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Contact angles in oil-in-water emulsions stabilized by ionic surfactants
Coltoi& ant1Surfaces. 4 (1982) 173-184 Elsevier Sclentitic Publishing Company. Amsterdam -Printed
CONTACX ANGLES
IN OIL-iN-WATER
EMUMIONS
173
in The Nethedsnds
STABILIZED
BY
IONIC SURFACTANTS
M.P. ARONSON*
and H.M. PRINCEN-
Lecer &o&hers Company. Research Diwikion. 45 Riuer Road, Edgewater. NJ07020 (Received August 24.1981;
(U.S.A.)
revised versfon accepted November 9.1961)
ABSTRACT (X-tact angles In aqueous films separating emulsion droplets have been measured for uarZausknk surr’actanb as a function of type and concentration OCadded ektroIyCe, and
OFtemperature.As fn foam GImsIn ak, appreclabfecontact angtesdevelop only with
npeciEiceA?ctrotytesand above a Grit&l electrolyte concentration, which increases with temperatut 9.The relative effectiveness OF different counter-tons depends on the nature of the surfactar& head group. When the contact angle Is significant, emuWon droplets deform when they fkxcufate. Thfa alters the structure and probably afso the mechanical properties of creams Pormrr rn emulsions by sedimentation. It was shown, on the bashcof their equilibrium volume fraction of ail. that the structure of these creams is sensitive to the type and concentration of the counter-ton, to temperature, and to the manner in which the creams are formed, i.e., by sedimentation under normaI gravity forces, or by centrifugation foIlawed by retaxation, The p-xeerence of a contact angIe dramatically aItem the rate of clearing of dilute emulsions cclnrposed of very small droplab. This rapid flocculation is highly taunter-ion specific arvl reversihfe by Increaait~gthe temperature,
LNTRODUiXlON
As a result of interaction forces, the tension of a thin liquid fihn can be lower than the sum of the tensions of the bulk surfaces bounding the Eilm [Il. To satisfy the conditions of mechanical equilibrium, a macroscopic contact angle forms between the film and its associated PIateau border [Z]. Princen [3] recognized that the presence of a contact angle in films separating emo~ian droplets, as depicted in Fig.1, affects the structure and packing of concentrati :I emulsions. Later studies [4,Sl revealed that quite we angles, in some cases as large as 70”. do indeed form with several common types of surfactanf soIutions, some of which are also known to give rise to contact angles in foam fiis in air [6-111. l Present address: Union CarbIde Corporation, Tarrytown TechnIcal Center, Tarrytown, NY 10591, U.S.A. l * Present ada: Exxon Research and Engineering Company, Corporate Research Lahotito~, Linden, NJ 67036. U.S.A.
0166-tj622l82~600~900~$02.75
a 1982 Ekevier Sckntific Fubfishing Company
173
WATER
0
OIL
Pig. 1, Spontaneous deformation of Rocculatsd emution droplets under the influence of allmctive forces.
In this article, contact angb studies in emulsions are reported and compared, when possible, to published results for foam films. Although the method used is not highly accurate, and the systems have not been studied in great detail, several conclusions can be drawn concarning the effects of electrolyte type, temperature and surffictant head group. Several consequences of contact angles in emulsions, particutnr~y as a resuft of their striking temperature dependence, are then discussed, RXPERfBfBNTAL Ma teriats
Dodecane and hexadecane were obtained from Wdom Chemicals {PIuka, Puriss) aild were used without further purification, Sodium dodecyl sulfate (NaDDS) was a BDH sample that was extracted with ether for 40 h, ft has a slight surface tension minImum in distiltrd water which diwppeared at the salt concentrations that were used in the exrJeriments described below. Sodium decyl sulfate (NaDS) was 1 laboratory sample dsscribcd elsewhere [Sl . C&y1 trim&hyi ammonium bromide (CL’AD) was from Merck, while dodecylamine [DA) was from Eae:‘Jnan Chemicals. Sodium undecyl benzene sulfonate (NaLAS) was kindly supplied by Conaco. Laurie (HL) and oIeic (KOl) acids were obtained from Tridom Chemicals (Fluka, Puriss) and neutralised with NaOH to yield sodium kurate (NaL) and sodium oh&e (NaOl), respectively. All saIts were analytical reagent grade and were! used as received, Water was obtained by fiItration of distilled water through a MU-Q system (MilEpore Corp.). Several experiments were later repeated with double distalled water and the agreement was excellent.
The method used to measure contact angles is described in detail in Ref. 4.
176 Briefly, a coarse emulsion (droplet size 50-100 cc) ia prepared by shaking several ml of oil in 25 ml of the appropriate surfactant solution. A drop of the emulsion is transferred to a microscope slide constructed to have a small well and covered with a glass slip. In later studies, we found it convenient to use capillary cells supplied by Vitrodynamics, The cell is fnndferr4 to a Mettler PP52 thermostatted stage and viewed under the microscope (100X ) at the desired temperature. Doublets (as in Fig. 1) of roughly equally sized droplets are selected and photographed. The contact angles are measured either directly by constructing tangents st the cusp or from the shape of the doublets as described previously [4]. At least four doublets were used for each measurement and the contact angles were generally reproducible to about 3”. In some systems, particularly at high salt concentrations, the surfactant precipitated. These systems are indicated in the text. Most of the meaurements were made at 25°C. In some systems, particularly those that had large contact angles, the influence of temperature was also studied. The stage was heated to the desired temperature and contact angles were recorded as a function of time. The contact angles reported below were stable for at least 10 min. In some systems, contact angles were measured both an heating and cooling, and were found to be in good agreement. lnterfaciat tensions were measured by the rotating drop technique [12,13] using a University af Texas Spinning Drop Tensiometer. Equilibrium was generally established within lU-15 min. The volume fraction of oil in creams, formed on top of emulsions that were initially 33% by volume, was studied by the methods described in detail in Ref. 4. The rate of sedimentation of dilute (0.1%) hexadecane-in-water emulsions stabilized by NaDDS was studied as a function of electrolyte concentration aa follows: A 1% stock emulsion (0.4% NaDDS + 0.3 M NaCl) wa. first prcpared, using a Brinkman. homogenizer fitted with a PTA-20TS proiJe generator. The emulsion was def’oamed in the manner described in Ref, 4, Appropriate aliquots of the stock emulsion, and of 2% NaDDS and 3 Ilf electrolyte solutions were added to a graduated cylinder and diluted to 250 ml with distilled water. ‘The emulsion was placed in a water bath and left for 12 h. Three samples were then removed from the middle one-third fraction of the emulsion and their absorbances were measured. The average value was taken aa a measure of the rate of sedimentation of the emulsion. RESULTS
AND DISCUSSION
Contact angZes between emulsion drops The contact angles formed between dadecane droplets ln various surfactaut-electrolyte solutions are shown in Table 1 along with the interfacial tension, row. Ail the anionic surfactanta were used aa the sodium salt so
176
0.2% NaDDS
0.25 0.4 9.6 0.6 l*O
df NaC! llf N&l 1cINaCl dl NaCt IA LiC!
