The adsorption of alkyl-dimethyl-benzyl-ammonium chloride onto cotton nonwoven hydroentangled substrates at the solid–liquid interface is minimized by additive chemistries

Abstract

Quaternary ammonium compounds, commonly referred to as quats, are cationic surfactants widely used as the active biocidal ingredient for disposable disinfecting wipes. The cationic nature of quats results in a strong ionic interaction and adsorption onto wipes materials that have an anionic surface charge, such as cellulosic materials, including cotton. The degree of adsorption of quats onto cotton nonwovens is affected by pretreatment of the substrate, more specifically whether it is a greige or a scoured and bleached fabric. This study examined the effect of varying the chemical and physical properties of solutions on the adsorption of the quat alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) onto greige and scoured and bleached cotton nonwoven fabrics produced by hydroentanglement. At a constant surfactant concentration, the liquor ratio, pH, temperature, and concentrations of various electrolytes in the solution were varied and the amount of ADBAC depleted from solution was determined over time. The results suggested that a more alkaline solution increased the amount of ADBAC adsorbed onto both cotton nonwoven fabrics, while a more acidic solution reduced ADBAC adsorption. Likewise, increasing the temperature and concentration of salts in the solution reduced the adsorption of ADBAC onto the cotton fabrics. The presence of nonionic surfactants or low molecular weight quats also reduced ADBAC adsorption onto cotton fabrics in a concentration-dependent manner. The results of this study will provide guidance for optimized chemical formulations compatible with disposable disinfecting cotton-based wipes, cloths, and other cotton-containing implements intended for use in cleaning and disinfecting applications.

Keywords

adsorption cationic surfactant greige cotton hydroentangle nonwovens quats

The wipes market represents one of the fastest growing areas in nonwoven textiles. Based on statistics from the Association of Nonwoven Fabrics Industry (INDA), in 2012 the North American wipes market represented 17.1% of the overall nonwovens market, which is broken down into 27% baby wipes; 26% household wipes; 10% personal care; and 36% industrial and institutional.¹ The overall wipes market is projected to grow at a compound annual growth rate (CAGR) of 6.65% over the period 2014–2019, with rising demands for personal hygiene products and disposable medical products as major drivers of the market.² Household wipes represent a broad range of uses, including consumer antibacterial, sanitizing, and antiseptic wipes, hospital antiseptic wipes, consumer disinfecting wipes, hospital disinfecting wipes, and indirect food contact wipes.

The predominant types of synthetic filaments cut to staple fibers that are used in the manufacturing of nonwovens are polypropylene (PP), polyester (PES), and rayon, which comprise approximately 25%, 30%, and 9% of global nonwoven fiber consumption, respectively.³ Cotton and other natural cellulosic fibers represent a small market share of the global nonwovens industry at approximately 3%,³ but have benefits compared to synthetic fibers, including high wet strength, high liquid absorptive capacity, biodegradability, sustainability as a renewable resource, comfort, and breathability. In textiles, breathability is the ability of a fabric to allow moisture vapor to be transmitted through the material. Staple fibers that are sustainable and biodegradable represent a growing trend in the disposable wipes market as consumers become more aware of the environmental impacts of household products and are also forecast to be a driving force in the future global wipes market.² The increased demand for flushable wipes is also a driving force for biodegradable wipes substrates. It was previously demonstrated that cotton and rayon nonwoven fabrics exhibited half-life values of only 12.6 and 7.6 days, respectively, when buried in an experimental plot consisting of Captina soil. This was confirmed in the laboratory using a modified American Association of Textile Chemists and Colorists (AATCC) Test Method.^4,5 Under the same soil and laboratory test conditions, nonwoven fabrics composed of polylactic acid (PLA) and PP exhibited some loss in strength due to oxidation, but no weight decrease associated with degradation.

