Natural Surfactants-Based Ag Nanofluids for Enhanced Wettability on

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Natural Surfactants-based Ag Nanofluids for Enhanced Wettability on Hair Surface Santosh Deb Barma, Barnali Banerjee, Krishnendu Chatterjee, and Santanu Paria ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03943 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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ACS Sustainable Chemistry & Engineering

Natural Surfactants-based Ag Nanofluids for Enhanced Wettability on Hair Surface Santosh Deb Barma, Barnali Banerjee, Krishnendu Chatterjee, Santanu Paria* Interfaces and Nanomaterials Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela - 769008, Orissa, India

*

To

whom

correspondence

should

be

addressed.

E–mail:

[email protected], Tel.: +91 661 246 2262

ACS Paragon Plus Environment

[email protected]

or

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The uses of natural products in personal care are increasing gradually in recent years because of their nontoxic nature compared to the conventional chemical compounds. The present study deals with the solution behaviours of three natural surfactants (Reetha, Shikakai, and Acacia) in the presence and absence of silver nanoparticles (Ag NPs) and their subsequent influence on the wettability of hair surface. Among all studied natural surfactants, Shikakai in the presence of Ag NPs showed lowest contact angle (~58.5º), which was comparable to that of widely used synthetic surfactant TX-100 (~57.2º). The Ag NPs were synthesized by the chemical as well as in-situ green routes in the presence of Acacia. The NPs and hair surface were characterized by transmission electron and atomic force microscopic techniques where the adherence of Acacia mediated Ag NPs (average size of 24.17 nm) were observed over the rough surface of hair strand. The reduction in surface tension from pure Acacia (~43.56 mN/m) to in-situ Ag NPs in Acacia media (~41.6 mN/m) followed the increasing wettability at virgin hair surface by lowering the contact angle from 105.5º to 66º. The chemically synthesized Ag NPs showed slight better wettability (~ 64.8º) as compared to in-situ Ag NPs in Acacia medium. However, the Acacia mediated green synthesized Ag NPs suspension, despite being marginal inferior in terms of surface tension reduction and wetting, may be important from the practical perspective.

Keywords: Acacia; Shikakai, Reetha, Hair wetting; Nanofluid; Silver Nanoparticles; Disjoining pressure, Surface tension.


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Introduction Contact angle of polar liquids on solid surfaces has many practical applications such as detergency, surface cleaning, shampoo etc.1-4 Gradual increasing uses of synthetic surfactants in these processes create acute environmental problem when the solutions are being discharged after the end use, as they are mostly non-biodegradable.5-7 As a result, the uses of biodegradable and natural surfactants, especially in personal care products are increasing day by day because of their non-toxic and biocompatible nature. In recent years many companies are trying to reduce the use of synthetic surfactants in shampoo, whereas as there is an increasing trend to application of the natural surfactants (saponins). In this regard, some of the natural surfactants with antimicrobial, antioxidant, and antidandruff activities in addition to surface activity may be useful.8,9 There are several saponins, which are highly surface active in aqueous media, some with antidandruff activity and act as useful ingredient in herbal/organic shampoos such as Sapindus mukorossi (Reetha),. Acacia auriculiformis (Acacia), Vernonia cinerea, Asparagus racemosus, Ricnus communis, Acacia concinna (Shikakai).10-14 Apart from plant-surfactants, bio-surfactants extracted from microbial sources have also drawn significant attention.15 While comparing the surfactants of microbial and plant origins, the plant surfactants are advantageous over the microbial ones because of various factors such as cheaper production cost, ease of industrial scale production, and easy separation/purification steps. However, plant surfactants have low shelf-life as these are quite susceptible to microbial (fungal and bacterial) attack and thus difficult be preserved in aqueous phase for prolong duration for commercial purposes. While the natural surfactants are used for wetting application, their inferior performance compared to the synthetic surfactant is mainly because of less surface activity or higher surface tension.16 Till now many researchers have used mixed surfactant system or added some additives to enhance the surface activity of the resultant solution. For increasing the shelf-life of a natural product generally antimicrobial agents are used. In this regard, apart from chemical reagents, Ag NPs are promising in several applications.15,17-21 The suspensions of nanoparticles are termed as nanofluids, which show distinctive features and remarkable properties in several applications. The nanofluids offer unprecedented potential for many applications in the fields wetting, enhanced oil recovery, thermal fluids for solar energy systems, heat transfer enhancement, and so on.22-24 The reduction of surface tension by the nanoparticles suspension or nanofluids similar to that of surfactants is an important phenomenon from the practical perspective.25 As a result, the wetting behaviour of nanofluids on the solid surfaces is also an important topic of research.26 The wetting behaviour nanofluid is attributed to the fact that spontaneous spreading of the



