Electrosteric Stabilization of Oil-in-Water Emulsions by

Chapter 6 Electrosteric Stabilization of Oil-in-Water Emulsions by Hydrophobically Modified Poly(acrylic acid) Thickeners 1

Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.ch006

R. Y. Lochhead

Technical Center, BFGoodrich Company, Moore and Walker Roads, Avon Lake, OH 44012

Hydrophobic modification of poly(acrylic acid) thickeners yields products that are useful as primary emulsifiers for oil-in-water sys tems. The resulting emulsions are stable for years, but they break and coalesce almost instantly when a sufficient concentration of electrolyte is introduced into the aqueous phase. The effect of electrolytes on these electrosterically stabilized emulsions is com plex. The polymer is anchored to the oil droplet by hydrophobic interaction. Such anchoring should theoretically be strengthened by the presence of water-structure-enhancing electrolytes. On the other hand, coulombic interaction between the electrolyte and the polyelectrolyte causes shrinkage of the overall polyelectrolyte confi guration and this should theoretically make coalescence more likely.

The objective of this study was to separate these two effects; namely enhanced anchoring of the polymer and collapse of the polymer chain, by studying the relative effects of electrolytes selected from the classical Hofmeister series. Expansion of the polyion was controlled by varying the pH of the system. Hydrophobically Modified Polyacrylic Acid In a previous communication (I), a unique polymer primary emulsifier was described. This polymeric emulsifier was a hydrophobically modified poly(acrylic acid) thickener. Unlike conventional emulsifiers which are usually designated by the CTFA name "Carbomers", this polymer was free of the constraints of HLB and therefore it displayed the ability to form 1

Current address: Department of Polymer Science, University of Southern Mississippi, Southern Station Box 10076, Hattiesburg, MS 39406-0076 0097-6156/91/0462-0101$06.00/0 © 1991 American Chemical Society

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


102

POLYMERS AS R H E O L O G Y MODIFIERS

stable emulsions with any oil (Figure 1). The emulsions were easily prepared at room temperature and were storage-stable for periods of years at ambient temperature. These novel polymeric emulsifiers displayed another unique property which set them apart from other hydrophobically modified polymers (2), such as cationic derivatives of hydroxyethyl cellulose (3). The uniqueness lies in the fact that emulsions prepared with the hydrophobically modified poly(acrylic acid) coalesced spontaneously when applied to surfaces, such as human skin, that contained low-molecular- weight electrolytes. The oil that was released quickly formed a thin-film hydrophobic barrier on the substrate. This property renders these polymers useful in a number of industrially important areas. A mechanism was proposed for the emulsion stabilization and the unique triggered release of oil (Figure 2). It was assumed that the polym eric microgels were anchored by hydrophobic interaction to the oil—water interface. This seemed reasonable, because polymers that were not hydro phobically modified gave unstable emulsions. The coalescence of the emulsion when it was exposed to electrolyte was explained by considera tion of the Donnan equilibrium of counterions in polyelectrolytes. Addi tion of salt causes collapse of the polyelectrolyte microgels that are adsorbed at the oil—water interface. Subsequent destabilization of the emulsion could be due to: 1.

Approach of the polyion chains to their unperturbed dimensions, when steric stabilization would be lost

2.

Collapse of the stabilizing electrical double layer around the droplets.

3.

A reduction in the surface coverage of the oil interface as a conse quence of rapid shrinkage of the polyelectrolyte microgels

However, another possibility exists. It is possible that the oil droplets are merely trapped within hydrophobic domain inside a gel. This study was aimed at distinguishing whether the hydrophobically modified poly(acrylic acid) was acting as a primary emulsifier or merely as a gel matrix with hydrophobic domains. Hydrophobic Interactions Hydrophobic substances are defined as substances which are readily soluble in nonpolar solvents, but only sparingly soluble in water. This definition draws an important distinction from "lyophobic" substances, which have low solubility in all solvents, in general, as a consequence of strong intermolecular cohesion within the substance itself (7). When oil separates from water, it is tempting to ascribe this phenomenon to "like attracting like." This explanation is incorrect. The mutual attraction of non-polar groups plays a minor role in hydrophobic interaction. Hydrophobic



LOCHHEAD

Electrosteric Stabilization of Oil-in-Water Emulsions

Figure 1. Schematic of emulsion stabilized with hydrophobically modified poly(acrylic acid).

Figure 2. Triggered collapse of the microgels upon contact with electrolyte causes emulsion instability.



