Physical Aspects of Cleaning Processes

Handbook for Cleaning/Decontamination of Surfaces I. Johansson and P. Somasundarau (Editors) 9 2007Elsevier B.V. All rights reserved.

-APhysical Aspects of Cleaning Processes Wolfgang von Rybinski Henkel KGaA, Dusseldorf, Germany

1. INTRODUCTION Cleaning processes are very complex due to the different parameters and effects that are involved in these systems. This becomes especially evident when one considers such different processes as household cleaning and detergency or personal care and institutional and industrial cleaning. Even processes like foam flotation in mineral processing or in waste paper treating can be regarded as cleaning steps. A very useful approach to study these processes in a more systematic way and to get more information about the physical chemical basics is given in Figure A.1. According to this the cleaning process can be divided into four different key factors: chemistry, mechanical action, thermal effects and time [1]. These factors are in most cases interdependent and guarantee the overall cleaning and washing results. Dependent on the substrate which has to be cleaned and the soil, one of these factors can be dominant or all factors can have similar influence. As the overall process is very complex due to these key factors, the different substrates, different solvents and the complex composition of cleansers and detergents, this chapter will mainly focus on the physical chemistry of cleaning processes in aqueous systems. As a majority of the published physical chemical studies deal with the washing of fabrics in aqueous detergent solutions, many of the shown examples will be in connection with this process but can be transferred also to other cleaning processes. The physical chemistry of cleaning processes is influenced by effects at the interfaces and in the bulk phases. Interfacial phenomena are the basis for all cleaning and washing processes. The effects range from the wetting of hard surfaces or fabrics and the dissolution of stains from hard surfaces or fabrics to the removal of ions from the washing liquor or the interaction of softeners with the fabric in the rinse cycle of a washing process [1]. Table A.1 shows the different types of interfacial processes

Handbook for Cleaning/Decontamination of Surfaces

Chem'str'

,,me /

/// Chemist~~

Automaticwasher

Vatwashing Figure A.1

\

Circular laundry chart (Sinner's circle) [1]

Table A.1 Interfacial processes in cleaning systems Air-liquid interface Wetting Surface tension Film elasticity Film viscosity Foam generation

Liquid-liquid interface Interfacial tension Interfacial viscosity Emulsification Electric charge Active ingredient penetration Rolling-up process

Solid-liquid interface Adsorption Dispersion Electric charge

Solid-solid interface Adhesion Flocculation Heterocoagulation Sedimentation

that are involved in the cleaning process. Besides this the components involved in the washing process can be very different including a variety of surfaces to be cleaned, liquid or solid stains with different structure and the ingredients of the cleaner and detergent [2]. Clustering of the different interfacial processes lead to the following main steps in washing or cleaning: - choice of the solvent - formulation of the cleansers and detergents

Physical Aspects of Cleaning Processes - dissolution of the detergent and cleanser formulation - wetting of the substrate to be cleaned or washed - removal of hardness ions of the cleaning solution by complexation precipitation or ion exchange - interaction of the detergent or cleanser with the stains - removal of the stains from surfaces - stabilization of the soil in the washing liquor - temporary or permanent modification of the substrate after or during the cleaning process (e.g. by softener in the rinse cycle). All of these processes occur in a consecutive row or simultaneously, and are influenced by the different interfacial parameters. In addition to these interfacial effects, the bulk properties of the liquid system play an important role. The cleaning liquid can be either a homogeneous one-phase system or a dispersion of two or more immiscible phases, e.g. a foam or an emulsion of two immiscible liquid. Also, viscosity or structures within the liquid may have a great impact.

2. C O M P O N E N T S IN CLEANING PROCESSES 2.1. Surfaces Table A.2 gives an overview on the different substrates and surfaces. The surfaces involved in cleaning processes can be very different ranging from fabrics or hair to metal surfaces or ceramics or skin. Therefore, the mechanisms of the cleaning process may vary, although the basic effects are similar. The surface properties of the substrates are decisive for any cleaning process. Important surface properties are surface area, polarity, surface charge and porosity.

Table A.2 Substrates and surfaces in cleaning processes Hard Surfaces

Fibers

Glass Ceramics Metal Polymers Teeth

Cotton Wool Polymers Glass fibers Hair

Handbook for Cleaning/Decontamination of Surfaces Besides this the interaction of the surfaces with the components of the bulk liquid plays an important role. For example, the adsorption of ions onto the surfaces changes the surface properties. Substrates that have a high content of multivalent cations - for example calcium ions etc. - on the surface behave different from surfaces that show a low adsorption of these ions. Due to these effects the different washing results of cotton (high adsorption) and synthetic fibers (low adsorption) can be explained.

2.2. Soils The soils involved in cleaning processes can vary significantly (Table A.3 [3]). The soils can either be solid pigments or a liquid-phase-like oils and fats. Usually they occur in mixtures, which may cause additional difficulties due to an interaction of the different soils. Difficult-to-remove-soils, e.g. in the washing process of fabrics, are pigments such as carbon black or inorganic oxides and fats and waxes or denatured proteins and certain dyes. The removal of soils can be either by temperature, mechanical force, interfacial processes or by chemical degradation, e.g. by enzymes, bleaching agent or alkali.

Table A.3 Soils in cleaning processes

Water-soluble materials Inorganic salts Sugar Urea Perspiration

Fats Animal fat Vegetable fat Sebum Mineral oil Wax

Bleachable dyes from the following Fruit Vegetables Wine Coffee Tea

Pigments

Metal oxides Carbonates Silicates Carbon black (soot)

Proteins from the following Blood Egg Milk Skin residues

Carbohydrates Starch

Physical Aspects of Cleaning Processes 2.3. Ingredients of Cleaners and Detergents The composition of a detergent or cleaner may be very complex, containing different types of substances. Tables A.4A-D show the typical major components of detergents and cleansers for household and institutional applications [4]. In addition to this complex formulation, the components themselves are mixtures as they are usually of technical grade. This makes the description and interpretation of the interfacial processes even more complex.

3. INTERFACIAL EFFECTS FOR THE CLEANING OF PARTICULATE AND OILY SOIL In the following sections, the major characteristics of the single interfacial processes of the cleaning process in aqueous media are summarized concentrating on the more general features applicable to different cleaner types. The major components of cleaners and detergents are the main focus. Minor but equally important ingredients such as enzymes, soilrepellents, perfume oils, etc. have also been studied to a certain extent regarding their interfacial effects, but are not included in this chapter.

3.1. Wetting Wetting of a surface by the solvent is the prerequisite for the performance of a cleaner solution. The wetting is caused by the interaction of the different interfacial tensions (see Figure A.2). For a droplet of a liquid on a solid surface, there are two limiting cases. Either the droplet forms a very thin film (complete wetting) or there is incomplete wetting of the solid by the liquid. In this case, a specific contact angle greater than zero describes the wetting process [5]. Characteristic contact angles are given in Table A.5 [6]. Young's equation gives a quantitative description of the wetting process: Ys -- Ysl + ~ cosO Ys Ysl 0 -

interfacial tension solid-gas interfacial tension solid-liquid surface tension liquid-gas contact angle

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Handbook for Cleaning/Decontamination of Surfaces

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Handbook for Cleaning/Decontamination of Surfaces

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Physical Aspects of Cleaning Processes Table A.4D Formulations of various types of detergents for institutional use

Components

Detergents Partially Built Products Base Specialty Surfactant Bleaching Enzyme Boosters Agents Boosters

Surfactants Sodium triphosphate or zeolite/ polycarboxylate Alkalies (soda ash, metasilicate) Bleaching agents Fluorescent whitening agents Enzymes Complexing agents (phosphonates) An tired epos itio n agents

x x

x x

x

x

X

X

x x x

x

x

x

Young's equation is seldom met in practice, as it is valid only for an ideal solid, chemically homogeneous, rigid and flat at an atomic scale. Most practical solid surfaces are rough to some extent, and may also be chemically heterogeneous. These features of practical solid surfaces lead to contact angle hysteresis. In contrast to the single value predicted by

~ r

0 ..........0

st ~

...........................................................

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.............: --- ~-

" : ' i :~"~----~

7

Figure A.2

Schematic sketch of the wetting of solid surfaces

Handbook for Cleaning/Decontamination of Surfaces

Table A.5 Contact angles of different solid-liquid systems [6] Solid

Liquid

Wool Paraffin Long-chain fatty acid Long-chain alcohols Glass

Water Water Water Water Water

Yc at Contact Angle | (o) 160 105 104 90 0

Young's equation for the contact angle, a wide range of stable contact angles m a y be observed [5]. In this case, the advancing and receding contact angles are the characteristic parameters. The so-called wetting tension j can be defined from this equation: j-

(2)

ys - Ys] - ~ cos 0

A complete wetting of a solid is only possible for spontaneous spreading of a drop of the liquid at the surface, i.e. for 0 = 0 or cos 0 = 1. For a specific solid surface of low surface energy, a linear correlation is observed between cos 0 and the surface tension. This is demonstrated for polytetrafluoro ethylene in Figure A.3 [7]. The limiting value cos 0 = 1 is a constant for a solid and is n a m e d critical surface tension of a solid yc.

cos 0 1.0

0.9

0.8

0.7 I

I

20

25

~'L (mN/m)

Figure A.3 Influence of the surface tension of various fluids on the wetting of polytetrafluoro ethylene [7] 10

Physical Aspects of Cleaning Processes Table A.6 Critical surface tension of polymer solids [8] Polymer

Yc at 20~

Polytetrafluoro ethylene Polytrifluoro ethylene Poly(vinyl fluoride) Polyethylene Polystyrene Poly(vinyl alcohol) Poly(vinyl chloride) Poly(ethylene terephthalate) Poly(hexamethylene adipamide)

18 22 28 31 33 37 39 43 46

(mN/m)

