rticle history:eceived 10 February 2012eceived in revised form 29 April 2013ccepted 30 April 2013vailable online 14 May 2013
a b s t r a c t
This paper compares the deposition of two aminofunctional silicones on hair that are both widelyused as textile finishes and hair conditioners. The two polymers had different structures: a linearamino-polyether-silicone block copolymer (ABn) with hydrophilic character and a conventional pendantaminodimethicone (AMD) with the same amine content. The deposition (irreversible adsorption) on ker-atin fibers as a function of pH, concentration, and treatment time was studied using streaming potential
measurements. The polymer deposition was also measured independently by analysis of the silicon ele-ment content of the treated hair samples. With ABn, the deposition reached a plateau after the reversalof the hair charge (overcompensation). In contrast, with AMD, the reversal of charge was not observedand the deposition was much higher, suggesting a more prominent role of hydrophobic interactions. Arecent model of streaming potential was used to interpret the data.
eratin
. Introduction
The deposition of silicones from aqueous dispersions on solidubstrates is of practical importance in textile and cosmeticndustries. Numerous studies have addressed the deposition ofncharged silicone, typically polydimethylsiloxane, from aque-
us dispersion containing anionic surfactants and organic cationicolymers where the deposition is controlled by the formation of
nsoluble coacervates [1,2]. Much less attention has been devoted
to the deposition of cationic silicone dispersions in water in theabsence of organic cationic polymers although this chemistry isincreasingly used in textile finishes and in hair conditioners [3,4].
Most cationic silicones are polydimethylsiloxanes containingamine groups which are either quaternized (permanent charge)or ionized as a function of pH. In conventional aminosilicone poly-mers, the amine functionality is present as side chains of the linearsiloxane backbone (Fig. 1a). The number of siloxane units betweenamino groups is usually kept high (m > 20), because longer siloxane
chains provide the enhanced spreading properties and a low coef-ficient of friction, which are beneficial for the finish performance.These copolymers remain very hydrophobic, requiring the additionof surfactants to disperse the aminosilicone in water.
A.D. Dussaud et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 434 (2013) 102– 109 103
Si
CH3
O Si O Si
CH3
CH3
CH3
CH3
H3C
CH3
CH2
nm
CH2
CH2
NH
CH2
CH2
NH2
a. Con ven tional aminos ili cone (AMD)
b. Amino sili con e cop olymer (A Bn)
Si O
CH3
CH3
Si C3H6 O C
x
CH3
CH3
N C C (PO)( EO)a b
RH
H
H
CH3
C
OH
C
H
H
H H
H
n
Fig. 1. Aminosilicone chemical structures. (a) Conventional aminosilicone (AMD). The parameters m and n denote the number of siloxane unit and amino functionalizedsiloxane unit respectively (b) AB copolymer. The parameters x, a, b denote the number of siloxane unit, the number of propylene oxide unit, the number of ethylene oxideu
doiotctsectbdamwplis
tiacc
Materials (Columbus, OH, USA).AMD was synthesized using the well-known equilibration
method [8]. An oil-in-water emulsion containing 20% silicone was
Table 1Relevant properties of silicone particles and keratin fibers.
Block copolymer ABn
Average molecular weight 50,000Amine content (meq/g) 0.2Siloxane block MW 7600Polyether block MW 2000Wt% of ethylene oxide (EO) unit in polyether block 83%Dispersion particle radius (nm) 68
Pendant aminosilicone AMDAverage molecular weight 38,000Amine content (meq/g) 0.2
n
nit respectively.