104


interaction actually arises from strong intermolecular forces between water molecules. In liquid water, the forces between water molecules are isotropically arranged, and when a solvent is dissolved in water, these forces must be distorted or disrupted. Strongly ionic or polar solutes can form strong bonds with the water molecules that more than compensate for the deformation of the water structure. Nonpolar substances are attracted to water molecules by weak dispersion forces such as van der Waals and dipole-induced-dipole interaction. The water molecules are attracted to each other by strong hydrogen bonds. This strong water structure between neighboring water molecules favors water-water interaction and eliminates the nonpolar molecules from the immediate vicinity. Thus, hydrophobic interaction arises from the water structure "pushing" the hydrophobic groups together to minimize the interfacial area (8). In the immediate vicinity of the nonpolar solute molecules, the hydro gen bonds between the molecules are maintained but distorted. This leads to enhanced structuring of the water molecules in the immediate vicinity of the nonpolar substance, and consequently, there is a loss in entropy of these water molecules. This loss of entropy, rather than the nonpolar substance's bond energy, leads to an unfavorable free energy change asso ciated with dissolution and results in the separation of oil from water. Hydrophobic Interaction: the Effect of Salts Salts can be classified as "water-structure makers" or "water-structure breakers" according to their position in the Hofmeister Series. Structuremaking salts enhance water structure and thus enhance the hydrophobic effect. This is why sodium chloride is often added to organic compounds in "aqueous dispersion," in order to "salt them out." Water-structure breakers, such as thiocyanate, "loosen" the water structure and reduce the hydrophobic effect, encouraging dissolution of hydrophobic solutes. Since hydrophobic interaction is the postulated mechanism of anchor ing the hydrophobically modified poly(acrylic acid) (HMPAA) molecules to the oil-water interface in an emulsion, it was of interest to examine the effect of added water-structure-making and water-structure-breaking salts on the stability of emulsions containing HMPAA as the primary emulsif ier. From this study, we hoped to be able to distinguish between the two possible mechanisms of emulsification using HMPAA, namely, true electrosteric stabilization or merely entrapment in hydrophobic domains within a hydrophilic gel. Experimental Procedures Materials. Hydrophobically modified poly(acrylic acid) (HMPAA-EX-231)


6. LOCHHEAD

Electrosteric Stabilization ofOil-in-Water Emulsions 105


has been described previously (i). This was a crosslinked poly(acrylic acid) that contained less than 1 mol % of a long-chain alkyl methacrylate. Carbopol EX-230 is identical to HMPAA-EX-231, except for the fact that it was not hydrophobically modified. Water was USP-grade, organicfree, deionized water. Potassium hydroxide was 45% w/w solution certified grade from Fisher Scientific. Potassium chloride was certified ACS-grade from Fisher Scientific. Potassium thiocyanate was certified ACS-grade from Fisher Scientific. Mineral Oil was Drakeol 19 Mineral Oil-USP from Penreco Corp. Mucilage Preparation. Each polymer was dispersed carefully in water to give the appropriate concentration. This dispersion of polymer in water was mixed for 20 min to ensure hydration of the polymer, then neutral ized to exactly a pH of 7 by addition of the 45% solution of potassium hydroxide. Viscosity and yield value were measured, and then each of the salts were added to separate mucilages in a series of additions. After each addition, viscosity and yield value were measured. Emulsion Preparation. Unneutralized polymer dispersions were prepared as described above. Sufficient oil was added to this unneutralized dispersion to yield a final oil content of 10% by weight. The oil was added to the rapidly mixed aqueous polymer dispersion. The emulsion was neutralized to a pH of 7 with potassium hydroxide (45%) and an aliquot was taken for observation and storage. Each of the salts were added sequentially to appropriate emulsions. After each addition, the emulsion was mixed for 30 min, and an aliquot was removed for observation and storage. All of the emulsions were stored at ambient temperature for 6 months before their stability was assessed. Yield value was determined using a B.P. Plastometer (4). The analysis of yield value, σ, was derived from the work of Voet and Brand (5), who derived their equation from the work of Houwink (6). Houwink showed that

12Wh

where a is the yield value in dynes cm , W is the effective weight of the top plate in grams, h is the distance between the plates in centime ters, and d is the mean zone diameter in centimeters. Assuming uniform spread, h can be expressed in terms of the volume of a cylinder (h = 4 V/d ), and the equation becomes y

