ironi the solutioii iz lov. The n o & of denetting is therefore little influenced by the protective colloid. For sol stabilization, however, not only the work of desorption but also the speed of desorption is important. For macromolecular protective colloids the speed of desorption may be so low that two colliding sol particles are separated by the Bron-ninii movement before a local desorption has occurred The stabilizing polver is maintained in this case in spite of the low work of desorption. The nearly identical $01 stabilizing power of methylcellulose and carboxymethylcellulose in 0.5 S S n C l clearly demonstrates the influence of suppressing the electrical interaction betlveen the sol particles by the high ionic strength. S t low ioiiic strength where the electrical interaction is not suppressed methj l ( d l u l o ~ eand carhoxymethyl cellu-
lose behave quite differently. I n Fig: S the increase of turbidity caused by addition of L a c 1 is plotted as a function of the SaCl concentration, aiid that without any addit'ive and with constant concentrations of met~hylcelluloseand of carborymethylcellulose. At' NaCl concentrations lo\\-er than 0.1 L$7 carboxymethylcellulose inhibits the coagulation whereas niethylcellu!ose causes n coagulation at S a C l concentrat'ions Tvhere the sol is stable in atiselice of any additive. This result is in accordance n-ith the fact, that the electrokinetic potential a t solid-aqueous solution interfaces is increased by carboxymethylcellulose and decreased by methylcellulose.9,10 ({I) I\-.liling a n d H, LanFe, Kolloid-2 , 127, I 9 (1952). (10). 11. v. Stackelberg, TV. Kling. \V. Iic~izrila n a l 1%'.TVilke. i b i d . , 135, ti7 (1934).
IYETTISG OF POLk'-(METH\-L JlETHilClEYLATE) ASD POLYST17KESE BY IYATER ASD SALIVA BY 11. G. CHAIG,G. C. BERRYAXD F. .I.~ ' E T T O N School of D e n t i s t r ~ C'niLerszty , of Jlrchzgan, A n n .libor, illzchzyaiz Recezved Xovember 1 1 . 1969
The wel,ting properties of poly-(methyl methacrylate) (PMRIA) arid polystyrcne IPS) in contact with distilled vxter and saliva were determilied by contact angle measurements. The advancing contact angles 8, for distilled \\ster and saliva u n PhIhlA were measured to be 78 & l o and 75 5 1O arid on PS were 86 f 1O and 79 & 1'. The receding contact angles 8, for distilled water and saliva on PMMA were 50 i. 1' and 0" and on PS {Tere 6-1 f 2" and 0 . Pigments arid opacifiers used in dental P1lAI-A or PS did not alter the contact angl s appreciably except in the case of 6, for water on PS xvhich was reduced auproxiniately 20". Certain components of the saliva appeared to be strongly adsorbed by PS and PlIMA since for water and saliva on the plast'ic surfaces stored in saliva for 48 hours decreased. Values for 9, on plastic surfaces contaminated with saliva were 57 to 65" althouch the values for 8, w r e 0". The force required to separate PMM.4 or PS from a glass surface ivith an interveiling film of water or saliva was dctermined and compared with values calculated on the basis of capillarity using the above data. The calculated and experimental values agreed within experimental error, supporting thc use of the capillary equation for liquids which form a finite contact with the solids.
