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PAPEROrlin D. Velev et al.Intense and selective coloration of foams stabilized with functionalized particles

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PAPER www.rsc.org/materials | Journal of Materials Chemistry

Intense and selective coloration of foams stabilized with functionalizedparticles†

Sejong Kim,a Harry Barrazab and Orlin D. Velev*a

Received 22nd April 2009, Accepted 8th July 2009

First published as an Advance Article on the web 20th August 2009

DOI: 10.1039/b908054f

We report a new method for intense and selective coloring of foams stabilized by particles from

cellulose derivatives. The addition of pH-sensitive dyes during the process of formation of

hydrophobically modified cellulose (HMC) particles by a pH-jump leads to co-precipitation and strong

adsorption of the pH-sensitive dyes on the cellulose particle surfaces at low pH. These strongly

colored HMC particles not only act as strong stabilizers of the foams, but also allow their intense and

selective coloration without any coloring of the solution medium. We characterized quantitatively

the color intensity of these foams, analyzed the adsorption behavior of pH-sensitive dyes on the

cellulose particles and showed that it follows well the Langmuir isotherm model. Lower pH of the

media leads to stronger adsorption of the pH-sensitive dyes on the HMC particles due to the hydrophobic

attraction under reduced electrostatic repulsion. The results illustrate how particle stabilizers can be used

to impart additional functionality to foams and prepare dispersion systems of unusual properties.

Introduction

Techniques for coloring of foams and bubbles are of large

industrial interest as foams have huge markets in personal care,

food and various other consumer products. It is difficult to

impart intense color to surfactant-based aqueous foams or

bubbles, because foaming in the presence of water-soluble dyes

typically results in weak color intensity of the foam. As the

majority of colorants are concentrated in the bulk solution, the

color intensity of the very thin films between the bubbles is

insufficient to impart intense color to the foam head where most

of the material is air entrapped in the foam. Coloring by surface

active molecules is also inefficient as the adsorption layer is

extremely thin. Hence, there is a need for a technique that

produces intense and selective coloration of foams and bubbles

in the personal care industry.

Structural color effects, where colors are developed by an optical

effect such as interference or diffraction, have been extensively

studied as an alternative way to make strongly colored disper-

sions.1–3 Dupin et al. reported a method for developing visible

colors in particle-stabilized foams due to light diffraction by latex

bilayers formed at the bubble shells.1 Ikkai prepared polystyrene

microcapsules, which developed structural colors depending on the

thickness of the shell around the bubble surface.2 Although the

structural colors are scientifically interesting, such techniques might

be impractical in large-volume industrial applications, due to the

higher cost of the monodisperse nanospheres.

Microparticle stabilizers for foams and emulsions have been

a subject of a significant research thrust recently, as they allow

aDepartment of Chemical and Biomolecular Engineering, North CarolinaState University, Raleigh, NC, 27695-7905, USA. E-mail: odvelev@unity.ncsu.edu; Fax: +1 919 515 3465; Tel: +1 919 513 4318bUnilever Research & Development, Bebington, Wirral, UK CH63 3JW

† Electronic supplementary information (ESI) available: RepresentativeEDX spectra for HMC–EB complex and Langmuir isotherm for EosinY. See DOI: 10.1039/b908054f

This journal is ª The Royal Society of Chemistry 2009

for long-term stability or phase inversion control.4–11 Particles of

intermediate surface hydrophobicity can strongly stabilize air–

water or oil–water interfaces.12–23 Various biomolecule-based

particles, such as protein–polysaccharide complexes, are

routinely used as stabilizers in the food industry.24,25 In our

previous work, we introduced a simple and efficient water-in-

water dispersion technique for the formation of superstable

foams from hydrophobically modified cellulose (HMC) parti-

cles.26,27 Here, we report how we can use such cellulose-based

particle superstabilizers as a means of making foams that are not

only very stable but are also intensely and selectively colored by

a dye component of the particles. We show that pH-sensitive

dyes can be strongly adsorbed onto HMC particles and interpret

the dye adsorption behavior by using Langmuir adsorption

isotherm. The dye–HMC complex particles adsorbed strongly

at the air–water interface of the bubbles, and made very

resilient foams with intense coloration of the foam–bubble head

without any coloring of the water media.

