T H E INTERFACE SYMPOSIUM
WALTER DROST-HANSEN
STRUCTURE O F
Water Near Solid Interfaces Existence of ordered structures at waterlsolid interfaces is postulated along with possible existence of long-range order vidence is presented for the existence of ordered eleE ments of water structure near certain aqueous/ solid interfaces. This evidence is based on various pieces of experimental observations by many authors, but particular importance is attached to the existence of thermal anomalies in the properties of vicinal water. A three-layer model for the structure of water near certain water/solid interfaces is discussed in some detail; in this model a layer of ordered (structured) water is expected to exist near the solid surface, the ordering extending into the bulk liquid and decreasing as a function of distance from the interface. As the degree of structuredness is decreased, the local disorder is increased. At sufficiently large distances from the surface, bulk water structure exists. Because of “lattice misfit” between the (unknown) structure of the bulk water (possessing local order) and the differently structured water near the interface a region of enhanced disorder may occur, akin to the disordered structure postulated in the Frank-Wen model of hydration. Indications are that the vicinal water is generally not Ice-I-like. T h e ordered structures near some types of interfaces may be clathrate cage-like (in particular, in the presence of clathrate-forming solutes) or resemble high pressure ice polymorphs. T h e primary distinguishing features are probably not the specific geometric aspects per se, but rather the expected lifetimes of the (possibly rather illdefined) structures. The properties of water in interfacial systems often exhibit notable thermal anomalies and these anomalies are interpreted as manifestations of structural transitions. This is consistent with the suggestions that the structures responsible for the anomalies possess welldefined thermal stability limits. They constitute evidence for higher-order phase transitions, effected through cooperative action within the structured units. The 10
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proposed model has several advantages: (a) it may explain why widely differing values are frequently obs tained for both thermodynamic and transport propertie of interfacial systems in different studies on supposedly similar materials; (b) by invoking clathrate cage formation (or the creation of some other structural entity possessing voids or “sites”) as the predominant feature of the structured vicinal water, the predeliction of some nonpolar solutes for many, otherwise relatively inert, surfaces may become readily understandable. The Pauling model for anesthesia is consistent with the notion of vicinal clathrate formation. The anomalous water described by Deryagin may possibly represent a new type of bonding in water, as recently proposed by Lippincott et al. If Lippincott is correct, the role of the quartz surface is probably to facilitate the rehybridization of the water molecule to allow for a completely new type of OH2 compound. Returning to the more general case, structured water near an interface may represent stabilized water (with a structure different from Ice-Ih); it may, for instance, be its own clathrate hydrate (possibly with the majority of the interstitial positions filled with water molecules), T h e water near an interface inducing such a structure will not serve as nuclei for Ice-I. The nucleating efficiency of certain materials (such as silver iodide) may instead be due to facilitated nucleation in the disordered water between the vicinal water and the bulk structure. T h e obvious difficulty with models of the type discussed here is how to make them quantitative. With as many different “phases” (phases in the sense of types of structures of water) as are involved here, it is obvious that a large number of adjustable parameters must be invoked. Hence, quantitative (or semiquantitative) agreement between calculated (estimated) and observed values is not likely to be sufficient proof of the
Photograph shows highly idealized model of ordering of water near solid surface. First f e w layers of molecules are highly ordered, while liquid, at some distance from surface, has normal undistrubed bulk structure. Between ordered layer and bulk structure may be disordered zone caused by d8erences in “lattice order” between diferent structures VOL. 6 1
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model if tested only on a small number of phenomena. For this reason, evidence for the correctness of the model must be sought in terms of the largest possible number of qualitative features which can be correctly explained.
I. INTRODUCTION A. Interfacial Water The properties of water have been systematically studied for more than 80 years. Unfortunately, these studies have resulted in only a modest increase in our understanding of water structure. Even less is known about the structure of water near an interface, although not for want of experimental information. O n e purpose of this article is to suggest that, in some respects, advances may be made more rapidly in the study of interfacial water than in bulk water. Two of the most natural questions to ask regarding water near an interface are: (1) How is the water structure near a solid interface different (if it is different at all) from bulk water? (2) If the structure of vicinal water is, indeed, different, how far from the interface does the altered structure penetrate into the bulk region? I n this paper, a brief review of some data is presented which suggests that the structure of water near interfaces is different from bulk water and a few comments are made on the thickness (the depth) of the changed layer (or layers). A specific model is proposed (based on an earlier suggestion by the present author) which invokes a “three-layer model” for the structure of water near certain interfaces. This model is conceptually similar to the hydration model for ions in (bulk) electrolyte solution suggested by Frank and Wen (46). T h e purpose of the present paper is not primarily to present a general review of the structure of water near a solid surface. Instead, the paper is intended to develop some possible conceptual models of vicinal water. T o develop such models, it is useful to review a part of the literature, and in many respects the present paper is an extensively annotated (although far from complete) summary (rather than a critical review) of the available evidence for long-range ordering of water near various interfaces. Such a presentation is justified by the current need to call attention to the large number of multifaceted types of surface chemical studies which contribute fairly substantial evidence for the existence of long-range ordering near interfaces. Though not specifically stated in each case, the reader is always admonished to scrutinize carefully for himself each of the examples mentioned. Also, it should be kept in mind that while some of the examples quoted may not, by themselves, be very strong contributions, one should seek to see the overall picture, rather than seek out the flaws in the minutiae. I t is my firm opinion that when all of the evidence is seen as a 12
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whole, there is good reason to believe that extensive ordering can, indeed, occur near interfaces. One facet of this study, however, is presented with no apologies or hesitation. I am quite firmly convincedand hope that the evidence will support this contentionthat thermal anomalies frequently occur in the properties of water near interfaces and that this may be taken as evidence for the existence of structured units of water near interfaces. As the thermal anomalies are not pronounced and have not been firmly demonstrated in the properties of bulk water, but seem readily discernible in the properties of vicinal water, it is reasonable to infer that the interface is able to induce or stabilize structures. I n other words, stable structures with a definite thermal stability limit may be induced by the proximity to a surface. It is recognized, on the other hand, that our knowledge of the depth of such surface-induced structures remains very uncertain. T h e author merely calls attention to various possible estimates rather than attempt to provide firm limits for the range of the ”deep surface orientation layers.” 6.
Previous Studies
T h e older literature regarding orientation near interfaces was reviewed by Henniker (56) in an article entitled “The Depth of the Surface Zone of a Liquid.” T h a t article has-for better or worse-become a classic in the field. While Henniker’s article is concerned with what he referred to as “deep surface orientation,” in general, the present article deals only with some of the available evidence for such surface orientation in aqueous systems. I t is primarily intended to be a summary of some of the experimental observations pertinent to this question and a contemplative discussion of various ideas which may well suggest the existence of ordered structures near aqueous interfaces. Introduced also is the notion that a separate zone of disorder may exist because of “lattice misfit’’ between one of the possible (“long-range”) ordered, vicinal water structures and the differently structured (but only ‘(short-range” ordered) bulk water. T h e older literature, on which the claim has been based that there exists at many water/solid interfaces, long-range ordering, was reviewed in the article by Henniker. Subsequently, arguments for and against ordering a t interfaces (especially clay surfaces) have been presented by many authors, notably by Low-see, for instance (82)-and by Martin (88). More recently, the possibility of significant structural changes in water near a particular interface (quartz) has been brought to culmination by the signal contribution of Derjaguin (25) in Russia. Derjaguin’s now well-known reports on highly anomalous water in narrow quartz capillaries (sometimes referred to as “ortho water,” “anomalous
water,” “super water,” “water 11,” or “polywater”) appear to be evidence that water may exist in “states” other than those classically recognized. C.