nanoparticles influenced by the structural disjoining force, which in turn, arises because of the ordering of nanoparticles in the confined wedge film.27 This disjoining pressure (∏h) is dominent at the vertex of the wedge film, and finally enhance spontaneous wetting process by shifting the position of three phase contact line. Several parameters such as van der Waals, electrostatic, and salvation forces of ∏h gradient are responsible for dynamic mechanism of spreading.28 In recent years, the effect of nanoparticles on surface or interfacial tension and contact angle has been an important topic of research in the fields nanoscience and interfacial phenomena because of several important applications.29-31 Since the wetting property of a solid surface depends on the interfacial property of both surfactant solution and nanofluid, the combined investigation of both may consequence a great practical attention. This study highlights an experimental analysis on hair wetting using three different aqueous natural surfactant solutions, namely Acacia (Ac), Reetha (Rh), and Shikakai (Sh) in the presence of Ag suspension as an additive. In-situ Ag NPs were synthesized in aqueous Ac media, whereas chemically synthesized Ag NPs were synthesized by sodium borohydride (NaBH4) reduction, which further mixed with the surfactant solutions for the surface tension and wetting study. The wettability of human hair surface by natural surfactants solutions and mixture of nanoparticles and natural surfactants was not reported elsewhere before. From the application perspective, this study may have the potential for the enhancement of the shelf-life of natural surfactant solutions as well as organic shampoo formulation.

Materials and Methods Materials Human hair of Indian women was used for the wettability experiment. The plant-based natural surfactants containing saponins used for this study were extracted from the dry pod of Ac, Rh and Sh. All the saponins extraction from Ac (MW-1765), Rh (MW-966) and Sh (MW-1056), were done according to our previously reported studies.16,32 In case of Ac saponin, some water insoluble components are present after methanol extract. To remove those water insoluble components the methanol extracted dry product was solubilized in the mixture of methanol and di-ethyl ether (1:5), and then the soluble part was vacuum dried to remove the water insoluble components. The chemical structures of the natural surfactants (saponins) used in this study are presented in Fig. S1 (Supporting Information).33 Other chemicals such as ethanol, methanol, diethyl ether, NaBH4, and AgNO3 were ordered from Merck. Ultrapure water (Milli-Q) was used


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from Merck Millipore (India) Pvt. Ltd., having 18.20 MΩ cm resistivity, 71.50 mNm-1 surface tension, and 6.5-7.0 pH for all experiments. Characterization of hair surface The hair strands used in this study was rinsed thoroughly before the experiment with deionized water, followed by ethanol, and finally with pure water again to remove the unwanted particulate or oily deposits accumulated on the hair cuticle over a time period. After the washing step, the hair strands were dried using a hot air blower for 3 min after each wash. The surface morphology in terms of roughness and diameter of hair strand was characterized using Atomic Force Microscope (AFM, Veeco, USA) and Field Emission Scanning Electron Microscope (FE-SEM, FEI, NOVA Nano-SEM 230) respectively. Further, the confirmation of Ac mediated in-situ Ag NPs formed and their adherence at hair surface was observed using a High Resolution TEM (FEI, Tecnai G2-F30 S-TWIN) and FE-SEM. Measurements of surface tension and contact angle To avoid fungal or microbial contamination and growth, the solutions were as prepared just before the measurements. Successive measurements of surface tension and contact angle were done at 25°C by using a surface tensiometer (DCAT-11EC) and a video based contact angle meter with a nano dosing system (OCA-30) respectively from Data Physics, Germany. For the surface tension and contact angle measurements Wilhelmy plate and sessile drop techniques were used respectively. The nano dosing system enables to maintain the drop volume of 500 nL at a pressure of 1.2 bar throughout the series of experiments. The Wilhelmy platinum plate was cleaned properly by washing with ethanol/acetone, and then burned in an gas burner to remove the adsorbed surfactants after each measurement of surface tension. The motor speed of the platinum plate of surface tensiometer was maintained to 1 mm/s and a 3 mm the immersion depth in the surfactant solution was maintained. The CMCs of the used surfactants (Ac, Rh, and Sh) were found to be 0.39, 0.52 and 0.50 mM/L respectively. Synthesis and characterization of Ag NPs The chemically synthesized Ag NPs were prepared by adding freshly prepared ice cooled reducing agent (NaBH4) into AgNO3 solution, and maintaining AgNO3 to NaBH4 molar ratio of 1:2, then different concentrations (0.001-1 mM/L) of surfactant solutions were added under stirring condition and continued to stir for 1 h at 35 ± 3°C.13 These NPs were also synthesized insitu by simply addition of AgNO3 solution in different natural surfactants media. The AgNO3 to surfactant stoichiometric ratios were obtained by trial and error experiments to 1:4 and 1:10 for