The netting properties of solids by liquids are of great importance in many fields of science. I n dentistr) the wetting properties of denture base plastics by human saliva is of interest since the extent of \tet,ting has a n effect on the retention of the denture. The denture base serves two purposes, first it supports the artificial teeth in a normal functional position and second it is contoured to the shape of the oral tissues to provide retention of the d e n t d structure during normal mastication. The retention of a denture base has been attributed to a number of factors w r h as physical, physiological, psycliological, ineclinnical aiid surgi(d. This study \\-as limited to an investigation of the physical fGictors related to denture retention. Force. 01' ~ohe,~ioii, adhesion aiid capillarity, a t one time or mother, have been cited a i the main reami for denture retention. It m$, therefore, the purpo5e of this itivestigntioii to study the fundnmental \,etting properties of water and saliva on plastic s u r h c ~ - . The suifarc tCtiqion of the saliva nas dete *milledby the riiig method and the wetting propertit s of saliva on poly-(methyl methacrylate) and polj styrene denture base plastics were evaluated by (sontact mgle measurements T n ndclition, the purpose \ m s to determilie the force required t o .?parate a plai;tic surface from a glass surface u i t h ,111 inter\ ciiiiig filni oi d v a aiicl to compare these
experimental values wit'h values calculated according to the equation expressing the force needed to separate tivo parallel solid surfaces with ai1 interveiling film of liquid. Experimental Materials.-Heat-activated poly-(methyl methxrylatc) and polystyrene (Jectron) denture base plastics as \Tell as c*ommercisl clear poly-(methyl methacrylate) and polystyrene, free from any solid lubricants such as zinc stearate, were used in this study. The distilled water for t'his study had n surface t,ension of 72.3 dynes/cni. at 25' as measured n i t h a tlu Noriy trnsiomet>er. The human sdive was rollcrted by chcwing p:tr:tHiii \Y:I,X :md only that portion collcct~cd:titer t,he first 100 nil. \V:W used. Thc si1rf:ac.e tension of the sulivu :ilso \viis c1vtc.rmined with :L du 5 6 u y tensiometer and v:diics from 5:1.-1 to 57.0 dj-rivs/cm. a t 25" n-ere obtained. Saliwt with :i surface tension of Z3.4 dynes/cm. was used in the contwt :ingle Incasureni(,nts. Fresh sdi\ ~ : t used s in :ill c:~sesand w:ts rtLfriger:tted when not in actii iise. It shoiiltl l)c notp that d i w is gpnernlly compost'cl of \ 1 9 . ; 3 r ~ w:iter, 0.2c inorg:tnic~srilts (nuinly Ii,i:d (nxtinly niuc:in, :t gliic-o-plotck, 0 . 4 ' 7 ) . Contact Angle Measurements. -~'I'hi:s h a d o ~box tecnlinique was used t,o determine t,he contact, anglrs of water and saliva on the p l a d c surfaces. Thc contart angles were measured dircct,ly from an enlargement, of :4 photographic negative of the drop profile. The ront:tl.t angles were nic:tsiirc:d ai 25' i i i the prcsenw of tlic ~ : t imtod t the liquid formiiig tlic drop. Tile stable d v a n t rccctling c o n t x t :in+ were nieasurcd in ;dl c.:~scs rcportcd in T:tblc I.
R. C. CRAIG,G. C. BEREYAXD I?. A.
542
~'EYPON
Vol. 64
BURET
'i
COUNTERBALANCE \
\
GLASS PLATE LIQUID FILM
I-
L
LOAD1NG BUCKET
L
PLASTIC \ CONTACT
INTERFERENCE
POINT
FRINGES
GLASS PLATE
Fig. l.-Siictch
of equipment used for sep:irating force values.
determining
TABLE I COXTACr .ih'GLES 01' \\-.\TER
AND
SALIVA" O X I'lflIA
AND
PS AT 2 5 O Liquid drop
8 2 0
Solid surface
Surface treatment
Paraffin wax Commercial wax Clean
88,
degrees
10'3 i 1
The attractive force F should also represent tlie force required to separate the surfaces if the receding contact angle cos 8, is substituted for cos e. These equations assume that the viscosity of the liquid is low and thus the rate of load application may be neglected. The equipment used to measure the separating forces is shown in Fig. 1. The plastic surface to be tested was placed on one of the pans of a trip balance and a liquid film formed between the plastic and a glass plate supported above the balance pan. The glass plate was used to simulate the hydrophilic oral tissues although it n-ab hard and unyielding compared to the tissues. The pans of a trip balance move in such a nianner that thc direction of movement is nearly a straight line over small distances and thus it may be assumed that movement was a straight line from the start of the load application until rupture ocmmed. The glass plate was aligned parallel t o the plastic surface by observing the interference fringes as indicated in Fig. 1. The separating force was measured by thv follon-ing