Jan., 1953
WETTINGCHAR.~CTE:IW~WS OF CJ~LLULOSE: DERIV.YYIVES
49
Discussion
continuous; and (4) capillary pores becoming filled with entrapped air. The results obtained in this investigation on the From the standpoint of the plants, rate of movecapillary rise of water in soils appear quite logical ment is more important than distance of movement. in the light of our present knowledge of soil moisThis is because the water requirements of plants ture relationships. are immediate, large and continuous. While in Capillary conductivity is fundamentally contime, water may rise to considerable distances ill trolled by the tension gradient which acts as the the soil; the rate of rise is too slow to benefit the driving force in the movement of mater. In immediate and large demands of the plants for actual practice, however, and especially under water. The conclusion is, therefore, that capillary field conditions, this tension gradient is beset with movement of water, especially from the deeper many difficulties and its force may be opposed or counterbalanced by various soil factors. Among water tables, supplies little, if any, water to the the most important of these factors are the follo~v- growing plants during the short seasoil of growth. Actually, the water that is the greatest determining ing. factor in plant growth, is that which the soil is (1) The tenacity with which the water is held by the soil increases with an increase in the moisture able to retain from rainfall and irrigation, or the tension. This increase of tenacity and tension available capillary water. The roots of plants, begins to operate from as high a moisture content therefore, search for this water, which they find with ease and rapidity. In, this connection it is of as field capacity, which is the maximum amount of interest t,o note the astounding .rate and extent of water soils can retain under normal field condi- root growth that plants make in a single season. tions. Movement of water as free energy increases Dittmer'* has estimated that a single rye plant is bound to become extremely slow or non-opera- under favorable conditions develops as a seasonal tive. total, a root surface, including root hairs, of 6875 ( 2 ) Even where theie is free water, as is the square feet and a total root length of 387 miles, case in a water table, capillary rise of water can be carrying 6603 miles of root hairs. The average tremendously impeded by: (1) the friction of the daily increase of root lengths alone was estimated capillaries to the passage of mater; ( 2 ) the trans- at more thaii 3 miles. Capillarity is of importance location and s\velling of soil colloids with the i n supplying plants with water only through very coiisequent clogging of the capillary pores; (3) short distances and in small amounts. the water films and capillary pores becoming dis(14) H. J. Dittnier, Am. J . Bofauy, 24, 417 (1927).
WETTING CHARACTERISTICS OF CELLULOSE DERIVATIVES. 11. INTERRELATIONS OF CONTACT BY
B. ROGERR A Y 3 AND F. E.
BARTELL
Contribution from the Department of Chemistry, Unizvrsity of Michigan, Ann Arbor, .lf idtiyou Received J u l y 8.3, 1966
The wetting characteristics of several cellulose derivatives were determined by measuring the advancing nnd reccding angles of contact formed upon them by each of four different water-organic liquid pairs. In general, the anglc of contact measured through the water phase was found to increase as the length arid number of substituted side chains increase in the cellulose derivatives. Hysteresis of contact angle of Considerable magnitude was shown t o occur in all systems studied. Calculations based upon contact angle equations taking into account thig hysteresis effect indicate (1) that the water advancing contact angle of a given solid-water-organic liquid system, OA,, 18 closely related to the water advancing contact angle, e:,,.*, of the corresponding solid-water-air system and to the organic liquid receding contact angle, e:,, of the corresponding solid-organic liquid-air system; and (2) that the factors responsible for hysteresis effects in the two solidliquid-air systems appear t o continue to be operative in the solid-liquid-liquid system, so that e uations can be combined in such a way aa t o cancel out these factors. Consequently, one can postulate modified Rartell-asterhof e uations as follows: yw. cos e& - yOscos e,',, = yav cos e:,.o and similarly ya. en8 e L - Y~~ cos eto, = Y~~ cos e&,. $hen these forinulations were applied to each of the fairly large number of systems studied, it was found that the observed and the calculated interfacial contact angles were in good agreement.