Ac and Rh to complete the reaction respectively. The equilibrium times for the particle growth were found to be 6 and 48 h for Ac and Rh solutions respectively at 35 ± 3°C temperature. The absorbance spectra of nanoparticles suspensions were measured by UV-Vis spectrophotometer (UV-3600, Shimadzu). The spectroscopic analysis using Fourier transform infrared (FT-IR) spectroscopy (Thermo Fisher Scientific, Nicolet iS10) was done to identify the functional groups of Ac, Rh, and Sh. The FT-IR spectra are compared with the previously reported study33 and presented in Fig. S2 (Supporting Information). The size change of chemically synthesized Ag NPs before and after dispersion in different surfactant media were observed using Dynamic light scattering (DLS, Malvern Nano ZS, UK) and TEM techniques. Results and discussion Microscopic characterization and surface energy of hair strand At the microscopic level, human hairs with various cellular structure and outermost cuticle surface is usually not even, hence the roughness and the diameter of the hair strand in this study was observed by the AFM and FE-SEM analyses. The diameter of a single hair strand used in this study is approximately 125-130 µm as seen from the FE-SEM image of Fig. 1(a), although the dimension may vary depending on the source as well as position.


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Fig. 1. (a) FE-SEM microstructure of a hair strand; (b, c) AFM images of hair strand at higher magnification showing the roughness along the length of hair; (d) Surface topography and height profile along z-axis, different coloured lines on the surface for different positions. Line positions are shown in inserted figure. The FE-SEM and magnified view of the AFM images (Fig. 1.b, c) clearly show the rough surface of hair. It can also be evidently visible from the FE-SEM image that the roughness is evenly scattered throughout the length of the hair strand similar to the AFM images. Subsequently, the surface roughness of hair was also analysed from the AFM image using SpmLab software and noted to be in the range of 36 to 39 nm as illustrated in Fig. 1(d). As the surface energy (work required to create unit area of a solid surface) of hair is an important parameter in the wetting process, the surface energy (γs) was determined experimentally. Owens-Wendt-Kaelble (OWK) model also termed as two liquid model, amongst various available models, is the most frequently used method for the calculation of solid surface energy.34-36 In the present study, the surface energy of hair surface in terms of polar (γsp) and dispersion (γsd) components were calculated experimentally using contact angle measurement



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technique by choosing ethylene glycol and water as test liquids. From the experiments we found the values of 0.21, 31.9, and 32.11 mJ/m2 for γsd, γsp, and γs respectively. The experimental γs value is similar to that of the reported values of 2437 and 32 mJ/m2.2

Solution behaviour of pure natural surfactants The reduction in surface tension was observed because of the adsorption of surfactant molecules at air-water interface by replacing the water molecules with the surfactant molecules. To know the adsorption density and the minimum surface tension at CMC, surface tension measurements were done for different concentrations using three natural surfactants, Ac, Rh and Sh. For all the three natural surfactants, the surface tension value depleting steadily with enhancing the concentration of surfactants until the CMC; beyond the CMC the surface tension remains constant. The values of CMC, surface tension at CMC ( ), surface excess ( ), and occupied areaper molecule (Amin) are presented in Table 1. Table 1. The CMC values and their respective γCMC) and Amin for three natural surfactant solutions.16