Therefore, only liquids with ~ < Yc are able to spontaneously spread on surfaces and to wet them completely. Table A.6 gives an overview of critical surface tension values of different polymer surfaces [8]. From these data, it is obvious that polytetrafluoro ethylene surfaces can only be wetted by specific surfactants with a very low surface tension, e.g. fluoro surfactants. Figure A.4 shows the wetting tension of two all-purpose cleaners for different surfaces [9]. For most surfaces the wetting tension is in very good agreement with the surface tension of the cleaners. Therefore a spreading of the cleaner solution on the surfaces and good wetting properties can be assumed. Only on polytetrafluoro ethylene surfaces an incomplete wetting is observed. In cleaning and washing, the situation becomes more complicated due to the presence of oily or fatty soil on the surface [10]. In this case there is a competition of the wetting by the surfactant solution and that of the oily soil (Figure A.5). When two d r o p l e t s - one of surfactant solution and one of the oily s o i l - are set on a solid surface, on the basal plane the two wetting tensions jA and jB will act [11]. When the two droplets approach each other a common interface is formed. At the contact line the difference of the wetting tensions will act. This parameter is called oil displacement tension: Aj -- jA + jB

(3)

By the adsorption of the surfactant from the phase A, jA is increased and thus Aj becomes bigger. In addition to this a fraction of the interfacial tension YAB acts in the basal plane with the value of YAB cos 0 with 0 being the contact angle in B, i.e. the oily phase. The resulting force R is 11

Handbook for Cleaning/Decontamination of Surfaces 3O ~A A

E z E 20

mB

C

O ...,, r

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10

(9

Teflon

Steel

Glass

China clay

Figure A.4 Wetting tension of two all-purpose cleaners vs different surfaces [9]

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Figure A.5 Two liquids A (detergent) and B (oily soil) on a solid surface, (a) separated, (b) in contact, jA and jB = wetting tensions, YAB = interracial tension, R = contact tension 12

Physical Aspects of Cleaning Processes called contact tension and is defined as: R - Aj +

(4)

}tAB C O S {9

When R becomes equal to zero, the equilibrium is reached. For the washing and cleaning process, the complete removal of the oil B by the surfactant solution A is the important step. This process is schematically shown in Figure A.6 [11]. The interfacial tension VAB supports for 900>0>0 ~ the contraction of the oil drop being the first step. For a contact angle 0>90 ~ this changes and the interfacial tension acts in an opposite way. Depend on Aj and Y A B a complete removal of the oil can occur. In practice, the rolling-up process is never complete, hence a support for the removal of the oil drop from a solid surface by mechanical forces is necessary for the cleaning step.

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A ...............................................

Figure A.6 Schematic view of the displacement phases of an oily drop B by a cleanser A 13

Handbook for Cleaning/Decontamination of Surfaces

3.2. Drainage The drainage of liquids from surfaces is connected with wetting effects. The drainage of liquid films is important for cleaners or dishwashing formulations for which the liquid should drain in a homogeneous film from surfaces avoiding the retention of insoluble soil residuals on the surface. This process is governed by wetting and non-equilibrium phenomena at the liquid-air interface. The drainage of surfactant-containing solutions under the gravity force is described only by few studies [12,13] despite the importance in applications. Drainage effects can be studied by gravimetric methods. A complete wettability of the substrate is a prerequisite for the validity of the experimental data. A typical equipment is shown in Figure A.7. A hollow glass cylinder is withdrawn from a surfactant solution. After withdrawal the weight of the residual liquid is measured dependent on time, yielding the so-called drainage curve. Figure A.8 shows a set of drainage curves for surfactant solutions with the gravimetric measuring technique [14]. The mass of the retained liquid M(t) is plotted as a function of t': t ' - 1/(td Jr-tw) 05

(5)

td -- time for drainage tw - time for withdrawal of cylinder

Data

"ecision balance

vlindor

Vessel

~

Figure A.7 Setup for measuring the drainage of surfactant solutions 14

Physical Aspects of Cleaning Processes 600

-

Concentration

- , - 0.05 500 - -o- 0.1 --~ ---

~ 400-

/

(mM/I) , /

9

Concentration (mM/I) -,1.0 _ -o-

0.2 0.5

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-Z

"~ 300- -0-- 1 0 . 0 /

2.0

5.0 10.0 50.0

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'10

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0

I

I

I

I

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0.3

0.4

CMC: 11 mM/I 0

ll(tw+td) ~

I

I

I

I

0.1

0.2

0.3

0.4

l ll tw+ td) ~

Figure A.8 Drainage of surfactant solutions on glass in deionized water, T = 40 ~ [14]

The drainage curve is known to be a linear function of t ~ for pure liquids [15]. The drainage curves of the surfactant solutions show a significant deviation from this linear dependence in t ~ at low surfactant concentrations. At high surfactant concentrations, the drainage curves resemble that of the pure water. Low and intermediate surfactant concentrations are seen to stabilize the draining film considerably due to Gibbs-Marangoni effects, yielding a substantial difference from the pure liquid case. Taking into account the diffusion of surfactant molecules from the solution into the surface in addition to this, the Gibbs-Marangoni can be suppressed at high bulk surfactant concentrations due to the diffusive transport of surfactant molecules to the surface. Thus the dynamic surface tension (see Section 3.3.) seems to be an important parameter for drainage effects, on solid surfaces for liquids containing surface-active components. For optimum drainage effects, the liquid should have both good wetting properties and high surfactant concentrations.

3.3. Adsorption at the Solid-Gas Interface The solid-gas interface can be modified by the presence of surfactants in the liquid phase which adsorb onto surfaces. At the gas-liquid interface this leads to a reduction in surface tension. Figure A.9 shows the dependence of the surface tension on the concentration for different surfactant types [16]. It is obvious from this figure that the nonionic surfactants have 15

Handbook for Cleaning/Decontamination of Surfaces 60

)3] Cle

C12H25OSO3Na

50E

"~ 40 30

2H25(OCH2CH2)6OH

-

o.o____o

i

10-5

10 -4

9

m"l

10 -3

i

i

10 -2

10 -1

c (mole/I)

Figure A.9 Surface tension of surfactants with the same chain length as a function of concentration [16]

a lower surface tension for the same alkyl chain length and concentration as the ionic surfactants. The reason for this is the repulsive interaction of ionic surfactants in the adsorption layer which leads to a lower surface coverage than for the nonionic surfactants. In cleaner formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. The second effect which can be seen from Figure A.9 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. This breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the cleaning solution has at least to be right above the cmc, if a dispersion ability of the liquid for soils is necessary for the cleaning process. The surface tension-concentration curves and therefore the cmc values strongly depend on structure parameters of the surfactants. Figure A.10 shows an additional example of this behavior for the surfactant class of the alkyl glycosides [17]. For longer alkyl chains, i.e. greater hydrophobicity of the molecule, the cmc is shifted to lower concentrations. The figure also demonstrates the effect of technical-grade surfactants with a minimum at the cmc (C8-monoglycoside) due to impurities which preferentially adsorb at the interface and therefore lower the surface tension. The cmc of the C12/14-alkyl glycoside is lower than 16

Physical Aspects of

Cleaning

Processes 9 C 8monoglycoside

45-

9C10 monoglycoside 9C12 monoglycoside

40-

[] C12/14APG ~" 35 - D Z E "~ 3 0 -

[] [] [] mm

2520-

I

~iillll

I I IIIIII

10 -4

10-5

9I I i

9O 0 ( O 0 0 /

I

I I IIIIII

10 -3

I

I I IIIIII

10 -2

10 -~

c (mole/I)

Figure A.10 Surface tension of alkyl monoglycosides and alkyl polyglycosides [17]

what can be calculated from the C12-compound and the C14-alkyl glycoside and the corresponding mixing ratio. This behavior has been observed for m a n y surfactants and can be explained by a preferential incorporation of the molecule with the longer alkyl chain in the micelle of the mixture [18]. The presence of electrolytes increases the adsorption of anionic surfactants at the gas-liquid interface as already mentioned. This leads to a reduction of the surface tension at an equal solution concentration [16] and to a strong decrease of the cmc (Figure A.11). The effect can be in several orders of magnitude. Similar to this are the effects of mixtures of surfactants with the same hydrophilic group and different alkyl chain length or mixtures of anionic and nonionic surfactants [19]. Mixtures of anionic and nonionic surfactants follow the mixing rule (equation 1) in the ideal case: 1

CmCmix

with

CmCmix cmcl cmc2 c~

=

=

c~

cmcl

t

1-c~

cmc2

(6)

cmc of surfactant mixtures cmc of surfactant 1 cmc of surfactant 2 mole fraction of surfactant 1 in bulk solution

According to a theory, based on the regular solution theory, a deviation from ideal behavior can be described by the introduction of the activity 17

Handbook for Cleaning/Decontamination of Surfaces 70

60 5040 ~ 30

20-

no additional electrolyte

+ 0.2 (mole/I) Na2SO 4

1010-5

I

I

I

I

10-4

10-3

10 -2

10 -1

c (mole/I)

Figure A.11 Influence of counter ions on the surface activity of a typical anionic surfactant dependent on the surfactant concentration [16]

coefficients fl and f2" 1

CmCmix

=

c~

+

fl cmcl

with fl = exp fl [1

1-~

f2 cmc2

-

Xl]2

and f2 - exp fl x 2 AHm

with

fl f2 fl Xl AHm

= = -

- - ,8

RT Xl

[1

(7) (8) (9)

-

Xl]

(10)

activity coefficient of c o m p o n e n t 1 activity coefficient of c o m p o n e n t 2 interaction p a r a m e t e r mole fraction of c o m p o n e n t 1 in the micelle micellization enthalpy

The interaction p a r a m e t e r fl characterizes the deviation from ideal behavior. If fi has negative values, there is an attractive interaction b e t w e e n the surfactants, and the cmc of the mixture is lower than expected for ideal behavior. For fl > 0, there is a repulsive interaction and the cmc is higher than that for ideal behavior. For highly negative values of fi and cmc of the surfactants which are quite similar, the cmc of the mixture is even lower than that of the single surfactants. The strongest interactions are observed 18

Physical Aspects of Cleaning Processes 10-2 ]

J

Calculated (ideal behavior) t

4

lml,Om

O

m m uu nn m u m m O m m m m m nu m m m O m

E

n m m m m m nmmO no

Measured

#

10-3 I

0

0.2

I

I

I

0.4 0.6 0.8 a (mole fraction in bulk)

I

1.0

Figure A.12 Critical micelle concentration of mixtures of sodium n-dodecyl sulfonate and n-octylnonaglycolether [16]

for mixtures of anionic and cationic surfactants due to the electrostatic forces between the head groups. An example of the influence of the interaction of the surfactant molecules on the cmc is shown in Figure A.12. The interaction between the surfactants has not only an influence on the cmc, but also on different properties which are relevant for cleaning. So a synergistic effect has been observed for foaming, emulsification and dispersing properties and even washing and cleaning efficiency for negative /~ parameters [19]. The kinetics of surface effects is an aspect which has been underestimated for a long time regarding the mechanisms of cleaning and washing. Especially at lower concentrations, there might be a strong influence of time on the surface and interfacial tension. Figure A.13 shows the dynamic surface tension of an anionic and a nonionic surfactant dependent on time for different concentrations [20]. For both surfactants, the time dependence of surface tension is greatly reduced when the concentration increases. This effect is especially pronounced when the cmc is reached. The reason for this dependence is the diffusion of surfactant molecules and micellar aggregates to the surface which influences the surface tension on newly generated surface. This dynamic effect of the surface tension can probably be attributed to the observation that an optimum of the washing efficiency usually occurs well above the cmc. The effect is also an important factor for cleaning and institutional washing where short process times are common. It can also influence foam formation and rinsing effects [14]. 19

Handbook for Cleaning/Decontamination of Surfaces (A)

(B) 70

70

1 mM

mM• ~ N

2 60

E

%.

