Defoaming effect of calcium soap

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Journal of Colloid and Interface Science 279 (2004) 539–547www.elsevier.com/locate/jcis

Defoaming effect of calcium soap

Hui Zhanga, Clarence A. Millera,∗, Peter R. Garrettb, Kirk H. Raneyc

a Department of Chemical Engineering, Rice University, Houston, TX 77251-1892, USAb Department of Chemical Engineering, UMIST, PO Box 88, Manchester, M60 1QD, UK

c Shell Chemical L.P., P.O. Box 1380, Houston, TX 77251-1380, USA

Received 27 January 2004; accepted 30 June 2004

Available online 28 July 2004

Abstract

The effect of calcium oleate on foam stability was studied for aqueous solutions of two commonly used surfactants(anionic and nonionic)under alkaline conditions in the absence of oil. For the anionic surfactant, defoaming by calcium oleate appears to involve two mechanismOne is that oleate and calcium ions are presumably incorporated into the surfactant monolayers with a resulting decrease in the maximof the disjoining pressure curve and therefore produces less stable thin films. The other is bridging of the films by calcium oleateThe latter mechanism was especially important in freshly made solutions where precipitation in the aqueous phase was still occuthe foam was generated. Foams generated after aging (hours) when precipitation was nearly complete were more stable even thoturbidities were greater. Foams of the nonionic surfactant were less stable than those of the anionic surfactant butwere also destabilizeby sufficient amounts of calcium oleate and exhibited a similar aging effect. A simplified model was developed for estimating theoleate concentration at which precipitation commences in solutions of the anionic surfactant containing dissolved calcium. It inchancement of calcium content in the electrical double layers of the surfactant micelles. Predictions of the model were in agreeexperiment. 2004 Elsevier Inc. All rights reserved.

Keywords:Foam; Thin film; Sodium soap; Calcium soap; Defoaming; Aging effect

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1. Introduction

Foam stability in the presence of soluble/insoluble sois of practical importance in applications such as ladry, personal and home cleaning, the potential use of ffor mobility control in alkaline/surfactant processes for iproved oil recovery, etc. In hard water the precipitation ocalcium and magnesium soaps of long chain fatty acids mayoccur and destabilize the foam. Soap is sometimes addlaundry products for defoaming action and can also formsitu during the detergency process due to the presencfatty acids in sebum-like soils. We have shown in a previstudy[1] that the combination of oil and calcium soap pduces a synergistic effect facilitating the bridging instabi

* Corresponding author. Fax: +1-713-348-5478.E-mail address:[email protected](C.A. Miller).

0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.06.103

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of foam films or Plateau borders and producing a substaantifoam effect. However, the defoaming effect of calciand magnesium in the absence of oils is not yet well unstood. Peper[2] proposed that rapid defoaming of detergsolutions by soap or fatty acid occurs when conditions arevorable for the formation of a solid monolayer by the actof calcium ion. He advanced the hypothesis that the surfacof the foam bubbles are heterogeneous and consist of atinuous film of adsorbed detergent in which there are islaof solid calcium soap film. He suggested that these islamake the film unstable because of their inflexible, brittleture. In this paper we describe a systematic study of oil-solutions containing calcium soap, which was formedadding sodium oleate to surfactant solutions with dissolcalcium chloride and pH adjusted to 9 by sodium hydroxAlkalinity was applied to limit the hydrolysis of oleateoleic acid, which would otherwise be significant due to eincorporation of oleic acid into micelles. Measurements o

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540 H. Zhang et al. / Journal of Colloid and Interface Science 279 (2004) 539–547

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the rate of collapse of a foam column were supplementemicroscopic observations of individual foam films andmeasurement of turbidity and surface tensions.

2. Materials

Two commercial surfactants supplied by Shell ChemL.P. (Houston, TX) were used as foaming agents. One ianionic surfactant, an alkyl ethoxy sulfate sodium salt wistraight C12–C15 hydrocarbon chain and an average of thethylene oxide (EO) groups, denoted as N25-3S later ontypical of surfactants used in hand dishwashing and shamproducts. The other is a nonionic surfactant, a linear alcethoxylate also with a C12–C15 hydrocarbon chain but witan average of seven EO groups, denoted as N25-7 lateIt is representative of nonionic surfactants which are usecombination with anionic surfactants in household launproducts.