Surfactant

CMC (mM/L)

γCMC (mN/m)

ΓLG (µmol/m2)

Amin (Å2)

Rh

0.52

38.29

1.70

97.48

Sh

0.50

38.71

1.71

97.02

Ac

0.39

43.56

1.54

107.23

TX-100

0.15

31.50

2.44

68.00

It is observed from Table 1, that the surface tension values for all three individual surfactants reduces from 71.5 mN/m (pure water) to a minimum value at CMC which are comparatively and significantly higher than synthetic single-chain cationic (CTAB, 32.99 mN/m), anionic (SDBS, 34.22 mN/m), and nonionic (TX-100, 31.01 mN/m, Igepal CO– 630, 31.02 mN/m) surfactants.16 These reported surface tension values indicate that the values for Rh and Sh are very close but lesser than Ac. It is also evident that the lowest area occupied by a surfactant molecule at the interface (Amin) is least for Sh as compared to that of Rh and Ac which indicates that Sh has


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higher adsorption density than Rh and Ac. Comparing with a widely used nonionic surfactant (TX-100) as a reference, it has been seen that the CMC and γCMC values of natural surfactants are inferior.38

Wettability of hair by natural surfactants The wettability of hair was studied in terms of contact angle measurement by a sessile drop technique. For the wettability study, along with three natural surfactants, TX-100 was also used for the comparison. Initially, contact angle on a virgin untreated hair surface (water washed) was done with ultrapure water and found the value of 105± 5º. The obtained value is fairly close with the reported value of 103± 4º.5

90 Reetha Acacia Shikakai TX-100

85 80

Contact angle, θ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


75 70 65 60 55 50 -1.5

-1.2

-0.9

-0.6

-0.3

0.0

Log c Fig. 2. Change in contact angle (θº) on hair surface with change in logarithmic of concentration (log c) of pure Rh, Ac, Sh, and TX-100 solutions. However, the contact angle of hair surface decreases further to 87.5º after ethanol wash, this is attributed to the removal of oily (hydrophobic) components from the hair strands. The alcohol washed hair surfaces were used further in this study for the better repeatability. The contact angle analysis results depicted in Fig. 2 shows that there is a gradual reduction in contact angle with increasing surfactant concentrations until it reaches the CMCs for all the surfactants, beyond which no further reduction in contact angle is observed on increasing surfactant concentration. The contact angle value reduces from 87.5º (ultrapure water) to 57.2, 64.4, 70.5, and 71.3º for TX-100, Sh, Rh, and Ac respectively at their respective CMC values. These results show that the difference in contact angles values of Rh and Ac solutions are not very significant; however, Shi ACS Paragon Plus Environment


solution has ~ 6º lower contact angle. While comparing with TX-100, it has ~ 7º lower contact angle than that of Sh, mainly because of lower surface tension achieved at CMC. Noteworthy to mention that, similar wetting behaviour was observed using same natural surfactants solutions on PTFE surface with contact angle values of 109.88º, 109.02º, and 98.13º for Rh, Ac and Sh respectively.16 Because of the lack of reported study on the wetting of natural surfactants on hair surface a comparison was made with the reported synthetic surfactant. A cationic surfactant, dimethylpabamidopropyl laurdimonium tosylate (CMC value of 0.174 mM/L at 40°C) was able to reduce the dynamic contact angle on hair surface 107 (pure water) to 78 in the presence of 0.05 mM/L surfactant solution.39 Generally, the wetting behaviour of surfactant solutions can be enhanced in several ways such as by the addition of additives such as electrolytes, alcohols etc. or using a mixed surfactant system. The present study shows the natural surfactants have inferior wetting property compared to that of synthetic one, to overcome this problem the influence of Ag NPs as an additive on the wetting property of natural surfactants over the hair surface was tested. From the practical perspective, some benefits of this study are improvement in shelf-life of the natural surfactants, prevention from microbial attack as they are very much susceptible to the microbial attack at room temperature, and may also act as an anti-dandruff agent for the application in shampoo, apart from the enhancement of wetting property.21 The wetting behaviour of natural surfactants in the presence of Ag NPs are discussed in the later sections. Synthesis and characterization of Ag nanofluids To see the effect of Ag NPs on hair wetting in the presence of natural surfactants the NPs were synthesized and characterized first. In general, the separation and purification of nanoparticles from the synthesis media are challenging job in any applications because of their small size. The separation of nanoparticles from the reaction mixture causes several problems such as increase in particle size, loss of particles, higher process cost, which limit their use in different applications. On the other hand, the synthesis of nanoparticles in-situ may reduce these problems in many applications. Since all natural surfactants are having some carboxylic and hydroxyl groups (in a sugar moiety) in their structure, we tested the in-situ reduction ability (Ag+ → Ag0) of these molecules to form Ag NPs in the absence of any chemical reducing agent. As Ac has more hydroxyl groups in its arabinose part of the structure, it is expected to show better reducing ability, and it has been confirmed after some preliminary trial experiments.18,19 The Ag NPsformation mechanism in the presence of saponin molecules of Ac is proposed in Scheme 1.