Evidence for Structure Based on Thermal Anomalies
T h e arguments in the present paper are based on a different approach from that pursued by previous authors. At its foundation is the observation that thermal anomalies exist in the properties of water and some aqueous solutions and that these anomalies are indicative clf structural transitions. I t is further noted that the thermal anomalies are far more readily observed and more pronounced in the properties of water near interfaces-e.g., the water/air interface. If thermal anomalies exist in the properties of vicinal water, they are most likely due to structural transitions, akin to higherorder phase transitions. The transitions thus announce the existence of stabilized structures near the interface, each structure possessing specified thermal stability limits. The possible existence of thermal anomalies in aqueous systems has been amply discussed elsewhere [Drost-Hansen (37-34, 38)1. A point to be stressed is that the structure of bulk water probably does not contain Ice-Ih-like elements, but some other type of structures-perhaps clathratelike; perhaps some type of clusters, or possibly highpressure ice polymorphs. This implies that the model for (bulk) water structure most likely to prove correct is a “mixture model” in which equilibrium exists between monomers and some structured units, be they clusters, ice polymorphs, or clathrate cage-like structures. Without detailed documentation, it is proposed at this time (but consistent with earlier proposals by the present author) that water near certain interfaces may possess a region of enhanced disorder between the two ordered zones-respectively, the zone adjacent to the solid on the one hand and the (locally ordered) bulk structure on the other hand. Some of the evidence for the existence of enhanced disorder (near certain solid surfaces) is presented later in the paper.
D.
Philip Low and his associates (who generally favor the idea of long-range ordering near interfaces) and Dr. van Olphen and Dr. Torrence Martin (both of whom have, with equal eloquence, argued against the notion that long-range ordering occurs at clay-mineral surfaces-or, at least, argued that it is not necessary to invoke such long-range order to explain the rheological properties of aqueous clay systems). Clays and clay suspensions have attracted much attention because of their practical importance-ranging from problems in soil mechanics, and the petroleum industry, to atmospheric nucleation problems. Even more intriguing are the remarkable properties which such systems possess, However, the present author has largely avoided discussing the water/clay systems in connection with the possible existence of induced water structures near interfaces: I t is difficult to separate structural phenomena from the geometric effects because of the packing of the clay platelets or the effects of the adsorbed (exchangeable) ions on the water structure. (Generally, the concentration of ions near the clay surface is high. This results in rather concentrated “local” solutions and such are theoretically far more difficult to treat than water itself or water in dilute solutions.) Furthermore, the clays in aqueous suspensions are obviously never in a true equilibrium state. Thus, their properties will continue to vary with time, because of such factors as the continued hydrolysis of aluminum and other degradations due to water/clay interactions. An additional difficulty, of course, with water in clay systems, is the interlamellar sorption resulting in lattice expansion. This particular effect may be monitored with precision by X-ray diffraction. However, this water must be so highly influenced by the confining surfaces that it becomes a matter of semantics if one wishes to refer to this as water or simply as an extension of a hydrate structure of a very complex chemical form. An excellent brief survey of the colloidal and surface properties of clays can be found in an article by Chambers ( 1 5 ) . This article also summarizes a few of the current writings regarding water structures near clay surfaces.
Problem of Clays
T h e properties of aqueous clay systems are mentioned only in passing. An impressive volume of literature exists concerning the problem of water/clay interactions, and the reader is referred to the writings of Professor
AUTHOR Walter Drost-Hansen is with the Institute of Marine and Atmospheric Sciences, University of Miami, Fla. 33149. This paper was presented at the Interfaces 11 Symposium on the Chemistry and Physics of Interfaces, Washington, D . C., June 7968. Contribution Number 7107 from the Institute of Marine and Atmospheric Sciences.
I I . T H E R M A L ANOMALIES A.
Anomalies in Surface Tension of Water
As outlined in the Introduction, part of the evidence for the existence of ordered structures near interfaces is built around the occurrence of thermal anomalies in the properties of water. I t has been suggested that these anomalies are evidence of higher-order phase transitions-such transitions, obviously, only occurring in partially ordered, structured units of a certain minimum size. Furthermore, the anomalies are more frequently fiound in the properties of interfacial systems than in bulk systems, although various solutions appear VOL. 6 1
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Figure 1. Surface tension of water as a function of temperature Data by Timmermans and Bodson
also to reveal thermal anomalies (the thermal anomalies are often referred to as “kinks”). As an example of a thermal anomaly, Figure 1 shows the surface tension of water as determined by Timmermans and Bodson (734). This example, together with a large number of other interfacial phenomena, has been discussed in detail in an article by the present author [Drost-Hansen ( 3 4 ) ] . I n Figure 1 the surface tension of water appears to possess an inflection point in the vicinity of 12 to 14°C. T h e data were obtained by several independent methods, all yielding the same quantitative results. I t should be noted that other investigators have measured the surface tension of water over this (and extended) range of temperature without finding corroborating evidence for the anomaly. [See, however, the classic study by Franks and Ives (48) on the interfacial tension between hexane and water. See also Note A4ddedin Proof. ] B.
Infrared Evidence for Thermal Anomalies
Some of the more spectacular demonstrations of thermal anomalies have come from Goring and coworkers. Elsewhere in this paper we discuss the observations made by Ramiah and Goring (772) on water near cellulose surfaces and the viscosity study by Ihnat (64,working in Goring’s laboratory. For the present, we focus attention on the spectroscopic observations made by Salama and Goring (718). These authors measured the intensity of infrared absorption of water at 2100 cm-l and gave their results in terms of per cent change in absorption as a function of temperature. Their graph shows a notable inflection point in the vicinity of 30”C., and that feature was stressed by the authors. A careful inspection of the data suggests also 14
the possible existence of a similar, but less pronounced, anomaly in the vicinity of 45°C. Salama and Goring applied a “moving quadratics” curve-fitting method of analysis. The result of this analysis to the infrared absorption data is shown in Figure 2. T h e very pronounced minimum a t 30°C. is one of the most convincing examples of a thermal anomaly that has been reported in this temperature range. However, it is unfortunate that in this otherwise impressive example, one must exert some caution in interpretation. The measurements were made using a high-resolution Unicam Vacuum Grading Spectrophotometer, calcium fluoride windows being used. However, the total cell thickness was only 12p. If one assumes that structures may exist near interfaces which conceivably could extend to a notable depth, it remains possible (although not highly probable) that the observed absorption changes may be caused, in part, by the water adjacent to the windows. Thus, the observed effect may not be directly applicable to bulk water. Another possibility is that the observed anomaly is related to the water associated with the hydration of dissolved ions (Ca2+, F-) either in solution or in the window material. Finally, because the measurements were made at a fixed frequency, a movement in band peaks with temperature could result in various degrees of superposition of different bands with rather different temperature coefficients. Hence, anomalous changes are conceivably possible because of this shift alone. O n the other hand, the abruptness of the transition appears to argue against this possibility, as does the coincidence of the temperature at which the change is observed with the temperature at which anomalies in a variety of other aqueous properties occur. The foregoing two examples illustrate quite typically the notion of more or less abrupt changes over a rela-
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Figure 2. Plot of temperature derivative of IR absorption of water (at2700cm-’) Data by Salama and Goring
tively narrow temperature interval. As discussed before by the present author, the occurrence of such narrow transition regions appears quite frequently. I n fact, a large number of such instances have been found and described although the existence of the transitions has been vigorously denied by other authors. A somewhat conciliatory notion, which may explain in part the divergence of opinion, has been proposed recently [DrostHansen (38)l.
