Competitive adsorption of surfactant foaming agents to nanoclays added to cement foams for enhanced strength.pdf

8/16/2019 Competitive adsorption of surfactant foaming agents to nanoclays added to cement foams for enhanced strength…

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O R I G I N A L A R T I C L E

Competitive adsorption of surfactant foaming agents

to nanoclays added to cement foams for enhanced strength

Benjamin J. Naden

Received: 28 January 2015 / Accepted: 18 March 2015

RILEM 2015

Abstract The adsorption affinity of a surfactant

foaming agent (SFA), a-olefin sulphonate (AOS)—

used for generation of foam for low density concrete—

to organically-modified montmorillonite (OMMT) has

been investigated. OMMT has been proposed as an

additive to cement and concrete for improved strength

and durability. Similar thermodynamic processes are

involved in the generation and stabilisation of foam

and in the compatibilisation and stabilisation of

organic particles in aqueous environments, so inter-

action between SFA and OMMT particles is likely.Association of foaming agent molecules with organ-

oclay may lead to poor foaming performance and

potential instability of the nanoparticles due to

displacement of dispersants from the particle surface

by foaming agent. Adsorption isotherms determined

using a combination of ion-pair reverse phase high

performance liquid chromatography (RP-HPLC) and

gravimetric methods revealed that there is a relatively

high affinity of AOS for the organoclay particles. This

is a dynamic process, with smaller molecules ad-

sorbing quickly but being displaced by largermolecules at higher surfactant loading. From the

adsorption isotherm it was possible to calculate the

minimum AOS addition that will ensure the full

foaming performance in the cement formulation.

Relative adsorption affinity and competitive adsorp-

tion at the particle surface of AOS with non-ionic and

anionic surfactants commonly used as wetting and

dispersing agents, was studied. The dispersants dis-

played considerably higher relative adsorption onto

the organoclay than AOS, particularly in the case of

the anionic species. There is evidence that some AOS

adsorption takes place in particle systems stabilised by

non-ionic dispersants; displacement of high adsorp-

tion affinity dispersants by the lower affinity AOSfrom the OMMT particle surface was not observed.

Keywords Surfactant foaming agent Organically-

modified montmorillonite Cement Adsorption

isotherm Competitive adsorption Nano-functional

additive

Abbreviations

SFA Surfactant foaming agent

AOS a-olefin sulphonateOMMT Organically-modified

montmorillonite

RP-HPLC Reverse phase high performance

liquid chromatography

HLB Hydrophilic–lipophilic balance

MB2HT-MMT Methylbenzyl di-hydrogenated

tallow ammonium chloride

modified montmorillonite

AE Fatty alcohol polyoxyethylene

B. J. Naden (&)

Chemistry & Biotechnology. Pera Technology Ltd, Pera

Business Park, Nottingham Road, Melton Mowbray,

Leicestershire LE13 0PB, UK

e-mail: [email protected]

Materials and Structures

DOI 10.1617/s11527-015-0603-9


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AAS Anionic alkyl-aryl sulphonate

UV Ultraviolet

MPI N -methylpyridinium iodide

1 Introduction

Surfactant foaming agents are added to cement and

concrete formulations for the production of low

density materials, often used in applications that

require low strength void filling. Surfactants are

amphiphilic molecules that reduce the interfacial

tension between hydrophilic and hydrophobic phases

and increase contact surface area, enabling association

of these otherwise incompatible phases. The be-

haviour of a surfactant at the interface can be predictedby the hydrophilic–lipophilic balance (HLB), a semi-

empirical scale based on the relative molecular mass

of hydrophilic functionality compared to lipophilic

groups. A surfactant with a high HLB will have a large

hydrophilic group compared to the hydrophobic

component and this will influence its behaviour at an

oil (or hydrophobic solid particle)/water (or hy-

drophilic particle) phase boundary, although this

partitioning is independent of the overall size of the

molecule.