Some of the challenges to integrating natural fibers into disposable wipes faced by the industry include the price volatility of both natural staple fibers and petroleum-based staple fibers and overhead costs associated with using one type of fiber over another. As the cost of petroleum-based fibers falls based on current oil prices, it becomes more cost prohibitive to integrate natural fibers such as cotton, especially scoured and bleached cotton fibers, that sell at much higher prices than raw cotton. Raw greige cotton is much more competitively priced relative to petroleum-based fibers, but contains visible foreign material referred to as trash content, which is comprised of plant material such as leafs, stems, and bract tissues. The trash content of cotton fiber was also demonstrated to serve as a primary source of microbial burden in raw cotton fibers,^6–9 which is restrictive for hygiene applications under US and International Pharmacopeia guidelines.^10–12 However, it was previously demonstrated that the hydroentanglement process utilized in producing nonwoven wipe substrates eliminates or reduces microbial burden levels below Pharmacopeia acceptance levels for non-sterile hygiene applications.¹³ Currently, two US companies are offering raw cotton fibers that are processed or reclaimed such that the majority of visible trash content is removed. The TJ Beall Company (Greenwood, MS) produced a mechanically cleaned raw cotton referred to as True Cotton®, while Barnhardt Natural Fibers Group (Charlotte, NC) offers a product named UltraClean Comber that are short staple raw cotton fibers that are reclaimed comber noils from the textile spinning process.^14,15

The antimicrobial agents used with nonwoven disposable disinfecting wipes must be registered as pesticides and are strictly regulated by the US Environmental Protection Agency (US EPA) under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA).¹⁶ The most commonly used and widely available antimicrobial agents approved by the EPA are quaternary ammonium-based compounds, known simply as quats. Quats are classified based on chemical and anti-microbiological characteristics and clustered into five categories as follows: monoalkonium halides, dialkonium halides, benzalkonium halides, diesteralkonium halides, and pyridalkonium halides.¹⁷ Quats, particularly those containing alkyl chain lengths of 12–18 carbon atoms, have been widely used as disinfectants and are considered lytic biocides based on their mode of action, and are effective against over 99% of microorganisms at typical application concentrations.¹⁷ Compared with other disinfecting compounds, quats are low cost, effective over a wide pH range from very acidic to very basic, have good cleaning capability, low odor, low skin irritation, and excellent storage stability.¹⁸ Quats are cationic surfactants and carry a positive charge at the N atom. Bacterial cell walls have net negative surface charges that result in ionic interactions with the quats. The generally accepted mechanism of action proposed for lytic biocides on microorganisms entails the following sequence of events: (1) adsorption and penetration of the cell wall; (2) interaction with the plasma membrane, reaction with membrane-associated proteins, and membrane disorganization; (3) leakage of low molecular weight metabolites and ions; (4) degradation of proteins and nucleic acids; and (5) cell lysis by autolytic enzymes.¹⁹ The optimal quat alkyl chain lengths for bactericidal activity are 12–16, with activity against gram-positive bacteria maximized at chain lengths of 12–14, and maximized at chain lengths of 14–16 for gram-negative bacteria.²⁰ Among the different types of quats available for use as disinfectants, alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) is regarded as a first-generation quat and considered by the US EPA to be the model compound for quat Reregistration Eligibility Decision (RED) data development.¹⁶

In addition to price volatility, another major hindrance to the use of cotton- and other cellulosic-based nonwovens with quats is a strong ionic attraction between the negative surface charge of cellulosic substrates and the positive charge of the cationic surfactant.²¹ This results in depletion or removal of the quat from bulk solution onto the substrate, effectively depleting the quat from solution and lowering the effective parts per million that reach a hard surface for disinfection. This depletion effect is exacerbated in the presence of a greige cotton wipe substrate due to the presence of non-cellulosic compounds such as pectin.²² Extensive research has been conducted on imparting functional antimicrobial finishes to cotton fabrics since the early 1960s. The majority of this research, however, has focused on durable and semi-durable antimicrobial finishes that utilize zirconium-based antimicrobial agents, telluride, zinc pyrithione, conjugated enzymes, or silver (Ag) compounds and nanoparticles (NPs).^23–30 Since these antimicrobial finishes are often bonded to the cotton, these materials cannot be used to disinfect hard surfaces as this requires depletion of the antimicrobial agent onto the surface. However, the novel mechanism of forming Ag NPs in situ in mercerized cotton fibers was shown to allow depletion of Ag(I) into aqueous surroundings, which merits further investigation into the possibly of using Ag NPs for hard surface disinfection.³¹ The relatively low cost and effectiveness of quats in disinfecting solutions continues to make them a mainstay for disinfecting wipe manufacturers. Developing cost effective chemical formulations that can minimize or nullify the ionic interaction between quats and cellulosic substrates has the potential to increase the use of natural staple fibers, such as cotton, in disposable disinfecting wipes.