0.4% NaDS
2.4 2.1 1.9 6.7 :.; 314 6.1 3.8
0.2% NaLbS
0.25% NaL + 0.25% rJa01
0.25% CTAB
0.26% DA HCI
-8A precipitate
-0 14.6 24.2 30.0 -0 14.0 21.4 26.5 40.0 89.0 48.6 <4
0.3 0.45 6.6 0.3 0.41 0.6 0.3 0.6
Jf NaCl Jf NaCl bf NaCP sr KC1 Lrf KC1 bf WX* sf LlCl bf UC1
0.3 0.46 0.6 0.7 0.6 1,O
M bf M Bf nf M
0.9 0.9 0.9
df NaCl 8f KI M Na sdicylate
-0 -0
0.75 1.6 0.75 1.5 0.76 1.6
M KCI M KC1 df KBr M KBti di Kf 8fK.p
-0
was present at 2VC.
N&l N&1 NaCl NaCI KC1 KC1
-0
0.47 0.47 0.73 0.66
0.60 0.78 0.76 0.74 I*1
2
-0 35 57 -0 -0 49 if 76
-0 w-0
-0 -0
24.0
177
that all the sotutions cant lin roughly 5 X f03 mob N:i*, along with the added electrolyte. DA was used as the KC1 salt, The soap soIutions w xd’zequal weight mixtures of NaOi and NaL. This mixture was used because the krafft points of these soap z are above 25°C while the Krafftpoints of tk.* Dlixture are betow room temperature. This procedure avoided cornpI&.’ rms due to insolubility. A simiIar procedure could probably have been used in tt. xe cases where other surfactants precipitated but this was not pursued. Several points emerge from Table 1. First, the inftuence of electrolyte type depends strongly on the nature uf the head group. With sulfates, the contact angle increases in the order Li*
OP LNTERACWON
IN THlN
LSQWID HLMS
The contact angle, 0, is governed by both the interfacial tension and the interaction free energy A F 161, as follows: -AF=270,(l-cosU)
(1)
AF represents the decrease in free energy accompanying the thinning of a unit area of film from infinity to an equilibrium thickness, he,. The interaction free energy computed from the- measured contact angles and interfacial tensions are shown in Fig, 2 as a function of the log of the ele&roIyte concentration. Abe included are results for foam fiIms in air, as reported by Princen [6] and DeFeijter [S,ll] for NaJX + NaCl and NaDDS + N&I, respectively, The interaction energies for N&IS and NaDDS with add& Na+ and for NaLAS with added Na’ <;r K’ fall on the same line, This indicates that the difference in interfacial tensions in these systems account for the differences in the contact angles observed in Table I, Sodium soaps fall on a different line and have higher interaction energies (more negative)
Fig. 2, T~~rzwt~am energy at ablecame in water emuMan firms as a function of &ctmIyte canceatra6lan at 26-G. 9.2% NaDDS In 6he gcesetlco of Nat3 (0); 0.2% NaDS fn the pcewnce of NaCI (a). KCI (A) or tiCI( 9.2% Na LA8 in the presence of NaCi (o), KC1 (-1 or LXX(o); O.ZS% NaL + 9.28% H&L fn the presence af Nat3 (6) or KCi I*)- Llna 1 and 2 are, reqwctively. Che resufts for 0.2% NaTI + NaCl (from Ref. 6) and O,OS% NaDS + NaCI (5~~1 Ref. 9,ll) for foam films,
than the sulfates and benzene sulfonato at the same electrolyte concentrations. l?o&sium alkyl sulfates have the highest idaaction energies of the systems studied, although surfactant precipitation occurred in this system, DeFeijter (S,IIJ found that the slope d (AF)/d(Iog C,,) is appsoximately equal to the difference in surface excess of the added electrolyte at the film and bulk surfaces. The dopes for the alkyl sulfates in emuIsion fihns and foam films are quite similar. Ln fact, the slopes of all the plots for the different sur&~ctantsare reasonably similar. Comparing again the resuIts for NaDS and NaDDS + NaCl, it is seen that the curves for emulsion fitms a~ displaced to higher salt concentrations reIative to foam films. As with foam films, Largecontact angles 8ce not produced until a critical salt concentration, C&, is reached, In emulsions, Cti is -0.35 M for alkyl sulfstes + NaCi and is higher than found by Princen and DeFeijter [9], in foam filras. This difference may be due to a basic difference in the electrolyte requirements to produce Newton black films in the oil/water/oil and air/water/air systems. However, both &incen tind DeFeijter studied large vertical soap fiIms while the fims in the present study are less than 60 pm in radius. Indeed, Kolarov IS] measured contact angles in microscopic foam films in air and also found Cd -. 0.34.M for NaDDS + NaCl. However, iibove Cd, Kolarov’s results rapidly merge with those of DeFeIjter.