Sodium oleate with a purity� 99% was purchasefrom Fluka Chemie. Calcium chloride (from Alfa Producreagent grade) was dissolved in deionized water to prehard water. The concentration of hardness was calcuas CaCO3 in accordance with the usual convention, i.e., theconcentration that would exist if the same amount ofcium had been added as CaCO3. Alkalinity was providedby sodium hydroxide purchased from Fisher Scientific wassay of NaOH� 98.5%. Water used for experiments wdistilled and deionized.

Mixtures of triolein (with a purity of 99%, from FlukaChemie) orn-hexadecane (with a purity of 99%, from thHumphrey Chemical Company) with a small amount of oacid (with a purity of 95%, from Fisher Scientific Companwere sometimes used as defoaming agents in experimconducted for comparison with the oil-free systems. Tmixture of hexadecane and oleic acid at weight ratio of 9to 1 is denoted as C16/HOl, and the mixture of triolein aoleic acid at weight ratio of9 to 1 is denoted as TO/HOl.

3. Methods

3.1. Foam stability tests

Foam stability was determined by measuring the ratcollapse of a vertical foam column formed by mixingwith a surfactant solution at the base of the column[1]. Theprocedure consisted of filling the apparatus with apprmately 440 ml of solution to the 0-cm mark of the gradated cylinder. Foam was generated while the solution wapumped through a recycle loop and air was allowed intorecycle line. The pump was shut off when the foam reacthe 20-cm mark, and foam decay was recorded as a funof time.

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3.2. Optical observation of horizontal foam film

Millimeter sized foam films were observed in reflectmonochromatic light with wavelength of 546 nm by usithe method of Sheludko and Exerowa[3,4]. The film wasformed from a biconcave drop placed in a capillary (3 mm, height 1.2 mm) by sucking out liquid from a siorifice. The film was illuminated and observed in a directperpendicular to its surfaces with a videomicroscopy system

3.3. Surface tension measurement

A University of Texas Model 300 spinning drop tesiometer was utilized to measure surface tensions. Meaments were made by introducing a small air bubble tosample tube which was filled with surfactant solution. Dwere collected after running the sample for at least 20 m

3.4. Turbidity measurement

Turbidity was measured by a Brinkmann PC 800 corimeter. The original measured datum was the percentaof transparency, T%, of the solution. Turbidity was then cculated from 100 T%. The colorimeter was calibratedfore each measurement with deionized water which g100− T%.

3.5. Ultrafiltration and titration

Ultrafiltration (UF) was used to separate free electrosolution from surfactant micelles. Hutchinson[5] showedthat this could be achieved at sufficiently high rates oftration through UF membrane. UF membranes used instudy were Amicon YM3 with 3000 nominal moleculweight limit (NMWL) from Millipore Corp. The materiawas regenerated cellulose. Surfactant and electrolyte solutions were charged to a stirred filtration cell of 50 to 55 mcapacity and forced through UF membrane with 330[48 psi] nitrogen pressure. Filtration rates were 0.480.53 g/min. During each filtration, successive samplesfiltrate were collected and analyzed for surfactant and cacium.

Concentration of anionic surfactant and calcium ion weranalyzed by potentiometric titration. The apparatusan automatic titrator, Metrohm Titrino Model 716, puchased from Brinkman Instruments. For analysis of aniosurfactant, the titrant solution is 0.004 M benzothoniumchloride (Hyamine 1622), reagent, ordered from GalloSchlesinger, Inc. The indicator is a combination anionicfactant specific electrode purchased from Phoenix ElectrodCo., Cat. No. SUR1502R. For analysis of calcium ion,titrant is reagent EDTA, a product of Fisher Chemicals. Tindicator is a calcium ion-specific electrode purchased froPhoenix Electrode Co., Cat. No. CAL1502R.

