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PHASE EQUILIBRIA IN THE SYSTEMS OF ACETIC ANDGLYCOLIC ACID WITH WATER AND LAURETH 4Abeer Al-Bawab a , Jennifer L. Barber a , Stig E. Friberg a & Patricia A. Aikens ba Department of Chemistry, Clarkson University, Potsdam, NY, 13699-5810b ICI Surfactants Concord Plaza, , Bedford BIdg, 3411 Silverside Road, Wilmington, DE,19850-5391|Version of record first published: 06 Apr 2007.

To cite this article: Abeer Al-Bawab , Jennifer L. Barber , Stig E. Friberg & Patricia A. Aikens (1998): PHASE EQUILIBRIA INTHE SYSTEMS OF ACETIC AND GLYCOLIC ACID WITH WATER AND LAURETH 4, Journal of Dispersion Science and Technology,19:2-3, 399-420

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J. DISPERSION SCIENCE AND TECHNOLOGY. 19(2&3). 399420 (1998)

PHASE EQUILIBRIA IN THE SYSTEMS OF ACETIC AND GLYCOLIC ACID WITH WATER AND LAURETH 4

Abeer Al-Bawab, Jennifer L. Barber, Stig E. Friberg Department of Chemistry

Clarkson University Potsdam, NY 13699-5810

and

Patricia A. Aikens ICI Surfactants

Concord Plaza, Bedford Bldg 3411 Silverside Road

Wilmington, DE 19850-5391

ABSTRACT

The effect of a-hydroxy group on the phase behavior in a skin lotion model

system was studied by determining the phase diagrams for the two systems acetic

acid / Laureth 4 /white oil / water and glycolic acid I Laureth 4 / white oil /water

using visual observation by optical microscopy. The two acids when combined

with surfactant and water gave an isotropic liquid solution, and a lamellar liquid

cryslal with different amounts of solubilization showing both acids to act as

hydrolropes. The result provided essential information about the differences in the

phase behavior which can attribute to the presence of a-hydroxy group in the

glycolic acid. Small angle x-ray diffraction was used to determine the boundaries

of the lamellar liquid crystal phases in the two systems, both acids were found to

be located in the water region in the liquid crystal,

Copyright 0 1998 by Marcel Dekker. In"

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AL-BAWAB ET A L

Keywords:

a-hydroxycarboxylic Acid Emulsion, Phase Behavior Liquid Crystals Glycolic Acid. Acetic Acid

In recent years the cosmetic industry has witnessed strongly enhanced use

of a-hydroxy acids (AHAs). They have been used in skin care already since the

days of Cleopatra, who is said to have bathed in sour milk. The% acids occur

naturally in fruit, sugar cane and milk and they are also produced synthetically.

Common fruit acids are glycolic (cane sugar), malic (apples), tartaric (grapes) and

citric acids. AHAS have been shown to reduce fine lines and wrinkles by

exfoliating the outer layer of skin. This new skin retains more water, which

plumps it up for a more youthful appearance (1-4).

Until recently, most studies were concerned with the functional changes in

the human stratum wrneum induced by the application of a-hydroxy acids (1-6)

and especially with the action of a-hydroxy acids on the skin evaluating the

relation between enhanced biological activity and irritancy (5,6). However, these

studies were concerned with simple solution of the acids and the estimation of the

action of the acids in more complex systems such as those found in commercial

lotions, is difficult. For a reliable estimate to be made, information is necessary

about structural changes during the evaporation of water and volatile organics,

which take place approximately during the first half hour after application (7,8).

The final structure of the lotion is radically different from the original one and it,

in turn, is the decisive factor for the transfer of the acids into the stratum wrneum

layer.

To make a reliable estimate of the final structure, the phase changes during

evaporation must be established and such information is not available except a few

studies on the phase behavior of a long chain a-hydroxy acid (9,lO).

With this fact in mind, we found an investigation on the effect of the a-

hydroxy group in a carhoxylic acid on the phase behavior in a skin lotion model

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PHASE BEHAVIOR I N ACIDS 401

system of a nonionic surfactant, Laureth 4, white oil, and distilled water to be

useful. With this article we present phase diagrams of four-component systems of

water, oil, surfactant, and short chain carboxylic acid with and without an a-

hydroxy group.

To some extent unexpectedly the results showed an effect of the glycolic

hydroxy group for the colloidal behavior also under water rich conditions. For the

water poor part of the system it is obvious that the hydroxy group should have a

decisive influence on the structures formed.

