Bioelectrochemistry: Ions, Surfaces, Membranes - ACS Publications

6 The Influence of Extremely Small Attractive As Well As of Repulsive van der Waals-

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London Forces on Cell Interactions C. J. V A N OSS—Department of Microbiology, State University of New York at Buffalo, Buffalo, New York 14214 A. W . NEUMANN—Department of Mechanical Engineering, University of Toronto, Toronto, Ontario M5S 1A4, Canada R. J. GOOD—Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14214 D. R. ABSOLOM—Department of Microbiology, State University of New York at Buffalo, Buffalo, New York 14214 The van der Waals interaction between two types of cells in a liquid (neglecting for the moment their zeta potentials) can be approached via the expression for the free energy of interaction of Cells 1 and 2 across Fluid 3: ∆F = —A /12πd for Solids 1 and 2 in Fluid Medium 3, sepa rated by a distance d. A is the Hamaker coefficient for the interacting system, which normally is positive; it is always positive if Fluid 3 is a gas. But when Fluid 3 is a liquid, under rather common conditions, A can be nega tive. Then Cells 1 and 2 will repel one another. Cells of the same kind always will attract one another, i.e. A is always positive. However, A can be very close to zero, and under the influence of even very low zeta potentials the cells also will repel each other. The intrinsic interfacial free energies of biological fluids have values close to those of the cells immersed in them, so that both effects can occur quite commonly. 132

132

2

132

132

131

131

hen the sign of the net van der Waals ( V D W ) interaction between W two different solid bodies (1) or between two different dissolved

polymers (2) i n liquids is negative, their interaction is repulsive. The possibility of the existence of such repulsive V D W interactions i n liquids 0-8412-9473-X/80/33-188-107$05.00/l © 1980 American Chemical Society

Blank; Bioelectrochemistry: Ions, Surfaces, Membranes Advances in Chemistry; American Chemical Society: Washington, DC, 1980.


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BIOELECTROCHEMISTRY: IONS, SURFACES, M E M B R A N E S

was already implicit i n Hamakers classical treatment (3), and was stated explicitly by Visser (4): "when two different materials are immersed i n a liquid medium and the interaction of each of these materials w i t h that of the liquid medium is larger than the interaction between these materials themselves, spontaneous separation can occur due to dispersion forces only." Such repulsive action was found for the system poly-(tetrafluorethylene)glycol-iron oxide (5). The V D W interaction between two different substances, 1 and 2, immersed or dissolved i n L i q u i d 3 w i l l be repulsive when the Hamaker coefficients of the two substances A n and A 2 stand i n either of the following relations to that of the liquid, A (4) 2

3 3

An < A

3 3

< A

2 2

(1)

An > A

3 3

> A22

(2)

Assuming for the moment that the interaction between particles and/or macromolecules in a liquid medium may be represented as occurring between semi-infinite, homogeneous slabs, the interaction energy A F , at separation distance d may be represented as AFi

3 2

— -Ai /12^d 3 2

2

(3)

if A i is the Hamaker coefficient of the system. Expressions 1 and 2 then can be written also as 3 2

AF

c

n

>

AF 33 > C

AF n < AF c

C

3 3

AF 22 C

< A F ^

(4)

(5)

as conditions for repulsion, where A F is the free energy of cohesion of Substance i . This may be restated as follows: in L i q u i d 3, Materials 1 and 2 w i l l undergo a V D W repulsion when the interfacial free energy of Material 1 is larger and that of Material 2 is smaller than that of the liquid medium or vice versa (1,2). The derivation of Equation 3 assumes that F l u i d 3, between Bodies 1 and 2, is chemically homogeneous. That is to say, if it is a solution there is no adsorption or orientation of any component at either the 1, 3 or 2, 3 interface. It also assumes that the intermolecular forces in the system obey an inverse sixth power law of attraction. c

t i

Not only can negative V D W forces be correlated with particle engulfment or rejection phenomena at solidification fronts ( I ) , and of the phenomenon of phase separation in solutions of pairs of polymers (2), but these repulsive forces also can be applied to novel separation meth-



6.