"~ 50

~

c

m

c

E 9,~50

11 mM I

=

20 mM I]ll

I

10-3

I

Illllll

10 -2

40 I

I Illllll

I

10-1

I IIIIlll

I

10 0

I

\ Icmc = 0.07 IIIII

1111111

101

I

10-3

t/s

I IIIIIII

10 -2

I

I IIIIIII

I

10-1

I IIIIIII

I

10 0

I

IIIIIII

101

t/s

Figure A.13 Dynamic surface tension of (A) C12SO3Na and (B) C12E6 as a function of concentration at 40~ [20]

3.4. Adsorption at the Solid-Liquid Interface For aqueous solutions, the physical separation of the soil from surfaces is based on the adsorption of ions and surfactants on the surface that will be cleaned. For a pigment soil, the separation is caused by an increased electrostatic charge due to the adsorption shown in Figure A.14 [21]. A theoretical understanding of the interaction forces causing a solid particle to adhere to a more or less smooth surface is based on the Derjaguin-Landau-Verwey-Overbeek theory (DLVO) [22]. Since this theory was developed to explain the phenomena of flocculation and coagulation, however, it can be applied to the washing process only in modified form [23]. A plot of potential energy as a function of the distance of a particle from a solid surface shows that the potential energy passes through a maximum (Figure A.15). The minimum in the potential energy curve corresponds to the closest possible approach, i.e. to the minimum distance that can be established between the particle and the surface to be cleaned. The maximum is a measure of the potential barrier that must be overcome if the particle is either removed from the surface or approaches the surface from a distance. Adhering particles are more easily removed if the potential barrier is small. Conversely, a soil particle already in the cleaning solution is less likely to establish renewed contact with the surface if the potential barrier is large. 20

Physical Aspects of Cleaning Processes (B) Disjoining pressure

(A) Electrostatic forces

(C) Rolling-up

Washing fiquor Air

iiiii!iiiiiFiiiiiiii ii;iii!iiiii~

|iiii iiiiiiii~~i~;ii~i!i i~

Washing liquor

JB

Substrate

.....t

i!iii!iiiiiiii!ii

i

~q

!

A = Detergent B = Oil

iii!!~Sii~!U

Figure A.14

i~iii~ii~iii!!iiii!!

Separation mechanisms in cleaning processes

With respect to the washing process, if a particle is b o u n d to a surface, only a single c o m m o n electrical double layer located at the overall external surface exists initially. None is present within the zone of contact. During the cleaning process, new diffuse double layers are created, which cause a reduction in the free energy of the system. The free energy of an

1 L_ e" m , tm~ C (U 4~ 0 Q.

Distance

=

Figure A.15 Calculated potential energy of attraction PA and repulsion PR as a function of the distance of a particle from a surface, along with the resultant potential P; predictions based on the DLVO theory 21

Handbook for Cleaning/Decontamination of Surfaces electrical double layer is a function of distance and diminishes asymptotically to a limiting value that corresponds to a condition of no interaction between two double layers. Twice as much effort must be expended to bring a particle into contact with a substrate because of the presence on both surfaces of a double layer (curve 2F in Figure A.16). The separation of two adhering surfaces is characterized initially only by van der Waals-London attractive forces PA and Born repulsion forces PB, since at this point no electrical double layer exists [23]. In Figure A.16, the equilibrium condition corresponding to the potential energy minimum has been taken as the zero point on the abscissa. With increasing distance between the particle and the contact surface, a diffuse double layer arises, which assists in the separation process by establishing an element of repulsion. Thus, the true potential curve P for the separation of an adhering particle in an electrolyte solution results from a combination of the van der Waals-Born potential and the free energy of formation of the electrical double layer. The important conclusion from Figures A.15 and 16 is that an increase in the potential of the electrical double layer increases the energy barrier for particle deposition but decreases for particle removal. The negative influence exerted by calcium ions in aqueous cleaning solutions originating from water hardness can also be explained with the help of potential theory. According to the Schulze-Hardy rule, compression of an electrical double layer increases rapidly as the valence of a cation increases. Therefore, high concentrations of calcium ions might

~_~

Distance

P,

Figure A.16 Potential energy diagram for the removal of an adhering particle; PB, Born repulsion; PA, van der Waals attraction; F, free energy of the double layer; P, resulting potential curve 22

Physical Aspects of Cleaning Processes -80 -60 E

._~ - 4 0 r ,,i-=

o -20

20 1

I

I

I

I

I

3

5

7

9

11

pH

Figure A.17 Zeta potential of various surfaces as a function of pH; (a) wool; (b) nylon; (c) silk; (d) cotton; (e) viscose

cause attractive forces to become the dominant factor, leading to significantly lower cleaning efficiency than would be achieved in distilled water. The foregoing theoretical treatment offers an explanation for the behavior that is actually observed. Surface potentials cannot be measured directly. Instead, the ~-potential or electrophoretic mobility of a particle is used as a measure of surface charge. As a rule, many solid substrates and pigments in an aqueous medium above pH 7 acquire negative charges, whereby the extent of charge increases with increasing pH. This is illustrated in Figure A.17, in which the ~-potential of various fibers is taken as a measure of electrical charge and is plotted against pH [24]. Essentially similar results are obtained for all major particulate soil components. This is one of the reasons for enhancement of cleaning performance by mere introduction of alkaline solutions. However, repulsive forces between soil and surfaces alone are insufficient to produce satisfactory cleaning even at high pH. Apart from changing pH, another way to significantly alter substrate and pigment surface charges is to introduce a surfactant. The sign of the resulting charge depends on the nature of the hydrophilic group of the surfactant. This can be shown in aqueous solutions of different surfactants with the same alkyl chain length by the change of electrophoretic mobility of pigments which is a measure for the surface charge (Figure A.18) [21]. The carbon black shown as an example has a negative surface charge in water at an alkaline pH value. As for most pigments present in cleaning 23

Handbook for Cleaning/Decontamination of Surfaces C14H290SO3Na

-60 -40 o

~

'T

>

"7

_ 0 ~

-

-20

~,

O

C14H230(CH2CH20)9H

0

O4

E

II

+20

\

I

I

I

I

10 -5

10 - 4

10 - 3

10 - 2

I=

c (mole/l)

+40 -

C14H29N(CH3)3Cl

+60 -

Figure A.18 Electrophoretic mobility u of carbon black in solutions of different surfactants at 308 K [21]

processes the isoelectric point is below pH 10. The nonionic surfactants show no influence on the electrophoretic mobility, whereas the anionic surfactant increases the negative surface charge of the pigment due to the adsorption. By the adsorption of cationic surfactant, the surface charge can be changed from a negative to a positive value during the adsorption process. This picture explains quite well the mode of action of different surfactant types for pigment removal in a cleaning process. As nonionic surfactants do not influence the electrostatic repulsion of pigment and fabric, their cleaning efficiency mainly is caused by the disjoining pressure of the adsorption layer. Anionic surfactants increase in addition to this electrostatic repulsion, but usually have lower amounts adsorbed than the nonionic surfactants. Cationic surfactants show similar effects in cleaning processes as anionic surfactants, but in spite of this they are not suited for most cleaning processes due to their adverse effects in the rinse cycles. In the rinse cycles, the positively charged surfaces (due to the adsorption of cationic surfactants) are recharged to negative values due to the dilution of the cleaning solution and the consecutive desorption of cationic surfactants. As the different fabrics and pigment soils have different isoelectric points, positively and negatively charged surfaces are present in the washing liquor. This leads to heterocoagulation processes and a redeposition of the already removed soil onto the fabric. Therefore cationic surfactants are not used in alkaline cleaning processes, only as softeners in the rinse cycle when no soil is present any more and a strong 24

Physical Aspects of Cleaning Processes adsorption of cationic softener on the negatively charged fabric is desired. Cationic surfactants are instead used in acidic cleaners, especially in metal treatment, where substrate and pigment soil have a positive charge and no charge reversal in the cleaning process occurs. Whereas surfactants are adsorbed non-specifically at all hydrophobic surfaces, complexing agents can undergo specific attraction to surfaces that have distinct localized charges. The main process is chemisorption and is especially characteristic of metal oxides and certain fibers [25]. As shown in Figure A.19, the adsorption of a complexing agent produces an effect similar to that of an anionic surfactant. The change in ~'-potential for hematite is taken as illustrative. The specificity of adsorption of complexing agents with respect to metal oxides is so great that even displacement of anionic surfactants from surfaces with lower adsorption energies is permitted [25]. Complexing agents suppress the adsorption of anionic surfactants on metal oxides. However, adsorption is enhanced on materials such as carbon black or synthetic fibers. This effect is due to the electrolyte character of the complexing agent. The cleaning process generally involves removing mixed soils that consist of both hydrophilic and hydrophobic matter from fiber surfaces. For this reason, the different specificities of complexing agents and surfactants give complementary functions to these two types of material.