EXPERIMENTAL

Materials:

The following chemicals were used without further purification. Glycolic

acid 99%. from Aldrich Chemical Co., Milwaukee, WI; acetic acid 99.7% from

VWR Scientific Inc., San Francisco, CA; polyoxyethylene-4-laureth ether (Laureth

4). ICI Surfactants, Wilmington, DE; white oil, Penrew, Karns City, PA; water

doubly distilled.

Phase Diagram Determination:

All phase diagrams were determined at room temperature (22+lQC). The

isotropic liquid solubility regions were determined by addition of either water or

white oil to combinations of Laureth 4 and carboxylic acid noticing the points of

clarity and turbidity. The extent of the solubility regions were mnfirmed at first

by centrifugation of samples at 5000 rpms for 30 minutes and for samples with

composition close to the solubility limit also by storage at room temperature for

several days.

The liquid crystalline phases were detected and identified by optical

microscopy with the samples between crossed polarizers, and their boundaries were

mnfirmed by the results of small-angle x-ray diffraction based on knick points in

the curves of interlayer spacing versus composition.

The three-phase areas were determined using the following procedure. A

series of samples with different ratios of carboxylic acid and surfactant in the two-

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402 ALBAWAB ET AL.

phase areas were prepared and water was added. The samples were observed

between crossed polarizers in a microscope and the appearance of a third phase was

detected from the appearance of a liquid crystal and the limit of three-phase area

was detected by the disappearance of one isotropic phase. Tie-lines were

determined by preparing a series of samples in the emulsion region and separating

them by centrifugation and analyzing the separated phases for composition. The

two-phase suspension region of the solid acid and oil surfactant solution was

separated by centrifugation at 5000 rpms for two minutes, the weight ratio of each

phase in equilibrium was taken and compared to those of calculated weight ratio.

The tie-lines of the emulsion of liquid crystal and surfactant solution were

determined by preparation of a series of samples with different ratios of the three

components in the one phase liquid region close to the demixing line and measured

the refractive index of the liquid using Baush and Lomb model ABBE-3L

refractometer. Another series of samples were prepared at the limit of the liquid

crystal region towards the liquid and measured for interlayer spacing using small

angle x-ray diffraction. Samples of the two phase region were allowed to separate

and centrifuged at 5000 rpms for two minutes. The liquid separated phase was

then measured for refractive index and compared to those of known refractive

index near the demixing line. The liquid crystal phase also separated and measured

for the interlayer spacing and compared to those of known interlayer spacing.

Small angle x-ray diffraction.

The samples for low angle x-ray diffraction were drawn into fine glass

capillaries of 0.7 mm diameter, sealed at both ends, and placed into a brass sample

holder with a hole of 2 mm diameter. The x-ray radiation is K a copper filtered

by a nickel foil to yield a wavelength of 0.1542 om at 40 KV and 18 mA. The x-

ray equipment, Siemens Crystallalloflex 4, is a Kiessig small angle camera from

Richard Seifert, Germany. The detector is an ORDELA detection system, which

allows the angle 20 to range between 5.7" and 0.8". The temperature in the x-ray

sample chamber is controlled to 21°C. The alignment of the instrument was

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PHASE BEHAVIOR IN ACIDS 403

checked by a standard lead stearate whose interlayer spacing is 4.82 om. With our

measuring instrument and graphical representation of the signal, we obtained paired

symmetric peaks on both sides of the center. Interlayer spacing was determined

using the Bragg equation.

RESULTS

The results are reported in the following order: the phase diagrams are first

described followed by the x-ray results.

The phase diagrams for compounds involved in the investigation arc

presented in Figure 1-7. Figure 1 shows the phase behavior for water / Laureth

4 /glycolic acid. There is one micellar aqueous solution (L,), one isotropic liquid

surfactant solution region (LJ, one lamellar liquid crystal phase (LLC), two three-

phase regions, and five two-phase regions. The acid is soluble up to 75% in the

water while ihe water is dissolved up to 15 wt.% in the acid. The micellar solution

solubilizes up to 46 wt.% of the surfactant. The isotropic liquid surfactant solution

(L3 shows a solubility of 5 wt.% of the glycolic acid in the surfactant solution,

while water is solubilized up to 13 wt.% in it. The liquid crystal phase is a

lamellar liquid crystal, LLC, consisting of amphiphilic bilayers separated by water

layers as shown from the optical microscope pattern, Figure 13. A maximum of

12 wt.% of the glycolic acid was accepted by the lamellar liquid phase. Of the two

three-phase regions in the diagram, one region connects the LLC, isotropic liquid

(b ) , and glycolic acid (LLC+b+acid) and the second one connects the LLC with

L, and acid (LLC+L,+acid). The five two-phase regions consist of b+acid,

b+LLC, LLC+acid, L,+acid, LLC+,.

