Our report on the study of the solution properties of toothpaste can be a benchmark for evaluating their effectiveness towards cleansing action. All investigated toothpaste showed higher surface tension values in distilled water than tap water. The decrease in surface tension is proportional to the adsorbed amount of surfactant, as shown by the Gibbs adsorption isotherm. As we know, tap water contains salts whereas distilled water does not contain any salts. Adding salt reduces electrostatic repulsion between ionized head groups of surfactants, so they can pack together at the interface. Therefore, more surfactant is adsorbed and causes more reduction of surface tension . Our data of high surface tension of all investigated toothpaste in distilled water and low surface tension in tap water also coincide with most investigated toothpaste in the literature .
At their lowest experimental concentration, toothpaste solutions have been found with relatively high surface tension values and reverse with high concentration. The results showed an increasing trend of surface tension upon dilution of the toothpaste solution. This may also be due to the increase of the water portion and the formation of monolayer and ionic micelles at low concentrations [17, 31]. Such variations regarding high and low concentrations of toothpaste at surface tension were also found by Gordon and Shand . It was claimed by Gordon and Shand that the effectiveness of toothpaste is mainly due to friction between teeth and brushes, and the support of other substances involved is of negligible importance . Even a toothpaste with the lowest surface tension value cannot remove all the particles of debris that have been lodged between the cervices only. Hattiangdi et al. concluded that soapless detergents are better than surface cleaning agents due to their low surface tension values compared to soaps with low concentrations .
Despite the differences, both soap and toothpaste have surfactants aimed at reducing surface tension and require friction for cleaning. And yet, without them, no better cleanup is achieved. Surface tension studies should, therefore, be of importance in the cleaning action. Some studies of surface tension on toothpaste prove that low surface tension helps in better teeth cleaning . Lower surface tension is possible only in an aqueous medium that combines with toothpaste containing the surfactant.
The specific conductivity of toothpaste in distilled water was found to be lower than that in tap water, and this is probably due to the presence of ionic impurities in tap water which increases the mobility of ions. All investigated toothpaste has a specific conductivity reduction with dilution. The decrease in conductivity, along with dilution, is due to the replacement of ions by the colloidal particles, which, when conducting, have lower conductivity than the ions from which they are formed. Colloidal particles are thus formed in very low concentrations. The variation in conductivity of different toothpaste solutions at a certain concentration is likely due to the presence of many sizes and types of colloidal particles . Toothpaste (DSS) has shown the highest specific conductivity. The higher the conductivity, the higher will be the charge in the pertaining ions and the higher will be the charge neutralized thereby increasing the cleansing behavior of the substances.
The positive slopes of the order DSS > PD > CU > DR > DHG > DH > ANH > SKH was found in the variations of specific conductivity with toothpaste concentration in distilled water at room temperature, whereas there was no reasonable trend in the intercept and almost negative values in the case of DR (Figure S3). The reason is that the very dilute composition of toothpaste in DW has a very low value of specific conductivity and therefore the cleansing action is low. Similarly, in tap water solutions, the positive slope indicates strong ion-ion interactions in the toothpaste solutions . The high positive values of the intercepts indicate that the very dilute composition of toothpaste in tap water also has a relatively high value of specific conductivity and therefore the cleaning action is high .
Acidic toothpaste spoils the oral environment causing aciduric bacteria to cause dental caries. Research on the effects of pH on the enamel surface indicates enamel erosion from the dental surface due to low pH and high acid concentrations. The pH of 6.5 can initiate tooth demineralization and 5.5 can do enamel erosion . Distilled water is often acidic and this nature is due to the dissolution of atmospheric CO2 which produces carbonic acid. Kulthanan et al. observed a pH of 5.7 for distilled water and 7.5 for water from home water filter which is similar to tap water . So, in our study on eight types of toothpaste, dissolved in distilled water was found to have low pH while higher for tap water in all the investigated compositions.
Distilled water, as well as tap water, has the highest pH for DSS in all compositions while CU has the lowest pH. The reason for the highest pH is due to the fact that DSS contains mostly alkaline ingredients (Table S1). The pH of calcium carbonate is around 9.91, and it is 8.35 for sodium bicarbonate regardless of the concentration. The reason for the lowest pH of CU is due to the presence of some acidic natural ingredients such as PEG-32 which has a pH of about 5 [36, 37]. The carbonated drinks can damage the tooth surface due to their low pH value. So it is recommended to use DSS, DR, PD, SKH, and DH toothpaste to remove acidic strains of the tooth surface .