-60

-

-40

a

E m

:= 0 r ~j~

-20

-

O- -// 5

pH

7

,

9

Figure A.19 Zeta potential of hematite as a function of pH at 25~ in the presence of (a) sodium chloride, (b) sodium triphosphate, (c) benzene hexacarboxylic acid and (d) 1-hydroxyethane-l,l-diphosphonic acid [25] 25

Handbook for Cleaning/Decontamination of Surfaces

r

/

~s

IIIIII S

Figure A.20 Schematic representation of adsorption-induced separation of a spherical particle from a hard surface; S - surface; P - particle; ~s = splitting pressure of the surfactant layer on the surface; 7rp --- splitting pressure of the surfactant layer on the particle

Figure A.20 is a schematic representation of the adsorption layers on substrate and soil particles. As can be seen from the diagram, both adsorption layers advance to the point of particle - surface contact. One consequence is the development of a disjoining pressure, which leads to separation of the soil particle from the surface. This effect is obviously present with anionic surfactants as well. However, this pressure is the decisive factor with nonionic surfactants, due to the absence of any repulsive components of electrostatic origin. In the aqueous cleaning solution, the fabric surface and the pigment soil are charged negatively due to the adsorption of OH--ions and anionic surfactants. This leads to an electrostatic repulsion. In addition to this effect, a disjoining pressure occurs in the adsorbed layer which supports the lift-off process of the soil from the surface. For a spherical particle with a radius r, the separation force is described by equation 11 [21]" f d - 2 rr r (7rs + yZ'p)

(11)

with rrs - disjoining pressure in the adsorption layer of the substrate yrp -- disjoining pressure in the adsorption layer of the particle The nonspecific adsorption of surfactants is based on the interaction of the hydrophilic head group and the hydrophobic alkyl chain with the 26

Physical Aspects of Cleaning Processes pigment and substrate surfaces as well as the solvent. For the adsorption of surfactants different models have been developed which take into account different types of interactions. A simple model which excludes lateral interactions of the adsorbed molecules is the Langmuir equation: 1

1

Q~

1

bQm c

1

(12)

Qm

with Q~ = equilibrium adsorbed amounts Q m = adsorbed amounts in a fully covered monolayer c = equilibrium concentration in solution b = constant This model is restricted to only very few systems. A more widely applicable model is presented in Figure A.21 with a visualization of the structure of the adsorbed molecules dependent on surface coverage [26]. Three different ranges are to be distinguished" in the low concentration range, single molecules are adsorbed on the surface with no interaction between the molecules. The molecules are preferably arranged on the surface in a flat structure or with a certain tilt angle. For ionic surfactants, the adsorption sites on the surface are determined by places of surface charge. When the surfactant concentration increases, a strong rise in the adsorbed amounts is observed by the lateral interaction of the hydrophobic parts of

< HMC x

> HMC

> HMC

(A)

+++

+§

I

+++

++++

I

+++

++++

I +++

++++

I

I+++

++++

++++

J

I+§

++§247I

§

I

+++

I

(B) +++

(c) I+++

++++

f

XHMC = hemimicelle concentration

Figure A.21 Adsorption models for surfactants [26] (A) model of Fuerstenau, (B) model of Scamehorn, Chandar, Dobias, (C) model of Harwell et al. 27

Handbook for Cleaning/Decontamination of Surfaces the surfactant molecules. The surfactant molecules have a perpendicular arrangement to the surface. There are different models for the structure of the adsorbed layer in this concentration range either assuming a flat monolayer or a hemimicellar structure, depending on the structure of the surfactants and the charge distribution on the solid surface. The hydrophilic groups of the surfactants can be directed either to the surface of the solid or the solution depending on the polarity of the solid surface. In the third part of the adsorption isotherm a plateau value is observed. During a further increase of the surfactant concentration a rise in the adsorbed amounts occurs. In this range of the adsorption isotherm a fully covered monolayer or double layer is adsorbed onto the surface, making the surface either hydrophilic or hydrophobic. Depending on the type of the surface in some cases micellar structures of the adsorbed surfactants have been postulated instead of flat double layers. Typical examples of adsorption isotherms of sodium dodecyl sulfate onto different surfaces are shown in Figure A.22 [21]. The adsorption isotherms for the carbon black and the graphitized carbon black (Graphon) are completely different. For graphitized carbon black, a step-like adsorption isotherm is observed which indicates the flat arrangement of the surfactant molecules on the surface at low concentrations with a perpendicular structure at higher concentrations (see Figure A.21). The adsorption process is exothermic with an adsorption enthalpy of about -128 to - 3 6 kJ mo1-1. The adsorption of sodium dodecyl sulfate on titanium dioxide is an example of the specific adsorption via the hydrophilic group

/

12

NaCI-4.10-2 (mole) ~

" TiO2 at pH 4

-15 04

OA

10

_

"O

O

~E

r O o

s

o-4

phon

4

0 I E 4306"2 I

I

I

I

I

I

2

4

6

8

10

C

.10 3 (mole/I)

Figure A.22 Equilibrium adsorption of sodium n-dodecyl sulfate on carbon black, Ti02 and Graphon at room temperature [21] 28

Physical Aspects of Cleaning Processes onto the polar pigment surface. A second adsorption layer is formed via hydrophobic interaction with the first adsorption layer which makes the pigment surface hydrophilic again in the range of the plateau of the adsorption isotherm. Figure A.22 also demonstrates the effect of the addition of electrolytes which are present in many cleaning processes. In the presence of ions the amounts adsorbed by the anionic surfactant are increased. This is due to a decreased electrostatic repulsion of the negatively charged hydrophilic groups of the anionic surfactant in presence of electrolytes. Therefore the adsorption density at equilibrium can be enhanced significantly. A similar effect can be observed in a comparison of an anionic and nonionic surfactant with the same alkyl chain length adsorbed onto a hydrophobic solid (Figure A.23) [21]. At the same concentration, the nonionic surfactant gives higher adsorbed amounts than the anionic surfactant. This is especially valid at low concentrations, whereas at very high concentrations both surfactants reach the same plateau value. For a hydrophilic solid surface, this effect can be just opposite due to a higher affinity of anionic surfactant to the surface via specific interactions. The electrolyte effect for the adsorption of anionic surfactants which leads to an enhancement of soil removal is valid only for low water hardness, i.e. low concentrations of calcium ions. High concentrations of calcium ions can lead to a precipitation of calcium surfactant salts and therefore to a reduction of concentration of active molecules. In addition to this, the electrical double layer is compressed that much, that

/ Q

~

O

C12H250(CH2CH20)12 H 9

0

I

O

O

C12H25OSO3Na

~, 4

6 2

0

i E 4306"31

I

I

I

I

2.5

5

7.5

10

C "10 3 (mole/I)

Figure A.23 Surfactant adsorption onto carbon black, T = 298 K, surface area 1150 m 2 g-1 (BET) [21]

29

Handbook for Cleaning/Decontamination of Surfaces the electrostatic repulsion between pigment soil and surface is reduced. Therefore, for many anionic surfactants the cleaning performance decreases with lower temperatures in the presence of calcium ions. This effect can be compensated for by the addition of complexing agents or ion exchangers (see chapter on ion complexation).

3.5. Adsorption at the Liquid-Liquid Interface The phenomena at the liquid-liquid interface are of outstanding importance for the removal of oily soil from the surface. As already shown in the chapter about wetting the interfacial tension is one of the decisive parameters in the rolling-up process. This parameter can be very different depending on the surfactant structure and the type of the oily soil [9]. Figure A.24 shows this for two different oils and two anionic surfactants. The interfacial tension has been recorded as a function of time. For both surfactants the interfacial tension is the same with lower values for the nonpolar decane. To demonstrate the influence of the polarity of the oil on the efficiency of the surfactant, a more polar oil is chosen (Figure A.25). In this case the interfacial tension is significantly lower when the fatty alcohol sulfate is used instead of linear alkylbenzene sulfonate. The increase of the interfacial tension dependent on time is probably caused by a solubility of the surfactant in the oil phase.

9 C12/14-FAS 9 0 [] LAS c = 1 g/I, dest. H20, T = 40~

2-Octyldodecanol

X

E Decane -O

0

I

I

I

I

I

5

10

15

20

25

I

30

Time (rain)

Figure A.24 Interfacial tension between a solution of C12/14-fatty alcohol sulfate (FAS) and linear alkylbenzene sulfonate (LAS) and two different oils as a function of time [9] 30

Physical Aspects of Cleaning Processes

c = 1 g/I, dest. H20, T= 40~ /

LAS

1.5

[]

E z E /9

0.5

9

-

0

1

4

9

~

C12/14.FAS

I

I

I

I

I

I

5

10

15

20

25

30

9

Time ( 9

Figure A.25 Dynamic interfacial tension of C12/14-fatty alcohol sulfate (FAS) and linear alkylbenzene sulfonate (LAS) for isopropyl myristate [9]

Figure A.26 shows the interfacial tension, of different detergent formulations against mineral oil. For overall low values of the interfacial tension, there are significant differences between the detergents which indicate a different performance against this nonpolar oil. As the interfacial tension should be minimized in cleaning processes, there is the need for a further decrease of the interfacial tension in

0.5 0.4

Mineral oil

i!i i i i i i i i i i ii!iiLiiiiiiii!!iiLiiiiiiiiiDiiiii!iii!!iiiiiiiiiiiiii

"

iiiiiiiiiii!iiiiiiii!ii~i~!iiiili~iiiiiiiii~iiiiiiiiiii

iiiiiiiiii!!iiii!ii

A

E 0.3

iiiii;ililiiiiiil;iiiii!i;ii!ililiiiiiiiiiiiiii!ilililiiiiiiiilililili!iiil

~.

iiiiiiiiiiiiili

z

~iiii~i~ii;ii!~!!ili? !ii!ii!ii:~iiii, iiiiiiiiiiiiiiiiiiii!iiiii!i@iii

0.2-

0,1

-

A

Figure A.26 Interfacial mineral oil [9]

li i i !i i i iliiii iili ili i B

C

tension

31

of

ii!i!i!!i!i!i!i!!!!ii!i!i!!!i!i4!!!i!!!!i!!