In hair conditioners, amino functional silicones are usuallyeposited on the hair during washing with shampoos or rinse-ff conditioners [5]. Jachowicz and Berthiaume [6] have studiedn detail by streaming potential measurements the depositionn hair fibers of amino functional silicones emulsified by surfac-ants. The authors demonstrated that due to the strong negativeharge of the hair surface, the electrostatic interactions con-rolled the deposition of the cationic silicone drops in the initialtage of the deposition process. However, it was observed thatven after the reversal of the hair surface charge (the surfaceharge became positive), the deposition of aminosilicones con-inued despite the electrostatic repulsion barrier. Due to thisehavior, conventional amino functional silicones have a ten-ency to accumulate on the hair after multiple washes, bringing
greasy feel to the hair. This problem can be solved by usingore hydrophilic linear aminosilicone [AB]n block copolymershich are comprised of alternating silicone blocks and hydrophilicolyether blocks (Fig. 1b) [7]. These copolymers do not accumu-
ate on the hair (small deposition) and an additional advantages that they can be dispersed in water without the use of emul-ifiers.
In this study, we used streaming potential measurementso understand the deposition of a hydrophilic linear aminosil-
cone [AB]n block copolymer on hair fibers. This study aimedlso at identifying the differences between the two aminosili-one structures, the conventional aminosilicone and the ABn blockopolymer.
2. Materials and methods
2.1. Copolymer emulsions
Two silicone polymers were studied: a conventional aminosil-icone (amodimethicone, AMD) (Fig. 1a) and an amino [AB]n blockcopolymer (ABn) (Fig. 1b). Their respective properties are given inTable 1. The samples were obtained from Momentive Performance
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2
4
6
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10
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0.1 1 10 100 1000 10000
% in
ten
sity
Size r (nm)
ABn
AMD
Fig. 2. Drop size distributions of the AMD emulsion and the ABn copolymer disper-s
pior
ipcdlagt(ww
2
pfwAapstss0o
2
y(2Asmtc
The hair sample used for the streaming potential measure-
ions obtained by dynamic light scattering.
roduced using 10% of a nonionic surfactant (Trideceth-6) accord-ng to the method described in [9]. The particle size distributionf AMD diluted emulsion is shown in Fig. 2. The average dropletadius was 120 nm.
The ABn was synthesized according to the method describedn [10] by reacting an epoxy end-blocked polysiloxane with aolyetheramine precursor. The epoxy end-blocked polysiloxaneontained an average of hundred siloxane units. The polyetheriamine precursor (Jeffamine ED-2003 from Huntsman, The Wood-
ands, TX, USA) was a linear chain containing 83% of ethylene oxidend had a pKa of 9.5. The ABn copolymer was blended in dipropylenelycol to form a 30% (w/w) clear dispersion which could then be fur-her diluted easily in water to form droplets of 68 nm average radiusFig. 2). The particle size (average radius) of the emulsion dropletsas measured by dynamic light scattering (DLS) at an angle of 173◦
ith a Malvern Nanosizer Nano ZS (Worcestershire, UK).
.2. Buffer solutions
Reagents were purchased from Fisher and used without furtherurification. In order to obtain buffers with low ionic strength, theollowing solutions were prepared. An electrolyte solution at pH 6as prepared by dissolving potassium chloride in deionized water.
pH 4 buffer stock solution was made to contain 0.06% (w/w) aceticcid and 0.0175% NaOH (w/w, to adjust pH) in deionized water. TheH 4 buffer was prepared by diluting 100 mL of the pH 4 buffer stockolution with 60 mL 0.33% (w/w) KCl solution and deionized watero a total volume of 1 l. The pH 8.7 buffer was prepared by dis-olving 1.2 g solid Tris(hydroxymethyl)aminomethane and 0.068 golid KCl in 900 mL water. The solution was titrated to pH 8.7 with.2 M NaOH, and deionized water was added to give a total volumef 1 l.