Figure 2 shows the phase behavior for water I Laureth 4 / acetic acid. There

is one solution (L,), one lamellar liquid crystal phase (LLC), one three-phase

region, and two two-phase regions. The liquid crystal phase is a lamellar liquid

crystal, LLC, consisting of amphiphilic bilayers separated by water layers as shown

from the optical microscope pattern, Figure 13. The solubilization of acid in the

lamellar phase has a maximum of 7 wt.%. There is one three-phase region in the

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Glvcolic Acid

ALBAWAB ET AL.

Figure 1 The phase diagram of water I glycolic acid I Laureth 4

L,: micellar solution

LLC: lamellar liquid crystal

: surfactant solution

diagram, one region connects the LLC, isotropic solution, and water

(LLC+solution+water). The two two-phase regions consist of micellar solution

(4) + water, LLC+solution.

The phase diagram of the system while oil / Laureth 4 1 glycolic acid is

shown in Figure 3. The white oil and Laureth 4 are totally miscible in each other,

and glycolic acid is neither soluble in the Laureth 4 nor in the white oil leaving the

rest of the phase diagram as two-phase region with simple tie lines as shown in

Figure 3. Figure 4 shows the phase diagram of the system while oil / Laureth 4

lacetic acid, where Laureth 4 is totally miscible in acetic acid and white oil, while

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PHASE BEHAVIOR IN ACIDS

Figure 2 The phase diagram of water I acetic acid 1 Laureth 4

LLC: lamellar liquid crystal

L,: micellar solution

acetic acid is insoluble in the white oil and vice versa. There is one one-phase

region and one two-phase region. The tie lines of the two phases are shown in

Figure 4. The phases in the water / white oil / glycolic acid phase diagram arc

shown in Figure 5. There was no significant solubility of the whire oil in the

glycolic acid nor in the water, as the solubility of water was 15 wt.% in the acid.

This lead to one three-phase region connecting the hydrate acid, white oil, and the

aqueous solution of the acid. Figure 6 shows the equilibrium between white oil

and aqueous solution of acetic acid in the system water / white oil I acetic acid

with tie lines between the two phases.

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ALBAWAB ETAL.

Figure 3 The phase diagram of white oil I glycolic acid / Laureth 4

The phase behavior of the system water 1 white oil 1 Laureth 4, Figure 7,

shows one isotropic region L,, where white oil and Laureth 4 are totally miscible

in each other with a maximum solubility of water of 15 wt.%. The liquid crystal

region solubilized up to 7.8 wt.% of white oil. There are two three-phase regions;

one region connects white oil to LLC and L, (b+LLC+white oil); second region

mnnects water, white oil and LLC phase, (LLC+H,O+white oil). There are three

two-phase regions in this diagram, (H,O+LLC), (LLC+white oil), and (LLC+b).

The tie lines of two phase regions are shown in Figure 7 connecting between LLC

and b. Figures 8 and 9 show essential features of the four components of the acid

I water 1 white oil I Laureth 4. In Figure 8, the microemulsion region solubilizes

water up to 58 wt.% upon the addition of the white oil, when the wt. ratio between

the glycolic acid and Laureth 4 is 713. Figure 9 shows that the one phase region

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PHASE BEHAVIOR IN ACIDS

Acetic Acid

White Oil Laureth 4

Figure 4 The phase diagram of white oil I acetic acid I Laureth 4

can also solubilize more water upon addition of the while oil maximum up to 64

wt.% when the wt. ratio between acetic acid and Laureth 4 is 25/75.

The interlayer spacing results for lamellar liquid crystal are given in Figures

10-12 for the systems; water / Laureth 4 / glycolic acid, water I Laureth 4 1 acetic

acid, water / Laureth 4 /white oil respectively.

The water can be dissolved up to 15 wt.% in the glycolic acid, in the range

of 15-25 wt.%, the hydrate acid and micellar solution (L,) are in equilibrium with

each other, Figure 1. When water was allowed to be adsorbed in the acid phase

it was found that time required for the acid to convert to hydrate acid is

independent on the initial weight and it follows a linear path, Figure 14, and Table

1.

