Honestly, you’re probably doing nothing wrong, other than perhaps having expectations for your soap that are a bit too high.
To start with, let’s look at how soap cleans, because that’s the crux of matter. Soap is made up of double-ended molecules; one end loves water, and the other end loves oil. The water loving end bonds with the water you’re washing with, and the oil loving end grabs the oils on your skin. When you rinse, the water loving end hitches a ride, taking the oil-loving end and the oil it’s scavenged up with it.
So, basically, soap cleanses by removing oil from your skin. This is how all surfactant/emulsifier powered cleansers work, but bar soap is unique in that its very concentrated and by definition has a very high pH. Because soap removes oil there is no way for this to not be at least a little drying to the skin. For this reason the idea of “moisturizing” soap has always been a bit baffling to me; the core function of soap is the opposite of moisturizing—it removes moisture. We can make gentle(r) soaps, but all they can really do is remove less oil. They will not be adding it. Fundamentally, soap is still soap, and if it’s working, it is removing moisture from your skin. If you have dry skin and/or live in a dry environment and wash frequently, you will need to be following up with something else to add moisture back into your skin. Soap is not a moisturizer.
The strength of soap can be diluted/tempered in a few ways. Superfatting is the most obvious, but it is typically not advised to exceed 5–7% superfat as that can create a bar of soap that will go rancid quickly, is too soft, or begins to sport “dreaded orange spots” (DOS). Superfatting gives the oil-loving end of the soap some fat to grab onto that isn’t oil that’s already on your skin. Some of the superfat may remain on your skin, but if the soap is doing its job and cleaning, not much will. We can also dilute to some degree by including ingredients like clay, herbs, or fruit/veg purees. These are typically very small dilutions, though—the bar will still typically be 90% soap. Liquid soap can be an effective way to create a more diluted soap, but in order to get the consistency most people want (something with some body), it’s typically still fairly concentrated. If you really love soap, though, this may be an angle to try!
The high pH of soap is also worth mentioning, as it disrupts our acid mantle. I’ve written quite a lot about this here, but the general gist of it is that high pH cleansers like soap damage our skin’s protective acid mantle, and frequent use impedes its ability to repair itself. This can create and exacerbate dry skin.
Surfactant-powered cleansers can be more gentle/less drying than soap because they can be pH adjusted to a more skin-friendly pH (attempting to acidify soap will cause the soap to convert into fatty acids and cease to be soap), and because the active matter of the cleanser can be more effectively diluted. Different types of surfactants can also be blended to create a milder final product.
Posted in: Soap