Surfactants promote the production of foams inaqueous media by stabilizing the interface of the

hydrophobic gas phase and the hydrophilic aqueous

phase (Fig. 1). A surface tension gradient is estab-

lished by the presence of the surfactant at the interface,

creating an elastic surface due to the flow of surfactant

molecules and liquid from the bulk to the interface.

This restoring Gibbs–Marangoni effect prevents thin-

ning of the liquid film between the gas bubbles and

stabilizes the foam [1]. Instability due to drainage of

liquid under gravity from the interfacial region can be

resisted by high viscosity liquid and the production of small, stiff-walled bubbles.

Foamed cement and concrete materials can provide

enhanced acoustic and thermal insulation properties

compared to non-foamed mineral structures [2] but are

generally inappropriate for load-bearing applications

or in circumstances where high mechanical strength is

required. This inherent weakness may be addressed by

increasing the strength of the foam cement concrete

matrix by the use of additives such as binders and

fibres. Foam pore walls can be reinforced, although

this requires the use of molecules that are significantlysmaller than the thickness of the gas-cement interfa-

cial region. One such approach investigated previous-

ly has been the addition of carbon nanotubes, the

inclusion of 0.05 wt% of which showed an increase in

compressive strength of non-autoclave cement foam

concrete of up to 70 % [3].

A number of studies have reported enhanced

structural properties of concrete with the addition of

nanoclays [4–7], organically-modified clay minerals

displaying nanostructural properties. The FibCem

project, an EU FP7 part-funded collaborative projectto develop nanotechnology-enhanced extruded fibre-

reinforced foam cement, intends to incorporate nan-

oclays to provide mechanical property improvements.

To achieve compatibility and stability of organoclays

with an aqueous cement environment it is necessary

that a surfactant is used, the absence of which will

result in particle agglomeration and the loss of any

reinforcing ‘nano effect’. Surfactants used to produce

emulsions and particle dispersions consist of a small

number of units and they mostly are reversibly

adsorbed at surfaces. Adsorbed amount is a functionof molecular weight, solubility in the solvent (and

therefore relative solubility parameters), affinity for

the surface of the adsorbing functional group(s), and

poor solubility and hence rejection from the solvent of

this functional group.

The adsorption of surfactants on hydrophobic

surfaces such as organically modified nanoclay is

governed by hydrophobic interaction between the

alkyl chain of the surfactant and the hydrophobicFig. 1 Optical micrograph of stabilised aqueous foam



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surface (Fig. 2). Immobilisation at the particle surface

results in a loss of entropy which must be compensated

for by the enthalpic and entropic gains implied by the

thermodynamics involved in association of functional

groups of the adsorbing molecule and those at the

particle surface and the liberation from the particle

surface of adsorbed solvent molecules. Since adsorp-tion depends on the magnitude of the hydrophobic

bonding free energy, the amount of surfactant ad-

sorbed increases directly with increasing alkyl chain

length [8].

In the application of particle dispersions, the

addition of other formulation ingredients, including

surface active components and dilution with solvent

can act to compromise dispersion stability. The same

mechanisms that are involved in the adsorption of the

stabilizing surfactant to the organic surface of the

nanoclay will also affect the amphiphilic surfactantfoaming agent. As a consequence, there is a very real

possibility of interaction between the foaming agent

and nanoclay, resulting in a reduction in foaming

capacity and foam stability. In addition, the adsorption

amount dependence upon molecular weight and the

reversible nature of the adsorption mechanism means

that competitive adsorption may occur in mixed

surfactant systems, involving the displacement of

stabilizing surfactant molecules by the foaming agent

and a consequent destabilizing effect. The result of the

competitive adsorption between two non-interactingmolecules depends on their relative affinity for the

surface, although other factors, including relative

solubility discussed above, need to be considered.

Thus, it is not sufficient simply to compare hydropho-

bic anchor chain composition of competing surfactant

molecules in order to predict displacement of adsorbed

stabilising molecules in the presence of, in this case, a

surfactant foaming agent.