VAN oss E T A L .

van der Waals-London Forces

109

ods. Antigen-antibody precipitates can be dissociated completely by lowering the interfacial free energy of the liquid medium to a value intermediate between those of the antigenic determinant and the antibody-active site ( 6 ) . The empirical separation method that is known as "hydrophobic chromatography" depends entirely upon adsorption under conditions where positive V D W attractions prevail between adsorbent and adsorbed compounds. It also depends on V D W repulsidn between these compounds and the adsorbent surface, when the interfacial free energy of the liquid medium is lowered to a value intermediate between those of the adsorbent and the adsorbed compounds i n the elution step ( 7 ) . Interactions Between Biological Materials Practically a l l biological materials have a lower surface free energy than water, and therefore w i l l always exert a V D W attraction on each other i n that medium An ^ A

2 2

< A

3 3

(6)

because Equation 6 disobeys Equations 1 and 2. Also, the same materials An = A

(7)

2 2

always w i l l exert a V D W attraction upon one another, i n our medium, as Equation 7 disobeys Equations 1 and 2. (These considerations neglect any interaction between such materials that may be caused by their f-potentials). However, when the interfacial free energies of the two materials and that of the liquid medium approximate being equal An « A

3 3

« A

2 2

(8)

A i , and thus the value of the V D W interaction, w i l l approach zero. 32

Interfacial Free Energies of Biological Liquids The liquid media that are most germane to studies on interactions between cells and/or biopolymers are blood plasma and serum. A closer investigation into the surface tensions of blood plasma and serum thus seems essential. As early as 1913 the surface tension of human blood serum at 37°C was reported as ^ 45.4 d y n / c m (measured by the fallingdrop method) ( 8 ) . More recently, L e w i n (using platinum ring torsiometry) found values of ^ 47.8 and ^ 50.5 d y n / c m at 37°C and 20°C re-


110


Table I. Surface Tensions of Normal Human Blood Serum and Plasma and Their Ultrafiltrates at 22°C Surface Tension in dyn/cm* Serum Plasma Serum ultrafiltrates Plasma ultrafiltrates Saline water (0.15M N a C l ) 6


6

45.7 47.8 70.3 70.5 73.8

° Measured by the pendant-drop method (10). "See Refs. 11 and 12. spectively for normal human serum (9). W i t h the pendant-drop method (10) we found, for one sample of human serum, 45.7 d y n / c m and 47.8 d y n / c m for heparinized plasma from the same donor at 22°C. Ultrafiltrates through membranes with a molecular weight cut-off of « 20,000 (11,12) of the same serum and plasma yielded values of 70.3 and 70.4 d y n / c m , respectively (see Table I ) . Now, while it is possible, by lowering the surface tension of the liquid medium to ^ 50 d y n / c m by means of solutes of low molecular weight, to dissociate antigen-antibody precipitates (of the pure V D W type) (6), whole mammalian serum ( w i t h a surface tension < 50 d y n / c m ) definitely has no such dissociating power. U p o n some reflection this is not as anomalous as it might seem. Table I shows that it is mainly because of the presence of proteins (of molecular weight > 20,000) that blood (or plasma) has a surface tension y < 50 (and not y « 70), and it cannot be expected that molecules with dimensions of the order of 100 A w i l l effect a separation by purely physical means between other proteins of the same approximate size that are only about 2 A apart at their site of interaction (13). F o r cell-protein interaction this reasoning also holds. However, for cell-cell interaction the dimensions of the proteins that cause the major decrease i n surface tensions is not ipso facto a hindrance i n contributing to the V D W attraction or repulsion between cells. Nevertheless, serum or plasma proteins (which take up roughly 5-6% of the total volume of the liquid) are not likely to play a significant role i n intracellular V D W interactions for reason(s) stated below. The surface tension of a pure liquid is a direct measure of the free energy or cohesion. F o r a solid, the surface free energy, which has the same dimensions as surface tension, is the measure of free energy of cohesion. Thus, for Substance i (9)


6.