procedure. The liquid film was formed b e t w e n the purallel surfaces and a set load of 150 g. was applied to the rountcrbalance pan. The excess liquid was removed and water added to the loading bucket until the surfnces separated. The load obtained was found to be essentially independent of the loading rate. The separating force F with a S L L ~ I Vfilm S ma) be calculated from equation 2 if the film thickness d is known, since all other quantities either are knoivri or have been determined. If the value of d is constant, and the same area il is used, then the separating forces measured for tn-o different liquids should be a function of the surface tension and cos Br. The separating forces for two liquids should be such that
e*,
degrees
'39 f 1
Poly-(methyl methacrylate) (?!ear Clean 78 =t 1 50fl l'igmc~itetl Clean 76&1 5 l i l Clear Clean 75 f 1 0 Clear Salivab 68 =t :J 0 Clear Saliva' 65 i 5 0 Polystyrene t i 2 0 ('ltw Cle:in 86 f 1 6-13~2 0 I'ignionteci Clc:111 83 i 1 45 = t 2 liva (2lear Clean 71) i 1 0 ;Saliva l'igmented Clean 80 i 3 0 ;Saliva Clear Saliva' 56 i 5 0 ISaliva I'igmcntetl Saliva* 58 i 5 0 H20 C1ea r Salivab 57 It 5 0 a Surf:m t.ension of s:iliva as 53.4 dynes/cm. b Thc :iiirf:tces were stored in saliva for 48 hours and superficially ~,v;tshrdwith distilled water just prior to milking the contact :tngle measurements. Retention Force Measurements.-The attractive force bctween two parallcl plates separ:tted by a liquid film which itstends to the edgc, of the p1:ttes and forms a zero contart :~nglewith the plates has been expressed by Bikerman' in the (quation
H20 H,O ;Saliva ;Saliva €120
here .f rtyresents the attractive force, y the surface tcnrion of the liquid film, -1 the surface area, and d the liquid film thickness. If the liquid forms a finite contact angle 3vith the solid, the attractive force would be reduced, pret:umably by a fartor representing the cosine of the contact mgle and, thereforc, equation 1 may be modified in the inanner 33
(1) J. J. Bilicrm;m, " S u r f a c e Cherrlistry for Industrial Research," icadcniic Prcss, Inc., X'ew Y n r l i , N. Y., 1948, pp. 31-32.
where F I and Fn, y1 and y 2 , and 81 and 61 rcpresent thc sepiirating forces, surface tensions and contact angles, respectively, for the two systems. The entire expression has been denoted as a ratio R . Thus, if the surfaces are aligned using one liquid for which it is known t,hnt the separating force is due to capillary attraction, then the film thickness can be computed from equation 2 and a sepnrat>ingforce predicted for some other liquid. Alternately, the separating forces can be substituted into equation 3, and the deviation of t,he value of R from unity used as a measure of the presence of some retention force other than the capillary attraction. To test the accuracy of this method, the separating forces for two glass plates were measured with methanol and water as the two liquids. It should be remembered that the crosssectional area of the film changes as the siirfaces part, as does the film thickness. If one computes d from the measured force and cross-sectiona! area by using equation 2 , the d computed is some effective film thickness which is a function of t,he rate of change of the film area n n d tohedegree to which the plates have been oriented. Thus, for Anids whose viscosities are similar, the measured forces should be in the ratio of equation 3 so long as the orientation of the surfacc is not varied from one force measurement to t'hc next. The measured forces for methanol and n.:tter with glass plates were such that R = 0.942. Since it can bo :&ssumcdthat! capillary attraction is the only force operating, this irdic:ited a 6T0crror in thc measurement,. The average values for I2 obtained with mttthnnol or w a k r as one liquid and saliva as the second liquid and using glass and PMRIA or PS surfaces are reported in Table 11. These values were within experimental error of unity, and therefore it was assumed that capillary attraction was the principal forcc. involved in the sepwating force mmsurements.
Results and Discussion The coiitact angles for dist,illed wnter :uid saliva oil poly-(methyl methacrylate) aiid polystyrene are list'ed in Table I. In addition, the advancing and receding aiigles for tlistilled water on paraffiii wax ivere determined as a check 011 the expeririieiitd
May, 1960
\VETTING OF 1'OLY- (METHYL METHACltYLATE) BY \17AThlt ASL) 8ALIV.i
TABLE I1 E(1RCh R h Q IRLU
S r a ~ k c sR I T H sol1d surface
TU
\N
IZ Glas 0 925 € ' ~ I ~ I - l 1 050 p .: 1 00(i
SULIUS FROM
sEI'kR.4TE
INTER d,
ING
A
p
g. force
17 3 21 7 25 3
176 140 117
FIB a t 25 force g. force/cm.
Fcrlc.