111.
EVIDENCEFORSURFACE STRUCTURES (Anomalous Temperature Dependencies in Various Properties) A. Disjoining Pressures Much of the evidence for long-range order near interfaces presented in the early years (especially by Derjaguin and coworkers) was criticized because of the possible spurious influences of such factors as dust, surface swelling, or electroviscous effects (in viscosity work). Recently, Peschel and Aldfinger (105)-see also Peschel ( 104)-have determined the disjoining pressure of water between quartz surfaces. These authors measured the force between two quartz plates in water (one planar plate, the other slightly curved; radius of curvature about 100 cm). At constant temperature, the pressure dependence on the separation can be represented by an equation in the form
Pd =
Figure 3. Disjoining pressure of water between two highly polished quartz surfaces, separated 700 A Data by Peschel and Aldjnger
C
-
dn
where Pd is the disjoining pressure, d is the separation, and C and n are parameters. T h e authors studied the disjoining pressure for separ?tion between the two quartz surfaces of a few thousand A or less. T h e parameters C and n are temperature dependent, and Figure 3 shows the observed temperature dependence of the disjoining pressure and of the parameters C and n. T h e results are seen to be highly unexpected and complex. Peschel and Aldfinger suggest that the extrema observed near 15 and 32OC., as well as the anomaly slightly above 6OoC., may be related to the anomalies discussed by the present author. More recently, Aldfinger (personal communication, 1968) has obtained additional measurements of disjoining pressures and these are shown in Figure 4. Again, one observes the persistent appearance of multiple, relative maxima in the disjoining pressure as a function of temperature. This example is of considerable interest because while earlier results used to demonstrate the existence of surface-induced structures in the water mighi have been influenced by spurious effects, there is no reason to suspect that any “mechanical” disturbance (for instance, the presence of dust or “swollen surface layers”) should give rise to such notable temperature-dependent variations. I n particular, one does
Figure 4. Additional data on the disjoining pressure of water Data by Aldjnger
not expect, by pure chance alone, that any variations in the temperature dependency would correlate with the anomalous temperatures, proposed independently on an entirely different basis. Thus, it seems reasonable to take the experiments by Peschel and Aldfinger as unequivocal evidence for the reality of ordered structures, possibly of considerable range, undergoing transitions at the various temperatures proposed by the present author. NMR Studies of Colloidal Suspensions Pethica, Clifford, and coworkers [see Johnson et al. (77)] have made a comprehensive study of colloid stability and the role of water structure in the interpretation of such stability. While the general topic of
B.
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Figure 5. Log rate of flocculation of polyuinyl toluene particles US. reciprocal absolute temperature Data by Johnson et al.
colloid stability is somewhat peripheral to the main subject pursued in the present paper, we stress here the fact that structured water has been proposed to exist surrounding micron-sized colloidal particles of polyvinyl acetate (PVA) and polyvinyl toluene (PVT). Pethica and coworkers prepared sols of these polymers and studied them by various techniques. Figure 5 shows the rate of flocculation of polyvinyl toluene particles us. reciprocal, absolute temperature. An abrupt anomaly is indicated in these data at approximately 42” or 43°C. Figure 6 shows the spin-spin relaxation time (T,) as a function of the absolute temperature for a suspension of 0 . 8 ~PVA particles. I t is seen that abrupt changes occur near 3°C. and near 31°C. Pethica, Clifford, and coworkers discuss the suggestion that there is a structural breakdown in the water between 30” and 32°C. This, indeed, is the temperature at which we have claimed the existence of a notable anomaly-in particular, in aqueous surface phenomena. Pethica and coworkers concluded that “the evidence presented (here) generally supports the views of Derjaguin on the longrange structuring of boundary layers of water.” We return to the papers by Derjaguin and by Pethica, Clifford et al. in the discussions in Sections VIII-D and X. C.
Figure 6. Log spin-spin relaxation time (Tz) as function of reciprocal absolute temperature in a suspension of polyvinyl alcohol particles Data bv Johnson et al.
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Dielectric Constant of Water as a Solute
I n connection with the colloid studies discussed above, it is of interest to note that dielectric constant measurements have shown rather abrupt anomalous changes in dielectric properties of water-saturated (and partially saturated) hydrocarbons at discrete temperatures. Figures 7 and 8 show some of the results obtained by the present author (unpublished data) where the capacitance of the dielectric cell is plotted as a function of temperature at very closely spaced intervals over the range from 20°C. (Figure 7) to 55°C. (Figure 8). I t is apparent that notable changes in the dielectric properties occur at approximately 45°C. I n fact, further measurements of this type on partly water-saturated solutions (water in benzene, cyclohexane, and carbon tetrachloride) revealed anomalies at or about all four temperatures where anomalies have been claimed to exist in purely aqueous systems. I t is possible that the observed effects are due “only” to water adsorbed from the solutions onto the walls of the capacitance cells. If this is the case, the results imply the existence of ordered water structures between a “wet” hydrocarbon phase and a glass surface. If the observed effects, however, are not due to such adsorbed water, the results suggest instead that at least Some of the water in the organic liquids is present in an associated (polymeric) structured form. I n either case, it is noteworthy that thermal anomalies are observed in the absence of a bulk phase of water, and this point is
Figure 7. Capacitance changes (in p p F ) of capacitance cell containing carbon tetrachloride: half saturated with water at 22°C. Drost-Hansen, unpublihhed data
essential to certain aspects of the discussion which follows. D.
Drost-Hansen, unflublished data
Micelle Sizes
Ottewill, Storer, and Walker (707) have studied the micellar weight of a nonionic surfactant (dodecyl-hexaoxyethylenc-glycol monoether) in DzO. T h e results showed a remarkable constancy in molecular weight (near approximately 50,000) in the range from 5" to 15°C. followed by a very abrupt increase in micelle size to approximately 840,000 at 35°C. (Figure 9). The method employed was the Archibold sedimentation equilibrium procedure, and the results were essentially similar (at temperatures above 15°C.) to the results obtained by Balmbra by a light scattering method. Ottewill and coworkers conclude that above 15°C. (and up to 35°C.) spherical micelles are aggregating to form larger units. T h e question of why the size remains constant at the lower temperature is not elucidated. However, it would again be reasonable to suppose that an explanation may be invoked in terms of differences in water structure. Thus, below 15"C., a water structure is stable which is conducive to stabilizing a certain kind of micelle, but as the temperature is increased above this point, the stability is more or less abruptly diminished and the micelles, as a result, find it energetically more favorable to rearrange into the larger units, consistent with that vicinal water structure which is stable in the region above 15°C. E.