179
Fig.3 InteractIon energy of dadecane-h-wateremuMoon films as c function of temperature. 0.2% NaDS l 1.S MN&l (A); 0.2WbNaL-t 0.2S% NaOl + 0.6 llf NaCt (0); 0,2% NaLAS + 0.6 M NaCl (e); Lhe results of DeFeIjter and Vcij [9,11] for foam films stabilized by 0.05% NmDDS + 0.95 ICINaCl I(.)*
Temperature has 8 iarge effect on the contact angle [Fi] . For anionic surfactants with added electrolyte, the contact angIe decreases with increasing temperature. The infiuence of temperature on the AF of emulsion fihns is shown in Fig. 3 for severaI systems. DeFeijter’s results for foam films are also innduded. It is first seen that dAF/dT, which is an interaction entropy, is remarkably similar for foam and em&ion films stabilized by similar surfactant-electrolyte systems, e.g., NaDDS + NaCI and NaDS f N&I. However, the sbpes for NaLAS and the soaps are substantially lower than for the s&fate, indicating smaller interaction entropies. There is general agreement, at present, that the large attractive interaction energies in films stabilized with ionic surfactants at high salt concentrations are due to the presence of an order~3 two dimensional structure within the Newton black fifm having strong electrostatic interactions [7,9,14]. However, one interesting implication of eq. (1) is that large contact angles may develop in an emulsion film &en if the interaction free energy is relatively smali provided that the interfacial tension is low, We have aIready seen several examples where two systems of similar AF have quite different values of 0, because of differences in the interfacial tension. This may have relevance to emuIs1onsstabilized by nonionic surfactants where additional attractive interactions of electrostatic origin are not expected. When the attractive forces are exclusively of the Van der Waals type and double iayer forces are absent, AF can be aperpximated by AF=-
A 12uh=
(2)
where A is the Hamaker constant and h is the equilibrium thickness [91_ The contac% angles computed from eqs. (1) and (2) are depicted in Fig. 4 for several v,llues of A/row _ It is clear that large contact angles can develop provided the films are sufficiently thin and the values of A and row fall in the correct range. Now, it is known that foam films in air that are stabilized by nonionic surf&tan@ have thicknesses in the range of 5Cl--fiO A (21,221 particularly in the absence of electrolyte, It is likety that films separating oil drops that are stabilized by such surfactants would be even thinner because the hydrocarbon chains =e, in this case, embedded in the oil phase. Furthermore, it is known from the work of Shinoda et al. j23,24] that the oil/water
8'
Figa 4, Theoretkal contact an&s as a furtctbn of PrimtMc&nesoof films in which OntY vm der WADISattractive farces are present.~~rve 1, Hamaker constantA divided W TOW (Al7,,)
AI-l,,
= 2 x
=2x
10-l’.
Curre
2,
A/y_
= 2 x
f0-l’.
Ctuv@
3, A/vow
= 2 X lo”*.
Curve
4,
m-“*
Werfacial tension can reach Iow vaXups (< 0.1 dyne/cm) at the phase inversbn temperature and the Hamaker constants are IikeIy to be in the range of 2 :A lo*” to 2 X lo-l4 ergs 1251. These circumstances could lead to substanWI contact angles in emulsions stabilized by nonionic surfactants under certain conditions. For example, Ingram [Zl] found that AF in foam films stabilized by decyl methyl sulfoxide were as high a~ -60 #N/m (0.06 ergs/cm’). If an emulsion film had this interaction energy and if the interfacial tension wxe 0.1 dyne/cm, the contact angle computed from eq. (1) would be 42*. Although decyt methyl suffoxide is not a characteristic nonionic stifactant, the above example Wustrates the behaviour that may arise. The contact angles in emulsions stabilizti by nonionic surfactants warrants further study.
INPLUENC%
OF THE CONTACT
ANGLl3 ON EMULSCON PROPERTHZS
To reiterate, when strong attractive interactions operate between emulsion droplets, ?he droplets nut onIy fIoccuFatebut aiso spontaxteoustydeform under the action of these forces. The degree of deformation as measured by the contact angie is reIated to the interaction energy and the interfacial tension. In a previous study [4], it was found that when appreciable contact angles are present, the droplets rapidly fIoccuIate and sediment. The structure and volume of the resultant cream at steady state depended on how it was formed. Creams formed under normal gravity setting had an expanded volume relative to the voIume occupied if the droplets were close packed spheres. As in unstable dispersions, the fIoc structure is sufficiently rigid to trap considerable amounts of external phase. However, if these pockets of external phase are removed by centrifugation and the cream is then allowed to retax under normal gravity, it uttimatefy occupies a vofume less than that for close packed undeformed spheres - in this case due to the spontaneous deformation of the dropbts in the cream [41_ KnRef. 4, we did not consider the effect of temperature on cream votume and structure through it..effect on the contact angte. In view of our present knowledge, it wou!d be expected that the structure and other properties of concentrated emufsions, in systems exhibiting contact angles, woutd be highly temperature dependent. Creams farmed from dodecane-in-water emulsions (# = 0.33). stabiIized by NaDDS with added t&G, were studied 8s a function of temperature. The results are shown in Fig. 5. The open cyznbois represent the “equilibrium” voiume fractions of dodecane in creams fonned by settling under normal gravity. The closed symbots refer to CT--S formed from similar emulsions by centrifugation followed by relaxatii>nunder gravity, which ailowed the creams to swell again to an equilibrium voIume. In these experiments* the respective creams were fit equilibrated in a water bath at 24OC. After an equilibrium cream volume was reached (after about 8 days) the temperature was raised and the emulsions were allowed to reach a new equilibrium. The droplet size distribution, measured by CouIter counter at the s&t and end of the experiment was the same within experimental error, indicating that no coarsening took pIace, It is seen from Fig. 5 that the volume fraction of oil in the creams formed by gravity settling decreases, while in those formed by centrifugation and retaxation it increaseswith increasingtemperature. The volume fraction of oil in both types of cream approaches the vaIue of O-74 at temperatures above about 40°C. over the whole range of NaCl concentrations considered. The contact angIe is then < 10”. and significant departures from the structure of hexagonally cIose pack@ spheres (i.e. 9 = 0.74) are not expected theokticalIy in this case 141. A second area where strong attractive forces, as indicated by appreciabte contact angles, are of importance, is in the clearing of dilute emuIsions with very small particles.
182
Fig. 5. Dependcncc of volume kct&on CICoil (#I an NaCt cottcenfration and temperature for creams formed on do&cane-in-water emulsions, e#Rer by gravity settfiilg (open aymbols) or by centrifugatian followed by relaxation (eked symbots). The ernubifiet Is 0,4% NaDDS. 0 l, 24°C: l , 30°C: a, 31°C; 0 +. 4!ZaC.