.3. Hair keratin fiber samples
The deposition studies with keratin fibers were performed usingak hair purchased from International Hair Importers & ProductsGlendale, New York, USA). Undamaged, clean hair was cut into–4 mm segments. 3 g of dry hair was used for each measurement.
plug of hair was formed by random packing of cylindrical fiberegments each having an average diameter of 70 �m. Using gravi-etric methods, the fiber geometrical parameters, and assuming
he hair surface as smooth, the estimated hair surface area wasalculated to be 500 cm2/g.
cochem. Eng. Aspects 434 (2013) 102– 109
2.4. Microelectrophoresis
The electrophoretic mobilities of the silicone droplets weremeasured in a capillary cell in a Malvern Nanosizer Nano ZS(Worcestershire, UK). A 10−3 M KCl buffer was used and the pHwas adjusted with dilute NaOH or HCl solution. The concentrationof silicone was 1 mg/g. This concentration was determined to beoptimal for measurement by the instrument, as concentrations toolow or high caused difficulty in fitting data to the experimentalmodel. The electrophoretic mobility was averaged over 100 runsfor each data point. Electrophoretic mobilities were converted to �potential using the Smoluchowski equation, � = 4���/ε where � isthe viscosity of the dispersion, � is the measured electrophoreticmobility, and ε is the dielectric constant.
2.5. Deposition treatment protocol
Concentrated silicone dispersions were diluted in buffer at con-stant ionic strength. The buffered silicone dispersion of diluteconcentration was poured into a 100 mL jar containing 3 g dryhair and immediately shaken on a table shaker at 200 min−1 fora time ranging from 5 min to 2 h (“deposition time”). The hair wasthen separated from the treatment solution by filtration using aceramic funnel. The treated hair sample was subsequently rinsedwith 500 mL fresh electrolyte and then placed into 100 mL freshelectrolyte. After sitting overnight, the hair was rinsed again with500 mL fresh electrolyte and packed into the streaming potentialcell.
This protocol insured that the deposition of silicone wasirreversible and produced stable signals of streaming potential.
2.6. Streaming potential measurement
The streaming potential was measured using a Zetacad appara-tus from CAD Instrumentation (Les Essarts le Roi, France). A glasscylinder of 10 cm length and 1.5 cm inner diameter was used. Aporous plug was formed by packing wet hair fiber snippets (3 gdry) into the cylindrical cell. The flow of the electrolyte, back andforth through the plug was controlled by the pressure appliedinto the electrolyte reservoirs by nitrogen gas. The voltage acrosstwo Ag/AgCl electrodes mounted at the inlet and outlet of the cellwas measured and recorded as a function of the applied pressure.The electrical conductivity and the temperature of the electrolytewere also measured. Prior to any measurement, the hair plug wasconditioned by flowing fresh electrolyte through the plug at lowpressure for 10 min. Then, the measurement was started, and theapplied pressure was increased incrementally from 5 to 300 mbar.At steady state, for a constant pressure applied, a constant potential(“streaming potential”) was observed. Provided that the potential(V) varied linearly with the applied pressure (P), the streamingpotential could be calculated from the slope of V versus P accordingto the Helmholtz–Smoluchowski equation:
� =(
V
P
).�.
�
εεo(I)
where � is the viscosity of the electrolyte solution, � is the con-ductivity of the solution, and ε and εo are the dielectric constantof water and the vacuum dielectric permittivity respectively [11].The plug pore size was much larger than the double layer thickness(∼10 nm), justifying the use of the above equation [11].
2.7. Total silicon content analysis
ment was collected after the measurement, dried in an oven, andanalyzed for total silicon element content. The silicone polymer
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AMD
Ff
weGcsos
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ig. 3. Zeta potential of silicone drops determined by microelectrophoresis as aunction of pH in a 0.001 M KCl electrolyte, at 1 mg/g concentration.
as depolymerized using boron trifluorodiethyl etherate to lib-rate volatile methyl fluorosilane, which was then quantified byC according to the method described in [12]. Since the siliconeopolymers were too large to penetrate the hair fiber [13], the totalilicon element content reflected the amount of silicone depositedn the surface of the fiber. The deposition will be expressed in mgilicone polymer per g hair.