Figure 8. Capacitance changes (in p p F ) of capacitance cell with benzene; one third saturated with water at 2 2 0 ~
Water and Hydrocarbon Dispersions
T h e absence of a sharp freezing point for supercooled water in intimate contact with various highly porous materials is a subject of active discussion. Thus, Mysels (98) felt that the phase rule would demand a sharp melting point for Derjaguin's water in narrow quartz capillaries (see the discussion in Section X-B), whereas the actual freezing appears to be reversibly spread out
Figure 9. Log micellar weight as a function of temperature Solid circles data by Ottewill et al., m's by Balbra, light-scattering methods
over the region from -15" to -30°C. Fowkes (45) commented on this phenomenon and reported measurements on water-saturated hydrocarbons, which were studied by a light-scattering method. O n cooling, water droplets appeared to occur (apparently fairly abruptly) in these wa ter-saturated and well-stirred hydrocarbons, at about +15"C. T h e scattering from these droplets was followed, upon cooling, to - 80°C. and again during reheating to +20"C. Fowkes points out that " . . . a sharp increase in scattering occurred at -20 to -31°C. VOL. 6 1
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(depending on hydrocarbon present), but no further change was seen in cooling to -80°C.” Fowkes suggests that these results could be interpreted as manifestations of “specific water” which froze and melted in the range from -20 to -31”C., but never freezing at 0°C. F.
Water on Silver Iodide
T h e problem of the state of water on a silver iodide surface has attracted considerable attention because of the importance of silver iodide as a nucleating agent in cloud-seeding experiments. Lyklema (85) has measured the surface charge (and thus the differential capacitance) on a silver iodide surface. One of the interesting results obtained by Lyklema is a notable decrease in the surface charge with an increase in temperature. Specifically, a definite inflection point is found in the vicinity of 50 to 55°C. for the differential capacitance as a function of temperature. Lyklema correctly infers that this implies a fairly sudden “melting” (at about 50°C.). T h a t this ‘(melting” is relatively sudden is further supported by some differential thermal analysis experiments performed by van der Plas and reported by Lyklerna. These experiments showed a phase transition at about 50”C., with a wet silver iodide. No such transition was observed with dry silver iodide; hence, the effect must be ascribed to a property of the water adjacent to the silver iodide surface. Further evidence for the abrupt change comes in experiments where flocculation concentrations for the silver iodide are determined as a function of temperature showing the disappearance of specific ion effects above 60” or 65°C. Unfortunately, here, as in so many other instances, measurements were made only at relatively widely spaced temperature intervals and far more details might h m e been revealed had measurements been made over temperature intervals of only a few degrees. Lyklema concluded that the silver iodide surface, at least pragmatically, may be described as “hydrophobic.” Romo ( 7 75) inquired if the observed phase transitions reported by Lyklema might not be due to a sudden change in the state of hydration of the lithium or rubidium ions employed in this study. I n reply, Lyklema stated that the change of about 20’% in thermal energy ( k T ) could hardly be sufficient to cause a dehydration of the alkali ions, more or less abruptly and completely, near 50°C. He also points out that the DTA experiment was performed in the absence of salt and in the absence of a bulk aqueous phase. Hence, the phenomenon is apparently an effect of the water at the water/ silver iodide interface and not of the bulk solution. I t should be noted in this connection that many ion properties do seem to go through notable changes in the vicinity of 50”C., such as the maximum in the B-coefficient for electrolytes [see, for instance, Millero, Drost-Hansen, and Korson (94)1. We postpone further 18
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discussion of the question of water near silver iodide particles to a subsequent note on nucleation phenomena as related to water structure models near interfaces. 0 . Volumetric Data for Water/Cellulose Sysfems
Ramiah and Goring (712) have dealt with water/ cellulose interaction in a very interesting paper. These authors measured dilatometrically the thermal expansion of various wood constituents and discovered large, anomalous changes in the expansion of‘ water-swollen materials (cellulose, hemicellulose, and lignin). They ascribed the changes to perturbations in the water structure caused by the hydrophilic surfaces of the woody macromolecules. Ramiah and Goring observed a series of transition temperatures, both for the “dried” compressed wood constituents, as well as for those in contact with water. The measurements by Ramiah and Goring were sufficiently precise that, not only was it possible to differentiate the volume changes with respect to temperature (to obtain plots of derivatives us. temperature), but the data could, in fact, profitably be differentiated twice to produce graphs which still showed remarkably little scatter. I n the case of the “dry” samples, the transition temperatures sometimes extended over several degrees. Ramiah and Goring, to aid the analysis of the data, used a “moving quadratics” method, which subsequently Salama and Goring ( 7 7 8 ) used to great advantage to study the intensity of infrared absorption at 2100 wave numbers for pure water (vide infra). When waterswollen samples were used, similar (but more pronounced) anomalies were observed. Although Ramiah and Goring had some reasonable suggestions for the possible origin of the transitions in the dried state of the cellulosic materials, it is possible those anomalies were also caused by residual water. I n spite of the vigorous drying efforts exercised by the authors, it seems possible that the last traces of water may not have been removed. (In general, the samples were degassed in a vacuum, 1- to 5-fi pressure, at relatively high temperatures.) O n the other hand, there is little doubt that the anomalous transitions in the water-swollen cellulosic components are due to structural changes in the water associated with the cellulose surfaces. [See also Note Added in Proof.] Ramiah and Goring proposed a model for such water, as shown in Figure 10. This model is essentially identical to the one advocated earlier by the present author and elaborated upon in this paper. T h e model is a three-layer model in which the structure of the vicinal water is perturbed by interaction with the polar groups on the surface of the solid. Sufficiently far removed from the surface is the unperturbed water structure while in-between exists a region which has elements of both unbonded and bonded water. I n our interpretation, this intermediate layer would be the more dis-
of from 40” to 50°C. for “unstabilized” cellulose. We return to studies of water on cellulose surfaces later in this paper (Sections VII-E and XI-C). J.
Figure 70. Model of water near cellulosic substances as proposed by Ramiah and Goring
ordered, broken-down structure, not conforming to either of the differently ordered structures of the vicinal water or the bulk water. H.