Adsorption of solutes at the solid–liquid interface is

usually described through isotherms, expressed as

amount of adsorbate on the adsorbent in relation to

equilibrium concentration in solution. Adsorption

isotherm curves can exhibit different shapes but are

generally composed of three different regions (Fig. 3):

(1) initially low adsorbate concentration is charac-terised by high adsorption rates of isolated molecules;

(2) this is followed by an increase in the surface

adsorption density and then (3) by a third plateau

region which is independent of the concentration of

adsorbate in solution and corresponds to complete

surface coverage.

The shape of the isotherm can provide insight into

the nature of the adsorption process. For example,

monodisperse molecular adsorption isotherms show a

sharp transition between a steep adsorption rate at

incomplete surface coverage to a plateau value at fullsurface coverage. A polydisperse distribution of

molecules produces a more rounded isotherm due to

the complete adsorption of molecules at low concen-

tration, but as concentration increases and the surface

becomes saturated the longer chains in solution are

able to displace the shorter chains on the surface [9].

Comparison of the adsorption isotherms for the

surfactant systems can provide useful information

regarding any likely competition at the surface. It may

also be possible to measure directly surfactant

displacement.The objective of this study was to investigate

the nature of the interaction between a typical

surfactant foaming agent and organically-modified

Fig. 2 Schematic of surfactant adsorption at organoclay

particle surface in aqueous medium

Fig. 3 Typical Langmuir-type adsorption isotherm curves

showing regions of adsorption behaviour



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montmorillonite nanoclay (OMMT) that has been

dispersed in water for incorporation into a cement

formulation as a nano-functional additive.

2 Experimental

2.1 Materials

The organoclay used in this study was a methylbenzyl

di-hydrogenated tallow ammonium chloride modified

montmorillonite (MB2HT-MMT) provided by Laviosa

Chimica Mineraria S.p.a. (Livorno, Italy). Powders

were dried at 70 C for a minimum of 24 h.

The following technical grade surfactants were

used for organoclay stabilisation in water:

Nonionic C10 fatty alcohol polyoxyethylene(6)

(AE) from Croda Chemicals, UK.

Nonionic emulsifier polyethylene(10) oleyl ether

(Brij O10 ) from Sigma Aldrich, UK.

Anionic alkyl-aryl sulphonate (AAS) from Croda

Chemicals, UK.

The surfactant foaming agent used was an anionic

a-olefin sulphonate (AOS) supplied by Sika S.A.U.,

Spain at actives concentration of 40 wt%.

2.2 Methods

To determine the adsorption isotherm for each of the

systems studied, various weights of surfactant were

diluted to approximately 9.5 g in deionised water in

screw-top 15 mL vials and fully dissolved. Ap-

proximately 0.5 g of nanoclay was added to each vial,

each of which was sealed and then sonicated for

45 min in a sonic bath. The samples were stored at

room temperature overnight. The contents of each

sample jar were transferred to centrifuge tubes and

centrifuged at 8500 rpm for 30 min. The supernatant

from each tube was carefully removed and filtered and

the equilibrium concentration of dispersant deter-

mined by the various techniques described below. The

amount of surfactant adsorbed at the particle surface

was derived from the concentration of the non-adsorbed surfactant in solution.

2.2.1 Reverse phase ion-pair UV detection HPLC

A method for analysis of the AOS surfactant was

developed based on a technique described by Zahrob-

sky and co-workers [10] using RP-HPLC with indirect

photometric detection. Pyridinium salt N -methylpyri-

dinium iodide (MPI) was used to form an ion-pair

complex with the sulphonate surfactant, allowing UV

detection. Surfactant solution samples of 25 lL wereinjected into a mobile phase comprising 60 vol%

water and 40 vol% acetonitrile and 0.15 mM MPI.