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ALBAWAB ETAL.

White Oil

Figure 5 The phase diagram of water / glycolic acid I white oil

DISCUSSION

The basic differencc between the two acids is the a-hydroxy group which

causes the distinction in phase behavior in systems with the two acids. The x-ray

structure of the glycolic acid in non-aqueous system is shown in Figure 15 (1 1).

This structure shows the orientation of the OH group in the glymlic acid with its

H-bond to the acid carbonyl group. This H-bond is the reason for the fact that

glymlic acid is in a solid state, while acetic acid is in liquid state at room

temperature. This difference is expected, but the present results show that the

presence of the OH group also gave rise to differences in the colloidal structures,

even at high water content.

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PHASE BEHAVIOR IN ACIDS

Figure 6 The phase diagram of water I acetic acid I white oil

The influence on colloidal structures is well illustrated by a pair-wise

comparison of the phase diagram, beginning with Figures 1 and 2. The most

mnspicious effect of the OH group in the glycolic acid is the fact that the acid and

the surfactant were mutually insoluble. Added water to such a system was

dissolved entirely into the surfactant leaving the solid acid in equilibrium with

surfactant-water combinations with up to 27% water.

m e cosmetic consequences of these equilibria are evident from the shape of

the solubility regions of the surfactant isotropic liquid solution and the region of

stability of the lamellar phasc in Figure 1. An initial formulation of water 89.5%,

surfactant 10% and glycolic acid 0.5% will at the end of water evaporation when

4% water is left of the original 89.5%, show a tendency for glycolic acid to

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AL-BAWAB ET AL.

LLC+Whlte 0WwJ

Water Laureih 4

Figure 7 The phase diagram of water I white oil I Laureth 4

: solubility area

LLC: lamellar liquid crystal

penetrate into skin equal to that of pure glycolic acid. In the same manner, an

initial formulation of waler 88.5%, Laureth 4 lo%, and glycolic acid 1.5% will

show such a tendency at an earlier slage, when 24% of the waler is left.

In contrast, the acetic acid and the surfactant are completely soluble in each

other, hence, always leaving a one-phase solution at the end of water evaporation

with a reduced chemical potential of the acid.

Both acids act as hydrotropes (12,13) i.e. addition of the acid causes a

stmctural change from a lamellar liquid crystal to a micellar solulion, with the

acetic acid being more efficient in this regard (Cfr Figures 1 and 2).

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PHASE BEHAVIOR IN ACIDS

Glycolic Acid

Figure 8 Three dimensional phase diagram of system:

water I glycolic acid I Laureth 4 / white oil

Black area is isotropic solution of the four compounds where:

region 1: isotropic solution where glycolic acid / Laureth 4

90110 wt.ratio

region 2: isotropic solution where glycolic acid I Laureth 4

70RO wl.ralio

region 3: isotropic solution where glycolic acid I Laureth 4

50150 wt.ratio

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ALBAWAB ET AL.

Acetic Acid

Water

Figure 9 Three dimensional phase diagram of system:

water / acetic acid / Laureth 4 / white oil

Black area is isotropic solution of the four compounds where:

region 1: isotropic solution where acetic acid / Laureth 4

50150 wt. ratio

region 2: isotropic solution where glycolic acid I Laureth 4

75/95 wt. ratio

However, the compatability of the glycolic acid and the surfactant cause

essential differences also at high water content. In the glycolic acid system, Figure

1, the aqueous solution of glycolic acid is in equilibrium with the liquid crystalline

phase even at the lowesl concentration of the surfactant; a sign of the lack of

interaction of the glycolic acid with the surfactant. In the acetic acid system,

addition of the surfactant leads to an equilibrium between the acid-surfactant

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PHASE BEHAVIOR IN ACIDS

Volume ratio of water I [Laureth 4+Glycolic acid)

Figure 10 Interlayer spacing versus volume ratio of water / (Laureth 4 + glycolic

acid)

glymlic acid / Laureth 4

4 0 / 100

X 5 / 9 5

1 0 1 9 0

A 12 / 98

15 1 85

micellar solution and water, i.e. an emulsion. The lowest surfactant concentration

to form the lamellar phase from the micellar soltuion is 12% by weight. This

means that the glycolic acid combination forms liposomes with water and Laureth

4 (14), while the acetic acid does not. The consequences from cosmetic

applications of the information in Figure 1, is obvious. In the glycolic acid system,

the liposomal solution with 5% surfactant would accepl3.4% glycolic acid without

passing through the micellar soluton when water is evaporated.