. Results and discussion
.1. Zeta potential of silicone drops
The zeta potential of the silicone drops (�p) in 10−3 M KCl elec-rolyte measured by microelectrophoresis is shown in Fig. 3. Forhe ABn drops, the zeta potential slowly decreased from +45 mVo +25 mV between pH 4 and pH 8, and then decreased sharplyetween pH 9 and pH 10. Electroneutrality was achieved aroundH 10. This pH dependence of zeta potential was likely relatedo the ionization of the amine groups present in the backbone of
he copolymer. At low pH (pH < 6.5), most of the amine groupsere protonated (maximum positive charge). At higher pH, the
mine groups were only partially protonated and the protonationecreased as the pH increased.
ig. 4. Typical plots of streaming potential voltage as function of pressure for untreated fibers (�o � 0), square symbols: silicone treated hair with �o < � < 0, triangle symbols: silic
cochem. Eng. Aspects 434 (2013) 102– 109 105
For AMD, two transitions were observed: at pH < 4, the zetapotential was highly positive with a value around +70 mV anddecreased sharply around pH 5.5. Between pH 6 and pH 9, thezeta potential decreased monotonically with pH. Electroneutral-ity was obtained at pH around 9.5. The two transition regions at pH5 and pH 9 probably corresponded to the pKa-s of the primary andsecondary amines of the aminosilicone side chains, respectively(Fig. 1a). The lower pKa value corresponded to the weaker base,which was the primary amine.
3.2. Streaming potential measurements
3.2.1. Voltage vs. applied pressureTypical plots of the voltage vs. pressure drop for an untreated
hair sample and two, silicone treated hair samples are shown inFig. 4. A linear behavior was observed, demonstrating that thedeposited polymer did not desorb under the shear flow conditionsof the streaming potential measurement. Curve a represents theuntreated hair sample exhibiting a highly negative surface charge(� ∼ −50 mV). The negative surface charge was probably carried byacidic amino acid residues of the hair fiber keratin proteins [13].Curve b represents a silicone-treated sample with a negative sur-face charge and a reduced streaming potential absolute value; thenegative surface charge of the substrate was partially screened bythe cationic groups of the deposited silicone polymer. Curve c rep-resents a silicone-treated hair sample with a positive surface charge(� > 0). The deposited polymer screened the surface charge of thehair and brought an excess of positive charge (overcompensation).In the following sections, only the streaming potential values cal-culated from the slope of voltage-pressure plot are reported.
3.2.2. Kinetic experiments with the ABn silicone copolymerThe polymer deposition and streaming potential are plotted as
a function of the deposition time in Figs. 5 and 6, respectively, fordifferent pH values and for a fixed concentration of silicone poly-mer (0.5 mg/g) in the treatment bath. At time t < 50 min, the silicone
deposition increased sharply with the deposition time (Fig. 5), then,it leveled off. The kinetics of polymer deposition did not changesignificantly as a function of pH. This behavior seems consistentwith the particle transfer mechanism by convection described by
bers and silicone treated fibers, respectively. Diamond symbols: untreated keratinone treated hair when � > 0.
106 A.D. Dussaud et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 434 (2013) 102– 109
00.10.20.30.40.50.60.70.80.9
1
0 20 40 60 80 10 0 12 0 14 0
Depo
si�o
n (m
g/g)
Deposi�on �me (min)
pH 4
pH 6
pH 8.7
Fig. 5. Polymer deposition vs. deposition time for hair fibers treated with amino[AB]n for C = 0.5 mg/g, I ∼ 0.001 M, at different pH values (diamond symbols: pH 4,square symbols: pH 6, triangle symbols: pH 8.7).