!~~.?!?i!i!~!~~ !i!4!~ i;!i!?!i~)~!~!~!~!~!i D

different

detergents

for

Handbook for Cleaning/Decontamination of Surfaces formulations. A suitable way is again to create mixed adsorption layers of suitable surfactants [10,19]. For example, the interfacial tension of the system water-olive oil as a function of composition for a surfactant mixture containing the anionic surfactant sodium n-dodecyl sulfate with the nonionic surfactant nonylphenol octaethylene glycol ether shows a pronounced minimum at a certain concentration ratio for a constant total surfactant concentration. Even small additions of one surfactant to another can lead to a significant reduction of the interfacial tension. For this specific example, a minimum value of the interfacial tension is reached with a ratio of anionic surfactant to nonionic surfactant of about 4 to 1. Kinetic effects play an important part in this process. The behavior of the mixtures can be completely different dependent on time, showing a minimum of the interfacial tension for a certain concentration ratio of the surfactants or not [19]. This has to be taken into account in the search for an effective surfactant system. Thus, the interfacial tension can be used to optimize cleaner formulations. The interfacial tension can be influenced by the penetration of the surfactant solution into the oily phase and the formation of new phases. A typical example is given in Figure A.27 [27]. The picture from a microscope with polarized light for oleic acid in contact with an aqueous solution of sodium dodecyl sulfate visualizes the formation of liquid crystalline mixed phases. These phases influence both the rolling-up and the emulsification of oil by surfactant solutions. For this model system an increased removal of oil from fabric surfaces was proven by this formation of mixed phases. The effects are described in more detail in this chapter on phase behavior of surfactant systems.

4. BULK PROPERTIES OF THE CLEANING SOLUTION 4.1. Phase Behavior of Surfactant Systems The phase behavior of the surfactant systems is decisive for the formulation of liquid and solid products and the mode of action of the surfactants in soil removal during the cleaning process. Due to the different phases of surfactant systems at higher concentrations e.g. the flow properties can vary very strongly depending on concentration and type of the surfactants. This is of crucial importance for the production and handling of liquid products. In addition to this, the phase behavior influences the dissolution properties of solid cleaners and detergents when water is added, forming or preventing high-viscous phases. One can distinguish 32

Physical Aspects of Cleaning Processes

Figure A.27 Polarized light microscopic photograph. Spontaneous formation of liquid crystalline mixed phase zones (bright areas) from sodium dodecylsulfate solution (2.5%) and oleic acid [27]. See Color Plate Section in the back of this book

between the phase behavior of surfactant-water-systems and multicomponent systems including an additional oil phase which occurs when the fatty or oily soil is released from surfaces. As an example of the different phases of surfactants, Figure A.28 shows the phase diagram of a pure nonionic surfactant of the alkyl polyglycol ether type [28]. Especially the phase behavior of nonionic surfactants with a low degree of ethoxylation is very complex. As the lower consolute boundary is shifted to lower temperatures with decreasing ethylene oxide (EO) number of the molecule, an overlapping of 33

Handbook for Cleaning/Decontamination of Surfaces

/

100

W+ L2 80-

L2

(J l.-

t~ Q. E p.

60-

W+Lo~

40-

W+L1

i s

\

!._

i

L1

i

V1

',

20i

f

H1

,

"'i', ii

ii

i

i|

I

I

I

25

50

75

100

[C12E5] (mass %)

Figure A.28 Phase diagram of the binary system water-pentaoxyethylene n-dodecanol (C12E5) [28]

this boundary with the mesophase region may result, as depicted in Figure A.28. At low surfactant concentrations in such systems, several two-phase areas are observed in addition to the single-phase isotropic L1 range: two coexisting liquid phases (W + L1), a dispersion of liquid crystals (W 4- L~) and a two-phase region of water and a surfactant liquid phase (W + L2). The phase behavior can have a significant impact on the cleaning process [29]. If there is no phase change for the surfactant water system, a linear dependence of the cleaning efficiency on temperature is observed which is shown as an example for household detergency in Figure A.29. The surfactant is in an isotropic micellar solution at all temperatures. The cloud point of the surfactant used here is 85~ at the given concentration (2 g/l), i.e. above the highest washing temperature. Tests with other pure ethoxylated surfactants have revealed that a discontinuity is observed with respect to oil removal vs temperature in cases of the existence of dispersions of liquid crystals in the binary system water/surfactant. Figure A.30 shows that the detergency values for mineral oil and olive oil, i.e. two oils with significantly different polarities, are at different levels. It also demonstrates that in both cases a similar reflectance vs temperature curve exists. In the region of the liquid crystal dispersion, i.e. between 20 and 40~ the oil removal increases significantly. Above the phase transition W + L~ --~ W + L3, between 40 and 70~ no further increase in oil removal takes place. For olive oil, 34

Physical Aspects of Cleaning Processes 6O

o~

2 g/I Surfactant

O

50-

l

i

v

e

~

l

~

n~

40-

o

i

l L1

0

--'-/i

I

40

0

Figure A.29

I

I

60 Temperature(~

80

Phase behavior of C12E9and detergency [29]

a small decrease in detergent performance is observed. The interfacial tensions between aqueous solutions of C12E3 and mineral oil lie at about 5 m N / m -1 at 30 and 50~ These relatively high values indicate that in this system the interfacial activity is not the decisive factor in oil removal from fabrics. The macroscopic properties of the liquid crystal dispersion seem to be responsible for the strong temperature dependence. It can be assumed that fragments of liquid crystals are adsorbed onto fabric and oily soil in the W + L~ range during washing. The local

60

. . ~ i l

5O

nera/oil

40 W + L~

o

2'o

- -

W + L3--~ ~--W + L2

4 60 Temperature(~ 0

I

80 I

Figure A.30 Phase behavior of the polyoxyethylene alcohol C12E3 and detergency, 2 g/I surfactant [29] 35

Handbook for Cleaning/Decontamination of Surfaces surfactant concentration is therefore substantially higher in comparison to the molecular surfactant layer that forms when surfactant monomers adsorb. As the viscosity of liquid crystals in the single phase range is strongly temperature dependent, it can be assumed that the viscosity of a fragment of a liquid crystal deposited on a fabric also significantly decreases with increasing temperature. Thus the penetration of surfactant into the oil phase and removal of oily soil are promoted. Technical grade surfactants are of specific interest for applications. As in the case of pure nonionic surfactants, definite ranges for technical grade surfactants exist in which there is only a slight dependence of oil removal on the temperature (Figure A.31). For C12/18E5, this is in the range of the two co-existing liquid phases (W + L1) and for C12/18E4 it is in the range of the surfactant liquid phase (W + L2). An unusually strong increase of oil removal with increasing temperature occurs in the region of the liquid crystal dispersion (W + L~). At 30 and 50~ the interfacial tensions between aqueous surfactant solutions and mineral oil and the contact angles on glass and polyester were determined for C12/18E4 . Whereas the values of interfacial tensions are practically identical (approximately 10 -1 m Nm -1) the contact angles on both substrates are slightly less advantageous at higher temperatures. Hence, the increased oil removal between 30 and 50~ cannot be attributed to an increase in the adsorbed amounts of surfactants. Rather in both cases, the decisive part is probably played by the macroscopic properties of the liquid crystal dispersion and their temperature dependence.

W + L~

70

6O

C12/18E4

W + L2 ,e --~

9

\

r

=: e

~o

\

50

40

W + L1 .-,,.--/t

0

I

I

30

40

~'

W + L~

I

50 Temperature

I

I

I

60

70

80

(~

Figure A.31 Phase behavior of the polyoxyethylene alcohols C12/18E4 and ClwleEs and detergency [29] 36

Physical Aspects of Cleaning Processes Nonionic

.20

Tu

o,,

j 1 ~ 1

\\

Ti Oil

Figure A.32 Schematic phase diagram of a ternary system consisting of water, oil and ethoxylated nonionic surfactant [30] During the oil removal from hard surfaces or fabrics ternary systems occur where three phases coexist in equilibrium. These systems are also referred to as three-phase microemulsions. These effects were studied in detail for alkyl polyglycol ethers [30]. Depending on temperature different phases exist, having a three-phase region between the temperature T1 and Tu (Figure A.32). When these three phases are formed, extremely low interfacial tensions between two phases are observed. Because the interfacial tension is generally the restraining force with respect to the removal of liquid soil in the cleaning process, it should be as low as possible for optimal soil removal. Other parameters such as the wetting energy and the contact angle on polyester, as well as the emulsifying ability of e.g. olive oil, also show optimum values at the same mixing ratio at which the minimum interfacial tension is observed. Figure A.33 (right) represents the three-phase temperature intervals for C12E4 and C12E5 vs the number n of carbon atoms of n-alkanes. The left part of Figure A.33 shows the detergency of these surfactants for hexadecane. Both parts of Figure A.33 indicate that the maximum oil removal is in the three-phase interval of the oil used (n-hexadecane) [31]. This means that not only the solubilization capacity of the concentrated surfactant phase, but probably also the minimum interfacial tension existing in the range of the three-phase body are responsible for the maximum oil removal. Further details about the influence of the polarity of the oil, the type of surfactant and the addition of salt are summarized in the review of Miller and Raney [32]. 37

Handbook for Cleaning/Decontamination of Surfaces 80-

80 A

oo

60-

~"

40-

(9 I., :3

-

"(. I--

60-

40-

Q.