Heat of Transport and Other Phenomena across Cellulosic Membranes
I n connection with the studies by Goring and coworkers, mention should be made also of the anomalous results obtained by Haase and Steinert (53) in a study of the heat of transport across a cellulosic membrane. I n a subsequent paper, Haase and deGreiff (52) noted that the anomalies reported earlier could not be reproduced. Recent work in the present author’s laboratory (unpublished) on the membrane properties of Millipore filters impregnated with toluene [“Ilani Membranes” (65-68)] has shown that thermal anomalies are sometimes encountered in various properties of these membranes, but are absent at other times. The properties in which the thermal anomalies have frequently, but not invariably, been observed include concentraticin potentials, bi-ionic potentials, and membrane conductances. Reasons for the lack of reproducibility are not currently known with certainty, but may be related to the penetration of water into the porous matrix (Gratin and Drost-Hansen, 1969, unpublished). 1.
Thermodynamic Properties of Adsorbed Water on Cellulose
Some interesting thermodynamic properties of adsorbed water on cellulose have been reported by Morrison and Dzieciuch (96). These authors found evidence for a transition from an immobile to a mobile film or the appearance of a new mobile film, above the immobile one, and a disordering effect, with increasing amounts of adsorbed water. These authors, incidentally, also discussed the observation that there seems to be a change: in the cellulose/water system at about 40” to 50°C. ‘They quote Collins as having found “a parallel break in the curves of the cross-sectional area changes and the regain when plotted against temperature, with a complete change in direction occurring in the range of 40” to 50°C.” They also quote Wahba and Nashed, who found the isosteric heat of adsorption to decrease markedly above an optimum temperature
Heat of Immersion of Clays
Another example of pronounced thermal anomalies was discussed previously by the present author (38)namely, the results obtained by Slabaugh from heat of immersion measurements on homo-ionic clays. I n the study by Slabaugh (728),a very sudden increase in heat of immersion occurred in preheated calcium bentonite, around 29” to 30°C. I n spite of the very small amount of water retained on the clay after the pretreatment, this finding may perhaps be interpreted in terms of a sudden change in structure of the (remaining) water structures on the preheated calcium clays, prior to immersion.
K.
Rates of Wetting of Fatty Acids
An interesting experiment with fatty acids in contact with water has been made by Yiannos (145),who studied the orientation of fatty acids on water surfaces. h’iannos determined the rate of approach to equilibrium of the contact angle as a function of temperature and concluded that two of his results (at 30” and 35°C.) were out of line with the remaining points and suggested that this could possibly be explained in terms of a transition taking place at the surface. Yiannos speculated that this might be related to a two-dimensional lattice stability for fatty acids on a surface [see also Section VII-B]. It seems reasonable to suggest that the results are another manifestation of the different water regimes at the interface, leading to different kinetic and equilibrium properties of water near interIaces. 1. Rates of Crystal Growth from Aqueous Solutiorr Sipyagin (127) has studied the rate of growth of different crystal faces of two alkali chlorates as a function of temperature at constant supersaturation. His results suggest that anomalous growth rates occur for both sodium chlorate and potassium chlorate in two temperature ranges. The ranges depend somewhat on the particular crystal face studied, but in general, fall in th:: vicinity of 1 ” to 13°C. and 31” to 42°C. T h e author concluded that these anomalies appear to reflect changes in the structure of the water near the growing crystal faces. Obviously we are dealing here with water in rather concentrated solutions. I t would be expected that the structure of the water would be considerably influenced by the force fields of the ions and, as a result, not show any evidence of thcse types of discrete stri ctures which would be expected in pure water or in dilute solutions of electrolytes. T h e present author has coiLimented on this problem on an earlier occasion (32). Another study of crystallization frLm supersaturated solutions of strong electrolytes has been made by Melia VOL. 6 1
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and Moffitt (90). These authors determined the number of nuclei produced in fresh and aged solutions of supersaturated ammonium bromide. Their data, for the number of crystallites formed, distinctly show the occurrence of thermal anomalies near 15’ and 30” C. Again, it is remarkable that structural effects should become manifested in such concentrated electrolyte solutions. M. Thermal Anomalies as Evidence of Surf ace Structures
The examples discussed in this section are intended to convey some of the evidence for more or less abrupt changes in many aqueous interfacial systems at discrete temperatures. Other examples are mentioned in a recent Note by the present author (33). The examples by Peschel and Aldfinger and the study by Pethica and coworkers clearly demonstrated the existence of thermal anomalies in the properties of water near solid interfaces. The data on the dielectric properties of water dissolved in organic solvents (or adsorbed from such organic solvents) suggest that the anomalies are found even for very low concentrations of water. The study by Ottewill and coworkers suggests that the anomalies may occur for water in interfacial systems where the “substrate” is highly dispersed (micelles), while the results by Fowkes suggest the existence of low temperature anomalies (somewhat similar in nature to the dielectric studies by the present author). The results by Lyklema on silver iodide and by Goring and coworkers on the water/cellulose interface, as well as the results by Haase and Steinert on cellulose, all suggest that water is structured at the interfaces studied and is capable of undergoing structural transitions at discrete temperatures. The example by Slabaugh of the heat of immersion of clays as a function of temperature shows definite effects of the temperature of pretreatment, but the results are probably too difficult to interpret in structural terms because of the extremely low concentration of residual water. Similarly, it is also difficult to interpret the results by Yiannos on the rate of wetting of fatty acids. A further complication is introduced in the studies by Sipyagin and by Melia and Moffitt, although, again, there appears to be little doubt but that anomalies do occur and it seems reasonable to propose that they are also related to water structure.
IV. DIELECTRIC AND NMR RESULTS A.
Dielectric Studies
Dielectric and nuclear magnetic resonance measurements have frequently been used for studies of structures near interfaces. We have already mentioned the studies by Pethica, Clifford, and coworkers, by Ottewill and coworkers, and by the present author. These studies were concerned with temperature effects on 20
INDUSTRIAL A N D ENGINEERING CHEMISTRY
the rate processes; we now turn our attention to further dielectric and nuclear magnetic resonance studies in general. First, we review briefly some of the available evidence obtained by dielectric measurements. One of the most interesting of these studies is one made by Palmer and coworkers (102)who measured the dielectric properties of water between sheets of mica. I n this study, very low values for the dielectric constant of water were obtained, indicating oriented water adjacent to the mica surfaces. The experiments by Palmer, Cunliffe, and Hough on mica are among the most convincing examples of evidence for long-range order in water near suitable inrerfaces. As a result of measurement at frequencies at two and three mHz, these authors were able to state: “ I t was found that the dielectric constant of the water decreased with the thinness of the film from more than 20 for films about 5p in thickness to less than 10 for films about 2 p in thickness (thickness calculated assuming no absorption). Even with very wet, composite dielectrics, the dielectric constant of the water did not exceed 60.” They also concluded that “The water film is tending to act not as ‘solid’ water, but rather as ‘liquid ice’.” I n this connection, they observe that while the distinction may be subtle, it may be important in connection with attempts to gain a better understanding of the abnormal mechanical properties of liquid films, and this is probably a particularly valuable point in connection with water in biologic systems. More recently, Afanas’ev and Mestik (2) have contributed very significantly by their dielectric studies on water near mica surfaces. These authors also interpret their data in terms of ordering of the water by the substrate. Many of the results obtained from dielectric studies, especially studies of water on hydrated aluminum oxide and similar adsorbents, have suggested the existence of very firmly bound water at interfaces. Transition layers appear to exist and these may extend for a certain depth away from the solid surface. This liquid resembles supercooled liquid at sufficiently low temperatures-where the liquid may go into an amorphous glass over a particular range of temperatures. I n connection with the question of dielectric properties, see also the studies by Windle and Shaw (141) on the hydration of wool, studied at very high frequencies (26 kilomega Hz). Various types of adsorbed, bound, mobile, and intermediate water are considered by these authors. Also of interest is the study of the Hall effect by Chai and Vogelhut (14). These authors studied the structure of bound water on crystalline hemoglobin by a microwave technique at 9.36 kilomega Hz. I t was observed that a hydration of 0.13 g of water per gram of dry hemoglobin appeared to be a critical number (to be compared with a value of 0.24 g per gram of hemoglobin, obtained by Rosen). T h e authors conclude that the
which one may assume an ordered water lattice ("Icelike") as the hydration shell, have dispersions of about l o 5 to lo7 Hz. [See also the article by Windle and Shaw ( 7 4 7 ) l . C.