The adsorption of a surfactant onto a substrate can occur through various mechanisms, including ion exchange, ion pairing, acid–base interaction, adsorption by polarization of electrons, adsorption by dispersion forces, and hydrophobic bonding.^32,33 The nature of this interaction is largely dependent upon the characteristics of both the substrate and the surfactant, such as surface charge of the substrate; charge associated with the surfactant (e.g. cationic, anionic, or nonionic); the nature of the surfactant hydrophobic group (i.e. chain length, straight or branched, aliphatic, or aromatic); and the nature of the aqueous phase (e.g. pH, electrolytes, short-chain polar solutes, and temperature).^32,33 These criteria were utilized in the selection of variables used to modify the quat surfactant solution with the objective of blocking the strong surfactant adsorption of quats onto cellulosic substrates, thereby minimizing or nullifying depletion of the quats from a disinfecting solution when used with a cellulosic-based wipe.

Here we report the effects of pH, liquor ratio, temperature, and the concentration of various electrolytes in solution on the adsorption of ADBAC onto greige and scoured/bleached cotton nonwovens. The effect on ADBAC adsorption was also evaluated in the presence of additional compounds, including the low molecular weight quat tetra-methyl-ammonium chloride (TMAC) ethanol, and nonionic surfactants. The surface excess adsorption of the biocidal compounds onto the cotton was determined by ultraviolet-visible (UV-Vis) spectroscopy. The results indicate it is feasible to modify the chemical composition of an ADBAC disinfecting solution in a manner that minimizes or nullifies the quat adsorption onto a cellulosic substrate, allowing the quat to effect biocidal activity onto a hard surface with no loss of efficacy.

Methods

Opening, carding, and needlepunching

The greige cotton fibers were processed through the opening line of the Southern Regional Research Center (USDA-ARS-SRRC, New Orleans, LA) textile mill, which consisted of a hopper, superior cleaner, fine opener, and reserve hopper manufactured by Fiber Controls (M&M Electric Service Inc., Gastonia, NC). Cotton fibers were chute fed to a Crosrol, Mark IV tandem card (Crosrol UK Limited, Bradford, UK), which delivered a fiber web of approximately 11 g m⁻² that fed directly into a commercial crosslapper (Technoplants srl., Pistoia, Italy). The card web was crosslapped 20 times and fed directly into a needlepunch machine (Technoplants srl.) for light needling. The needles used were 3 barb, 9 cm conical needles (Groz-Beckert KG, Albstadt, Germany). The needling impact on the crosslapped fiber web was 130 points cm⁻² and the strokes were 490 min⁻¹. The resulting NP fabrics were 1 m in width with a fabric weight of approximately 70 g m⁻².

Hydroentanglement

The lightly needlepunched fabrics were converted to hydroentangled (H-E) fabrics on a 1 m wide Fleissner pilot-scale H-E system (Trützschler Nonwovens GmbH, Dülmen, Germany) running at a constant production speed of 5 m min⁻¹. The H-E system utilized three pressure heads: one low pressure for fabric wet-out maintained at a constant pressure of 50 bar during roll goods production; and two high pressure heads both maintained at 100 bar during roll goods production. The specific energy of H-E was 6.8 MJ kg⁻¹, which was calculated as previously described.³⁴

Scouring and bleaching

A portion of the nonwoven fabrics produced from the H-E process were subjected to scouring and bleaching using the following procedures. Fabrics were placed in an overflow jet dyeing system type JFO (Werner Mathis AG, Oberhasli, Switzerland) with deionized water with 1.93⁻⁴ M Triton™ X-100, 0.05 M NaOH, 6.60e⁻³ M Na₂CO₃, 1.30e⁻³ % (v/v) sodium silicate 42^BE, and 0.07 M H₂O₂ at a liquor ratio of 22:1 (ml liquid:g fabric) and circulated for 90 min at 100℃. Fabrics were then rinsed for 20 min three times with deionized water then boiled off for 20 min at 100℃ and neutralized with 4.16e⁻³ M glacial acetic acid for 10 min, then rinsed three more times with deionized water. Excess liquid was removed from the fabrics with a HVF horizontal/vertical padder (Werner Mathis AG).