Many of the substituted derivatives of cellulose have properties particularly suited for wetting studies. They can be obtained in satisfactory purity and can be formed into smooth films, foils (1) Presented before the twenty-sixth National Colloid Symposium which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society i n LOS Angeles, California, June 16-18, 1952. (2) The d a t a i n this paper were taken from a portion of a thesis of B. Roger Ray, submitted t o the School of Graduate Studies of t h e University of hlichigan in partial fulfillment of the requirements for the Ph.D. degree, December, 1948. (3) Minnesota Mining and RIanufacturing Co. Fellow, 1943-1945. Present address: Cheiiiistry Department, University of Illinois, Urbana.
and fibers which have readily reproducible and stable surface properties. They have low free surface energies and can be classed as typical soft solids; that is, adsorptive effects appear to be at a minimum and finite and measurable contact angles i n air are produced 011 them with several organic liquids as well as with water. Although several of these polymers are of great technical interest, there is scant information available on their wetting characteristics. It was deemed worthwhile to mnlce a study of each of the three systems of contact angles, solidwater-air, solid-organic liquid-air, and solid-
5u
B. ROGER RAYAND F. E. BARTELL
water-orgaiiic liquid, on the surfaces of each of a number of cellulose derivatives, using derivatives of known composition with surfaces prepared in known and reproducible manners. In additioii. to establishing correlations betweeii wetting behavior and the physical and chemical characteristics, it was hoped that quantitative interrelationships could be shown among t,he three contact angle systems. Significant hysteresis effects were found to occur on these solids, ie., two characteristic contact angles were found for each system depeiidiiig upon whether, in the formation of the angle, a giveii liquid phase had been caused to advance or to recede over the solid surface. The first contribution4 of this series presented the results pertaining to the cantact angles of water aiid of several different organic liquids upon the solids in air. Correlations were found between wettilie; and chemical properties (degree of substitution and nature of substituent group) aiid also between wetting and the physical properties (degree of polymerization, density, tensile strength and mode of formation of the surface). The present contribution gives the results obtained with. the interfacial contact angles of water against organic liquids upon portions of the same samples of derivatives. Experimental Details Measurement of Contact Angles.-The interfacial contact angles were measured by the vertical-rod, the verticalplat,e, the tilting-plate or the controlled-drop-volume method depending upon the form of the solid surface under study. Details of these tnet.hods, as well as ot.her details of measurements, have previously been given. ,411 t,he solid-water-organic liquid systems studied gave finite and reproducible angles except systems in which the particular organic liquid caused swelling or a partial dissolving of the solid. The initially formed angles quickly reached constant values which remained unaltered for the observation period of 10 to 30 minutes. The reproducibility was good, t.he average doeviation of the individual measurements bcing about =tl . Each solid was placed in contact with the first liquid, i.e., the liquid which was to be receded, two minutes before the angle was formed by displacing this liquid with the second liquid. The surface properties of a sample appeared to be the same whether the Sample was fi,eshly prepared or had been subjected to two years of exposure to laboratory air. The contact angles, both the angles in air and thr interfacial angles, were usually found to be significantly different on the opposite sides of foils of t.he derivatives. These foils had been cast from concentrated solutions spread on metal plates then later stripped after evaporation of the solvent. I t was necessary, therefore, to distinguish between the data obtained on each of the two sides of a foil. Mutually saturated pairs of liquids were used for the contact angle measurements. Wat'er had a negligible solubility and a' negligible effect upon the surface tensions of the oi,ganic liquids used. Likewise, these organic liquids had little effect upon the surface tension of water, a8 can be seen f r o m Table I . ,4s wported previously' thcb various solid-water-air contact angles were the same, wit,hin the accuracy of the measurements, for pure water as for water saturated with any one of the organic liquids. Materials.-& has been stated, for the interfacial nieasurements reported herein portions of the same samples of the cellulose derivatives were used as were used for the previously reported measurements in ais.4 Information was given in the previous report as to the sources, methods of synthesis and compositions of the solid materials. The carefully purified liquids were stored in dark bottles over activated silica gel. The methylene iodide discolored slightly when exposed t,o light and the last traces of surface(4) 11'. E, Bartell and B. H. Ray, J . A m . Ckem. Soc., 74, 778 (1952).
Vol. 57
active contaniinants could be removed from the broniobenzeiie only by repeated treatments with silica gel. The wat.er used was freshly redistilled. I t was stored in Pyrex bottles and was delivered by siphon from the interior of the liquid. The surface and interfacial tension values of the liquids arc given in Table I . The values shown in columns 2 and 4 were used in the calculations to follow.
TAELE I SURFACETENSIONSOF PURE LIQUIDSA N D OF \VAT& S A T ~ R A TWITH E D ORGANIC LIQUIDS;INTERFACIAL TENSIONS O F WATERm a ORGANIC Liqurns Temperature 25 i 0.1" Surface tension of liquid, L i cl I I i ds
?!a,*
dynes/oin.