Mobile phase elution rate was set at 1 mL/min through

a 5 lm C18 reverse phase column fitted to an Agilent

1100 series HPLC using diode array detector at

258 nm wavelength.

2.2.2 Gravimetric analysis

Non-adsorbed surfactant concentration was deter-

mined by loss of mass of the centrifuge supernatantat 150 C. Approximately 5–10 g of sample was

placed in the pan of a thermal balance (MS-70

moisture analyzer, A&D Weighing) and reduction of

mass was monitored until rate of loss was less than

0.1 % per minute. The remaining weight was used to

determine equilibrium concentration and subsequently

adsorbed amount using mass balance calculations.

3 Results and discussion

3.1 RP-HPLC calibration

The elution chromatograms for standard solutions of

AOS surfactant are presented in Fig. 4. A large

negative peak was observed early in the run, with the

same retention time as the ion-pair reagent (ap-

proximately 2 min). Later in the chromatogram another

large, broad, negative peak was observed; these

negative system peaks were present in all surfactant



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elution chromatograms and may be caused by depletion

of the UV-active reagent from the mobile phase when

the sample is injected. When the surfactant material

was eluted from the column, the second system peak

area increased with surfactant concentration.

A number of retention peaks that are representative

of the surfactant composition appear in the elution

profile. These peaks can be attributed to the range of molecular weights of compounds within the sample.

The larger the adsorbing alkyl tail of the surfactant, the

more lipophilic the molecule and the longer the

residence time in the reverse phase column. Any

branching of the tail results in a more compact alkyl

chain and may be expected to reduce lipophilicity of a

compound compared to the linear equivalent with the

same number of carbon atoms [11].

At high surfactant concentration, the large negative

system peak at *6 min dominated the chromatogram

at higher retention times, rendering any peaks occur-ring in this region unsuitable as representative peaks

for surfactant concentration determination. It was

found that the peak appearing at elution time between

3.0 and 3.2 min provided a linear response in the

detector with respect to the concentration of surfactant

in the injected sample over a range of more than two

orders of magnitude (Fig. 5). This peak was present

over the full range of surfactant concentration

measured and was therefore used as being represen-

tative of the concentration of the surfactant sample.

3.2 Adsorption isotherms

Isotherms for the adsorption of AOS to MB2HT-

MMT, determined by RP-HPLC and by gravimetric

Fig. 4 HPLC elution

spectra of a-olefin

sulphonate surfactant with

MPI ion-pair reagent

Fig. 5 Calibration curve for peak area at retention time

between 3.0 and 3.2 min with concentration of a-olefin

sulphonate



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methods are presented in Fig. 6. Samples were

prepared with ratios of AOS to organoclay up to

0.11 for RP-HPLC measurement and 0.13 for the

gravimetric method. Similar adsorption per unit mass

of nanoclay and maximum adsorbed amount were

observed for both gravimetric and RP-HPLC tech-

niques. This correlation at low surfactant:nanoclayratio of the two techniques provides confidence that

there is no desorption of MB2HT modifier from the

nanoclay which would influence the results of the

gravimetric measurement. Beyond an adsorption ratio

of approximately 0.08 the gravimetric method showed

independence of adsorbed amount to concentration of

surfactant remaining in solution, indicating that

adsorption saturation was reached. The RP-HPLC

method, on the other hand, showed a reduction in

adsorbed amount with increasing equilibrium concen-

tration beyond this point. Taken in isolation, the RP-HPLC result would indicate desorption of AOS from

the particle surface. However, the gravimetric results

for the same experiment do not show this behaviour.