Figure 7 7 . Temperature dependence of T-tan 6 for water adsorbed on chondroitin-4-sulfate (various amounts of water, in milligrams, per gram polymer) Data by Lubezsky et al.
angle of rotation of the initially adsorbed water begins to increase above the critical hydration level and take this to indicate the formation of hydration layers, due to hydrogen bonded clusters of water, on the surface of the hemoglobin molecule. B.
Advantages and Intrinsic Difficulties of Dielectric Studies
Unfortunately, one of the problems in the interpretation of dielectric measurements on mixed systems is the nature of the mathematical models required. I t is difficult to separate the individual (dielectric) properties of the bulk phases from those effects which are derived from the particular and unique aspects of the interfacial regions. Thus, Debye dipolar relaxation and Maxwell-Wagner conductivity absorption will contribute to the overall dielectric behavior of heterogeneous samples. As a result, complex curves are usually obtained for e ' and e" in, say, clay suspensions, as a function of frequency. Among the authors who have contributed notably to this field are Ebert, Frisch and Wood, Stewart, Palmer, Muir, Windle and Shaw. Essentially, the diagnostic usefulness of dielectric measurements lies in the fact that bulk water has a dielectric dispersion of about 1O1O Hz while, for ice (depending on the temperature), the dispersion occurs between 100 and 10,000 Hz. As pointed out-for instance, by Jacobson (69)-many macromolecular solutions of molecules, for
Water on Chondroitin-4-Sulfate
An example of unexpected temperature dependencies for water comes from a recent study by Lubezky et al. (84). These authors studied the dielectric properties of water adsorbed on chondroitin-4-sulfate. Among the interesting results observed in this microwave study was the persistent occurrence of peaks in the loss tangent of water adsorbed on the polymer in the vicinity of 35°C. (or slightly below) and the notable drop in absorption above approximately 60" to 61°C. (Figure 11). The authors recognized that the phenomenon observed cannot be explained in terms of capillary condensation in the conventional sense. Instead, an explanation was attempted in terms of specific hydrogen bonding to slightly different sites for binding (which were presumed energetically close to each other) on the hydrophilic surface of the substrate. The present author, however, prefers to interpret the results in terms of specific thermal stability limits for different, discrete water structures adsorbed onto the solid surface. If this is the case, it may obviously be of great importance in connection with the states of water in connective tissues. Chondroitin-4-sulfate is apparently, to a large extent, responsible for the water retention in the connective tissues of animals (see Section V re water in biological systems). D.
Other Dielectric Studies
Another interesting dielectric study is reported by Johnson and Neal (72). These authors concluded from a study of the dielectric properties of alumina, fibers, and carborundum (in aqueous systems) that an orienting effect likely existed, caused by the solid surface. This orientation extended into the liquid to a considerable distance from the surface. They also called attention to some dielectric studies on polyelectrolyte solutions which tend to support such a model. The authors, however, did not propose any specific depths of orientation of the water adjacent to the solids.
E.
Water Adsorbed on Alumina
Subsequent to the important studies by Palmer and coworkers [see Palmer (702) and Cownie and Palmer (23)],Ebert and Langhammer (39) have pursued the problem of the dielectric properties of water adsorbed on various solids and, in particular, dealt with water adsorbed on aluminum oxide. While it is not possible here to summarize all the significant findings of Ebert and coworkers, we call particular attention to one feature. Ebert and Langhammer observed remarkable peaks in the temperature derivative of the dielectric VOL. 6 1
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constant of their systems at a discrete temperaturenamely, in the vicinity of -30°C. (within, say, =!=3 to 5 degrees). T h e occurrence of anomalous rate phenomena in porous matrices at low temperatures is also mentioned in connection with Antoniou’s (5) study of water on Vycor (see below). Again, such phenomena likely reflect unusual properties of relatively thick structured layers of water, stabilized by the proximity to the solid. Another interesting finding by Ebert and Langhammer is the notable maxima in the loss tangent for water adsorbed on y-alumina (measured at 100 kHz) when plotted as a function of the amount of water adsorbed. T h e position of the maxima (i.e., maximum for loss tangent us. amount of water per gram alumina) shifts to lower concentrations at higher temperatures. As an example, at -lO”C., the maximum appears at approximately 56 mg HzO/g alumina while at +2OoC., the maximum occurs at approximately 30 mg HzO/g alumina. This suggests the thermal breakdown, at higher temperatures, of the thickness of a structured layer, McIntosh (87) has discussed Ebert and Langhammer’s experiments in detail, and suggests that “ I t is reasonable to suppose that the first amounts adsorbed are firmly bound and have little orientational freedom. As amount of adsorbate is increased, successive additions have greater and greater freedom of movement and they also influence the condition of the first quantities adsorbed.” F.
Nuclear Magnetic Resonance Studies
Resing ( I 14) has recently presented a careful review of the present state of nuclear magnetic resonance relaxation as a tool for the study of the state of adsorbed molecules on surfaces. Among several other aspects, Resing discusses the molecular motion of adsorbed water on various specific adsorbants. 0. Water on Silica
T h e question of water near silica surfaces and near alumina surfaces has been studied by nuclear magnetic resonance techniques and reported on in detail by Michel (93) and by Reball and Winkler (113). T h e study by Michel suggests that for water adsorbed on silica gel, two types of water exist in samples with only a 3/4 statistical monolayer. T h e water structures are presumed to be organized into clusters (containing as much as 95% of the adsorbed molecules) with a correlation time of 2.7 X 10-l0 sec (at 0°C.) and individually adsorbed molecules, with a correlation time of larger than 2.3 X 10-8 sec. H. Water and Cyclohexane on High Surface Area Silica An interesting N M R study was made by Pickett and Rogers (107). These authors were particularly concerned with the nature of adsorption of liquids on 22
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Figure 12. Observed line widths as a function of calculated number of surface layers of cyclohexane ( A ) and water ( B ) on Cab-0-Si1 M - 5 Data
61. Pickett
and Rogers
pyrogenic silica as well as on other silica preparations. Figure 12 shows a graph of the observed line-width us. the (calculated) number of surface layers for water and cyclohexane adsorbed on Cab-0-Si1 M-5. They proposed that the narrow line-width of the resonance lines observed on Cab-0-Si1 M-5 for low surface coverages may best be explained by suggesting highly mobile molecules on the surface. For the higher surface coverages (20 to 54 layers), the signal broadening may be related to polarization and orientation of the first few layers and these, in turn, causing further layers to become built up with lattice-like order. The authors conclude: “Above about 50 layers, the ordered structure apparently breaks down and a more mobile, gel-like structure may be formed.” Results obtained with cyclohexane and water were similar. T h e similarity between the N M R results obtained with water and with cyclohexane on Cab-0-Si1 indicate that while aqueous systems likely do possess anomalous properties specifically related to the nature of the water molecule, general surface effects (such as those due to van der Waal and London disperson forces) will, obviously, be found in any interfacial system. Using NMR, Karagounis et al. ( 7 4 , for instance, have shown a lowering of the melting points of various organic compounds (such as 9-ethylanthracene) adsorbed on silver. I n this connection, recall that Henniker, as well as Derjaguin, have long advocated long-range order for both water and organic liquids (including oils) on a number of solid surfaces. See also the Note by A. D. Bangham and D. R. Bangham ( 8 ) .
