA, 1975;pp 131-132. (3)Mysels, K. J. Langmuir 1986,2,423-428.
Comments On “An Unusual Gel without a Gelling Agent“
The enthusiasm for the peculiar rheological properties and potential applications of “polyaphron” gels, expressed in a recent letter in this journal,’ is understandable. It is shared by many others, including myself, who have worked with, or have been exposed to, these intriguing materials. It must be pointed out, however, that the paper lacks novelty, except for the name that the authors have attached to what more prosaic workers have referred to simply as “high-internal-phase-ratio emulsions”‘ (HIPREs) or “highly concentrated emulsions” (e.g., ref 2-14). These
(1)Bergeron, V.;Sebba, F. Langmuir 1987,3,857,858. (2)Lissant, K. J. J. Colloid Interface Sci. 1966,22,462;1973,42,201; 1974,47,416. (3) Lissant, K. J. SOC. Cosmet. Chem. 1970,21,141. (4)Nixon, J.; Beerbower, A. Prepr. Am. Chem. SOC.,Diu. Pet. Chem. 1969,14(1), 49,62. (5)Mannheimer, R.J. J. Colloid Interface Sci. 1972,40,370. (6)Princen, H.M. J. Colloid Interface Sci. 1979,71, 55. (7)Princen, H.M.;Aronson, M. P.; Moser, J. C. J . Colloid Interface Sci. 1980,75,246. (8) Prudhomme, R. K., 53rd Annual Society of Rheology Meeting, Louisville, KY, 1981. (9)Princen, H. M. J. Colloid Interface Sci. 1983,91, 160; 1986,105, 150. (10)Princen, H.M.;Kiss, A. D. J. Colloid Interface Sci. 1986,112,427. (11)Princen, H. M. Langmuir 1986,2,519; 1988,4,164-169. (12)Princen, H.M.;Kiss, A. D. Langmuir 1987,3,36. (13)Schwartz, L. W.;Princen, H. M. J . Colloid Interface Sci. 1987, 118, 201. (14)Princen, H. M.; Kiss, A. D. J . Colloid Interface Sci., submitted.
are emulsions (oil-in-water or water-in-oil) in which the volume fraction of the dispersed phase, 4, approaches or exceeds that of the close-packed-sphere packing, 4o= 0.74. As the volume fraction approaches unity, the droplets assume an increasingly pronounced polyhedral shape, while remaining separated (and protected against coalescence) by thin films of continuous phase. With the right kind and concentration of surfactant in the continuous phase, emulsions with 4 as high as 0.99 can be readily prepared. The presence of a second surfactant in the dispersed phase . may or may not be helpful. The structural and rheological properties of such emulsions are described by essentially the same laws as those of gas-liquid foams of 4 > 40. Because of the crowding, the deformable dispersed units (drops or bubbles) cannot move freely past each other when a small stress or strain is applied to the system. Therefore, up to , systems behave as viscoelastic the yield stress, T ~ these solids (“gels”),characterized by a shear modulus, G. Above the yield stress, they are shear-thinning fluids. This behavior has nothing to do with the presence of a “web of thin and, therefore, strong water which is ice-like and which has to be broken before the gel can flow”.’ The behavior is indeed determined by the three-dimensional network of films of continuous phase, but in a different way that is well understood from two-dimensional modeling.8~9J3J6-19~21-23 These models, and careful measure(15)Khan, s. A., Ph.D. Thesis, MIT, 1985. (16)Khan, S.A.; Armstrong, R. C. J . Non-Newtonian Fluid Mech. 1986,22, 1; 1987,25,61. (17)Khan, S. A. Rheologica Acta 1987,26,78. (18)Kraynik, A. M.;Hansen, M. G. J . Rheology (N.Y.)1986,30,409.
0743-7463/88/2404-0486$01.50/0 , 0 1988 American Chemical Society I
,
Book Reviews
Langmuir, Vol. 4, No. 2, 1988 487
ments on real, three-dimensional oil-in-water emulsions, have established that the rheology of these systems is governed by the relevant physical parameters approximately as follows:
G = 1.77(~/R32)4’/~(4 - 40) for the shear modulus (ref 9) 70 = ( U / R ~ ~ ) $ J ~ ’ ~ Y ( ~ ) for the yield stress (ref 8 and 13)
II,=
+ 32(4 for the viscosity (ref 13)
TO/-?
where c is the interfacial tension, R32 is the surface-volume or Sauter mean drop (or bubble) radius, Y ( 0 )is a function that has been established experimentally, i/ is the shear rate, Ca ( ~ R 3 2 q ) is / ~the capillary number, and p is the viscosity of the continuous phase. Thus, high gel strength and viscosity are promoted by small drop size, high volume fraction, and high interfacial tension. (19) Kraynik, A. M.; Hansen, M. G. J. Rheology (N.Y.)1987,31,175. (20) Yoshimura, A.; Prud’homme, R. K.; Princen, H. M.; Kiss, A. D. J. Rheology (N.Y.) 1987, 31, 699. (21) Weaire, D.; Fu, T.-L.;Kermode, J. P. Philos. Mug., [Part]B 1986, 54, L39. (22) Weaire, D.; Kermode, J. P. Philos. Mug. [Part]B 1984,50, 379. (23) Weaire, D.; Fu, T.-L. J.Rheology (N.Y.) 1988, 32.