H. Zhang et al. / Journal of Colloid and Interface Science 279 (2004) 539–547 541

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Table 1Turbidity measurements with 0.01wt% N25-3S, 300 ppm (0.03%) Ca, pH

Transmittance, T%

0.0001% NaOl 0.0004% NaOl 0.001% NaO

Fresh 99.9 98.8 96.34 h 99.9 97.7 92.623 h 99.8 96.8 89.629 h 99.9 96.6 89.6

Table 2Turbidity measurements with 0.05wt% N25-3S, 300 ppm (0.03%) Ca, pH

Transmittance, T%

0.0005% NaOl 0.001% NaO

Fresh 99.8 97.92 h 99.9 96.525 h 99.7 95.850 h 99.6 96.1

4. Results and discussion

4.1. Turbidity

As indicated previously, the main objective of this stuwas to determine the effect of calcium soaps on foambility. For this purpose it is important to know whether sosoap is present. Most calcium soaps have low solubilitwater, e.g., the solubility product of calcium oleate is tenthe power of−15.4 at 295 K and zero ionic strength[6]. Sowhen sodium oleate (NaOl) is added to an alkaline aquephase containing hardness, turbidity of the solution ispected to increase. Accordingly, a series of turbidity studiewas carried out by measuring the transmittance as a funof the concentration of NaOl and aging time. Samples wblanketed by nitrogen during aging.

Transmittances of solutions containing N25-3S at tconcentrations, 0.01 wt% (cmc) and 0.05 wt%, are giveTables 1 and 2, respectively. Water hardness was 300 ppRegarding the stoichiometry of calcium oleate, CaOl2, cal-cium is in excess for all compositions of NaOl studied. Hefresh solution means that the measurements were condwithin 30 min after the samples were prepared.

Clearly, turbidity increased with aging time over perioof hours except for the first solutions ofTables 1 and 2.This increase shows that solid calcium oleate particlesgrowing. Turbidity increased as the concentration of Naincreased, indicating more precipitate formed in the solutAs concentration of N25-3S increased, turbidity decreaseindicating fewer particles were formed because more oland calcium were incorporated in micelles.

For solutions of the nonionic surfactant N25-7 at 0.1 wtransmittances were measured at 300 ppm hardness anferent concentrations of sodium oleate, as shown inTable 3.The solutions are clear up to 0.002% NaOl. Beyondpoint, transmittance decreased rapidly with increasing c

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Table 3Turbidity measurements with 0.1 wt% N25-7, 300 ppm (0.03%) Ca, pH 9

Transmittance, T%

NaOl 0.001% 0.002% 0.003% 0.004% 0.005% 0.0

Fresh 99.9 99.6 99.4 91.4 88.7 69.54 h 99.9 99.7 2 h 95.0 83.0 7 h 59.3 34.623 h 99.9 99.3 25 h 92.2 77.329 h 99.9 99.5 34 h 93.1 79.7

Fig. 1. Foam stability with 0.01% N25-3S at different concentrationssodium oleate.

centration. Again aging effects over periods of hours wobserved.

4.2. Foam stability

Foam stability was tested by the method described in Stion 3.1. Reproducibility was confirmed by running eacsystem twice or more.

4.2.1. Experiments with freshly mixed solutionsA series of fresh solutions containing 0.01 wt% N25-

and different amounts of sodium oleate and hardnesstested at pH 9 for foam stability.Fig. 1 illustrates the resultat 300 ppm hardness. At the lowest studied concentratioNaOl, 0.0001 wt%, foam stability was not influenced by theaddition of NaOl. Nor was turbidity, according toTable 1,which indicates that calcium oleate particles were absenthe concentration of sodium oleate increased, foam stity decreased. The lower the concentration of sodium olethe longer the delay before the initiation of foam decay,the larger the final stable foam height. Substantial decrin foam stability was found for NaOl concentration up to0.005% with little additional effect beyond that point. Solutions containing 0.0004 and 0.0008 wt% sodium olewere visually transparent, although transmittance wasthan 100% as shown inTable 1. With 0.001% NaOl, the solution was bluish. When the concentration of sodium oleincreased to 0.005% and 0.01%, solutions were quite tu

Foam stability was tested for a hardness range of300 ppm, the range of interest for laundry, dishwashing,other cleaning applications. Trends similar to those ofFig. 1were observed for other calcium concentrations, as showTable 4, which gives foam heights after 20 min when foastability curves reached equilibrium heights. Since calc


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Table 4Foam height (cm) after 20 min

NaOl (wt%) Hardness

20 ppm 50 ppm 100 ppm 300 pp

0.0004 8.0 7.7 7.4 7.90.001 6.2 6.1 6.1 6.0

Fig. 2. Effect of pH on foam stability with solution containing 0.01N25-3S and 300 ppm hardness.

is in excess, foam stability is relatively insensitive to haness.