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vwtdt8io
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pH 8.7
Fig. 7. Streaming potential as a function of deposition for hair fibers treated with
TCb
ig. 6. Streaming potential vs. deposition time for hair fibers treated with the ABn
opolymer for C = 0.5 mg/g, I ∼ 0.001 M, at different pH values (triangle symbols: pH, square symbols: pH 6, diamond symbols: pH 8.7).
damczyk et al. [14]. In this convective-diffusion model, the typ-cal deposition kinetic was linear in the early stage of depositionnd could be described by the relation N = kcnb t, where N is theeposition, kc is the mass transfer rate, nb is the particle concentra-ion in the bulk and t is the deposition time. The mass transfer rateepended on the flow condition and the diffusion coefficient of therops. In our case, the slope of the plot gave a mass transfer ratef 2.5 × 10−6 cm s−1. According to this model, the initial depositionate should not depend on the particle charge, as observed in theresent study.
However, the kinetics of the streaming potential showed a clearariation with pH (Fig. 6). At time zero, the streaming potentialas highly negative (�o ∼ −45 mV). Shortly thereafter (t = 5 min),
he absolute values of the streaming potential dropped due to theeposition of the positively charged polymer. The streaming poten-
ial change was much more pronounced at pH 4 and pH 6 than at pH.7. At pH 4 and pH 6, the kinetic curve leveled off and the stream-
ng potential reached a positive value. At pH 8.7, a maximum wasbserved at around t = 10 min (overshoot).
able 2omparison of the plateau values of streaming potentials obtained on keratin fibers treaulk and the fitting parameter values of model (Eq. (V)).
pH Untreated hair zetapotential �o (mV)
Plateau value of streamingpotential �s (mV)
�s/�o Silicone papotential �
ABn 4 −44 22 −0.50 44
6 −45 9 −0.20 37
8.7 −48 −27 0.56 22
AMD 4 −44 −8 0.18 68
the ABn copolymer at different pH values and I ∼ 0.001 M (triangle symbols: pH 4,square symbols: pH 6, diamond symbols: pH 8.7, dashed lines: calculated curvesfrom Eq. (V)).
The change of streaming potential with pH at fixed adsorbedamount could be explained by the dependence of the polymercharge on pH. At high pH, the amount of ionized amine groups perpolymer was reduced, and therefore, for a fixed deposited amount,the charge screening by the polymer was less. In contrast, at pH4, most of the amine groups were ionized, leading to not only ahigher polymer charge and streaming potential change, but alsomore charge deposition than necessary to neutralize the substratecharge. This overcompensation has been often observed with thedeposition of solid particles or soluble polyelectrolytes on oppo-sitely charged substrates [15,16]. This will be discussed further inthe next section.
The overshoot effect at pH 8.7 could be probably explained bythe poor polymer anchoring due to the small portion of chargedsegments. We observed that for hair treated at short contact timeand left overnight in quiescent electrolyte, the deposited polymerdid not desorb, whereas at long contact time (t > 60 min), the poly-mer that was loosely attached to the hair surface, desorbed underthe action of the vigorous mixing.
3.2.3. Streaming potential as a function of the ABn siliconecopolymer deposition
In Fig. 7, each data point represents a different hair sample forwhich both the streaming potential and the ABn deposition wereindependently measured at various times. The streaming poten-tial (Y-axis) is plotted as function of deposition (X-axis) at varyingpH values. The streaming potential increased monotonically withthe deposition and leveled off for all pH values studied (pH 4, 6,and 8.7). The plateau values of streaming potential at differentpHs are shown in Table 2 (column #4). At pH 6, the streamingpotential plateau corresponded to deposition in the range of about0.6–1.2 mg/g.