E

E a) 2 0 -

9 20-

r

I

I

I

I

I

40

50

60

70

80

]~jt

I

6

R(%)

I

8

I

I

10

12

I

14

16

n

Figure A.33 Detergency of C12E4 and C12E5 against hexadecane as a function of temperature (left side) and the corresponding threephase ranges for these surfactants as a function of the number n of carbon atoms of alkanes [31]

Studies of diffusional phenomena have direct relevance to detergency processes. Experiments are reported which investigate the effects of changes in temperature on the dynamic phenomena, which occur when aqueous solutions of pure nonionic surfactants contact hydrocarbons such as tetradecane and hexadecane. These oils can be considered to be models of nonpolar soils such as lubricating oils. The dynamic contacting phenomena, at least immediately after contact, are representative of those which occur when a cleaner solution contacts an oily soil on a polymer surface. With C12E5 as the nonionic surfactant at a 1 wt.% level in water, quite different phenomena were observed below, above and well above the cloud point when tetradecane or hexadecane was carefully layered on top of the aqueous solution. Below the cloud point temperature of 31~ very slow solubilization of oil into the one-phase micellar solution occurred. At 35~ which is just above the cloud point, a much different behavior was observed. The surfactant-rich L1 phase separated to the top of the aqueous phase prior to the addition of hexadecane. Upon addition of the oil, the L1 phase rapidly solubilizes the hydrocarbon to form an oil-in-water microemulsion containing an appreciable amount of the nonpolar oil. After depletion of the larger surfactant-containing drops, a front developed as smaller drops were incorporated into the microemulsion phase. Unlike the experiments carried out below the cloud point temperature, appreciable solubilization of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion 38

Physical Aspects of Cleaning Processes interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the temperature of the system was raised to just below the phase inversion temperatures of the hydrocarbons with C12E5 (45~ for tetradecane and 50~ for hexadecane), two intermediate phases formed when the initial dispersion of L1 drops in the water contacted the oil. One was the lamellar liquid crystalline phase L~ (probably containing some dispersed water). Above it was a middlephase microemulsion. In contrast to the studies below the cloud point temperature, there was appreciable solubilization of hydrocarbon into the two intermediate phases. A similar progression of phases was found at 35~ using n-decane as the hydrocarbon. At this temperature, which is near the phase inversion temperature of the water-C12E5-decane system, the existence of a two-phase dispersion of L~ and water below the middle-phase microemulsion was clearly evident. These results can be utilized to optimize surfactant systems in cleaners, and in particular to improve the removal of oily soils. The formation of microemulsions is also described in the context of the pre-treatment of oil-stained textiles with a mixture of water, surfactants and cosurfactants. Besides cleaning efficiency, the liquid crystalline phases of surfactant systems at higher concentrations are of crucial importance for the processing of concentrated surfactant systems and the formulation as well as the application of liquid products. This is demonstrated with the help of the phase diagram of anionic surfactants for the example of fatty alcohol sulfates. Figure A.34 shows the complete phase diagram of sodium dodecyl sulfate [33]. At higher concentration of the surfactant a multitude of

100 / I

I

Micellar '. +H~

~

80 A

Qa+TaQa H~

9- 60 4-1

/

M /I

Micellar I

!._

G} O. E 40 I-

20

Ro~

Mc~+C2 i

/

I

_ - _-/

I

II

I

~,'-

Ta

,,

,/11 I L(z+C2

"-

30

-"L +c

..../

0(,

X

-

I

"_~111

j~_~C- 2-

_l~ i

H~+C 2 Micellar+C 2

Jt--r

L,

m

r

40

50

Figure A.34

D20+C2 T

02+0 x ~ f

60 70 SDS (weight%)

I

C2I' -

-I

+

I

: : Cx :

Micellar + C_2

I

I

Cx+C

"

\ / L }l.Jl- L'-:-----'Z:;T-:

--

/

t

Cx+C1 T

80

~

01 8

r' ' L _ J ' -,I- -~,. . . .

I i,

I

T, ,, 018

90

Phase diagram of sodium dodecyl sulfate [33] 39

lO0

Handbook for Cleaning/Decontamination of Surfaces viscosity (Pa s) 1000 --= m

100

Temperature 70~

L1

I

Hexagonal

7 c'"FS* I !.. o

pH 11.5 ,

Yield point (Pa) = 105 Lamellar

e~

1

0.1

viscosity D = 307s

0.01

10 4

I /~

~

C16FAS yield point i 20

10 3

~ . /

~ i 40 60 c (weight%)

J 80

102 100

Figure A.35 Liquid crystalline phases in comparison with viscosity and yield point for C16-fatty alcohol sulfate as a function of concentration [18] different liquid crystalline phases occurs. These liquid crystalline phases significantly influence the rheological properties of the surfactant systems [34]. This is demonstrated by a comparison of the simplified phase diagram of hexadecylsulfate and both the viscosity at a constant shear rate and the yield point (Figure A.35). With increasing surfactant concentration and a transition from the micellar solution to the hexagonal phase a strong increase in viscosity is observed. At even higher concentration a lamellar liquid crystalline phase occurs which leads to a decrease in viscosity again. This high viscous region of many surfactants in the medium concentration range has a strong impact on the formulation and production of concentrated surfactant systems. The same is valid for the dissolution of concentrated solid cleaners and detergents where intermediate high-viscous phases have to be avoided. The addition of nonionic surfactants to the anionic surfactants may have a strong influence of the rheological behavior (Figure A.36). A decrease is observed both in viscosity and yield point, which leads to improved flow properties.

4.2. Ion Complexation Water-soluble complexing agents or water-insoluble ion exchangers are part of cleaners or detergent formulations in order to remove especially calcium ions from the liquid [2]. These calcium ions have a 40

Physical Aspects of Cleaning Processes 30 7,

1 25

"

o.J.

I /\

..xaon..

" ~"

20 A

t

15 O O O

10

9

10 1 911

812

C12.14-FAS

7/3

6/4

515

416

3/7

218

Temperature = 30~ (AS" 50 weight%)

1/9

0/10

C12.18-E07

Figure A.36 Zero shear viscosity ~/o and elastic shear modulus G' for mixtures of C12/14-fatty alcohol sulfate and C12/14-fatty alcohol ethoxylate (7 EO) as a function of the concentration ratio at a constant concentration [18] disadvantageous effect in the cleaning process due to interaction with soils or the formation of insoluble calcium salts, especially calcium carbonate which precipitates on surfaces of the substrate to be cleaned or the cleaning equipment. In addition the solubility of anionic surfactants is decreased by calcium ions. Beside these primary effects of complexing agents and ion exchangers, they enhance the cleaning efficiency by their interaction with interfaces and modify the physical properties of cleaner formulations. Therefore in detergents they are often named builders. Some typically used complexing agents and ion exchangers are given in Table A.7. Water-soluble complexing agents show a specific adsorption onto substrates like hard surfaces and fabrics and pigment soil. If one considers the adsorption of ions onto aluminum oxide, the adsorption of sodium sulfate for example follows the Langmuir-type isotherm. Especially efficient

Table A.7 Typical builders for detergents Penta sodium triphosphate Sodium aluminum silicate (zeolite A and X) Sodium nitrilo triacetate Sodium polycarboxylate Sodium citrate 41

Handbook for Cleaning/Decontamination of Surfaces

(A)

~

i

OH2""OH!+H2P30130 ~H2 ',

' ~

i-

HP30~0"-'H+i+H20+OHOH2 i

-OH

-

NOH

(B) ~ O H + HP30~0

,

N-

OH

+OH-

-

(C) ~~OH O_._H

j

+HP3040

, ~~_OH O-----H§

+OH-

Figure A.37 Chemisorption of the triphosphate anion on aluminum oxide, (A) pH < isoelectric point, (B) pH = isoelectric point, (C) pH > isoelectric point [16]

builders have an isotherm of the high-affinity type, i.e. there are high amounts adsorbed at very low concentrations. This indicates high adsorption energy, which is characteristic for chemisorption. A well-studied system is sodium tripolyphosphate (STP) which is used in different types of detergents and cleaners. Figure A.37 visualizes the interaction of STP with y-A1203 at different pH-values at, above and below the isoelectric point [16]. Below the isoelectric point OH as well as OH~ groups are substituted by the polyanions. At pH-values above the isoelectric point the surface of aluminum oxide has a negative charge. The electrostatic interaction between the surface and the polyanions interferes with the adsorption. Ions like sulfate are not adsorbed any more due to their only possible physical adsorption. Complexing agents like STP or 1-hydroxyethane-l,2-di-phosphonic acid (HEDP) are still adsorbed. The adsorption of complexing agents decreases in the sequence: HEDP > STP > citrate The adsorption of the complexing agents has a significant impact on the dispersion properties. This can be shown for the sedimentation of graphitized carbon black and kaolinite in solutions of STP (Figure A.38) [21]. As a specific development for detergents, zeolites have been used since the 1980s to replace phosphate in many detergents to prevent 42

Physical Aspects of Cleaning Processes Graphite

2.0

--0

1.5r

E O

~m 1 . 0 0.5

0

--.-//

I E 4306.171

I

I

I

I

I

0.15

0.3

0.6

1.0

3.0

c (ga)

Figure A.38 Settling volume of graphitized carbon black and kaolinite in sodium triphosphate solutions at 16~ water hardness, 0.30 g/10 ml graphitized carbon black, 0.50 g/10 ml kaolinite [21] eutrophication of stagnant and slowly flowing surface waters. The main type of zeolites used in detergents is zeolite A. This substance is a water insoluble, finely dispersed ion exchanger which differs regarding its properties from water-soluble complexing agents. The general formula of sodium aluminum silicates with a zeolite structure is x N a 2 0 . A1203 ySiO2 9 zH20 9 The main properties of zeolites in the washing process besides the ion exchange of the calcium and magnesium ions of the water hardness are - a d s o r p t i o n of water-soluble substances, e.g. dyes on the zeolite particles - heterocoagulation of pigments and solid fats with zeolite - action as crystallization nucleus of sparingly soluble salts. All these effects support the mode of action of zeolites in the washing process. The most characteristic feature of zeolites is the ion exchange of the sodium ions in the crystal structure by calcium and magnesium ions. Figure A.39 shows the ion exchange kinetics of zeolite A and X for calcium and magnesium ions [35]. Calcium ions diffuse with a high rate into both types of zeolite with a slight preference for the wider pore zeolite X. These differences are only evident for short times which are not of practical importance for the washing process. The ion exchange kinetics 43

Handbook for Cleaning/Decontamination of Surfaces 1.0

0.8 o

0.6

o

0.4

_

zeolite - A/Mg 2+ --,--- 9

z e o l i t e - X/Ca 2+ zeolite - AJCa 2+ z e o l i t e - X/Mg 2+

0.2

I

I

10

20

I

30

I

40 t (min)

I

I

I

50

60

70

Figure A.39 Kinetics of ion exchange of calcium and magnesium ions for zeolite A and zeolite X, T = 25~ ion concentration 536.10 -3 mole/I, zeolite concentration 1 g/I [35] is more strongly dependent on the pore size of the ion exchanger for magnesium ions. Despite the smaller ion radius at 25~ the magnesium ion has a more stable and bigger hydration shell than the calcium ion and therefore more slowly penetrates into the pore system of the zeolite. A comparison between the decrease of water hardness by ion exchange and washing performance is given in Figure A.40 [36]. A decrease of the (ppm)