Quat solution parameters

The ADBAC was purchased as an aqueous solution containing 50% ADBAC (Sigma-Aldrich, St. Louis, MO). A stock solution of 0.625 g L⁻¹ ADBAC was prepared by adding 1.25 g of the ADBAC solution to a 1 liter volumetric flask and brought to volume by adding deionized water. Unless otherwise specified, the nonwoven cotton fabrics were immersed into surfactant solutions for a total immersion time of 2 hours. The liquor ratio, pH, temperature, and electrolyte concentration of the solutions were varied while the ADBAC concentration remained constant at 0.625 g L⁻¹. To vary the liquor ratio, the mass of fabric placed in a 5 mL solution of 0.625 g L⁻¹ ADBAC was varied from 0.5 to 0.05 g to give a liquor ratio range of 1:10–1:100. The pH values of the surfactant solution were adjusted over the pH range of 3–11 by the addition of acetic acid or sodium carbonate. The temperature of the solutions was varied from 5℃ to 90℃ using an ice bath or a model PC-420D hot plate with digital temperature control (Corning Incorporated Life Sciences, Tewksbury, MA). To examine the effect of added electrolyte, several different monovalent and divalent electrolytes were added to the surfactant solution in varying amounts before the fabric was immersed into the bath to obtain a range of electrolyte concentrations of 0–1.0 weight percent electrolyte. The electrolytes used in this research include potassium chloride, sodium chloride, magnesium sulfate, sodium sulfate, calcium chloride, magnesium chloride, sodium citrate, and potassium citrate (Sigma-Aldrich). The three nonionic surfactants, polyoxyethylene (12) tridecyl ether (PEO-12, Sigma-Aldrich), polyoxyethylene (18) tridecyl ether (PEO-18, Sigma-Aldrich), and Triton™ X-100 (Sigma-Aldrich) were received as neat solutions. The concentration of nonionic surfactant in a 0.652 g L⁻¹ solution of ADBAC was varied from 0 to 0.1 M for each nonionic surfactant. To investigate the effects of low molecular weight quats on the adsorption of ADBAC, the concentration TMAC (Sigma-Aldrich) was varied from 0 to 20 mM in the standard 0.625 g L⁻¹ ADBAC formulation.

Quat depletion from the aqueous solutions

The concentration of ADBAC in the bulk solutions was determined by comparing the UV spectra of the solution at the end of each experimental run with that of a standard solution. Calibration curves were constructed using a Cary® 50 UV-Vis spectrophotometer (Agilent Technologies, Santa Clara, CA) to measure the absorbance at 263 nm of dilute solutions of ADBAC in deionized water. The percentage of ADBAC depleted from the bulk solution, and thus absorbed onto the cotton nonwoven fabric, was calculated by Equation (1):

% E = (\frac{A_{0} - A_{t}}{A_{0}}) \times 100

(1)

where %E is the percentage depletion of ADBAC at time t, A₀ is the absorbance of the ADBAC solutions at the beginning, and A_t the absorbance of the ADBAC solution at time t.

Results and discussion

Previous research conducted on ADBAC adsorption onto nonwoven cotton fabrics indicated that greige cotton substrates adsorb more of the cationic surfactant than scoured and bleached cotton substrates of similar weight and structural composition.^22,35 As the concentration of ADBAC in the bulk solution was increased at a constant liquor ratio of 20:1, the total fraction of ADBAC adsorbed per gram of cotton material increased before reaching a plateau at approximately 1.25 g L⁻¹ ADBAC coinciding with the critical micelle concentration (cmc) of ADBAC.³⁵ Another important variable to be considered in the adsorption process is the liquor ratio. The effect of liquor ratio on the adsorption of ADBAC onto cotton nonwovens was investigated by varying the weight of substrate added to a constant volume of 0.625 g L⁻¹ ADBAC in deionized water. Data showing the percentage of ADBAC depleted from the immersion bath onto the nonwovens as a function of liquor ratios, ranging from 10:1 to 100:1, are presented in Figure 1. As the liquor ratio is increased, the total percent of ADBAC depleted from the solution after 2 hours of immersion decreases. A total immersion time of 2 hours was chosen based on the results of previous work that showed no significant change in the adsorption of ADBAC was observed after this time for a range of experimental runs.³⁵ The fact that decreasing the liquor ratio increases the amount of ADBAC depleted from the bath is predictable, since at lower liquor ratios more of the substrate is present in the standard aqueous solution of 0.625 g L⁻¹ ADBAC. When less substrate is added to the surfactant solution, as is the case with increasing liquor ratio, less ADBAC will be depleted from the bulk solution since there is less of the substrate available to adsorb the quat from solution.

Figure 1.

Percent of alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) depleted from the bath at a constant concentration of 0.625 g L⁻¹ ADBAC as a function of liquor ratio for cotton nonwoven fabrics. Data was collected after the substrate was immersed for 2 hours at 25 ℃ with a pH of 7.