Water 72.1 Methylene iodide 50.2 a-Brornonaphtlialeuc 44.0 a-Chloi.onaphthalet~e 41.9" Heptane 19.8 Broniohenzene 35.9 a Values are from the literature
Burfaue tension of water satd. with org. liquid, 7,
'&,
dynes/cin.
71.6" 70.6"
70.2" 71.3"
Interfacial t,ension.
c
YWO,
dynes/ciir.
48.0 41.6 40.5" 50.8" 38.4
Results Experimental Data.-In Table II are given the observed values of the interfacial contact angles formed on surfaces of the derivatives by several water-organic liquid pairs. For each system the advancing angle, aiid the receding angle, e;wo, are given, as well as the hysteresis effect, A0.5 Values enclosed in parentheses indicate systems in which sivelhig effect's \rere observed but in which the angles did not change appreciably with time. Values enclosed in parentheses and with an arrow, designate cases of swelling or softening in which the angles did change more or less slowly with time, finally reaching the stable values recorded iri Table 11. Each of the four derivatives studied ill the form of a foil had been produced 011 commercial equipment by flowing a 20% solution onto a metal roll and evaporating the mixed solveiits.4 I t caniiot be said with certainty which side in cadi case had solidified in contJact with air. The two sides are therefore distinguished iu the tables merely as sides A aud B. For comparison a film of AcetateFM2' was formed on a glass rod from a 20% solution in acetone-ethanol-chloroform. The "airside" of this film was found to have wetting characterist.ics more like side A of the foil than of side B, but significantly different than either. The films of the fatty acid t'riesters were formed by coat'ing glass rods from 10% chloroform solutions. 'Hie evaporation rate and humidity were controlled atid tinally the coated rods were heated at TO" for three hours. Both the advaiiciiig a i d receding interfacial angles increased in magiiitude with increase in the organophilic character of the derivatives, i e . , water wet the solids progressively less as the ( 5 ) The 8abscpiQb s, I, a, w and o stand for solid, liquid, air, water aqd organic liqiiid, respectively., y represeJits tension values and 8
the contact angle. All contact angle8 were measured tlirough the water phase. The superscriyb a or r is used with e tu denote air sdr.ancing or receding rendition, again with respect t o the wateiv phase f u r interfacial systeuth.
'
51
WNTTINGCHAKAC'TERIMTICS OF CELLULOSE DERIVATIVES
Jan., lC3.X
l'AB1.E:
I1
ISTERFACIAL C0NT.IC.r i\NULEK OF \\',4'Tb;R .iSI) C)Rl:ASIC LIQUIDSON SURFACES Ob' CELLULOSE L)tiRlV.4TIVES Heptane lletliylene iodide u-Rrottionaphthalene Broinobenaene e:,,,, A9 Cellulose e.:,,,, e;,,,, An n".,,,, n,L, AB ,e: d'wo A' dW0 degrees degrees derivati1.e degrees degrees
*"
.haet:tte-FM2 Foil, Side A" Foil, Side B" Film Triacetate Film .%cetopropioiiate- I3 Foil, Side A . Foil, Side U r , ,
1ripropioiiate
44
10:;
ti; 5!l TO
101 3
81
20.5
Ill IO5
1 IO Ilti
46
100 I05
ti9 55
50
119.5 115
64.5 46
55 69
106
82
24
113
81
32
122.5 123
75
57.5 67
37
50 4'4
55
104
1!4
75
39
110
53
57
105
--
( l . 5
41.5
128
,!it
1;
124,s
80 18 ti5 59.5 + (120)
5ti -t
133.5 120 84.5 35.5 (180) 154 Filrn (180) (180) .hetohutyritte-H 70 112 Foil, Side A 79 141.5 i2.5 69 110 67.5 Foil, Side U 'l'iil )ut !,rate Filin 1 i,ivaproatcA Filin Ethyl uellulosr-ll Foil, Sitlc A 121 65 5 55 5 (180) (10.5) --P ( T I 5) 129.5 67.5 Foil, Side 1% 118 58 ti0 (180) (68 5 ) 123 65 The system cellulose acetrtte-~~atei,-o-chloroiiahthalrne gave :tiigles of 1 I O " :itid 67.5" for side A and 104.5" for side B. The A0 values WI'C then 42.5 and 50.5 d-', respectivrly. r
1
ti0
:U
20.5 42 42.5
.