I.
Water on Various Clays
Wu (744) has studied the clay-water bonding by a nuclear magnetic resonance technique. Spin-spin and spin-lattice relaxation times were determined with water and deuterium oxide adsorbed on kaolinite, grundite, and montomorillonite. Wu concluded from this study that below O’C., the water adjacent to the clay surface has a structure different from that of ice. Relating the inverse of the relaxation time to the viscosity, Wu finds apparent viscosity values for water near sodium montmorillonite (at a water content of 100%) of 110 P. At a water content of 4oY0, an apparent viscosity of 330 P is obtained. These values are in reasonable agreement with the values estimated by Oster and Low (700) based on an energy of activation of about 8 kcal/gmol for ion movement in bentonite with two molecular layers of water. If we assume the energy of activation for viscous flow is close to the value for ionic conduction, the result would correspond to a viscosity of these water layers of 700 times that of ordinary water. Wu compared his measurements also with earlier measurements by Rosenqvist ( 7 77). I t must be concluded that the various estimates for viscosity of water near clay surfaces show very large scatter, and the concept of laminar Poiseuille flow for water layers, only two molecular layers thick, seems highly inappropriate (the classical difficulty with the application of continuum mechanics to molecular, discrete happenings). At the same time, it appears definitely as if anomalous and, indeed, extremely high values of apparent viscosity are obtained in studies of water near clay surfaces. Further Studies of Water on Silica Gel Zimmerman and coworkers (747) have studied the nature of water on silica gel by nuclear magnetic resonance spectrometry. These contributions are among the most authoritative studies of their kind. I n connection with the present paper, particular interest attaches to the studies by Woessner and Zimmerman (743). These authors, among many other observations, noted a transition in the vicinity of 15OC. for the apparent proton longitudinal relaxation time as a function of temperature for water adsorbed on silica gel at very low surface coverages.
J.
K.
Utility of Dielectric and NMR Data I n this section we have discussed both dielectric and nuclear magnetic resonance studies of aqueous interfacial phenomena. The difficulties encountered here are the traditional difficulties of interpreting rate processes in general and, particularly, under circumstances where no specific models are available for the quantitative treatment of the data. Thus, all dielectric data suffer from the possibility of spurious influences of various polarization mechanisms while the N M R
results are complicated by the underlying physical processes and their quantitative descriptions. However, while none of the studies per se may prove conclusively the existence of ordered aqueous structures near interfaces, some of the data do strongly tend to suggest the possibility. Perhaps this, in particular, goes for the dielectric studies by Palmer and coworkers as well as by Ebert. Undoubtedly, the most sophisticated studies are those by Zimmerman and coworkers, Resing, Michel, and Reball and Winkler. However, it is here that the difficulties of interpretation become the greatest.
V.
BIOCHEMICAL AND BIOPHYSICAL STUD I ES
A.
NMR Studies of TMV I n no field is the problem of water near interfaces more pertinent and crucial than in the general field of biology. Biological systems per se fall entirely outside the scope of the present article; however, we briefly describe a very small fraction of the evidence for the existence of ordered water structures in biologic systems, as suggested by biochemical and biophysical studies. Even though relatively simple biological systems in vitro may be complex beyond description compared to, say, the air/water interface or a solid/water interface, biologic systems must eventually be interpreted in terms of the physics and chemistry of the underlying molecular processes. As an example of systems of biologic interest which have been studied in detail are solutions of tobacco mosaic virus. The results from these studies amply demonstrate the contradictory results which have been obtained so far. I n particular, very different results have been obtained, and antithetical conclusions have been drawn from self-diffusion coefficient measurements (for water) in such solutions. Jardetsky and Jardetsky (70) concluded on the basis of their N M R studies that as much as 20% of the water in a l.9yotobacco mosaic virus solution exists in an ordered (“ice-like”) structure. However, Douglass et al. (29) concluded from selfdiffusion measurements that the diffusion coefficient of water in such solutions is very close to the diffusion coefficient of water in pure water. I n fact, it was estimated that no more than 3% of the water (in a 4% tobacco mosaic virus solution) is in an ordered state (compare Section IX). Other studies of similar nature have been made by Balazs, Bothner-By, and Gergely (7). These authors studied solutions of DNA, hyaluronic acid, collagen, gelatin, and myosin. I n none of these solutions was evidence obtained for the existence of ordered (“icelike”) domains. However, a broadening of the proton signal was observed in the DNA solutions. Similar broadening was observed in the presence of graphite particles while the presence of sodium chloride had no VOL. 6 1
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effect on the broadening. Later, Hechter, Wittstruck, McNiven, and Lester (55) studied water in agar gels by high resolution N M R methods. These authors concluded that “The N M R data can best be explained at present on the basis that water in an agar gel is in a modified state with properties of structural rigidity and mobility intermediate between ‘free’ water and ice.” Collison and McDonald (78) studied the broadening of the water proton line in the NMR spectra of starch gels and concluded that the width of the water line in starch gels and suspensions depended upon the hydration of the starch, suggesting a more highly ordered lattice than exists in pure water (while part of the line width was determined by the mobility of segments of the macromolecules). NMR in Nerve Fibers Other evidence for the orientation of water molecules by proximity to a biologically important interface has come from proton magnetic resonance studies by Chapman and McLaughlin (76) on the sciatic nerve of rabbits. These authors measured the proton magnetic resonance spectra of a 4-mm-long cylindrical sample of nerve, the orientation of which could be varied with respect to the applied magnetic field. T h e results of the experiment suggested that the “bulk” water inside the nerve is in a partially ordered state. Furthermore, other spectral peaks showed orientation-dependent shifts which were larger than could be accounted for owing to susceptibility shifts. Again, this was interpreted as indicating partially oriented water. T h e authors considered four possibilities for such ordering: (A) The water might simply be constrained by van der Waals forces to move in annuli of approximate (B) molecular dimensions (quoted to be about 10 T h e orientation could be produced as a secondary effect caused by the orientation of the protein molecules, which by themselves, would be aligned in the pores of the nerve. (C) Orientatioll effects were suggested caused by electrostatic interactions near the fiber structure. However, calculations apparently suggested that such field strengths would be too weak to explain the observed effects. (D) I t was suggested that the water could be arranged in a chain structure as postulated for the collagen system. T h e authors dismiss this latter possibility as having no justification, claiming the degree of orientation observed to be explicable completely in terms of normal water. They concluded: “ I n view of the normally accepted pore sizes in nerves which are much too large to cause orientation, it is surprising that most of the water is not in its normal state.” I n view of the large number of examples discussed in the present paper, it may not be too surprising a t all to discover that a large fraction of the water is in an “abnormal” state. B.