e:
162 148 117
y
50 f 3 43 1 2 45 5 1
procedure and also are reported in Table I. The contact angle> for water on paraffin wax of 109 and '39" for Oa :uid Or, respectively, mere in close agreement with the values of 110 and 99' obtained by Bartell and Shepard2 on smooth paraffin wax surfaces. The adImic>ingcontact angles of 78 and 76' for water on clear and pigmented (dental grade) poly(methyl methacrylate) , PnInL4, were within experimental error. The receding contact angles of 50 a i d 51' for m t e r on clear aiid pigmented PAIAIX also agreed and were approximateiy 25' less than the advancing angIes. The advancing angles of 75 aiid 68" for human saliva on clean and saliva contaminated clear PRINA indicated that the wetting by saliva was very similar to distilled water and that the contaminated surface ~ r a more s easily wetted by saliva. The receding coiitact angles for saliva on I ' X L i could not be measured since complete Fpre:tding occurred and thus the angles are denoted as zero angles. The advancing aiid rereding angles for water on saliva contaminated P X M A w r e approkimately the same as for saliva 011 the coiitniiiinated PML4, indicating that an adsorbed film I\ us controlling the system. The advaucing contact angles for water and saliva on clean surfaces of clear and pigmented (dental grade) polystyrene. PS, indicated that the wetting of the two liquids was very similar. The advancing angle of SG" for ivater on a clean surface of clear 1's is soniewh:tt loner but close to the value of 91 " reported by Elliwi and Zisman.' The receding angle for w-ntcr on clear 1% Thas approximately 20' lower and on pigmented PS was about 40" lower than the ( orresponding advancing contact angles. L i b Irith E'SIAILi, the receding contact angles for saliva on PS were aln-ays zero. The advancing :ingles for d i v a or water on saliva contaminated 1%were \Tithin experimental error, having an average value of 57'. Thus, it appears that some component of the Saliva such as mucin is adsorbed on the PR.IM.1and PS surfaces and results in the receding contact :uigle being zero. In gene1al, the contact :ingle reiults shower1 that tlie wetting properticy of PlIlIA and PS b y \rater ::iid saliva were iiniilar in Spite of the fact that the lornier h:is citer groups while the latter has phenyl group5 exleiithg off the main polymer chain. Since it i gciirwdly recognized that the phenyl group is rimre c.asil,v wetted b y water than the methyl groiip, thr. +iniilarity in wetting properties of P A I l L 2 :tiit1 1'3 may l)e d u to ~ an nlteratioii of (2) E. 1 . B irtell mil J n SLepard, Trrrs J O L B ~57, ~L 211 (1953). _I
('3)
4 H Ell son .ind W .\ Yisinan zbid., 68, .503 ( 1 9 i l )
the wetting properties of the methyl group by the The moderately large hysteresis effect noted with water on PNIlIX and 1's suggests that adsorption effects as well as frictional effects, surface heterogeneity or surface roughness are responsible for the contact angle hysteresis of 20 to 25". The large hysteresis effect observed nith saliva on PMAIX and P S is of particular interest and indicates the formation of strongly adsorbed fiIms on the plastic surfaces. The fact that the advancing angles for water on plastic surfaces having an adsorbed film of some saliva component were 57 to 65", while the receding angles were zero is possibly due to an additional adsorbed layer inverting the nature of the film. In regard to the use of these plastics for denture base materials, it was of considerable importance that the hysteresis observed with saliva mas 75 to 79" and that the rereding angles for saliva on these plastics were always zero since the recediiig angle is the angle fornied when a force tends to remove a denture base. Thus, cos 0, has the maxinium attainable value of 1 and contributes its maximum effect to the separating force F in equation 2. The experimental separating forces F o b s required to separate glass, PAInIA and PS from a glass surface with a n intervening film of saliva are recorded in Table 11. Also listed are the ratios K . the film thickness d, the calculated separating force Fcalc., and the separating force per unit area at a corrected film thickness of 25 p . The film thickness d as determined by using methanol and water as liquids and substituting into equation 2 the experimental separating forces and the recediiig contact angles for the liquids against the solid. The values of the film thicknesses obtained were between 17 and 25 u which are within the experimental error. The fact that approximately the same film thickness was obtained with different liquids and solids indicates that surface irregularities of the various types of zurfaces were similar or alignment conditions were consistent. The values for tlie ratio R shown in equation 3 xvere calculated usiiig water or methanol as ffuid 1 arid saliva as fluid 2. The values oht:iined for R were within experimental error of unity and the observed separating forces were in close agreement n-ith the corresponding separating forces calculated from equatios 2 . The separating force per unit area for t hc system glass-saliva-ghsb was little different from the values for the systems glasssaliva-PAINA and glass-saliva-13. would be expected from the similarity of the n-cttiiig properties of I'lISLi and 1'8. no difference n in the beparating force per unit area for t1ie.e t n o plastics. These results show that capillary forces are the main physical forces involwd in the separation of parallel plates with an intervening liquid film. The scpnratiiig forre measurements alco wbstantiate the use of equation 2 for liquids forming a finite contnct with the solid wrfare.
GLASS attached oxygen.
SALIVA FILMAT 25'
Fob,
543