The deposition of spherical drops (particles) on a solid surfacecan be estimated by assuming that the drops deposit randomly untilthey form a single layer of close packed spheres. For hard spheres,the maximum surface coverage is �j = 0.547 (jamming coverage)
ted with silicone dispersion with the zeta potential of silicone drops measured in
rticle zetap (mV)
�p/�o Fitting parameter a Fitting parameter b Regressioncoefficient R2
Fig. 8. Reduced streaming potential, �/�o as a function of deposition for hair fibers
hair [6]. The reason for this effect is that small cations screened thecharges on the hair substrate whereas small anions screened thecharge of the silicone particle, and this way decreased the depo-sition. This screening-reduced deposition has also been observed
y = 0.0869x -0.457
R² = 0.9318
0.1
1
Depo
si�o
n (m
g/g)
A.D. Dussaud et al. / Colloids and Surfaces A:
hen the electrostatic repulsion between the spheres is negligible.n the presence of lateral electrostatic interactions, the maximumoverage is smaller and depends on the Debye layer thickness. Here,ith an ionic strength of 0.001 M, the double layer length is equal
o 9.6 nm and the maximum coverage, �max predicted by the ran-om sequential model is around �max = 0.35 [14]. Assuming thathe ABn dispersion is monodisperse (r = 68 nm) and the spreadingf the droplets on the keratin surface is negligible, the number ofBn copolymer drops in a saturated layer (per gram of hair) ns is:
s ≈ �maxAh
� · r2(II)
here Ah is the hair surface area (per gram of hair) and r is theadius of the spherical droplet. The ABn copolymer deposition N inram per gram of hair can be estimated by
≈ n · v. ≈ 43
� · Ah.r · (III)
here n is the number of ABn drops per gram of hair, v is the ABn
rop volume and is the density of the copolymer ( = 0.96 g/cm3).hen, from (II), the deposition at saturation is:
s ≈ ns · v. ≈ 43
�max · Ah.r · (IV)
With an estimated hair surface area of 500 cm2/g, the depositionhould be in the order of 1.5 mg/g. This estimated value is reason-bly close to the observed deposition at saturation (∼1.2 mg/g).One always has to remember that such polymers are never uni-orm and always contain a distribution of species with a range of, b and x values – see Fig. 1b, and therefore a close agreementannot be expected.) In contrast, if we assume that the ABn copoly-er deposits as individual random coils, using the polymer density
nd molecular weight to estimate the polymer random coil radiusrp ∼ 2.7 nm), a single layer of individual polymer coils would leado a deposition of about 0.1 mg/g, which is one order of magnitudemaller than the measured value. These calculations indicate thathe silicone polymer probably deposited as whole droplets on theolid substrate, as also suggested by Jachowicz and Berthiaume [6],ather than as individual molecules. Therefore, it seems reasonableo compare our data with the streaming potential model proposedy Adamczyk et al. [14], who postulated a three-dimensional chargeistribution near the solid surface, induced by deposition of isolatedpherical particles. This model predicting the streaming potentials a function of the surface coverage could be approximated by theollowing exponential equation [14]:
ς
ςo=
(ςs
ςo
)+ e−a� −
(ςs
ςo
)e−b� (V)
here �o is the streaming potential of the untreated hair, �s is thelateau value of the streaming potential obtained at saturation,
is the surface coverage, the constants a and b reflect the elec-rostatic contributions from the interface and from the adsorbedrops, respectively. The surface coverage � can be calculated fromhe deposition data using the relation (III). The experimental dataf streaming potential versus deposition (Fig. 7) and the calculatedurves are plotted using the reduced streaming potential �/�o as aunction of surface coverage in Fig. 8. The pH dependence of theurves resembles the experimental data obtained with the depo-ition of amidine polystyrene latex on mica [16]. To fit the modelEq. (V)), �s was assumed to be equal to the experimental valuef the streaming potential plateau. The parameters a and b in (V)ere determined by a regression algorithm (Sigmaplot). The fitted
alue of the parameters a, b and the regression coefficient of the
ts are shown in Table 2. The values of a and b have a similar orderf magnitude as the theoretical values calculated in [14].