(~

100

250 (9 2 0 0 -

80

1

t-

"O I,. t~

,L_ (9 t~

A (9 O

Without NaCI 60

150-

m (U

40

10050-

(9

20

0.04 mole/I NaCI

00

1

2

3

4

5

6

Czeolite A (g/I)

Figure A.40 Influence of NaCI on the water-softening effect and the washing performance of zeolite A. Water-softening effect at 90~ after 15 min, washing performance at 90~ and 285 ppm water hardness on particulate-sebum-soiled cotton [36] 44

Physical Aspects of Cleaning Processes water hardness from 16~ down to 3-4~ only slightly influences the detergency. Only a further decrease of the calcium ion concentration leads to a significant increase of soil removal from the fabric. Due to the fact of zeolite A being an ion exchanger the calcium ion exchange is decreased by a high concentration of sodium ions despite of the high selectivity of the ion exchange process. According to this the detergency in the presence of sodium ions slightly decreases. The ion exchange of the zeolite can be described by equation 13:

QCa2+ Qca2+ Qm

Qrn CCa2+ b2 , CCa2+4- 2 Fll (CNa -ff 2 Qca2+)

l

(13)

= exchanged amount of calcium ions

- maximum exchanged amount of calcium ions CCa2+ = equilibrium concentration of calcium ions initial concentration of sodium ions CNa bl and b2 - constants '

1

Figure A.41 shows a comparison of experimental data of the ion exchange with the calculated curves [37]. Data and calculated curves are in good agreement. With increasing sodium concentration not only

QCaO

(mg/g)

QCa 2+- 1 0 3 (mole/g) .

It . . . . . . . . . . . . .

"

""

150 -

"

"

It'""

100 -

50-

-

i

N

CCa2+

i 91 0 3

~

;

N

(mole/I)

l

l

!

I

5

10

15

20

CCa0 ( ~

Figure A.41 Comparison of calculated and measured isotherms of calcium ion exchange by zeolite A, T = 22~ 1 h exchange time [37] 45

Handbook for Cleaning/Decontamination of Surfaces Methylene blue

-8 o

-6 ..~ -~

Zeolite A extracted

from detergent

e-

~,~" 4 r

-4

O

/

~o =o

-2

Benzopurpurine (zeoliteA)

I

I

I

I

0

10

20

30

c .10 6 (mole/I)

Figure A.42

Adsorption of dyes on zeolite A, T = 23~

[38]

the maximum exchanged amounts of calcium ions decrease, but also a higher calcium ion concentration is necessary to reach the equilibrium values. Zeolites show significant adsorption properties of importance for the washing process and for waste water conditions. Figure A.42 demonstrates the adsorption of a cationic dye (methylene blue) and an anionic dye (benzopurpurine) onto zeolite A [38]. The cationic dye is strongly adsorbed on the negatively charged surface of zeolite A whereas the anionic dye is only adsorbed on zeolite A which is extracted from a detergent formulation produced on a technical scale. This is due to a hydrophobization of the zeolite surface in the production process, which increases the interaction of the dye and the zeolite surface. Due to the negatively charged zeolite surface at alkaline pH values, cationic surfactants are strongly adsorbed onto zeolite A (Figure A.43). For mixtures of cationic and nonionic surfactants a strong increase of the adsorbed amounts is observed in a certain concentration range [39]. Because of hydrophobic interaction between the adsorbed cationic surfactants and nonionic surfactant molecules additional nonionic surfactant molecules are adsorbed probably in a second layer from mixtures. These effects have an impact on the behavior of zeolites in waste water. In detergents zeolites are used in combination with water-soluble complexing agents or polycarboxylates. The dissolution of calcium by zeolite A is enhanced by complexing agents which specifically adsorb on calcium-containing particles and subsequently desorb after sequestering calcium ions. Even small amounts of water-soluble complexing agents increase the dissolution rate of calcium carbonate by zeolites to the extent 46

Physical Aspects of Cleaning Processes

30

--~ 20 0 E (.0

o

DAC + NP 8, CNP8= 3.5.10 -4

(mole/I)

10

DAC, CNp 8 =

0

2

0

I

I

I

I

4

6

8

10

CDAC

I~

10 9 5 (mole/I)

Figure A.43 Mixed adsorption of cationic and nonionic surfactants onto zeolite A, T - 25~ DAC - ditallow dimethylammonium chloride, NP8 - nonylphenol octaglycolether [39] that the dissolution rate approaches that of the water-soluble complexing agent alone. The increase is particularly pronounced in the range of small complexing agent concentrations and with short reaction times. As the water-soluble complexing agents act as carriers for the transfer of calcium from the precipitate to the water insoluble ion exchanger this process is called "carrier effect" in the literature. A different effect occurs with the use of polycarboxylates in combination with zeolite. Small amounts of polycarboxylates or phosphonates can retard the precipitation of sparingly soluble calcium salts such as CaCO3 ("threshold effect"). As anionic polyelectrolytes they bind cations (counter ion condensation), and multivalent cations are strongly preferred. Whereas the pure calcium salt of the polymer is nearly insoluble in water, mixed C a / N a salts are soluble, i.e. only stoichiometric excess amounts of calcium ions can cause precipitation. Polycarboxylates are also able to disperse many solids in aqueous solutions. Both dispersion and threshold effect result from the adsorption of the polymer on the surface of soil and CaCO3-particles, respectively. The stabilization of sparingly soluble salts such as CaCO3 in a colloidal state is one of the possible effects of polycarboxylates in detergents. The advantage is that, in contrast to ion exchange or complexation, the concentration of the cobuilder can be much lower than the calcium concentration in the washing liquor. Thus, small amounts of threshold-active 47

Handbook for Cleaning/Decontamination of Surfaces

,~176 t 0 o.., 8 0 m !__

E I--

60-

40-

0.5

1

1.5

2

Na2CO 3 (g/I)

Figure A.44 Precipitation inhibition of calcium carbonate by polycarboxylates as a function of temperature and soda concentration, 3.04.10 -3 mole/I calcium ions, (1) 105 mg/I polycarboxylate, (2) 210 mg/I polycarboxylate [40]

compounds could be used as cobuilders even in soda-based laundry detergents. The effect, however, is strongly dependent on the experimental (or washing) conditions, i.e. temperature, soda and cobuilder concentration. Figure A.44 [40] illustrates the range of effectiveness of polycarboxylates in a carbonate-containing system for typical central European conditions of water hardness (3.04.10 -3 mol/1Ca2+). The results are based on turbidity measurements. The appearance of a CaCO3 particle larger than approximately 0.2 ~m within 30 min was taken as an indicator of the threshold effect. The soda concentrations in the test include the hydrogen carbonate content of the tap water as well as the soda content of the detergent. The results show that for typical German phosphate-free, heavy-duty detergents, polycarboxylate is no longer threshold active at temperatures above 40~ This is valid even more for higher carbonate concentrations, i.e. purely soda-based detergents. For zeolite A and soda-containing products, the participation of zeolite A in the elimination of calcium ions during the washing process has to be taken into account. For typical test concentrations, the amount of coarsely dispersed CaCO3 is reduced in the presence of zeolite A over the whole range of washing temperatures. The effect of polycarboxylate on the total amount of precipitation is strongly dependent on the presence of zeolite A. In absence of zeolite, precipitation is inhibited 48

Physical Aspects of Cleaning Processes only below 40~ With increasing temperature, the precipitated amounts strongly increase. In this case, on addition of CaCO3, polycarboxylate is precipitated as calcium salt, as can be seen from the respective measurements of the residual concentrations of water-soluble polycarboxylate. In contrast, the amount of precipitates in the presence of zeolite A and polycarboxylate is negligibly low, and the residual concentration of water-soluble polycarboxylate is as high as in zeolite A/polycarboxylate systems without soda. These results can be explained by the binding of calcium ions by zeolite A and by polycarboxylate in its water-soluble form. This is possible because the calcium ion concentration of the water is lowered by zeolite A. Thus, Ca 2+ is no longer in excess of polycarboxylate and formation of the insoluble calcium salt of polycarboxylate is no longer possible.

5. DISPERSED SYSTEMS IN CLEANING 5.1. Foams Foaming and the control of foam is an important factor in the application of cleaners. This regards high-foaming systems for e.g. manual dish-washing detergents or shampoos in hair care as well as low-foaming systems for use in textile or dish washing machines or institutional and industrial cleaning. The foam properties of the products are mainly governed by the surfactant system and the use of anti-foams. Besides this the chemical composition of the product or the washing liquor, e.g. electrolyte content and soil strongly influences the foam properties. Physical parameters like temperature and pH value or mechanical input in the system have additionally to be taken into account. The basis for the foam properties is given by interfacial parameters. An overview of these parameters and the correlation to foam properties is shown in Figure A.45 [20]. All these parameters influence the foam properties in a complex way and have been studied in detail. Although there have been shown correlations between a single parameter and foam properties, there is still a lack in a general correlation between interfacial properties and the foam behavior of complex systems like in cleaning systems. As foam is not the specific subject of this chapter, the influence of the single parameters will not be discussed in detail here, only a specific example regarding detergency will be given. The simplest approach to correlate interfacial parameters to foam properties is the comparison of the surface activity measured by the surface tension 49

Handbook for Cleaning/Decontamination of Surfaces Foaming of surfactant-water systems Correlation of experimantal data with fundamental parameters