A).
24
INDUSTRIAL AND ENGINEERING CHEMISTRY
C.
Studies of Muscles
Bratton, Hopkins, and Weinberg (13) measured the change in the transverse relaxation time for protons in the water of a living skeletal muscle (from a frog). These authors found that “A fraction of the intracellular water molecules have restricted rotational freedom and this fraction decreases when contraction occurs.” They conclude that the available evidence points to the existence of (at least) a two-phase system of free and bound water in this system. D.
Evidence for Extensive fflce-Like” Restructuring
Cope (27) used the N M R spectrum of sodium ions to study the solubility of sodium in the interstitial water of actomyosin gel. T h e solubility of sodium in this (gelled) water is considerably lower than the solubility in bulk water, suggesting that the water in the gel is organized (structured) into an (‘ice-like” state as a result of the actomyosin molecule/water interface interactions. Cope concluded: “If a major fraction of intracellular sodium ions exists in a complex state, then major revisions in most theoretical treatments of equilibria, diffusion, and transport of cellular sodium ions have become appropriate.” More recently, Cope (79) extended his N M R studies by the use of deuturated water (administered to the live animals for several days prior to isolation of the brain and muscle tissue studied). Cope was able to demonstrate quantitatively that the results obtained were not artifacts caused by paramagnetic ions, and he concluded, on the basis of all the available evidence, that his data could only be explained by assuming extensive structural ordering of the water in tissues. The two articles by Cope are likely to become classic examples of the attempts to demonstrate evidence for ordered water structures in living systems. The results clearly are consistent with the overall picture which has been developed 0~7erthe past several years by the present author regarding the ability of interfaces to induce pronounced structural changes in vicinal water-changes which may extend over considerable distances, and which could, in fact, “restructure” the water contents of the entire cell. E.
Variolus Biologically Important Examples; Biologic Aggregation Phenomena
Regarding the implications for biology of structured water, attention is called to the rather inspired small book by Szent-Gyorgyi (729) and the Symposium Proceedings on “Water in Biological Systems” edited by Kayushin (77). See also the Proceedings from the Conference on “Forms of Water in Biologic Systems” [see New York Academy of Sciences (Q9)]. Regarding water within membranes, attention is called to recent
articles by Coldman and Good (77) and by Scheuplein (727), the study by Thorhaug and Drost-Hansen (733) on temperature-induced, anomalous changes in membrane properties. Further reference is made to the monograph by Webb (740), the articles by Cope (79, 20), and to the recent study by Fuller and Brey (49). T h e latter article will be discussed in some detail in the section on Structural Interpretations and Some Supporting Evidence (Section XI-G). Suffice it here to note that these authors observed greater freedom of motion of protons in the secondary water layer than in the “solid,” initially adsorbed water and a much less regular structure. They further discuss the occurrence of a “kind of second order phase transition in which increasing temperature disorders the arrangement of the liquid bond by the protein.” Very low energies of activation are calculated for the NMR absorption line width (about 1 to 2 kcal/ mol for ordinary water) at high temperatures. This leads the authors to propose that the correlation time is determined by transfers between the primary and secondary phases of water, rather than by either of the phases alone. Fuller and Brey also observed a transition below 2OoC., in which a fairly abrupt decrease was noted in line width with increasing amounts of water. Compare this finding with the types of phase transitions discussed in the present paper. I t is, indeed, this type of temperature effect which is employed in the present article as an important part of the evidence for the existence of structures, near interfaces, which are different from bulk water and which have definite ordering-i.e., structures which are capable of undergoing specific, thermal transitions. We return to the study by Fuller and Brey in the abovementioned section. An interesting study of the effect of water structure in a biologic system was reported by Abdulla (7). This author studied the aggregation of platelets of human blood; specifically, he measured the effect on rate of aggregation of platelets due to the presence of very dilute solutions of pentanol, hexanol, octanol, and decanol. The higher the number of carbon atoms in the alcohol (all at very low concentrations: 5 mg/lOO-ml solution), the larger the effect. Abdulla suggests that the hydroxyl groups of the alcohols tend to disrupt the “ice-lattice” surrounding each platelet while the nonpolar, hydrocarbon part enhanced the structuredness of the vicinal water. Abdulla also measured the effects of argon and xenon on the platelet aggregation and again observed notable effects. As the result, Abdulla suggests that “Ordered water acts as an entropic trigger that trips the balance and carries the system (platelets) over a small potential energy barrier, so that the internal energy of the system
can drop and its entropy can increase.” Thus, the effect of the inert gases, in this case, is to enhance the water structure surrounding the platelet.
F. Cell Adhesion in General I n connection with the problems of cell adhesion, an earlier contribution by Pethica (706) is also of interest. In outlining the factors which will affect adhesion and repulsion between cell surfaces, Pethica lists specifically the repulsion between cell membranes which may be due to “hindrance to attraction due to steric barriers, such as inert capsules and solvated layers.” Incidentally, Pethica pointed out, “This hindrance is not a ‘force’ of repulsion, except that the entropy effect due to the mutual disordering of adsorbed layers as the surfaces approach might be regarded as a force. T h e effect of adsorbed inert layers may more usually be to increase the range between otherwise active groups, and to attenuate the attractions between the surfaces to the point where reversible collisions can take place.” Indeed, if the three-layer model discussed elsewhere in this paper is correct, there may be “steric interferences,” determining the minimum distance of approach between aggregating cells (or other types of aggregating, colloidal particles), where the minimum distance of approach between cells corresponds to the maximum exclusion of the disordered layer between adjacent particles. The effects of structural changes of water in and near biologically interesting interfaces have been discussed in some detail by the present author (33) and by Thorhaug and the present author (33) and by Thorhaug (732). 0. Structural Transitions Affecting DNA
I n a recent review (30), some pertinent remarks were made regarding temperature effects on biologically important systems. Thus, it was observed that interfacial polarization effects, due to accumulated ions associated with the surfaces of DNA, may explain low frequency dispersion phenomena (for frequencies smaller than 1 kHz). The authors, however, note that r‘On the other hand, it is difficult to understand why this nonlinearity of ionic origin disappears at about 60°C.” I n a number of experiments, obtained over wide frequency ranges, the capacity of DNA samples (with 40% water content) is shown in Figure 13, part B. Douzou and Sadron (30) suggest that the changes in slope at the temperatures indicated are due to some