CaCO3 as an abrasive in the toothpaste has a profound effect in forming alkaline medium and acts as a natural buffer in the oral environment while the others have a neutral effect. Baking soda as an abrasive has a beneficial role in neutralizing pH changes in acid-forming plaque after exposure to sucrose and in the caries inhibition [39, 40]. Manufacturers formulate toothpaste to adjust pH value in the oral environment. The primary objective of toothpaste is to clean the approachable tooth surfaces with minimal damage to enamel, dentin, root surfaces, and gingival tissues due to mechanical abrasion of those surfaces. Relative abrasivity comparison of toothpaste is done by radioactive dentin abrasion (RDA) number [41, 42]. The lower the RDA number the lower will be the toothpaste’s abrasion. One cannot assume that whitening toothpaste has high RDA numbers. Many whitening kinds of toothpaste are less abrasive than anti-tartar and smoker’s toothpaste. Common ingredients of toothpaste include hydroxyapatite, xylitol, erythritol, and baking soda, which increase the pH in the mouth cavity.
The dietary acids from carbonated drinks and other beverage foods have higher potential for causing tooth erosion. The erosive effect is due to low pH, low phosphate and high fluoride concentration, and also high buffering capacity [43, 44]. On the other hand, a high pH value of the surrounding environment can cause irritation of pulp tissue and develop superficial necrosis on the exposed pulp and provoke mineralization on the necrotic zone . It also creates a hostile environment for the proliferation and survival of the bacterial pathogen. Therefore, the detection of the pH value of toothpaste products is an important part of quality monitoring. And, we can ensure the stability of the toothpaste by controlling the pH in a reasonable range to control the dissociation rate. National and international toothpaste companies often pay more attention to this point, and they consider the pH value to be an important parameter to identify the quality of the toothpaste products . Long exposure of the acidic environment gradually comes into contact with the dentine, the pain of which increases the gingival recession [47, 48]. Also, root desorption and enamel demineralization have been reported at a pH of about 5.2 .
We have also reported the antibacterial sensitivity of toothpaste against two bacterial species, S. aureus and E. coli. While members of common flora of the oral cavity such as Streptococcus mutans, Streptococcus sobrinus, and some species of Lactobacillus are the usual causative agents of dental caries and gingivitis, several members of the “non-oral” microbiota inhabit the oral microenvironment due to ecological diversification in the oral cavity . Besides, pathogenic “non-oral” bacteria such as S. aureus and E. coli are prevalent in subgingival biofilms of chronic periodontitis patients [50,51,52]. A high prevalence of periodontitis in non-smokers has also been associated with S. aureus . Therefore, the roles of these bacteria as part of the etiology of caries and periodontal diseases cannot be ruled out. S. aureus is a nosocomial pathogen most commonly associated with hospital-acquired infections as well as community-acquired infections. This study focuses on testing the inhibitory effects of the toothpaste formulations on the common but “non-oral” pathogens that are associated with chronic dental infections. S. aureus and E. coli were chosen as the common representative species of Gram-positive and Gram-negative groups, respectively. A future direction for the study may be to determine the antimicrobial effect of toothpaste formulations on specific “oral” pathogens associated with dental caries and periodontitis.
In this study, we investigated the active antimicrobial ingredients of toothpaste which can be formulated to reduce the deleterious effects of bacteria in oral flora. Most herbal-based toothpaste contains active antimicrobial ingredients to enhance antibacterial activity and is listed in Table S2. The addition of herbal extracts to toothpaste can enhance the antibacterial spectrum and can be enhanced by mixing with non-natural antibacterial agents . Fluoride-containing toothpaste such as SKH, PD, CU, and ANH enhance their effect on oral bacteria by direct inhibition of cellular enzymes and by easy permeation of hydrogen fluoride through cell membrane . In general, the toothpaste showed similar patterns of inhibition with both S. aureus and E. coli bacteria in their variable compositional concentrations.