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AL-BAWAB ET AL.

204 . , . , . , . . . , . 0.0 0.2 0.4 0.8 0.8 1 .O

Volume ratio of water flLaureth4 +Acetic Acid]

Figure 11 Interlayer spacing versus volume ratio of water / (Laureth 4 t acetic

acid)

acetic acid I Laureth 4

0 I 1 0 0

4 / 9 6

6 / 9 4

V 8 / 9 2

The favorable interaction between acetic acid and the surfactant is also well

illustrated in Figures 3 and 4. In Figure 3, the tie lines all are between the pure

acid and the white oil-surfactant solution; the interaction between the oil and the

surfactant is supcrior lo that between the acid and the surfactant. In the system

with acetic acid, the conditions are opposite; the tie lines are now converging

towards the white oil corner reflecting the relative. strength of the molecular

nteractions with the surfactant.

Finally, the difference in the molecular interactions is also illustrated by the

action of the acids as cosurfactants to form water-in-oil (W/O) microemulsions,

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PHASE BEHAVIOR IN ACIDS 415

0.0 0.1 0.Z 0.3 0.1 0.5 0.8 0.7 0.8 0 9

Volume ratio of water1 [Laureth 4tWhite oil]

Figure 12 Interlayer spacing versus volume ratio of water / (Laureth 4 + white

oil)

white oil / Laureth 4

t 0 / 1 m

X 5 / 9 5

1 10 / 90

Figures 8 and 9. The glycolic acid at optimal concentrations, 36%, gives W/O

microemulsion with a modest increase in water content, from 48.8 to 57.5% by

weight, while the acetic acid is extremely efficient; at 18% the water content is

increased from 27% to 60%.

These differences are indirectly reflected in the small angle x-ray results

Figure 10 and 11. Their interpretation is made according to the following relations.

Small angle x-ray diffraction patterns for samples with varied content of water

but constant ratio of the compounds provide information about the gwmetric

dimension d, and do, Figure 10-12, and also the water penetration into zone B from

zone A, Figure 13. The penetration of water from A to B is defined by the

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Figure 13 7he lamellar liquid crystal is described by three zones:

A: water layer plus polar groups

B: hydrogen chains methylene groups

C: hydrocarbon chains terminal methyl group and space

between them

20-1 . , . , . , . , . , 0 20 40 80 80 ID0 120 140 160

Time (h)

Figure 14 Plotting of the weight of glycolic acid and water (g) versus time (h)

W initial weight: 2.32g

initial weight: 3.21g

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PHASE BEHAVIOR IN ACIDS 417

TABLE I

Wcight change for glycolic acid due to absorption of water

'.. Thc acid was kept under vacuum for one wcek to dry it complelely b f o n using

.'. Pcncnt of weight changc was 0.7%

following expression,

d = d, {(I+R)/(l+aR)} [I]

where a represent the fraction of water that has penetrated the amphiphilic part of

the system, d, the interlayer spacing, do is its value at zero water mntent and R

represents the volume ratio of water to amphiphile. This equation leads to d = do

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AL-BAWAB ET AL.

Molewle I Molecule 2 8 (H:J 1

(G2)

Figure 15 Microscopic drawings showing the linkings of molecules in glycolic

acid by hydrogen bonds.

TABLE !I

Interlayer spacing, 4, pcnctration, a, values in LLC

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PHASE BEHAVIOR Pi ACIDS 419

for a = 1, e.g. complete penetration and d = do (1+R) for a = 0, e.g. the water is

localized to the space between the polar group A, Figure 13. R, d, and d, are all

known and a may be calculated for each R value, Table 11, (7,8). Upon

rearrangement of [I] when R -. 0 gives

a = 1 - [(dd/JR)/d,] 121

The results, Table 11, illustrate the behavior of the two acids. The glycolic

acid causes less water to enter the region B, Figure 13, that is the space between

the polyoxyethylene groups. The acetic acid on the other hand having greater

affivity for the surfactant causes more water to enter this region, causing

destabilization at lower acid content according to Figures 10 and 11.

The white oil has a different location, the do values show it to locate between

the methyl group layers, C in Figure 13.

CONCLUSION

The presence of the OH group in glycolic acid has been shown to influence

the colloidal behavior of a skin care model system also at high water content.

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420 AL-BAWAB ET AL

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