Without any sodium oleate, it was observed that alkapH influences the stability of foam of 0.01% N25-3S cotaining hardness (CaCl2). As shown inFig. 2, foam is lessstable at pH 9 than at neutral pH for a freshly mixed sotion. The difference in stability occurs after 10 min whthe dry foam consists of very thin film lamellae. It is fouthat pH of the initially alkaline solution decreased as thelution aged. When the solution was tested again after a43 h, pH was around 7.5 and foam stability was similar tothat of the freshly mixed solution at neutral pH. On the ctrary, pH did not change in the absence of N25-3S, implythat some component in this commercial surfactant seemslowly consume the hydroxide ions.

Behavior similar to that ofFig. 2 was observed at othehardness levels besides 300 ppm, e.g., 50 ppm. A pble explanation is suggested by the work of RutlandPugh[7]. They made measurements with surface forcesparatus of forces between negatively charged mica pseparated by alkaline aqueous solutions. When thecounterion in solution was Na+, there was a strong shorrange repulsion, i.e., hydration force. But when calciumpresent at pH 9, they reported that substantial Ca(OH)+ wasspecifically adsorbed at the negatively charged surfaceshydration forces were minimal. This is because the wateof hydration of Ca(OH)+ was less strongly bound than thof Na+ and Ca2+. The relevance of our case to their study(1) the two surfaces of our foam films are negatively chardue to presence of anionic surfactants; (2) Na+, Ca2+ andCa(OH)+ are the only cations present; (3) a strong shrange repulsion helps stabilize thin film; (4) instabilityfoam at alkaline pH occurs only when foam lamellae becovery thin. Thus, the decrease of foam stability as pH chanfrom 7 to 9 may be related to Rutland and Pugh’s findingthe solution ages, pH decreases with the result that con

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Fig. 3. Foam stability with N25-7 at different concentrations of sodoleate.

tration of Ca(OH)+ also decreases. This may be why fostability returns to that for solutions formed at neutral pH

For studies with nonionic surfactant N25-7, surfactconcentration was kept at 0.1 wt%. Foam stability of pHsolution containing 300 ppm hardness and different amoof sodium oleate was tested. The results are shown inFig. 3.No significant decrease in foam stability was seen up0.002 wt% NaOl. Indeed, foam stability seemed to increslightly, probably owing to the surface charge introduby adsorbed oleate ions. At this concentration the soluhas virtually 100% transmittance (perTable 3). Decreasein foam stability was noticeable with 0.003% NaOl, whimade the solution a little bit bluish. At higher concentratioof sodium oleate, which made the solutions turbid, foambility was further decreased. Substantial decrease in fstability was found up to 0.005% NaOl with little additioneffect for further increases.

4.2.2. Experiments with aged solutions—aging effect infoam stability

We saw previously that turbidity changed with timesolutions containing calcium soap particles. Now, we loat how aging affects foam stability. When the solutions wmixed, foam stability was not tested until after overnighlonger. During the aging process, the solutions were bketed with nitrogen. Results are compared with those opreceding section for fresh solutions.

With concentration of the anionic surfactant N25-3Sing fixed at 0.01 wt% and hardness at 300 ppm, foambility was tested at several concentrations of sodium oleFig. 4shows the results at 0.0004% and 0.01% NaOl.Fig. 5shows the results at 0.001% NaOl. Compared with the fsolutions, foam stability increased after aging. The chawas significant with 0.0004% and 0.01% NaOl, while evlarger at the intermediate concentration of 0.001% NaThe solution with 0.0004% NaOl looked transparent eafter aging (refer toTable 1). The solution with 0.01% NaOwas quite turbid from the beginning, obviously with substtial amount of insoluble soap particles. The solution w0.001% NaOl looked bluish, indicating the existence of sparticles, but they must have been small. The pH of theslutions decreased from 9 to about 7.5 after aging.


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Fig. 4. Foam stability of fresh and aged solutions of N25-3S contain0.0004% and 0.01% NaOl.

Fig. 5. Foam stability of fresh and aged solutions of N25-3S contain0.001% NaOl.

Fig. 6. Comparison of foam stability between fresh and aged solutionN23-3S in the presence of oil.