When the depletion data was normalized to the amount of substrate in the bath, the amount of surfactant adsorbed per gram of cotton substrate initially increased as the liquor ratio increased (Figure 2). The increase was much more significant in the greige cotton nonwoven than in the scoured and bleached cotton nonwoven. The solid phase surfactant concentration of the bleached samples began to level out at a liquor ratio of 40:1; however, the greige samples did not appear to reach a plateau value until after a liquor ratio of 70:1. Interestingly, previous work indicated the adsorption of ADBAC reached an equilibrium after 30 minutes at a concentration of 0.625 g L⁻¹ ADBAC and a liquor ratio of 20:1; however, the adsorption data for 0.625 g L⁻¹ ADBAC at a liquor ratio of 100:1 did not approach equilibrium until after 60 minutes in both scoured and bleached and greige cotton nonwovens.³⁵ The data in Figure 2 indicates that changing the liquor ratio shifted the equilibrium of the system so that more ADBAC was depleted onto the nonwovens at equilibrium when the liquor ratio was increased. The adsorption of ADBAC onto bleached cotton nonwovens appeared to be less dependent on the liquor ratio than the greige samples. In a previous study, it was observed that adsorption kinetics for both greige and scoured and bleached cotton substrates followed a pseudo-first-order kinetic model; however, greige cotton nonwovens lacked a distinguishable Region I typically observed on adsorption isotherms, presumably due to high equilibrium adsorption capacity.³⁶ Although further investigations are needed, a plausible explanation for this observation and for the trend observed in Figure 2 is the unique surface properties of greige cotton H-E nonwovens resulting from the energy exerted on the substrate by the H-E process. This includes visible fiber fibrillation, which results in increased surface area of the substrate, partial removal of epicuticular waxes, and exposure of retained pectin.^36,37 The retained surface wax on the greige cotton could also contribute to ADBAC adsorption through hydrophobic interaction with the alkyl chain of the quat. The main components found in mature cotton fiber cuticular waxes include mixtures of primary n-alcohols (C26–C36) and fatty acids (C16–C36).³⁸ Since nonpolar substances tend to aggregate in an aqueous solution (hydrophobic effect), these alkyl chains would have a strong hydrophobic interaction with the alkyl chain of the ADBAC, which is a mixture 12–16 carbons in length.³²

Figure 2.

Weight of alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) per gram of nonwoven fabrics exposed to a constant concentration of 0.625 g L⁻¹ ADBAC at varying liquor ratios. Data was collected after the substrate was immersed for 2 hours at 25 ℃ with a pH of 7.

When immersed in aqueous solutions, cotton exhibits a negative electric surface charge and, as a result, the nonwovens in this study readily adsorbed the positively charged ADBAC molecules. This electric surface charge, however, is partially dependent on the pH of the solution. In solutions of lower pH, the negative surface charge of cotton becomes less pronounced, whereas increasing the pH increases the magnitude of the negative surface charge.²³ The results indicated that as the pH of the solution is increased, more of the ADBAC is depleted from the immersion bath onto the nonwoven fabrics (Figure 3). Lowering the pH of the solution minimized the overall negative charge of the cotton surface, resulting in less of the ADBAC being depleted from the immersion bath. According to the data in Figure 3, varying the pH of the solution had a greater effect on scoured and bleached cotton than greige cotton. Increasing the pH from 3 to 11 increased the amount of ADBAC depleted from the bath by a factor of 5, whereas the same increase only results in a factor 2 difference in greige cotton substrates. This observation was attributed to greige cotton's waxy surface coating and a surface charge that is less susceptible to changes in pH. Exposed carboxyl functional groups of the retained pectin on greige cotton could also require higher levels of protonation under the same acidic pH conditions to neutralize the overall negative surface charge.

Figure 3.

Percent of alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) depleted from the bath at a constant concentration of 0.625 g L⁻¹ ADBAC as a function of solution pH for cotton nonwoven fabrics. Data was collected after the substrate was immersed for 2 hours at 25 ℃ with a liquor ratio of 20:1.

In addition to pH, temperature is an important factor affecting the diffusion, adsorption, and possibly the affinity of the quat on the cotton nonwoven substrates. Figure 4 shows the percent of ADBAC depleted from each solution after 2 hours of immersion time as a function of temperature. The maximum amount of ADBAC depleted onto both greige and bleached substrates was found to occur at 5 ℃ and the efficiency decreased with increasing temperature. One explanation for this observation is that as the temperature was increased, the equilibrium of the system was shifted.³² As a result, the amount of ADBAC depleted from the bath is lower when the system is run at high temperatures. Bleached cotton adsorbs 60% less ADBAC at 90 ℃ than it does at 5 ℃, while greige cotton absorbs 85% less ADBAC over the same temperature range. The temperature having a greater effect on the greige substrate may be a result of the surface waxes of the primary wall of greige cotton melting at higher temperatures. Depending on species and composition, plant cuticular waxes can melt completely from 58 ℃ to 90 ℃, which was within the temperature range examined.³⁹

Figure 4.