U-
----__+_ 0
U-
Pig. 6, Creaming of dilute (6.1%) hexadecane-in-O.Z% NaDDS emubbnn as a functfon of electrolyte concentration ad temperature. NaCl at 25*C (0); NaCI at 40°C (8): LICI at 25°C (A): Dashed line ia the initial abaoebartce of the emulsbn,
Emulsions generally have a siz&Ie fractiou of fine particks that are well below 1 ym. When such emulsions flocculate into a secondary minimum and are allowed to cream, the Iower external phase generally retains these small droplets which resist sedimentation. The exphtition for this is that fbc-
183
culation into a secondary minimum is a more or less reversible process 1261, with small droplets tending to remain as singlets. However, if more electroiyte is added so tha; C,, is exceeded, the diiute emulsion rapidly flocculates and the em&ion clears. An exampie of both the temperature sensitivity and the cation specific nature of the process is shown in Fig. 6, for a hexadecane-in-water emulsion stabilized by 0.2% NaDDS (Q = 0.001) and having an average particte size well beIow 0.8 pm, The figure shows the average absorbance of the middle third of a 12 cm high emulsion column after standing for 12 h at the desired temperature. The dashed line represents the initia1 absorbancrt of the emutsion. It is first noted that tiCI, even at 0.6 M, at 25*C or 40°C. does not cause any significant creaming of the emulsion, which is consistent with the fact that LiCl does not lead to significant contact angles in these systems, i.e., does not promote strong attractive interactions. However, N&I above C&t (whi& depends on temperature) causes total clearing of the emution by sedimentation. It was observe<1 that when the temperature of a totally coagulated emuIsion was raised such that the electrolyk concentration in the emulsion fell beiow &tit at the new temperature, the vibration caused by the water bath was sufficient to redispem the cream. Ct i3 likely that setieral other emulsion properties, especially those related to its viscosity and fiow behavior, are also sensitive to the presence of huge contact angles. SUMMARY
AND CONCLUStONS
Contact ang& in aqueous films separating em&ion droplets have been measured for various ionic surfactant solutions as a function of added eiectroByte and temperature, The response of the contact angle to these variables is similar to that in foam films, With the ionic surfactants that have been studied, large contact angles oniy devetop with specific electrolytes and onty above a critical concentration which increases with temperature. The counter-ion spscificity depends on the nature of the head group. Where comparisons could be made, it was found that the magnitude of the interaction free energy, AF, as well as of its electrolyte and temperature dependence, are alf similar to the
vaIues found for aqueous foam fitms in air. The contact angle depends both on the free energy of interaction and on the interfacial tension. Uecause different surfactants can lead to relatively large differences in row, they can, by this factor alone, produce differences in 0. It is predicted that surfactants generating low interfacial tensions can produce substantial contact angles, even if they do not give rise to large free energies of interaction. When the interaction energy is significant, relative to 2 row, emulsion dropIek deform when they fiocculate. This alters the structure and probably aiso the mechanical properties of creams formed on emubions as a result of sedimentation. It was shown that the structure of these creams is sensitive to the
type and concentration of the counter-ion as welt as to the tempenlttie. Finally, it was found that the presence of a contact angle dramatically alters the rate of clearing of diIute emulsions composed of fine droplets. The interesting feature here is that flocculation of the oil droplets into a deep energy minimum is highly counter-ion specific and readiSyreversible by increa&ng the temperature. ACKNOWLZXIGEMENTS
The authors thank Dr. E.D. Goddard for several helpful suggestions,and Lever Brothers Company for permission to publish this paper. REFERENCES
14 16 I6 17 18 i9 20 21 22
E4.V.Derjaguin, Acta Phiskochim URSB, 12 (1840) X81. H .M. Princsm and S.G. Mason, J. colbrd fnt-face Sci., a9 (1965) 166. ELM. Frincen, 3. Collofd Interface Sci., 71(1979) 66. H.&l. Princen. BZJ?, Arorwrn and J.C. Moser, J. CoItofd kterface Sci., 76 (19SO) 246. M.P. Aroman and H.&f. Rfmen, Nature 286 (1980) 370. H.&l. Princea,J. Phys. Chem., 72 (1968) 3342. F. Mulsman and K.J. Myseta, J. Phya. Glum., 73 (1969) 469. T, Kofarov, A. Scbtudko and D. Exerowa, l&uw. Faraday Sac., 64 (1968) 2864. J.A. DeFeijter.Ph, D. Thesb, University of Utrecht. 1973. &A DeFeijter, J.B. RiJrbout and A. Vrij, J. ColMd Interface sd., 64 (1978) 25% J.A. DeFeijter s nd A. Vrii, J, Collofd Interf§cL, 64 (le78) 269. H-M. Primem, I.Y.Z. Zia and S.G. Mason, J. Calbid Maface Sci., 23 (lg67199. J.L., Cayah, R.S. Sch~hter and W.H. Wade, in ‘*Adsotptkn at mterfaer”, p234, Aa Sympclsium Series, Vol.8, Am. Chem. Sac. WashIngtan, D.C., 1976. M.N. Jones, KJ. Mysels and P.C. Schotten, Trams Fara&y Sac., 62 (1966) 1336. P. Mukerjee, K.J. My&s and P, Kapauan, J. Phys. Chem., 71(1967) 4168. I. Weil, d. Phya. Cbm., 70 (1966) 133. ED. Goddard, Croat. Chem. Acta, 42 [1970) 143. 8. GravshoIt, In R.M. Fitch, (KU.) Polymer Coltokk II, Plenrrm, New York, 1980. E.D. Goddard, G.H. Matteson and G&E, Totten, J. Callui-l Interface Ski.. fn press. H.M. Princen, Ph. D. Thesis, University of Utmht, 1865. B.T. In-, J. Chem. Sm., Faraday I, 63 (19i2) 2230. J. Cfunle. J. Co&ill, J. Qoodman and B. Tngram, Spec. Dtsc. Faraday Sac., 111970)
23 21 23 86
?S&o and K. Shinoda J CoIlaid rnterface Sci 32 (lC70) 647 K. Shinoda, M. Hmrin, k i
1 2 3 4 5 6 7 8 9 10 11 12 13
CONTACX ANGLES
IN OIL-iN-WATER
EMUMIONS
173
in The Nethedsnds
STABILIZED
BY
IONIC SURFACTANTS
M.P. ARONSON*
and H.M. PRINCEN-
Lecer &o&hers Company. Research Diwikion. 45 Riuer Road, Edgewater. NJ07020 (Received August 24.1981;
(U.S.A.)