As a comparison, solutions containing both oils and ccium soap were also tested after aging. The systems stuwere pH 9 solutions of 0.01 wt% N25-3S plus 300 ppm haness and 0.01 wt% oil mixtures containingn-hexadecane otriolein with 10 wt% oleic acid, C16/HOl or TO/HOl. Ademonstrated in our previous work[1], calcium soaps formin situ at the oil–water interface in these systems.Fig. 6shows that foams were still quite unstable after aging inpresence of these oils.

Results with N25-7 are given inFig. 7. Foam stability offresh and aged solutions were compared at two concetions of NaOl, 0.005 and 0.01 wt%. Clearly, stability of tfoams increased after the solutions aged. In contrast tosystem of anionic surfactant N25-3S, alkalinity of theselutions did not change as the solutions aged.

When an oil mixture was present, that is for pH 9 sotions containing 0.05 wt% C16/HOl and 300 ppm hardnefoam was even less stable than without oil, as indicatedthe curve marked by ‘+’ for a fresh solution, and the antifoam effect actually increased with aging, as indicatedthe curve marked by dots inFig. 7.

d

-

Fig. 7. Foam stability of fresh and aged solutions of N25-7 containing N

4.3. Horizontal foam film

In order to obtain a better understanding of the defoing mechanism of calcium soapand of the intriguing agingeffect on foam stability, we studied individual foam filmwith diameters between 0.6 and 1 mm in Sheludko cA film produced from the pH 9 solution containing 0.01 wtN25-3S and 300 ppm hardness but no NaOl was takenreference. It went through the thinning process illustrain Fig. 8. Asymmetric drainage of the dimple took plawithin 1–2 s after the film was formed (Fig. 8a). Channelsdisappeared and a white planar film formed at around 1(Fig. 8b). After further continuous drainage, a uniform sble black film formed at around 3 min (Fig. 8c).

Film thinning behavior was not influenced by additionNaOl at concentrations lower than 0.001 wt%. Ten films pduced from a freshly mixed pH 9 solution containing 0.01N25-3S, 300 ppm hardness, and 0.001% NaOl were stuEight of them behaved similarly to the film with no NaOadditive. Two of them showed a different thinning patteAfter the initial asymmetric dimple drainage (within 2 sa new dimple-like pattern formed, as shown inFig. 9. A par-ticle was observed in the middle of the newly formed dimpFilm drainage was somewhat slowed down due to the dple. No white planar film was observed. A uniform blafilm finally formed after approximately 3 min.

At a higher concentration of sodium oleate, 0.005%, rainitial asymmetric dimple drainage was again observHowever, at around 1 min the white film was not unifo(Fig. 10a); at around 3 min, the black film was not uniforeither (Fig. 10b).

At an even higher concentration of sodium oleate, 0.0we saw stable films and less stable films with similar fquency. After the initial quick asymmetric dimple drainaga new dimple formed, and it was still there after 1 min. Sofilms broke after several minutes. Some films were still sble after 8 min with effect of particles visible, as seenFig. 11. Many particles were seen in these films.

When the solutions containing the above three concentions of NaOl were tested again after aging 40 h, the pnomena described above were not observed. The thinprocess was the same as for solutions without any ol(Fig. 8). Although the presence of solid soap particles


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Fig. 8. Thinning process of a film produced from solution containing 0.01N25-3S and 300 ppm hardness. (a) Asymmetric drainage of the diwithin 1–2 s after the film was formed; (b) white planar film formedaround 48 s; (c) a uniform stable black film formed at around 3 min.

Fig. 9. Foam film with pH 9 solution containing 0.01% N25-3S, 300 phardness, and 0.001% NaOl.

the solution was obvious, particles were all expelled dudrainage and unable to stay in the film.

(a)

(b)

Fig. 10. Foam film with pH 9 solution containing 0.01% N25-3S, 300 phardness, and 0.005% NaOl. (a) Nonuniform white film formed at aroun59 s; (b) nonuniform black film formed at around 3 min.

Fig. 11. Foam film with pH 9 solution containing 0.01% N25-3S, 300 phardness, and 0.01% NaOl.

4.4. Mechanism of defoaming by calcium soap

For foams made from alkaline solutions contain0.01% N25-3S and 300 ppm Ca2+, foam stability decreasedaccording to the results presented above, whenever enNaOl was added to decrease light transmittance throthe solution, i.e., whenever some calcium oleate partiformed. In contrast, at the lowest NaOl content investiga0.0001%, both transmittance and foam stability weresame as in the absence of NaOl.