Percent of alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) depleted from the bath at a constant concentration of 0.625 g L⁻¹ ADBAC as a function of solution temperature for cotton nonwoven fabrics. Data was collected after the substrate was immersed for 2 hours in a bath with pH of 7 and a liquor ratio of 20:1.

At a low amount of adsorbed cationic surfactant, decreased adsorption onto a cellulose substrate with increasing salt concentration was previously demonstrated.⁴⁰ This was explained as a screening effect on the negative cellulose surface charge and the positive surfactant head group charge as the ionic strength of the solution increased with added electrolytes, in this case KCl.³² Similar results were observed with adsorption of the cationic surfactant sodium p-3-nonylbenzene sulfonate (SNBS) onto rutile (TiO₂) at various concentrations of the electrolyte NaCl. In this case increasing the electrolyte concentration of the bulk solution exerted a screening effect as previously mentioned and resulted in a decrease in SNBS adsorption.⁴¹ In order to examine the effect of added salts on the adsorption of ADBAC onto cotton substrates, various electrolytes with mono- and divalent cations were included. Figures 5 and 6 show the effect of varying the weight percent of salts added to aqueous solutions of 0.625 g L⁻¹ ADBAC on percent of ADBAC depleted from the bath when bleached and greige cotton nonwovens, respectively, were immersed in the solution for 2 hours. Figures 5 and 6 clearly illustrate how the screening effects of added electrolytes reduce the total amount of ADBAC depleted from the bath. In aqueous-based systems, the added electrolyte disassociates into positively (cationic) and negatively (anionic) charged ions. The cationic ion of the added electrolyte readily interacts with the negatively charged cotton fiber, decreasing the overall negative potential of the cotton nonwoven surface. As seen in Figures 5 and 6, this decrease in negative potential reduces the amount of ADBAC depleted onto the substrates by screening the electrostatic interactions between the negatively charged cellulose surface and the positively charged head group of the surfactant. In addition to the screening effect at the surface, the anionic ions of the added electrolyte interact with the positively charged ADBAC, thus reducing the quats' affinity for the cotton nonwoven.

Figure 5.

Percent of alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) depleted from the bath at a constant concentration of 0.625 g L⁻¹ ADBAC as a function of added electrolyte concentration for scoured/bleached cotton nonwoven fabrics. Data was collected after the substrate was immersed for 2 hours at 25 ℃ with a pH of 7 and a liquor ratio of 20:1. KCl: potassium chloride; NaCl: sodium chloride; MgSO₄: magnesium sulfate; Na₂SO₄: sodium sulfate; CaCl₂: calcium chloride; MgCl₂: magnesium chloride; K₃C₆H₅O₇: potassium citrate; Na₃C₆H₅O₇: sodium citrate.

Figure 6.

Percent of alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) depleted from the bath at a constant concentration of 0.625 g L⁻¹ ADBAC as a function of added electrolyte concentration for greige cotton nonwoven fabrics. Data was collected after the substrate was immersed for 2 hours at 25 ℃ with a pH of 7 and a liquor ratio of 20:1. KCl: potassium chloride; NaCl: sodium chloride; MgSO₄: magnesium sulfate; Na₂SO₄: sodium sulfate; CaCl₂: calcium chloride; MgCl₂: magnesium chloride; K₃C₆H₅O₇: potassium citrate; Na₃C₆H₅O₇: sodium citrate.