revised versfon accepted November 9.1961)
ABSTRACT (X-tact angles In aqueous films separating emulsion droplets have been measured for uarZausknk surr’actanb as a function of type and concentration OCadded ektroIyCe, and
OFtemperature.As fn foam GImsIn ak, appreclabfecontact angtesdevelop only with
npeciEiceA?ctrotytesand above a Grit&l electrolyte concentration, which increases with temperatut 9.The relative effectiveness OF different counter-tons depends on the nature of the surfactar& head group. When the contact angle Is significant, emuWon droplets deform when they fkxcufate. Thfa alters the structure and probably afso the mechanical properties of creams Pormrr rn emulsions by sedimentation. It was shown, on the bashcof their equilibrium volume fraction of ail. that the structure of these creams is sensitive to the type and concentration of the counter-ton, to temperature, and to the manner in which the creams are formed, i.e., by sedimentation under normaI gravity forces, or by centrifugation foIlawed by retaxation, The p-xeerence of a contact angIe dramatically aItem the rate of clearing of dilute emulsions cclnrposed of very small droplab. This rapid flocculation is highly taunter-ion specific arvl reversihfe by Increaait~gthe temperature,
LNTRODUiXlON
As a result of interaction forces, the tension of a thin liquid fihn can be lower than the sum of the tensions of the bulk surfaces bounding the Eilm [Il. To satisfy the conditions of mechanical equilibrium, a macroscopic contact angle forms between the film and its associated PIateau border [Z]. Princen [3] recognized that the presence of a contact angle in films separating emo~ian droplets, as depicted in Fig.1, affects the structure and packing of concentrati :I emulsions. Later studies [4,Sl revealed that quite we angles, in some cases as large as 70”. do indeed form with several common types of surfactanf soIutions, some of which are also known to give rise to contact angles in foam fiis in air [6-111. l Present address: Union CarbIde Corporation, Tarrytown TechnIcal Center, Tarrytown, NY 10591, U.S.A. l * Present ada: Exxon Research and Engineering Company, Corporate Research Lahotito~, Linden, NJ 67036. U.S.A.
0166-tj622l82~600~900~$02.75
a 1982 Ekevier Sckntific Fubfishing Company
173
WATER
0
OIL
Pig. 1, Spontaneous deformation of Rocculatsd emution droplets under the influence of allmctive forces.
In this article, contact angb studies in emulsions are reported and compared, when possible, to published results for foam films. Although the method used is not highly accurate, and the systems have not been studied in great detail, several conclusions can be drawn concarning the effects of electrolyte type, temperature and surffictant head group. Several consequences of contact angles in emulsions, particutnr~y as a resuft of their striking temperature dependence, are then discussed, RXPERfBfBNTAL Ma teriats
Dodecane and hexadecane were obtained from Wdom Chemicals {PIuka, Puriss) aild were used without further purification, Sodium dodecyl sulfate (NaDDS) was a BDH sample that was extracted with ether for 40 h, ft has a slight surface tension minImum in distiltrd water which diwppeared at the salt concentrations that were used in the exrJeriments described below. Sodium decyl sulfate (NaDS) was 1 laboratory sample dsscribcd elsewhere [Sl . C&y1 trim&hyi ammonium bromide (CL’AD) was from Merck, while dodecylamine [DA) was from Eae:‘Jnan Chemicals. Sodium undecyl benzene sulfonate (NaLAS) was kindly supplied by Conaco. Laurie (HL) and oIeic (KOl) acids were obtained from Tridom Chemicals (Fluka, Puriss) and neutralised with NaOH to yield sodium kurate (NaL) and sodium oh&e (NaOl), respectively. All saIts were analytical reagent grade and were! used as received, Water was obtained by fiItration of distilled water through a MU-Q system (MilEpore Corp.). Several experiments were later repeated with double distalled water and the agreement was excellent.
The method used to measure contact angles is described in detail in Ref. 4.
176 Briefly, a coarse emulsion (droplet size 50-100 cc) ia prepared by shaking several ml of oil in 25 ml of the appropriate surfactant solution. A drop of the emulsion is transferred to a microscope slide constructed to have a small well and covered with a glass slip. In later studies, we found it convenient to use capillary cells supplied by Vitrodynamics, The cell is fnndferr4 to a Mettler PP52 thermostatted stage and viewed under the microscope (100X ) at the desired temperature. Doublets (as in Fig. 1) of roughly equally sized droplets are selected and photographed. The contact angles are measured either directly by constructing tangents st the cusp or from the shape of the doublets as described previously [4]. At least four doublets were used for each measurement and the contact angles were generally reproducible to about 3”. In some systems, particularly at high salt concentrations, the surfactant precipitated. These systems are indicated in the text. Most of the meaurements were made at 25°C. In some systems, particularly those that had large contact angles, the influence of temperature was also studied. The stage was heated to the desired temperature and contact angles were recorded as a function of time. The contact angles reported below were stable for at least 10 min. In some systems, contact angles were measured both an heating and cooling, and were found to be in good agreement. lnterfaciat tensions were measured by the rotating drop technique [12,13] using a University af Texas Spinning Drop Tensiometer. Equilibrium was generally established within lU-15 min. The volume fraction of oil in creams, formed on top of emulsions that were initially 33% by volume, was studied by the methods described in detail in Ref. 4. The rate of sedimentation of dilute (0.1%) hexadecane-in-water emulsions stabilized by NaDDS was studied as a function of electrolyte concentration aa follows: A 1% stock emulsion (0.4% NaDDS + 0.3 M NaCl) wa. first prcpared, using a Brinkman. homogenizer fitted with a PTA-20TS proiJe generator. The emulsion was def’oamed in the manner described in Ref, 4, Appropriate aliquots of the stock emulsion, and of 2% NaDDS and 3 Ilf electrolyte solutions were added to a graduated cylinder and diluted to 250 ml with distilled water. ‘The emulsion was placed in a water bath and left for 12 h. Three samples were then removed from the middle one-third fraction of the emulsion and their absorbances were measured. The average value was taken aa a measure of the rate of sedimentation of the emulsion. RESULTS
AND DISCUSSION
Contact angZes between emulsion drops The contact angles formed between dadecane droplets ln various surfactaut-electrolyte solutions are shown in Table 1 along with the interfacial tension, row. Ail the anionic surfactanta were used aa the sodium salt so
176
0.2% NaDDS
0.25 0.4 9.6 0.6 l*O
df NaC! llf N&l 1cINaCl dl NaCt IA LiC!