Experimental

Materials

The hydrophobically modified cellulose used in this study was

hypromellose phthalate (hydroxypropylmethylcellulose phtha-

late, grade HP-55), provided from Shin Etsu Chemical Co., Ltd.

(Tokyo, Japan). Hydrochloric acid solution (1 N) and sodium

hydroxide solution (1 N) were procured from Aldrich. Erythro-

sine B (EB), Eosin Y (EY), Allura red (AR), and FD&C Blue No.

1 (Brilliant Blue FCF) dye were purchased from Aldrich.

Deionized water was obtained from a Millipore water purifica-

tion system (Millipore Milli-Q+, Billerica, MA).

Stock solution preparation

Hypromellose phthalate stock solution (10 w/v% in water, pH

5.6) was prepared by mixing 10 g of HP-55 in 70 mL of deionized

J. Mater. Chem., 2009, 19, 7043–7049 | 7043

(DI) water, followed by the addition of 1 N NaOH solution to

adjust the pH to 5.6. This mixture was stirred for 12 h to obtain

a homogeneous clear solution, and finally the total volume was

adjusted to 100 mL by adding DI water.

Colored foam preparation

The cellulose particle foams were prepared in situ using a high-

speed blender (Oster Model 4242, Sunbeam Products, Inc., Boca

Raton, FL). Varying amounts of HMC stock solution and dye

solution were slowly poured into the blender running at

�15 000 rpm containing 140 mL of DI water where hydrochloric

acid was premixed to adjust the pH of the final foam suspension.

The foams formed immediately during the blending process for

60 s, and then the foams were transferred into a 250 mL

graduated cylinder for observation.

Characterization

The concentration and/or color intensity of dyes in aqueous

solution were determined using a UV-Vis spectrophotometer

(V550, Jasco Corp., Japan). The color intensities of foams were

determined by the same UV-Vis spectrophotometer equipped

with an integrating sphere accessory. The integrating sphere

acquires most of the light diffuse-reflected from the sample within

a wide range of angles and focuses it to the detector, measuring the

intensity of both the directly reflected light and the light back-

scattered from the dispersed system. Thus, it provides a good

measure of the visual appearance of the foams to an observer at an

arbitrary angle with regards to samples and illuminators.

Foam samples containing various dye amounts were carefully

transferred to a quartz cuvette (10 mm of light-path and 3 mL of

volume) followed by measurements of the reflected/backscattered

intensity with the integrating sphere (the measurements were

performed with wet foam samples from the bottom half of the

foam head rather than dry ones at the top of the foam head, which

ensured more reproducible data). Subsequently, the normalized

absorbance of the colored foam was determined from eqn (1).

Normalized absorbance ¼Reflectance of pure foam�Reflectance of colored foam

Reflectance of pure foam

(1)

The distribution of dyes in foams was analyzed with an optical/

fluorescence microscope (Olympus BX-61, Tokyo, Japan).

Scanning electron microscopy (model S3200, Hitachi Ltd.,

Tokyo, Japan) was performed at 5 kV with vacuum-dried

particles of HMC and EB dye. To improve the imaging quality,

a 60 A thick gold–palladium layer was pre-deposited on the

sample surface. Elemental analysis by energy dispersive X-ray

(EDX) spectroscopy was done using the same scanning electron

microscope at 20 kV. The pKa values of the hypromellose

phthalate and EB were determined by titration with NaOH

solution.28

Fig. 1 Schematics of the formation of colored foam stabilized with

HMC–dye complex particles. During the foaming process, dyes precipi-

tate to form HMC–dye particles, which subsequently adsorb onto the

bubble surfaces to form colored foams.

Adsorption isotherm experiments

Experiments of the adsorption isotherm of dye were conducted

using a batch method by forming colored foams as described

7044 | J. Mater. Chem., 2009, 19, 7043–7049

above with varying concentration of HMC (0–8 g L�1) and fixed

amount of dye (C0¼ 0.125 g L�1). The solution was adjusted with

1 N hydrochloric acid to obtain pH of 2.5, 3.5 or 4.5. Subse-

quently, the aqueous solutions were separated from the foams,

and the concentration (Ce) of dye in solution was determined by

UV-Vis absorption. The fraction of dye adsorbed on particles

was calculated as (C0 � Ce)/C0, and the concentration of

dye retained in the adsorbent phase (qe, mg g�1) was obtained

from eqn (2)

qe ¼ (C0 � Ce)V/Ws (2)

where C0 is the initial dye concentration and Ce is the equilibrium

dye concentration (mg L�1) after adsorption, V is the volume of

solution (L), and Ws is the mass of the adsorbent (g).