[ ii iQuantity

parameters

I/Foam ~ ~

Dynamic surface tension

Foam I J kinetics ~

~

...o'"'"'~.~

(staticanddynamic) Interracial rheology

Dynamic surface elasticity

9

Foam stability ~

Interfacial potential/ intermolecular cohesion

~Oil

Figure A.45

particle size

Foam properties and interfacial parameters [20]

of a surfactant systems and the foam stability. This has been done for a series of pure surfactants. Within a specific class of surfactants the surface tension directly correlates to the foam stability of the surfactant-water system. A more general approach of this concept is not possible due to the influence of other parameters summarized in Figure A.45. As foam generation and also foam stability is a dynamic for process for generating and reducing surface area, in a surfactant-water system the diffusion of the surfactant to the surface and the change in surface coverage, at least locally during bubble generation and drainage of the film, is a more useful way of explaining foam properties. If one distinguishes between foam formation and foam stability, a good correlation has been found between the relative dynamic surface pressure derived from the time-dependent dynamic surface tension and the rate of foam formation (Figure A.46). The specific time for the relative dynamic surface pressure was chosen empirically. The correlation of the two parameters is valid for different surfactant types and addition of electrolyte. The effect can be explained by the micellar kinetics of the surfactant solution and the diffusion of the molecules and micelles to the surface. Different approaches for the correlation between dynamic interfacial parameters and foam properties have been shown for single lamellae studies [41] and surface viscosity and elasticity parameters [42]. The importance of these effects for finding the optimum surfactant system in detergents is shown in Figure A.47. For high-foaming detergents the foam stability of the products is shown in the presence of oily soils which usually suppress the foam formation. It can be demonstrated that foam stability strongly depends on the formulation, i.e. the surfactant system and can be adjusted 50

Physical Aspects of Cleaning Processes

0.06 -

,-

9 C12SO3Na 9C12E6 A 2 mM C12SO3Na + NaCI <> C16SO3Na

0.04 -

A

I00

E .~ 0 . O 2 -

_

0'.2

014

016

018

1'.0

/7 (100 ms)///eq

Figure A.46 Correlation of relative dynamic surface pressures with foam kinetics data dh/dt as a function of the type of surfactant, alkyl chain length and salt concentration [20]

4el, o

i i i t ,~Oo

30

r

Light-duty detergents Soil: 0.2 g/I lipstick o product A 9product B

9 %.%% ,

;;..:"

._~ ~ 20E o i1_

,'';

10-

I

0 -O"

I

~

0

1

2

8

~

9

"

O

]

I

I

f

3

4

5

6

P

Time (min)

Figure A.47 Foam stability of fine textile detergents, T = 40~ 160 mg/I CaO, recommended dosage of detergent [20]

51

Handbook for Cleaning/Decontamination of Surfaces

at a high level in this case for care aspects of the detergents towards sensitive textiles.

5.2. Microemulsions In contrast to the formation of microemulsions from aqueous surfactant systems and oily soils during the cleaning process, less basic research has been carried out on microemulsions as a cleaning medium [43]. Initial studies of textile cleaning with microemulsions by Solans et al. [44] were published in 1985. At washing temperatures between 296 and 307 K, homogeneous microemulsions obtained from the system water/C12E4/ n-hexadecane and systems with technical nonionic surfactant mixtures remove 1.5 to 2 times more soil from wool, cotton and cotton-polyester blended fabrics stained with oily and particulate soils than a highly concentrated liquid detergent (Figure A.48). Soil removal by the microemulsions was increased by 20-25% by adding 0.05 M of the electrolytes, sodium triphosphate and sodium citrate, which act as builders. The microemulsions also proved superior to the liquid detergent, in that they could be used seven times without losing any of their cleaning effectiveness. D6rfler et al. [45] systematically studied the phase behavior of quaternary systems, consisting of water, nonionic surfactants, a cosurfactant and a hydrocarbon, with regard to possible applications in the textile

30

00

T= 29~

20

o~ 10

N~N ME

ME L. Det. Polyester-cotton

L. Det. Cotton

Figure A.48 Soil removal (S) by a surfactant phase microemuision (ME) and by a 1% aqueous liquid detergent solution (L. Det) from different fabrics [44] 52

Physical Aspects of Cleaning Processes (B)

(A) T (K)

343-

2 2 ~ 2~

3332.

T(K)

~

~

I

CC

2~

343333

1

323 -

323 -

313 -

313 -

303 -

303 -

293

, 0102O

,

, , 304050

3~

293

o ;o 2'0 3'0 4:0 50

w t % C12.14E 6

C12.14E 6

wt%

(c)

(D) T(K)

T(K)

343-

343-

333

333

323 -

323

313 -

313 -

303 -

303 -

293 0

I

I

I

I

10

20

30

40

2, -

c

'

293 0

50

w t % C12.14E6

10

20

30

40

50

w t % C12.14E 6

Figure A.49 Phase behavior of water-oil-C12/14E6 mixtures (A) without cosurfactant, (B) with 2 wt% n-pentanol, (C) with 4 wt% n-pentanol and (D) with 6 wt% n-pentanol; water-oil ratio = 1-1 [45] cleaning sector. As an example, Figure A.49 shows the influence of the cosurfactant on the phase behavior of the water-oil-surfactant system. In this case the phase inversion range decreases by an average of about 5 K per added mol% cosurfactant. The extent of the three-phase zone is scarcely affected. A detailed review about the application of microemulsions in cleaning is given in [46].

REFERENCES 1. E. Smulders, in Ullmann's Encyclopedia of Industrial Chemistry. Laundry Detergents, Weinheim: Wiley, 2002. 2. W.G. Cutler and E. Kissa, Detergency- Theory and Applications, New York: Marcel Dekker, 1987.

53

Handbook for Cleaning/Decontamination of Surfaces K.R. Lange, Detergents and Cleaners, Munich: Hanser, 1994. 4. H.G. Hauthal and G. Wagner, Household Cleaning, Care and Maintenance Products, Verlag f. chemische Industrie, Germany: Augsburg, 2003. K.L. Mittal, Contact Angle, Wettability and Adhesion, Zeist: VSP BV, The Netherlands, 1993. H.-D. D6rfler, Grenzfl~ichen und colloid-disperse Systeme, Berlin: Springer, Germany, 2002. H.W. Fox and W.A. Zisman, J. Colloid Sci. 5:514 (1950). 8. E.G. Shafrin and W.A. Zisman, J. Phys. Chem. 64:519 (1960). 9. D. Nickel, H.D. Speckmann and W. von Rybinski, Tenside Surfactants Det. 32:470 (1995). 10. G. Jakobi and A. L6hr, Detergents and Textile Washing, Weinheim: VCH, 1986. 11. W. Kling, Kolloid-Z. 115:37 (1949). 12. L.W. Schwartz, R.V. Roy, J. Colloid Int. Sci. 218:309 (1999). 13. S. Whitaker, AIChEJ 15:527 (1969). 14. M. Buzzacchi, P. Schmiedel, W. von Rybinski, Colloids Surfaces A 273: 47 (2006). 15. V.G. Levich, Physicochemical Hydrodynamics, Prentice Hall, New York: Englewoods Cliffs, 1962. 16. P. Berth and M.J. Schwuger, Tenside Det. 16:3 (1979). 17. D. Nickel, C. Nitsch, P. Kurzendoerfer and W. von Rybinski, Progr. Colloid Polymer Sci. 89:249 (1992). 18. R. Hofmann, D. Nickel and W. von Rybinski, Tenside Surfactants Det. 31:63 (1994). 19. F. Jost, H. Leiter and M.J. Schwuger, Colloids Polym. Sci. 266:554 (1988). 20. T. Engels, W. von Rybinski and P. Schmiedel, Progr. Colloid Polymer Sci. 111:117 (1998). 21. M.J. Schwuger, Ber. Bunsenges. Phys. Chem. 83:1193 (1979). 22. E.J.W. Verwey and J.T.G. Overbeek, Theory of the Stability of Hydrophobic Colloids, Amsterdam: Elsevier, The Netherlands, 1948. 23. H. Lange, in Adsorption at Interfaces, (K.H. Mittal, ed.), ACS Symp. Ser. No. 8:270 (1975). 24. E. Hageb6cke, Dissertation, Bonn, Germany, 1956. 25. G. Jakobi and M.J. Schwuger, Chem. Ztg. 99:182 (1975). 26. B. Dobias, X. Qiu and W. von Rybinski, Solid-Liquid Dispersions, New York: Marcel Dekker, 1999. 27. C.P. Kurzend6rfer and H. Lange, Fette, Seifen, Anstrichmittel 71: 561 (1969). 28. D.J. Mitchell, G.J.T. Tiddy, L. Warring, T. Bostock and M.P. Mc Donald, J. Chem. Soc. Faraday Trans. 1 79:975 (1983). 29. F. Schambil and M.J. Schwuger, Colloid Polymer Sci. 265:1009 (1987). 30. M. Kahlweit, Tenside Surfactants Det. 30:83 (1993). 31. H.L. Benson, K.R. Cox and J.E. Zweig, Happi, 50 (1985) and M. Kahlweit and R. Strey, in Proc. Vth Int. Conf. Surface Colloid Sci. Potsdam, New York, 1985. 32. C.A. Miller and K.H. Raney, Colloids Surf. A 74:169 (1993). 33. K. Fontell, Mol. Cryst. Liq. Cryst. 63:59 (1981). ~

~

.

o

54

Physical Aspects of Cleaning Processes 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.

H. Hoffmann and W. Ulbricht, in Handbook of Applied Surface and Colloid Chemistry, Vol. 2 (K. Holmberg, ed.), Chichester: Wiley, England 2002, p. 189. M.J. Schwuger and H.G. Smolka, Colloid Polymer Sci. 254:1062 (1976). M.J. Schwuger and H.G. Smolka, Tenside Det. 16:233 (1979). M.J. Schwuger and H.G. Smolka, Colloid Polymer Sci. 256:1014 (1978). M.J. Schwuger, J. Amer. Oil Chem. Soc. 59:265 (1982). M.J. Schwuger, W. von Rybinski and P. Krings, Progr. Colloid Polymer Sci. 69:167 (1984). M.J. Schwuger and M. Liphard, Colloid Polymer Sci. 267:336 (1989). C. Stubenrauch and R. Strey, Langmuir 20:5185 (2004). P. Koelsch and H. Motschmann, Langmuir, 21:6265 (2005). T. Foerster and W. von Rybinski, in Modern Aspects of Emulsion Science (B.P. Binks, ed.), The Royal Society of Chemistry, UK: Cambridge, 1998, p. 418. C. Solans, J. Garcia Dominguez and S.E. Friberg, J. Disp. Sci. Technol. 6:523 (1985). H.D. D6rfler, A. Grosse and H. Kr6gmann, Tenside Surfactants Det. 32:484 (1995). P. Kumar and K.L. Mittal, Handbook of Microemulsion Science and Technology, New York: Marcel. Dekker, USA, 1999.

55