Visual observations in this system revealed that atconcentrations of NaOl (0.0004–0.0008%) where turbiwas low and relatively few soap particles were formed,foams had quite uniform bubble size and decayed fthe top of the foam column. Moreover, the height ofresidual relatively stable foam decreased as NaOl con


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Fig. 12. Foam stability of 0.01% NaOl at pH 9, 0.01% N25-3S, and tmixture.

tration increased (Fig. 1). At high concentrations of NaOl—at least 0.005%—where solutions were much more tuindicating that many solid particles formed, some coacence of air bubbles was observed during foam generaand film breakage was observed throughout the foam dudrainage.

It is suggested that two mechanisms contribute to thecrease in foam stability produced by added NaOl. Onincorporation of oleate and calcium ions into the surfactanmonolayers with a resulting decrease in foam film stability. Even in the absence of calcium, NaOl foams arestable than those of N25-3S (Fig. 12). In the present experiments the oleate which adsorbs with N25-3S likely promoadditional calcium adsorption with a resulting decreasesurface charge. Whatever the precise monolayer comtion this mechanism is supported by the observation thathe aged solutions where the Sheludko cell results indithat few particles enter the film surfaces, foam stabilityless than in solutions containing N25-3S alone. Also pviding support is that the nearly constant heights of foafor the fresh solutions after 20–30 min, when most partichave left the films, are only slightly less than the correspoing heights for the aged solutions (Figs. 4 and 5). The smalldifference is due to the higher supersaturation of oleatthe fresh solutions, as also indicated by their slightly higsurface tensions (Table 5).

The second mechanism is bridging by calcium oleate particles. That hydrophobic particles can destabilize foam filmis well known[8]. The Sheludko cell results show that mamore particles are present in the film surfaces in the frthan in the aged solutions. This behavior suggests that pcle bridging is likely responsible for much of the defoamiaction observed at short times for fresh solutions. Perhsome particles actually nucleate in the film surfaces wfoam is generated during the early stages of precipitaas suggested by Raghavachari et al.[9]. If so, they would

,

-

be less easily swept from the film during drainage, therebproviding more time for them to enter the opposite surfand bridge the film. Whether or not such nucleation occthe higher supersaturation of oleate in the fresh solutshould facilitate particle entry. For instance, entry mightfavored by a lower surface charge of the surfactant molayers (see above). For the fresh solutions bridging cleoccurs more rapidly at higher NaOl concentrations, whthe greater turbidity suggests that there are enough largeticles to destabilize the films even during foam generatio

It is noteworthy that the capillary pressures reached infoam column (some 1000 Pa in a column 10 cm high)considerably higher than those in our Sheludko cell (40This difference is one possible reason that films in the Sludko cell with NaOl concentrations up to 0.005% were sble, whereas the corresponding foams were not. MoreoMonin et al. found that foam films in bulk foams breaklower capillary pressure than do isolated films due tovery different perturbations in the two situations[10]. Highercapillary pressures also favor particle entry into the film sfaces. Thus, even though we did not observe particle efor the aged solutions in the Sheludko cell, we cannotclude the possibility of entry for the same solutions infoam column. What we can conclude, as indicated abovthat entry is facilitated in the fresh solutions.

For solutions containing the nonionic surfactant N2no significant adverse effect on foam stability was seencept at concentrations of NaOl of at least 0.003% (Fig. 3).Indeed, at lower concentrations of NaOl the turbidity resindicate that no calcium soap particles formed. Moreoadsorption of oleate ions provided a surface charge, wactually increased foam stability slightly. Particle bridgingalso an important defoaming mechanism, as indicated byservations of coalescence during foam generation for fsolutions with NaOl concentrations of at least 0.004%. Tincrease in foam stability after aging is similar to thatthe anionic surfactant and presumably occurs for the sreason.

When oils were present, foam stability did not increaafter aging. As discussed previously[1], the defoamingmechanism when both oil and insoluble soap are preis a synergistic effect. The small soap particles formedthe surfaces of the much larger oil drops help destabthe pseudoemulsion films between the drops and thesurfaces, thereby facilitating entry of the drops into the sfaces. Then bridging by the drops destabilizes the films. Tmechanism is not expected to change after aging.