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Ag → Ag Scheme 1: Ag nanoparticles formation mechanism in the presence of Ac. To find the stoichiometric ratio for complete reduction of AgNO3, previously published protocol21 was followed and found 1:4 AgNO3: Ac is required to complete the reaction. To ascertain the equilibrium time, the UV-Vis absorption spectra of Ag NPs were noted at different time intervals and the resultant spectra are shown in Fig. 3.

Fig. 3. (a) UV-Vis absorbance spectral pattern of Ag NPs with timefor Ac mediated in-situ synthesis and chemically synthesized Ag NPs (AgNO3: NaBH4 =1 : 2). The colour changes in all three surfactants’ media (b) before and (c) after the addition of chemically synthesized Ag NPs. The absorption spectra show a prominent characteristic peak at 420 nm wavelength because of the formation of Ag NPs in the presence of Ac solutions, as shown in Fig.3. A gradual increase in the intensity of the absorbance spectra at 420 nm wavelength with the increasing reaction time from 5 m to 6 h indicates that the reaction is slow. No significant change was observed between 6 h to 24 h time interval, confirms the completion of reaction within 6 h. Further, Fig. 4 (a) illustrates the TEM image of Ag NPs synthesized in Ac media, where the calculated average particle size was



found to be 24.17 nm. The in- situ synthesis of nanoparticles in Rh and Sh media were also conducted at a different AgNO3 to surfactant molar ratios varying from 1:1 to 1:10. It was found that, Rh can reduce AgNO3 to Ag NPs within 48 h in the presence of 1:10 molar ratio. However, unlike Ac and Rh, Sh was not able to produce Ag NPs in-situ. Therefore, our experimental studies on wetting was limited only to Ac mediated in-situ synthesis of Ag NPs.

Fig. 4. TEM images of Ag NPs (a) by in-situ synthesis method, in Acmedium, (b) by chemically synthesis (AgNO3: NaBH4 =1mM : 2mM) [inset shows histogram for particle size distribution], (c, d) High magnification TEM images showing lattice fringe pattern of Ag NPs [inset shows SAED pattern of Ag] The results of chemically synthesized Ag NPs in the absence of natural surfactants were also compared with that of in-situ. The formed Ag NPs were then mixed with natural surfactant for wetting study. To ensure the nanoparticle formation, the spectroscopic analysis of silver nanofluid was done and the surface plasmon resonance bands of Ag NPs were detected at around 420 nm wavelength as shown in Fig.3. The TEM image of chemically synthesized particles with average size of 30 nm is shown in Fig. 4(b). The formation of smaller sized particles in the presence of Ac


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is mainly because of capping effect, which prevent the particles growth. The particles synthesised in both routes were also characterized by FE-SEM and presented in Fig. S3 (Supporting Information). The size changes of chemically synthesized Ag NPs before and after dispersion in different surfactant media were observed using DLS (Fig. S4, Supporting Information) and TEM (Fig. S5, Supporting Information) after 12 h. From these analyses it can be seen that the size change of NPs in different surfactants media were negligible. Solution behaviour of Ag nanofluid The solution behaviour of silver nanofluids in different surfactant media were studied by the measurement of surface tension. The parameters that affect the physical properties of nanofluids and surface wettability include the type of material, concentration, shape and size of nanoparticle, along with the nature of the dispersing media.26-29 The presence of nanoparticles as well as surfactants can effectively reduces the solid-liquid and gas-liquid interfacial tensions which in turn increases the wettability of solid surface.25 The changes in surface tension values are plotted against log c of surfactant solutions in the presence of Ag NPs (in-situ and chemically synthesized), illustrated in Fig. 5.