Fig. 8 shows that Adamczyk’s model describes reasonably wellhe silicone copolymer particle deposition data. However, some
treated with [AB]n copolymer at different pH (I ∼ 0.001 M) and comparison with themodel described by Eq. (V) (triangle symbols: pH 4, square symbols: pH 6, diamondsymbols: pH 8.7).
aspects of this model are in disagreement with our results. In Adam-czyk’s model [14], at very high coverage, the streaming potential(�) of the solid substrate converged toward a constant value whichwas equal to 0.7�p, where �p is the zeta potential of the particle,measured by microelectrophoresis. In our case, however, at satura-tion the streaming potential converged toward a much lower valuethan �p. This could indicate that the amine ionization of the copoly-mer deposited on the hair fiber surface was significantly reducedcompared to the ionization of the copolymer inside the droplets dis-persed in the bulk. A similar decrease of amine ionization inducedby adsorption on a solid surface has also been observed for thehydrophilic polyelectrolyte polydimethylaminoethyl methacrylatepolymer adsorbed onto a TiO2 wafer [17]. In addition, at pH 8.7the streaming potential leveled off and stayed negative whereas inthe theoretical model, for uncharged drops, the streaming potentialshould converge toward 0. This may indicate that when a particlehad a very low charge, it coalesced with another deposited dropletinstead of depositing on unoccupied surface.
3.3. ABn copolymer deposition as a function of ionic strength
Fig. 9 shows that the deposition decreased as the ionic strengthincreased for a fixed ABn copolymer concentration of 0.5 mg/g, atpH 6. These data are consistent with the data reported by Jachowiczand Berthiaume for the deposition of emulsified aminosilicones on
0.001 0.01 0.1 1Ionic strength (M)
Fig. 9. Polymer deposition as a function of ionic strength for the ABn copolymer atpH 6 and C = 0.5 mg/g.
108 A.D. Dussaud et al. / Colloids and Surfaces A: Physi
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Fig. 10. Streaming potential as a function of the treatment bath concentration at pH4ra
e[tsppo
4A
catw0mbvatmnm9stbtssdbpioTn
5
tsi
and ionic strength 0.001 M for the ABn copolymer (diamond) and AMD (square),espectively. Depositions at 0.5 mg/g treatment bath concentration for ABn and AMDre shown below their respective plots.
xperimentally with the adsorption of low charge density polymers18]. This result was also predicted by Dobrynin and Rubinstein inheir model of polyelectrolyte adsorption with high charge solidubstrate and low charge density polymers [19,20]. However, theirredicted power law of the dependence of ionic strength had aower of −0.75, whereas our experimental data indicated a powerf −0.45.
. Comparison of streaming potential and deposition ofBn and AMD polymers
In Fig. 10, the streaming potential as a function of copolymeroncentration at pH 4 is shown after 1 h deposition time for ABn
nd AMD polymers. The deposited amount was measured for a bathreatment polymer concentration of 0.5 mg/g. AMD amine contentas identical to the one of ABn (0.2 meq/g). At concentrations below
.1 mg/g, the streaming potential increased sharply with the poly-er concentration of the treatment bath, and then leveled off for
oth polymers. However, the two streaming potential curves areery different. For the ABn, the streaming potential changed signnd reached a quasi-steady plateau value of +20 mV, whereas withhe AMD, the streaming potential kept increasing with the poly-
er concentration, but was lower than with the ABn and stayedegative (� � 0 mV). Remarkably, for a fixed bath treatment poly-er concentration of 0.5 mg/g, with the AMD, the deposition was
mg/g, which was more than ten times higher than the ABn depo-ition (0.8 mg/g). Since the particle size of AMD was almost twicehe size of the ABn particle, according to (III), the deposition woulde expected rather to be twice as much, if we assume a deposi-ion of a single layer of drops (with r = 120 nm). From the particleize we can estimate the deposition of AMD as 4 mg/g, which isignificantly lower than the measured value. In addition, for AMD,espite the very high deposition, there was a strong discrepancyetween the zeta potential (+68 mV) of the AMD drops and thelateau value of the streaming potential (∼−8 mV) (Table 2). This is
n disagreement with Adamczyk’s model, where the plateau valuef streaming potential should be close to the particle zeta potential.his may indicate that the deposition of AMD was very heteroge-eous, leading to many uncompensated charges on the hair surface.