0.4% NaDS
2.4 2.1 1.9 6.7 :.; 314 6.1 3.8
0.2% NaLbS
0.25% NaL + 0.25% rJa01
0.25% CTAB
0.26% DA HCI
-8A precipitate
-0 14.6 24.2 30.0 -0 14.0 21.4 26.5 40.0 89.0 48.6 <4
0.3 0.45 6.6 0.3 0.41 0.6 0.3 0.6
Jf NaCl Jf NaCl bf NaCP sr KC1 Lrf KC1 bf WX* sf LlCl bf UC1
0.3 0.46 0.6 0.7 0.6 1,O
M bf M Bf nf M
0.9 0.9 0.9
df NaCl 8f KI M Na sdicylate
-0 -0
0.75 1.6 0.75 1.5 0.76 1.6
M KCI M KC1 df KBr M KBti di Kf 8fK.p
-0
was present at 2VC.
N&l N&1 NaCl NaCI KC1 KC1
-0
0.47 0.47 0.73 0.66
0.60 0.78 0.76 0.74 I*1
2
-0 35 57 -0 -0 49 if 76
-0 w-0
-0 -0
24.0
177
that all the sotutions cant lin roughly 5 X f03 mob N:i*, along with the added electrolyte. DA was used as the KC1 salt, The soap soIutions w xd’zequal weight mixtures of NaOi and NaL. This mixture was used because the krafft points of these soap z are above 25°C while the Krafftpoints of tk.* Dlixture are betow room temperature. This procedure avoided cornpI&.’ rms due to insolubility. A simiIar procedure could probably have been used in tt. xe cases where other surfactants precipitated but this was not pursued. Several points emerge from Table 1. First, the inftuence of electrolyte type depends strongly on the nature uf the head group. With sulfates, the contact angle increases in the order Li*
OP LNTERACWON
IN THlN
LSQWID HLMS
The contact angle, 0, is governed by both the interfacial tension and the interaction free energy A F 161, as follows: -AF=270,(l-cosU)
(1)
AF represents the decrease in free energy accompanying the thinning of a unit area of film from infinity to an equilibrium thickness, he,. The interaction free energy computed from the- measured contact angles and interfacial tensions are shown in Fig, 2 as a function of the log of the ele&roIyte concentration. Abe included are results for foam fiIms in air, as reported by Princen [6] and DeFeijter [S,ll] for NaJX + NaCl and NaDDS + N&I, respectively, The interaction energies for N&IS and NaDDS with add& Na+ and for NaLAS with added Na’ <;r K’ fall on the same line, This indicates that the difference in interfacial tensions in these systems account for the differences in the contact angles observed in Table I, Sodium soaps fall on a different line and have higher interaction energies (more negative)
Fig. 2, T~~rzwt~am energy at ablecame in water emuMan firms as a function of &ctmIyte canceatra6lan at 26-G. 9.2% NaDDS In 6he gcesetlco of Nat3 (0); 0.2% NaDS fn the pcewnce of NaCI (a). KCI (A) or tiCI( 9.2% Na LA8 in the presence of NaCi (o), KC1 (-1 or LXX(o); O.ZS% NaL + 9.28% H&L fn the presence af Nat3 (6) or KCi I*)- Llna 1 and 2 are, reqwctively. Che resufts for 0.2% NaTI + NaCl (from Ref. 6) and O,OS% NaDS + NaCI (5~~1 Ref. 9,ll) for foam films,
than the sulfates and benzene sulfonato at the same electrolyte concentrations. l?o&sium alkyl sulfates have the highest idaaction energies of the systems studied, although surfactant precipitation occurred in this system, DeFeijter (S,IIJ found that the slope d (AF)/d(Iog C,,) is appsoximately equal to the difference in surface excess of the added electrolyte at the film and bulk surfaces. The dopes for the alkyl sulfates in emuIsion fihns and foam films are quite similar. Ln fact, the slopes of all the plots for the different sur&~ctantsare reasonably similar. Comparing again the resuIts for NaDS and NaDDS + NaCl, it is seen that the curves for emulsion fitms a~ displaced to higher salt concentrations reIative to foam films. As with foam films, Largecontact angles 8ce not produced until a critical salt concentration, C&, is reached, In emulsions, Cti is -0.35 M for alkyl sulfstes + NaCi and is higher than found by Princen and DeFeijter [9], in foam filras. This difference may be due to a basic difference in the electrolyte requirements to produce Newton black films in the oil/water/oil and air/water/air systems. However, both &incen tind DeFeijter studied large vertical soap fiIms while the fims in the present study are less than 60 pm in radius. Indeed, Kolarov IS] measured contact angles in microscopic foam films in air and also found Cd -. 0.34.M for NaDDS + NaCl. However, iibove Cd, Kolarov’s results rapidly merge with those of DeFeIjter.