This discrepancy in the shape of the isotherm provides

evidence that the surfactant sample is composed of an

homologous series of molecules with varying alkyl

chain length. This polydispersity has been observed

elsewhere for similar technical grade surfactants

[12] and the multiple peaks observed in the HPLC

chromatogram (Fig. 4) implies a distribution of

molecular weights within the sample; surfactants withmonodispersed well-defined chain length are only

synthesized for research purposes. The gravimetric

method measured directly the mass of surfactant

molecules remaining in solution after adsorption had

occurred, allowing the surface-adsorbed component to

be calculated from simple mass balance. By contrast,

the chromatographic method assumed that the mole-

cules eluting from the column at 3.1 min were

representative of all surfactant molecules in the

sample. This was true at low surfactant-to-nanoclayratio, when all of the surfactant molecules adsorbed

onto the surface. However, at high surfactant concen-

tration when the number of molecules exceeded

particle surface saturation, competition between

molecules for surface adsorption resulted in the

displacement of the small molecules (represented by

the 3.1 min elution time) by larger molecules from

solution, as predicted by the thermodynamic adsorp-

tion process described earlier. Thus, at high surfactant

addition, desorption of these smaller molecules from

the surface increased their relative concentration insolution, giving the impression of reduced surface

adsorption at high surfactant concentration; the larger

molecules were not detected due to the limitations of

the method.

Knowledge of the amount of surfactant adsorbing at

the organoclay surface can provide guidance on the

quantity of additional AOS foaming agent required to

generate the desired foaming effect. In this case

saturation of AOS at the particle surface was reached

at equilibrium concentration *500 mg/L. It can be

determined that, to achieve the full effect from thefoaming agent, AOS addition should exceed *80 mg

per gram of MB2HT-MMT organoclay in the cement

formulation. An understanding of how adsorption at

the organoclay surface is affected by the molecular

weight distribution of the AOS foaming agent can also

help select surfactant concentration to balance rate of

foam formation (low molecular weight surfactant

molecules adsorb at a higher rate at the foam

interfacial region than their larger counterparts) and

enhanced foam stability provided by larger molecules.

Adsorption affinity of the surfactants selected toprovide stabilisation of the organoclay in water was

investigated and the isotherms are presented in Fig. 7.

AE showed higher adsorbed amount onto MB2HT-

MMT than AOS. Brij O10, like AE, is non-ionic but is

a larger molecule than, although its behaviour at the

phase boundary can be expected to be similar (HLB

12.4 for Brij O10 compared to 12.6 for AE). This was

confirmed by the similarity of the adsorption iso-

therms, indicating that adsorption affinity is a function

Fig. 6 Comparison of a-olefin sulphonate/nanoclay adsorption

isotherms measured by RP-HPLC and by gravimetric methods.

Connecting lines have been added to guide the eye



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of overall molecular properties, rather than simply that

of the adsorbing moiety. All of the stabilising surfac-

tants displayed lower rate of adsorption at low addition

than AOS. This was particularly pronounced for AAS

which displayed relatively low mass adsorption at low

surfactant concentration, producing a comparatively

gentle slope to the isotherm, but overall adsorbed

amount at maximum surface coverage was far greater

than that of the non-ionic surfactants or AOS.

3.3 Competitive adsorption

Competition between AOS foaming agent and stabil-

ising surfactants AE and AAS for adsorption at the

surface of MB2HT was investigated. Particle suspen-

sions were prepared using each of the stabilising

surfactants at a mass ratio to organoclay of 0.25, to

which AOS was added at a ratio of foaming agent to

organoclay of 0.20.

For competition with AE, the RP-HPLC elution

chromatograms of suspension samples containing AOSin the absence and in the presence of AE were compared

(Fig. 8). The higher free AOS concentration for the

sample prepared in the absence of AE suggests that the

presence of dispersant promotes AOS adsorption at the

particle surface. This may be a result of the molecular

weight selective feature of the RP-HPLC method.

Adsorption of AOS does not mean that AE was

desorbed; this was not demonstrated by this experiment.