VAN oss E T A L .


111

where v stands for vapor. The expression relating free energy of cohesion, and hence surface tension, to the Hamaker coefficient is of a form that is analogous to Equation 3 (14,15)

^

= -l6^r-

( 1 0 )

where d is the equilibrium intermolecular distance for Substance i. ( I n Ref. 15, the symbol A is used to represent a quantity that is proportional to the Hamaker coefficient as used i n this chapter, but smaller b y the factor, (n /?r) , where n is the concentration of i i n molecules/cubic centimeters.) F o r a pure liquid, A can be estimated from the molecular properties of Substance i (4,15). The restrictions indicated earlier on the derivation of Equation 3 also apply to Equation 10. The situation is somewhat different for a solution, particularly if a surface-active component is present. The measured surface tension is strongly influenced by adsorption at the liquid-vapor surface (16). Yet adsorption does not influence the values of the Hamaker coefficients that must be used i n Equations 3 and 10, which are related to those of the pure substances by the volume fraction weighted averages. Thus, for solution /


oii

(

2

f

r t

Ay^XqpiAii

m j

where (p» is the volume fraction of the component whose Hamaker coefficient is At* in Solution /. W e now may examine the relevance of surface tension measurements to interactions of cells i n contact with serum or plasma. The surface tensions reported i n Table I for the unfiltered liquids are much lower than those of the ultrafiltrates, at least in part, because of the adsorption of proteins having molecular weight > 20,000 at the l i q u i d - a i r interface. A n d the measurements made on the ultrafiltrates are likely to be a good approximation to the zero-time surface tensions of the whole serum and plasma. The solutes that remain i n the solution after ultrafiltration evidently are of a relatively low level of surface activity, and do not affect the surface tension to any greater extent than would be expected from their volume fractions. Finally, we can estimate the comparative influence of plasma and serum proteins with molecular weights > 20,000 on the cell-serum and cell-plasma interfaces, as opposed to surface-active solutes such as ethylene glycol or dimethylsulfoxide ( 6 ) . The latter w i l l adsorb at the interfaces to an extent that is comparable with their adsorption at the l i q u i d -


112


vapor surface, i.e. they w i l l form an oriented monolayer. O n the other hand the cells are bathed normally in serum or plasma and their surfaces already are at a condition of saturation with the serum or plasma proteins. There w i l l be little tendency for additional adsorption at the cell-fluid interfaces of protein components of serum or plasma which are stable solutes i n the natural environment of the cells. Hence these proteins w i l l not cause the dissociation of cell-cell aggregates.


Intrinsic Interfacial Free Energies of Biological Liquids Thus it would seem that the actual intrinsic surface tension of biological liquids, i.e. the one that plays a role in the interactions between cells among one another, and between cells and biopolymers, must be close to that of serum or plasma ultrafiltrates, i.e. y « 70 d y n / c m or the intrinsic interfacial free energy of the interstitial mammalian liquid, serum or plasma, or AF* « —140 ergs/cm . 2

Interfacial Free Energies of Blood Cells F r o m contact angle measurements (17,18), interfacial free energies for blood cells can be derived easily (17,19) (see Table II). Clearly, the interfacial free energies of blood cells all are exceedingly close to the —140ergs/cm of the surrounding liquid (disregarding the proteins, see Table I). Thus, for all blood cells i n their surrounding liquids, the Hamaker coefficient A i i , according to Visser (4) 2

3

Ai3i = A n + A

3 3

— 2A

i 3

« An + A

3 3

— 2 VAnA

(12)

33

is extremely close to zero. Also, with the slightest negative zeta potentials, all these cells i n their medium should be i n very stable suspension, as indeed they are.