179
Fig.3 InteractIon energy of dadecane-h-wateremuMoon films as c function of temperature. 0.2% NaDS l 1.S MN&l (A); 0.2WbNaL-t 0.2S% NaOl + 0.6 llf NaCt (0); 0,2% NaLAS + 0.6 M NaCl (e); Lhe results of DeFeIjter and Vcij [9,11] for foam films stabilized by 0.05% NmDDS + 0.95 ICINaCl I(.)*
Temperature has 8 iarge effect on the contact angle [Fi] . For anionic surfactants with added electrolyte, the contact angIe decreases with increasing temperature. The infiuence of temperature on the AF of emulsion fihns is shown in Fig. 3 for severaI systems. DeFeijter’s results for foam films are also innduded. It is first seen that dAF/dT, which is an interaction entropy, is remarkably similar for foam and em&ion films stabilized by similar surfactant-electrolyte systems, e.g., NaDDS + NaCI and NaDS f N&I. However, the sbpes for NaLAS and the soaps are substantially lower than for the s&fate, indicating smaller interaction entropies. There is general agreement, at present, that the large attractive interaction energies in films stabilized with ionic surfactants at high salt concentrations are due to the presence of an order~3 two dimensional structure within the Newton black fifm having strong electrostatic interactions [7,9,14]. However, one interesting implication of eq. (1) is that large contact angles may develop in an emulsion film &en if the interaction free energy is relatively smali provided that the interfacial tension is low, We have aIready seen several examples where two systems of similar AF have quite different values of 0, because of differences in the interfacial tension. This may have relevance to emuIs1onsstabilized by nonionic surfactants where additional attractive interactions of electrostatic origin are not expected. When the attractive forces are exclusively of the Van der Waals type and double iayer forces are absent, AF can be aperpximated by AF=-
A 12uh=
(2)
where A is the Hamaker constant and h is the equilibrium thickness [91_ The contac% angles computed from eqs. (1) and (2) are depicted in Fig. 4 for several v,llues of A/row _ It is clear that large contact angles can develop provided the films are sufficiently thin and the values of A and row fall in the correct range. Now, it is known that foam films in air that are stabilized by nonionic surf&tan@ have thicknesses in the range of 5Cl--fiO A (21,221 particularly in the absence of electrolyte, It is likety that films separating oil drops that are stabilized by such surfactants would be even thinner because the hydrocarbon chains =e, in this case, embedded in the oil phase. Furthermore, it is known from the work of Shinoda et al. j23,24] that the oil/water
8'
Figa 4, Theoretkal contact an&s as a furtctbn of PrimtMc&nesoof films in which OntY vm der WADISattractive farces are present.~~rve 1, Hamaker constantA divided W TOW (Al7,,)
AI-l,,
= 2 x
=2x
10-l’.
Curre
2,
A/y_
= 2 x
f0-l’.
Ctuv@
3, A/vow
= 2 X lo”*.
Curve
4,
m-“*
Werfacial tension can reach Iow vaXups (< 0.1 dyne/cm) at the phase inversbn temperature and the Hamaker constants are IikeIy to be in the range of 2 :A lo*” to 2 X lo-l4 ergs 1251. These circumstances could lead to substanWI contact angles in emulsions stabilized by nonionic surfactants under certain conditions. For example, Ingram [Zl] found that AF in foam films stabilized by decyl methyl sulfoxide were as high a~ -60 #N/m (0.06 ergs/cm’). If an emulsion film had this interaction energy and if the interfacial tension wxe 0.1 dyne/cm, the contact angle computed from eq. (1) would be 42*. Although decyt methyl suffoxide is not a characteristic nonionic stifactant, the above example Wustrates the behaviour that may arise. The contact angles in emulsions stabilizti by nonionic surfactants warrants further study.
INPLUENC%
OF THE CONTACT
ANGLl3 ON EMULSCON PROPERTHZS
To reiterate, when strong attractive interactions operate between emulsion droplets, ?he droplets nut onIy fIoccuFatebut aiso spontaxteoustydeform under the action of these forces. The degree of deformation as measured by the contact angie is reIated to the interaction energy and the interfacial tension. In a previous study [4], it was found that when appreciable contact angles are present, the droplets rapidly fIoccuIate and sediment. The structure and volume of the resultant cream at steady state depended on how it was formed. Creams formed under normal gravity setting had an expanded volume relative to the voIume occupied if the droplets were close packed spheres. As in unstable dispersions, the fIoc structure is sufficiently rigid to trap considerable amounts of external phase. However, if these pockets of external phase are removed by centrifugation and the cream is then allowed to retax under normal gravity, it uttimatefy occupies a vofume less than that for close packed undeformed spheres - in this case due to the spontaneous deformation of the dropbts in the cream [41_ KnRef. 4, we did not consider the effect of temperature on cream votume and structure through it..effect on the contact angte. In view of our present knowledge, it wou!d be expected that the structure and other properties of concentrated emufsions, in systems exhibiting contact angles, woutd be highly temperature dependent. Creams farmed from dodecane-in-water emulsions (# = 0.33). stabiIized by NaDDS with added t&G, were studied 8s a function of temperature. The results are shown in Fig. 5. The open cyznbois represent the “equilibrium” voiume fractions of dodecane in creams fonned by settling under normal gravity. The closed symbots refer to CT--S formed from similar emulsions by centrifugation followed by relaxatii>nunder gravity, which ailowed the creams to swell again to an equilibrium voIume. In these experiments* the respective creams were fit equilibrated in a water bath at 24OC. After an equilibrium cream volume was reached (after about 8 days) the temperature was raised and the emulsions were allowed to reach a new equilibrium. The droplet size distribution, measured by CouIter counter at the s&t and end of the experiment was the same within experimental error, indicating that no coarsening took pIace, It is seen from Fig. 5 that the volume fraction of oil in the creams formed by gravity settling decreases, while in those formed by centrifugation and retaxation it increaseswith increasingtemperature. The volume fraction of oil in both types of cream approaches the vaIue of O-74 at temperatures above about 40°C. over the whole range of NaCl concentrations considered. The contact angIe is then < 10”. and significant departures from the structure of hexagonally cIose pack@ spheres (i.e. 9 = 0.74) are not expected theokticalIy in this case 141. A second area where strong attractive forces, as indicated by appreciabte contact angles, are of importance, is in the clearing of dilute emuIsions with very small particles.
182
Fig. 5. Dependcncc of volume kct&on CICoil (#I an NaCt cottcenfration and temperature for creams formed on do&cane-in-water emulsions, e#Rer by gravity settfiilg (open aymbols) or by centrifugatian followed by relaxation (eked symbots). The ernubifiet Is 0,4% NaDDS. 0 l, 24°C: l , 30°C: a, 31°C; 0 +. 4!ZaC.