. Concluding remarks
This study showed clear differences between the deposi-ion of the emulsified conventional aminosilicone AMD and theurfactant-free dispersed aminosilicone block copolymer ABn. Sim-lar to Jachowicz and Berthiaume [6], we found a large amount of
cochem. Eng. Aspects 434 (2013) 102– 109
aminosilicone (AMD) deposited on the hair surface, although ourAMD did not cause charge reversal. This may be due to the lowamine content of our AMD (0.2 meq/g). In other experiments, notshown here, we also observed charge reversal with an aminosili-cone with a much greater amine content (0.8 meq/g amino).
Treating with the ABn copolymer, we not only observed chargereversal (at low polymer concentrations), but also an order of mag-nitude lower deposition than with the AMD polymer. The streamingpotential data, as a function of deposition at pH 4 and pH 6, was con-sistent with near monolayer adsorption of ABn copolymer drops.At saturation, the hair surface charge was overcompensated bythe charged amine groups and became positive. After a positivestreaming potential plateau value was reached, further depositionwas significantly reduced, due to the formation of an electrostaticrepulsive barrier. In spite of its lower adsorption, the ABn copolymerworks well as a hair conditioner because the long siloxane sectionsare sufficient to cause a silky feel and reduce combing force [7].
In contrast to the results with ABn, AMD did not reverse thesurface charge of the hair fiber, despite the AMD droplets having ahigh positive charge (+68 mV) and depositing much higher amountthan ABn. This behavior is not consistent with the Adamczyk’ model[14], which postulates that higher particle charge should lead togreater overcompensation of the substrate surface charge.
A major difference between the two silicone polymers is that theABn copolymer contains significant amounts of polar (polyethyle-neoxide – see Fig. 1b) groups, and therefore it self-dispersed inwater. The AMD polymer, however, contains no hydrophilic groups,apart from the amines, which are themselves low (Fig. 1a). Conse-quently, a significant amount of non-ionic surfactant (Trideceth-6)was necessary to form an emulsion. During the deposition of theAMD polymer emulsion, the silicone polymer drops and the surfac-tant molecules/micelles are competing to bind to the hydrophobichair surface. Therefore it is likely that the aminosilicone dropletscan only cover part of the hair surface and can only compensate asubset of the negative charges. The high deposition of AMD poly-mers can be explained only if multiple drops deposit on the samespot.
The conformation model of hydrophobic polyelectrolytesrecently elucidated in the literature [20] may offer additional expla-nation for the differences between the surface charge of AMD andABn droplets. At low charge density (fraction of charge f ∼ 0.01),in the absence of surfactant, a hydrophobic polyelectrolyte poly-mer, such as AMD, may adopt a globular conformation burying afraction of the amine groups in the collapsed siloxane backbone,and thereby reducing ionization. In contrast, for ABn, the differentsolubilities of the silicone and the polyethylene glycol blocks prob-ably led to block segregation, allowing the ABn copolymer to adopta more unfolded conformation than AMD. The ABn, for example,could adopt a necklace conformation, such as proposed by Halderin[21] for multiblock copolymers, allowing the polymer amine groupsgreater access to the negative charges of the solid substrate.
This study confirms the advantage of a more hydrophilic ABn
copolymer structure vs. the AMD polymer as a mean to avoid exces-sive accumulation of silicone conditioners on hair. The formationof a repulsive electrostatic barrier can explain why the ABn siliconecopolymer did not have the tendency to build up on hair when usedin conditioners whereas AMD polymer interaction with hair wasdriven more by hydrophobic interactions and consequently AMDtended to accumulate on hair.
Further studies would be necessary to reveal the structure of theadsorbed aminosilicone drops.
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