Based on the results shown in Figures 5 and 6, NaCl had the least effect on the depletion of ADBAC in solutions containing bleached and greige cotton nonwovens, while potassium citrate had the greatest effect. Increasing the ionic strength of the solution also greatly reduced the overall amount of ADBAC depletion from the bath in the presence of cotton nonwovens. Increasing the concentration of NaCl in solution to 1.0 wt. % virtually eliminated the adsorption of ADBAC by bleached cotton and decreased adsorption in greige cotton by a factor of 1.6. The addition of potassium chloride eliminated the adsorption of ADBAC in bleached samples at a concentration below 0.75 wt. % and reduced the adsorption onto greige samples by a factor of 4 at 1.0 wt. %. Comparing Figure 5 to Figure 6, it is clear that the effect of electrolyte screening was less pronounced for the bleached substrate. This result further supports the theory that the hydroentanglement process affects adsorption equilibrium onto the greige cotton substrate by changing the surface nature of the cotton fibers through fibrillation, partial wax removal, and exposure of pectin.^36,37 The resultant higher surface potential of H-E greige cotton nonwoven fabrics compared to scoured and bleached cotton nonwoven fabrics is a function of increased surface area and exposure of charged carboxyl and hydroxyl functional groups in the pectin structure causing a more rapid initial adsorption and the lack of a clearly defined Region I on adsorption isotherms.³⁶ Region I is typically defined by electrostatic interactions between the quat and the substrate surface whereby the quats are rapidly adsorbed onto the surface as monomers. This is followed by neutralization of the surface charge in Region II. The hemimicelle concentration (hmc) of the quat denotes the onset of Region III. At this point in the isotherm, hydrophobic interactions between the quat in solution and the quats adsorbed on the surface leads to a rapid increase in the rate of adsorption per added quat. The higher electrostatic surface potential of the greige cotton fabric may result in more rapid adsorption in Region I that transitions quickly to Region II. This was previously observed using the kinetics of adsorption study in the regions with the constant adsorption acceleration (KASRA) model in which the magnitude of adsorption acceleration on greige cotton was greater than on scoured and bleached cotton by a magnitude of 5.6.³⁶

The effect of short-chain polar compounds on ADBAC adsorption onto cotton nonwovens was examined using the low molecular weight quat TMAC, which has a positively charged headgroup that can interact with the negatively charged surface of the cotton substrate by ion pairing. This interaction would theoretically result in competing adsorption with the ADBAC onto the cotton substrate. In addition, studies have shown that adding a charged molecule to an ionic surfactant solution generally lowers the cmc of the ionic surfactant in solution, making it less likely to adsorb onto the surface since adsorption is a function of individual surfactant molecules, not micelles. The screening effect of TMAC on adsorption of ADBAC onto greige and scoured and bleached cotton nonwoven fabrics was even more pronounced at low concentrations (<2 mM) of the small quat (Figure 7). The ADBAC adsorption onto scoured and bleached cotton substrates was completely nullified at ∼4 mM TMAC, while much higher concentrations (∼18 mM) were required to eliminate adsorption of ADBAC onto greige cotton nonwoven substrates. This could once again be explained by the unique surface properties and higher negative surface charge of the greige cotton H-E substrates.³⁷

Figure 7.

Percent of alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) depleted from the bath at a constant concentration of 0.625 g L⁻¹ ADBAC as a function of tetra-methyl-ammonium chloride (TMAC) concentration for cotton nonwoven fabrics. Data was collected after the substrate was immersed for 2 hours in a bath with pH of 7 and a liquor ratio of 20:1.

Cationic surfactants, such as the quat ADBAC, are known to be incompatible with anionic surfactants in terms of biocidal activity, but ADBAC maintains biocidal activity when co-formulated with nonionic surfactants.⁴² Nonionic surfactants can also add improved cleaning and degreasing capabilities to a quat-containing disinfecting formulation and potentially inhibit loss of quat efficacy by high organic soil loads on a hard surface.^42–44 Specific nonionic surfactants, including polyoxyethylene derivatives, also exhibit excellent biocidal activity on their own and could add to the efficacy of a disinfecting solution.⁴⁵ In addition, nonionic surfactants were shown to reduce the adsorption of cationic surfactants onto charged surfaces^46,47 and would theoretically reduce ADBAC depletion onto cotton nonwovens substrates. In one study, adsorption of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) onto silica in aqueous solution was decreased in the presence of polyoxyethylene, and further decreased with higher molecular weight PEOs.⁴⁷ This observation was attributed to competitive adsorption onto silica by both surfactants. In a second study, a decrease in the adsorption of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) onto polytetra fluoroethylene (PTFE) was observed when co-formulated with nonyl phenyl ethoxylates (NP-n) nonionic surfactant containing either 13, 20, or 30 ethoxylate units (NP-13, NP-20, and NP-30).⁴⁶ The greatest reduction in adsorption occurred in CTAB/NP-30 co-formulations at CTAB concentrations above the cmc and was attributed to mixed micellization with the nonionic surfactant and a consequent decrease in monomer concentration. However, it was also noted that CTAB adsorption onto PTFE was enhanced at concentrations below the cmc in the presence of the nonionic surfactant.⁴⁶ Extensive research on the synergistic effects of the adsorption of cationic–nonionic mixtures onto various surfaces has been conducted and thorough reviews of the literature are available.^48,49 In the current study, three nonionic surfactants were selected for co-formulation with ADBAC, all of which are polyoxyethylene derivatives (PEO-12, PEO-18, and Triton™ X-100). In all three co-formulations of ADBAC with the nonionic surfactants, the adsorption of ADBAC on both greige and scoured and bleached nonwoven H-E fabrics decreased with increasing concentrations of the nonionic surfactant, as indicated by the reduction in percent ADBAC depleted from the bulk solution (Figures 8 –10). Co-formulation of ADBAC with PEO-12 and PEO-18 resulted in the greatest initial reduction in the depletion of the cationic surfactant from bulk solution at low concentration (1.0E⁻⁴ M) of the cationic surfactants. Co-formulations of ADBAC with either PEO-12 or PEO-18 resulted in complete nullification of ADBAC depletion from bulk solution onto both cotton substrates at 0.01 M concentration (Figures 8 and 9).