U-
----__+_ 0
U-
Pig. 6, Creaming of dilute (6.1%) hexadecane-in-O.Z% NaDDS emubbnn as a functfon of electrolyte concentration ad temperature. NaCl at 25*C (0); NaCI at 40°C (8): LICI at 25°C (A): Dashed line ia the initial abaoebartce of the emulsbn,
Emulsions generally have a siz&Ie fractiou of fine particks that are well below 1 ym. When such emulsions flocculate into a secondary minimum and are allowed to cream, the Iower external phase generally retains these small droplets which resist sedimentation. The exphtition for this is that fbc-
183
culation into a secondary minimum is a more or less reversible process 1261, with small droplets tending to remain as singlets. However, if more electroiyte is added so tha; C,, is exceeded, the diiute emulsion rapidly flocculates and the em&ion clears. An exampie of both the temperature sensitivity and the cation specific nature of the process is shown in Fig. 6, for a hexadecane-in-water emulsion stabilized by 0.2% NaDDS (Q = 0.001) and having an average particte size well beIow 0.8 pm, The figure shows the average absorbance of the middle third of a 12 cm high emulsion column after standing for 12 h at the desired temperature. The dashed line represents the initia1 absorbancrt of the emutsion. It is first noted that tiCI, even at 0.6 M, at 25*C or 40°C. does not cause any significant creaming of the emulsion, which is consistent with the fact that LiCl does not lead to significant contact angles in these systems, i.e., does not promote strong attractive interactions. However, N&I above C&t (whi& depends on temperature) causes total clearing of the emution by sedimentation. It was observe<1 that when the temperature of a totally coagulated emuIsion was raised such that the electrolyk concentration in the emulsion fell beiow &tit at the new temperature, the vibration caused by the water bath was sufficient to redispem the cream. Ct i3 likely that setieral other emulsion properties, especially those related to its viscosity and fiow behavior, are also sensitive to the presence of huge contact angles. SUMMARY
AND CONCLUStONS
Contact ang& in aqueous films separating em&ion droplets have been measured for various ionic surfactant solutions as a function of added eiectroByte and temperature, The response of the contact angle to these variables is similar to that in foam films, With the ionic surfactants that have been studied, large contact angles oniy devetop with specific electrolytes and onty above a critical concentration which increases with temperature. The counter-ion spscificity depends on the nature of the head group. Where comparisons could be made, it was found that the magnitude of the interaction free energy, AF, as well as of its electrolyte and temperature dependence, are alf similar to the
vaIues found for aqueous foam fitms in air. The contact angle depends both on the free energy of interaction and on the interfacial tension. Uecause different surfactants can lead to relatively large differences in row, they can, by this factor alone, produce differences in 0. It is predicted that surfactants generating low interfacial tensions can produce substantial contact angles, even if they do not give rise to large free energies of interaction. When the interaction energy is significant, relative to 2 row, emulsion dropIek deform when they fiocculate. This alters the structure and probably aiso the mechanical properties of creams formed on emubions as a result of sedimentation. It was shown that the structure of these creams is sensitive to the
type and concentration of the counter-ion as welt as to the tempenlttie. Finally, it was found that the presence of a contact angle dramatically alters the rate of clearing of diIute emulsions composed of fine droplets. The interesting feature here is that flocculation of the oil droplets into a deep energy minimum is highly counter-ion specific and readiSyreversible by increa&ng the temperature. ACKNOWLZXIGEMENTS
The authors thank Dr. E.D. Goddard for several helpful suggestions,and Lever Brothers Company for permission to publish this paper. REFERENCES
14 16 I6 17 18 i9 20 21 22
E4.V.Derjaguin, Acta Phiskochim URSB, 12 (1840) X81. H .M. Princsm and S.G. Mason, J. colbrd fnt-face Sci., a9 (1965) 166. ELM. Frincen, 3. Collofd Interface Sci., 71(1979) 66. H.&l. Princen. BZJ?, Arorwrn and J.C. Moser, J. CoItofd kterface Sci., 76 (19SO) 246. M.P. Aroman and H.&f. Rfmen, Nature 286 (1980) 370. H.&l. Princea,J. Phys. Chem., 72 (1968) 3342. F. Mulsman and K.J. Myseta, J. Phya. Glum., 73 (1969) 469. T, Kofarov, A. Scbtudko and D. Exerowa, l&uw. Faraday Sac., 64 (1968) 2864. J.A. DeFeijter.Ph, D. Thesb, University of Utrecht. 1973. &A DeFeijter, J.B. RiJrbout and A. Vrij, J. ColMd Interface sd., 64 (1978) 25% J.A. DeFeijter s nd A. Vrii, J, Collofd Interf§cL, 64 (le78) 269. H-M. Primem, I.Y.Z. Zia and S.G. Mason, J. Calbid Maface Sci., 23 (lg67199. J.L., Cayah, R.S. Sch~hter and W.H. Wade, in ‘*Adsotptkn at mterfaer”, p234, Aa Sympclsium Series, Vol.8, Am. Chem. Sac. WashIngtan, D.C., 1976. M.N. Jones, KJ. Mysels and P.C. Schotten, Trams Fara&y Sac., 62 (1966) 1336. P. Mukerjee, K.J. My&s and P, Kapauan, J. Phys. Chem., 71(1967) 4168. I. Weil, d. Phya. Cbm., 70 (1966) 133. ED. Goddard, Croat. Chem. Acta, 42 [1970) 143. 8. GravshoIt, In R.M. Fitch, (KU.) Polymer Coltokk II, Plenrrm, New York, 1980. E.D. Goddard, G.H. Matteson and G&E, Totten, J. Callui-l Interface Ski.. fn press. H.M. Princen, Ph. D. Thesis, University of Utmht, 1865. B.T. In-, J. Chem. Sm., Faraday I, 63 (19i2) 2230. J. Cfunle. J. Co&ill, J. Qoodman and B. Tngram, Spec. Dtsc. Faraday Sac., 111970)
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?S&o and K. Shinoda J CoIlaid rnterface Sci 32 (lC70) 647 K. Shinoda, M. Hmrin, k i
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