References 2-25 form a list, by no means exhaustive, of papers that deal with structural, rheological, and other properties of such systems, including foams. It is clear that, over the last 20 years or so, the art and science of “polyaphrons” have matured to a level far beyond that suggested by Bergeron and Sebba.’ This includes the area of practical applications. In fact, the impetus for the early workers, i.e., Lissant2J and Nixon and B e e r b ~ w e rwas ,~ provided by the search for safer aviation (Vietnam!) and rocket fuels. Many patents were issued in this, as well as in the cosmetics and foods areas. Finally, I would like to suggest that the term “polyaphrons”, as well as “foamulsions” encountered elsewhere, be abandoned. There appears t o be no need to further mystify what are, after all, “simply” highly concentrated fluid/fluid dispersions, be they foams or emulsions. Previous attempts to demonstrate otherwise, e.g., in ref 26, have been singularly unconvincing. (24) Kraynik, A. D. Annu. Reu. Fluid Mech. 1988,20, 325. (25) Heller, J. P.; Kuntamukkula, M. S. Ind. Eng. Chem. Res. 1987, 26, 318. (26) Sebba, F. Chem. Ind. (London) 1984, 367.
H. M. Princen General Foods Corp., Technical Center, 555 South Broadway, Tarrytown, New York 10591 Received November 30, 1987
Book Reviews Nonionic Surfactants: Physical Chemistry. Surfactant Science Series Vol. 23. M. J. Schick, Editor. Marcel Dekker: New York, 1987. 1135 pages. We read, studied, and discussed the book over a 6-week period in our research group andfound the exercise most rewarding. It is unquestionably both a master key t o a vast and important literature as well as a book that can be read for a good grasp of current understanding. The book is indeed =a new, updated, and vastly expanded version of P a r t I1 of the original edition” (1967), which also contained sections on organic chemistry, analytical chemistry, and biology, of nonionic surfactants. The updated core of the book is summarized by the key words: monolayers, adsorption and wetting, aqueous and nonaqueous micelles, thermodynamics, solubilization, microemulsions, macroemulsions, stability of dispersions, detergency, and foaming. New insights into structure and dynamics are given by chapters on NMR and SANS (small-angle neutron scattering). Three chapters deal with the interactions, configuration, hydrodynamics, and stability of the poly(oxyethy1ene) chain. Other new chapters deepen our understanding of multiple emulsions (W/O/W type), HLB (hydrophile-lipophile balance), PIT (phase inversion termperature), and EIP (emulsion inversion point). The Preface by Schick is an excellent guide to the book for the nonspecialist. A forward by Pethica offers great insight, complements the preface, and nearly amounts to a review in itself. We tend t o agree with the view that the subject remains phenomenological, described by phase diagrams and adsorption isotherms, but the many revised treatments along with the new insights from NMR and SANS add a depth that makes the book a most worthy reference work. Some duplication is unavoidable with 18 chapters by 29 contributors, but we found this to be helpful in highlighting important points and exposing different points of view. The editor has
demonstrated a deft hand in assembling the presentation, which is abundant with references, and includes very valuable author and subject indexes.
R. L. Rowell, University of Massachusetts at Amherst Phenomena in Mixed Surfactant Systems. John F. Scamehorn, Editor. ACS Symposium Series 311; American Chemical Society: Washington, DC, 1986.
It is a well-known fact that pure surfactants are not useful for application as stabilizers for foams, emulsions, or dispersions. However, the majority of investigations in the area are concerned with surfactants of high purity, and articles on the exact shape of a micelle of pure surfactants have appeared ad nauseam. It is refreshing t o find a compilation of competent authors treating the properties of micelles and macromolecular layers of mixed surfactants, and the editor should be highly commended for this initiative. The book is divided into a fairly extensive introductory part (100pages) discussing structure and thermodynamics of mixed micelles, followed by a few articles on macromolecular layers, treatments of combined monolayer/micellar systems, and solid surfaces and finally by a few chapters of varia. The strength of the book lies with the fact that it clarifies the foundation for the formation of mixed micelles and that the micelle/monolayer interactions are given adequate space. The weakness of the book is as obvious; it almost completely neglects the more ordered systems of mesomorphic character. These are technologically extremely important, and the fact that they are not treated limits the value of the book to some extent. For those whose interest is limited to micellar systems and traditional monolayers a t the air/water or solid/water interface, the book provides excellent information and is extremely useful. Stig E. Friberg, Clarkson University