2


106


vw π α Replacing W by the downward force, P, yields 48PV


π α From this equation, the yield force can easily be calculated from measure ment of the mean zone diameter of a sample placed between the glass plates and allowed to reach equilibrium stress. Results and Discussion Tables 1-6 show clearly that emulsion prepared with conventional poly(acrylic acid) thickeners in the absence of conventional emulsifier, are incapable of preventing coalescence of the emulsion. This result, which has been shown previously (7), persists despite polymer concentration, oil loading or conferred viscosity or yield value on the continuous phase of the original emulsion. Emulsions prepared using hydrophobically modified poly(acrylic acid) are stable even in many cases where the viscosity and yield value of the continuous phase is low. (Tables 7—9). In this series, emulsion instability is observed only at high salt concentrations or low polymer concentrations. Chloride is a water-structure maker and thiocyanate is a water-structure breaker. The effect of each of these salts on measured viscosity is, to all intents and purposes, identical, regardless of which anion is present. Thus, the potassium counterion dominates in determining the overall con figuration of these polyelectrolyte molecules (Figures 3 and 4). The effect of these ions on emulsion stability can be seen most clearly from phase diagrams which were constructed after storage of the composi tions for 6 months at ambient temperature (Figures 5—7). The overall compositional extent of emulsion stability decreases with added salts in the order KC1 > K O H > KSCN. The addition of potassium thiocyanate causes most rapid loss of emulsion stability. It is significant that a large area of the KC1 phase diagram represents creamed emulsion, whereas the area of creamed emulsion in the KSCN diagram is much smaller and is removed to lower salt levels. In these creamed emulsions, the droplets are stable, even though they are in "inti mate" contact. The stability of these creamed emulsions must be due to the presence of a polymeric barrier between the droplets. As noted ear lier, for such steric stabilization to be effective, it is essential that the polymers be anchored firmly to the interface. The polymers used in this investigation can be anchored to the oil—water interface only by



6. LOCHHEAD

Electrosteric Stabilization of Oil-in-Water Emulsions 107

Table 1. Effect of pH on Mucilage Viscocity, Yield Value, and Emulsion Stability of an Emulsion Prepared with 2% Poly(acrylic acid) Thickener PAA EX-230 and with Potassium Hydroxide as Neutralizing Agent pH

Brookfield Yield Value (NmJ viscosity (mPa.s) (2.5 rpm) (Model RVT)

Emulsion Appearance (6 months)

3.1

7350

9.0

Coalesced

3.8

42,000

8.5

Coalesced

4.0

44,000

14.0

Coalesced

4.5

45,600

17.0

Coalesced

6.9

56,000

12.0

Coalesced

12.5

50,000

9.0

Coalesced

13.0

38,400

8.5

Coalesced

13.3

32,000

4.9

Coalesced

13.5

26,800

4.1

Coalesced

13.8

14,000

2.3

Coalesced



108


Table 2. Effect of p H on Mucilage Viscocity, Yield Value, and Emulsion Stability of an Emulsion Prepared with 1% Poly(acrylic acid) Thickener PAA EX-230 and with Potassium Hydroxide as Neutralizing Agent 1% w/w. PAA EX-230 pH

Brookfield viscosity (mPa β) (2.5 rpm. Modtl RVT)

Yield Value (Nm*)

2.9

13,528

3.2

Coalesced

3.6

28,000

4.9

Coalesced

3.9

29,600

6.5

Coalesced

4.2

29,600

6.0

Coalesced

4.7

30,000

5.6

Coalesced

6.6

32,000

5.2

Coalesced

11.9

31,300

3.0

Coalesced

12.7

20,000

2.4

Coalesced

13.3

12,400

0.93

Coalesced

13.2

8,400

0.6

Coalesced

2




6. LOCHHEAD


Table 3. Effect of p H on Mucilage Viscocity, Yield Value, and Emulsion Stability of an Emulsion Prepared with 0.5% Poly(acrylic acid) Thickener PAA EX-230 and with Potassium Hydroxide as Neutralizing Agent

pH

Brookfield viscosity (mPa β) (2.5 rpm. Model RVT)

3.0

6400

4.0

Yield Value (Nm*) 2


1.6

Coalesced

19,200

1.65

Coalesced

4.4

19,200

3.25

Coalesced

4.7

19,600

3.0

Coalesced

5.0

20,000

2.7

Coalesced

10.0

19,200

3.25

Coalesced

12.0

14,000

2.3

Coalesced

12.5

9,000

1.15

Coalesced

13.1

2,800

0.44

Coalesced

13.2

1040

0.31

Coalesced



110


Table 4. Effect of pH on Mucilage Viscocity, Yield Value, and Emulsion Stability of an Emulsion Prepared with 0.25% Poly (acrylic acid) Thickener PAA EX-230 and with Potassium Hydroxide as Neutralizing Agent pH

Brookfield viscosity (mPa s) (2.5 rpm. Model RVT)

Yield Value (Nm ) 2


3.15

3040

0.43

Coalesced

4.3

13000

0.93

Coalesced

4.6

13,200

2.3

Coalesced

4.8

13,400

3.0

Coalesced

6.1

14,600

1.95

Coalesced

11.5

10,400

1.95

Coalesced

11.8

7,560

1.7

Coalesced

12.0

5,600

0.87

Coalesced

12.8

560

0.67

Coalesced



6. LOCHHEAD


Table 5. Effect of pH on Mucilage Viscocity, Yield Value, and Emulsion Stability of an Emulsion Prepared with 0.1% Poly(acrylic acid) Thickener PAA EX-230 and with Potassium Hydroxide as Neutralizing Agent pH

Brookfield viscosity (mPa s) (2.5 rpm. Model RVT)

Yield Value (Nm ) 2


Coalesced

3.5

688

4.4

4190

Electrosteric Stabilization of Oil-in-Water Emulsions by

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