Results and discussion

Selective and intense coloring of foams using dyes with

pH-dependent solubility

The full details of the efficient water-based method used to form

superstable foams with hydrophobically modified cellulose

microparticles are described in our previous publication.26

Briefly, the microparticles are formed by a water-based liquid–

liquid dispersion process, in which hydrophobic cellulose

dissolved at high pH is sheared and precipitated in acidic

aqueous media. The resulting HMC particles immediately

adsorb onto the water–air interfaces created during shear

This journal is ª The Royal Society of Chemistry 2009

Fig. 2 Images of foams colored by a pH-sensitive dye (EB) and

a pH-insensitive dye (AR). Foams colored with the pH-sensitive dye

(A) show intense and selective color of the foam phase. Typical foams

with the pH-insensitive dye (B) show more intense color in the bottom

solution than in the foam phase.

blending, resulting in particulate-stabilized foams with excep-

tional long-term stability.26,27 Since these foams are stabilized by

particles, one can easily endow them with additional function-

ality (in this case color) by modifying the particles, which would

be difficult for typical foams formed by molecular surfactants.

The method that we designed to confine dye molecules to the

foam phase is illustrated in Fig. 1.

We dissolved a pH-sensitive dye (EB, eqn (3); insoluble in

water at pH < 5) in the cellulose solution at high pH. The result

of the process was particle-stabilized foam with intense colora-

tion of the foam head only (Fig. 2 and 3). Since the EB dye is not

soluble in acidic media, during the particle formation phase the

dye molecules precipitated and then adsorbed onto the HMC

particles to form an HMC–dye complex structure as illustrated in

Fig. 1. During the foaming process, these complex particles

adsorb around the bubbles and stabilize them. Since the dye is

not in the solution but on the HMC particles, the final foam is

intensely colored, while the solution is colorless. On the other

hand, an addition of a pH-insensitive dye (Allura Red, AR,

eqn (4)) results only in weak coloration of the foam, but intense

color of the medium (Fig. 2 and 3), which is the typical

appearance of most foams made of colored liquid. Notably,

neither of the dyes on its own is able to stabilize the foam at this

concentration without the presence of the HMC particle matrix.

(3)

(4)

To illustrate our hypothesis of the selective adsorption of

pH-sensitive dyes, we tested a mixed solution containing both red

pH-sensitive dye (EB) and blue pH-insensitive dye (FD&C Blue

No. 1) in the foaming process. After the foaming process the dye

mixture was completely separated to form a red foam head above

a blue solution (Fig. 4). This points out that the EB dye adsorbs

to the HMC particles, which form the red foam at the top of the

cylinder (Fig. 4B), leaving behind the blue dye in the bottom

solution. The unique bi-colored foam–solution system demon-

strated in Fig. 4 cannot be obtained with molecular surfactants

alone and illustrates how the functionalization of particles in

Pickering systems can endow them with useful features beyond

simple stabilization.

Quantitative analysis of foam color

Measuring the color intensity of foams by common transmitted

light spectrometry is difficult because foams diffuse and scatter

This journal is ª The Royal Society of Chemistry 2009

the incident light from the spectrometer. We performed quanti-

tative analysis of the foam color intensity in reflectance mode by

using a UV-Vis spectrometer equipped with an integrating sphere

attachment. This accessory collects most of the light diffuse-

reflected by the sample into the integrating sphere and redirects it

to the detector.29 The intensity of the integral reflected and

scattered light is likely a good measure of the human perception

of ‘‘color’’ of a disperse system such as foam.