Fig. 5. Change in surface tension (γ) with logarithmic concentration (log c) of Ac for in-situ synthesized Ag NPs; and Ac, Rh and Shfor chemically synthesized Ag NPs on hair surface. It can be comprehended from figure that the minimum surface tension of in-situ synthesized Ag NPs in Ac medium is 41.6 mN/m at 0.7 mM of Ac solution, which is comparatively lower than



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that of the pure Ac solution. Similarly, reduction in surface tension of different natural surfactants solutions in the presence of Ag NPs synthesized chemically are shown in Fig. 5. The comparative studies on surface tension data for both in-situ and chemically synthesized methodology are presented in the Table 2. Table 2. Minimum surface tension (γ) and minimum contact angle (θº) of natural surfactants’ (Ac, Rh, Sh) solution in the presence of 0.25 mM/L Ag NPs for in-situ and chemically synthesized approach. Minimun Contact Methodology

Sample

Minumum Surface

Angle

Tension, γmin (mN/m) (θºmin)

in-situ

Ac+Ag NPs

41.6 (at 0.7 mM/L)

66.1º

Chemical

Ac+Ag NPs

39.81 (at 0.6 mM/L)

64.8º

Sh+Ag NPs

36.24 (at 0.6 mM/L)

58.5º

Rh+Ag NPs

35.21 (at 0.7 mM/L)

63.3º

From Table 2, it can be depicted that the surface tension of in-situ synthesized Ag NPs in Ac media is slight higher to that of suspension of chemically synthesized Ag NPs, but the difference is less. Additionally, it can also be concluded that Sh and Rh are better in terms of lowering of surface tenstion than that of Ac. Rh shows the lowest surface tension (35.2 mN/m), and it has a marginal difference (~ 1 mN/m) with Sh. In case of in-situ synthesized suspension of slightly higher surface tension value can be attributed to the fact of incomplete reduction reaction which eventually leads to lesser particle density in the suspension. The particle size also have a role in reduction of surface tension, and in general smaller particles should be more active at air-water interface. However, as the particle sizes are not significantly different in these two cases, the size effect may not be significant on interfacial behavior. As surfacetension is the additivity of the inftermolecular forces, the reduction in surface tension in the presence of NPs depends on several parameters such as material property, particles concentration size, density, etc. When the nanoparticles are in suspension Brownian motion dominates because of weak gravitational force, which drives in the mobility of nanoparticles towards the interface (air-water) as well as the bulk


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phase. Under this situation, if significant number of NPs move to the air-water interface, the presence of NPs affect the cohesive force between the water molecules at the interface, which eventually reduce the surface tension of nanofluids. The particles at interface may be of two types: (i) floating (floating at the interface), (ii) immersed (partially submerged into the liquid layer). In both cases, particles at interface covered by a liquid layer experience attractive capillary force. It has been reported that the immersion capillary force is very significant for nanoparticles suspension (particle diameter ≥ 2 nm), however the floatation force is important mainly for micron size particles.25 When the hydrophilic particles come close at the interface, (i) the capillary force will try to attract the particles and (ii) at the same time electrical repulsive force will oppose the particles to move to a larger distance, and finally there will be balance between these two forces. As the natural surfactants are nonionic in nature, after adsorption on the NPs surface, it is expected that the particles present at the interface can move to a closer distance because of capillary force and shows additional reduction in surface tension than that of the pure surfactant solution.

Wettability of hair surface by nanofluids Unlike the wetting of solid surfaces by pure liquids, that by the nanofluids is a complex phenomena because of the presences of intermolecular forces between the surfactant molecules surface, particles – surfactant molecules, and particles – surface; these interactions finally varry the surface tension and ∏h of the droplet on solid surface. In the present study, the wettability of the nanofluids in the presence of natural surfactants on hair surface was studied and presented in Fig. 6. The equilibrium surface tension values in the presence of nanoparticles are already presented in Table 2.