If AE molecules remain in place at the surface, the

approach of large AOS molecules is sterically hindered;

adsorption of the small molecules represented by the

selected elution peak will be less impeded. Consequent-

ly, adsorption of these small AOS molecules between

molecules of AE will be favoured over the higher

molecular weight components. In the absence of AE at

the surface, the free energy balance favours the

adsorption of large AOS molecules at this surfactant-

to-organoclay ratio, resulting in a higher relative

concentration of low molecular weight moleculesremaining in solution, as described above. Further

investigation into this phenomenon is advised as

selective adsorption of low molecular weight foaming

agent molecules may affect foaming characteristics

with regards to rate of foam formation and foam

stability, as discussed above. An alternative RP-HPLC

detection method, such as refractive index detection,

may allow for the characterization of the full molecular

weight distribution of the surfactants and hence the

adsorption behaviour. The effect this has on foaming

effects should be investigated also.Competitive adsorption of AOS foaming agent

with AAS stabilizer to MB2HT-MMT was investigat-

ed by comparison of RP-HPLC elution profiles of

AAS prior to and following the addition of AOS

(Fig. 9). The aromatic functionality of AAS is chro-

mophoric with UV absorption over a wide spectral

range, including the 258 nm wavelength used to detect

ion-pair modified AOS. Adsorption behaviour of the

AAS stabilizer to organoclay appeared to be consistent

Fig. 7 Adsorption isotherms for particle stabilising surfactants

to MB2HT-MMT organoclay. Also plotted is the isotherm fora-

olefin sulphonate foaming agent for comparison. Lines have

been drawn through the points to guide the eye

Fig. 8 Comparative elution spectra for a-olefin sulphonate;

peak magnitude is inversely proportional to adsorption amount

of the foaming agent to MB2HT-MMT organoclay in the

presence and absence of AE stabilizer at the particle surface



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regardless of AOS presence. It was not possible to

determine if the adsorption of AOS to MB2HT-MMT

occurred using this technique. The structural simila-

rities of the AOS and AAS molecules means that the

residence time in a chromatography column will be

similar (and also the adsorption to the particle

surface), making differentiation between them diffi-

cult using this technique. Radiolabelling of the AOS

molecules by synthesizing with e.g. the 14C isotope

will allow identification of those molecules by

radioactive beta decay detection. The concentrationof non-adsorbed AOS molecules can then be deter-

mined and hence the adsorbed amount by mass

balance calculation. To determine the influence of

molecular weight upon adsorption it will be neces-

sary to carefully synthesize a range of labelled

monodisperse AOS molecules and carry out a number

of experiments to determine adsorbed amount for each

size of molecule.

4 Conclusions

A-olefin sulphonate, a surfactant frequently used as a

foaming agent for cement and concrete, displays a

relatively high adsorption affinity for organically

modified nanoclay that is added to the cement

formulation for enhanced strength of the cured mate-

rial. Although the maximum adsorbed amount of AOS

at the organoclay surface is considerably less than

those of the surfactants used as dispersing agents, this

will inevitably have implications for foaming efficacy

when organoclays are present and should be taken into

consideration when setting surfactant foaming agent

concentration. Increasing foaming agent loading to

account for the non-availability of the organoclay-

adsorbed component will certainly increase material

costs. Desorption of dispersing agents due to com-petitive adsorption of the foaming agent will lead to

instability of dispersed particles; there is no explicit

evidence of dispersant displacement in the ex-

periments described here, however. A minimum

addition ratio of AOS to organoclay was determined

to account for any adsorption at the particle surface

and thus ensure the full foaming performance effect.

Acknowledgments The author would like to acknowledge the

European Commission funding under the 7th Framework

Programme for the project FIBCEM—Nanotechnology Enhanced

Extruded Fibre Reinforced Cement Based Environmentally

Friendly Sandwich Material for Building Applications (Grant

agreementno. 262954).Thanksto projectpartners LaviosaChimica

Mineraria and to the Lithuanian Energy Institute for powder supply

and characterisation, and Sika SAU for foaming agent supply.

Thanks also to Croda Chemicals Europe for supply of surfactants.

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Competitive adsorption of surfactant foaming agents to nanoclays added to cement foams for enhanced strength.pdf