The normalized absorbance spectra of the foams containing

varying concentrations of EB dye are plotted in Fig. 5, where the

peaks of spectra at 540 nm are identical to the characteristic

adsorption peak of EB dye in solution. The intensities of foams’

normalized absorption at varying concentrations of EB and AR

dyes are also plotted in Fig. 6A, which demonstrates quantita-

tively the stronger color intensity of foams with EB dye (enriched

with particles) as compared to the case of AR dye. Although the

curves are non-linear, which appears to stem from scattering in

the foam, this methodology provides a simple solution to

quantify the color intensity of foams. In addition, we analyzed

the color intensity of the solution below the foam head by

transmission spectrometry. As shown in Fig. 6B, the solution

with EB dye is virtually colorless, whereas in the case of AR dye it

is strongly colored. These results indicate that pH-sensitive dyes

are not located in the bulk solution but only at the foams, leading

to selective foam coloration without coloring the bottom media.

Structure and composition of the dye-infused HMC particles

The key to engineering Pickering-type colored foams and emul-

sions is to understand and quantify the partitioning of the dyes

into the synthesized particles. The first question that we investi-

gated is whether the dye is uniformly distributed and adsorbed

on the cellulose matrix, or existing in a phase-separated state

where smaller dye particles are entrapped within the larger HMC

ones. We observed by scanning electron microscopy the mixed

HMC–EB particles, as well as the ones from the pure HMC and

J. Mater. Chem., 2009, 19, 7043–7049 | 7045

Fig. 4 Example of the formation of a bi-colored foam–solution system

stabilized by dye-containing particles. (A) Modified cellulose stock

solution containing a mixture of two dyes—pH-sensitive (EB red) and

pH-insensitive (FD&C Blue No. 1) before foaming. (B) After particle-

stabilized foam is formed and separated on top, the EB dye on the

particles colors the foam head red above a blue-colored solution on the

bottom.

Fig. 5 UV-Vis spectra of colored foams with varying concentrations of

EB dye. The normalized absorbances of the colored foams were deter-

mined from the diffuse-reflectance spectra of the pure and colored foams

by using eqn (1).

Fig. 6 (A) Comparison of the normalized absorbances of foams

containing EB and AR dyes acquired at 540 nm and 500 nm, respectively.

The normalized absorbances of the colored foams were determined from

the diffuse-reflectance spectra of the pure and colored foams by using

eqn (1). (B) Maximum absorbance of the aqueous media below the foams

at pH 3.5.

Fig. 3 Micrographs of bubbles in various colored systems. (A) Fluo-

rescence micrograph of bubbles in EB dye foam indicates that the colored

particles are adsorbed around air bubble surfaces. (B) Bright field optical

micrographs of bubbles with EB dye do not show red color in the

surrounding medium whereas (C) bubbles with AR dye are surrounded

by a medium with red color.

7046 | J. Mater. Chem., 2009, 19, 7043–7049

EB. The morphology of HMC–EB complex particles formed at

pH 3.5 is shown in Fig. 7A. Examples of HMC-only and EB dye-

only particles are shown in Fig. 7B and C. The dyed HMC–EB

complex particles (Fig. 7A) are very similar to the HMC-only

This journal is ª The Royal Society of Chemistry 2009

Fig. 7 Scanning electron micrographs of (A) HMC–EB dye complex

particles, (B) HMC-only particles, and (C) EB dye-only particles formed

at pH 3.5.

Fig. 8 The fraction of EB and AR dyes adsorbed on different HMC

particle concentrations at varying pH.

particles (Fig. 7B) in shape and morphology, forming networks

of strands and flakes, while EB dye alone forms small spherical

and needle shaped crystals (Fig. 7C). These results indicate that

EB dyes are uniformly distributed in the composite HMC–EB

particles, rather than being phase-separated and entrapped

mechanically. This conclusion was confirmed by elemental

analysis using energy dispersive X-ray (EDX) performed at

various spots on HMC–EB dye complex particles and detecting

uniform dye distribution (see ESI†).