.



Fig. 6. The plot of contact angle (θº) with logarithmic concentration (log c) of Acfor in-situ synthesized Ag NPs; and Ac, Rh, and Sh in the presence of chemically synthesized Ag NPs on hair surface. The variation of θº vs. the logarithm of the total concentration of surfactants in aqueous solution in the presence of Ag NPs is depicted in Fig. 6. For in-situ synthesized Ag NPs in Ac, the contact angle reduces from 84.53º to 66.0º with the increasing concentration of Ac solution till 0.7 mM/L (log c = 0.15), for chemically synthesized Ag NPs, the contact angle reduces to 64.8º untill 0.6 mM/L (log c = 0.22) concentration of Ac solution. In the range of log c from -0.25 to 0, the values of the contact angles are almost constant, and they are minimal for a given surfactant. The wetting behaviour is consistent with that of the solution behaviour. The minimum contact angles with their corresponding natural surfactant concentrations in the presence of both in-situ and chemically synthesized Ag NPs are shown in Table 2. In case of wetting of solid surfaces by nanofluids, surface tension, surface roughness, substrate-liquid adhesion tension, and ∏h are crucial parameters which in turn directly or indirectly influence the wetting process. The drop views on hair surface under different conditions and the TEM images of hair surface with Ac mediated in-situ Ag NPs are presented in Fig. 7. The TEM images (Fig. 7e-g) clearly show that the Ag NPs are deposited on hair surface from the suspension.


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Fig. 7. Optical contact angle images of water droplet (a) on untreated hair, (b) on ethanol treated hair, (c) droplet of pure Ac solution at CMC (0.53 mM/L) on ethanol treated hair, (d) droplet of Ac solution (0.7 mM/L) in the presence of in-situ synthesized Ag NPs, (e) TEM image of hair surface after drying of a droplet of Ac solution (0.7 mM/L) in the presence of in-situ Ag NPs, (f,g,h) Dark field TEM image, Ag elemental mapping and EDX spectra at hair surface with Ac mediated Ag NPs , respectively. In the presence of surfactant solutions reduction of contact angle compared to pure water is mainly because of lowering of surface tension as well as adsorption of surfactant molecules on hair surface; when the surfactant molecules adsorbed on the hair surface, the surface become more hydrophilic. The natural surfactant based Ag nanofluids lead to a further decrease in contact angle on the hair surface. In the Ac media chemically synthesized Ag NPs show slight better wettability compared to that of in-situ, and the results are also consistent with the surface tension. The reasons of lowering of contact angles in the presence of nanoparticles compared to that of pure surfactants are attributed to: (i) lowering of surface tension, which in turn reduce the contact angle as per Young’s equation, (ii) deposition of NPs on the hair surface because of favorable van der Waals attactive force that may increase the roughness(as shown in Fig. 7e) and subsequently the wetting property, and (iii) finally the ∏h at the three phase contact line. ∏h is the distance dependent interaction (attractive or repulsive) between two surfaces. A negative ∏h attracts two interfaces. Among electrical (e), molecular (m), and structural (s) components of ∏h,the last component is significant here. The structural disjoining pressure (∏s) depicts the long range structural forces originates from the organization of the suspended nanofluid’s particles in the wedge of the droplet, mainly because of the van der Waals force. According to theoretical predictions, ∏s varies on the size and the number of the deposited particle layer near to the triple line, which in turn depends on the particles concentration within the suspension. In general, the ∏h is maximum when the deposited particles form as a single layer which favors the wetting process by decreasing contact angle. In this study as the particle concentration is low, there is a chance of formation single layer at the wedge of the droplet. So, all these factors collectively help in reduction of contact angle on hair surface using natural surfactant + Ag NPs mixture compared to that of individual pure surfactants. The decreasing order of contact angle values in different systems are: Sh (chem) < Rh (chem) < Ac (chem) < Ac (in-situ). As the natural surfactants are having inferior interfacial property compared to that of widely used synthetic surfactants and their shelf-life is also short in aqueous media because of microbial attack, both of these properties can be improved in the presence of Ag NPs. The contact angle value of Sh in the presence of Ag NPs