Figure 8.

Percent of alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) depleted from the bath at a constant concentration of 0.625 g L⁻¹ ADBAC as a function of polyoxyethylene (12) tridecyl ether (PEO-12) concentration for cotton nonwoven fabrics. Data was collected after the substrate was immersed for 2 hours in a bath with pH of 7 and a liquor ratio of 20:1.

Figure 9.

Percent of alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) depleted from the bath at a constant concentration of 0.625 g L⁻¹ ADBAC as a function of polyoxyethylene (18) tridecyl ether (PEO-18) concentration for cotton nonwoven fabrics. Data was collected after the substrate was immersed for 2 hours in a bath with pH of 7 and a liquor ratio of 20:1.

Figure 10.

Percent of alkyl-dimethyl-benzyl-ammonium chloride (ADBAC) depleted from the bath at a constant concentration of 0.625 g L⁻¹ ADBAC as a function of Triton™ X-100 concentration for cotton nonwoven fabrics. Data was collected after the substrate was immersed for 2 hours in a bath with pH of 7 and a liquor ratio of 20:1.

Conclusions and outlook

ADBAC is a cationic surfactant commonly used in the production of disposable nonwoven antimicrobial wipes capable of disinfecting hard surfaces. The use of cotton in such products is significantly hindered by the fact that cotton fibers adsorb quats and do not release the antimicrobial agents when in contact with hard surfaces. The depletion of the quat from bulk solution by adsorbtion is nearly double for greige cotton compared to scoured and bleached cotton substrates. This work has shown that the adsorption of ADBAC onto greige and scoured and bleached cotton nonwoven substrates can be controlled by varying the chemical and physical properties of the surfactant formulation. Decreasing the ratio of fabric to solution was found to decrease the total amount of ADBAC adsorbed onto the substrates. Increasing the temperature or decreasing the pH reduced the adsorption of ADBAC. Adding electrolyte to the surfactant formulation significantly reduced ADBAC adsorption onto both of the nonwoven substrates; however, this effect was more pronounced for the bleached substrate. Co-formulation with polyoxyethylene-derived nonionic surfactants also reduced the adsorption of ADBAC onto both cotton substrates in a concentration-dependent manner. The information presented in this work will guide the development of quat co-formulations that are compatible with disposable cotton-based nonwoven disinfecting wipes, cloths, and other cotton-containing implements intended for use in cleaning and disinfecting applications.

In order to minimize the potential costs associated with the additive chemistries, possible synergism in quat co-formulations with multiple added chemicals, and their effect on quat adsorption onto cotton substrates at the solid–liquid interface, are being examined. Studies are currently in the process of modeling ADBAC adsorption onto various wipe substrates using response surface methodology to develop optimized chemical formulations for use with cellulosic materials. A combination of electrolyte, low molecular weight quat, and nonionic surfactant concentrations in the same co-formulation with ADBAC is being examined during long-term storage using response surface methodology to develop an optimized co-formulation for cellulosic wipes. This research aims to pass standardized efficacy testing for pre-saturated towlettes for hard surface disinfection under the scrutiny of Good Laboratory Practice (GLP) standards required for US EPA registration, thereby providing guidelines for the wipes industry to utilize a large volume of cotton fibers with quat-based disinfecting solutions.

Footnotes

Acknowledgements

Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Agriculture (USDA). USDA is an equal opportunity provider and employer.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

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