Adsorption isotherm of dye on HMC particles

The observations from particle morphology lead us to assume

that the amount of EB dye contained in the particles as a function

of the bulk molecular dye concentration can be described by an

adsorption isotherm model.30,31 Such adsorption processes may

be classified as physical or chemical, and can be influenced by the

chemistry of the surface, adsorbate concentration and charge,

effect of other ions, pH, temperature, electrolyte concentration

This journal is ª The Royal Society of Chemistry 2009

and others. We characterized the adsorption behavior of pH-

sensitive dyes (adsorbate) on the HMC particles (adsorbent) as

a function of solution pH. The effect of HMC particle concen-

tration on the fraction of EB dye adsorbed on HMC particles at

different pH is illustrated in Fig. 8. After forming colored foams

with varying HMC concentration, the bottom aqueous solutions

were separated from the foams, and then the concentration (Ce)

of dye in solution was determined by UV-Vis absorption. The

fraction of adsorbed dye increases as the HMC particle

concentration increases. It is also evident that the adsorption

capacity increases as the pH decreases. At low pH (2.5 or 3.5), the

carboxyl groups of the pH-sensitive dyes lose their negative

charges, resulting in precipitation by loss of dye solubility in

water. These precipitated dyes adsorb on the HMC particle

surface probably due to hydrophobic attraction. At the higher

pH of 4.5, on the other hand, a significant amount of dye is still

dissolved in the solution. Since hypromellose phthalate is

a polymer with negatively charged carboxyl groups, the nega-

tively charged dye molecules in the solution would not adsorb to

these HMC particles due to the electrostatic and hydration

repulsion. Similarly, no significant adsorption at any pH was

observed in the experiments with pH-insensitive dye (AR), which

is charged and soluble over the full pH range (Fig. 8).

The adsorption data were interpreted on the basis of the

Langmuir isotherm.32,33 The Langmuir isotherm is the simplest

model describing many processes, including adsorption of low

molecular compounds on solids.31–37 The linear form of the

Langmuir isotherm equation is given by

1

qe

¼ 1

qmax

þ�

1

qmaxKL

�1

Ce

(5)

where qe is the equilibrium dye concentration on the adsorbent,

Ce the equilibrium dye concentration in the solution, qmax the

monolayer adsorption capacity of the adsorbent, and KL is the

Langmuir adsorption constant, which is related to the free energy

of adsorption. The plots of 1/qe versus 1/Ce for the adsorption of

EB dye onto HMC particle give a straight line of slope 1/qmaxKL

and intercept 1/qmax, from which qmax and KL can be determined.

The isotherm type reflecting ‘‘favorable’’ or ‘‘unfavorable’’

adsorption can be described by the dimensionless constant

separation factor RL:

J. Mater. Chem., 2009, 19, 7043–7049 | 7047

Table 2 Estimated degree of ionization of the acidic groups at varyingpH conditions

Fraction of ionizedcarboxyl groupsfor HMC(pKa ¼ 7.6)

Fraction of ionizedcarboxyl groupsfor EB(pKa ¼ 4.8)

Fraction of ionizedsulfonate groupsfor AR(pKa ¼ 1)

pH ¼ 2.5 0.0008% 0.5% 96.9%pH ¼ 3.5 0.008% 4.8% 99.7%pH ¼ 4.5 0.08% 33.3% 99.9%

Fig. 10 Schematics of the hypothesis for the relation between the

charged state of the species and dye adsorption on HMC particles.

RL ¼1

1þ KLC0

(6)

where C0 is the initial concentration of dye solution.30,33,37 RL

values in the range 0 < RL < 1 indicate that the adsorption is

energetically favorable. Lower values point out to stronger

binding of the adsorbate, which becomes irreversible at RL ¼ 0.

The Langmuir adsorption isotherms for the EB dye are plotted

in Fig. 9. The parameters determined from the plots in Fig. 9 are

summarized in Table 1. The Langmuir adsorption constants (KL)

and the separation factors (RL) indicate that the adsorption

process of EB at the three pH conditions is always favorable (i.e.,

RL is in between 0 and 1). However, the results prove that more

favorable (greater KL and smaller RL) adsorption takes place at

lower pH conditions.