(58.5) is close to that of TX-100 (57.2). In spite of the fact that Ac is slight inferior in terms of surface tension reduction and wetting, in-situ Ag NPs synthesis ability of this surfactant may be advantageous by eliminating the chemical synthesis stem in the process. Since Ag NPs have very good inhibitory activity against Malassezia furfur, a dandruff causing fungus, the inferences of this this report may be helpful for the formulation of plant surfactant based anti-dandruff shampoo. The effect of particle size on wetting was not studied here extensively, however, the reported literatures proved that the surface tension decreases and contact angle increases with the decreasing particle size.25,40 In general, wetting behaviour of nanofluids is a complex phenomenon, and depends on the types of particles (hydrophilic/hydrophobic), types of surfaces, and particles surface modification, etc. The ∏h is very important among several factors affecting the wetting by nanofluids. In case of larger size particles, the chances of formation of single layer of deposited particles are more near to the triple line of the droplet; and for smaller size particles the deposited particles may form a multi-layer. As mentioned before, the development of ∏h is maximum in the area of the single layer deposited particles, which in turn favors wetting process by lowering the contact angle. Because of this reason, better wetting of chemically synthesized Ac mediated Ag nanofluid was observed compared to that of in-situ.

Conclusions The solution behaviours of three natural surfactants, Ac, Rh and Sh showed a substantial reduction in surface tension (71.5 to 43.56, 38.29, 38.71 mN/m) till their respective CMC values. The change in contact angle values on human hair surface using these surfactants and a nonionic synthetic surfactant (TX-100) were studied and found the changes from 105± 5º (pure water) to 57.2º, 64.4º, 70.5º, and 71.3º for TX-100, Sh, Rh, and Ac respectively at their respective CMC values. The present study showed inferior wetting properties of pure natural surfactants as compared to that of synthetic one, so the wettability of human hair was studied experimentally using different natural surfactants solutions in the presence of Ag NPs. Among these surfactants, Ac has ability to produce Ag NPs (24.17 nm) in-situ from AgNO3 of smaller particle size than that of the chemically synthesized particles (30 nm). In the Ac media, chemically synthesized Ag NPs confirmed slight better wettability (~ 64.8º) compared to that of in-situ (~ 66º), where the results were consistent with the surface tension. Among three studied natural surfactants, Sh in the presence of Ag NPs has shown lowest contact angle value (~58.5º), which is comparable to that of widely used synthetic surfactant TX-100 (~57.2º).


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The combination of both natural surfactant and nanofluids showed a significant change in the wetting properties on hair surface by the influence of surface tension, surface roughness, adhesion tension between substrate and liquid, and ∏h at solid-liquid interface. In spite of the fact that Ac is slight inferior in terms of surface tension reduction and wetting, in-situ Ag NPs synthesis ability in Ac solution is advantageous by eliminating the chemical reducing agent in the process. In addition to the better interfacial and wetting behaviors of natural surfactants based Ag nanofluids, the enhancement in shelf-life of the natural surfactants by inhibiting the susceptibility towards microbial attack, and anti-dandruff activity of Ag NPs are advantageous from the practical application perspectives. Thus, the present study may be useful for the natural surfactant based anti-dandruff shampoo formulation.

Associated Content Supporting Information Structures of three plant surfactants, Reetha, Shikakai and Acacia, FT-IR spectra of pure extracts of Reetha, Shikakai and Acacia, FE-SEM images of in-situ Ag NPs, chemically synthesized Ag NPs, hair surface with Acacia mediated in-situ Ag NPs and its high magnification FE-SEM image, DLS analysis of chemically synthesized AgNPs before and after dispersing with surfactant solutions), HTEM images of chemically synthesized Ag NPs and after dispersing in surfactants medium with corresponding histograms of particle size distribution, and nomenclature list



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TOC Graphic:

Synopsis: The enhanced shelf-life and wettability of natural surfactants on human hair surface in the presence of Ag NPs.


Natural Surfactants-Based Ag Nanofluids for Enhanced Wettability on

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