Ol

Table 5Surface tension (mN/m) of solutions containing soap and hardness

0.01 wt% N25-3S, 300 ppm hardness, pH 9 plus

0.0% NaOl 0.001% NaOl 0.005% NaOl 0.05% Na

Freshly mixed solution 27.5 27.8 28.8 30.1Aged 40 h 27.5 27.6 27.9Aged 140 h 27.4 27.7 27.5


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4.5. Precipitation of calcium oleate in micellar solutionsof N25-3S

In micellar solutions, precipitation of calcium soap ismore complex process than that in the absence of miceSince some calcium and oleate are incorporated in the mcelles, concentrations of molecularly dissolved calcium anoleate are less than the total amounts added. This eshould be taken into account in the analysis to determthe conditions for precipitation. In this section, a simplifiapproach is applied to model such behavior. The objectito estimate the precipitation boundary of calcium oleate,the amount of sodium oleate which must be added to a stion with given concentrations of N25-3S and CaCl2 beforecalcium oleate starts to precipitate.

Our interest here lies in systems where the molar contration of oleate is much less than that of N25-3S andcium. Owing to the low solubility product of calcium oleatethe monomer concentration of oleate ion will be very lfor significant amounts of calcium in bulk solution. Thefore, there should be very little oleate in the micelles,their behavior should be similar to N25-3S micelles. Soassume that binding of calciumwith micelles in the presencof a small amount sodium oleate and alkalinity can be emated by the same model as for a solution without sodoleate. Hirasaki and Lawson[11] have studied the assocition of calcium with micelles of N25-3S and justified theelectrostatic model, which does not consider the fine poof ion binding interaction, by data obtained with an ultratration (UF) membrane. Weperformed UF when NaOl waalso present, determined the concentration of free calciumions, C0

Ca2+ , in the filtrate, and used this value to appromate free calcium concentration in the micellar solution. Ifthe measured concentration of free calcium ions is not aable, it can be estimated from the model of HirasakiLawson[11].

Anionic surfactants have been found to mix ideallymicelles. Accordingly, we assume here ideal mixing of N3S and oleate, even though the former is ethoxylated. Twith a solubility product Ksp of calcium oleate of 10−15 [6],the monomer concentration of oleate,C0 − , is calculated by

Ol

.

t

,

C0Ol− =

√Ksp/C0

Ca2+ . With cmc of NaOl at pH 9 (CcmcOl )

as 0.052 mM[12], the composition of oleate in the mixemicelle is calculated byx = C0

Ol−/CcmcOl . The concentration

of N25-3S in micelles is the difference between total sfactant concentration,CT

S, and monomer concentration,C0S.

The latter is based on measuredsurfactant concentration ithe filtrate at the same total calcium concentration. Thmicelles are expected to contain oleate in the amounCm

Ol− = x(CTS − C0

S). Therefore, the total oleate added at

precipitation boundary isCmOl− + C0

Ol− .The precipitation boundary calculated this way was co

pared with that found experimentally, which is the boundbetween the isotropic region (100% transmittance) andregion with turbidity (with transmittance less than 99.8%Results for surfactant concentrations between 0.010.05 wt% are given inFig. 13. The calculated and expermental boundaries match well in spite of the limitationsour simplified model. AsFig. 13 shows, the effect of thmicelles must certainly be included.Fig. 13is based on experiments and calculations summarized inTable 6. It is clearfrom the fifth column inTable 6that the percentage of oleain mixed micelles was as low as 1%. This justifies oursumption that there was very little oleate in the micelles,that binding of calcium should be similar to that of N23S micelles. In fact, experimental values of calcium bindwere compared for solutions containing 0.05% N25-3S

Fig. 13. Precipitation boundary of calcium oleate in micellar solution oN25-3S.

rfactant

Table 6Calculation of precipitationboundary at 300 ppm hardness (CaCl2)

Conc. ofN25-3S(wt%)

Conc. ofN25-3SCT

S

Conc. offree Ca2+C0

Ca2+ a

Conc. ofmono. Ol−C0

Ol− × 104

mol% ofOl− inmicelles

Conc. ofOl− in micelleCm

Ol− × 104

Total oleateCm

Ol− + C0Ol−

(mM × 104)

Calculatedboundary(wt%×105)

Experimentboundary(wt% ×105)