The results from fitting the isotherms can be interpreted on the

basis of molecular change and interplay between electrostatic

repulsion and hydrophobic attraction. The degree of ionization

of the acid groups (carboxyl for HMC and EB; sulfonate for AR)

can be estimated by the Henderson–Hasselbalch equation:

pH ¼ pKa þ log

�A��

½HA�

!(7)

where [A�] and [HA] are the concentrations of ionized and non-

ionized acid groups, respectively.38 The pKa values of hypro-

mellose phthalate and EB were determined as 7.6 and 4.8,

respectively, by titration with NaOH solution.28 The pKa of AR

was assumed as 1, which is typical for the strongly ionizable

sulfonate group.39 The calculated degrees of ionization of the

Fig. 9 Langmuir adsorption isotherms for EB dye on HMC particles at

different pH conditions. Solid lines are linear fits for each experimental

data set (symbols). Adsorption isotherms for Eosin Y dye are presented

in ESI†, where the adsorption isotherms of Eosin Y have similar trend

with the ones for EB dye.

Table 1 Langmuir isotherm constants for EB dye adsorption on HMCparticles at different pH conditions

pH KL/mL mg�1 RL/mg g�1 Type of adsorption

2.5 156.4 0.11 More favorable3.5 88.9 0.17 Favorable4.5 4.0 0.64 Less favorable

7048 | J. Mater. Chem., 2009, 19, 7043–7049

acid groups of the three compounds at the pH values in the

experiments are listed in Table 2. These values match well with

the hypothesis that in order to promote binding by hydrophobic

interactions both the HMC and the dye molecules have to be in

the low ionization state. The overall mechanism of the adsorp-

tion process that emerges from these results is schematically

illustrated in Fig. 10. The majority of the HMC groups are not

ionized at the given pHs. For the case of EB, the non-ionized

form is dominant at pH 2.5–3.5, resulting in strong adsorption by

hydrophobic interactions between dye and HMC particles.

However, only 66% of EB molecules are non-ionized at pH¼ 4.5,

where the adsorption of the dyes on the HMC particles is much

weaker. Interestingly, though perhaps somewhat coincidentally,

the fraction of EB dye adsorbed on the particles at a large excess

of HMC at pH 4.5 is close to 66% (Fig. 8). On the other hand, the

major fraction of the pH-insensitive dye AR remains negatively

charged at any pH. These dye molecules do not adsorb on the

HMC particles due to the electrostatic repulsion and hydration,

and remain dispersed in the water media.

Conclusions

We report a new method that imparts intense and selective color

to foams stabilized with cellulose particles. The intense colora-

tion results from embedding of dye molecules in the particles,

leading to high enrichment of the foam with coloring agent. The

dye-enriched particles are synthesized by co-precipitation and

adsorption of pH-sensitive dyes on the HMC particle surfaces at

low pH during the particle formation process. The degree of

adsorption of the pH-sensitive dye on the cellulose particles is

well described quantitatively by a Langmuir isotherm model.

This journal is ª The Royal Society of Chemistry 2009

More favorable adsorption at lower pH can be explained by the

charge interaction of carboxyl groups of the dye and HMC. On

the other hand, the adsorption of pH-insensitive dye was unfa-

vorable over the entire pH range due to the strong charge of the

sulfonate group of the AR dye.

The color intensity of EB-dyed foam was 3–4 times stronger

than the intensity of AR-dyed foam. We also demonstrated

a unique multi-color foam system (Fig. 4) by using a mixture of

pH-sensitive and pH-insensitive dyes. This foam coloring tech-

nique is simple and compatible with the HMC particle containing

superstable foams that we reported earlier.26 The surface prop-

erties and stabilization efficiency of the HMC particles appear to

be largely unaffected by the presence of surfactants and other

amphiphilic molecules.27 Since both the modified cellulose and

the dyes used are non-toxic, these colored particles could be used

in a wide variety of personal care or cosmetic products. In

addition, this study reveals the potential of particle-stabilized

dispersions as multi-functional materials. Unlike molecular

surfactants, solid particles are amenable to easy functionaliza-

tion including adsorption and encapsulation. Particles with

designed properties, therefore, can be used not only to stabilize

foams and emulsions, but also to endow them with other useful

properties. Currently, we are investigating means of embedding

other components such as fragrances, nutraceuticals or drugs

into particles from modified cellulose, which can be used in

foams with high applied value.

Acknowledgements

The authors thank Unilever Research & Development and US

Army Research Office for financial support. We also thank

Mr J. T. Loftis and Dr S. Smoukov for assistance with the

experiments and useful discussions. The HMC samples were

generously provided by Shin Etsu Chemical Co., Ltd.

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