Table II. Contact Angles with Sessile Drops of Saline Water (17, 18) and Interfacial Free Energies of Various Blood Cells Derived from These (17)

Cell Type

Contact Angle (in degrees)

Interfacial Free Energy A F i (in ergs/cm )

Polymorphonuclear leukocytes Lymphocytes Erythrocytes Platelets

18° 15°-16° 15°-16° 16.3°

-138.3 - 1 4 0 . 3 - -139.6 -140.3 139.6 -139.4

2


6.

V A X OSS E T A L .


113

Some virus (herpes simplex)-transformed cells become quite hydrophilic (17,20), with contact angles as low as 13° (AF* « —141.4 ergs/ c m ) , while many tissue surfaces have interfacial free energies of about — 136 ergs/cm (20). I n such cases A i w i l l be negative and the cells w i l l be repelled by the tissues (even without a negative zeta potential). Such cells clearly would be free to circulate and penetrate i n most tissues without adhesion and may suggest a model for metastatic cells. 2

2

3 2


Glossary of Symbols A = effective Hamaker coefficient d =» separation distance d = equilibrium separation distance A F = free energy difference mol wt = molecular weight n = concentration, i n molecules/cm i, j, 1, 2 = different materials 3 = liquid medium v = vapor c = cohesion 0

3

Greek L e t t e r s

y = surface tension £ = electrokinetic potential qp = volume fraction of a given component Literature Cited 1. Neumann, A. W.; Omenyi, S. N.; van Oss, C. J. Colloid Polym. Sci. 1978, 257, 413. 2. van Oss, C. J.; Omenyi, S. N.; Neumann, A. W. Colloid Polym. Sci. 1978, 257, 737 3. Hamaker, H. C. Physica 1937, 4, 1058. 4. Visser, J. Adv. Colloid Interface Sci. 1972, 3, 331. 5. Fowkes, C. F. M. In "Surfaces and Interfaces"; Burke, J. J., Ed.; Syracuse University Press: New York, 1967; pp. 197-244. 6. van Oss, C. J.; Absolom, D. R.; Grossberg, A. L.; Neumann, A. W. Immunol. Commun. 1979, 8, 11. 7. van Oss, C. J.; Absolom, D. R.; Neumann, A. W. Sep. Sci. Technol. 1979, 14, 305. 8. Morgan, J. L. R.; Woodward, H. E.; J. Am. Chem. Soc. 1913, 35, 1249. 9. Lewin, S. Br.J.Haematol 1972, 22, 561 10. Padday, J. F. In "Surface and Colloid Science";Matijević,E., Ed.; WileyInterscience: New York, 1969; Vol. 1, pp. 101-149. 11. van Oss, C. J.; Bronson, P. M. Sep. Sci. 1970, 5, 63. 12. van Oss, C. J.; Bronson, P. M. In "Membrane Science and Technology"; Flinn, J. E., Ed.; Plenum: New York, 1970; pp. 139-149. 13. van Oss, C. J.; Neumann, A. W. Immunol. Commun. 1977, 6, 341. 14. Girifalco, L. A.; Good, R. J. J. Phys. Chem. 1957, 61, 904. 15. Good, R.J.;Elbing, E. Ind. Eng. Chem. 1970, 62, 54.


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IONS, SURFACES, M E M B R A N E S

16. Gibbs, J. W. "The Equilibrium of Heterogeneous Substances," "The Collected Papers of J. W. Gibbs"; Dover Publications: New York, 1961; Vol. 1, pp. 219-328. 17. van Oss, C. J.; Gillman, C. F.; Neumann, A. W. "Phagocytic Engulfment and Cell Adhesiveness"; Marcel Dekker: New York, 1975; pp. 7-19. 18. van Oss, C. J. Annu. Rev. Microbiol.1978,32,19. 19. Neumann, A. W.; Good, R. J.; Hope, C. J.; Sejpal, M. J. Colloid Interface Sci.1974,49,291. 20. van Oss, C. J., unpublished data.


RECEIVED October 17, 1978.


Bioelectrochemistry: Ions, Surfaces, Membranes - ACS Publications

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