18
leiigth aiid number of substituted side chains increased. Although there were large individual deviations, the contact angles increased in the approximate order of acetate, triacetate, acetopropionate, ethyl cellulose, acetobutyrate, tripropionate, tributyrate atid tricaproate. The solid-water-air contact augles measured in the earlier work were found to iiicrease approximately in this same order. Theoretical Considerations When a liquid drop rest's upon a solid surface t'here will exist about the line of iiitersection a balance of interfacial teiisioii forces resiiltiiig iii a clefiiiit8eaiigle of coiit,act. The relattive magnitudes of these differeiit teiisioiis determine the magnitudc of the angle of cotittact of the liquid. For an ideal case the ititert,elatioiiships can be expressed by tthe Young equatioii which for a solid-liquid-air (01. vapor) syst,em is
62 58
and 54"
the measured angles themselves has been called "hysteresis of the cont'act angle." Two factors most generally responsible for "hysteresis of the contact angle" are: (1) surface roughness and (2) changes in the interfacial tension relationships. a surface is not (1) Surface Roughness.-If perfectly plane and homogeneous hysteresis effects of considerable magnitude may be obtained. For rough surfaces the magnitude of t,he apparent hysteresis appears to be largely dependent upon t,heslopes of the sides of the asperities on the surface which cause the ro~ighiiess.~,~ The cellulose derivat ives studied had \very smooth surfaces and possible roiighiiess effects haire iiot been considered. (2) Changes in Interfacial Tension Relationships.-Consider the case of a liquid drop, say water, on a solid with a second fluid phase present. The interfacial tension between water and the ot,her fluid phase will a t all times remain constant. - = COB eels (1s Nevertheless, the contact angle, measured through This equation holds for a system in true eduilihri~m water, is larger \\-hen the water drop is caused atid call be applied with assurance only to a system to advance that] when the water drop is caused to i l l which t'he surface of the solid is rierfect,ly plane recede. .This difference in angle value can h e atid homogeneous ill character and in \\-hi(*'?tJhe explained by assuming that the tension of water tension (or the free surface energy) of each iiiter- agaiiist solid or t'he tension of the other fluid against face remains constant,. \'ery few, if ally, systems solid, or botJh,do not' remain the same for a receding have beeti studied which were of certaiilty i t 1 it. (11-opas for ail acl\.aiicing drop. One or both of the condition of true equilibriiim, itniclrie tor the solid-fluid iiiterfacial tensions must have become system. altered by movemeiit of the drop. Perhaps this For practically every' system studied including alteration may have occurred only in the area solid-liquid-air (or vapor) aiid solid-liqriitl-liquid immediately adjacent to the drop periphery where systems two characteristic angles, an advancing water advances over a solid previously in contact angle and a receding angle, were obtained. Rlore- with organic liquid but recedes over solid preover, angles (at, least temporarily stable) of any viously in contact with water. The distance over d u e between these limiting vahes could have (6) B. Roger Ray and F. E. Bartell, J. Colloid Sci., in press. heen obtained. Such effect's have beeti referred to (7) F. E. Bartell and J. \V. Shepard, THIS JOURS.AL,67, Feb. as "hysteresis" effects and the act'ual difference in (1933).
"
62
.
WNTTING CHARACTERISTICS OF CELLULOSE DERIV.ZTIVES
.Jan., 1053
\vhicli alteration occurrccl \vould be relatively l'halLcratioii may be caused by uuimportaiil. ' sorption or hy layer formatioii effect,s. i n the present i nvcsi8igatiori t,lie celliilose derivatives iised \\'err ( h ~ s o I)nr.aiw ~t t,ticy had low free siirfacc energies ut i d w r e Iiot espcctcd to eshil)it, marked sorptive or layer formatioil effects. Pievertheless, since hysteresis of contact angle of considerable inagnitude occurred on the surface of each cellulose derivati\.e, possible sorptioii iititl layer efiects (wI I lot be disregarded, Calculations.-lf no hysteresis of contact angle occurred :i,t ti given solid surface, the t h e e Young equations for the solid-water-air, solid-organic liquid-air and solid-water-organic liquid systems could be used without modification. It has been shown8 that tliese unmodified equations can be combined into a single expression coiitaining oiily directly measurable quantities for systems in which all three contact angles are finite atid measurable. Where hysteresis does occur it is iiecessary to take iiito coiisideration both advancing aiid receding angles and any alteratioii of surface and interfacial t