0.0 0.0 3.00 5.77 0.0 0.0 5.77 1.76 –0.01 0.22 2.87 5.90 1.13 7.9 13.8 4.22 90.02 0.44 2.79 5.99 1.15 33.4 39.4 12.0 150.03 0.66 2.73 6.05 1.16 59.4 65.4 19.9 220.04 0.88 2.59 6.21 1.19 87.2 93.4 28.5 300.05 1.10 2.47 6.36 1.22 128.5 134.9 41.1 42

Note. Unit of concentration is mM if not otherwise specified; conc. of mono. is monomeric concentration.a Obtained by direct measurement of filtrate composition;C0

S (cmc of N25-3S at 300 ppm hardness) used is 0.15 mM based on the measured suconcentration in the filtrate.


dif-

haneo-this

w asfor

ard-n inthe

romsti-

nd-

oam

-in-

ting

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in

ic

d-themi-hismi-

um

aticdeln

nda-

ce

s-

l

-

300 ppm hardness with and without 0.0003% NaOl. Noference was found.

Although precipitation boundaries for hardness less t300 ppm were not actually determined experimentally, thretical calculation shows that the assumptions made formodel are reasonable for calcium concentrations as lo50 ppm for N25-3S concentrations up to 0.05 wt%. Thus,other situations such as low concentration of NaOl at hness less than 300 ppm, a frequently occurring situatiolaundry applications, this model can be used to predictamount of N25-3S needed to prevent calcium oleate fprecipitating. Also, this same method could be used to emate the precipitation boundary (or at least turbidity bouary) for other anionic surfactants.

5. Summary

Calcium soap, without dispersed oil, can decrease fstability for both anionic (N25-3S) and nonionic (N25-7)surfactants. For the anionic surfactant, defoaming by calcium oleate appears to involve two mechanisms. One iscorporation of oleate and calcium ions into the surfactanmonolayers, which decreases the maximum of the disjoinpressure curve and therefore produces less stable thin filmThe other is bridging of the films by calcium oleate particlFoams generated from freshly mixed solutions were lessble than those generated hours later when precipitationgrowth of calcium oleate particles were nearly compleSince observations of individual foam films showed that pticle entry into the film surfaces was facilitated for the fresolutions, we conclude that particle bridging was mainlysponsible for the reduced stability of their foams.

Foams of the nonionic surfactant were also destabilby sufficient amounts of calcium oleate. Particle bridgingagain an important defoaming mechanism. The increase

foam stability after aging is similar to that for the anionsurfactant and presumably occurs for the same reason.

A simplified model for estimating the precipitation bounary including the enhancement of calcium content inelectrical double layers of anionic surfactant (N25-3S)celles yielded results in agreement with experiment. Tmodel assumes that mixing of N25-3S and oleate in thecelle is ideal; that micelles with a small amount of sodioleate behave similarly to N25-3S micelles; and that calciumbinding with micelle can be described by an electrostapproach. It is demonstrated that predictions of this mogave reasonable agreement withthe measured precipitatioboundary.

Acknowledgment

The authors acknowledge the National Science Foution for supporting this work under grant CTS-9911954.

References

[1] H. Zhang, C.A. Miller, P.R. Garrett, K.H. Raney, J. Colloid InterfaSci. 263 (2003) 633.

[2] H. Peper, J. Colloid Sci. 13 (1958) 199.[3] A. Sheludko, D. Exerowa, Kolloid-Z. 155 (1957) 39.[4] A. Sheludko, Adv. Colloid Interface Sci. 1 (1967) 391.[5] E. Hutchinson, Z. Phys. Chem. Neue Folge 21 (1959) 38.[6] R.R. Irani, C.F. Callis, J. Phys. Chem. 64 (1960) 1741.[7] M. Rutland, R.J. Pugh, Colloids Surf. A Physicochem. Eng. A

pects 125 (1997) 33.[8] P.R. Garrett, in: P.R. Garrett (Ed.), Defoaming: Theory and Industria

Application, Dekker, New York, 1993, chap. 1.[9] R. Raghavachari, K.S. Narayan, N.Nayaar, in: P.R. Garrett (Ed.), Ab

stracts of Eurofoam Conference, 2002.[10] D. Monin, A. Espert, A. Colin, Langmuir 16 (2000) 3873.[11] G.J. Hirasaki, J.B. Lawson, SPE Reservoir Eng. (March 1986) 119.[12] K. Theander, R.J. Pugh, J. Colloid Interface Sci. 239 (2001) 209.

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Defoaming effect of calcium soap